Birefringence measuring device and birefringence measuring method

Abstract

The present invention provides a birefringence measuring device for measuring the birefringence of a polarization-maintaining fiber and an electro-optic crystal, using an integrated electro-optic probe in which the retard axis of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle. [Solution] The birefringence measuring device 1 measures the birefringence of an electro-optic probe 2, which consists of a polarization-maintaining fiber 11 and an electro-optic crystal 17 having natural birefringence. The birefringence measuring device includes an optical circulator 16 that inputs CW light to the polarization-maintaining fiber and outputs light output from the polarization-maintaining fiber, a half-wave plate 19 with an angle θ1 / 2 into which the light output from the optical circulator is input, photodetectors 13 and 14 that detect the difference in intensity between the 0-degree polarized component and the 90-degree polarized component of the light output from the half-wave plate, and a signal processing unit 20 that determines the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal from the output of the photodetectors when the wavelength of the CW light is swept.

Description

[Technical Field]

[0001] This invention relates to a technique for measuring the birefringence of an electro-optic probe consisting of an electro-optic crystal having natural birefringence and a polarization-maintaining fiber. [Background technology]

[0002] (Method for measuring birefringence of polarization-maintaining fibers) Polarization-maintaining fibers exhibit birefringence, where the propagation delay time differs between linearly polarized light in the slow axis direction and linearly polarized light in the fast axis direction. A wavelength sweep method, described in Non-Patent Literature 1, is known as a method for measuring the birefringence of polarization-maintaining fibers. Figure 1 shows an example configuration of a wavelength sweep method using a narrowband light source.

[0003] In the configuration example shown in Figure 1, the CW light source 10 is a tunable light source and outputs 0-degree linearly polarized CW light (narrowband light). The polarization-maintaining fiber 11 (PMF1) has its lagging axis positioned at 45 degrees, and 0-degree linearly polarized CW light is input to the polarization-maintaining fiber 11. The CW light output from the polarization-maintaining fiber 11 is input to the polarization beam splitter 12 (PBS1), which transmits the 0-degree polarized component of the input light and outputs it to the photodetector 13 (PD1), and reflects the 90-degree polarized component and outputs it to the photodetector 14 (PD2). Photodetectors 13 and 14 each output an electrical signal proportional to the intensity of the input light. By taking the difference between the output of photodetector 13 and the output of photodetector 14 using the differential amplifier 15, 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 polarization-maintaining fiber 11 is obtained.

[0004] In other words, the polarization direction of the CW light input to the polarization-maintaining fiber 11 is at a 45-degree angle to the slow axis and fast axis of the polarization-maintaining fiber 11, so CW light with the same phase is input to the slow axis and fast axis of the polarization-maintaining fiber 11. Then, the polarization beam splitter 12 extracts the 0-degree polarization component and the 90-degree polarization component of the light output from the polarization-maintaining fiber 11, which means that the sum and difference of the CW light waves output from the slow axis and fast axis of the polarization-maintaining fiber 11 are output. Due to the birefringence of the polarization-maintaining fiber 11, a phase difference is generated in the CW light waves output from the slow axis and fast axis, and by taking the sum and difference of the CW light waves, the phase difference is converted into intensity. Since the slow axis and fast axis of the polarization-maintaining fiber 11 have a constant delay time difference, the phase difference between the slow axis and fast axis changes by sweeping the wavelength of the CW light, and the intensity of the light output from the polarization beam splitter 12 changes periodically. Therefore, by sweeping the wavelength of CW light and measuring the period of change in the photodetector output, the birefringence of the polarization-maintaining fiber 11 can be determined.

[0005] Expressed quantitatively using mathematical formulas, the following can be obtained. The electric field of CW 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 output from the polarization-maintaining fiber 11. In this specification, the imaginary number +j in the Jones vector indicates that the phase is 90 degrees ahead of the real number +1. Then, in a right-handed coordinate system, the direction of light propagation is defined as the positive z-axis, and the angles of the optical elements are defined as 0 degrees in the positive x-axis direction and 90 degrees in the positive y-axis direction. The electric field of the light output from the CW light source 10 is E0, and the electric field of the light output from the polarization-maintaining fiber 11 is E 11 , the delay time difference due to birefringence of the polarization-maintaining fiber 11 is τ p If we let the angular frequency of light be ω, then the following equation holds.

[0006]

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[0007] From the above equation, by sweeping the wavelength of the CW light and measuring the difference |E 11x | 2 -|E 11y | 2 between the outputs of the photoreceivers 13 and 14, it varies sinusoidally with respect to the sweep of the optical angular frequency ω. If the period of the sine wave expressed in terms of the optical angular frequency is Δω, then Δωτ p = 2π, so the reciprocal of the period of the photoreceiver output expressed in terms of the optical frequency is τ p , and the delay time difference τ p due to the birefringence of the polarization-maintaining fiber 11 can be obtained.

[0008] In the above configuration, a CW light (narrow-band light) and a photoreceiver without wavelength selectivity are used. However, the same measurement is possible by using a broadband light and a spectrometer with wavelength selectivity.

[0009] (Electro-Optic Probe and Electric Field Measuring Device Using the Same) An electric field measurement method using an electro-optic crystal whose birefringence changes due to an electric field has been proposed. In Patent Document 1, a reflective electro-optic probe is disclosed in which the electrical principal axis of the electro-optic crystal makes an angle of 45 degrees with the slow axis of the polarization-maintaining fiber and a mirror is provided at the tip of the electro-optic crystal. A configuration example of an electric field measuring device using this electro-optic probe is shown in FIG. 2.

[0010] In FIG. 2, the CW light source 10 outputs CW light (narrow-band light) with 0-degree linear polarization. The CW light is input to the first end of the optical circulator 16 (CIR). The optical circulator 16 is polarization-maintaining. The light input to the first end is output from the second end while maintaining its polarization. The 0-degree linearly polarized light output from the second end of the optical circulator 16 is input to the left side of the polarization-maintaining fiber 11.

[0011] The polarization-maintaining fiber 11 has its lagging axis in the 0-degree direction, and since the polarization direction of the input light coincides with the direction of its lagging axis, the 0-degree linearly polarized light propagates to the right side of the diagram while maintaining its polarization. The 0-degree linearly polarized light output from the right side of the polarization-maintaining fiber 11 is input to one end of the electro-optic crystal 17 (EO) whose electrical principal axis is positioned at 45 degrees.

[0012] Light propagating through the electro-optic crystal 17 is reflected by a mirror 18 located at the other end of the electro-optic crystal 17, propagates in the reverse direction through the electro-optic crystal 17 and the polarization-maintaining fiber 11, and is input to the second end of the optical circulator 16. The light input to the second end of the optical circulator 16 maintains its polarization and is output from the third end, and input to the polarized beam splitter 12.

[0013] The polarizing beam splitter 12 transmits the 0-degree polarized component of the input light and outputs it towards the photodetector 13, and reflects the 90-degree polarized component and outputs it towards the photodetector 14. The photodetectors 13 and 14 each output an electrical signal proportional to the intensity of the input light. The differential amplifier 15 takes the difference between the output of the photodetector 13 and the output of the photodetector 14, thereby obtaining 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 polarizing beam splitter 12.

[0014] When the electric field to be measured is applied to the electro-optic crystal 17, the birefringence of the electro-optic crystal 17 changes, which changes the polarization of the light that has traveled back and forth through the electro-optic crystal 17. This changes the intensity ratio of the 0-degree polarized component and the 90-degree polarized component, and the change in the output of the photodetector is used to measure the electric field to be measured.

[0015] To measure the electric field at any given position, it is desirable to make the electro-optic crystal portion small and lightweight. Therefore, an electro-optic probe 2 is often used, which integrates the polarization-maintaining fiber 11, the electro-optic crystal 17, and the mirror 18. The left side of the polarization-maintaining fiber 11 and the optical circulator 16 are connected by an optical connector (not shown), allowing the electro-optic probe 2 to be easily attached and detached. Here, the electro-optic crystal 17 is assumed to have natural birefringence (birefringence when the applied electric field is zero).

[0016] Expressed quantitatively using mathematical formulas, the following can be obtained. The electric field of CW 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 polarization beam splitter 12. The electric field of the light output from the CW light source 10 is E0, and the delay time difference due to birefringence of the polarization-maintaining fiber 11 is τ. p The phase difference of light due to birefringence of the polarization-maintaining fiber 11 is φ p The delay time difference (round trip) due to the natural birefringence of the electro-optic crystal 17 is τ d The phase difference (round trip) of light due to the natural birefringence of the electro-optic crystal 17 is φ d The phase difference (round trip) of light due to the application of an electric field to the electro-optic crystal 17 is φ e The electric field of the light input to the polarizing beam splitter 12 is E 12 If the angular frequency of light is ω, then it can be expressed as follows:

[0017]

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number

number

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[0018] Here, the electric field under measurement is zero (φ e At =0) ωτ d +φ d If the wavelength of the CW light is adjusted so that it equals -π / 2, the difference in output between receiver 13 and receiver 14 is |E 12x | 2 -|E 12y | 2 =sin(φ e ) Therefore, |φ e |≪1|E 12x | 2 -|E 12y | 2 ≒φ e Therefore, differential detection is performed by φ eA proportional output is obtained, making it possible to measure the electric field.

[0019] Also, the electric field under measurement is set to zero (φ e When the wavelength of the CW light is swept (=0) and the difference in output between receiver 13 and receiver 14 is measured, |E 12x | 2 -|E 12y | 2 =cos(ωτ d +φ d ) and changes sinusoidally with respect to the sweep of the optical angular frequency ω. If Δω is the period of the sine wave expressed in terms of optical angular frequency, then Δωτ d Since = 2π, the reciprocal of the period of the photodetector output expressed in terms of optical frequency is τ. d As a result of this configuration, the delay time difference τ due to the natural birefringence of the electro-optic crystal 17 is achieved. d It can be measured.

[0020] In this configuration as well, the natural birefringence of the electro-optic crystal 17 can be measured in the same way by using broadband light and a wavelength-selective spectrometer instead of CW light (narrowband light) and a non-wavelength-selective photodetector. [Prior art documents] [Patent Documents]

[0021] [Patent Document 1] Japanese Patent Publication No. 2005-214892 [Non-patent literature]

[0022] [Non-Patent Document 1] JIS C 6872:2008, "Test Method for Polarization Plane Preservation Optical Fiber Beat Length" [Overview of the Initiative] [Problems that the invention aims to solve]

[0023] The birefringence measurement method in Non-Patent Literature 1 is a method for measuring a single birefringent medium. That is, it is possible to measure the birefringence of a polarization-maintaining fiber alone or an electro-optic crystal alone, but it was not possible to measure the birefringence of an electro-optic probe in which an electro-optic crystal with natural birefringence and a polarization-maintaining fiber are integrated. More specifically, if the angle between the slow axis or fast axis of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal coincides, it becomes equivalent to a single birefringent medium with the sum of the birefringences of both, and therefore it is possible to measure the sum of the birefringences of both using the method in Non-Patent Literature 1. However, as mentioned above, the electro-optic probe used for electric field measurement has a structure in which the slow axis of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle, so unlike a single birefringent medium, the birefringence measurement method in Non-Patent Literature 1 could not be applied.

[0024] Furthermore, in the configuration of Patent Document 1, it is possible to measure the natural birefringence of the electro-optic crystal 17 by wavelength sweeping using an integrated electro-optic probe 2 in which the retard axis of the polarization-maintaining fiber 11 and the electrical principal axis of the electro-optic crystal 17 form a 45-degree angle. However, in the configuration of Patent Document 1, it was not possible to measure the birefringence of the polarization-maintaining fiber 11.

