Electric field measuring device and electric field measuring method

Abstract

This invention provides an electric field measuring device and method that enable the realization of a small and lightweight electric field probe, even if the electro-optic crystal of the electric field probe has natural birefringence, and that can measure high-frequency electric fields by equivalent time sampling using pulsed light. [Solution] In an electric field measuring device 1 that measures an electric field to be measured using an electro-optic crystal 28 with a polarization-maintaining fiber and photodetectors 18, 19 that detect the difference in intensity between a 0-degree polarization component and a 90-degree polarization component, the electro-optic crystal is a crystal having natural birefringence, and first and second birefringent media 29, 31 that compensate for the birefringence of the polarization-maintaining fiber and the natural birefringence of the electro-optic crystal, respectively, and first and second polarization controllers 30, 32 that adjust the phase are arranged on the light source / polarization detection unit side, and a high-frequency electric field is measured by equivalent time sampling using pulsed light.

Description

[Technical Field]

[0001] This invention relates to a technique for measuring high-frequency electric fields by an equivalent time sampling method using an electro-optic crystal having natural birefringence and pulsed light. [Background technology]

[0002] It is possible to measure electric fields by utilizing the fact that the refractive index of light changes when an electric field is applied to an electro-optic crystal. Electro-optic crystals have advantages such as being small, having high spatial resolution for electric field measurement, and having little electromagnetic disturbance because they do not contain metal parts. To measure electric fields using an electro-optic crystal, an optical system is required that converts the change in the refractive index of the electro-optic crystal into a change in the intensity of light. One method for doing this is to utilize birefringence, where the refractive index differs depending on the crystal axis of the electro-optic crystal, and convert the change in polarization due to birefringence into a change in the intensity of light.

[0003] Furthermore, if the electric field under measurement is a repeating waveform, the time waveform of the electric field can be measured by an equivalent time sampling method using pulsed light with a predetermined repetition frequency.

[0004] Specifically, by setting the repetition period of the pulsed light to be slightly different from an integer multiple of the repetition period of the electric field under test, or by setting the repetition period of the pulsed light to an integer multiple of the repetition period of the electric field under test and sweeping the relative time difference between the pulsed light and the electric field under test using a variable delay device, the time axis of the waveform of the electric field under test is expanded, and the time waveform of a high-frequency electric field, such as a millimeter wave or terahertz wave, can be obtained using a low-speed photodetector. For example, a technique has been established to generate short-pulse light with a pulse width of about 100 fs using a mode-locked fiber laser, and the waveform of an electric field in the 300 GHz band can be measured using the equivalent time sampling method.

[0005] This invention relates to electro-optic sampling, in which short pulse light is input to an electro-optic crystal, the change in polarization due to birefringence of the electro-optic crystal is converted into a change in light intensity, and the electric field is measured by the equivalent time sampling method.

[0006] In this specification, the polarization of light is represented by a Jones vector, where the imaginary number +j indicates that the phase is 90 degrees ahead of the real number +1. In a right-handed coordinate system, the direction of light propagation is defined as the positive z-axis, and the angle of the optical element is defined as 0 degrees in the positive x-axis direction and 90 degrees in the positive y-axis direction. When light travels back and forth through the same optical element, the signs of the z-axis and y-axis are reversed between the outward and return paths, so the sign of the optical element's angle is also reversed between the outward and return paths. However, the angles of the optical element shown in the figures represent the values ​​in the outward path.

[0007] Figure 1 shows the optical system for electro-optical sampling described in Non-Patent Literature 1. The pulse light source 10 outputs pulsed light with 0-degree linear polarization. The polarization beam splitter 17 (PBS1) is positioned to transmit the 0-degree linearly polarized light from the pulse light source 10, and the 0-degree linearly polarized pulsed light is transmitted from the left to the right side of the polarization beam splitter 17 and input to the left side of the polarization-maintaining fiber 12 (PMF1) whose lagging axis is in the 0-degree direction. Since the polarization direction of the pulsed light and the lagging axis direction of the polarization-maintaining fiber 12 are the same, the pulsed light propagates to the right through the polarization-maintaining fiber 12 while maintaining its 0-degree linear polarization.

[0008] The pulsed light output from the right side of the polarization-maintaining fiber 12 passes through the half-wave plate 13 (HWP1) with a lagging axis of 11.25 degrees and the quarter-wave plate 14 (QWP1) with a lagging axis of 45 degrees, and is input to the left side of the electro-optic crystal 15 (EO1) with its electrical principal axis at 0 degrees, and propagates to the right through the electro-optic crystal 15. The birefringence of the electro-optic crystal 15 changes depending on the electric field being measured applied to the electro-optic crystal 15. There is a mirror 16 that reflects light on the right end face of the electro-optic crystal 15, and the pulsed light is reflected by the mirror 16 and propagates in the reverse direction through the electro-optic crystal 15, passing through the quarter-wave plate 14 and the half-wave plate 13, propagating to the left through the polarization-maintaining fiber 12, and reaching the polarization beam splitter 17.

[0009] The polarizing beam splitter 17 reflects the 90-degree polarized component, and the reflected 90-degree polarized component is input to the photodetector 18 (PD1). In other words, the polarization separation unit 11, consisting of the polarizing beam splitter 17, outputs the 0-degree linearly polarized light input to the first end from the second end, and outputs the 90-degree polarized component of the light input to the second end from the third end. Since the photodetector 18 outputs an electrical signal proportional to the intensity of the input light, the intensity of the 90-degree polarized component of the light output from the left side of the polarization-maintaining fiber 12 can be obtained.

[0010] Figure 2 shows the configuration of differential detection for electro-optic sampling described in Non-Patent Literature 1. The pulse light source 10 outputs pulsed light with 0-degree linear polarization. The polarizing beam splitter 20 (PBS2) is positioned to transmit the 0-degree linearly polarized light from the pulse light source 10. The 0-degree linearly polarized pulsed light passes through the polarizing beam splitter 20 from left to right and is input to the Faraday rotator 21 (FR). As the pulsed light passes through the Faraday rotator 21 from left to right, the polarization rotates by +45 degrees and is returned to 0-degree linear polarization by the half-wave plate 22 (HWP2) with a lagging axis in the 22.5-degree direction.

[0011] The configuration to the right of the polarization beam splitter 17 is the same as in Figure 1. A pulse of 0-degree linearly polarized light is transmitted from the left to the right of the polarization beam splitter 17, and is input to the electro-optic crystal 15 via the polarization-maintaining fiber 12, half-wave plate 13, and quarter-wave plate 14. The pulse of light reflected by the mirror 16 propagates in the reverse direction through the electro-optic crystal 15, as in Figure 1, and reaches the polarization beam splitter 17 via the quarter-wave plate 14, half-wave plate 13, and polarization-maintaining fiber 12.

[0012] In the polarizing beam splitter 17, the 90-degree polarized component is reflected and input to the photodetector 18, while the 0-degree polarized component is transmitted to the left. The 0-degree polarized component that has passed through the polarizing beam splitter 17 is linearly polarized to -45 degrees by the half-wave plate 22 with a lagging axis of -22.5 degrees (return path), and as it passes through the Faraday rotator 21 from right to left, the polarization rotates by -45 degrees, becoming linearly polarized to -90 degrees.

[0013] The polarization beam splitter 20 reflects the -90-degree polarization component, and the reflected -90-degree polarization component is input to the photodetector 19 (PD2). In other words, the polarization separation unit 11A, consisting of the polarization beam splitter 20, Faraday rotator 21, half-wave plate 22, and polarization beam splitter 17, outputs the 0-degree linearly polarized light input to the first end from the second end, outputs the 0-degree polarization component of the light input to the second end from the fourth end, and outputs the 90-degree polarization component of the light input to the second end from the fifth end.

[0014] The photodetectors 18 and 19 each output electrical signals proportional to the intensity of the input light. By taking the difference between the output of photodetector 19 and the output of photodetector 18 using the differential amplifier 23, the difference between the intensity of the 0-degree polarized component and the intensity of the 90-degree polarized component of the light output from the left side of the polarization-maintaining fiber 12 is obtained.

[0015] As shown in Figure 3, differential detection can also be achieved using a polarization-maintaining optical circulator 24 (CIR). 0-degree linearly polarized pulsed light output from the pulse light source 10 is input to the first end of the optical circulator 24, and is output from the second end of the optical circulator 24 while maintaining its polarization. The 0-degree linearly polarized pulsed light is input to the left side of the polarization-maintaining fiber 12. The configuration to the right of the polarization-maintaining fiber 12 is the same as in Figure 2. The pulsed light is input to the electro-optic crystal 15 via the polarization-maintaining fiber 12, half-wave plate 13, and quarter-wave plate 14. It is reflected by the mirror 16 and propagates in the reverse direction through the electro-optic crystal 15, passing through the quarter-wave plate 14, half-wave plate 13, and polarization-maintaining fiber 12 before being input to the second end of the optical circulator 24. The pulsed light input to the second end of the optical circulator 24 is output from the third end of the optical circulator 24 while maintaining its polarization and reaches the first end of the polarization beam splitter 17.

[0016] The polarization beam splitter 17 reflects the 90-degree polarization component of the light input to the first end, outputs it from the fourth end, and inputs it to the light receiver 18, and transmits the 0-degree polarization component, outputs it from the third end, and inputs it to the light receiver 19. The light receivers 18 and 19 output electrical signals proportional to the intensity of the input light, respectively. By taking the difference between the output of the light receiver 19 and the output of the light receiver 18 by the differential amplifier 23, the difference between the intensity of the 0-degree polarization component and the intensity of the 90-degree polarization component of the light output from the left side of the polarization-maintaining fiber 12 can be obtained as in FIG. 2.

[0017] The electric field of the pulsed light is represented by the Jones vector, and the propagation characteristics of each optical component are represented by the Jones matrix to obtain the electric field of the light output from the left side of the polarization-maintaining fiber 12. Let the electric field of the pulsed light output from the pulsed light source 10 be E0, the delay time difference between the slow axis and the fast axis due to the birefringence of the polarization-maintaining fiber 12 be τ , , , , , , 11y , , , , 11x , , 11 , , , e , , ,

[0019] , 2 , , 2 , , , e , , ,

[0018] , , e 、the optical phase difference between the slow axis and the fast axis due to the birefringence of the polarization-maintaining fiber 12 be φ p 、the optical phase difference (round trip) between the slow axis and the fast axis due to the application of the electric field to the electro-optic crystal 15 be φ e 、assuming the angular frequency of the light is ω, the electric field E of the light output from the left side of the polarization-maintaining fiber 12 11 is as follows.

[0018]

Equation

Equation

Equation

Equation

[0019] |φ e |≪1, then |E 11x | 2 -|E 11y | 2 ≒φ e and φ can be obtained by the differential detection in FIG. 2 or FIG. 3 eAn output proportional to φ is obtained. In the single-ended configuration shown in Figure 1, φ e The result is a value obtained by adding an offset to a term that is proportional to [the given value].

[0020] Furthermore, as shown in Figure 4, Patent Document 1 describes a method using a quarter-wave plate 25 (QWP2) with a lagging axis of 22.5 degrees and an electro-optic crystal 26 (EO2) with an electrical principal axis of -22.5 degrees. Similarly, the electric field E of the light output from the left side of the polarization-maintaining fiber 12 is described in the same manner. 12 When we find this, we get the following equation.

[0021]

number

number

number

[0022] |φ e |≪1|E 12x | 2 -|E 12y | 2 ≒φ e Thus, similar to the configuration in Non-Patent Document 1, differential detection is used to obtain φ e A proportional output is obtained, and the number of half-wave plates can be reduced compared to the configuration in Non-Patent Document 1.

[0023] The methods in Figures 1, 2, 3, and 4 assume an electro-optic crystal that does not exhibit natural birefringence. The maximum slope of the sine function in equations (4) and (7), i.e., the maximum sensitivity, occurs when the natural birefringence of the electro-optic crystal is zero. When the natural birefringence of the electro-optic crystal is zero, the light intensity |E 11 | 2 ,|E 12 | 2 Since it does not exhibit wavelength dependence, similar output can be obtained even with short-pulse light having numerous line spectral components, making it possible to measure high-frequency electric fields using the equivalent time sampling method with short-pulse light.

[0024] On the other hand, as a method assuming an electro-optic crystal with natural birefringence, the configuration shown in Figure 5 is described in Patent Document 2. In the configuration of Figure 5, a wavelength-tunable CW light source 27 is used, and an electro-optic crystal 28 (EO3) with its electrical principal axis at a 45-degree angle is arranged. The delay time difference (round trip) between the slow axis and the fast axis due to the natural birefringence of the electro-optic crystal 28 is τ d The phase difference (round trip) of light between the slow axis and the fast axis due to the natural birefringence of the electro-optic crystal 28 is φ d The phase difference (round trip) of light between the slow axis and the fast axis due to the application of an electric field to the electro-optic crystal 28 is φ e Therefore, the electric field E of the light output from the left side of the polarization-maintaining fiber 12 13 The equation is as follows:

[0025]

number

number

number

[0026] Here, ωτ d +φ d If we adjust the wavelength of light so that it equals -π / 2, then |E 13x | 2 -|E 13y | 2 =sin(φ e ) Therefore, |φ e |≪1|E 13x | 2 -|E 13y | 2 ≒φ e Therefore, differential detection is performed by φ e An output proportional to this is obtained.

[0027] Furthermore, if an electro-optic crystal that does not exhibit natural birefringence is used in the configuration shown in Figure 5, |E 13x | 2 -|E 13y | 2 =cos(φ e) Therefore, the slope of the cosine function is zero, meaning the sensitivity is zero, and φ e The output is not proportional to the value.

[0028] In Figures 1 and 2, a polarization-maintaining fiber 12 with a lagging axis of 0 degrees is inserted between the polarizing beam splitter 17 and the half-wave plate 13; in Figure 3, between the optical circulator 24 and the half-wave plate 13; in Figure 4, between the optical circulator 24 and the quarter-wave plate 25; and in Figure 5, between the optical circulator 24 and the electro-optic crystal 28. The forward pulse light is linearly polarized at 0 degrees, so its polarization state does not change even when passing through the polarization-maintaining fiber 12. The return pulse light is not linearly polarized at 0 degrees, so when it passes through the polarization-maintaining fiber 12, the relative phase between the 0-degree polarized component and the 90-degree polarized component changes due to the birefringence of the polarization-maintaining fiber 12. However, since the photodetectors 18 and 19 only detect the intensity of the 90-degree polarized component and the 0-degree polarized component, respectively, no problems arise due to the birefringence of the polarization-maintaining fiber 12. This allows the polarizing beam splitter 17 or optical circulator 24 to be separated into a light source / polarization detection unit on the left and an electric field probe unit on the right from the polarization-maintaining fiber 12. The two are connected by a flexible polarization-maintaining fiber 12, and the electric field probe unit is small and lightweight, allowing it to be placed at any position for electric field measurement. [Prior art documents] [Patent Documents]

[0029] [Patent Document 1] Japanese Patent Publication No. 2010-14579 [Patent Document 2] Japanese Patent Publication No. 2005-214892 [Non-patent literature]

[0030] [Non-Patent Document 1] A. Sasaki and T. Nagatsuma, "Millimeter-Wave Imaging Using an Electrooptic Detector as a Harmonic Mixer," IEEE Journal of Selected Topics in Quantum Electronics, vol. 6, no. 5, pp. 735-740, 2000. [Overview of the project] [Problems that the invention aims to solve]

[0031] In conventional methods (Figures 2, 3, and 4 for differential detection) that assume an electro-optic crystal without natural birefringence, when an electro-optic crystal with natural birefringence is used, ωτ d +φ d With the addition of the term, the output signal becomes as follows:

[0032]

number

[0033] Short pulses of light with a predetermined repetition frequency consist of numerous line spectral components with different optical frequencies, and since equation (11) is a function of the optical angular frequency ω, the output signal with respect to the electric field differs depending on each line spectral component. For example, if the spectral width of the short pulse light is 3 THz, and the delay time difference τ due to the natural birefringence of the electro-optic crystal is... d If we define this as 1 ps, then the spectral width Δω expressed in terms of angular frequency and the delay time difference τ are given. d The product Δωτ d The frequency is approximately 19 rad, and because the output signal relative to the electric field takes on various positive and negative values ​​based on trigonometric functions due to each line spectral component, they cancel each other out, and an output signal proportional to the electric field is almost impossible to obtain. In order to obtain a sufficient output signal proportional to the electric field, the spectral width Δω must be Δωτ d The pulse width needs to be increased or converted to CW light to satisfy π. And ω0τ d +φ dThe center frequency ω0 needs to be adjusted so that it equals nπ (where n is an integer).

[0034] In the configuration shown in Figure 5, the output signal is expressed as a trigonometric function including the optical angular frequency ω, as in equation (10). When short-pulse light is used, an output signal proportional to the electric field is almost lost, and similarly, the spectral width Δω becomes Δωτ. d <<Increase the pulse width to satisfy π, or use CW light, ω0τ d +φ d The center frequency ω0 needs to be adjusted so that it equals nπ + π / 2 (where n is an integer).

[0035] The pulse width of pulsed light determines the time resolution in equivalent time sampling. Therefore, increasing the pulse width lowers the upper limit of the frequency of the electric field under measurement, making it impossible to measure high-frequency electric fields. Furthermore, using CW light instead of pulsed light eliminates the need for equivalent time sampling, limiting the measurement bandwidth to that of the photodetector and the subsequent electrical system, making high-frequency electric field measurement difficult.

[0036] It is conceivable to incorporate an optical element into the electric field probe to compensate for the natural birefringence of the electro-optic crystal. However, while it is possible to roughly compensate for the delay time difference due to natural birefringence, it is difficult to perfectly match the natural birefringence of the electro-optic crystal with the birefringence of the compensating optical element, making it difficult to compensate for the phase difference of light due to natural birefringence. Delay time difference τ due to natural birefringence d Even if we roughly compensate for this, the phase difference of light φ d If it remains, φ d This changes the sensitivity of the electric field measurement, for example, in equation (11) ωτ d =0,φ d If =π / 2, then |E 14x | 2 -|E 14y | 2 =cos(φ e This resulted in a problem where the slope of the cosine function became zero, meaning the sensitivity of the electric field measurement became zero.

[0037] Therefore, in addition to an optical element that roughly compensates for the delay time difference due to natural birefringence, a mechanism to adjust the phase difference of light is necessary. However, incorporating these into an electric field probe makes the structure complex, which prevents the creation of a small and lightweight electric field probe.

[0038] The present invention has been made to solve the above-mentioned problems, and aims to provide an electric field measuring device and electric field measuring method that can realize a small and lightweight electric field probe even if the electro-optic crystal of the electric field probe has natural birefringence, and that can measure high-frequency electric fields by equivalent time sampling with pulsed light. [Means for solving the problem]

[0039] To achieve the above objective, the electric field measuring device according to the present invention includes a light source (10) that outputs linearly polarized light, an optical circulator (24) that outputs the linearly polarized light input to the first end from the second end and outputs the light input to the second end from the third end, a polarization-maintaining fiber (12) into which the light output from the second end of the optical circulator is input to one end such that the slow axis or fast axis coincides with the polarization direction of the light output from the second end of the optical circulator, an electro-optic crystal (28) into which the light output from the other end of the polarization-maintaining fiber is input to one end such that the electrical principal axis is at a 45-degree angle with the slow axis of the polarization-maintaining fiber, and a mirror (16) provided at the other end of the electro-optic crystal that reflects the light input to one end of the electro-optic crystal. A polarizing beam splitter (17) that outputs either the linear polarization component in the slow axis direction of the polarization-maintaining fiber or the linear polarization component in the fast axis direction of the polarization-maintaining fiber from the second end, or both from the third and fourth ends, respectively, of the light output from the second end of the polarization-maintaining fiber and the linear polarization component in the fast axis direction of the polarization-maintaining fiber of the light output from the second end, and a photodetector (18) that detects the intensity of the light output from the second end of the polarizing beam splitter, or the difference in intensity of the light output from the third and fourth ends of the polarizing beam splitter.19) An electric field measuring device having and for measuring an electric field applied to an electro-optic crystal, wherein the light source is a pulse light source that outputs pulse light of a predetermined repetition frequency, the electro-optic crystal is a crystal having natural birefringence, and between the third end of the optical circulator and the first end of the polarizing beam splitter, there is a first birefringent medium (29) that compensates for the delay time difference between the slow axis and the fast axis due to the birefringence of the polarization-holding fiber, a first polarization controller (30) that adjusts the phase difference of light between the slow axis and the fast axis of the first birefringent medium, and the slow axis of the second birefringent medium is at a 45-degree angle with the slow axis of the first birefringent medium and compensates for the delay time difference between the slow axis and the fast axis due to the natural birefringence of the round trip portion of the electro-optic crystal, and the slow axis of the second birefringent medium The optical circulator comprises a second polarization controller (32) for adjusting the phase difference of light between the optical axis and the speed axis, a quarter-wave plate (33) whose slow axis is in the same direction as the slow axis or speed axis of the polarization-maintaining fiber, and a half-wave plate (34) whose slow axis is at a 22.5-degree angle with the slow axis or speed axis of the polarization-maintaining fiber, wherein the first birefringent medium and the first polarization controller are positioned on the third end side of the optical circulator than the second birefringent medium and the second polarization controller, the quarter-wave plate is positioned on the first end side of the polarizing beam splitter than the second birefringent medium and the second polarization controller, and the half-wave plate is positioned on the first end side of the polarizing beam splitter than the quarter-wave plate, and the electric field is measured by equivalent time sampling.

