Error signal generation device and error signal generation method

Abstract

To realize an error signal generation device that obtains an appropriate error signal for stabilizing the oscillation frequency of a laser light source. [Solution] The error signal generation device (3) comprises a resonator (16) into which laser light emitted from a laser light source (2) is incident, a first photodetector (21) that measures first interference light obtained when a first laser light that has passed through the resonator and a second laser light that has not passed through the resonator interfere with each other, and a signal generation unit (23) that generates an error signal based on the output of the first photodetector.

Description

[Technical Field]

[0001] This invention relates to an error signal generation device. [Background technology]

[0002] The oscillation frequency of a laser light source can change due to factors such as the heat generated by the laser light source itself. Methods for stabilizing the oscillation frequency of a laser light source by obtaining an error signal representing the deviation of the oscillation frequency from a reference frequency have been conventionally known. One such method using frequency modulation of laser light is the PDH (Pound-Drever-Hall) method. The PDH method requires a modulator and demodulator, and its applicable range is limited.

[0003] Non-patent document 1 discloses a method for stabilizing the oscillation frequency of a laser light source using a Mach-Zehnder interferometer (MZI) without using modulation. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] MH Idjadi, K. Kim, and NK Fontaine, “Modulation-free laser stabilization technique using integrated cavity coupled Mach-Zehnder interferometer”, Nature Communications, 15(1), 1922, (2024) [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] However, in methods like the one described in Non-Patent Document 1 (MZI method), for example, the range of controllable frequency fluctuations is not sufficiently wide.

[0006] One aspect of the present invention aims to realize an error signal generation device that obtains an appropriate error signal for stabilizing the oscillation frequency of a laser light source.

Means for Solving the Problem

[0007] The error signal generation device according to Aspect 1 of the present invention includes a resonator into which laser light emitted from a laser light source is incident, a first laser light that has passed through the resonator, and a second laser light that has not passed through the resonator, and a first photodetector that measures first interference light in which the first laser light and the second laser light interfere with each other, and a signal generation unit that generates an error signal based on the output of the first photodetector.

[0008] The error signal generation device according to Aspect 2 of the present invention, in the above Aspect 1, further includes a second photodetector that measures second interference light in which the first laser light and the second laser light interfere with each other with a phase difference different from that of the first interference light, and the signal generation unit may be configured to generate the error signal based on the difference between the output of the first photodetector and the output of the second photodetector.

[0009] The error signal generation device according to Aspect 3 of the present invention, in the above Aspect 2, may be configured to include a beam splitter into which the first laser light and the second laser light are incident and that emits the first interference light and the second interference light.

[0010] The error signal generation device according to Aspect 4 of the present invention, in the above Aspect 1, further includes a second photodetector that measures second interference light in which a third laser light that has passed through the resonator in a direction opposite to that of the first laser light and a fourth laser light that has not passed through the resonator interfere with each other, and the signal generation unit may be configured to generate the error signal based on the difference between the output of the first photodetector and the output of the second photodetector.

[0011] An error signal generation device according to embodiment 5 of the present invention may be configured such that, in embodiment 4 above, the laser light emitted from the laser light source is divided into a fifth laser beam and a sixth laser beam, and of the fifth laser beam incident on the resonator from the first side, the portion that passes through the resonator is the first laser beam, and the portion that does not pass through the resonator is the fourth laser beam, and of the sixth laser beam incident on the resonator from a second side different from the first side, the portion that passes through the resonator is the third laser beam, and the portion that does not pass through the resonator is the second laser beam.

[0012] In the error signal generation device according to embodiment 6 of the present invention, the resonator may be configured to be a ring resonator in any of embodiments 1 to 5 described above.

[0013] In the error signal generation device according to embodiment 7 of the present invention, in any of embodiments 1 to 5 described above, the resonator may be configured to have two gratings provided in an optical fiber or optical waveguide.

[0014] An error signal generation device according to embodiment 8 of the present invention may, in embodiment 7 above, include an optical circulator connected to the resonator, wherein the optical circulator is configured to emit light from the laser light source side to the resonator and emit light from the resonator to the first photodetector side.

[0015] An error signal generating device according to embodiment 9 of the present invention may be configured in embodiment 1 above to include a polarizing beam splitter, a first wave plate which is a quarter wave plate disposed between the polarizing beam splitter and the first side of the resonator, and a second wave plate which is a quarter wave plate disposed between the polarizing beam splitter and a second side of the resonator different from the first side.