[0025] The present invention has been made to solve the above-mentioned conventional problems, and aims to provide a birefringence measuring device and method for measuring both the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal in an integrated electro-optic probe in which the retard axis of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle. When the electric field applied to the electro-optic crystal is zero, the natural birefringence will be measured. [Means for solving the problem]

[0026] The birefringence measuring device according to the present invention measures the birefringence of an electro-optic probe, comprising: a polarization-maintaining fiber (11) whose retard axis is positioned at 0 or 90 degrees; an electro-optic crystal (17) having natural birefringence, one end of which is connected to one end of the polarization-maintaining fiber such that its electrical principal axis is at a 45-degree angle to the retard axis of the polarization-maintaining fiber; and a mirror (18) provided at the other end of the electro-optic crystal. The device measures the birefringence of an electro-optic probe, comprising: a CW light source (10) that outputs CW light with 0-degree linear polarization; and an optical input / output light that inputs the CW light to the other end of the polarization-maintaining fiber and outputs light that is output from one end of the polarization-maintaining fiber, input to one end of the electro-optic crystal, reflected by the mirror and output from one end of the electro-optic crystal, input to one end of the polarization-maintaining fiber, and output from the other end of the polarization-maintaining fiber. The device comprises a power unit (16), a half-wave plate (19) to which light output from the optical input / output unit is input, a polarization separation unit (12) to which light output from the half-wave plate is input to a first end and which outputs either the 0-degree polarization component or the 90-degree polarization component of the light input to the first end from a second end, or both from a third end and a fourth end, respectively, photodetectors (13, 14) that detect the intensity of the light output from the second end of the polarization separation unit, or the difference in intensity of the light output from the third end and the fourth end of the polarization separation unit, and a signal processing unit (20) that detects the frequency component of the output of the photodetectors when the wavelength of the CW light is swept to determine the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal, wherein if the angle of the lagging axis of the half-wave plate is θ / 2, then θ is set to satisfy sin(2θ)≠0.

[0027] As described above, the birefringence measuring device of the present invention includes a half-wave plate into which light output from the optical input / output unit is input, and the frequency component (|τ) of the output of the photodetector when the wavelength of CW light is swept. d ±τ p |,τ d ) detects the birefringence τ of the polarization-maintaining fiber p and the birefringence τ of electro-optic crystals dThe system includes a signal processing unit that determines |τ, and if the angle of the lagging axis of the half-wave plate is θ / 2, then θ is set to satisfy sin(2θ)≠0. With this configuration, the |τ detected by the signal processing unit is d ±τ p The amplitude of the frequency component of | becomes zero τ p This avoids the situation where it becomes impossible to find τ p and τ d Both can be determined. Therefore, the birefringence measuring device of the present invention can simultaneously measure the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal in a single wavelength sweep for an integrated electro-optic probe in which the retard axis of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle.

[0028] The birefringence measuring apparatus according to the present invention includes, for achieving the above object, a polarization maintaining fiber (11) having a slow axis disposed at 0 degrees or 90 degrees, an electro-optic crystal (17) having natural birefringence, one end of which is connected to one end of the polarization maintaining fiber such that the electric major axis forms a 45-degree angle with the slow axis of the polarization maintaining fiber, and a mirror (18) provided at the other end of the electro-optic crystal. The birefringence measuring apparatus measures the birefringence of an electro-optic probe including these components. The apparatus further includes a CW light source (10) that outputs CW light of linearly polarized light at 0 degrees, an optical input / output unit (16) that inputs the CW light to the other end of the polarization maintaining fiber, outputs the light from one end of the polarization maintaining fiber, inputs the light to one end of the electro-optic crystal, outputs the light reflected by the mirror from one end of the electro-optic crystal, inputs the light to one end of the polarization maintaining fiber, and outputs the light from the other end of the polarization maintaining fiber. The apparatus also includes a polarization separation unit (24) that inputs the light output from the optical input / output unit to a first end and outputs either the polarization component of the light input to the first end at an angle θ or both the polarization components at angles θ and θ + 90 degrees from a second end or from a third end and a fourth end respectively. The apparatus further includes a light receiver (13, 14) that detects the intensity of the light output from the second end of the polarization separation unit or the difference in intensity of the light output from the third end and the fourth end of the polarization separation unit, and a signal processing unit (20) that detects the frequency component of the output of the light receiver when the wavelength of the CW light is swept to obtain the birefringence of the polarization maintaining fiber and the birefringence of the electro-optic crystal. The θ is set to satisfy sin(2θ) ≠ 0.

[0029] As described above, the birefringence measuring apparatus of the present invention includes a polarization separation unit that inputs the light output from the optical input / output unit to a first end and outputs either the polarization component of the light input to the first end at an angle θ or both the polarization components at angles θ and θ + 90 degrees from a second end or from a third end and a fourth end respectively, and a signal processing unit that detects the frequency component (|τ d ±τ p |, τ d ) of the output of the light receiver when the wavelength of the CW light is swept to obtain the birefringence τ p of the polarization maintaining fiber and the birefringence τ dThe system includes a signal processing unit that determines |τ, and the angle θ is set to satisfy sin(2θ)≠0. With this configuration, the |τ detected by the signal processing unit d ±τ p The amplitude of the frequency component of | becomes zero τ p This avoids the situation where it becomes impossible to find τ p and τ d Both can be determined. Therefore, the birefringence measuring device of the present invention can simultaneously measure the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal in a single wavelength sweep for an integrated electro-optic probe in which the retard axis of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle.

[0030] The birefringence measuring device according to the present invention measures the birefringence of an electro-optic probe, comprising: a polarization-maintaining fiber (11) with a retard axis of 0 or 90 degrees; an electro-optic crystal (17) having natural birefringence, one end of which is connected to one end of the polarization-maintaining fiber such that its electrical principal axis is at a 45-degree angle with the retard axis of the polarization-maintaining fiber; and a mirror (18) provided at the other end of the electro-optic crystal, wherein the device measures the birefringence of an electro-optic probe, and comprises a CW light source (25) that outputs linearly polarized CW light at an angle of -θ; and inputting the CW light to the other end of the polarization-maintaining fiber, outputting from one end of the polarization-maintaining fiber, inputting to one end of the electro-optic crystal, reflecting off the mirror, outputting from one end of the electro-optic crystal, and inputting to one end of the polarization-maintaining fiber. The system includes an optical input / output unit (16) that outputs light output from the other end of the polarization-maintaining fiber, a polarization separation unit (12) that receives the light output from the optical input / output unit to a first end and outputs either the 0-degree polarization component or the 90-degree polarization component of the light input to the first end from a second end, or both from a third end and a fourth end, respectively, photodetectors (13, 14) that detect the intensity of the light output from the second end of the polarization separation unit, or the difference in intensity of the light output from the third end and the fourth end of the polarization separation unit, and a signal processing unit (20) that detects the frequency component of the output of the photodetectors when the wavelength of the CW light is swept to determine the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal, wherein θ is set to satisfy sin(2θ)≠0.

[0031] As described above, the birefringence measuring apparatus of the present invention includes a CW light source that outputs CW light with linearly polarized light at an angle -θ, and the frequency component (|τ d ±τ p |, τ d ) of the output of the light receiver when the wavelength of the CW light is swept is detected to obtain the birefringence τ p of the polarization-maintaining fiber and the birefringence τ d of the electro-optic crystal. The angle θ is set so as to satisfy sin(2θ)≠0. With this configuration, the amplitude of the frequency component of |τ d ±τ p | detected by the signal processing unit becomes zero, and it is possible to avoid the situation where τ p cannot be obtained, and both τ p and τ d can be obtained. Therefore, the birefringence measuring apparatus of the present invention can simultaneously measure the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal with one wavelength sweep for an integrated electro-optic probe in which the slow axis of the polarization-maintaining fiber and the electrical main axis of the electro-optic crystal form an angle of 45 degrees.

[0032] In the birefringence measuring apparatus according to the present invention, θ is set so as to satisfy cos(2θ)=0, and when the magnitude relationship between the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal is known, the signal processing unit detects the frequencies of two frequency components output from the light receiver when the wavelength of the CW light is swept, and obtains the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal. This may be the configuration.

[0033] As described above, when θ is set so that cos(2θ)=0, the amplitude of the frequency component of τ d becomes zero, and only the frequency component of |τ d ±τ p | is detected. When the magnitude relationship between the birefringence τ d of the electro-optic crystal and the birefringence τ p of the polarization-maintaining fiber is known, τ d -τ pIn order to determine the sign of d and p τ can be obtained. Therefore, under the above conditions, the birefringence measuring device of the present invention can simultaneously measure the birefringence of the polarization maintaining fiber and the birefringence of the electro-optic crystal by one wavelength sweep for an integrated electro-optic probe in which the slow axis of the polarization maintaining fiber and the electrical main axis of the electro-optic crystal form an angle of 45 degrees.

[0034] The birefringence measuring device according to the present invention is set so that θ further satisfies cos(2θ)≠0, and the signal processing unit calculates the birefringence τ p of the polarization maintaining fiber and the birefringence τ d of the electro-optic crystal, τ d >τ p 2τ d >τ p >τ d 2τ d <τ p When it is known which magnitude relationship is satisfied among the three frequency components output from the light receiver when the wavelength of the CW light is swept, the frequencies of any two of the three frequency components are detected to obtain the birefringence of the polarization maintaining fiber and the birefringence of the electro-optic crystal, which may also be a configuration.

[0035] As described above, when θ is set so that cos(2θ)≠0, three frequency components of |τ d ±τ p |,τ d are detected. When the magnitude relationship between the birefringence τ d of the electro-optic crystal and the birefringence τ p of the polarization maintaining fiber is known, it is determined which of the three frequency components τ1, τ2, τ3 corresponds to |τ d ±τ p | and τ d and τ d -τ pThe sign of is also determined, so the signal processing unit detects the frequencies of any two of the three frequency components τ1, τ2, and τ3 and then τ d and τ p Therefore, the birefringence measuring device of the present invention can simultaneously measure the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal in a single wavelength sweep for an integrated electro-optic probe in which the retard axis of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle under the above conditions.

[0036] The birefringence measuring device according to the present invention is set such that θ further satisfies cos(2θ)≠0, and the signal processing unit measures the birefringence τ of the polarization-maintaining fiber. p and the birefringence τ of the electro-optic crystal d and, 2τ d >τ p When it is known that the magnitude relationship is satisfied, the frequencies of the highest frequency component and the second highest frequency component among the three frequency components output from the photodetector when the wavelength of the CW light is swept are detected to determine the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal, and the birefringence τ of the polarization-maintaining fiber is determined. p and the birefringence τ of the electro-optic crystal d and, 2τ d <τ p In cases where the relationship between magnitudes is known, the configuration may involve detecting the frequencies of any two of the three frequency components output from the photodetector when the wavelength of the CW light is swept, thereby determining the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal.

[0037] As mentioned above, if we set θ such that cos(2θ)≠0, then |τ d ±τ p |,τ d The following three frequency components are detected. Birefringence τ of the electro-optic crystal d and the birefringence τ of polarization-maintaining fiber p Regarding 2τ d and τ p If the relative magnitudes are known, then which of the three frequency components τ1, τ2, τ3 is |τd ±τ p | and τ d It can identify whether it corresponds to 2τ d <τ p In the case of τ d -τ p The sign of is also determined, so the signal processing unit detects the frequencies of the two frequency components mentioned above out of the three frequency components τ1, τ2, and τ3 and τ d and τ p Therefore, the birefringence measuring device of the present invention can simultaneously measure the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal in a single wavelength sweep for an integrated electro-optic probe in which the retard axis of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle under the above conditions.