[0040] With this configuration, the electric field measuring device of the present invention, when using an electro-optic crystal having natural birefringence, compensates for the natural birefringence of the electro-optic crystal, enabling equivalent time sampling with pulsed light of a wide spectral width, i.e., a short pulse width. As a result, the upper limit of the frequency of the electric field under measurement is increased, allowing for high-frequency electric field measurements.

[0041] Furthermore, by arranging an optical element that compensates for the natural birefringence of the electro-optic crystal on the light source / polarization detection unit side, it becomes easy to incorporate a second polarization controller that adjusts the phase difference of light, in addition to a second birefringent medium that roughly compensates for the delay time difference due to natural birefringence. This allows for maximum sensitivity of electric field measurement by compensating for the phase difference of light due to natural birefringence, while the electric field probe unit remains small and lightweight, enabling electric field measurement at any position, similar to conventional technology.

[0042] To achieve the above objective, the electric field measuring device according to the present invention includes a light source (10) that outputs linearly polarized light, an optical circulator (24) that outputs the linearly polarized light input to the first end from the second end and outputs the light input to the second end from the third end, a polarization-maintaining fiber (12) into which the light output from the second end of the optical circulator is input to one end such that the slow axis or fast axis coincides with the polarization direction of the light output from the second end of the optical circulator, an electro-optic crystal (28) into which the light output from the other end of the polarization-maintaining fiber is input to one end such that the electrical principal axis is at a 45-degree angle with the slow axis of the polarization-maintaining fiber, and a mirror (16) provided at the other end of the electro-optic crystal that reflects the light input to one end of the electro-optic crystal. A polarizing beam splitter (17) that outputs either the linear polarization component in the slow axis direction of the polarization-maintaining fiber or the linear polarization component in the fast axis direction of the polarization-maintaining fiber from the second end, or both from the third and fourth ends, respectively, of the light output from the second end of the polarization-maintaining fiber and the linear polarization component in the fast axis direction of the polarization-maintaining fiber of the light output from the second end, and a photodetector (18) that detects the intensity of the light output from the second end of the polarizing beam splitter, or the difference in intensity of the light output from the third and fourth ends of the polarizing beam splitter.19) An electric field measuring device having and for measuring an electric field applied to an electro-optic crystal, wherein the light source is a pulse light source that outputs pulse light of a predetermined repetition frequency, the electro-optic crystal is a crystal having natural birefringence, and between the third end of the optical circulator and the first end of the polarizing beam splitter, there is a first birefringent medium (29) that compensates for the delay time difference between the slow axis and the fast axis due to the birefringence of the polarization-holding fiber, and a first polarization controller (30) that adjusts the phase difference of light between the slow axis and the fast axis of the first birefringent medium, and the slow axis is at a 45-degree angle with the slow axis of the first birefringent medium, and the delay time difference between the slow axis and the fast axis due to the natural birefringence of the round trip portion of the electro-optic crystal The optical circulator comprises a second birefringent medium (31) that compensates for the first birefringent medium, and a second polarization controller (38) that adjusts the phase difference of light between the slow axis and the fast axis of the second birefringent medium, wherein the first birefringent medium and the first polarization controller are positioned on the third end side of the optical circulator than the second birefringent medium and the second polarization controller, and the second polarization controller is set such that when the electric field is zero, the intensity of the light output from the second end of the polarizing beam splitter is half of the maximum value, or the difference in intensity of the light output from the third and fourth ends of the polarizing beam splitter is zero, and the electric field is measured by equivalent time sampling.

[0043] With this configuration, the electric field measuring device of the present invention, when using an electro-optic crystal having natural birefringence, compensates for the natural birefringence of the electro-optic crystal, enabling equivalent time sampling with pulsed light of a wide spectral width, i.e., a short pulse width. As a result, the upper limit of the frequency of the electric field under measurement is increased, allowing for high-frequency electric field measurements.

[0044] Furthermore, by arranging an optical element that compensates for the natural birefringence of the electro-optic crystal on the light source / polarization detection unit side, it becomes easy to incorporate a second polarization controller that adjusts the phase difference of light, in addition to a second birefringent medium that roughly compensates for the delay time difference due to natural birefringence. This allows for maximum sensitivity of electric field measurement by compensating for the phase difference of light due to natural birefringence, while the electric field probe unit remains small and lightweight, enabling electric field measurement at any position, similar to conventional technology.

[0045] Furthermore, since the compensation for the phase difference of light due to the natural birefringence of the electro-optic crystal and the polarization change for differential detection are realized by a single polarization controller 38, the above effects can be obtained with a simple configuration.

[0046] Either or both of the first birefringent medium and the first polarization controller may be arranged between the second end of the optical circulator and one end of the polarization-maintaining fiber.

[0047] With this configuration, the electric field measuring device of the present invention includes, for example, a first birefringent medium as the electric field probe section, and when the first birefringent medium and the first polarization controller are connected by an optical connector, the birefringence of the polarization-maintaining fiber is largely compensated in the electric field probe section, making it easy to replace the electric field probe section with one having a polarization-maintaining fiber of various lengths.

[0048] To achieve the above objective, the electric field measuring device according to the present invention includes a light source (10) that outputs linearly polarized light, a polarization separation unit (11A) that outputs the linearly polarized light input to the first end from the second end, and outputs either the linearly polarized component in the slow axis direction of the polarization-maintaining fiber and the linearly polarized component in the fast axis direction of the polarization-maintaining fiber of the light input to the second end from the third end, or both from the fourth and fifth ends, respectively, and a polarization-maintaining fiber (12) into which the light output from the second end of the polarization separation unit is input to one end, such that the slow axis or fast axis matches the polarization direction of the light output from the second end of the polarization separation unit, and An electro-optic crystal (28) into which light output from the other end of the polarization-maintaining fiber is input to one end, such that the target principal axis forms a 45-degree angle with the retard axis of the polarization-maintaining fiber; a mirror (16) provided at the other end of the electro-optic crystal that reflects the light input to one end of the electro-optic crystal; and a light receiver (18,19) An electric field measuring device having and for measuring an electric field applied to an electro-optic crystal, wherein the light source is a pulse light source that outputs pulse light of a predetermined repetition frequency, the electro-optic crystal is a crystal having natural birefringence, and between the second end of the polarization separation unit and one end of the polarization-maintaining fiber, there is a first birefringent medium (29) that compensates for the delay time difference between the slow axis and the fast axis due to the birefringence of the polarization-maintaining fiber, and a first polarization controller (30) that adjusts the phase difference of light between the slow axis and the fast axis of the first birefringent medium, and the slow axis is at a 45-degree angle with the slow axis of the first birefringent medium, and the delay time between the slow axis and the fast axis due to the natural birefringence of the electro-optic crystal for one path The first polarization-maintaining fiber is characterized by comprising a second birefringent medium (31) for compensating for the difference and a second polarization controller (38) for adjusting the phase difference of light between the slow axis and the fast axis of the second birefringent medium, wherein the first birefringent medium and the first polarization controller are positioned on one end side of the polarization-maintaining fiber than the second birefringent medium and the second polarization controller, and the second polarization controller is set such that when the electric field is zero, the intensity of the light output from the third end of the polarization-separating unit is half of the maximum value, or the difference in intensity of the light output from the fourth and fifth ends of the polarization-separating unit is zero, and the electric field is measured by equivalent time sampling.

[0049] With this configuration, the electric field measuring device of the present invention, when using an electro-optic crystal having natural birefringence, compensates for the natural birefringence of the electro-optic crystal, enabling equivalent time sampling with pulsed light of a wide spectral width, i.e., a short pulse width. As a result, the upper limit of the frequency of the electric field under measurement is increased, allowing for high-frequency electric field measurements.

[0050] Furthermore, by arranging an optical element that compensates for the natural birefringence of the electro-optic crystal on the light source / polarization detection unit side, it becomes easy to incorporate a second polarization controller that adjusts the phase difference of light, in addition to a second birefringent medium that roughly compensates for the delay time difference due to natural birefringence. This allows for maximum sensitivity of electric field measurement by compensating for the phase difference of light due to natural birefringence, while the electric field probe unit remains small and lightweight, enabling electric field measurement at any position, similar to conventional technology.

[0051] Furthermore, if the field probe section includes, for example, a first birefringent medium, and the first birefringent medium and the first polarization controller are connected by an optical connector, the birefringence of the polarization-maintaining fiber is largely compensated for in the field probe section, making it easy to replace the field probe section with one that has polarization-maintaining fibers of various lengths.

[0052] To achieve the above objective, the electric field measurement method according to the present invention outputs a first linearly polarized light, inputs the first light to one end of the polarization-maintaining fiber (12) so that the polarization direction of the first light coincides with the slow axis or fast axis of the polarization-maintaining fiber (12), inputs the light output from the other end of the polarization-maintaining fiber to one end of the electro-optic crystal (28) so that the polarization direction of the light output from the other end of the polarization-maintaining fiber coincides with the electrical principal axis of the electro-optic crystal (28), and reflects the light input to one end of the electro-optic crystal by a mirror (16) provided at the other end of the electro-optic crystal. In an electric field measurement method, the electric field applied to the electro-optic crystal is measured by outputting light from one end of the electro-optic crystal, inputting the light output from one end of the electro-optic crystal to the other end of the polarization-maintaining fiber, and detecting the difference between either or both of the intensity of the linear polarization component in the slow axis direction of the polarization-maintaining fiber and the intensity of the linear polarization component in the fast axis direction of the polarization-maintaining fiber of the light output from one end of the polarization-maintaining fiber, wherein the first light is pulsed light with a predetermined repetition frequency, the electro-optic crystal is a crystal having natural birefringence, and the polarization-maintaining Before detecting the difference between the intensity of the linear polarization component in the slow axis direction and the linear polarization component in the fast axis direction of the polarization-maintaining fiber, or both, for light output from one end of the polarization-maintaining fiber, a first birefringent medium (29) is used to compensate for the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber, a first polarization controller (30) is used to adjust the phase difference of the light between the slow axis and the fast axis of the first birefringent medium, and a second birefringent medium (31) whose slow axis is at a 45-degree angle with the slow axis of the first birefringent medium is used for the round trip portion of the electro-optic crystal. The delay time difference between the slow axis and the fast axis due to natural birefringence is compensated, and the phase difference of the light between the slow axis and the fast axis of the second birefringent medium is adjusted using a second polarization controller (32), and the compensation of the delay time difference by the first birefringent medium and the adjustment of the phase difference by the first polarization controller are performed before the compensation of the delay time difference by the second birefringent medium and the adjustment of the phase difference by the second polarization controller, and the light output from one end of the polarization-maintaining fiber passes through the first birefringent medium, the first polarization controller, the second birefringent medium, and the second polarization controller,The method is characterized by measuring the electric field by equivalent time sampling, passing sequentially through a quarter-wave plate (33) whose slow axis is in the same direction as the slow or fast axis of the polarization-maintaining fiber, and a half-wave plate (34) whose slow axis is at a 22.5-degree angle with the slow or fast axis of the polarization-maintaining fiber.

[0053] This configuration allows the electric field measurement method of the present invention to compensate for the natural birefringence of the electro-optic crystal when using an electro-optic crystal with natural birefringence, thereby enabling equivalent time sampling with pulsed light of a wide spectral width, i.e., a short pulse width. As a result, the upper limit of the frequency of the electric field under measurement is increased, enabling high-frequency electric field measurements.

[0054] Furthermore, by compensating for the natural birefringence of the electro-optic crystal after the light is output from one end of the polarization-maintaining fiber, in addition to coarse compensation for the delay time difference by the second birefringent medium, the adjustment of the phase difference of the light by the second polarization controller becomes easier. This compensates for the phase difference of the light due to natural birefringence, achieving maximum sensitivity for electric field measurement, while the electric field probe side remains small and lightweight, enabling electric field measurement at any position, similar to conventional technology.

[0055] The electric field measurement method according to the present invention outputs linearly polarized CW light, inputs the CW light to one end of the polarization-maintaining fiber so that the polarization direction of the CW light coincides with the slow axis or speed axis of the polarization-maintaining fiber, inputs the light output from the other end of the polarization-maintaining fiber to one end of the electro-optic crystal so that the polarization direction of the light output from the other end of the polarization-maintaining fiber forms a 45-degree angle with the electrical principal axis of the electro-optic crystal, reflects the light input to one end of the electro-optic crystal using the mirror and outputs it from one end of the electro-optic crystal, inputs the light output from one end of the electro-optic crystal to the other end of the polarization-maintaining fiber, and adjusts the wavelength of the CW light so that the intensity of the linear polarization component in the slow axis direction of the polarization-maintaining fiber is equal to the intensity of the linear polarization component in the speed axis direction of the polarization-maintaining fiber, and the polarization direction of the CW light coincides with the slow axis or speed axis of the polarization-maintaining fiber. As described above, the wavelength-adjusted CW light is input to one end of the polarization-maintaining fiber, and the light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle, the light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal, and the light output from one end of the electro-optic crystal is The light input to the other end of the polarization-maintaining fiber and output from one end of the polarization-maintaining fiber is compensated for by the first birefringent medium to compensate for the delay time difference between the slow axis and the fast axis due to the birefringence of the polarization-maintaining fiber, and the phase difference of the light between the slow axis and the fast axis of the first birefringent medium is changed by the first polarization controller, and then the slow axis is in the same direction as the slow axis or fast axis of the polarization-maintaining fiber, and the quarter-wave plate is in the same direction as the slow axis or fast axis of the polarization-maintaining fiber.The first polarization controller is adjusted so that the light passing through the half-wave plate forming a 5-degree angle becomes linearly polarized in the same direction as the slow axis or fast axis of the polarization-maintaining fiber, and a second linearly polarized light is output. The second light is input to one end of the polarization-maintaining fiber so that the polarization direction of the second light coincides with the slow axis or fast axis of the polarization-maintaining fiber. The light output from the other end of the polarization-maintaining fiber is then processed so that the polarization direction of the light output from the other end of the polarization-maintaining fiber forms a 45-degree angle with the electrical principal axis of the electro-optic crystal. Light is input to one end of an optical crystal, the light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal, the light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber, the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is compensated for with respect to the light output from one end of the polarization-maintaining fiber using the first birefringent medium, and the phase difference of light between the slow axis and the fast axis of the first birefringent medium is compensated for with respect to the light output from one end of the polarization-maintaining fiber using the first birefringent medium according to the result of adjusting the first polarization controller The following changes are made: the delay time difference between the slow axis and the fast axis due to the natural birefringence of the electro-optic crystal for the round trip is compensated using the second birefringent medium; the phase difference of the light between the slow axis and the fast axis of the second birefringent medium is changed using the second polarization controller; the compensation for the delay time difference by the first birefringent medium and the change of the phase difference by the first polarization controller are performed before the compensation for the delay time difference by the second birefringent medium and the change of the phase difference by the second polarization controller; and the light output from one end of the polarization-maintaining fiber is compared with the first birefringent medium. Alternatively, the configuration may involve adjusting the second polarization controller so that, after passing through the first polarization controller, the second birefringent medium, and the second polarization controller, the light becomes linearly polarized in the same direction as the slow axis or fast axis of the polarization-maintaining fiber. The adjustment of the phase difference of light between the slow axis and fast axis of the first birefringent medium using the first polarization controller follows the result of the adjustment of the first polarization controller, and the adjustment of the phase difference of light between the slow axis and fast axis of the second birefringent medium using the second polarization controller follows the result of the adjustment of the second polarization controller.

[0056] With this configuration, the electric field measurement method of the present invention makes it possible to easily adjust the polarization controller, which is used to obtain the maximum sensitivity of electric field measurement by compensating for the phase difference of light due to the natural birefringence of the electro-optic crystal, by adjusting it based on the light intensity when the electric field under measurement is zero.

[0057] To achieve the above objective, the electric field measurement method according to the present invention outputs a first linearly polarized light, inputs the first light to one end of the polarization-maintaining fiber (12) so that the polarization direction of the first light coincides with the slow axis or fast axis of the polarization-maintaining fiber (12), inputs the light output from the other end of the polarization-maintaining fiber to one end of the electro-optic crystal (28) so that the polarization direction of the light output from the other end of the polarization-maintaining fiber coincides with the electrical principal axis of the electro-optic crystal (28), and uses a mirror (16) provided at the other end of the electro-optic crystal to measure the electro-optic crystal. In an electric field measurement method, the electric field applied to the electro-optic crystal is measured by reflecting light input to one end and outputting it from one end of the electro-optic crystal, inputting the light output from one end of the electro-optic crystal to the other end of the polarization-maintaining fiber, and detecting the difference between either or both of the intensity of the linear polarization component in the slow axis direction of the polarization-maintaining fiber and the linear polarization component in the fast axis direction of the polarization-maintaining fiber of the light output from one end of the polarization-maintaining fiber, the first light is pulsed light with a predetermined repetition frequency, and the electro-optic The crystal is a crystal that exhibits natural birefringence. Before detecting the difference between either or both the intensity of the linear polarization component in the slow axis direction and the linear polarization component in the fast axis direction of the polarization-maintaining fiber with respect to the light output from one end of the polarization-maintaining fiber, the first birefringent medium (29) is used to compensate for the delay time difference between the slow axis and the fast axis due to the birefringence of the polarization-maintaining fiber. The first polarization controller (30) is used to adjust the phase difference of the light between the slow axis and the fast axis of the first birefringent medium, so that the slow axis is the same as the slow axis of the first birefringent medium. The method is characterized in that a second birefringent medium (31) forming a 45-degree angle is used to compensate for the delay time difference between the slow axis and the fast axis due to the natural birefringence of the electro-optic crystal during the round trip, and a second polarization controller (38) is used to adjust the phase difference of light between the slow axis and the fast axis of the second birefringent medium, and the compensation for the delay time difference by the first birefringent medium and the adjustment of the phase difference by the first polarization controller are performed before the compensation for the delay time difference by the second birefringent medium and the adjustment of the phase difference by the second polarization controller, and the electric field is measured by equivalent time sampling.

[0058] This configuration allows the electric field measurement method of the present invention to compensate for the natural birefringence of the electro-optic crystal when using an electro-optic crystal with natural birefringence, thereby enabling equivalent time sampling with pulsed light of a wide spectral width, i.e., a short pulse width. As a result, the upper limit of the frequency of the electric field under measurement is increased, enabling high-frequency electric field measurements.

[0059] Furthermore, by compensating for the natural birefringence of the electro-optic crystal after the light is output from one end of the polarization-maintaining fiber, in addition to coarse compensation for the delay time difference by the second birefringent medium, the adjustment of the phase difference of the light by the second polarization controller becomes easier. This compensates for the phase difference of the light due to natural birefringence, achieving maximum sensitivity for electric field measurement, while the electric field probe side remains small and lightweight, enabling electric field measurement at any position, similar to conventional technology.

[0060] Furthermore, since the compensation for the phase difference of light due to the natural birefringence of the electro-optic crystal and the polarization change for differential detection are performed simultaneously by a single polarization controller 38, the above effects can be obtained in a simple manner.