[0016] An error signal generation device according to embodiment 10 of the present invention, in embodiment 9 above, the polarization beam splitter divides the laser light from the laser light source into a first polarization laser light and a second polarization laser light having different polarization directions, the first polarization laser light is the portion of the first polarization laser light that passes through the resonator, the second polarization laser light is the portion of the second polarization laser light that is reflected by the resonator, the third polarization laser light is the portion of the second polarization laser light that passes through the resonator, and the fourth polarization laser light is the portion of the first polarization laser light that is reflected by the resonator, and the device includes a second photodetector that measures a second interference light obtained when the third polarization laser light that has passed through the resonator in the opposite direction to the first polarization laser light and the fourth polarization laser light that has not passed through the resonator interfere with each other, and the signal generation unit may be configured to generate the error signal based on the difference between the output of the first photodetector and the output of the second photodetector.

[0017] An error signal generation device according to embodiment 11 of the present invention may be configured to include a third wave plate, which is a quarter-wave plate that emits the laser light from the laser light source to the polarizing beam splitter in an elliptical polarized state, as described in embodiment 9 or 10 above.

[0018] An error signal generation device according to embodiment 12 of the present invention, in embodiment 9 above, comprises a second photodetector that measures a second interference light obtained when the first laser light and the second laser light interfere with each other with a phase difference different from that of the first interference light, and a third wave plate which is a quarter-wave plate into which the first laser light and the second laser light are incident, disposed between the polarizing beam splitter and the first and second photodetectors, wherein the signal generation unit may be configured to generate the error signal based on the difference between the output of the first photodetector and the output of the second photodetector.

[0019] An error signal generation method according to embodiment 13 of the present invention is a method comprising: generating a first laser beam that has passed through a resonator and a second laser beam that has not passed through a resonator from laser light emitted from a laser light source using a resonator; measuring a first interference light obtained when the first laser beam and the second laser beam interfere with each other; and generating a signal based on the intensity of the first interference light. [Effects of the Invention]

[0020] According to one aspect of the present invention, an appropriate error signal can be obtained for stabilizing the oscillation frequency of a laser light source. [Brief explanation of the drawing]

[0021] [Figure 1] This figure shows the configuration of a laser oscillation frequency control device according to one embodiment of the present invention. [Figure 2] This diagram shows the error signal generation flow in an error signal generation device. [Figure 3] This is a schematic diagram showing transmitted and reflected light through a resonator. [Figure 4] This figure shows the error signal S obtained by the error signal generation device. [Figure 5] This figure shows the error signal S obtained by the error signal generator over a wider frequency range. [Figure 6] This figure shows the error signal S obtained by the error signal generator when Δ = ±π / 3. [Figure 7] This figure shows the error signal SAS obtained by an error signal generator when the resonator is asymmetrical. [Figure 8] This figure shows the configuration of a laser oscillation frequency control device according to one embodiment of the present invention. [Figure 9] This diagram shows the configuration of an error signal generation device according to one embodiment of the present invention. [Figure 10] This diagram shows the configuration of an error signal generation device according to one embodiment of the present invention. [Figure 11]This diagram shows the configuration of an error signal generation device according to one embodiment of the present invention. [Figure 12] This diagram shows the configuration of an error signal generation device according to one embodiment of the present invention. [Modes for carrying out the invention]

[0022] [Embodiment 1] Figure 1 shows the configuration of the laser oscillation frequency control device 1 according to this embodiment. The laser oscillation frequency control device 1 comprises a laser light source 2, an error signal generation device 3, a frequency control unit 4, and an adjustment element 5. The laser light source 2 may be any laser light source. A portion of the laser light emitted from the laser light source 2 is incident on the error signal generation device 3 to stabilize the oscillation frequency, and the remaining laser light (not shown) is used for its original purpose.

[0023] The error signal generator 3 is a circuit that generates an error signal based on the laser light emitted from the laser light source 2. The error signal represents the deviation of the oscillation frequency of the laser light source 2 from the reference frequency (target frequency). The error signal generator 3 outputs the error signal to the frequency control unit 4.

[0024] The frequency control unit 4 is a circuit that controls the adjustment element 5 based on the error signal. The adjustment element 5 may be an element that adjusts the oscillation frequency of the laser light source 2 by adjusting the resonator length of the laser light source 2. The adjustment element 5 may be, for example, a piezoelectric element that displaces the mirror of the laser light source 2. However, the adjustment element 5 may also be an element that adjusts the oscillation frequency of the laser light source 2 by adjusting the current of the laser light source 2, etc.