[0038] The birefringence measuring device according to the present invention is set such that θ further satisfies cos(2θ)≠0, and the signal processing unit measures the birefringence τ of the polarization-maintaining fiber. p and the birefringence τ of the electro-optic crystal d Toga, τ d >τ p 2τ d >τ p >τ d 2τ d <τ p An alternative configuration may be one in which, assuming that one of the following magnitude relationships is satisfied, the frequencies of any two of the three frequency components output from the photodetector when the wavelength of the CW light is swept are detected to determine the birefringence of the polarization-maintaining fiber and the electro-optic crystal, the assumption of the magnitude relationship is verified using the frequency of the remaining one of the three frequency components, and the birefringence of the polarization-maintaining fiber and the electro-optic crystal when the assumption of the magnitude relationship is correct is output.

[0039] As mentioned above, if we set θ such that cos(2θ)≠0, then |τ d ±τ p |,τ d The following three frequency components are detected. Birefringence τ of the electro-optic crystald and the birefringence τ of polarization-maintaining fiber p Assuming one of the above magnitude relationships, τ is derived from the frequencies of any two frequency components. d and τ p By finding τ and using the frequency of the remaining frequency component to verify whether the assumption is correct, d and τ p Even when the relative magnitudes are unknown, τ d and τ p This can be determined. Therefore, the birefringence measuring device of the present invention can simultaneously measure the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal in a single wavelength sweep for an integrated electro-optic probe in which the slow axis of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle.

[0040] The birefringence measuring device according to the present invention is set such that θ further satisfies cos(2θ)≠0, and the signal processing unit measures the birefringence τ of the polarization-maintaining fiber. p and the birefringence τ of the electro-optic crystal d and, 2τ d >τ p Assuming that the first magnitude relationship is satisfied, the frequencies of the highest frequency component and the second highest frequency component among the three frequency components output from the photodetector when the wavelength of the CW light is swept are detected to obtain a first candidate for the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal. The assumption of the first magnitude relationship is verified using the frequency of the lowest frequency component among the three frequency components. If the assumption of the first magnitude relationship is correct, the first candidate for the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal is output, and the birefringence τ of the polarization-maintaining fiber is determined. p and the birefringence τ of the electro-optic crystal d and, 2τ d <τ pThe system may also be configured to assume that the second magnitude relationship is satisfied, detect the frequencies of any two of the three frequency components to obtain a second candidate for the birefringence of the polarization-maintaining fiber and the electro-optic crystal, verify the assumption of the second magnitude relationship using the frequency of the remaining one of the three frequency components, and output the second candidate for the birefringence of the polarization-maintaining fiber and the electro-optic crystal if the assumption of the second magnitude relationship is correct.

[0041] As mentioned above, if we set θ such that cos(2θ)≠0, then |τ d ±τ p |,τ d The following three frequency components are detected. Birefringence τ of the electro-optic crystal d and the birefringence τ of polarization-maintaining fiber p Regarding 2τ d and τ p Assuming a relative magnitude, τ is derived from the frequencies of the two frequency components mentioned above. d and τ p By finding τ and using the frequency of the remaining frequency component to verify whether the assumption is correct, d and τ p Even when the relative magnitudes are unknown, τ d and τ p This can be determined. Therefore, the birefringence measuring device of the present invention can simultaneously measure the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal in a single wavelength sweep for an integrated electro-optic probe in which the slow axis of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle.

[0042] The birefringence measuring device according to the present invention is such that the θ is cos(2θ)≠0 |sin(2θ)|≠|2cos(2θ)| The signal processing unit may be configured to further satisfy both of the above equations, and the signal processing unit may be configured to identify, from two or three frequency components of the output of the photodetector when the wavelength of the CW light is swept, a first frequency component whose frequency corresponds to the sum of the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal, and a second frequency component whose frequency corresponds to the birefringence of the electro-optic crystal, based on the amplitude of the frequency components, and to determine the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal from the frequencies of the first frequency component and the frequencies of the second frequency component.

[0043] As mentioned above, if we set θ such that cos(2θ)≠0 and |sin(2θ)|≠|2cos(2θ)|, then |τ d ±τ p |,τ d Three frequency components were detected, |τ d ±τ p | Amplitude and τ of the frequency components d The amplitudes of the frequency components will be different. Therefore, which of the three or two frequency components is τ d +τ p and τ d By identifying whether it corresponds to τ by amplitude, p and τ d Even when the relative magnitudes are unknown, τ p and τ d Therefore, the birefringence measuring device of the present invention can determine τ p and τ d Even when the relative magnitudes are unknown, for an integrated electro-optic probe in which the retard axis of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle, it is possible to simultaneously measure the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal in a single wavelength sweep.

[0044] The present invention provides a method for measuring the birefringence of an electro-optic probe, comprising: a polarization-maintaining fiber (11) with a retard axis of 0 degrees or 90 degrees; an electro-optic crystal (17) having natural birefringence, one end of which is connected to one end of the polarization-maintaining fiber such that its electrical principal axis is at a 45-degree angle with the retard axis of the polarization-maintaining fiber; and a mirror (18) provided at the other end of the electro-optic crystal, wherein the method outputs 0-degree linearly polarized CW light and the CW light The light is input to the other end of the polarization-maintaining fiber, output from one end of the polarization-maintaining fiber, input to one end of the electro-optic crystal, reflected by the mirror and output from one end of the electro-optic crystal, input to one end of the polarization-maintaining fiber, output from the other end of the polarization-maintaining fiber, input to a half-wave plate (19) with a retard angle of θ / 2, and the intensity difference of either the 0-degree polarization component or the 90-degree polarization component of the light output from the half-wave plate, or both, is detected. sin(2θ)≠0 cos(2θ)≠0 |sin(2θ)|≠|2cos(2θ)| The θ is set to satisfy all of the equations, two or three frequency components are detected from the intensity or intensity difference when the wavelength of the CW light is swept, the frequency component with the highest frequency among the two or three frequency components is identified as the first frequency component corresponding to the sum of the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal, the frequency component with the largest amplitude or the frequency component with the smallest amplitude among the two or three frequency components is identified as the second frequency component corresponding to the birefringence of the electro-optic crystal, and the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal are determined from the frequencies of the first frequency component and the second frequency component.

[0045] With this configuration, the birefringence measurement method of the present invention uses a half-wave plate set to the above angle, τ p and τ d Even if the relative magnitudes are unknown, which of the three or two detected frequency components is τ d +τp and τ d By discriminating whether it corresponds to ω and τ by the amplitude, it is possible to simultaneously measure the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal in one wavelength sweep.

[0046] The birefringence measurement method according to the present invention is for achieving the above object. A polarization-maintaining fiber (11) in which the slow axis is arranged at 0 degrees or 90 degrees, and an electro-optic crystal (17) having natural birefringence, one end of which is connected to one end of the polarization-maintaining fiber such that the electrical principal axis forms a 45-degree angle with the slow axis of the polarization-maintaining fiber, and a mirror (18) provided at the other end of the electro-optic crystal. A birefringence measurement method for measuring the birefringence of an electro-optic probe including: outputting CW light of linearly polarized light at 0 degrees, inputting the CW light to the other end of the polarization-maintaining fiber, outputting from one end of the polarization-maintaining fiber, inputting to one end of the electro-optic crystal, reflected by the mirror and output from one end of the electro-optic crystal, input to one end of the polarization-maintaining fiber, and detecting the intensity of either one or both of the polarization components of the angle θ of the light output from the other end of the polarization-maintaining fiber and the polarization component of the angle θ + 90 degrees, or the intensity difference, sin(2θ)≠0 cos(2θ)≠0 |sin(2θ)|≠|2cos(2θ)| Set θ so as to satisfy all of the following equations, detect two or three frequency components from the intensity or the intensity difference when the wavelength of the CW light is swept, and identify the highest-frequency component among the two or three frequency components as a first frequency component corresponding to the sum of the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal. Excluding the highest-frequency component among the two or three frequency components, identify the frequency component with the largest amplitude or the smallest amplitude as a second frequency component corresponding to the birefringence of the electro-optic crystal, and obtain the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal from the frequency of the first frequency component and the frequency of the second frequency component.

[0047] With this configuration, the birefringence measurement method of the present invention detects the intensity of the polarization component at the above angle, τ p and τ d Even if the relative magnitudes are unknown, which of the three or two frequency components is τ d +τ p and τ d By identifying the corresponding amplitude, it is possible to simultaneously measure the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal in a single wavelength sweep.

[0048] The birefringence measurement method according to the present invention, in order to achieve the above objective, measures the birefringence of an electro-optic probe comprising: a polarization-maintaining fiber (11) whose retard axis is positioned at 0 degrees or 90 degrees; an electro-optic crystal (17) having natural birefringence, one end of which is connected to one end of the polarization-maintaining fiber such that its electrical principal axis is at a 45-degree angle with the retard axis of the polarization-maintaining fiber; and a mirror (18) provided at the other end of the electro-optic crystal, wherein linearly polarized CW light with an angle of -θ is output, the CW light is input to the other end of the polarization-maintaining fiber, the light is output from one end of the polarization-maintaining fiber, input to one end of the electro-optic crystal, reflected by the mirror, output from one end of the electro-optic crystal, input to one end of the polarization-maintaining fiber, and the intensity difference of either one or both of the 0-degree polarized component and the 90-degree polarized component of the light output from the other end of the polarization-maintaining fiber, sin(2θ)≠0 cos(2θ)≠0 |sin(2θ)|≠|2cos(2θ)| The θ is set to satisfy all of the equations, two or three frequency components are detected from the intensity or intensity difference when the wavelength of the CW light is swept, the frequency component with the highest frequency among the two or three frequency components is identified as the first frequency component corresponding to the sum of the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal, the frequency component with the largest amplitude or the frequency component with the smallest amplitude among the two or three frequency components is identified as the second frequency component corresponding to the birefringence of the electro-optic crystal, and the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal are determined from the frequencies of the first frequency component and the second frequency component.

[0049] With this configuration, the birefringence measurement method of the present invention uses linearly polarized CW light of the above angle, τ p and τ d Even if the relative magnitudes are unknown, which of the three or two detected frequency components is τ d +τ p and τ d By identifying the corresponding amplitude, it is possible to simultaneously measure the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal in a single wavelength sweep. [Effects of the Invention]

[0050] According to the present invention, a birefringence measuring device and method can be provided for measuring the birefringence of a polarization-maintaining fiber and an electro-optic crystal in an integrated electro-optic probe in which the retard axis of a polarization-maintaining fiber and the electrical principal axis of an electro-optic crystal form a 45-degree angle. In particular, according to the present invention, the birefringence of a polarization-maintaining fiber and the birefringence of an electro-optic crystal can be measured simultaneously in a single wavelength sweep. [Brief explanation of the drawing]

[0051] [Figure 1] This is a diagram illustrating the configuration of a conventional method for measuring the birefringence of polarization-maintaining fibers. [Figure 2]This is a diagram illustrating the configuration of a conventional electric field measurement device using an electro-optic probe. [Figure 3] This is a configuration diagram of a birefringence measuring device according to the first embodiment of the present invention. [Figure 4] Figure 3 shows the configuration of a birefringence measuring device using an optical circulator with polarization-maintaining fibers. [Figure 5] This is a configuration diagram of a birefringence measuring device according to a second embodiment of the present invention. [Figure 6] Figure 5 shows the configuration of a birefringence measuring device when an optical circulator with polarization-maintaining fibers is used. [Figure 7] This is a diagram showing the configuration of a birefringence measuring device according to a third embodiment of the present invention. [Figure 8] Figure 7 shows the configuration of a birefringence measuring device when an optical circulator with polarization-maintaining fibers is used. [Figure 9] This graph shows the relationships between each frequency component. [Figure 10] This graph shows the relationship between the angle θ and the amplitude of each frequency component. [Modes for carrying out the invention]

[0052] Embodiments of the present invention will be described below with reference to the drawings.