[0061] The electric field measurement method according to the present invention outputs linearly polarized CW light, inputs the CW light to one end of the polarization-maintaining fiber so that the polarization direction of the CW light matches the slow axis or speed axis of the polarization-maintaining fiber, inputs the light output from the other end of the polarization-maintaining fiber to one end of the electro-optic crystal so that the polarization direction of the light output from the other end of the polarization-maintaining fiber matches the electrical principal axis of the electro-optic crystal at a 45-degree angle, reflects the light input to one end of the electro-optic crystal using the mirror and outputs it from one end of the electro-optic crystal, inputs the light output from one end of the electro-optic crystal to the other end of the polarization-maintaining fiber, and adjusts the wavelength of the CW light so that the intensity of the linear polarization component in the slow axis direction of the polarization-maintaining fiber is equal to the intensity of the linear polarization component in the speed axis direction of the polarization-maintaining fiber, so that the polarization direction of the CW light matches the slow axis or speed axis of the polarization-maintaining fiber. The wavelength-adjusted CW light is input to one end of the polarization-maintaining fiber, and the light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle, the light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal, and the light output from one end of the electro-optic crystal is polarized The light input to the other end of the fiber and output from one end of the polarization-maintaining fiber is compensated for by the first birefringent medium to compensate for the delay time difference between the slow axis and the fast axis due to the birefringence of the polarization-maintaining fiber, and the phase difference of the light between the slow axis and the fast axis of the first birefringent medium is changed by the first polarization controller. Then, the slow axis is in the same direction as the slow axis or fast axis of the polarization-maintaining fiber, and the slow axis is in the same direction as the slow axis or fast axis of the polarization-maintaining fiber.The first polarization controller is adjusted so that the light passing through the first half-wave plate (34) forming a 5-degree angle becomes linearly polarized in the same direction as the slow axis or fast axis of the polarization-maintaining fiber, and a second linearly polarized light is output. The second light is input to one end of the polarization-maintaining fiber so that the polarization direction of the second light coincides with the slow axis or fast axis of the polarization-maintaining fiber, and the polarization direction of the light output from the other end of the polarization-maintaining fiber coincides with the electrical principal axis of the electro-optic crystal. The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal, the light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal, the light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber, and the delay time difference between the slow axis and the fast axis due to the birefringence of the polarization-maintaining fiber is applied to the light output from one end of the polarization-maintaining fiber using the first birefringent medium. Compensation is performed, the phase difference of light between the slow axis and the fast axis of the first birefringent medium is changed according to the result of the adjustment of the first polarization controller using the first polarization controller, the delay time difference between the slow axis and the fast axis due to the natural birefringence of the electro-optic crystal for round trip is compensated using the second birefringent medium, the phase difference of light between the slow axis and the fast axis of the second birefringent medium is changed using the second polarization controller, the delay time difference by the first birefringent medium and by the first polarization controller The phase difference change occurs before the compensation for the delay time difference by the second birefringent medium and the phase difference change by the second polarization controller, and the light output from one end of the polarization-maintaining fiber passes through the first birefringent medium, the first polarization controller, the second birefringent medium, and the second polarization controller, and then the slow axis is in the same direction as the slow axis or fast axis of the polarization-maintaining fiber, and the slow axis is in the same direction as the slow axis or fast axis of the polarization-maintaining fiber 22.The second polarization controller may be adjusted so that light passing sequentially through the second half-wave plate (40) forming a 5-degree angle becomes linearly polarized in the same direction as the slow axis or fast axis of the polarization-maintaining fiber. The adjustment of the phase difference of light between the slow axis and fast axis of the first birefringent medium using the first polarization controller follows the result of the adjustment of the first polarization controller, and the adjustment of the phase difference of light between the slow axis and fast axis of the second birefringent medium using the second polarization controller follows the result of the adjustment of the second polarization controller.

[0062] With this configuration, the electric field measurement method of the present invention makes it possible to easily adjust the polarization controller, which is used to obtain the maximum sensitivity of electric field measurement by compensating for the phase difference of light due to the natural birefringence of the electro-optic crystal, by adjusting it based on the light intensity when the electric field under measurement is zero.

[0063] The electric field measurement method according to the present invention outputs linearly polarized CW light, inputs the CW light to one end of the polarization-maintaining fiber so that the polarization direction of the CW light matches the slow axis or speed axis of the polarization-maintaining fiber, inputs the light output from the other end of the polarization-maintaining fiber to one end of the electro-optic crystal so that the polarization direction of the light output from the other end of the polarization-maintaining fiber matches the electrical principal axis of the electro-optic crystal at a 45-degree angle, reflects the light input to one end of the electro-optic crystal using the mirror and outputs it from one end of the electro-optic crystal, inputs the light output from one end of the electro-optic crystal to the other end of the polarization-maintaining fiber, and adjusts the wavelength of the CW light so that the intensity of the linear polarization component in the slow axis direction of the polarization-maintaining fiber is equal to the intensity of the linear polarization component in the speed axis direction of the polarization-maintaining fiber, so that the polarization direction of the CW light matches the slow axis or speed axis of the polarization-maintaining fiber. The wavelength-adjusted CW light is input to one end of the polarization-maintaining fiber, and the light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle, the light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal, and the light output from one end of the electro-optic crystal is polarized The light input to the other end of the polarization-maintaining fiber and output from one end of the polarization-maintaining fiber is compensated for by the first birefringent medium to compensate for the delay time difference between the slow axis and the fast axis due to the birefringence of the polarization-maintaining fiber, and the phase difference of the light between the slow axis and the fast axis of the first birefringent medium is changed by the first polarization controller, and then the slow axis is in the same direction as the slow axis or fast axis of the polarization-maintaining fiber, and the slow axis is in the same direction as the slow axis or fast axis of the polarization-maintaining fiber, and 22.The first polarization controller is adjusted so that the light passing through the 5-degree half-wave plate (34) in sequence becomes linearly polarized in the same direction as the slow axis or fast axis of the polarization-maintaining fiber, and a second linearly polarized light is output. The second light is input to one end of the polarization-maintaining fiber so that the polarization direction of the second light coincides with the slow axis or fast axis of the polarization-maintaining fiber. The light output from the other end of the polarization-maintaining fiber is input to the one end of the electro-optic crystal so that the polarization direction of the light output from the other end of the polarization-maintaining fiber coincides with the electrical principal axis of the electro-optic crystal. The light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal, the light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber, the delay time difference between the slow axis and the fast axis due to the birefringence of the polarization-maintaining fiber is compensated for with respect to the light output from one end of the polarization-maintaining fiber using the first birefringent medium, the phase difference of the light between the slow axis and the fast axis of the first birefringent medium is changed according to the result of the adjustment of the first polarization controller using the first polarization controller, and the second birefringence The medium is used to compensate for the delay time difference between the slow axis and the fast axis due to the natural birefringence of the electro-optic crystal for the round trip, and the second polarization controller is used to change the phase difference of the light between the slow axis and the fast axis of the second birefringent medium, and the compensation of the delay time difference by the first birefringent medium and the change of the phase difference by the first polarization controller are performed before the compensation of the delay time difference by the second birefringent medium and the change of the phase difference by the second polarization controller, and the light output from one end of the polarization-maintaining fiber is used by the first birefringent medium, the first polarization controller and the second birefringent medium The optical phase difference between the slow axis and the fast axis of the first birefringent medium may be adjusted so that, after passing through the folded medium and the second polarization controller, the intensity of the linear polarization component in the slow axis direction of the polarization-maintaining fiber is equal to the intensity of the linear polarization component in the fast axis direction of the polarization-maintaining fiber. The adjustment of the optical phase difference between the slow axis and the fast axis of the first birefringent medium using the first polarization controller follows the result of the adjustment of the first polarization controller, and the adjustment of the optical phase difference between the slow axis and the fast axis of the second birefringent medium using the second polarization controller follows the result of the adjustment of the second polarization controller.

[0064] With this configuration, the electric field measurement method of the present invention makes it possible to easily adjust the polarization controller, which is used to obtain the maximum sensitivity of electric field measurement by compensating for the phase difference of light due to the natural birefringence of the electro-optic crystal, by adjusting it with fewer optical components based on the light intensity when the electric field under measurement is zero.

[0065] To achieve the above objective, the electric field measurement method according to the present invention outputs a first linearly polarized light, inputs the first light to one end of the polarization-maintaining fiber (12) so that the polarization direction of the first light coincides with the slow axis or fast axis of the polarization-maintaining fiber (12), inputs the light output from the other end of the polarization-maintaining fiber to one end of the electro-optic crystal (28) so that the polarization direction of the light output from the other end of the polarization-maintaining fiber coincides with the electrical principal axis of the electro-optic crystal (28), and reflects the light input to one end of the electro-optic crystal using a mirror (16) provided at the other end of the electro-optic crystal. In an electric field measurement method, the electric field applied to the electro-optic crystal is measured by outputting light from one end of the electro-optic crystal, inputting the light output from one end of the electro-optic crystal to the other end of the polarization-maintaining fiber, and detecting the difference between either or both of the intensity of the linear polarization component in the slow axis direction of the polarization-maintaining fiber and the intensity of the linear polarization component in the fast axis direction of the polarization-maintaining fiber of the light output from one end of the polarization-maintaining fiber, wherein the first light is pulsed light with a predetermined repetition frequency, the electro-optic crystal is a crystal having natural birefringence, and the polarization-maintaining fiber Before detecting the difference between the intensity of the linear polarization component in the slow axis direction and the linear polarization component in the fast axis direction of the polarization-maintaining fiber, or both, for light output from one end of the fiber, a first birefringent medium (29) is used to compensate for the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber, a first polarization controller (30) is used to adjust the phase difference of the light between the slow axis and the fast axis of the first birefringent medium, and a second birefringent medium (31) whose slow axis is at a 45-degree angle with the slow axis of the first birefringent medium is used to compensate for the natural birefringence of the electro-optic crystal for one path. The delay time difference between the slow axis and the fast axis due to refraction is compensated, and the phase difference of the light between the slow axis and the fast axis of the second birefringent medium is adjusted using a second polarization controller (38). The compensation of the delay time difference by the first birefringent medium and the adjustment of the phase difference by the first polarization controller are performed before the compensation of the delay time difference by the second birefringent medium and the adjustment of the phase difference by the second polarization controller. Furthermore, before the pulsed light is input to one end of the polarization-maintaining fiber, the first birefringent medium, the first polarization controller, the second birefringent medium, and the second polarization controller are configured as follows:The method is characterized by measuring the electric field by equivalent time sampling, with the light output from one end of the polarization-maintaining fiber propagating in the opposite direction to the light output.

[0066] This configuration allows the electric field measurement method of the present invention to compensate for the natural birefringence of the electro-optic crystal when using an electro-optic crystal with natural birefringence, thereby enabling equivalent time sampling with pulsed light of a wide spectral width, i.e., a short pulse width. As a result, the upper limit of the frequency of the electric field under measurement is increased, enabling high-frequency electric field measurements.

[0067] Furthermore, by compensating for the natural birefringence of the electro-optic crystal before the light is input to one end of the polarization-maintaining fiber and after the light is output from one end of the polarization-maintaining fiber, in addition to coarse compensation for the delay time difference by the second birefringent medium, the adjustment of the phase difference of the light by the second polarization controller becomes easier. This compensates for the phase difference of the light due to natural birefringence, achieving maximum sensitivity for electric field measurement, while the electric field probe side remains small and lightweight, enabling electric field measurement at any position, similar to conventional technology.

[0068] The electric field measurement method according to the present invention outputs linearly polarized CW light, inputs the CW light to one end of the polarization-maintaining fiber such that the polarization direction of the CW light coincides with the slow axis or fast axis of the polarization-maintaining fiber, inputs the light output from the other end of the polarization-maintaining fiber to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber forms a 45-degree angle with the electrical principal axis of the electro-optic crystal, reflects the light input to one end of the electro-optic crystal with the mirror and outputs it from one end of the electro-optic crystal, and the light output from one end of the electro-optic crystal The CW light is input to the other end of the polarization-maintaining fiber, and the wavelength of the CW light is adjusted so that the intensity of the linear polarization component in the slow axis direction of the polarization-maintaining fiber and the intensity of the linear polarization component in the fast axis direction of the polarization-maintaining fiber are equal to the intensity of the linear polarization component in the fast axis direction of the polarization-maintaining fiber of the light output from one end of the polarization-maintaining fiber, and the wavelength-adjusted CW light is input to one end of the polarization-maintaining fiber so that the polarization direction of the CW light coincides with the slow axis or fast axis of the polarization-maintaining fiber, and the polarization of the light output from the other end of the polarization-maintaining fiber is adjusted so that the polarization direction of the light and the electrical principal axis of the electro-optic crystal form a 45-degree angle. Light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal, the light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal, the light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber, the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is compensated for with respect to the light output from one end of the polarization-maintaining fiber using the first birefringent medium, and after changing the phase difference of the light between the slow axis and the fast axis of the first birefringent medium using the first polarization controller, the slow axis becomes the polarity The CW light, whose wavelength has been adjusted, is passed sequentially through a quarter-wave plate (41) in the same direction as the slow axis or speed axis of the wave-maintaining fiber, and a half-wave plate (42) whose slow axis is at an angle of 11.25 degrees to the slow axis or speed axis of the polarization-maintaining fiber. Before the CW light, whose wavelength has been adjusted, is input to one end of the polarization-maintaining fiber, the first birefringent medium, the first polarization controller, the quarter-wave plate, and the half-wave plate are propagated in the opposite direction to the light output from one end of the polarization-maintaining fiber. The light output from one end of the polarization-maintaining fiber passes through the first birefringent medium and the first polarization controller.After passing through the quarter-wave plate and the half-wave plate, the first polarization controller is adjusted so that the polarization is linearly polarized in the same direction as the slow axis or speed axis of the polarization-maintaining fiber, and a second linearly polarized light is output. The second light is input to one end of the polarization-maintaining fiber so that the polarization direction of the second light coincides with the slow axis or speed axis of the polarization-maintaining fiber. The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal so that the polarization direction of the light output from the other end of the polarization-maintaining fiber forms a 45-degree angle with the electrical principal axis of the electro-optic crystal. The light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal, the light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber, the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is compensated for with respect to the light output from one end of the polarization-maintaining fiber using the first birefringent medium, the phase difference of the light between the slow axis and the fast axis of the first birefringent medium is changed according to the result of the adjustment of the first polarization controller using the first polarization controller, and the second birefringent medium is used to... The delay time difference between the slow axis and the fast axis due to the natural birefringence of the gas-optic crystal for round trip is compensated, and the phase difference of the light between the slow axis and the fast axis of the second birefringent medium is changed using the second polarization controller, and the compensation of the delay time difference by the first birefringent medium and the change of the phase difference by the first polarization controller are performed before the compensation of the delay time difference by the second birefringent medium and the change of the phase difference by the second polarization controller, and before the second light is input to one end of the polarization-maintaining fiber, the first birefringent medium, the first polarization controller and the second birefringent medium The second polarization controller is propagated in the opposite direction to the light output from one end of the polarization-maintaining fiber, and after the light output from one end of the polarization-maintaining fiber passes through the first birefringent medium, the first polarization controller, the second birefringent medium, and the second polarization controller, the second polarization controller is adjusted so that the intensity of the linear polarization component in the slow axis direction of the polarization-maintaining fiber is equal to the intensity of the linear polarization component in the fast axis direction of the polarization-maintaining fiber, and the adjustment of the phase difference of light between the slow axis and the fast axis of the first birefringent medium using the first polarization controller is,The adjustment of the phase difference of light between the slow axis and the fast axis of the second birefringent medium using the second polarization controller may be configured to follow the results of the adjustment of the second polarization controller, in accordance with the results of the adjustment of the first polarization controller.

[0069] With this configuration, the electric field measurement method of the present invention makes it possible to easily adjust the polarization controller, which is used to obtain the maximum sensitivity of electric field measurement by compensating for the phase difference of light due to the natural birefringence of the electro-optic crystal, by adjusting it based on the light intensity when the electric field under measurement is zero. [Effects of the Invention]

[0070] According to the present invention, even if the electro-optic crystal of the electric field probe has natural birefringence, it is possible to realize a small and lightweight electric field probe and to provide an electric field measuring device and electric field measuring method that can measure high-frequency electric fields by equivalent time sampling with pulsed light.

[0071] In detail, the present invention enables equivalent time sampling with pulsed light of a wide spectral width, i.e., a short pulse width, by compensating for the natural birefringence of the electro-optic crystal when using an electro-optic crystal having natural birefringence. This increases the upper limit of the frequency of the electric field under measurement, enabling high-frequency electric field measurement. Furthermore, by arranging the optical element that compensates for the natural birefringence of the electro-optic crystal on the light source / polarization detection unit side, it becomes easy to incorporate a second polarization controller that adjusts the phase difference of light in addition to a second birefringent medium that roughly compensates for the delay time difference due to natural birefringence. This allows for maximum sensitivity of electric field measurement by compensating for the phase difference of light due to natural birefringence, while the electric field probe side remains small and lightweight, enabling electric field measurement at any position, similar to the conventional technology. In addition, regarding the adjustment of the polarization controller to obtain maximum sensitivity of electric field measurement by compensating for the phase difference of light due to the natural birefringence of the electro-optic crystal, the proposed method of adjusting based on light intensity with the electric field under measurement at zero eliminates the need for an electric field under measurement for adjustment, making it possible to easily adjust the polarization controller. [Brief explanation of the drawing]

[0072] [Figure 1] This diagram shows the configuration of a conventional technique for measuring electric fields using electro-optic sampling (single-ended). [Figure 2] This diagram shows the configuration of a conventional technique for measuring electric fields using electro-optical sampling (differential). [Figure 3] This diagram shows the configuration of a conventional technique for measuring electric fields using electro-optic sampling (with an optical circulator). [Figure 4] This diagram shows the configuration of a conventional technique for measuring electric fields using electro-optic sampling (with the half-wave plate omitted from Figure 3). [Figure 5] This diagram shows the configuration of a conventional technique for measuring electric fields using an electro-optic crystal with natural birefringence. [Figure 6] This figure shows the configuration of the first embodiment of the present invention. [Figure 7] This figure shows the configuration during wavelength adjustment in the first embodiment. [Figure 8] This diagram shows the configuration during adjustment of the polarization controller (PC1) in the first embodiment. [Figure 9] This diagram shows the configuration during adjustment of the polarization controller (PC2) in the first embodiment. [Figure 10] This figure shows another configuration of the first embodiment. [Figure 11] This figure shows the configuration when an optical circulator with polarization-maintaining fibers is used in the first embodiment. [Figure 12] This figure shows the configuration of a second embodiment of the present invention. [Figure 13] This figure shows the configuration during adjustment of the polarization controller (PC3) in the second embodiment. [Figure 14] This figure shows another configuration for adjusting the polarization controller (PC3) in the second embodiment. [Figure 15] This figure shows another configuration of the second embodiment. [Figure 16] This figure shows the configuration when an optical circulator with polarization-maintaining fibers is used in the second embodiment. [Figure 17] This figure shows the configuration of a third embodiment of the present invention. [Figure 18] This figure shows the configuration during wavelength adjustment in the third embodiment. [Figure 19] This diagram shows the configuration during adjustment of the polarization controller (PC1) in the third embodiment. [Figure 20] This graph shows the relationship between φc and |E24x|² and |E24y|². [Figure 21] This figure shows the configuration during adjustment of the polarization controller (PC3) in the third embodiment. [Figure 22] This figure shows another configuration of the third embodiment. [Figure 23] This figure shows the configuration when a polarization separation unit with a polarization-maintaining fiber is used in the third embodiment. [Modes for carrying out the invention]

[0073] Embodiments of the present invention will be described below with reference to the drawings.

[0074] [First Embodiment] Figure 6 shows the configuration of the first embodiment of the present invention. The pulse light source 10 outputs pulsed light with 0-degree linear polarization. The pulsed light with 0-degree linear polarization output from the pulse light source 10 is input to the first end of the optical circulator 24 (CIR), output from the second end of the optical circulator 24, and input to the left side of the polarization-maintaining fiber 12 (PMF1). The optical circulator 24 is a polarization-maintaining type, and the pulsed light output from the second end of the optical circulator 24 is 0-degree linearly polarized. The polarization-maintaining fiber 12 has a lagging axis in the 0-degree direction, and the polarization direction of the pulsed light and the lagging axis direction of the polarization-maintaining fiber 12 coincide. Therefore, the pulsed light input to the left side of the polarization-maintaining fiber 12 propagates to the right side of the polarization-maintaining fiber 12 while maintaining 0-degree linear polarization, and the 0-degree linearly polarized pulsed light is output from the right side of the polarization-maintaining fiber 12.

[0075] A pulse of 0-degree linearly polarized light output from the right side of the polarization-maintaining fiber 12 is input to the left side of the electro-optic crystal 28 (EO3), whose electrical principal axis is in the 45-degree direction, and propagates to the right through the electro-optic crystal 28. The birefringence of the electro-optic crystal 28 changes depending on the electric field applied to the electro-optic crystal 28. A mirror 16 that reflects light is located on the right end face of the electro-optic crystal 28, and the pulse light is reflected by the mirror 16 and propagates in the opposite direction through the electro-optic crystal 28, propagates to the left through the polarization-maintaining fiber 12, and reaches the second end of the optical circulator 24.

[0076] A collimating lens (not shown) may be included between the polarization-maintaining fiber 12 and the electro-optic crystal 28, which aligns the light output from the polarization-maintaining fiber 12 into parallel light and inputs the parallel light reflected by the mirror 16 back into the polarization-maintaining fiber 12.