[0025] (Configuration of Error Signal Generator 3) The error signal generation device 3 comprises a first polarizing beam splitter 11, a half-wave plate 12, a first wave plate 13, a second polarizing beam splitter 14, a second wave plate 15, a resonator 16, a third wave plate 17, a third polarizing beam splitter 18, a first photodetector 21, a second photodetector 22, and a signal generation unit 23. In the figure, some mirrors are omitted from the illustration.

[0026] The first polarizing beam splitter (PBS) 11 transmits only light whose polarization direction is in a predetermined direction.

[0027] The half-wave plate (HWP) 12 changes the polarization direction of linearly polarized light. The half-wave plate 12 is positioned closer to the laser light source 2 than the first wave plate 13.

[0028] The first wave plate 13 is a quarter wave plate (QWP). The first wave plate 13 converts linearly polarized light into circularly polarized light. The first wave plate 13 is positioned on the laser light source 2 side of the second polarization beam splitter 14.

[0029] The second polarizing beam splitter 14 transmits light with a polarization direction in the first direction and reflects light with a polarization direction perpendicular to the first direction in the second direction. The second polarizing beam splitter 14 splits the laser light from the laser light source 2 incident on the second polarizing beam splitter 14 into first polarized laser light and second polarized laser light, which have different polarization directions from each other.

[0030] The second wave plate 15 and the third wave plate 17 are quarter-wave plates. The second wave plate 15 and the third wave plate 17 convert incident linearly polarized light into circularly polarized light. Also, the second wave plate 15 and the third wave plate 17 convert incident circularly polarized light into linearly polarized light. The lagging axis of the second wave plate 15 is tilted 90 degrees with respect to the lagging axis of the third wave plate 17.

[0031] The resonator 16 is a resonator that can obtain both transmitted and reflected light. Here, the resonator 16 is a Fabry-Perot resonator (FP Cavity). The resonator length of the resonator 16 is the length over which the reference frequency laser light resonates. The resonator 16 is located between the second wave plate 15 and the third wave plate 17.

[0032] The second waveplate 15 is positioned in the laser beam path between the second polarizing beam splitter 14 and the first side of the resonator 16. The third waveplate 17 is positioned in the laser beam path between the second polarizing beam splitter 14 and the second side of the resonator 16. The second side is different (opposite) to the first side.

[0033] The third polarizing beam splitter 18 reflects light with a polarization direction in the first direction and transmits light with a polarization direction in the second direction. The third polarizing beam splitter 18 is positioned in the path through which the laser light emitted from the first side of the resonator 16 passes through the second polarizing beam splitter 14.

[0034] The first photodetector 21 and the second photodetector 22 measure the intensity of incident light. The first photodetector 21 measures the intensity of light with a polarization direction of the first direction that has been reflected by the third polarizing beam splitter 18. The second photodetector 22 measures the intensity of light with a polarization direction of the second direction that has been transmitted through the third polarizing beam splitter 18. The first photodetector 21 and the second photodetector 22 output intensity signals indicating the measured light intensity to the signal generation unit 23.

[0035] The signal generation unit 23 is a circuit that generates an error signal based on the input intensity signal. Here, the signal generation unit 23 generates the error signal based on the outputs of the first photodetector 21 and the second photodetector 22. For example, the signal generation unit 23 generates the error signal based on the difference between the output of the first photodetector 21 and the output of the second photodetector 22. For example, the signal generation unit 23 may use the result of subtracting the output of the second photodetector 22 from the output of the first photodetector 21 as the error signal. The signal generation unit 23 outputs the error signal to the frequency control unit 4.

[0036] (Operation of error signal generator 3) Figure 2 shows the error signal generation flow in the error signal generation device 3.

[0037] The laser light emitted from the laser light source 2 is incident on the first polarization beam splitter 11. The laser light that has passed through the first polarization beam splitter 11 is incident on the half-wave plate 12. The half-wave plate 12 is provided to adjust the polarization direction of the laser light. The linearly polarized laser light that has passed through the half-wave plate 12 is incident on the first wave plate 13. The circularly polarized laser light that has passed through the first wave plate 13 is incident on the second polarization beam splitter 14.