[0053] [First Embodiment] Figure 3 shows a birefringence measuring device 1 according to the first embodiment of the present invention. The electro-optic probe 2 to be measured consists of a polarization-maintaining fiber 11 (PMF1), an electro-optic crystal 17 (EO), and a mirror 18. The polarization-maintaining fiber 11 has birefringence, where the light propagation delay time differs between the slow axis and the fast axis, and the slow axis is positioned in the 0-degree or 90-degree direction. The following description will be given for the case where the slow axis of the polarization-maintaining fiber 11 is 0 degrees, but when the slow axis of the polarization-maintaining fiber 11 is 90 degrees, the sign of the delay time difference of the polarization-maintaining fiber 11 is reversed, and the absolute value of the delay time difference of the polarization-maintaining fiber 11 can be measured in the same manner.

[0054] The electro-optic crystal 17 has natural birefringence, where the light propagation delay time differs between its electrical principal axis and an axis perpendicular to it when the electric field is zero. The electrical principal axis of the electro-optic crystal 17 is positioned at a 45-degree angle to the retard axis of the polarization-maintaining fiber 11. The following describes the case where the retard axis of the electro-optic crystal 17 is 45 degrees. However, if the retard axis of the electro-optic crystal 17 is -45 degrees, the sign of the delay time difference of the electro-optic crystal 17 is reversed, and the absolute value of the delay time difference of the electro-optic crystal 17 can be measured in the same way. A mirror 18 is placed at the tip of the electro-optic crystal 17 to reflect light. A collimating lens (not shown) may be included between the polarization-maintaining fiber 11 and the electro-optic crystal 17 to make the light output from the polarization-maintaining fiber 11 into parallel light and to input the parallel light reflected by the mirror 18 back into the polarization-maintaining fiber 11.

[0055] The CW light source 10 outputs 0-degree linearly polarized CW light (narrowband light), and the output CW light is input to the first end of the optical circulator 16 (CIR), which serves as the optical input / output unit. The optical circulator 16 is polarization-maintaining, and the light input to the first end maintains its polarization and is output from the second end. The 0-degree linearly polarized light output from the second end of the optical circulator 16 is input to the left side of the polarization-maintaining fiber 11. Since the polarization direction of the light input to the left side of the polarization-maintaining fiber 11 coincides with the lagging axis of the polarization-maintaining fiber 11, the light propagates to the right side of the diagram while maintaining its polarization. In other words, in the forward path, it is not affected by the birefringence of the polarization-maintaining fiber 11.

[0056] Light with 0-degree linear polarization output from the right side of the polarization-maintaining fiber 11 is input to one end of the electro-optic crystal 17, whose electrical principal axis is positioned at 45 degrees. The light propagates within the electro-optic crystal 17, is reflected by a mirror 18 at the other end of the electro-optic crystal 17, propagates in the opposite direction through the electro-optic crystal 17 and the polarization-maintaining fiber 11, and is input to the second end of the optical circulator 16. At this time, the polarization of the light changes due to birefringence of the electro-optic crystal 17 and the polarization-maintaining fiber 11.

[0057] It is desirable to integrate the polarization-maintaining fiber 11, the electro-optic crystal 17, and the mirror 18 to make the electro-optic probe 2 smaller and lighter, and to connect the left side of the polarization-maintaining fiber 11 and the optical circulator 16 with an optical connector (not shown) so that the electro-optic probe 2 can be easily attached and detached.

[0058] Light input to the second end of the optical circulator 16 maintains its polarization and is output from the third end of the optical circulator 16, and input to the half-wave plate 19 (HWP1). It is also possible to use an optical directional coupler instead of the optical circulator 16 to separate the input light to the left side of the polarization-maintaining fiber 11 from the output light from the left side of the polarization-maintaining fiber 11.

[0059] The half-wave plate 19 is positioned so that its lagging axis is at a predetermined angle θ1 / 2. The light output from the half-wave plate 19 is input to the first end of the polarizing beam splitter 12 (PBS1). The polarizing beam splitter 12 transmits the 0-degree polarized component of the light input to the first end and outputs it from the third end toward the photodetector 13 (PD1), and reflects the 90-degree polarized component and outputs it from the fourth end toward the photodetector 14 (PD2). The photodetectors 13 and 14 each output an electrical signal proportional to the intensity of the input light.

[0060] Here, a differential configuration is shown in which 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 polarizing beam splitter 12 is detected by taking the difference between the output of the photodetector 13 and the output of the photodetector 14. However, it is also possible to place only the photodetector 13, which receives light transmitted through the polarizing beam splitter 12, to detect the intensity of the 0-degree polarized component of the light input to the polarizing beam splitter 12, or to place only the photodetector 14, which receives light reflected from the polarizing beam splitter 12, to detect the intensity of the 90-degree polarized component of the light input to the polarizing beam splitter 12. In the case of a single-ended configuration in which either the photodetector 13 or the photodetector 14 is placed, it is also possible to use a polarizer that extracts the 0-degree polarized component or the 90-degree polarized component instead of the polarizing beam splitter 12.

[0061] In this embodiment, CW light (narrowband light) and a non-wavelength-selective photodetector were used, but similar measurements can be performed using broadband light and a wavelength-selective spectrometer.

[0062] Expressed quantitatively using mathematical formulas, the following can be obtained: The electric field of CW 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 12. The birefringence (delay time difference, phase difference) of the electro-optic crystal 17 is shown as the round-trip value.

[0063] E0 is the electric field of the light output from the CW light source 10, and τ is the delay time difference due to birefringence of the polarization-maintaining fiber 11. p The phase difference of light due to birefringence of the polarization-maintaining fiber 11 is φ p The delay time difference (round trip) due to the natural birefringence of the electro-optic crystal 17 is τ d The phase difference (round trip) of light due to the natural birefringence of the electro-optic crystal 17 is φ d The phase difference (round trip) of light due to the application of an electric field to the electro-optic crystal 17 is φ e The electric field of the light input to the first end of the polarizing beam splitter 12 is E 21 If we let the angular frequency of light be ω, then the following equation holds.

[0064]

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[0065] Sweep the optical angular frequency ω to E 21The intensity of the 0-degree polarization component or the intensity of the 90-degree polarization component, or the difference between them, is obtained as the photodetector output. The method of sweeping ω may be to obtain the photodetector output while changing ω in a stepwise manner at predetermined angular frequency intervals, or to obtain the photodetector output at predetermined time intervals while continuously changing ω. When the photodetector output is obtained and represented with ω on the horizontal axis and the photodetector output on the vertical axis, three sinusoidal components are included from equations (11) and (12). The sweep of the optical angular frequency ω should be performed within the range in which the frequencies of these sinusoidal components can be measured.

[0066] For more details, see E 21 In a single-ended configuration where either the intensity of the 0-degree polarized component or the intensity of the 90-degree polarized component is obtained, equation (11) shows that there are three sinusoidal components and a constant (offset), E 21 In the case of a differential configuration that obtains the difference between the intensity of the 0-degree polarized component and the intensity of the 90-degree polarized component, only three sinusoidal components exist according to equation (12). From the frequencies of these sinusoidal components, the delay time difference τ due to birefringence of the polarization-maintaining fiber 11 is obtained. p and the delay time difference τ due to the natural birefringence of the electro-optic crystal 17 d The method for finding this will be explained later.

[0067] If an optical circulator 16 with a polarization-maintaining fiber is used, the measured value is the sum of the delay time difference of the polarization-maintaining fiber 11 of the electro-optic probe 2 and the delay time difference of the polarization-maintaining fiber of the optical circulator 16. Therefore, the delay time difference of the polarization-maintaining fiber of the optical circulator 16 can be determined in advance using a conventional birefringence measurement method, and the delay time difference of only the polarization-maintaining fiber 11 of the electro-optic probe 2, after subtracting the delay time difference of the polarization-maintaining fiber of the optical circulator 16, can be output.

[0068] More specifically, as shown in Figure 4(a), when a polarization-maintaining fiber 21 (PMF21) is attached to the first end of the optical circulator 16, a polarization-maintaining fiber 22 (PMF22) to the second end, and a polarization-maintaining fiber 23 (PMF23) to the third end, the forward path of polarization-maintaining fibers 21 and 22 is input with 0-degree linearly polarized light aligned with the lagging axis of polarization-maintaining fibers 21 and 22, so it is not affected by birefringence, and the birefringence of the return path of polarization-maintaining fiber 22 and polarization-maintaining fiber 23 is added to the birefringence measurement value of polarization-maintaining fiber 11. For this reason, as shown in Figure 4(b), the lagging axis angle of polarization-maintaining fibers 22 and 23 can be set at 45 degrees, and the birefringence of the return path of polarization-maintaining fiber 22 and polarization-maintaining fiber 23 can be measured in advance using a conventional method for measuring the birefringence of polarization-maintaining fibers, and then subtracted from the measurement value of the electro-optic probe 2.

[0069] [Second Embodiment] The second embodiment, as shown in Figure 5, is a configuration in which the half-wave plate 19 (HWP1) is removed from the first embodiment and the polarizing beam splitter 24 (PBS2) is rotated by an angle θ2. The other configurations are the same as in the first embodiment and will be omitted from the explanation as appropriate.

[0070] The 0-degree linearly polarized CW light (narrowband light) output from the CW light source 10 is input to the first end of the optical circulator 16, output from the second end of the optical circulator 16, propagates through the polarization-maintaining fiber 11 from left to right in the figure, is input to one end of the electro-optic crystal 17, is reflected by the mirror 18 provided at the other end of the electro-optic crystal 17, propagates through the electro-optic crystal 17 and the polarization-maintaining fiber 11 in the reverse direction, is input to the second end of the optical circulator 16, and is output from the third end of the optical circulator 16, as in the first embodiment.

[0071] Light output from the third end of the optical circulator 16 is input to the first end of the polarizing beam splitter 24. Since the polarizing beam splitter 24 is positioned rotated by an angle θ2, the polarization component of the light input to the first end of the polarizing beam splitter 24 at angle θ2 is transmitted and output from the third end toward the photodetector 13, and the polarization component at angle θ2+90 degrees is reflected and output from the fourth end toward the photodetector 14. The photodetectors 13 and 14 each output an electrical signal proportional to the intensity of the input light.

[0072] Here, a differential configuration is shown in which the difference between the intensity of the polarization component at angle θ2 and the intensity of the polarization component at angle θ2+90 degrees of light input to the polarizing beam splitter 24 is detected by taking the difference between the output of the photodetector 13 and the output of the photodetector 14. However, it is also possible to place only the photodetector 13, which receives light transmitted through the polarizing beam splitter 24, to detect the intensity of the polarization component at angle θ2 of the light input to the polarizing beam splitter 24, or to place only the photodetector 14, which receives light reflected from the polarizing beam splitter 24, to detect the intensity of the polarization component at angle θ2+90 degrees of the light input to the polarizing beam splitter 24. In the case of a single-ended configuration in which either the photodetector 13 or the photodetector 14 is placed, it is also possible to use a polarizer that extracts the polarization component at angle θ2 or the polarization component at angle θ2+90 degrees instead of the polarizing beam splitter 24.