[0077] The pulsed light input to the second end of the optical circulator 24 is output from the third end of the optical circulator 24 while maintaining its polarization, and input to the polarization-maintaining fiber 29 (PMF2). It is also possible to use an optical directional coupler instead of the optical circulator 24 to separate the input light to the left side of the polarization-maintaining fiber 12 from the output light from the left side of the polarization-maintaining fiber 12.

[0078] The polarization-maintaining fiber 29 is set to a length such that its lagging axis is 90 degrees and it exhibits approximately the same birefringence as the polarization-maintaining fiber 12. If the polarization-maintaining fiber 12 and the polarization-maintaining fiber 29 are polarization-maintaining fibers of the same specifications, then they should be made to the same length. The polarization-maintaining fiber 29 is intended to compensate for the birefringence of the polarization-maintaining fiber 12, and is not limited to a polarization-maintaining fiber; any optical element with approximately the same birefringence as the polarization-maintaining fiber 12 would suffice.

[0079] Furthermore, in this embodiment, the retard axis of the polarization-maintaining fiber 12 is set to the 0-degree direction and the retard axis of the polarization-maintaining fiber 29 is set to the 90-degree direction. However, even if the retard axis of the polarization-maintaining fiber 12 is set to the 90-degree direction and the retard axis of the polarization-maintaining fiber 29 is set to the 0-degree direction, the birefringence of the polarization-maintaining fiber 12 is compensated. The pulsed light output from the polarization-maintaining fiber 29 is input to the polarization controller 30 (PC1).

[0080] The polarization-maintaining fiber 29 is set to have almost the same birefringence as the polarization-maintaining fiber 12, and the delay time difference between the slow axis and the fast axis due to the birefringence of the polarization-maintaining fiber 12 is almost compensated. However, it is difficult to perfectly match the birefringence of the polarization-maintaining fiber 29 and the polarization-maintaining fiber 12, so it is difficult to compensate for the phase difference of light between the slow axis and the fast axis due to birefringence. For this reason, the polarization controller 30 adjusts the phase difference of light between the slow axis and the fast axis. The polarization controller 30 is a variable waveplate that can adjust the phase difference between 0-degree polarization and 90-degree polarization, or a polarization controller that can be adjusted to any polarization state. The order of the polarization-maintaining fiber 29 and the polarization controller 30 may be reversed.

[0081] Furthermore, in this embodiment, the direction of the polarization controller 30 is set to 0 degrees, but even if the direction of the polarization controller 30 is set to 90 degrees, the phase difference between the slow axis and the fast axis of the polarization-maintaining fiber 29 can be adjusted in the same way, only the sign of the phase difference is reversed. The pulsed light output from the polarization controller 30 is input to the polarization-maintaining fiber 31 (PMF3). The polarization-maintaining fiber 31 has a length set so that its slow axis is in the 45-degree direction and its birefringence is approximately the same as that of the birefringence of the electro-optic crystal 28 round trip. The polarization-maintaining fiber 31 is for compensating for the birefringence of the electro-optic crystal 28, and is not limited to a polarization-maintaining fiber; any optical element with approximately the same birefringence as that of the electro-optic crystal 28 round trip will suffice.

[0082] Furthermore, in this embodiment, the retard axis of the electro-optic crystal 28 is set to the 45-degree direction (forward path) and the retard axis of the polarization-maintaining fiber 31 is set to the 45-degree direction (return path). However, even if the retard axis of the electro-optic crystal 28 is set to the -45-degree direction (forward path) and the retard axis of the polarization-maintaining fiber 31 is set to the -45-degree direction (return path), the birefringence of the electro-optic crystal 28 is compensated.

[0083] The pulsed light output from the polarization-maintaining fiber 31 is input to the polarization controller 32 (PC2). The polarization-maintaining fiber 31 is set to have birefringence that is almost the same as that of 28 round trips of the electro-optic crystal, and the delay time difference between the slow axis and the fast axis due to the birefringence of 28 round trips of the electro-optic crystal is almost compensated. However, it is difficult to perfectly match the birefringence of the polarization-maintaining fiber 31 with that of 28 round trips of the electro-optic crystal, so it is difficult to compensate for the phase difference of the light between the slow axis and the fast axis due to birefringence. For this reason, the polarization controller 32 adjusts the phase difference of the light between the slow axis and the fast axis. The polarization controller 32 is a variable waveplate that can adjust the phase difference between 45-degree polarization and 135-degree polarization, or a polarization controller that can be adjusted to any polarization state. The order of the polarization-maintaining fiber 31 and the polarization controller 32 may be reversed.

[0084] Furthermore, although the polarization controller 32 is set to 45 degrees in this embodiment, even if the polarization controller 32 is set to -45 degrees, the phase difference between the slow axis and the fast axis of the polarization-maintaining fiber 31 can be adjusted in the same way, only the sign of the phase difference is reversed.

[0085] The pulsed light output from the polarization controller 32 is input to the first end of the polarization beam splitter 17 via a quarter-wave plate 33 (QWP3) with a lagging axis of 0 degrees and a half-wave plate 34 (HWP3) with a lagging axis of -22.5 degrees. In this embodiment, the lagging axis of the quarter-wave plate 33 is set to 0 degrees, but even if the lagging axis of the quarter-wave plate 33 is set to 90 degrees, the electric field measurement can be performed in the same way, only the sign of the electric field measurement is reversed. Similarly, in this embodiment, the lagging axis of the half-wave plate 34 is set to -22.5 degrees, but even if the lagging axis of the half-wave plate 34 is set to 22.5 degrees, -67.5 degrees, or 67.5 degrees (i.e., the lagging axis of the half-wave plate 34 is 22.5 degrees from the lagging axis or speed axis of the polarization-maintaining fiber 12), the electric field measurement can be performed in the same way, only the sign of the electric field measurement is reversed.

[0086] The polarizing beam splitter 17 reflects the 90-degree polarized component of the light input to the first end and outputs it from the fourth end to the receiver 18, while transmitting the 0-degree polarized component of the light input to the first end and outputting it from the third end to the receiver 19. Receivers 18 and 19 each output an electrical signal proportional to the intensity of the input light. By taking the difference between the output of receiver 19 and the output of receiver 18, the difference between the intensity of the 0-degree polarized component and the intensity of the 90-degree polarized component of the light input to the first end of the polarizing beam splitter 17 can be obtained. Here, an example of differential detection using two receivers is shown, but a single-ended configuration using either receiver 18 or receiver 19 is also possible. In the case of a single-ended configuration, a polarizer that transmits either the 0-degree or 90-degree polarized component can be used instead of the polarizing beam splitter 17.

[0087] Next, a method for adjusting the polarization controllers 30 and 32 in the first embodiment is shown. First, as shown in Figure 7, a tunable CW light source 27 is used instead of a pulsed light source, and the output light from the polarization controller 30 is input to the first end of the polarizing beam splitter 17. The optical circulator 24, polarization-maintaining fiber 12, electro-optic crystal 28, mirror 16, polarization-maintaining fiber 29, polarization controller 30, polarizing beam splitter 17, and photodetectors 18, 19 are the same as in Figure 6. The CW light output from the CW light source 27 is 0-degree linearly polarized, and the electric field applied to the electro-optic crystal 28 is zero. The electric field of the light is represented by a Jones vector, and the propagation characteristics of each optical component are represented by a Jones matrix to determine the electric field of the light input to the first end of the polarizing beam splitter 17.

[0088] E0 is the electric field of the CW light output from the CW light source 27, and τ is the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber 12. p The phase difference of light between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber 12 is φ p The delay time difference (round trip) between the slow axis and the fast axis due to the natural birefringence of the electro-optic crystal 28 is τ d The phase difference (round trip) of light between the slow axis and the fast axis due to the natural birefringence of the electro-optic crystal 28 is φ dThe phase difference (round trip) of light between the slow axis and the fast axis due to the application of an electric field to the electro-optic crystal 28 is φ e The delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber 29 is τ p The phase difference of light between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber 29 is φ p ′, the phase difference of light between the slow axis and the fast axis by the polarization controller 30 is φ c Let ω be the angular frequency of light.

[0089] The delay time difference due to birefringence in polarization-maintaining fiber 12 includes the phase difference of light, but here we consider it separately as a delay time difference that can be compensated by polarization-maintaining fiber 29 and a phase difference that cannot be compensated. In other words, the delay time difference between the slow axis and the fast axis due to birefringence in polarization-maintaining fiber 12 and polarization-maintaining fiber 29 is equal to (τ p ), the phase difference of light between the slow axis and the fast axis due to birefringence of polarization-maintaining fiber 12 and polarization-maintaining fiber 29 is different (φ p ≠φ p Let's assume it's '). The electric field E2 of the light output from the polarization controller 30 is expressed as follows:

[0090]

number

number

number

number

[0091] The wavelength of the CW light source 27 is adjusted so that the intensities of the 0-degree polarization component and the 90-degree polarization component of the electric field E2 are equal, that is, so that the difference in output between the photodetector 19 and the photodetector 18 is zero. 2x | 2 -|E 2y | 2 The condition for =0 is cos(ωτ) d +φ d +φ e )=0 therefore ωτd +φ d +φ e There are two points in one cycle such as ωτ + φ + φ = ±π / 2, but for ω, the slope of |E 2x | 2 - |E 2y | 2 is negative. Selecting this side makes ωτ + φ + φ = π / 2. In the case of a single - end configuration, adjust the wavelength of the CW light source 27 so that the output of the photoreceiver becomes 1 / 2 of the maximum value, and select the side where the slope of |E d +φ d +φ e is negative with respect to ω, or the side where the slope of |E 2x | 2 is positive with respect to ω. This wavelength adjustment can obtain the same result even when performed with the configuration of FIG. 5 without the polarization - maintaining fiber 29 and the polarization controller 30. The electric field E of the light output from the polarization controller 30 after wavelength adjustment 2y | 2 becomes as follows by substituting ωτ + φ + φ = π / 2 into equation (13). 21 is substituted into (13) formula, and ωτ d +φ d +φ e = π / 2, and it becomes as follows:

[0092]

Equation

[0093] Next, at the wavelength adjusted above, as shown in FIG. 8, input the output light from the polarization controller 30 to the first end of the polarization beam splitter 17 through a quarter - wave plate 33 with the slow axis in the 0 - degree direction and a half - wave plate 34 with the slow axis in the - 22.5 - degree direction. The CW light source 27, the optical circulator 24, the polarization - maintaining fiber 12, the electro - optic crystal 28, the mirror 16, the polarization - maintaining fiber 29, the polarization controller 30, the polarization beam splitter 17, the photoreceivers 18, 19 are the same as those in FIG. 7. Adjust the polarization controller 30 so that the light input to the first end of the polarization beam splitter 17 becomes linearly polarized light at 0 degrees, that is, the output of the photoreceiver 18 becomes zero, or the output of the photoreceiver 19 becomes maximum, or the output of the photoreceiver 19 - the output of the photoreceiver 18 becomes maximum. The electric field E of the light output from the half - wave plate 34 22 is expressed as follows:

[0094]

number

[0095] Electric field E 22 The field E is such that the polarization is 0 degrees linear. 22 The 90-degree polarization component of φ becomes zero. c Adjusting this gives -1+e j(φp-φp′+φc) = 0 therefore φ p -φ p ′+φ c = 0, and the phase difference of light between the slow axis and the fast axis due to birefringence in the polarization-maintaining fiber 12 is compensated. c The field of light E of the light output from the polarization controller 30 at any adjusted wavelength. 23 In equation (13), φ p -φ p ′+φ c Substituting =0, we get the following equation.

[0096]

number

[0097] Furthermore, remove the half-wave plate 34 from Figure 8 and rotate the polarizing beam splitter 17 by -45 degrees so that the light output from the quarter-wave plate 33 becomes linearly polarized at -45 degrees φ c The same result can be obtained by adjusting the settings.

[0098] Next, the φ adjusted above cIn this configuration, as shown in Figure 9, the output light from the polarization controller 30 is input to the first end of the polarization beam splitter 17 via a polarization-maintaining fiber 31 with a 45-degree lagging axis and a polarization controller 32 with a 45-degree lagging axis. The CW light source 27, optical circulator 24, polarization-maintaining fiber 12, electro-optic crystal 28, mirror 16, polarization-maintaining fiber 29, polarization controller 30, polarization beam splitter 17, and photodetectors 18 and 19 are the same as in Figure 8. The following equation holds true at any wavelength, so the wavelength of the CW light source 27 in Figure 9 does not have to be the wavelength adjusted above, and the CW light source 27 in Figure 9 may also be a pulsed light source.

[0099] The polarization controller 32 is adjusted so that the light input to the first end of the polarizing beam splitter 17 becomes 0-degree linearly polarized, that is, so that the output of the photodetector 18 becomes zero, or so that the output of the photodetector 19 becomes maximum, or so that the output of the photodetector 19 minus the output of the photodetector 18 becomes maximum. The delay time difference between the slow axis and the fast axis due to the birefringence of the polarization-maintaining fiber 31 is τ d The phase difference of light between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber 31 is φ d ′, the phase difference of light between the slow axis and the fast axis by the polarization controller 32 is φ c Let's assume '. The delay time difference due to the birefringence of the electro-optic crystal 28 includes the phase difference of light, but as above, we consider it separately as a delay time difference that can be compensated by the polarization-maintaining fiber 31 and a phase difference of light that cannot be compensated. In other words, the delay time difference between the slow axis and the fast axis due to the natural birefringence of the electro-optic crystal 28 (round trip) and the delay time difference between the slow axis and the fast axis due to the birefringence of the polarization-maintaining fiber 31 are equal to (τ d ), the phase difference of light between the slow axis and the fast axis due to the natural birefringence of the electro-optic crystal 28 (round trip) and the phase difference of light between the slow axis and the fast axis due to the birefringence of the polarization-maintaining fiber 31 are different (φ d ≠φ d Let's assume it's ′). The electric field E3 of the light output from the polarization controller 32 is given by the following equation.

[0100]

number

number

number

[0101] The electric field under measurement is zero (φ e At =0), the electric field E3 becomes 0-degree linearly polarized, that is, the 90-degree polarization component of the electric field E3 becomes zero. c Adjusting ′ gives -1 / 2+(1 / 2)e j(φd-φd′-φc′+φe) = 0 therefore φ d -φ d ′-φ c '=0, and the phase difference of light between the slow axis and the fast axis due to the birefringence of the electro-optic crystal 28 is compensated. φ c and φ c ′The field of light E output from the adjusted polarization controller 32 31 In equation (19), φ d -φ d ′-φ c Substituting '=0, we get the following equation.

[0102]

number

[0103] With the adjustment of polarization controllers 30 and 32 completed as described above, as shown in Figure 6, the pulse light source 10 is used, and the output light from polarization controller 32 is input to the first end of the polarization beam splitter 17 via a quarter-wave plate 33 with a lagging axis of 0 degrees and a half-wave plate 34 with a lagging axis of -22.5 degrees. The output light E4 from the half-wave plate 34 is given by the following equation.

[0104]

number

number

number

[0105] In this way, the birefringence of the polarization-maintaining fiber 12 and the electro-optic crystal 28 is compensated, including the phase difference of light between the slow axis and the fast axis, |φ e |≪1|E 4x | 2 -|E 4y | 2 ≒φ e Therefore, differential detection is performed by φ e An output proportional to φ is obtained, resulting in the maximum slope of the sine function, i.e., maximum sensitivity. In the case of a single-ended configuration, φ e The result is a value obtained by adding an offset to a term that is proportional to [the given value].

[0106] The configuration shown in Figure 6 allows for separation of the optical circulator 24 into a light source / polarization detection unit 3 on the left and a polarization-maintaining fiber 12 into an electric field probe unit 2 on the right, with the two connected by a flexible polarization-maintaining fiber 12. Optical elements for compensating for birefringence, particularly polarization controllers 30 and 32 with movable parts for adjusting the phase difference, are located on the light source / polarization detection unit side. As a result, the electric field probe unit 2 is small, lightweight, and can be positioned at any location to enable electric field measurement. It is also possible to easily attach and detach the electric field probe unit 2 by connecting the optical circulator 24 and the polarization-maintaining fiber 12 with an optical connector (not shown).

[0107] In the first embodiment, as shown in Figure 10, the polarization-maintaining fiber 29 and the polarization controller 30 can also be placed between the optical circulator 24 and the polarization-maintaining fiber 12. For example, if the polarization-maintaining fiber 29 is included in the field probe section 2A, and the polarization-maintaining fiber 29 and the polarization controller 30 are connected with an optical connector (not shown), the birefringence of the polarization-maintaining fiber 12 is largely compensated for in the field probe section 2A, making it easy to replace the field probe section 2A with polarization-maintaining fibers of various lengths. Since the forward light is linearly polarized at 0 degrees, the polarization does not change even when passing through the polarization-maintaining fiber 29 and the polarization controller 30, and it acts on the return light in the same way as in Figure 6, so the same results as in Figure 6 can be obtained. Similarly, it is also possible to place either the polarization-maintaining fiber 29 or the polarization controller 30 between the optical circulator 24 and the polarization-maintaining fiber 12. However, in the configuration shown in Figure 10, light travels back and forth between the polarization-maintaining fiber 29 and the polarization controller 30, resulting in two instances of light attenuation due to losses in the polarization-maintaining fiber 29 and the polarization controller 30. Therefore, the configuration shown in Figure 6 results in less light attenuation and a higher output.

[0108] If an optical circulator 24 with a polarization-maintaining fiber is used, the birefringence of the polarization-maintaining fiber of the optical circulator 24 is added to the birefringence of the polarization-maintaining fiber 12 of the electro-optic probe. Therefore, the birefringence of the polarization-maintaining fiber 29 must be set to compensate for the birefringence of both polarization-maintaining fibers.

[0109] Specifically, as shown in Figure 11, if a polarization-maintaining fiber 35 (PMF41) is attached to the first end of the optical circulator 24, a polarization-maintaining fiber 36 (PMF42) is attached to the second end, and a polarization-maintaining fiber 37 (PMF43) is attached to the third end, then the forward path of polarization-maintaining fibers 35 and 36 receives 0-degree linearly polarized light that is aligned with the lagging axis of polarization-maintaining fibers 35 and 36, and is therefore unaffected by birefringence. The birefringence of the return path of polarization-maintaining fiber 36 and polarization-maintaining fiber 37 is added to the birefringence of polarization-maintaining fiber 12. Therefore, the length of polarization-maintaining fiber 29 should be set so that the birefringence is approximately the same as the sum of the birefringences of polarization-maintaining fiber 12, the return path of polarization-maintaining fiber 36, and polarization-maintaining fiber 37.

[0110] Furthermore, by aligning the speed axes of polarization-maintaining fibers 36 and 37 with the slow axis of polarization-maintaining fiber 12, the sum of each birefringence becomes smaller, allowing the length of polarization-maintaining fiber 29 to be shortened.

[0111] [Second Embodiment] In the first embodiment shown in Figure 6, a polarization controller 32 was provided to compensate for the phase difference of light between the slow axis and the fast axis due to the birefringence of the electro-optic crystal 28, and quarter-wave plates 33 and half-wave plates 34 were provided to change the polarization for differential detection. However, in the second embodiment shown in Figure 12, these functions are combined into a single polarization controller 38 (PC3). The polarization controller 38 is a variable waveplate capable of adjusting the phase difference between 45-degree polarization and 135-degree polarization, or a polarization controller capable of adjusting to any polarization state. The order of the polarization-maintaining fiber 31 and the polarization controller 38 may be reversed. In this embodiment, the direction of the polarization controller 38 is set to 45 degrees, but even if the direction of the polarization controller 38 is set to -45 degrees, only the sign of the phase difference is reversed, and the phase difference between the slow axis and the fast axis of the polarization-maintaining fiber 31 can be adjusted in the same way.

[0112] Figure 12 shows the same configuration from the pulse light source 10 to the polarization-maintaining fiber 31 as in Figure 6, and the method for adjusting the polarization controller 30 is the same as in the first embodiment. That is, the wavelength of the CW light source 27 is adjusted in the configuration of Figure 7, and the polarization controller 30 is adjusted in the configuration of Figure 8. After adjusting the polarization controller 30, as shown in Figure 13, the output light from the polarization controller 30 is input to the first end of the polarization beam splitter 17 via the polarization-maintaining fiber 31 with a 45-degree lagging axis, the polarization controller 38 with a 45-degree lagging axis, the quarter-wave plate 39 (QWP4) with a 0-degree lagging axis, and the half-wave plate 40 (HWP4) with a 22.5-degree lagging axis. The optical circulator 24, polarization-maintaining fiber 12, electro-optic crystal 28, mirror 16, polarization-maintaining fiber 29, polarization controller 30, polarization-maintaining fiber 31, polarization controller 38, polarization beam splitter 17, and photodetectors 18 and 19 are the same as in Figure 12. Since the following equation holds true at any wavelength, the wavelength of the CW light source 27 in Figure 13 does not have to be the wavelength adjusted in the configuration of Figure 7, and the CW light source 27 in Figure 13 may also be a pulsed light source.