[0038] The second polarization beam splitter 14 separates the laser light from the laser light source 2 into first polarization laser light (V polarization) and second polarization laser light (H polarization) (S1). Of the laser light from the laser light source 2 incident on the second polarization beam splitter 14, the component with a polarization direction perpendicular to the laser light (V polarization) is reflected by the second polarization beam splitter 14. The reflected first polarization laser light (V polarization) is incident on the second wave plate 15. Of the laser light from the laser light source 2 incident on the second polarization beam splitter 14, the component with a polarization direction horizontal to the laser light (H polarization) is transmitted through the second polarization beam splitter 14. The transmitted second polarization laser light (H polarization) is incident on the third wave plate 17. The phase difference between the first polarization laser light and the second polarization laser light is φ. Since the laser light incident on the second polarization beam splitter 14 from the laser light source 2 is circularly polarized, the phase difference φ is π / 2.

[0039] The first polarized laser light (V-polarized) is incident on the second wave plate 15 and becomes circularly polarized. The first polarized laser light (circularly polarized) that has passed through the second wave plate 15 is incident on the resonator 16 from the first side. Of the first polarized laser light (circularly polarized), a portion passes through the resonator 16 and is emitted from the second side, and the remainder is reflected by the resonator 16 and emitted from the first side. If the frequency of the first polarized laser light is the resonant frequency of the resonator 16, then almost all of it passes through the resonator 16. If the frequency of the first polarized laser light deviates from the reference frequency, the proportion that passes through the resonator 16 decreases, and the proportion that is reflected by the resonator 16 increases.

[0040] Of the first polarized laser light (circularly polarized), the first laser light (circularly polarized) that passes through the resonator 16 is converted to linearly polarized (V-polarized) by the third wave plate 17. The first laser light (V-polarized) that passes through the third wave plate 17 is reflected by the second polarized beam splitter 14 and heads towards the third polarized beam splitter 18.

[0041] Of the first polarized laser light (circularly polarized), the fourth laser light (circularly polarized) reflected by the resonator 16 is converted by the second waveplate 15 into linearly polarized light (H-polarized), which has a different polarization direction from the original linearly polarized light. The fourth laser light (H-polarized) that has passed through the second waveplate 15 passes through the second polarized beam splitter 14 and heads towards the third polarized beam splitter 18.

[0042] The second polarized laser light (H-polarized) is incident on the third wave plate 17 and becomes circularly polarized. The second polarized laser light (circularly polarized) that has passed through the third wave plate 17 is incident on the resonator 16 from the second side. Of the second polarized laser light (circularly polarized), a portion passes through the resonator 16 and is emitted from the first side, and the remainder is reflected by the resonator 16 and emitted from the second side. If the frequency of the second polarized laser light is the resonant frequency of the resonator 16, then almost all of it passes through the resonator 16. If the frequency of the second polarized laser light deviates from the reference frequency, the proportion that passes through the resonator 16 decreases, and the proportion that is reflected by the resonator 16 increases.

[0043] Of the second polarized laser light (circularly polarized), the third laser light (circularly polarized) that passes through the resonator 16 is converted to linearly polarized (H-polarized) by the second waveplate 15. The third laser light passes through the resonator 16 in the opposite direction to the first laser light. The third laser light (H-polarized) that passes through the second waveplate 15 passes through the second polarized beam splitter 14 and heads towards the third polarized beam splitter 18.

[0044] Of the second polarized laser light (circularly polarized), the second laser light (circularly polarized) reflected by the resonator 16 is converted by the third wave plate 17 into linearly polarized light (V-polarized), which has a different polarization direction from the original linearly polarized light. The second laser light (V-polarized) that has passed through the third wave plate 17 is reflected by the second polarizing beam splitter 14 and heads towards the third polarizing beam splitter 18.

[0045] In other words, the resonator 16 generates first and third laser beams that have passed through the resonator 16, and second and fourth laser beams that have not passed through the resonator, from the laser light emitted from the laser light source 2 (S2).

[0046] The first laser beam (V-polarized) and the second laser beam (V-polarized) are reflected by the third polarization beam splitter 18 and incident on the first photodetector 21. The first photodetector 21 measures the intensity of the first interference light, which is formed when the first laser beam and the second laser beam interfere with each other (S3). The first interference light is formed when the first laser beam that has passed through the resonator 16 from the first side and the second laser beam that has been reflected on the second side without passing through the resonator 16 interfere with each other.

[0047] The third laser beam (H-polarized) and the fourth laser beam (H-polarized) pass through the third polarizing beam splitter 18 and are incident on the second photodetector 22. The second photodetector 22 measures the intensity of the second interference light, which is formed when the third and fourth laser beams interfere with each other (S4). The second interference light is formed when the third laser beam, which has passed through the resonator 16 from the second side in the opposite direction to the first laser beam, and the fourth laser beam, which has been reflected on the first side without passing through the resonator 16, interfere with each other.