[0073] In this embodiment, CW light (narrowband light) and a non-wavelength-selective photodetector were used, but similar measurements can be performed using broadband light and a wavelength-selective spectrometer.

[0074] Expressed quantitatively using mathematical formulas, it is as follows: The electric field E obtained by rotating the light input to the first end of the polarizing beam splitter 24 by an angle of -θ2 is... 22 When we find the electric field E, 22 The 0-degree polarized component is the component that passes through the polarizing beam splitter 24 and is input to the photodetector 13 from the third end, electric field E 22 The 90-degree polarized component is reflected by the polarizing beam splitter 24 and becomes the component input to the photodetector 14 from the fourth end.

[0075]

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[0076] The intensity of the polarization component of the light at angle θ2 or the polarization component at angle θ2+90 degrees, or the difference between them, i.e., the electric field E, is determined by sweeping the optical angular frequency ω and inputting it to the first end of the polarization beam splitter 24. 22 The intensity of the 0-degree polarization component or the intensity of the 90-degree polarization component, or the difference between them, is obtained as the photodetector output. The method of sweeping ω may be to obtain the photodetector output while changing ω in a stepwise manner at predetermined angular frequency intervals, or to obtain the photodetector output at predetermined time intervals while continuously changing ω. When the photodetector output is obtained and represented with ω on the horizontal axis and the photodetector output on the vertical axis, three sinusoidal components are included from equations (14) and (15). The sweep of the optical angular frequency ω should be performed within the range in which the frequencies of these sinusoidal components can be measured.

[0077] More specifically, the electric field E 22 In a single-ended configuration where either the intensity of the 0-degree polarized component or the intensity of the 90-degree polarized component is obtained, equation (14) shows that there are three sinusoidal components and a constant (offset), and the electric field E 22 In the case of a differential configuration that obtains the difference between the intensity of the 0-degree polarized component and the intensity of the 90-degree polarized component, only three sinusoidal components exist according to equation (15). From the frequencies of these sinusoidal components, the delay time difference τ due to birefringence of the polarization-maintaining fiber 11 is obtained. p and the delay time difference τ due to the natural birefringence of the electro-optic crystal 17 d The method for finding this will be explained later.

[0078] If an optical circulator 16 with a polarization-maintaining fiber is used, the measured value is the sum of the delay time difference of the polarization-maintaining fiber 11 of the electro-optic probe 2 and the delay time difference of the polarization-maintaining fiber of the optical circulator 16. Therefore, the delay time difference of the polarization-maintaining fiber of the optical circulator 16 can be determined in advance using a conventional birefringence measurement method, and the delay time difference of only the polarization-maintaining fiber 11 of the electro-optic probe 2, after subtracting the delay time difference of the polarization-maintaining fiber of the optical circulator 16, can be output.

[0079] More specifically, as shown in Figure 6(a), when a polarization-maintaining fiber 21 (PMF21) is attached to the first end of the optical circulator 16, a polarization-maintaining fiber 22 (PMF22) to the second end, and a polarization-maintaining fiber 23 (PMF23) to the third end, the forward path of polarization-maintaining fibers 21 and 22 is input with 0-degree linearly polarized light aligned with the lagging axis of polarization-maintaining fibers 21 and 22, so it is not affected by birefringence, and the birefringence of the return path of polarization-maintaining fiber 22 and polarization-maintaining fiber 23 is added to the birefringence measurement value of polarization-maintaining fiber 11. For this reason, as shown in Figure 6(b), the lagging axis angle of polarization-maintaining fibers 22 and 23 can be set at 45 degrees, and the birefringence of the return path of polarization-maintaining fiber 22 and polarization-maintaining fiber 23 can be measured in advance using a conventional method for measuring the birefringence of polarization-maintaining fibers, and then subtracted from the measurement value of the electro-optic probe 2.

[0080] [Third Embodiment] The third embodiment, as shown in Figure 7, is a configuration in which the half-wave plate 19 is removed from the first embodiment and the polarization direction of the CW light source 25 is changed to linear polarization at an angle of -θ3. The other configurations are the same as in the first embodiment and will be omitted from the explanation as appropriate.

[0081] The CW light source 25 outputs linearly polarized CW light (narrowband light) with an angle of -θ3, and the output CW light is input to the first end of the optical circulator 16. It is also possible to generate linearly polarized CW light with an angle of -θ3 using a CW light source (not shown) that outputs 0-degree linearly polarized CW light and a half-wave plate (not shown) with a lagging axis angle of -θ3 / 2.

[0082] The optical circulator 16 is polarization-maintaining, and the light input to the first end maintains its polarization and is output from the second end. The linearly polarized light with an angle of -θ3 output from the second end of the optical circulator 16 is input to the left side of the polarization-maintaining fiber 11.

[0083] Light input to the left side of the polarization-maintaining fiber 11 is linearly polarized at an angle of -θ3. Unlike the first embodiment, as it propagates through the polarization-maintaining fiber 11 to the right side of the diagram, its polarization changes due to the birefringence of the polarization-maintaining fiber 11. Light output from the right side of the polarization-maintaining fiber 11 is input to one end of the electro-optic crystal 17, whose electrical principal axis is positioned at 45 degrees.

[0084] Light propagating through the electro-optic crystal 17 is reflected by a mirror 18 located at the other end of the electro-optic crystal 17, propagates in the reverse direction through the electro-optic crystal 17 and the polarization-maintaining fiber 11, and is input to the second end of the optical circulator 16.

[0085] It is desirable to integrate the polarization-maintaining fiber 11, the electro-optic crystal 17, and the mirror 18 to make the electro-optic probe 2 smaller and lighter, and to connect the left side of the polarization-maintaining fiber 11 and the optical circulator 16 with an optical connector (not shown) so that the electro-optic probe 2 can be easily attached and detached.

[0086] The light input to the second end of the optical circulator 16 maintains its polarization and is output from the third end of the optical circulator 16, and input to the first end of the polarized beam splitter 12. It is also possible to use an optical directional coupler instead of the optical circulator 16 to separate the input light to the left side of the polarization-maintaining fiber 11 from the output light from the left side of the polarization-maintaining fiber 11.

[0087] The polarizing beam splitter 12 transmits the 0-degree polarized component of the light input to the first end and outputs it from the third end toward the photodetector 13, and reflects the 90-degree polarized component and outputs it from the fourth end toward the photodetector 14. The photodetectors 13 and 14 each output an electrical signal proportional to the intensity of the input light.

[0088] Here, a differential configuration is shown in which 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 polarizing beam splitter 12 is detected by taking the difference between the output of the photodetector 13 and the output of the photodetector 14. However, it is also possible to place only the photodetector 13, which receives light transmitted through the polarizing beam splitter 12, to detect the intensity of the 0-degree polarized component of the light input to the polarizing beam splitter 12, or to place only the photodetector 14, which receives light reflected from the polarizing beam splitter 12, to detect the intensity of the 90-degree polarized component of the light input to the polarizing beam splitter 12.

[0089] In a single-ended configuration where either the photodetector 13 or the photodetector 14 is placed, a polarizer that extracts either a 0-degree polarized component or a 90-degree polarized component can be used instead of the polarization beam splitter 12. Since the polarization-maintaining fiber 11 has a lagging axis in the 0-degree direction, and the polarization beam splitter 12 and photodetectors 13 and 14 detect the intensity of the 0-degree and 90-degree polarized components of the output light of the polarization-maintaining fiber 11 on the return path, the birefringence of the polarization-maintaining fiber 11 does not affect the output of the photodetectors 13 and 14 on the return path.

[0090] In this embodiment, CW light (narrowband light) and a non-wavelength-selective photodetector were used, but similar measurements can be performed using broadband light and a wavelength-selective spectrometer.

[0091] Expressed quantitatively using a mathematical formula, it is as follows: The electric field of the light output from the CW light source 25 is E 03 The electric field of the light input to the first end of the polarizing beam splitter 12 is E 23 Then, the following equation holds true.

[0092]

number

number

number

number

[0093] Sweep the optical angular frequency ω to E 23 The intensity of the 0-degree polarization component or the intensity of the 90-degree polarization component, or the difference between them, is obtained as the photodetector output. The method of sweeping ω may be to obtain the photodetector output while changing ω in a stepwise manner at predetermined angular frequency intervals, or to obtain the photodetector output at predetermined time intervals while continuously changing ω. When the photodetector output is obtained and represented with ω on the horizontal axis and the photodetector output on the vertical axis, equations (18) and (19) show that it contains three sinusoidal components. The sweep of the optical angular frequency ω should be performed within the range in which the frequencies of these sinusoidal components can be measured.

[0094] For more details, see E 23 In a single-ended configuration where either the intensity of the 0-degree polarized component or the intensity of the 90-degree polarized component is obtained, equation (18) shows that there are three sinusoidal components and a constant (offset), E 23 In the case of a differential configuration that obtains the difference between the intensity of the 0-degree polarized component and the intensity of the 90-degree polarized component, only three sinusoidal components exist according to equation (19). From the frequencies of these sinusoidal components, the delay time difference τ due to birefringence of the polarization-maintaining fiber 11 is obtained. p and the delay time difference τ due to the natural birefringence of the electro-optic crystal 17 d The method for finding this will be explained later.

[0095] If an optical circulator 16 with a polarization-maintaining fiber is used, the measured value is the sum of the delay time difference of the polarization-maintaining fiber 11 of the electro-optic probe 2 and the delay time difference of the polarization-maintaining fiber of the optical circulator 16. Therefore, the delay time difference of the polarization-maintaining fiber of the optical circulator 16 can be determined in advance using a conventional birefringence measurement method, and the delay time difference of only the polarization-maintaining fiber 11 of the electro-optic probe 2, after subtracting the delay time difference of the polarization-maintaining fiber of the optical circulator 16, can be output.

[0096] More specifically, as shown in Figure 8(a), when a polarization-maintaining fiber 21 (PMF21) is attached to the first end of the optical circulator 16, a polarization-maintaining fiber 22 (PMF22) to the second end, and a polarization-maintaining fiber 23 (PMF23) to the third end, the birefringence of the forward paths of polarization-maintaining fibers 21 and 22 is added to the birefringence measurement value of polarization-maintaining fiber 11. For the return path of polarization-maintaining fiber 22 and polarization-maintaining fiber 23, only the intensity of the 0-degree polarization component and the 90-degree polarization component are detected by the polarization beam splitter 12 and photodetectors 13 and 14, and are not affected by birefringence. Therefore, as shown in Figure 8(b), the angle of the lagging axis of polarization-maintaining fibers 21 and 22 is set at 45 degrees, and the birefringence of the forward paths of polarization-maintaining fibers 21 and 22 is measured in advance using a conventional method for measuring the birefringence of polarization-maintaining fibers, and then subtracted from the measurement value of the electro-optic probe 2.

[0097] [τ p and τ d [How to find it] Next, the delay time difference τ due to birefringence of the polarization-maintaining fiber 11 is calculated from the frequency of the sinusoidal component of the photodetector output. p and the delay time difference τ due to the natural birefringence of the electro-optic crystal 17 d We will now describe how to find τ. If we set θ=θ1=θ2=θ3, then equations (11), (14), and (18), and equations (12), (15), and (19) become the same, so the first, second, and third embodiments can be found in a similar way. p and τ d It is possible to find this.