[0113] The polarization controller 38 is adjusted so that the light input to the first end of the polarization beam splitter 17 becomes 0-degree linearly polarized, that is, so that the output of the photodetector 18 becomes zero, or so that the output of the photodetector 19 becomes maximum, or so that the output of the photodetector 19 minus the output of the photodetector 18 becomes maximum. The delay time difference between the slow axis and the fast axis due to the birefringence of the polarization-maintaining fiber 31 is τ d The phase difference of light between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber 31 is φ d ′, the phase difference of light between the slow axis and the fast axis by the polarization controller 38 is φ c If we consider this to be the case, then the electric field E of the light output from the half-wave plate 40 32 This can be expressed using E3 in equation (19) as follows:

[0114]

number

[0115] The electric field under measurement is zero (φ e At (=0), the electric field E 32 The field E is such that the polarization is 0 degrees linear. 32The 90-degree polarization component of φ becomes zero. c If you adjust ′, 1-je j(φd-φd′-φc′+φe) = 0 therefore φ d -φ d ′-φ c ′ = -π / 2.

[0116] Alternatively, in the configuration of Figure 14, the polarization controller 38 is adjusted so that the intensities of the 0-degree polarized component and the 90-degree polarized component of the light input to the first end of the polarizing beam splitter 17 are equal, that is, so that the difference between the outputs of the photodetector 19 and the photodetector 18 is zero, or so that the output of the photodetector 19 is half of its maximum value, or so that the output of the photodetector 18 is half of its maximum value. The optical circulator 24, polarization-maintaining fiber 12, electro-optic crystal 28, mirror 16, polarization-maintaining fiber 29, polarization controller 30, polarization-maintaining fiber 31, polarization controller 38, polarizing beam splitter 17, and photodetectors 18 and 19 are the same as in Figure 12. The following equation holds true at any wavelength, so the wavelength of the CW light source 27 in Figure 14 does not have to be the wavelength adjusted in the configuration of Figure 7, and the CW light source 27 in Figure 14 may also be a pulsed light source.

[0117] Light intensity |E3| from polarization controller 38 2 It is expressed by equation (20), where the electric field under measurement is zero (φ e At |E = 0) 3x | 2 -|E 3y | 2 φ = 0 c Adjusting ′ gives cos(φ d -φ d ′-φ c ′+φ e ) = 0, therefore φ d -φ d ′-φ c ' = ±π / 2. c ′ for |E 3x | 2 -|E 3y | 2 The one where the slope is negative, or |E 3x | 2 The one where the slope is negative, or |E 3y | 2If you choose the option where the slope is positive, the sign is determined and φ d -φ d ′-φ c ′ = -π / 2.

[0118] With the adjustment of polarization controllers 30 and 38 completed as described above, the pulse light source 10 is used, and the output light from polarization controller 38 is input to the first end of polarization beam splitter 17. φ d -φ d ′-φ c When ′=±π / 2, the optical field E of the light output from the polarization controller 38 33 In equation (19), φ d -φ d ′-φ c Substituting '=±π / 2, we get the following equation.

[0119]

number

number

number

[0120] In this way, the birefringence of the polarization-maintaining fiber 12 and the electro-optic crystal 28 is compensated, including the phase difference of light between the slow axis and the fast axis, |φ e |≪1

number

[0121] When adjusting the polarization controller 38 in the configuration of FIG. 14, if the positive or negative of the slope of |E c ′ with respect to 3x | 2 -|E 3y | 2 is not discriminated, the sign of the measured electric field becomes indefinite, but in the measurement of a high-frequency radio signal, it is not necessary to obtain the sign of the normal electric field and it does not pose a problem. The configuration of FIG. 14 is only different in the light source from the configuration of FIG. 12 when performing electric field measurement. And when adjusting φ c ′ in the configuration of FIG. 14, it is also possible to use a pulse light source instead of a CW light source. In that case, it becomes the same as the configuration of FIG. 12 and has the advantage that the polarization controller 38 can be adjusted with a simpler configuration than FIG. 13. The second embodiment of FIG. 12 can obtain the same result with a simpler configuration than the first embodiment of FIG. 6 and has the same features as the first embodiment.

[0122] ​In the second embodiment, as shown in FIG. 15, it is also possible to arrange the polarization maintaining fiber 29 and the polarization controller 30 between the optical circulator 24 and the polarization maintaining fiber 12. For example, if the electric field probe unit 2A includes the polarization maintaining fiber 29 and the space between the polarization maintaining fiber 29 and the polarization controller 30 is connected by an optical connector (not shown), the birefringence of the polarization maintaining fiber 12 is substantially compensated in the electric field probe unit 2A, so that it becomes easy to replace the electric field probe unit with polarization maintaining fibers of various lengths. Since the light in the forward path is linearly polarized at 0 degrees, the polarization does not change even when passing through the polarization maintaining fiber 29 and the polarization controller 30, and it acts on the light in the return path in the same manner as in FIG. 12, so the same result as in FIG. 12 can be obtained. Similarly, it is also possible to arrange either the polarization maintaining fiber 29 or the polarization controller 30 between the optical circulator 24 and the polarization maintaining fiber 12. However, in the configuration of FIG. 15, light travels back and forth between the polarization maintaining fiber 29 and the polarization controller 30, and optical attenuation due to the loss of the polarization maintaining fiber 29 and the polarization controller 30 occurs twice. Therefore, the configuration of FIG. 12 has less optical attenuation and a larger output.

[0123] When using the optical circulator 24 with a polarization maintaining fiber, since the birefringence of the polarization maintaining fiber of the optical circulator 24 is added to the birefringence of the polarization maintaining fiber 12 of the electro-optic probe, it is necessary to set the birefringence of the polarization maintaining fiber 29 so as to compensate for the birefringence of both polarization maintaining fibers.

[0124] Specifically, as shown in FIG. 16, when the polarization maintaining fiber 35 is attached to the first end of the optical circulator 24, the polarization maintaining fiber 36 is attached to the second end, and the polarization maintaining fiber 37 is attached to the third end, the forward paths of the polarization maintaining fiber 35 and the polarization maintaining fiber 36 are input with light linearly polarized at 0 degrees aligned with the slow axes of the polarization maintaining fiber 35 and the polarization maintaining fiber 36, so they are not affected by birefringence. Since the birefringence of the return path of the polarization maintaining fiber 36 and the birefringence of the polarization maintaining fiber 37 are added to the birefringence of the polarization maintaining fiber 12, the length of the polarization maintaining fiber 29 may be set so that it has substantially the same birefringence as the sum of the birefringences of the polarization maintaining fiber 12, the return path of the polarization maintaining fiber 36, and the polarization maintaining fiber 37.

[0125] Furthermore, by aligning the speed axes of polarization-maintaining fibers 36 and 37 with the slow axis of polarization-maintaining fiber 12, the sum of each birefringence becomes smaller, allowing the length of polarization-maintaining fiber 29 to be shortened.

[0126] [Third Embodiment] Figure 17 shows the configuration of a third embodiment of the present invention. The pulse light source 10 outputs pulse light with 0-degree linear polarization. The polarizing beam splitter 20 (PBS2) is arranged to transmit the 0-degree linear polarization from the pulse light source 10, and the 0-degree linear polarization pulse light passes through the polarizing beam splitter 20 from left to right and is input to the Faraday rotator 21 (FR). As the pulse light passes through the Faraday rotator 21 from left to right, the polarization rotates by +45 degrees, and the lagging axis returns to 0-degree linear polarization by the half-wave plate 22 in the 22.5-degree direction. The polarizing beam splitter 17 (PBS1) is arranged to transmit the 0-degree linear polarization, and the 0-degree linear polarization pulse light passes through the polarizing beam splitter 17 from left to right.

[0127] The 0-degree linearly polarized pulsed light output from the right side of the polarizing beam splitter 17 passes through the polarization controller 38 (PC3) in the -45-degree direction, the polarization-maintaining fiber 31 (PMF3) with a lagging axis of -45 degrees, the polarization controller 30 (PC1) in the 0-degree direction, the polarization-maintaining fiber 29 (PMF2) with a lagging axis of 90 degrees, and the polarization-maintaining fiber 12 (PMF1) with a lagging axis of 0 degrees, before being input to the left side of the electro-optic crystal 28 (EO3) with its electrical principal axis in the 45-degree direction, and propagating through the electro-optic crystal 28 to the right. The right end face of the electro-optic crystal 28 has a mirror 16 that reflects light, and pulsed light is reflected by the mirror 16 and propagates in the reverse direction through the electro-optic crystal 28, passing through a polarization-maintaining fiber 12 with a 0-degree lagging axis, a polarization-maintaining fiber 29 with a 90-degree lagging axis, a polarization controller 30 in the 0-degree direction, a polarization-maintaining fiber 31 with a 45-degree lagging axis, and a polarization controller 38 in the 45-degree direction before being input to the polarization beam splitter 17.

[0128] Because the sign of the z and y axes is reversed between the forward and return paths of light, the angle of the optical element is also reversed in sign between the forward and return paths, and the angles of the polarization controller 38 and polarization-maintaining fiber 31 are -45 degrees in the forward path and 45 degrees in the return path. The length of the polarization-maintaining fiber 29 is set so that it has approximately the same birefringence as the polarization-maintaining fiber 12. If the polarization-maintaining fiber 12 and the polarization-maintaining fiber 29 are polarization-maintaining fibers of the same specifications, then the polarization-maintaining fiber 12 and the polarization-maintaining fiber 29 should be made to the same length. The polarization-maintaining fiber 29 is intended to compensate for the birefringence of the polarization-maintaining fiber 12, and is not limited to a polarization-maintaining fiber; any optical element with approximately the same birefringence as the polarization-maintaining fiber 12 will suffice.

[0129] Furthermore, in this embodiment, the retard axis of the polarization-maintaining fiber 12 is set to the 0-degree direction and the retard axis of the polarization-maintaining fiber 29 is set to the 90-degree direction. However, even if the retard axis of the polarization-maintaining fiber 12 is set to the 90-degree direction and the retard axis of the polarization-maintaining fiber 29 is set to the 0-degree direction, the birefringence of the polarization-maintaining fiber 12 is compensated. The polarization-maintaining fiber 29 is set to have almost the same birefringence as the polarization-maintaining fiber 12, and the delay time difference due to the birefringence of the polarization-maintaining fiber 12 is almost compensated. However, since it is difficult to perfectly match the birefringence of the polarization-maintaining fiber 29 and the polarization-maintaining fiber 12, it is difficult to compensate for the phase difference of light between the retard axis and the fast axis due to birefringence. For this reason, the polarization controller 30 adjusts the phase difference of light between the retard axis and the fast axis. The polarization controller 30 is a variable waveplate that can adjust the phase difference between 0-degree polarization and 90-degree polarization, or a polarization controller that can be adjusted to any polarization state. The order of the polarization-maintaining fiber 29 and the polarization controller 30 can be reversed.

[0130] Furthermore, although the polarization controller 30 is set to 0 degrees in this embodiment, setting the polarization controller 30 to 90 degrees only reverses the sign of the phase difference, and similarly, the phase difference between the slow axis and the fast axis of the polarization-maintaining fiber 29 can be adjusted. The polarization-maintaining fiber 31 is set to a length such that it has approximately the same birefringence as the birefringence of the electro-optic crystal 28 for one path. The polarization-maintaining fiber 31 is for compensating for the birefringence of the electro-optic crystal 28, and is not limited to a polarization-maintaining fiber; any optical element with approximately the same birefringence as the birefringence of the electro-optic crystal 28 for one path is acceptable.

[0131] Furthermore, in this embodiment, the retard axis of the electro-optic crystal 28 is set to the 45-degree direction (forward path), and the retard axis of the polarization-maintaining fiber 31 is set to the -45-degree direction (forward path). However, even if the retard axis of the electro-optic crystal 28 is set to the -45-degree direction (forward path) and the retard axis of the polarization-maintaining fiber 31 is set to the 45-degree direction (forward path), the birefringence of the electro-optic crystal 28 is compensated. The polarization-maintaining fiber 31 is set to have birefringence that is almost the same as the refractive index of the electro-optic crystal 28 for one path, and the delay time difference due to the birefringence of the electro-optic crystal 28 for one path is almost compensated. However, it is difficult to perfectly match the birefringence of the polarization-maintaining fiber 31 and the birefringence of the electro-optic crystal 28 for one path, so it is difficult to compensate for the phase difference of light between the retard axis and the fast axis due to birefringence. For this reason, the polarization controller 38 adjusts the phase difference of light between the retard axis and the fast axis. The polarization controller 38 is a variable waveplate capable of adjusting the phase difference between 45-degree polarization and 135-degree polarization, or a polarization controller capable of adjusting to any polarization state. The order of the polarization-maintaining fiber 31 and the polarization controller 38 may be reversed.

[0132] Furthermore, although the polarization controller 38 is set to a direction of -45 degrees in this embodiment, even if the polarization controller 38 is set to a direction of 45 degrees, the phase difference between the slow axis and the fast axis of the polarization-maintaining fiber 31 can be adjusted in the same way, only the sign of the phase difference is reversed. The polarization beam splitter 17 reflects the 90-degree polarization component and inputs it to the photodetector 18, and transmits the 0-degree polarization component to the left. The 0-degree polarization component transmitted through the polarization beam splitter 17 becomes -45-degree linear polarization by the half-wave plate 22 with the slow axis in the -22.5-degree direction (return path), and when it passes through the Faraday rotator 21 from right to left, the polarization rotates by -45 degrees, becoming -90-degree linear polarization. The polarization beam splitter 20 reflects the -90-degree polarization component and inputs it to the photodetector 19. In other words, the polarization separation unit 11A, consisting of a polarizing beam splitter 20, a Faraday rotator 21, a half-wave plate 22, and a polarizing beam splitter 17, outputs 0-degree linearly polarized light input to the first end from the second end, outputs the 0-degree polarized component of the light input to the second end from the fourth end, and outputs the 90-degree polarized component of the light input to the second end from the fifth end. The photodetectors 18 and 19 each output electrical signals proportional to the intensity of the input light.

[0133] By taking the difference between the output of photodetector 19 and the output of photodetector 18, the difference between the intensity of the 0-degree polarized component and the intensity of the 90-degree polarized component of the return path light input to the right side of the polarizing beam splitter 17 can be obtained. Here, an example of differential detection using two photodetectors 18 and 19 is shown, but a single-ended configuration using either photodetector 18 or photodetector 19 is also possible. In the case of a single-ended configuration using only photodetector 18, the polarizing beam splitter 20, Faraday rotator 21, and half-wave plate 22 become unnecessary, and only the polarization separation section of the polarizing beam splitter 17 is required.

[0134] Next, a method for adjusting the polarization controllers 30 and 38 in the third embodiment is shown. First, as shown in Figure 18, a tunable CW light source 27 is used instead of the pulse light source 10, and the output light from the second end of the polarization separation unit 11A is input to the polarization controller 30. The polarization separation unit 11A, polarization controller 30, polarization-maintaining fiber 29, polarization-maintaining fiber 12, electro-optic crystal 28, mirror 16, and photodetectors 18 and 19 are the same as in Figure 17. The CW light output from the CW light source 27 is 0-degree linearly polarized, and the electric field applied to the electro-optic crystal 28 is zero.

[0135] E0 is the electric field of the CW light output from the CW light source 27, and τ is the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber 12. p The phase difference of light between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber 12 is φ p The delay time difference (round trip) between the slow axis and the fast axis due to the natural birefringence of the electro-optic crystal 28 is τ d The phase difference (round trip) of light between the slow axis and the fast axis due to the natural birefringence of the electro-optic crystal 28 is φ d The phase difference (round trip) of light between the slow axis and the fast axis due to the application of an electric field to the electro-optic crystal 28 is φ e The delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber 29 is τ p The phase difference of light between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber 29 is φ p ′, the phase difference of light between the slow axis and the fast axis by the polarization controller 30 is φ c Let ω be the angular frequency of the light. The delay time difference due to birefringence in the polarization-maintaining fiber 12 includes the phase difference of the light, but here we consider it separately as a delay time difference that can be compensated by the polarization-maintaining fiber 29 and a phase difference of the light that cannot be compensated.

[0136] In other words, the delay time difference between the slow axis and the fast axis due to birefringence of polarization-maintaining fiber 12 and polarization-maintaining fiber 29 is equal to (τ p ), the phase difference of light between the slow axis and the fast axis due to birefringence of polarization-maintaining fiber 12 and polarization-maintaining fiber 29 is different (φ p ≠φ p Let's assume it's '). The electric field E2 of the light output from the polarization controller 30 on the return path is given by the following equation.

[0137] [Number] [Number] [Number] [Number]

[0138] Adjust the wavelength of the CW light source 27 so that the intensities of the 0-degree polarization component and the 90-degree polarization component of the electric field E2 are equal, that is, so that the difference in the outputs of the photoreceptors 19 and 18 becomes zero. |E 2x | 2 -|E 2y [[ID=3i]]| 2 The condition for = 0 is cos(ωτ d +φ d +φ e ) = 0, so ωτ d +φ d +φ e = ±π / 2, and there are two points in one cycle, but when selecting the direction in which the slope of |E 2x | 2 -|E 2y | 2 is negative with respect to ω, ωτ d +φ d [[ID=5i]]+φ e = π / 2. In the case of a single-ended configuration, adjust the wavelength of the CW light source 27 so that the photoreceptor output becomes 1 / 2 of the maximum value, and select the direction in which the slope of |E 2x | 2 is negative with respect to ω, or the direction in which the slope of |E 2y | 2 is positive with respect to ω. This wavelength adjustment can obtain similar results even when performed in a configuration without the polarization-maintaining fiber 29 and the polarization controller 30.

[0139] Next, at the wavelength adjusted above, a half-wave plate 42 (HWP5) with a retard axis of -11.25 degrees and a quarter-wave plate 41 (QWP5) with a retard axis of 0 degrees are inserted between the second end of the polarization separation unit 11A and the polarization controller 30, as shown in Figure 19. The CW light source 27, polarization separation unit 11A, polarization controller 30, polarization-maintaining fiber 29, polarization-maintaining fiber 12, electro-optic crystal 28, mirror 16, and photodetectors 18,19 are the same as in Figure 18. In other words, the output light from the second end of the polarization separation unit 11A on the outward path is input to the polarization controller 30 via the half-wave plate 42 with a lagging axis of -11.25 degrees and the quarter-wave plate 41 with a lagging axis of 0 degrees, and the output light from the polarization controller 30 on the return path is input to the second end of the polarization separation unit 11A via the quarter-wave plate 41 with a lagging axis of 0 degrees and the half-wave plate 42 with a lagging axis of 11.25 degrees (return path). The polarization controller 30 is adjusted so that the light input to the second end of the polarization separation unit 11A is 90-degree linearly polarized, that is, so that the output of the photodetector 19 becomes zero, or the output of the photodetector 18 becomes maximum, or the output of the photodetector 19 minus the output of the photodetector 18 becomes minimum. The electric field E of the light output from the half-wave plate 42 on the return path 24 The equation is as follows:

[0140]

number

[0141] φ c and |E 24x | 2 ,|E 24y | 2 The relationship is as shown in Figure 20, and the solid line |E 24x | 2 φ becomes zero in one period. p -φ p ′+φ c The only possible value is =0. Therefore, |E 24x | 2 φ = 0 c Adjusting φ p -φ p ′+φ c This becomes 0, and the phase difference of the light due to birefringence in the polarization-maintaining fiber 12 is compensated for.

[0142] Next, the φ adjusted above c In this configuration, as shown in Figure 21, a polarization controller 38 with a -45 degree direction and a polarization-maintaining fiber 31 with a -45 degree lagging axis are inserted between the second end of the polarization separation unit 11A and the polarization controller 30. The CW light source 27, polarization separation unit 11A, polarization controller 30, polarization-maintaining fiber 29, polarization-maintaining fiber 12, electro-optic crystal 28, mirror 16, and photodetectors 18 and 19 are the same as in Figure 19. In other words, the output light from the second end of the polarization separation unit 11A on the forward path is input to the polarization controller 30 via the polarization controller 38 with a -45 degree direction and the polarization-maintaining fiber 31 with a -45 degree lagging axis, and the output light from the polarization controller 30 on the return path is input to the second end of the polarization separation unit 11A via the polarization-maintaining fiber 31 with a 45 degree lagging axis (return path) and the polarization controller 38 with a 45 degree direction (return path). Since the following equation holds true at any wavelength, the wavelength of the CW light source 27 in Figure 21 does not have to be the wavelength adjusted above, and the CW light source 27 in Figure 21 may also be a pulsed light source.