[0048] The signal generation unit 23 generates an error signal based on the intensity of the first interference light and the intensity of the second interference light (S5). Specifically, the signal generation unit 23 uses the signal obtained by subtracting the intensity signal output by the second photodetector 22 from the intensity signal output by the first photodetector 21 as the error signal. The error signal represents the difference between the intensity of the first interference light and the intensity of the second interference light.

[0049] (error signal) The error signal obtained by the error signal generator 3 will now be described.

[0050] Figure 3 is a schematic diagram showing the transmitted and reflected light from the resonator 16. The arrows indicate the direction of laser light propagation. M1 and M2 represent the first and second side mirrors of the resonator 16, respectively. E1 is the complex representation of the amplitude of the laser light incident on the resonator 16 from the first side. E2 is the complex representation of the amplitude of the laser light incident on the resonator 16 from the second side. e1 is the complex representation of the amplitude of the laser light emitted from the first side of the resonator 16. e2 is the complex representation of the amplitude of the laser light emitted from the second side of the resonator 16. The transfer matrix U of the resonator 16 is expressed as follows.

[0051]

number

[0052] E1, E2, e1, and e2 are expressed as follows:

[0053]

number

[0054]

Number

[0055] When the resonator 16 is a symmetric resonator, \(r1 = r2 = r\) and \(t1 = t2 = t\) hold, and \(u\) 11 * \(u\) 12 becomes a pure imaginary number. At this time, \(S\) is expressed as follows.

[0056]

Number

[0057] Figure 4 shows the error signal S obtained by the error signal generator 3. The vertical axis shows the value of the normalized error signal. The horizontal axis shows the frequency. The horizontal axis is drawn so that θ / π is 0 at the reference frequency. In this graph, adjacent resonant frequencies are 2π away in terms of θ. An ideal case is assumed, and Δ=0. The parameters of mirrors M1 and M2 are r1=r2=0.999 and t1=t2=√(1-r1 2 ) was used. S PDH This shows the error signal by the PDH method, S MZI This shows the error signal obtained by the MZI method. In the PDH method, the modulation index β = 1 / 2 and the phase shift θp corresponding to the sideband frequency is π / 10. 2 The dashed line represents the transmission spectrum in resonator 16. The transmission spectrum peaks at the resonant frequency.

[0058] In the error signal generation device 3 of this embodiment, an error signal S with a peak approximately twice as large as that obtained by the MZI method and the PDH method is obtained. The error signal S is symmetrical with respect to the resonant frequency (reference frequency) and changes sharply (has a large slope) near the resonant frequency. This means that an error signal sensitive to deviations from the reference frequency is obtained. PDH The signal strength inevitably decreases with modulation. MZI Therefore, since half of the total intensity escapes as transmitted light in the resonator, the sensitivity is halved. The error signal generator 3 measures the interference light resulting from the interference of transmitted and reflected light. As a result, the error signal generator 3 can obtain an error signal S with higher sensitivity compared to the conventional method.

[0059] Figure 5 shows the error signal S obtained by the error signal generator 3 over a wider frequency range. The vertical and horizontal axes are the same as in Figure 4. However, the parameters of mirrors M1 and M2 were set to r1=r2=0.9. The phase period of the reflected light is the same as the interval of the resonant frequencies (FSR). In contrast, the phase period of the transmitted light is twice the FSR. Therefore, the period of the error signal S obtained by the error signal generator 3 is twice the FSR. Thus, the error signal S obtained by the error signal generator 3 is the same as the error signal S obtained by the MZI method. MZI It has twice the period compared to the previous one. In the error signal S, MZI Compared to that, the range in which the value is positive (or negative) around the reference frequency is twice as long. For example, the frequency control unit 4 adjusts to increase the oscillation frequency when the error signal S is positive. The frequency control unit 4 can use the error signal S to control the error signal S MZI Compared to that, it can adjust for a wider range of frequency deviations to return to the reference frequency.

[0060] Also, error signal S MZI It crosses zero at θ / π = -1. In this vicinity, the error signal S MZI As the value of becomes smaller, the error signal S MZI Using this method means that the oscillation frequency cannot be stabilized.