[0098] From equations (11), (12), (14), (15), (18), and (19), the frequencies of the three sinusoidal components (the angular frequencies of the sinusoidal components when ω is on the horizontal axis, the photodetector output is on the vertical axis, and the horizontal axis is considered to be time) are |τ d ±τ p |,τ d Therefore, if we denote the frequencies of the sinusoidal components detected from the photodetector output as τ1, τ2, and τ3 in descending order, then τ1, τ2, and τ3 are |τ d ±τ p |,τ d It corresponds to one of the following, but τ p and τd If it is unknown, which of τ1, τ2, or τ3 is |τ d ±τ p | and τ d It is unclear whether this corresponds to [the relevant issue].

[0099] For more details, τ p and τ d If we take τ as a positive value, d +τ p The largest value is τ1 = τ d +τ p However, |τ d -τ p | and τ d It is unclear which is larger, and τ2 is |τ d -τ p | and τ d It is impossible to determine which of the following it corresponds to. Also, the frequency of the sinusoidal component is observed in absolute value, so τ d -τ p The sign of |τ is unknown, that is, d -τ p |=τ d -τ p ka|τ d -τ p |=τ p -τ d It is unclear whether so. Therefore, simply from τ1, τ2, τ3, τ p ,τ d Since it is not possible to derive τ by the following method p and τ d We seek.

[0100] (Method 1) τ p and τ d We will show the first method for finding τ. If we set θ such that cos(2θ)=0, then τ d The amplitude of the sinusoidal component becomes zero, |τ d ±τ p Two sinusoidal components of | are output from the receiver. If τ d and τ p If the relationship between magnitudes is known, τ d -τ p Since the sign of is determined, τ is obtained from the frequencies of the two sinusoidal components. p and τd It is possible to find this.

[0101] Specifically, τ d >τ p In the case of |τ d -τ p |=τ d -τ p Therefore, as shown in Figure 9(a), τ1 = τ d +τ p τ²=τ d -τ p Therefore, τ1 and τ2 are detected and τ d =(τ1+τ2) / 2,τ p = (τ1-τ2) / 2 d and τ p It is possible to find this.

[0102] τ d <τ p In the case of |τ d -τ p |=τ p -τ d Therefore, as shown in Figure 9(b), τ1 = τ d +τ p τ²=τ p -τ d Therefore, τ1 and τ2 are detected and τ d =(τ1-τ2) / 2,τ p = (τ1+τ2) / 2 d and τ p It is possible to find this.

[0103] If τ d =τ p In this case, as shown in Figure 9(c), |τ d -τ p The sinusoidal component of | has a frequency of zero and a phase of φ. d +φ e -φ p Depending on the value of |τ d -τ p The | component may also disappear. In this case, τ1 is detected and τ d =τ p =τ1 / 2 d and τ p It is possible to find this.

[0104] As described above, in the first method, τ d and τ p When the relative magnitudes of the two components are known, set θ such that cos(2θ)=0, and detect the two sinusoidal components τ1 and τ2, and then τ d and τ p We seek.

[0105] (Second method) τ p and τ d A second method for finding 2τ is presented. d and τ p If the relative magnitudes are known, then which of the three sinusoidal components τ1, τ2, and τ3 is |τ d ±τ p | and τ d It can identify whether it corresponds to [a certain condition].

[0106] 2τ d >τ p In this case, τ1 = τ as shown in Figure 9(d) or (e). d +τ p τ²=τ d τ3=|τ d -τ p To correspond to |, τ1 and τ2 are detected and τ d =τ2,τ p =τ1-τ2 d and τ p It is possible to find this.

[0107] 2τ d <τ p In this case, as shown in Figure 9(f), τ1 = τ d +τ p τ2=|τ d -τ p |,τ3=τ d To address this, τ1 and τ3 are detected and τ d =τ3,τ p =τ1-τ3 d and τ p It is possible to find this.

[0108] 2τ d >τ p In the case of τd and τ p If the relationship between magnitudes is known, τ d -τ p The sign of the number is determined.

[0109] τ d >τ p In the case of |τ d -τ p |=τ d -τ p Therefore, as shown in Figure 9(d), τ1 = τ d +τ p τ²=τ d τ3=τ d -τ p Therefore, τ1 and τ3 are detected and τ d =(τ1+τ3) / 2,τ p = (τ1-τ3) / 2 d and τ p It is possible to determine τ2 and τ3 and τ d =τ2,τ p =τ2-τ3 d and τ p It is possible to find this.

[0110] 2τ d >τ p >τ d In the case of |τ d -τ p |=τ p -τ d Therefore, as shown in Figure 9(e), τ1 = τ d +τ p τ²=τ d τ3=τ p -τ d Therefore, τ1 and τ3 are detected and τ d =(τ1-τ3) / 2,τ p = (τ1+τ3) / 2 d and τ p It is possible to determine τ2 and τ3 and τ d =τ2,τ p =τ² + τ³ d and τ p It is possible to find this.

[0111] 2τd <τ p In the case of |τ d -τ p |=τ p -τ d Therefore, as shown in Figure 9(f), τ1 = τ d +τ p τ²=τ p -τ d τ3=τ d Therefore, τ1 and τ2 are detected and τ d =(τ1-τ2) / 2,τ p = (τ1+τ2) / 2 d and τ p It is possible to determine τ2 and τ3 and τ d =τ3,τ p =τ² + τ³ d and τ p It is possible to find this.

[0112] If τ d =τ p In this case, as shown in Figure 9(g), |τ d -τ p The sinusoidal component of | has a frequency of zero and a phase of φ. d +φ e -φ p Depending on the value of |τ d -τ p The | component may also disappear. In this case, τ1 and τ2 are detected and τ d =τ2,τ p =τ1-τ2 d and τ p It is possible to determine τ1 and τ d =τ p =τ1 / 2 d and τ p It is possible to determine τ2 and τ d =τ p =τ² d and τ p It is possible to find this.

[0113] If τ p =2τ d In this case, as shown in Figure 9(h), |τ d -τ pThe sinusoidal component of | is τ d Since the sine wave component and the absolute value of the frequency are equal, the two sine wave components overlap on the frequency axis, |τ d -τ p | and the sinusoidal component and τ d If the amplitudes of the sinusoidal components are equal, then the phase φ p The amplitude of the superimposed sinusoidal components may become zero. In this case, τ1 is detected and τ d =τ1 / 3,τ p = 2τ¹ / 3 d and τ p It is possible to find this.

[0114] Therefore, in the second method, if the magnitude relationship described above is known, two of the three sinusoidal components τ1, τ2, τ3 are detected and τ d and τ p This can be determined. In detail, τ d >τ p , 2τ d >τ p >τ d , 2τ d <τ p If it is known which of the following magnitude relationships is satisfied, then any two of the three sinusoidal components τ1, τ2, τ3 can be detected and τ d and τ p It is possible to find . Alternatively, 2τ d >τ p If the relationship between magnitudes is known to be satisfied, then τ1 and τ2 are detected from the three sinusoidal components τ1, τ2, τ3. d and τ p We can find 2τ d <τ p If the relationship between magnitudes is known to be satisfied, then any two of the three sinusoidal components τ1, τ2, τ3 can be detected and τ d and τ p It is possible to find this.

[0115] (Third method) τ p and τ d A third method for finding τ is p and τ d This method is used when the relative sizes of two things are unknown.

[0116] First, 2τ d >τ p Assuming this, τ1 = τ as shown in Figure 9(d) or (e). d +τ p τ²=τ d τ3=|τ d -τ p To correspond to |, τ1 and τ2 are detected and τ d =τ2,τ p =τ1-τ2 d and τ p We find |τ3 and detect it. d -τ p If it matches when compared to |, then the assumption is judged to be correct and this τ d and τ p We will adopt this.

[0117] And 2τ d <τ p Assuming this, τ1 = τ d +τ p τ2=|τ d -τ p |,τ3=τ d To address this, τ1 and τ3 are detected and τ d =τ3,τ p =τ1-τ3 d and τ p We find |τ and detect τ2. d -τ p If it matches when compared to |, then the assumption is judged to be correct and this τ d and τ p We will adopt this.

[0118] Alternatively, first τ d >τ p Assuming that, |τ d -τ p |=τ d -τ p Therefore, as shown in Figure 9(d), τ1 = τ d +τ p τ²=τ d τ3=τ d -τ p Therefore, τ1 and τ3 are detected and τ d=(τ1+τ3) / 2,τ p = (τ1-τ3) / 2 d and τ p We find τ2 and detect τ d If it matches when compared, then the assumption is judged to be correct and this τ d and τ p Either adopt or detect τ2 and τ3 and τ d =τ2,τ p =τ2-τ3 d and τ p We determine τ1 and detect τ d +τ p If it matches when compared, then the assumption is judged to be correct and this τ d and τ p We will adopt this.

[0119] And 2τ d >τ p >τ d Assuming that, |τ d -τ p |=τ p -τ d Therefore, as shown in Figure 9(e), τ1 = τ d +τ p τ²=τ d τ3=τ p -τ d Therefore, τ1 and τ3 are detected and τ d =(τ1-τ3) / 2,τ p = (τ1+τ3) / 2 d and τ p We find τ2 and detect τ d If it matches when compared, then the assumption is judged to be correct and this τ d and τ p Either adopt or detect τ2 and τ3 and τ d =τ2,τ p =τ² + τ³ d and τ p We determine τ1 and detect τ d +τ p If it matches when compared, then the assumption is judged to be correct and this τ d and τ p We will adopt this.

[0120] And 2τd <τ p Assuming that, |τ d -τ p |=τ p -τ d Therefore, as shown in Figure 9(f), τ1 = τ d +τ p τ²=τ p -τ d τ3=τ d Therefore, τ1 and τ2 are detected and τ d =(τ1-τ2) / 2,τ p = (τ1+τ2) / 2 d and τ p We determine τ3 and detect τ d If it matches when compared, then the assumption is judged to be correct and this τ d and τ p Either adopt or detect τ2 and τ3 and τ d =τ3,τ p =τ² + τ³ d and τ p We determine τ1 and detect τ d +τ p If it matches when compared, then the assumption is judged to be correct and this τ d and τ p We will adopt this.

[0121] If τ p =τ d In this case, as shown in Figure 9(g), |τ d -τ p The sinusoidal component of | has a frequency of zero and a phase of φ. d +φ e -φ p Depending on the value of |τ d -τ p The | component may also disappear. p =2τ d In this case, as shown in Figure 9(h), |τ d -τ p The sinusoidal component of | is τ d Since the sine wave component and the absolute value of the frequency are equal, the two sine wave components overlap on the frequency axis, |τ d -τ p | and the sinusoidal component and τ d If the amplitudes of the sinusoidal components are equal, then the phase φp The amplitude of the superimposed sinusoidal components may become zero. Therefore, when there are two sinusoidal components, τ p =τ d In the case of τ p =2τ d This is one possible scenario, and in either case, τ1 = τ d +τ p τ²=τ d Therefore τ d =τ2,τ p =τ1-τ2 d and τ p We can find the following, and if the number of sinusoidal components is one, τ p =2τ d Limited to the case where τ1 = τ d +τ p =3τ d Therefore, τ d =τ1 / 3,τ p = 2τ¹ / 3 d and τ p It is possible to find this.