[0143] The polarization controller 38 is adjusted so that the intensity of the 0-degree polarized component and the 90-degree polarized component of the light input to the second end of the polarization separation unit 11A are equal, that is, so that the difference between the outputs of the photodetector 19 and the photodetector 18 is zero, or so that the output of the photodetector 19 is half of its maximum value, or so that the output of the photodetector 18 is half of its maximum value. The delay time difference between the slow axis and the fast axis due to the birefringence of the polarization-maintaining fiber 31 is τ d / 2, the phase difference of light between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber 31 is φ d ' / 2, the phase difference of light between the slow axis and the fast axis by the polarization controller 38 is φ c Let's assume it's ' / 2. The delay time difference due to the birefringence of the electro-optic crystal 28 includes the phase difference of light, but as above, we consider it separately as a delay time difference that can be compensated by the polarization-maintaining fiber 31 and a phase difference of light that cannot be compensated.

[0144] In other words, the delay time difference (one way) between the slow axis and the fast axis due to the natural birefringence of the electro-optic crystal 28 and the delay time difference (one way) between the slow axis and the fast axis due to the birefringence of the polarization-maintaining fiber 31 are equal to (τ d / 2) The phase difference of light between the slow axis and the fast axis due to the natural birefringence of the electro-optic crystal 28 (one way) and the phase difference of light between the slow axis and the fast axis due to the birefringence of the polarization-maintaining fiber 31 (one way) are different (φ d / 2≠φ d Let's assume it's ' / 2). The electric field E3 of the light output from the polarization controller 38 on the return path is given by the following equation.

[0145]

number

number

number

[0146] The electric field under measurement is zero (φ e At |E = 0) 3x | 2 -|E 3y | 2 φ = 0 c Adjusting ′ gives cos(φ d -φ d ′+φ e -φ c ') = 0, therefore φ c ′=φ d -φ d '±π / 2. c ′ for |E 3x | 2 -|E 3y | 2 The one where the slope is negative, or |E 3x | 2 The one where the slope is negative, or |E 3y | 2 When you select the option where the slope is positive, the sign is determined, and φ c ′=φ d -φ d It becomes ' + π / 2.

[0147] With the adjustment of polarization controllers 30 and 38 completed as described above, the pulse light source 10 is used, and the output light from polarization controller 38 on the return path is input to the second end of polarization separation unit 11A.c and φ c ′The optical field E output from the polarization controller 38 on the return path after adjustment. 34 In equation (35), φ c ′=φ d -φ d Substituting '±π / 2, we get the following equation.

[0148]

number

number

number

[0149] In this way, the birefringence of the polarization-maintaining fiber 12 and the electro-optic crystal 28 is compensated, including the phase difference of light between the slow axis and the fast axis, |φ e |≪1|E 34x | 2 -|E 34y | 2 ≒±φ e Therefore, differential detection is performed by φ e An output proportional to φ is obtained. In the case of a single-ended configuration, φ e The value obtained is the sum of the terms proportional to φ and the offset. c 'When adjusting φ c |E for ' 3x | 2 -|E 3y | 2 If the sign of the slope of φ is determined, the sign of the above equation is determined. c ′=φ d -φ d If you select the sign of '+π / 2, then |E 34x | 2 -|E 34y | 2 =sin(φ e ) and |φ e |≪1|E 34x | 2 -|E 34y | 2 ≒φ eThis makes it possible to determine the electric field under measurement, including its sign. In either sign, the absolute value of the slope of the sine function is maximized, i.e., maximum sensitivity is obtained. In this embodiment, φ c 'When adjusting φ c |E for ' 3x | 2 -|E 3y | 2 If the sign of the slope is not determined, the sign of the electric field being measured will be undefined. However, in the measurement of high-frequency wireless signals, it is usually not necessary to determine the sign of the electric field, so this is not a problem.

[0150] The third embodiment shown in Figure 17 is a configuration that enables differential detection without using an optical circulator. Similar to the first embodiment, the polarization-maintaining fiber 29, polarization controller 30, polarization-maintaining fiber 31, and polarization controller 38 are arranged on the light source / polarization detection unit side, and it has the same features as the first embodiment. Furthermore, it is possible to include the polarization-maintaining fiber 29 in the electric field probe section, and by connecting the polarization-maintaining fiber 29 and the polarization controller 30 with an optical connector (not shown), the birefringence of the polarization-maintaining fiber 12 is largely compensated in the electric field probe section, making it easy to replace the electric field probe section with polarization-maintaining fibers of various lengths.

[0151] The internal configuration of the polarization separation unit 11A in Figure 17 is the same as that of the polarization separation unit 11A in Figure 2 of the conventional technology. It is also possible to use a polarization separation unit 11B that uses a polarization-maintaining optical circulator 24 and a polarization beam splitter 17 as shown in Figure 22. 0-degree linearly polarized pulsed light from the pulse light source 10 is input to the first end of the optical circulator 24, and 0-degree linearly polarized pulsed light is output from the second end of the optical circulator 24 as the forward path light. The right side from the polarization controller 38 is the same as in Figure 17. The return path light is input to the second end of the optical circulator 24, output from the third end of the optical circulator 24, and input to the first end of the polarization beam splitter 17. The 0-degree polarized component is output from the third end of the polarization beam splitter 17, and the 90-degree polarized component is output from the fourth end of the polarization beam splitter 17. The difference between the intensity of the 0-degree polarized component and the intensity of the 90-degree polarized component is detected by the difference between the output of the photodetector 19 and the output of the photodetector 18.

[0152] Similar to Figure 17, a pulse of 0-degree linearly polarized light is input to the first terminal of the polarization separator 11B, a pulse of 0-degree linearly polarized light is output from the second end of the polarization separator 11B as the forward light, and the 0-degree polarized component and the 90-degree polarized component of the return light input to the second end of the polarization separator 11B are output from the fourth and fifth ends of the polarization separator 11B, respectively, and input to the photodetectors 19 and 18. Thus, Figure 22 yields the same results as Figure 17. However, in the configuration of Figure 22, light travels back and forth through the polarization-maintaining fiber 29, polarization controller 30, polarization-maintaining fiber 31, and polarization controller 38, and light attenuation occurs twice due to losses in the polarization-maintaining fiber 29, polarization controller 30, polarization-maintaining fiber 31, and polarization controller 38. Therefore, when using the optical circulator 24, the configuration of Figure 12 results in less light attenuation and a larger output.

[0153] In the third embodiment, it is also possible to use a polarization separator 11C with polarization-maintaining fibers. Specifically, as shown in Figure 23, when a polarization-maintaining fiber 35 is attached to the first end of the polarization separator 11C and a polarization-maintaining fiber 36 is attached to the second end, the forward path of the polarization-maintaining fibers 35 and 36 is input with 0-degree linearly polarized light aligned with the lagging axis of the polarization-maintaining fibers 35 and 36, so it is not affected by birefringence. For the return path of the light, the polarization separator 11C only detects the intensity of the 0-degree polarized component and the intensity of the 90-degree polarized component, so it is not affected by the birefringence of the return path of the polarization-maintaining fiber 36, and thus the same results as in the case of the polarization separator 11A without polarization-maintaining fibers can be obtained. Furthermore, for the fourth and fifth ends of the polarization separator 11C, only the intensity of the light is detected by the photodetectors 18 and 19, so it is not affected by birefringence and there is no problem even if the polarization changes, so it is also possible to use polarization-maintaining fibers or non-polarization-maintaining fibers 43, 44 (SMF44, SMF45). [Industrial applicability]

[0154] As described above, the present invention has the effect of realizing a small and lightweight electric field probe even if the electro-optic crystal of the electric field probe has natural birefringence, and measuring high-frequency electric fields by equivalent time sampling with pulsed light, and is useful for electric field measuring devices and electric field measuring methods in general. [Explanation of symbols]

[0155] 1, 1A, 1B, 1C, 1D, 1E, 1F, 1G, 1H Electric field measuring device 2. 2A field probe section 3, 3A, 3B, 3C, 3D, 3E, 3F, 3G, 3H Light source / polarization detection section 10 pulse light sources 11, 11A, 11B, 11C Polarization separation section 12, 29, 31, 35, 36, 37 Polarization-maintaining fibers 43, 44 Fibers 13, 22, 34, 40, 42 1 / 2 wave plate 14, 25, 33, 39, 41 1 / 4 wave plate 15, 26, 28 Electro-optic crystals 16 Mirror 17, 20 Polarizing Beam Splitter 18, 19 Receiver 21 Faraday Roteta 23 Differential Amplifier 24 Light Circulator 27 CW light source 30, 32, 38 Polarization controllers

Claims

1. A light source (10) that emits linearly polarized light, An optical circulator (24) that outputs the linearly polarized light input to the first end from the second end, and outputs the light input to the second end from the third end, A polarization-maintaining fiber (12) into which light output from the second end of the optical circulator is input to one end, such that the slow axis or fast axis matches the polarization direction of the light output from the second end of the optical circulator, An electro-optic crystal (28) into which light output from the other end of the polarization-maintaining fiber is input to one end, such that the electrical principal axis is at a 45-degree angle with the retard axis of the polarization-maintaining fiber, A mirror (16) is provided at the other end of the electro-optic crystal and reflects light input to one end of the electro-optic crystal, A polarization beam splitter (17) that reflects the light from the mirror, outputs it from one end of the electro-optic crystal, inputs it to the other end of the polarization-maintaining fiber, outputs it from one end of the polarization-maintaining fiber, inputs it to the second end of the optical circulator, inputs the light output from the third end of the optical circulator to the first end, and outputs either the linear polarization component in the slow axis direction of the polarization-maintaining fiber and the linear polarization component in the fast axis direction of the polarization-maintaining fiber of the light input to the first end from the second end, or both from the third and fourth ends, respectively. A light detector (18, 19) detects the intensity of light output from the second end of the polarizing beam splitter, or the difference in intensity of light output from the third and fourth ends of the polarizing beam splitter. In an electric field measuring device having the following characteristics, for measuring the electric field applied to the electro-optic crystal, The light source is a pulse light source that outputs pulsed light with a predetermined repetition frequency, The aforementioned electro-optic crystal is a crystal that exhibits spontaneous birefringence. Between the third end of the optical circulator and the first end of the polarizing beam splitter, The slow axis is at a 90-degree angle to the slow axis of the polarization-maintaining fiber, the delay time difference between the slow axis and the fast axis due to birefringence is set to be equal to the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber, and a first birefringent medium (29) compensates for the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber, A first polarization controller (30) adjusts the phase difference of light between the slow axis and the fast axis of the first birefringent medium, The retard axis is at a 90-degree angle to the retard axis of the electro-optic crystal, and the delay time difference between the retard axis and the fast axis due to birefringence is set to be equal to the round-trip delay time difference between the retard axis and the fast axis due to the natural birefringence of the electro-optic crystal, and a second birefringence medium (31) that compensates for the delay time difference between the retard axis and the fast axis due to the natural birefringence of the electro-optic crystal for the round trip, A second polarization controller (32) adjusts the phase difference of light between the slow axis and the fast axis of the second birefringent medium, A quarter-wave plate (33) whose slow axis is in the same direction as the slow axis or speed axis of the polarization-maintaining fiber, A half-wave plate (34) whose slow axis makes a 22.5-degree angle with the slow axis or fast axis of the polarization-maintaining fiber, It has, The first birefringent medium and the first polarization controller are positioned on the third end side of the optical circulator, compared to the second birefringent medium and the second polarization controller. The quarter-wave plate is positioned on the first end side of the polarization beam splitter, relative to the second birefringent medium and the second polarization controller. The half-wave plate is positioned closer to the first end of the polarizing beam splitter than the quarter-wave plate. An electric field measuring device that measures the electric field by equivalent time sampling, It outputs linearly polarized CW light, The CW light is input to one end of the polarization-maintaining fiber such that the polarization direction of the CW light matches the slow axis or fast axis of the polarization-maintaining fiber. The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle. The light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. The wavelength of the CW light is adjusted so that the intensity of the linear polarization component in the slow axis direction of the polarization-maintaining fiber and the intensity of the linear polarization component in the fast axis direction of the polarization-maintaining fiber are equal, The wavelength-adjusted CW light is input to one end of the polarization-maintaining fiber so that the polarization direction of the CW light matches the slow axis or fast axis of the polarization-maintaining fiber. The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle. The light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. With respect to the light output from one end of the polarization-maintaining fiber, The delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is compensated using the first birefringent medium, in which the slow axis is at a 90-degree angle to the slow axis of the polarization-maintaining fiber, and the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is set to be equal to the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber. If φp is the phase difference of light between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber, and φp' is the phase difference of light between the slow axis and the fast axis due to birefringence of the first birefringent medium, The phase difference φc of the light between the slow axis and the fast axis of the first polarization controller is adjusted to φc = -φp + φp' such that the light passing through the first polarization controller, the quarter-wave plate whose slow axis is in the same direction as the slow axis or fast axis of the polarization-maintaining fiber, and the half-wave plate whose slow axis is at a 22.5-degree angle with the slow axis or fast axis of the polarization-maintaining fiber becomes linearly polarized in the same direction as the slow axis or fast axis of the polarization-maintaining fiber. It outputs a second light with linear polarization, The second light is input to one end of the polarization-maintaining fiber such that the polarization direction of the second light coincides with the slow axis or speed axis of the polarization-maintaining fiber. The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle. The light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. With respect to the light output from one end of the polarization-maintaining fiber, The delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is compensated using the first birefringent medium, in which the slow axis is at a 90-degree angle to the slow axis of the polarization-maintaining fiber, and the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is set to be equal to the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber. Using the first polarization controller and the result of adjusting the first polarization controller, the phase difference of light between the slow axis and the fast axis of the first birefringent medium is changed by φ c = -φ p + φ p'. The second birefringent medium, in which the retard axis is at a 90-degree angle to the retard axis of the electro-optic crystal, and the delay time difference between the retard axis and the rapid axis due to birefringence is set to be equal to the round-trip delay time difference between the retard axis and the rapid axis due to the natural birefringence of the electro-optic crystal, is used to compensate for the delay time difference between the retard axis and the rapid axis due to the natural birefringence of the electro-optic crystal for the round trip. If φd is the phase difference of light traveling back and forth between the slow axis and the fast axis due to the natural birefringence of the electro-optic crystal, and φd' is the phase difference of light between the slow axis and the fast axis due to the birefringence of the second birefringent medium, The light output from one end of the polarization-maintaining fiber passes through the first birefringent medium, the first polarization controller, the second birefringent medium, and the second polarization controller, and then the phase difference φ c ' of the light between the slow axis and the fast axis of the second polarization controller is adjusted to φ c ' = φ d - φ d ' so that the light becomes linearly polarized in the same direction as the slow axis or fast axis of the polarization-maintaining fiber. Adjusting the phase difference of light between the slow axis and the fast axis of the first birefringent medium using the first polarization controller involves changing the phase difference of light between the slow axis and the fast axis of the first birefringent medium by φ c = -φ p + φ p' using the result of adjusting the first polarization controller. The adjustment of the phase difference of light between the slow axis and the fast axis of the second birefringent medium using the second polarization controller involves changing the phase difference of light between the slow axis and the fast axis of the second birefringent medium by φ c' = φ d - φ d' using the result of the adjustment of the second polarization controller. The compensation for the delay time difference by the first birefringent medium and the change of the phase difference by the first polarization controller are performed before the compensation for the delay time difference by the second birefringent medium and the change of the phase difference by the second polarization controller. An electric field measuring device characterized by the following features.

2. A light source (10) that emits linearly polarized light, An optical circulator (24) that outputs the linearly polarized light input to the first end from the second end, and outputs the light input to the second end from the third end, A polarization-maintaining fiber (12) into which light output from the second end of the optical circulator is input to one end, such that the slow axis or fast axis matches the polarization direction of the light output from the second end of the optical circulator, An electro-optic crystal (28) into which light output from the other end of the polarization-maintaining fiber is input to one end, such that the electrical principal axis is at a 45-degree angle with the retard axis of the polarization-maintaining fiber, A mirror (16) is provided at the other end of the electro-optic crystal and reflects light input to one end of the electro-optic crystal, A polarization beam splitter (17) that reflects the light from the mirror, outputs it from one end of the electro-optic crystal, inputs it to the other end of the polarization-maintaining fiber, outputs it from one end of the polarization-maintaining fiber, inputs it to the second end of the optical circulator, inputs the light output from the third end of the optical circulator to the first end, and outputs either the linear polarization component in the slow axis direction of the polarization-maintaining fiber and the linear polarization component in the fast axis direction of the polarization-maintaining fiber of the light input to the first end from the second end, or both from the third and fourth ends, respectively. A light detector (18, 19) detects the intensity of light output from the second end of the polarizing beam splitter, or the difference in intensity of light output from the third and fourth ends of the polarizing beam splitter. In an electric field measuring device having the following characteristics, for measuring the electric field applied to the electro-optic crystal, The light source is a pulse light source that outputs pulsed light with a predetermined repetition frequency, The aforementioned electro-optic crystal is a crystal that exhibits spontaneous birefringence. Between the third end of the optical circulator and the first end of the polarizing beam splitter, The slow axis is at a 90-degree angle to the slow axis of the polarization-maintaining fiber, the delay time difference between the slow axis and the fast axis due to birefringence is set to be equal to the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber, and a first birefringent medium (29) compensates for the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber, A first polarization controller (30) adjusts the phase difference of light between the slow axis and the fast axis of the first birefringent medium, The retard axis is at a 90-degree angle to the retard axis of the electro-optic crystal, and the delay time difference between the retard axis and the fast axis due to birefringence is set to be equal to the round-trip delay time difference between the retard axis and the fast axis due to the natural birefringence of the electro-optic crystal, and a second birefringence medium (31) that compensates for the delay time difference between the retard axis and the fast axis due to the natural birefringence of the electro-optic crystal for the round trip, A second polarization controller (38) adjusts the phase difference of light between the slow axis and the fast axis of the second birefringent medium, It has, The first birefringent medium and the first polarization controller are positioned on the third end side of the optical circulator, compared to the second birefringent medium and the second polarization controller. An electric field measuring device that measures the electric field by equivalent time sampling, It outputs linearly polarized CW light, The CW light is input to one end of the polarization-maintaining fiber such that the polarization direction of the CW light matches the slow axis or fast axis of the polarization-maintaining fiber. The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle. The light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. The wavelength of the CW light is adjusted so that the intensity of the linear polarization component in the slow axis direction of the polarization-maintaining fiber and the intensity of the linear polarization component in the fast axis direction of the polarization-maintaining fiber are equal, The wavelength-adjusted CW light is input to one end of the polarization-maintaining fiber so that the polarization direction of the CW light matches the slow axis or fast axis of the polarization-maintaining fiber. The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle. The light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. With respect to the light output from one end of the polarization-maintaining fiber, The delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is compensated using the first birefringent medium, in which the slow axis is at a 90-degree angle to the slow axis of the polarization-maintaining fiber, and the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is set to be equal to the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber. If φp is the phase difference of light between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber, and φp' is the phase difference of light between the slow axis and the fast axis due to birefringence of the first birefringent medium, The phase difference φc of the light between the slow axis and the fast axis of the first polarization controller is adjusted to φc = -φp + φp' such that the light passing through the first polarization controller, the first quarter-wave plate (33) whose slow axis is in the same direction as the slow axis or fast axis of the polarization-maintaining fiber, and the first half-wave plate (34) whose slow axis is at a 22.5-degree angle with the slow axis or fast axis of the polarization-maintaining fiber becomes linearly polarized in the same direction as the slow axis or fast axis of the polarization-maintaining fiber. It outputs a second light with linear polarization, The second light is input to one end of the polarization-maintaining fiber such that the polarization direction of the second light coincides with the slow axis or speed axis of the polarization-maintaining fiber. The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle. The light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. With respect to the light output from one end of the polarization-maintaining fiber, The delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is compensated using the first birefringent medium, in which the slow axis is at a 90-degree angle to the slow axis of the polarization-maintaining fiber, and the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is set to be equal to the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber. Using the first polarization controller and the result of adjusting the first polarization controller, the phase difference of light between the slow axis and the fast axis of the first birefringent medium is changed by φ c = -φ p + φ p'. The second birefringent medium, in which the retard axis is at a 90-degree angle to the retard axis of the electro-optic crystal, and the delay time difference between the retard axis and the rapid axis due to birefringence is set to be equal to the round-trip delay time difference between the retard axis and the rapid axis due to the natural birefringence of the electro-optic crystal, is used to compensate for the delay time difference between the retard axis and the rapid axis due to the natural birefringence of the electro-optic crystal for the round trip. If φd is the phase difference of light traveling back and forth between the slow axis and the fast axis due to the natural birefringence of the electro-optic crystal, and φd' is the phase difference of light between the slow axis and the fast axis due to the birefringence of the second birefringent medium, Light output from one end of the polarization-maintaining fiber passes through the first birefringent medium, the first polarization controller, the second birefringent medium, and the second polarization controller, A second quarter-wave plate (39) whose slow axis is in the same direction as the slow axis or speed axis of the polarization-maintaining fiber, The phase difference φ c ' of the light between the slow axis and the fast axis of the second polarization controller is adjusted to φ c ' = φ d - φ d ' + π / 2 such that the light passing through the second half-wave plate (40), whose slow axis makes a 22.5-degree angle with the slow axis or fast axis of the polarization-maintaining fiber, becomes linearly polarized in the same direction as the slow axis or fast axis of the polarization-maintaining fiber. Adjusting the phase difference of light between the slow axis and the fast axis of the first birefringent medium using the first polarization controller involves changing the phase difference of light between the slow axis and the fast axis of the first birefringent medium by φ c = -φ p + φ p' using the result of adjusting the first polarization controller. The adjustment of the phase difference of light between the slow axis and the fast axis of the second birefringent medium using the second polarization controller involves changing the phase difference of light between the slow axis and the fast axis of the second birefringent medium by φ c' = φ d - φ d' + π / 2 using the result of the adjustment of the second polarization controller. The compensation for the delay time difference by the first birefringent medium and the change of the phase difference by the first polarization controller are performed before the compensation for the delay time difference by the second birefringent medium and the change of the phase difference by the second polarization controller. An electric field measuring device characterized by the following features.