[0122] Therefore, the third method detects three sinusoidal components τ1, τ2, and τ3, and assuming a magnitude relationship, calculates τ from the frequencies of two sinusoidal components. d and τ p This method involves determining τ and then using the frequency of the remaining sinusoidal component to verify whether the assumption is correct. In detail, d >τ p , 2τ d >τ p >τ d , 2τ d <τ p Assuming that one of the following magnitude relationships is satisfied, from the frequencies of any two of the three sinusoidal components τ1, τ2, τ3, τ d and τ p We find this and use the frequency of the remaining sinusoidal component to verify if the assumption is correct. Alternatively, 2τ d >τ p Assuming that the relationship of magnitude is satisfied, the frequencies of the sinusoidal components τ1 and τ2 out of the three sinusoidal components τ1, τ2, and τ3 are used to determine τ d and τ pWe find the value of τ3 and verify whether the assumption is correct using the frequency of the sinusoidal component of τ3, and then 2τ d <τ p Assuming that the relationship between magnitudes is satisfied, from the frequencies of any two of the three sinusoidal components τ1, τ2, τ3, τ d and τ p We find this and verify whether the assumption is correct using the frequency of the remaining sinusoidal component. However, this method is 4τ d =τ p In the case of (τ d =x,τ p =4x) and 2τ d =3τ p In the case of (τ d =3x,τ p =2x) In both cases, τ1=5x, τ2=3x, τ3=x, and there is no contradiction in the frequencies of the three sinusoidal components, so the two cannot be distinguished, that is, 4τ d =τ p and 2τ d =3τ p There is a problem in that it cannot be measured correctly under certain conditions.

[0123] (Fourth method) τ p and τ d In the fourth method for finding τ, p and τ d When the relative magnitudes of the three sinusoidal components are unknown, which of τ1, τ2, and τ3 is |τ d ±τ p | and τ d Determine if it corresponds to |τ d ±τ p The amplitude of the sinusoidal component of | is |(1 / 2)sin(2θ)|, τ d The amplitude of the sinusoidal component is |cos(2θ)|. For example, if θ is 22.5 degrees, then |τ d ±τ p The amplitude of the sinusoidal component of | is 1 / (2√2), τ d Since the amplitude of the sinusoidal component is 1 / √2, as shown in Figures 9(i), (j), and (k), the sinusoidal component with the largest amplitude among the three sinusoidal components is τ d Corresponding to this, the frequency of its sinusoidal component is τ dThis can be determined. And in either case, τ1 is τ d +τ p To address this, τ p =τ1-τ d by τ p This can be determined.

[0124] If τ p =τ d In this case, as shown in Figure 9(l), |τ d -τ p The sinusoidal component of | has a frequency of zero and a phase of φ. d +φ e -φ p Depending on the value of |τ d -τ p The | component may also disappear. p =2τ d In this case, as shown in Figure 9(m), |τ d -τ p The sinusoidal component of | is τ d Since the sine wave component and the absolute value of the frequency are equal, the two sine wave components overlap on the frequency axis, |τ d -τ p | and the sinusoidal component and τ d Since the sinusoidal components have different amplitudes, the amplitudes of the superimposed sinusoidal components are in phase φ. p It changes depending on the factor, but the amplitude never becomes zero.

[0125] Therefore, from the highest frequency sinusoidal component, τ d +τ p We find the τ, and after removing the highest frequency sine wave component, we take the τ from the sine wave component with the largest amplitude. d If we find τ, p =τ d In the case of or τ p =2τ d This is applicable to all cases, including the case mentioned above.

[0126] Therefore, in the fourth method, which of the three or two sinusoidal components is τ d +τ p and τ d By identifying whether it corresponds to τ by amplitude, p and τ dEven when the relative magnitudes are unknown, τ p and τ d This can be determined. The fourth method is 4τ, which is indistinguishable by the frequency of the sinusoidal component. d =τ p In the case of 2τ d =3τ p In this case as well, it can be distinguished by the amplitude of the sinusoidal component, so 4τ cannot be measured by the third method. d =τ p and 2τ d =3τ p Measurement is possible even in this case.

[0127] Methods for determining the frequency of a sinusoidal component include performing a Fourier transform and detecting peaks on the frequency axis, extracting a desired sinusoidal component, detecting the period from the zero-crossing point of the sinusoid and determining the frequency from the reciprocal of the period, or extracting a desired sinusoidal component, performing a Hilbert transform, calculating the phase using the arctangent function, and determining the phase slope. The above processing is performed in the signal processing unit 20, and the delay time difference τ of the polarization-maintaining fiber 11 is obtained as the birefringence measurement result. p and the delay time difference τ of the electro-optic crystal 17 d Outputs.

[0128] Figure 10 illustrates the relationship between the angle θ and the amplitude of each sinusoidal component. The solid line in Figure 10 represents |τ d ±τ p |The amplitude of the sinusoidal component, the dashed line represents τ d This represents the amplitude of the sinusoidal component. The characteristic is that the angle θ repeats with a period of 90 degrees. In Figure 10, θ is shown between 0 and 180 degrees, but similar results can be obtained for other angles as well, as the period of 90 degrees repeats.

[0129] For θ (0 degrees, 90 degrees, 180 degrees in the range from 0 degrees to 180 degrees) that satisfies sin(2θ)=0, |τ d ±τ p The amplitude of the frequency component of | becomes zero τ p Because it becomes impossible to find τ p and τ d This must be avoided in any method used to obtain it.p and τ d In the first method for finding θ, we set θ to satisfy cos(2θ)=0 (45°, 135° in the range of 0° to 180°), and τ d The amplitude of the sinusoidal component of the waveform is set to zero. Incidentally, for any θ that satisfies cos(2θ)=0, sin(2θ)≠0 is always satisfied.

[0130] τ p and τ d In the second and third methods for finding |τ d ±τ p | and τ d The preferred θ values ​​(approximately 31.72 degrees, 58.28 degrees, 121.72 degrees, and 148.28 degrees in the range of 0 to 180 degrees) that satisfy |sin(2θ)|=|2cos(2θ)|, where the amplitudes of the sinusoidal components are equal, are |τ d ±τ p When the sinusoidal component of | becomes zero (sin(2θ)=0), and τ d It is necessary to avoid the case where the sinusoidal component of is zero (cos(2θ)=0). Therefore, θ is not limited to approximately 31.72 degrees, approximately 58.28 degrees, approximately 121.72 degrees, and approximately 148.28 degrees, but |τ d ±τ p The sinusoidal component of | is zero at 0 degrees, 90 degrees, 180 degrees, and τ d It is possible to set the angle to any angle except 45 degrees and 135 degrees, where the sinusoidal component of the waveform becomes zero.

[0131] τ p and τ d In the fourth method for finding |τ d ±τ p | and τ d Since the amplitudes of each sinusoidal component are relatively large and the amplitude differences are large enough to be reliably distinguishable, θ (22.5 degrees, 67.5 degrees, 112.5 degrees, 157.5 degrees in the range of 0 to 180 degrees) that satisfy |sin(2θ)|=|cos(2θ)| are preferred choices. And |τ d ±τ p When the sinusoidal component of | becomes zero (sin(2θ)=0), τ dIn addition to the case where the sinusoidal component of |τ is zero (cos(2θ)=0), d ±τ p | and the sinusoidal component and τ d The case where the sinusoidal components of the waveform have equal amplitude (|sin(2θ)|=|2cos(2θ)|) also needs to be avoided.

[0132] Therefore, θ is not limited to 22.5 degrees, 67.5 degrees, 112.5 degrees, and 157.5 degrees, |τ d ±τ p | is zero at 0 degrees, 90 degrees, 180 degrees, and τ. d The sinusoidal component of |τ| becomes zero at 45 degrees, 135 degrees, and |τ|. d ±τ p | and the sinusoidal component and τ d It is possible to set the angle to any angle except for approximately 31.72 degrees, 58.28 degrees, 121.72 degrees, and 148.28 degrees, where the sinusoidal components have equal amplitude.

[0133] Also, the vicinity of these angles to be excluded is |τ d ±τ p | The sinusoidal component or τ d The sinusoidal component of |τ approaches zero, which degrades the measurement accuracy, or |τ d ±τ p | and the sinusoidal component and τ d It is desirable to avoid this because the sinusoidal component of the waveform becomes close to equal amplitude, making misjudgment more likely. Note that between θ of approximately 31.72 degrees and approximately 58.28 degrees, and between approximately 121.72 degrees and approximately 148.28 degrees, τ d |τ d ±τ p Since the amplitude of the sinusoidal component of | is larger, τ p and τ d In the fourth method for determining τ, the sinusoidal component with the smallest amplitude is detected. d We seek. [Industrial applicability]

[0134] As described above, the present invention provides an integrated electro-optic probe in which the retard axis of the polarization-maintaining fiber and the electrical principal axis of the electro-optic crystal form a 45-degree angle, and has the effect of simultaneously measuring the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal in a single wavelength sweep, making it useful for birefringence measuring devices and birefringence measuring methods in general. [Explanation of Symbols]

[0135] 1, 1A, 1B, 1C, 1D, 1E Birefringence measuring device 2. Electro-optic probe 10, 25 CW light source 11, 21, 22, 23 Polarization-maintaining fibers 12, 24 Polarizing beam splitter (polarization separation unit) 13, 14 Receiver 15 Differential Amplifier 16. Optical Circulator (Optical Input / Output Section) 17 Electro-optic crystals 18 Mirror 19 1 / 2 wave plate 20 Signal Processing Unit

Claims

1. A polarization-maintaining fiber (11) with a retard axis positioned at 0 or 90 degrees, An electro-optic crystal (17) having natural birefringence is connected at one end to the other end of the polarization-maintaining fiber, such that its electrical principal axis is at a 45-degree angle to the retard axis of the polarization-maintaining fiber. A mirror (18) is provided at the other end of the aforementioned electro-optic crystal, A birefringence measuring device for measuring the birefringence of an electro-optic probe, including, A CW light source (10) that outputs CW light with 0 degree linear polarization, An optical input / output unit (16) inputs the CW light to the other end of the polarization-maintaining fiber and outputs light that is output from one end of the polarization-maintaining fiber, input to one end of the electro-optic crystal, reflected by the mirror and output from one end of the electro-optic crystal, input to one end of the polarization-maintaining fiber, and output from the other end of the polarization-maintaining fiber, A half-wave plate (19) to which light output from the optical input / output unit is input, A polarization separation unit (12) receives light output from the half-wave plate at the first end, and outputs either the 0-degree polarized component or the 90-degree polarized component of the light input to the first end from the second end, or both from the third and fourth ends, respectively. A light receiver (13, 14) detects the intensity of light output from the second end of the polarization separation unit, or the difference in intensity of light output from the third and fourth ends of the polarization separation unit, A signal processing unit (20) detects the frequency components of the output of the photodetector when the wavelength of the CW light is swept, and determines the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal, It has, If the angle of the retard axis of the half-wave plate is θ / 2, then θ is sin(2θ) ≠ 0 A birefringence measuring device configured to satisfy the following conditions.