3. A light source (10) that outputs linearly polarized light, An optical circulator (24) that outputs the linearly polarized light input to the first end from the second end, and outputs the light input to the second end from the third end, A polarization-maintaining fiber (12) into which light output from the second end of the optical circulator is input to one end, such that the slow axis or fast axis matches the polarization direction of the light output from the second end of the optical circulator, An electro-optic crystal (28) into which light output from the other end of the polarization-maintaining fiber is input to one end, such that the electrical principal axis is at a 45-degree angle with the retard axis of the polarization-maintaining fiber, A mirror (16) is provided at the other end of the electro-optic crystal and reflects light input to one end of the electro-optic crystal, A polarization beam splitter (17) that reflects the light from the mirror, outputs it from one end of the electro-optic crystal, inputs it to the other end of the polarization-maintaining fiber, outputs it from one end of the polarization-maintaining fiber, inputs it to the second end of the optical circulator, inputs the light output from the third end of the optical circulator to the first end, and outputs either the linear polarization component in the slow axis direction of the polarization-maintaining fiber and the linear polarization component in the fast axis direction of the polarization-maintaining fiber of the light input to the first end from the second end, or both from the third and fourth ends, respectively. A light detector (18, 19) detects the intensity of light output from the second end of the polarizing beam splitter, or the difference in intensity of light output from the third and fourth ends of the polarizing beam splitter. In an electric field measuring device having the following characteristics, for measuring the electric field applied to the electro-optic crystal, The light source is a pulse light source that outputs pulsed light with a predetermined repetition frequency, The aforementioned electro-optic crystal is a crystal that exhibits spontaneous birefringence. Between the third end of the optical circulator and the first end of the polarizing beam splitter, The slow axis is at a 90-degree angle to the slow axis of the polarization-maintaining fiber, the delay time difference between the slow axis and the fast axis due to birefringence is set to be equal to the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber, and a first birefringent medium (29) compensates for the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber, A first polarization controller (30) adjusts the phase difference of light between the slow axis and the fast axis of the first birefringent medium, The retard axis is at a 90-degree angle to the retard axis of the electro-optic crystal, and the delay time difference between the retard axis and the fast axis due to birefringence is set to be equal to the round-trip delay time difference between the retard axis and the fast axis due to the natural birefringence of the electro-optic crystal, and a second birefringence medium (31) that compensates for the delay time difference between the retard axis and the fast axis due to the natural birefringence of the electro-optic crystal for the round trip, A second polarization controller (38) adjusts the phase difference of light between the slow axis and the fast axis of the second birefringent medium, It has, The first birefringent medium and the first polarization controller are positioned on the third end side of the optical circulator, compared to the second birefringent medium and the second polarization controller. An electric field measuring device that measures the electric field by equivalent time sampling, It outputs linearly polarized CW light, The CW light is input to one end of the polarization-maintaining fiber such that the polarization direction of the CW light matches the slow axis or fast axis of the polarization-maintaining fiber. The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle. The light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. The wavelength of the CW light is adjusted so that the intensity of the linear polarization component in the slow axis direction of the polarization-maintaining fiber and the intensity of the linear polarization component in the fast axis direction of the polarization-maintaining fiber are equal, The wavelength-adjusted CW light is input to one end of the polarization-maintaining fiber so that the polarization direction of the CW light matches the slow axis or fast axis of the polarization-maintaining fiber. The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle. The light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. With respect to the light output from one end of the polarization-maintaining fiber, The delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is compensated using the first birefringent medium, in which the slow axis is at a 90-degree angle to the slow axis of the polarization-maintaining fiber, and the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is set to be equal to the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber. If φp is the phase difference of light between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber, and φp' is the phase difference of light between the slow axis and the fast axis due to birefringence of the first birefringent medium, The phase difference φc of the light between the slow axis and the fast axis of the first polarization controller is adjusted to φc = -φp + φp' such that the light passing through the first polarization controller, a quarter-wave plate (33) whose slow axis is in the same direction as the slow axis or fast axis of the polarization-maintaining fiber, and a half-wave plate (34) whose slow axis is at a 22.5-degree angle with the slow axis or fast axis of the polarization-maintaining fiber becomes linearly polarized in the same direction as the slow axis or fast axis of the polarization-maintaining fiber. It outputs a second light with linear polarization, The second light is input to one end of the polarization-maintaining fiber such that the polarization direction of the second light coincides with the slow axis or speed axis of the polarization-maintaining fiber. The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle. The light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. With respect to the light output from one end of the polarization-maintaining fiber, The delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is compensated using the first birefringent medium, in which the slow axis is at a 90-degree angle to the slow axis of the polarization-maintaining fiber, and the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is set to be equal to the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber. Using the first polarization controller and the result of adjusting the first polarization controller, the phase difference of light between the slow axis and the fast axis of the first birefringent medium is changed by φ c = -φ p + φ p'. The second birefringent medium, in which the retard axis is at a 90-degree angle to the retard axis of the electro-optic crystal, and the delay time difference between the retard axis and the rapid axis due to birefringence is set to be equal to the round-trip delay time difference between the retard axis and the rapid axis due to the natural birefringence of the electro-optic crystal, is used to compensate for the delay time difference between the retard axis and the rapid axis due to the natural birefringence of the electro-optic crystal for the round trip. If φd is the phase difference of light traveling back and forth between the slow axis and the fast axis due to the natural birefringence of the electro-optic crystal, and φd' is the phase difference of light between the slow axis and the fast axis due to the birefringence of the second birefringent medium, Light output from one end of the polarization-maintaining fiber passes through the first birefringent medium, the first polarization controller, the second birefringent medium, and the second polarization controller, and then the phase difference of light φ c ' between the slow axis and the fast axis of the second polarization controller is adjusted to φ c ' = φ d - φ d ' ± π / 2 so that the intensity of the linear polarization component in the slow axis direction of the polarization-maintaining fiber is equal to the intensity of the linear polarization component in the fast axis direction of the polarization-maintaining fiber. Adjusting the phase difference of light between the slow axis and the fast axis of the first birefringent medium using the first polarization controller involves changing the phase difference of light between the slow axis and the fast axis of the first birefringent medium by φ c = -φ p + φ p' using the result of adjusting the first polarization controller. The adjustment of the phase difference of light between the slow axis and the fast axis of the second birefringent medium using the second polarization controller involves changing the phase difference of light between the slow axis and the fast axis of the second birefringent medium by φ c' = φ d - φ d' ± π / 2 using the result of the adjustment of the second polarization controller. The compensation for the delay time difference by the first birefringent medium and the change of the phase difference by the first polarization controller are performed before the compensation for the delay time difference by the second birefringent medium and the change of the phase difference by the second polarization controller. An electric field measuring device characterized by the following features.

4. Either the first birefringent medium or the first polarization controller, or both, are arranged between the second end of the optical circulator and one end of the polarization-maintaining fiber. The electric field measuring device according to any one of claims 1 to 3.

5. A light source (10) that emits linearly polarized light, A polarization separation unit (11A) outputs the linearly polarized light input to the first end from the second end, and outputs either the linearly polarized component in the slow axis direction of the polarization-maintaining fiber and the linearly polarized component in the fast axis direction of the polarization-maintaining fiber of the light input to the second end from the third end, or both from the fourth and fifth ends, respectively. The polarization-maintaining fiber (12) to which the light output from the second end of the polarization-separating unit is input to one end, such that the slow axis or fast axis matches the polarization direction of the light output from the second end of the polarization-separating unit, An electro-optic crystal (28) into which light output from the other end of the polarization-maintaining fiber is input to one end, such that the electrical principal axis is at a 45-degree angle with the retard axis of the polarization-maintaining fiber, A mirror (16) is provided at the other end of the electro-optic crystal and reflects light input to one end of the electro-optic crystal, A light receiver (18, 19) detects the intensity of light reflected by the mirror, output from one end of the electro-optic crystal, input to the other end of the polarization-maintaining fiber, output from one end of the polarization-maintaining fiber, input to the second end of the polarization-separating unit, and output from the third end of the polarization-separating unit, or the difference in intensity of light output from the fourth and fifth ends of the polarization-separating unit. In an electric field measuring device having the following characteristics, for measuring the electric field applied to the electro-optic crystal, The light source is a pulse light source that outputs pulsed light with a predetermined repetition frequency, The aforementioned electro-optic crystal is a crystal that exhibits spontaneous birefringence. Between the second end of the polarization separation unit and one end of the polarization-maintaining fiber, The slow axis is at a 90-degree angle to the slow axis of the polarization-maintaining fiber, the delay time difference between the slow axis and the fast axis due to birefringence is set to be equal to the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber, and a first birefringent medium (29) compensates for the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber, A first polarization controller (30) adjusts the phase difference of light between the slow axis and the fast axis of the first birefringent medium, The retard axis is at a 90-degree angle to the retard axis of the electro-optic crystal, and the delay time difference between the retard axis and the fast axis due to birefringence is set to be equal to the delay time difference between the retard axis and the fast axis due to the natural birefringence of the electro-optic crystal, and a second birefringence medium (31) that compensates for the delay time difference between the retard axis and the fast axis due to the natural birefringence of the electro-optic crystal for one path, A second polarization controller (38) adjusts the phase difference of light between the slow axis and the fast axis of the second birefringent medium, It has, The first birefringent medium and the first polarization controller are positioned on one end of the polarization-maintaining fiber, compared to the second birefringent medium and the second polarization controller. An electric field measuring device that measures the electric field by equivalent time sampling, It outputs linearly polarized CW light, The CW light is input to one end of the polarization-maintaining fiber such that the polarization direction of the CW light matches the slow axis or fast axis of the polarization-maintaining fiber. The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle. The light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. The wavelength of the CW light is adjusted so that the intensity of the linear polarization component in the slow axis direction of the polarization-maintaining fiber and the intensity of the linear polarization component in the fast axis direction of the polarization-maintaining fiber are equal, The wavelength-adjusted CW light is input to one end of the polarization-maintaining fiber so that the polarization direction of the CW light matches the slow axis or fast axis of the polarization-maintaining fiber. The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle. The light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. With respect to the light output from one end of the polarization-maintaining fiber, The delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is compensated using the first birefringent medium, in which the slow axis is at a 90-degree angle to the slow axis of the polarization-maintaining fiber, and the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is set to be equal to the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber. If φp is the phase difference of light between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber, and φp' is the phase difference of light between the slow axis and the fast axis due to birefringence of the first birefringent medium, The first polarization controller, a quarter-wave plate (41) whose slow axis is in the same direction as the slow axis or speed axis of the polarization-maintaining fiber, and a half-wave plate (42) whose slow axis is at an angle of 11.25 degrees to the slow axis or speed axis of the polarization-maintaining fiber are passed through in order, Before inputting the wavelength-adjusted CW light to one end of the polarization-maintaining fiber, the first birefringent medium, the first polarization controller, the quarter-wave plate, and the half-wave plate are propagated in the opposite direction to the light output from one end of the polarization-maintaining fiber. The phase difference φc of the light between the slow axis and the fast axis of the first polarization controller is adjusted to φc = -φp + φp' so that the light output from one end of the polarization-maintaining fiber, after passing through the first birefringent medium, the first polarization controller, the quarter-wave plate, and the half-wave plate, becomes linearly polarized in the same direction as the slow axis or fast axis of the polarization-maintaining fiber. It outputs a second light with linear polarization, The second light is input to one end of the polarization-maintaining fiber such that the polarization direction of the second light coincides with the slow axis or speed axis of the polarization-maintaining fiber. The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle. The light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. With respect to the light output from one end of the polarization-maintaining fiber, The delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is compensated using the first birefringent medium, in which the slow axis is at a 90-degree angle to the slow axis of the polarization-maintaining fiber, and the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is set to be equal to the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber. Using the first polarization controller and the result of adjusting the first polarization controller, the phase difference of light between the slow axis and the fast axis of the first birefringent medium is changed by φ c = -φ p + φ p'. The second birefringent medium, in which the retard axis is at a 90-degree angle to the retard axis of the electro-optic crystal, and the delay time difference between the retard axis and the fast axis due to birefringence is set to be equal to the delay time difference between the retard axis and the fast axis due to the natural birefringence of the electro-optic crystal, is used to compensate for the delay time difference between the retard axis and the fast axis due to the natural birefringence of the electro-optic crystal for one path. Before inputting the second light to one end of the polarization-maintaining fiber, the first birefringent medium, the first polarization controller, the second birefringent medium, and the second polarization controller are propagated in the opposite direction to the light output from one end of the polarization-maintaining fiber. If we denote the phase difference of light traveling back and forth between the slow axis and the fast axis due to the natural birefringence of the electro-optic crystal as φd, and the phase difference of light between the slow axis and the fast axis due to the birefringence of the second birefringent medium as φd' / 2, The light output from one end of the polarization-maintaining fiber passes through the first birefringent medium, the first polarization controller, the second birefringent medium, and the second polarization controller, and then the second polarization controller adjusts the phase difference of light between the slow axis and the fast axis of the second polarization controller to φ c ' = φ d - φ d ' ± π / 2 so that the intensity of the linear polarization component in the slow axis direction of the polarization-maintaining fiber is equal to the intensity of the linear polarization component in the fast axis direction of the polarization-maintaining fiber. Adjusting the phase difference of light between the slow axis and the fast axis of the first birefringent medium using the first polarization controller involves changing the phase difference of light between the slow axis and the fast axis of the first birefringent medium by φ c = -φ p + φ p' using the result of adjusting the first polarization controller. The adjustment of the phase difference of light between the slow axis and the fast axis of the second birefringent medium using the second polarization controller involves changing twice the phase difference of light between the slow axis and the fast axis of the second birefringent medium by φ c' = φ d - φ d' ± π / 2 using the result of the adjustment of the second polarization controller. The compensation for the delay time difference by the first birefringent medium and the change of the phase difference by the first polarization controller are performed before the compensation for the delay time difference by the second birefringent medium and the change of the phase difference by the second polarization controller. An electric field measuring device characterized by the following features.

6. It outputs the first light with linear polarization, The first light is input to one end of the polarization-maintaining fiber (12) such that the polarization direction of the first light coincides with the slow axis or fast axis of the polarization-maintaining fiber (12). The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal (28) such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal (28) form a 45-degree angle. A mirror (16) provided at the other end of the electro-optic crystal reflects the light input to one end of the electro-optic crystal and outputs it from the other end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. By detecting the difference between the intensity of the linear polarization component in the slow axis direction of the polarization-maintaining fiber and the intensity of the linear polarization component in the fast axis direction of the polarization-maintaining fiber, or both, of the light output from one end of the polarization-maintaining fiber, In an electric field measurement method for measuring the electric field applied to the aforementioned electro-optic crystal, The first light is pulsed light with a predetermined repetition frequency, The aforementioned electro-optic crystal is a crystal that exhibits spontaneous birefringence. Before detecting the difference between either or both of the intensity of the linear polarization component in the slow axis direction and the linear polarization component in the fast axis direction of the polarization-maintaining fiber with respect to the light output from one end of the polarization-maintaining fiber, The delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is compensated using a first birefringent medium (29) in which the slow axis is at a 90-degree angle to the slow axis of the polarization-maintaining fiber, and the delay time difference between the slow axis and the fast axis due to birefringence is set to be equal to the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber. The first polarization controller (30) is used to adjust the phase difference of light between the slow axis and the fast axis of the first birefringent medium. A second birefringent medium (31) is used, in which the retard axis is at a 90-degree angle to the retard axis of the electro-optic crystal, and the delay time difference between the retard axis and the fast axis due to birefringence is set to be equal to the round-trip delay time difference between the retard axis and the fast axis due to the natural birefringence of the electro-optic crystal, thereby compensating for the delay time difference between the retard axis and the fast axis due to the natural birefringence of the electro-optic crystal for the round trip. The second polarization controller (32) is used to adjust the phase difference of light between the slow axis and the fast axis of the second birefringent medium. The compensation for the delay time difference by the first birefringent medium and the adjustment of the phase difference by the first polarization controller are performed before the compensation for the delay time difference by the second birefringent medium and the adjustment of the phase difference by the second polarization controller. Light output from one end of the polarization-maintaining fiber passes through the first birefringent medium, the first polarization controller, the second birefringent medium, and the second polarization controller, A quarter-wave plate (33) whose slow axis is in the same direction as the slow axis or speed axis of the polarization-maintaining fiber, The slow axis passes sequentially through a half-wave plate (34) that makes a 22.5-degree angle with the slow axis or fast axis of the polarization-maintaining fiber, An electric field measurement method for measuring the electric field by equivalent time sampling, It outputs linearly polarized CW light, The CW light is input to one end of the polarization-maintaining fiber such that the polarization direction of the CW light matches the slow axis or fast axis of the polarization-maintaining fiber. The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle. The light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. The wavelength of the CW light is adjusted so that the intensity of the linear polarization component in the slow axis direction of the polarization-maintaining fiber and the intensity of the linear polarization component in the fast axis direction of the polarization-maintaining fiber are equal, The wavelength-adjusted CW light is input to one end of the polarization-maintaining fiber so that the polarization direction of the CW light matches the slow axis or fast axis of the polarization-maintaining fiber. The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle. The light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. With respect to the light output from one end of the polarization-maintaining fiber, The delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is compensated using the first birefringent medium, in which the slow axis is at a 90-degree angle to the slow axis of the polarization-maintaining fiber, and the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is set to be equal to the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber. If φp is the phase difference of light between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber, and φp' is the phase difference of light between the slow axis and the fast axis due to birefringence of the first birefringent medium, The phase difference φc of the light between the slow axis and the fast axis of the first polarization controller is adjusted to φc = -φp + φp' such that the light passing through the first polarization controller, the quarter-wave plate whose slow axis is in the same direction as the slow axis or fast axis of the polarization-maintaining fiber, and the half-wave plate whose slow axis is at a 22.5-degree angle with the slow axis or fast axis of the polarization-maintaining fiber becomes linearly polarized in the same direction as the slow axis or fast axis of the polarization-maintaining fiber. It outputs a second light with linear polarization, The second light is input to one end of the polarization-maintaining fiber such that the polarization direction of the second light coincides with the slow axis or speed axis of the polarization-maintaining fiber. The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle. The light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. With respect to the light output from one end of the polarization-maintaining fiber, The delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is compensated using the first birefringent medium, in which the slow axis is at a 90-degree angle to the slow axis of the polarization-maintaining fiber, and the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is set to be equal to the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber. Using the first polarization controller and the result of adjusting the first polarization controller, the phase difference of light between the slow axis and the fast axis of the first birefringent medium is changed by φ c = -φ p + φ p'. The second birefringent medium, in which the retard axis is at a 90-degree angle to the retard axis of the electro-optic crystal, and the delay time difference between the retard axis and the rapid axis due to birefringence is set to be equal to the round-trip delay time difference between the retard axis and the rapid axis due to the natural birefringence of the electro-optic crystal, is used to compensate for the delay time difference between the retard axis and the rapid axis due to the natural birefringence of the electro-optic crystal for the round trip. If φd is the phase difference of light traveling back and forth between the slow axis and the fast axis due to the natural birefringence of the electro-optic crystal, and φd' is the phase difference of light between the slow axis and the fast axis due to the birefringence of the second birefringent medium, The light output from one end of the polarization-maintaining fiber passes through the first birefringent medium, the first polarization controller, the second birefringent medium, and the second polarization controller, and then the phase difference φ c ' of the light between the slow axis and the fast axis of the second polarization controller is adjusted to φ c ' = φ d - φ d ' so that the light becomes linearly polarized in the same direction as the slow axis or fast axis of the polarization-maintaining fiber. Adjusting the phase difference of light between the slow axis and the fast axis of the first birefringent medium using the first polarization controller involves changing the phase difference of light between the slow axis and the fast axis of the first birefringent medium by φ c = -φ p + φ p' using the result of adjusting the first polarization controller. The adjustment of the phase difference of light between the slow axis and the fast axis of the second birefringent medium using the second polarization controller involves changing the phase difference of light between the slow axis and the fast axis of the second birefringent medium by φ c' = φ d - φ d' using the result of the adjustment of the second polarization controller. The compensation for the delay time difference by the first birefringent medium and the change of the phase difference by the first polarization controller are performed before the compensation for the delay time difference by the second birefringent medium and the change of the phase difference by the second polarization controller. A method for measuring electric fields characterized by the following features.