2. A polarization-maintaining fiber (11) with a retard axis positioned at 0 or 90 degrees, An electro-optic crystal (17) having natural birefringence is connected at one end to the other end of the polarization-maintaining fiber, such that its electrical principal axis is at a 45-degree angle to the retard axis of the polarization-maintaining fiber. A mirror (18) is provided at the other end of the aforementioned electro-optic crystal, A birefringence measuring device for measuring the birefringence of an electro-optic probe, including, A CW light source (10) that outputs CW light with 0 degree linear polarization, An optical input / output unit (16) inputs the CW light to the other end of the polarization-maintaining fiber and outputs light that is output from one end of the polarization-maintaining fiber, input to one end of the electro-optic crystal, reflected by the mirror and output from one end of the electro-optic crystal, input to one end of the polarization-maintaining fiber, and output from the other end of the polarization-maintaining fiber, A polarization separation unit (24) receives light output from the optical input / output unit at a first end, and outputs either the polarization component of the light input to the first end at an angle θ and the polarization component at an angle of θ + 90 degrees from the second end, or both from the third and fourth ends, respectively. A light receiver (13, 14) detects the intensity of light output from the second end of the polarization separation unit, or the difference in intensity of light output from the third and fourth ends of the polarization separation unit, A signal processing unit (20) detects the frequency components of the output of the photodetector when the wavelength of the CW light is swept, and determines the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal, It has, The aforementioned θ is sin(2θ) ≠ 0 A birefringence measuring device configured to satisfy the following conditions.

3. A polarization-maintaining fiber (11) with a retard axis positioned at 0 or 90 degrees, An electro-optic crystal (17) having natural birefringence is connected at one end to the other end of the polarization-maintaining fiber, such that its electrical principal axis is at a 45-degree angle to the retard axis of the polarization-maintaining fiber. A mirror (18) is provided at the other end of the aforementioned electro-optic crystal, A birefringence measuring device for measuring the birefringence of an electro-optic probe, including, A CW light source (25) that outputs linearly polarized CW light with an angle of -θ, An optical input / output unit (16) inputs the CW light to the other end of the polarization-maintaining fiber and outputs light that is output from one end of the polarization-maintaining fiber, input to one end of the electro-optic crystal, reflected by the mirror and output from one end of the electro-optic crystal, input to one end of the polarization-maintaining fiber, and output from the other end of the polarization-maintaining fiber, A polarization separation unit (12) receives light output from the optical input / output unit at the first end, and outputs either the 0-degree polarization component or the 90-degree polarization component of the light input to the first end from the second end, or both from the third and fourth ends, respectively. A light receiver (13, 14) detects the intensity of light output from the second end of the polarization separation unit, or the difference in intensity of light output from the third and fourth ends of the polarization separation unit, A signal processing unit (20) detects the frequency components of the output of the photodetector when the wavelength of the CW light is swept, and determines the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal, It has, The aforementioned θ is sin(2θ) ≠ 0 A birefringence measuring device configured to satisfy the following conditions.

4. The aforementioned θ is cos(2θ) = 0 Set to satisfy, The signal processing unit, when the relative magnitudes of the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal are known, detects the frequencies of the two frequency components output from the photodetector when the wavelength of the CW light is swept, and determines the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal. The birefringence measuring device according to any one of claims 1 to 3.

5. The aforementioned θ is cos(2θ) ≠ 0 It is set to further satisfy the following conditions: The signal processing unit, Birefringence τ of the polarization-maintaining fiber ｐ and the birefringence τ of the electro-optic crystal ｄ Toga, t ｄ >t ｐ 2t ｄ >t ｐ >t ｄ 2t ｄ <t ｐ When it is known which of the following magnitude relationships is satisfied, the frequencies of any two of the three frequency components output from the photodetector when the wavelength of the CW light is swept are detected to determine the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal. The birefringence measuring device according to any one of claims 1 to 3.

6. The aforementioned θ is cos(2θ) ≠ 0 It is set to further satisfy the following conditions: The signal processing unit, Birefringence τ of the polarization-maintaining fiber ｐ and the birefringence τ of the electro-optic crystal ｄ Toga, 2t ｄ >t ｐ When it is known that the magnitude relationship is satisfied, the frequencies of the highest frequency component and the second highest frequency component among the three frequency components output from the photodetector when the wavelength of the CW light is swept are detected to determine the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal. Birefringence τ of the polarization-maintaining fiber ｐ and the birefringence τ of the electro-optic crystal ｄ Toga, 2t ｄ <t ｐ When it is known that the magnitude relationship is satisfied, the frequencies of any two of the three frequency components output from the photodetector when the wavelength of the CW light is swept are detected to determine the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal. The birefringence measuring device according to any one of claims 1 to 3.

7. The aforementioned θ is cos(2θ) ≠ 0 It is set to further satisfy the following conditions: The signal processing unit, Birefringence τ of the polarization-maintaining fiber ｐ and the birefringence τ of the electro-optic crystal ｄ Toga, t ｄ >t ｐ 2t ｄ >t ｐ >t ｄ 2t ｄ <t ｐ Assuming that one of the following inequalities is satisfied, When the wavelength of the CW light is swept, the frequencies of any two of the three frequency components output from the photodetector are detected to determine the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal. The assumption of the magnitude relationship is verified using the frequency of the remaining frequency component among the three frequency components, The birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal are output when the assumption of the magnitude relationship is correct. The birefringence measuring device according to any one of claims 1 to 3.

8. The aforementioned θ is cos(2θ) ≠ 0 It is set to further satisfy the following conditions: The signal processing unit, Birefringence τ of the polarization-maintaining fiber ｐ and the birefringence τ of the electro-optic crystal ｄ Toga, 2t ｄ >t ｐ Assuming that the first inequality relationship is satisfied, When the wavelength of the CW light is swept, the frequencies of the highest frequency component and the second highest frequency component among the three frequency components output from the photodetector are detected to determine a first candidate for the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal. The first assumption of magnitude relationship is verified using the frequency of the lowest frequency component among the three frequency components. If the assumption of the first magnitude relationship is correct, the first candidate for the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal is output. Birefringence τ of the polarization-maintaining fiber ｐ and the birefringence τ of the electro-optic crystal ｄ Toga, 2t ｄ <t ｐ Assuming that the second inequality is satisfied, The frequencies of any two of the three frequency components are detected to determine a second candidate for the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal. The assumption of the second magnitude relationship is verified using the frequency of the remaining frequency component among the three frequency components, If the assumption of the second magnitude relationship is correct, the second candidate for the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal is output. The birefringence measuring device according to any one of claims 1 to 3.

9. The aforementioned θ is cos(2θ) ≠ 0 |sin(2θ)|≠|2cos(2θ)| It is set to further satisfy both of the following equations, The signal processing unit, From two or three frequency components of the output of the photodetector when the wavelength of the CW light is swept, a first frequency component whose frequency corresponds to the sum of the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal, and a second frequency component whose frequency corresponds to the birefringence of the electro-optic crystal are identified based on the amplitude of the frequency components. The birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal are determined from the frequencies of the first frequency component and the second frequency component. The birefringence measuring device according to any one of claims 1 to 3.

10. A polarization-maintaining fiber (11) with a retard axis positioned at 0 or 90 degrees, An electro-optic crystal (17) having natural birefringence is connected at one end to the other end of the polarization-maintaining fiber, such that its electrical principal axis is at a 45-degree angle to the retard axis of the polarization-maintaining fiber. A mirror (18) is provided at the other end of the aforementioned electro-optic crystal, A method for measuring the birefringence of an electro-optic probe, including, It outputs CW light with 0-degree linear polarization, The CW light is input to the other end of the polarization-maintaining fiber, The light is output from one end of the polarization-maintaining fiber, input to one end of the electro-optic crystal, reflected by the mirror and output from one end of the electro-optic crystal, input to one end of the polarization-maintaining fiber, output from the other end of the polarization-maintaining fiber, input to a half-wave plate (19) with a retarded axis angle of θ / 2, and the intensity difference of either the 0-degree polarization component or the 90-degree polarization component of the light output from the half-wave plate, or both, is detected. sin(2θ) ≠ 0 cos(2θ) ≠ 0 |sin(2θ)|≠|2cos(2θ)| Set θ such that it satisfies all of the following equations: Two or three frequency components are detected from the intensity or intensity difference when the wavelength of the CW light is swept. The frequency component with the highest frequency among the two or three frequency components is identified as a first frequency component corresponding to the sum of the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal. Excluding the frequency component with the highest frequency among the two or three frequency components, the frequency component with the largest amplitude or the frequency component with the smallest amplitude is identified as a second frequency component corresponding to the birefringence of the electro-optic crystal. The birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal are determined from the frequencies of the first frequency component and the second frequency component. Method for measuring birefringence.

11. A polarization-maintaining fiber (11) with a retard axis positioned at 0 or 90 degrees, An electro-optic crystal (17) having natural birefringence is connected at one end to the other end of the polarization-maintaining fiber, such that its electrical principal axis is at a 45-degree angle to the retard axis of the polarization-maintaining fiber. A mirror (18) is provided at the other end of the aforementioned electro-optic crystal, A method for measuring the birefringence of an electro-optic probe, including, It outputs CW light with 0-degree linear polarization, The CW light is input to the other end of the polarization-maintaining fiber, The intensity difference between the polarization component at angle θ and the polarization component at angle θ + 90 degrees, or both, of the light output from one end of the polarization-maintaining fiber, input to one end of the electro-optic crystal, reflected by the mirror and output from one end of the electro-optic crystal, input to one end of the polarization-maintaining fiber, and output from the other end of the polarization-maintaining fiber is detected. sin(2θ) ≠ 0 cos(2θ) ≠ 0 |sin(2θ)|≠|2cos(2θ)| Set θ such that it satisfies all of the following equations: Two or three frequency components are detected from the intensity or intensity difference when the wavelength of the CW light is swept. The frequency component with the highest frequency among the two or three frequency components is identified as a first frequency component corresponding to the sum of the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal. Excluding the frequency component with the highest frequency among the two or three frequency components, the frequency component with the largest amplitude or the frequency component with the smallest amplitude is identified as a second frequency component corresponding to the birefringence of the electro-optic crystal. The birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal are determined from the frequencies of the first frequency component and the second frequency component. Method for measuring birefringence.

12. A polarization-maintaining fiber (11) with a retard axis positioned at 0 or 90 degrees, An electro-optic crystal (17) having natural birefringence is connected at one end to the other end of the polarization-maintaining fiber, such that its electrical principal axis is at a 45-degree angle to the retard axis of the polarization-maintaining fiber. A mirror (18) is provided at the other end of the aforementioned electro-optic crystal, A method for measuring the birefringence of an electro-optic probe, including, It outputs linearly polarized CW light with an angle of -θ, The CW light is input to the other end of the polarization-maintaining fiber, The intensity difference between the intensity of either the 0-degree polarization component or the 90-degree polarization component of the light output from one end of the polarization-maintaining fiber, input to one end of the electro-optic crystal, reflected by the mirror and output from one end of the electro-optic crystal, input to one end of the polarization-maintaining fiber, and output from the other end of the polarization-maintaining fiber, or the intensity difference between both, is detected. sin(2θ) ≠ 0 cos(2θ) ≠ 0 |sin(2θ)|≠|2cos(2θ)| Set θ such that it satisfies all of the following equations: Two or three frequency components are detected from the intensity or intensity difference when the wavelength of the CW light is swept. The frequency component with the highest frequency among the two or three frequency components is identified as a first frequency component corresponding to the sum of the birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal. Excluding the frequency component with the highest frequency among the two or three frequency components, the frequency component with the largest amplitude or the frequency component with the smallest amplitude is identified as a second frequency component corresponding to the birefringence of the electro-optic crystal. The birefringence of the polarization-maintaining fiber and the birefringence of the electro-optic crystal are determined from the frequencies of the first frequency component and the second frequency component. Method for measuring birefringence.

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