7. It outputs the first light with linear polarization, The first light is input to one end of the polarization-maintaining fiber (12) such that the polarization direction of the first light coincides with the slow axis or fast axis of the polarization-maintaining fiber (12). The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal (28) such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal (28) form a 45-degree angle. A mirror (16) provided at the other end of the electro-optic crystal reflects the light input to one end of the electro-optic crystal and outputs it from the other end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. By detecting the difference between the intensity of the linear polarization component in the slow axis direction of the polarization-maintaining fiber and the intensity of the linear polarization component in the fast axis direction of the polarization-maintaining fiber, or both, of the light output from one end of the polarization-maintaining fiber, In an electric field measurement method for measuring the electric field applied to the aforementioned electro-optic crystal, The first light is pulsed light with a predetermined repetition frequency, The aforementioned electro-optic crystal is a crystal that exhibits spontaneous birefringence. Before detecting the difference between either or both of the intensity of the linear polarization component in the slow axis direction and the linear polarization component in the fast axis direction of the polarization-maintaining fiber with respect to the light output from one end of the polarization-maintaining fiber, The delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is compensated using a first birefringent medium (29) in which the slow axis is at a 90-degree angle to the slow axis of the polarization-maintaining fiber, and the delay time difference between the slow axis and the fast axis due to birefringence is set to be equal to the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber. The first polarization controller (30) is used to adjust the phase difference of light between the slow axis and the fast axis of the first birefringent medium. A second birefringent medium (31) is used, in which the retard axis is at a 90-degree angle to the retard axis of the electro-optic crystal, and the delay time difference between the retard axis and the fast axis due to birefringence is set to be equal to the round-trip delay time difference between the retard axis and the fast axis due to the natural birefringence of the electro-optic crystal, thereby compensating for the delay time difference between the retard axis and the fast axis due to the natural birefringence of the electro-optic crystal for the round trip. The second polarization controller (38) is used to adjust the phase difference of light between the slow axis and the fast axis of the second birefringent medium. The compensation for the delay time difference by the first birefringent medium and the adjustment of the phase difference by the first polarization controller are performed before the compensation for the delay time difference by the second birefringent medium and the adjustment of the phase difference by the second polarization controller. An electric field measurement method for measuring the electric field by equivalent time sampling, It outputs linearly polarized CW light, The CW light is input to one end of the polarization-maintaining fiber such that the polarization direction of the CW light matches the slow axis or fast axis of the polarization-maintaining fiber. The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle. The light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. The wavelength of the CW light is adjusted so that the intensity of the linear polarization component in the slow axis direction of the polarization-maintaining fiber and the intensity of the linear polarization component in the fast axis direction of the polarization-maintaining fiber are equal, The wavelength-adjusted CW light is input to one end of the polarization-maintaining fiber so that the polarization direction of the CW light matches the slow axis or fast axis of the polarization-maintaining fiber. The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle. The light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. With respect to the light output from one end of the polarization-maintaining fiber, The delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is compensated using the first birefringent medium, in which the slow axis is at a 90-degree angle to the slow axis of the polarization-maintaining fiber, and the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is set to be equal to the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber. If φp is the phase difference of light between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber, and φp' is the phase difference of light between the slow axis and the fast axis due to birefringence of the first birefringent medium, The phase difference φc of the light between the slow axis and the fast axis of the first polarization controller is adjusted to φc = -φp + φp' such that the light passing through the first polarization controller, the first quarter-wave plate (33) whose slow axis is in the same direction as the slow axis or fast axis of the polarization-maintaining fiber, and the first half-wave plate (34) whose slow axis is at a 22.5-degree angle with the slow axis or fast axis of the polarization-maintaining fiber becomes linearly polarized in the same direction as the slow axis or fast axis of the polarization-maintaining fiber. It outputs a second light with linear polarization, The second light is input to one end of the polarization-maintaining fiber such that the polarization direction of the second light coincides with the slow axis or speed axis of the polarization-maintaining fiber. The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle. The light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. With respect to the light output from one end of the polarization-maintaining fiber, The delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is compensated using the first birefringent medium, in which the slow axis is at a 90-degree angle to the slow axis of the polarization-maintaining fiber, and the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is set to be equal to the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber. Using the first polarization controller and the result of adjusting the first polarization controller, the phase difference of light between the slow axis and the fast axis of the first birefringent medium is changed by φ c = -φ p + φ p'. The second birefringent medium, in which the retard axis is at a 90-degree angle to the retard axis of the electro-optic crystal, and the delay time difference between the retard axis and the rapid axis due to birefringence is set to be equal to the round-trip delay time difference between the retard axis and the rapid axis due to the natural birefringence of the electro-optic crystal, is used to compensate for the delay time difference between the retard axis and the rapid axis due to the natural birefringence of the electro-optic crystal for the round trip. If φd is the phase difference of light traveling back and forth between the slow axis and the fast axis due to the natural birefringence of the electro-optic crystal, and φd' is the phase difference of light between the slow axis and the fast axis due to the birefringence of the second birefringent medium, Light output from one end of the polarization-maintaining fiber passes through the first birefringent medium, the first polarization controller, the second birefringent medium, and the second polarization controller, A second quarter-wave plate (39) whose slow axis is in the same direction as the slow axis or speed axis of the polarization-maintaining fiber, The phase difference φ c ' of the light between the slow axis and the fast axis of the second polarization controller is adjusted to φ c ' = φ d - φ d ' + π / 2 such that the light passing through the second half-wave plate (40), whose slow axis makes a 22.5-degree angle with the slow axis or fast axis of the polarization-maintaining fiber, becomes linearly polarized in the same direction as the slow axis or fast axis of the polarization-maintaining fiber. Adjusting the phase difference of light between the slow axis and the fast axis of the first birefringent medium using the first polarization controller involves changing the phase difference of light between the slow axis and the fast axis of the first birefringent medium by φ c = -φ p + φ p' using the result of adjusting the first polarization controller. The adjustment of the phase difference of light between the slow axis and the fast axis of the second birefringent medium using the second polarization controller involves changing the phase difference of light between the slow axis and the fast axis of the second birefringent medium by φ c' = φ d - φ d' + π / 2 using the result of the adjustment of the second polarization controller. The compensation for the delay time difference by the first birefringent medium and the change of the phase difference by the first polarization controller are performed before the compensation for the delay time difference by the second birefringent medium and the change of the phase difference by the second polarization controller. A method for measuring electric fields characterized by the following features.

8. Outputting a first light with linear polarization, The first light is input to one end of the polarization-maintaining fiber (12) such that the polarization direction of the first light coincides with the slow axis or fast axis of the polarization-maintaining fiber (12). The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal (28) such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal (28) form a 45-degree angle. A mirror (16) provided at the other end of the electro-optic crystal reflects the light input to one end of the electro-optic crystal and outputs it from the other end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. By detecting the difference between the intensity of the linear polarization component in the slow axis direction of the polarization-maintaining fiber and the intensity of the linear polarization component in the fast axis direction of the polarization-maintaining fiber, or both, of the light output from one end of the polarization-maintaining fiber, In an electric field measurement method for measuring the electric field applied to the aforementioned electro-optic crystal, The first light is pulsed light with a predetermined repetition frequency, The aforementioned electro-optic crystal is a crystal that exhibits spontaneous birefringence. Before detecting the difference between either or both of the intensity of the linear polarization component in the slow axis direction and the linear polarization component in the fast axis direction of the polarization-maintaining fiber with respect to the light output from one end of the polarization-maintaining fiber, The delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is compensated using a first birefringent medium (29) in which the slow axis is at a 90-degree angle to the slow axis of the polarization-maintaining fiber, and the delay time difference between the slow axis and the fast axis due to birefringence is set to be equal to the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber. The first polarization controller (30) is used to adjust the phase difference of light between the slow axis and the fast axis of the first birefringent medium. A second birefringent medium (31) is used, in which the retard axis is at a 90-degree angle to the retard axis of the electro-optic crystal, and the delay time difference between the retard axis and the fast axis due to birefringence is set to be equal to the round-trip delay time difference between the retard axis and the fast axis due to the natural birefringence of the electro-optic crystal, thereby compensating for the delay time difference between the retard axis and the fast axis due to the natural birefringence of the electro-optic crystal for the round trip. The second polarization controller (38) is used to adjust the phase difference of light between the slow axis and the fast axis of the second birefringent medium. The compensation for the delay time difference by the first birefringent medium and the adjustment of the phase difference by the first polarization controller are performed before the compensation for the delay time difference by the second birefringent medium and the adjustment of the phase difference by the second polarization controller. An electric field measurement method for measuring the electric field by equivalent time sampling, It outputs linearly polarized CW light, The CW light is input to one end of the polarization-maintaining fiber such that the polarization direction of the CW light matches the slow axis or fast axis of the polarization-maintaining fiber. The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle. The light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. The wavelength of the CW light is adjusted so that the intensity of the linear polarization component in the slow axis direction of the polarization-maintaining fiber and the intensity of the linear polarization component in the fast axis direction of the polarization-maintaining fiber are equal, The wavelength-adjusted CW light is input to one end of the polarization-maintaining fiber so that the polarization direction of the CW light matches the slow axis or fast axis of the polarization-maintaining fiber. The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle. The light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. With respect to the light output from one end of the polarization-maintaining fiber, The delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is compensated using the first birefringent medium, in which the slow axis is at a 90-degree angle to the slow axis of the polarization-maintaining fiber, and the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is set to be equal to the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber. If φp is the phase difference of light between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber, and φp' is the phase difference of light between the slow axis and the fast axis due to birefringence of the first birefringent medium, The phase difference φc of the light between the slow axis and the fast axis of the first polarization controller is adjusted to φc = -φp + φp' such that the light passing through the first polarization controller, a quarter-wave plate (33) whose slow axis is in the same direction as the slow axis or fast axis of the polarization-maintaining fiber, and a half-wave plate (34) whose slow axis is at a 22.5-degree angle with the slow axis or fast axis of the polarization-maintaining fiber becomes linearly polarized in the same direction as the slow axis or fast axis of the polarization-maintaining fiber. It outputs a second light with linear polarization, The second light is input to one end of the polarization-maintaining fiber such that the polarization direction of the second light coincides with the slow axis or speed axis of the polarization-maintaining fiber. The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle. The light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. With respect to the light output from one end of the polarization-maintaining fiber, The delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is compensated using the first birefringent medium, in which the slow axis is at a 90-degree angle to the slow axis of the polarization-maintaining fiber, and the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is set to be equal to the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber. Using the first polarization controller and the result of adjusting the first polarization controller, the phase difference of light between the slow axis and the fast axis of the first birefringent medium is changed by φ c = -φ p + φ p'. The second birefringent medium, in which the retard axis is at a 90-degree angle to the retard axis of the electro-optic crystal, and the delay time difference between the retard axis and the rapid axis due to birefringence is set to be equal to the round-trip delay time difference between the retard axis and the rapid axis due to the natural birefringence of the electro-optic crystal, is used to compensate for the delay time difference between the retard axis and the rapid axis due to the natural birefringence of the electro-optic crystal for the round trip. If φd is the phase difference of light traveling back and forth between the slow axis and the fast axis due to the natural birefringence of the electro-optic crystal, and φd' is the phase difference of light between the slow axis and the fast axis due to the birefringence of the second birefringent medium, Light output from one end of the polarization-maintaining fiber passes through the first birefringent medium, the first polarization controller, the second birefringent medium, and the second polarization controller, and then the phase difference of light φ c ' between the slow axis and the fast axis of the second polarization controller is adjusted to φ c ' = φ d - φ d ' ± π / 2 so that the intensity of the linear polarization component in the slow axis direction of the polarization-maintaining fiber is equal to the intensity of the linear polarization component in the fast axis direction of the polarization-maintaining fiber. Adjusting the phase difference of light between the slow axis and the fast axis of the first birefringent medium using the first polarization controller involves changing the phase difference of light between the slow axis and the fast axis of the first birefringent medium by φ c = -φ p + φ p' using the result of adjusting the first polarization controller. The adjustment of the phase difference of light between the slow axis and the fast axis of the second birefringent medium using the second polarization controller involves changing the phase difference of light between the slow axis and the fast axis of the second birefringent medium by φ c' = φ d - φ d' ± π / 2 using the result of the adjustment of the second polarization controller. The compensation for the delay time difference by the first birefringent medium and the change of the phase difference by the first polarization controller are performed before the compensation for the delay time difference by the second birefringent medium and the change of the phase difference by the second polarization controller. A method for measuring electric fields characterized by the following features.

9. It outputs the first light with linear polarization, The first light is input to one end of the polarization-maintaining fiber (12) such that the polarization direction of the first light coincides with the slow axis or fast axis of the polarization-maintaining fiber (12). The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal (28) such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal (28) form a 45-degree angle. A mirror (16) provided at the other end of the electro-optic crystal reflects the light input to one end of the electro-optic crystal and outputs it from the other end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. By detecting the difference between the intensity of the linear polarization component in the slow axis direction of the polarization-maintaining fiber and the intensity of the linear polarization component in the fast axis direction of the polarization-maintaining fiber, or both, of the light output from one end of the polarization-maintaining fiber, In an electric field measurement method for measuring the electric field applied to the aforementioned electro-optic crystal, The first light is pulsed light with a predetermined repetition frequency, The aforementioned electro-optic crystal is a crystal that exhibits spontaneous birefringence. Before detecting the difference between either or both of the intensity of the linear polarization component in the slow axis direction and the linear polarization component in the fast axis direction of the polarization-maintaining fiber with respect to the light output from one end of the polarization-maintaining fiber, The delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is compensated using a first birefringent medium (29) in which the slow axis is at a 90-degree angle to the slow axis of the polarization-maintaining fiber, and the delay time difference between the slow axis and the fast axis due to birefringence is set to be equal to the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber. The first polarization controller (30) is used to adjust the phase difference of light between the slow axis and the fast axis of the first birefringent medium. A second birefringent medium (31) is used, in which the retard axis is at a 90-degree angle to the retard axis of the electro-optic crystal, and the delay time difference between the retard axis and the fast axis due to birefringence is set to be equal to the delay time difference between the retard axis and the fast axis due to the natural birefringence of the electro-optic crystal, thereby compensating for the delay time difference between the retard axis and the fast axis due to the natural birefringence of the electro-optic crystal for one path. The second polarization controller (38) is used to adjust the phase difference of light between the slow axis and the fast axis of the second birefringent medium. The compensation for the delay time difference by the first birefringent medium and the adjustment of the phase difference by the first polarization controller are performed before the compensation for the delay time difference by the second birefringent medium and the adjustment of the phase difference by the second polarization controller. Before inputting the pulsed light to one end of the polarization-maintaining fiber, the first birefringent medium, the first polarization controller, the second birefringent medium, and the second polarization controller are propagated in the opposite direction to the light output from one end of the polarization-maintaining fiber. An electric field measurement method for measuring the electric field by equivalent time sampling, It outputs linearly polarized CW light, The CW light is input to one end of the polarization-maintaining fiber such that the polarization direction of the CW light matches the slow axis or fast axis of the polarization-maintaining fiber. The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle. The light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. The wavelength of the CW light is adjusted so that the intensity of the linear polarization component in the slow axis direction of the polarization-maintaining fiber and the intensity of the linear polarization component in the fast axis direction of the polarization-maintaining fiber are equal, The wavelength-adjusted CW light is input to one end of the polarization-maintaining fiber so that the polarization direction of the CW light matches the slow axis or fast axis of the polarization-maintaining fiber. The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle. The light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. With respect to the light output from one end of the polarization-maintaining fiber, The delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is compensated using the first birefringent medium, in which the slow axis is at a 90-degree angle to the slow axis of the polarization-maintaining fiber, and the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is set to be equal to the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber. If φp is the phase difference of light between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber, and φp' is the phase difference of light between the slow axis and the fast axis due to birefringence of the first birefringent medium, The first polarization controller, a quarter-wave plate (41) whose slow axis is in the same direction as the slow axis or speed axis of the polarization-maintaining fiber, and a half-wave plate (42) whose slow axis is at an angle of 11.25 degrees to the slow axis or speed axis of the polarization-maintaining fiber are passed through in order, Before inputting the wavelength-adjusted CW light to one end of the polarization-maintaining fiber, the first birefringent medium, the first polarization controller, the quarter-wave plate, and the half-wave plate are propagated in the opposite direction to the light output from one end of the polarization-maintaining fiber. The phase difference φc of the light between the slow axis and the fast axis of the first polarization controller is adjusted to φc = -φp + φp' so that the light output from one end of the polarization-maintaining fiber, after passing through the first birefringent medium, the first polarization controller, the quarter-wave plate, and the half-wave plate, becomes linearly polarized in the same direction as the slow axis or fast axis of the polarization-maintaining fiber. It outputs a second light with linear polarization, The second light is input to one end of the polarization-maintaining fiber such that the polarization direction of the second light coincides with the slow axis or speed axis of the polarization-maintaining fiber. The light output from the other end of the polarization-maintaining fiber is input to one end of the electro-optic crystal such that the polarization direction of the light output from the other end of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle. The light input to one end of the electro-optic crystal is reflected by the mirror and output from one end of the electro-optic crystal. The light output from one end of the electro-optic crystal is input to the other end of the polarization-maintaining fiber. With respect to the light output from one end of the polarization-maintaining fiber, The delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is compensated using the first birefringent medium, in which the slow axis is at a 90-degree angle to the slow axis of the polarization-maintaining fiber, and the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber is set to be equal to the delay time difference between the slow axis and the fast axis due to birefringence of the polarization-maintaining fiber. Using the first polarization controller and the result of adjusting the first polarization controller, the phase difference of light between the slow axis and the fast axis of the first birefringent medium is changed by φ c = -φ p + φ p'. The second birefringent medium, in which the retard axis is at a 90-degree angle to the retard axis of the electro-optic crystal, and the delay time difference between the retard axis and the fast axis due to birefringence is set to be equal to the delay time difference between the retard axis and the fast axis due to the natural birefringence of the electro-optic crystal, is used to compensate for the delay time difference between the retard axis and the fast axis due to the natural birefringence of the electro-optic crystal for one path. Before inputting the second light to one end of the polarization-maintaining fiber, the first birefringent medium, the first polarization controller, the second birefringent medium, and the second polarization controller are propagated in the opposite direction to the light output from one end of the polarization-maintaining fiber. If we denote the phase difference of light traveling back and forth between the slow axis and the fast axis due to the natural birefringence of the electro-optic crystal as φd, and the phase difference of light between the slow axis and the fast axis due to the birefringence of the second birefringent medium as φd' / 2, The light output from one end of the polarization-maintaining fiber passes through the first birefringent medium, the first polarization controller, the second birefringent medium, and the second polarization controller, and then the second polarization controller adjusts the phase difference of light between the slow axis and the fast axis of the second polarization controller to φ c ' = φ d - φ d ' ± π / 2 so that the intensity of the linear polarization component in the slow axis direction of the polarization-maintaining fiber is equal to the intensity of the linear polarization component in the fast axis direction of the polarization-maintaining fiber. Adjusting the phase difference of light between the slow axis and the fast axis of the first birefringent medium using the first polarization controller involves changing the phase difference of light between the slow axis and the fast axis of the first birefringent medium by φ c = -φ p + φ p' using the result of adjusting the first polarization controller. The adjustment of the phase difference of light between the slow axis and the fast axis of the second birefringent medium using the second polarization controller involves changing twice the phase difference of light between the slow axis and the fast axis of the second birefringent medium by φ c' = φ d - φ d' ± π / 2 using the result of the adjustment of the second polarization controller. The compensation for the delay time difference by the first birefringent medium and the change of the phase difference by the first polarization controller are performed before the compensation for the delay time difference by the second birefringent medium and the change of the phase difference by the second polarization controller. A method for measuring electric fields characterized by the following features.

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