[0061] On the other hand, the error signal S takes large values ​​across all frequency ranges, crosses zero only at periodically occurring resonant frequencies, and has a large slope near the zero crossing. Therefore, the frequency control unit 4 can adjust the oscillation frequency toward θ / π=0 when the frequency deviation is between -2 and 2 using the error signal S. Furthermore, the frequency control unit 4 can adjust the oscillation frequency toward θ / π=2 when the frequency deviation is between 2 and 6 using the error signal S. In other words, by using the error signal S, the oscillation frequency can be locked to one of the resonant frequencies even when the frequency deviation is very large. The laser oscillation frequency control device 1 has a wider capture range than the FSR. This is useful in applications where it is sufficient for the oscillation frequency to stabilize at one of the resonant frequencies.

[0062] Figure 6 shows the error signal S obtained by the error signal generator 3 when Δ = ±π / 3. The parameters, vertical axis, and horizontal axis are the same as in Figure 4. Even when Δ deviates from 0, the error signal S obtained by the error signal generator 3 shows a decrease in peak height, but crosses zero at the resonant frequency and maintains symmetrical characteristics with respect to the resonant frequency.

[0063] On the other hand, the error signal S obtained by the MZI method MZI The level changes significantly and becomes asymmetrical with respect to the resonant frequency. This means that even a slight shift in frequency will prevent the oscillation frequency from being locked.

[0064] u included in error signal S 11 and u 12 These shift in opposite directions due to the misalignment of the resonator 16. Therefore, the error signal generator 3 has robust characteristics against optical system misalignment.

[0065] Figure 7 shows the error signal S obtained by the error signal generator 3 when the resonator 16 is asymmetrical. AS This figure shows the same configuration as in Figure 4. However, the parameters of the mirrors M1 and M2 were set to r1=0.93 and r2=0.87, and Δ=π / 3.

[0066] When the resonator 16 is asymmetric, the error signal S obtained by the MZI method MZI,AS This can result in the signal not crossing zero near the resonant frequency.

[0067] On the other hand, the error signal S obtained by the error signal generation device 3 AS Even if the resonator 16 is asymmetric, it maintains the characteristic of zero-crossing near the resonant frequency and having a period twice that of the FSR. In particular, the error signal S AS The sign of the signal changes almost symmetrically with respect to the resonant frequency. Therefore, the error signal generator 3 has robust characteristics even with respect to the non-uniformity of the resonator 16.

[0068] Thus, according to the error signal generation device 3 of this embodiment, a more appropriate error signal S can be obtained for stabilizing the oscillation frequency of the laser light source 2.

[0069] [Embodiment 2] Other embodiments of the present invention are described below. For the sake of clarity, components having the same function as those described in the above embodiments will be denoted by the same reference numerals, and their descriptions will not be repeated.

[0070] Figure 8 shows the configuration of the laser oscillation frequency control device 1a according to this embodiment. The laser oscillation frequency control device 1a comprises a laser light source 2, an error signal generation device 3a, a frequency control unit 4, and an adjustment element 5.

[0071] In the error signal generation device 3a, the arrangement of the half-wave plate 12 and the first wave plate 13 differs from that of the error signal generation device 3 in Embodiment 1. The first wave plate 13 is positioned between the second polarizing beam splitter 14 and the first photodetector 21 and the second photodetector 22. Specifically, the first wave plate 13 is positioned between the second polarizing beam splitter 14 and the third polarizing beam splitter 18.

[0072] The half-wave plate 12 is positioned between the first wave plate 13 and the third polarizing beam splitter 18.

[0073] In the error signal generation device 3a, linearly polarized (V-polarized or H-polarized) laser light from the laser light source 2 is incident on the second polarization beam splitter 14. The laser light from the laser light source 2 (e.g., V-polarized) incident on the second polarization beam splitter 14 is reflected by the second polarization beam splitter 14. The reflected laser light (V-polarized) is incident on the second waveplate 15.

[0074] The laser light (V-polarized) is incident on the second wave plate 15 and becomes circularly polarized. The laser light (circularly polarized) that has passed through the second wave plate 15 is incident on the resonator 16 from the first side. Of the laser light (circularly polarized), a portion passes through the resonator 16 and is emitted from the second side, while the remainder is reflected by the resonator 16 and emitted from the first side.

[0075] Of the circularly polarized laser light, the first laser light (circularly polarized) that passes through the resonator 16 is converted to linearly polarized (V-polarized) light by the third wave plate 17. The first laser light (V-polarized) that passes through the third wave plate 17 is reflected by the second polarizing beam splitter 14 and incident on the first wave plate 13.

[0076] Of the circularly polarized laser light, the second laser light (circularly polarized) reflected by the resonator 16 is converted by the second waveplate 15 into linearly polarized light (H-polarized), which has a different polarization direction from the original linearly polarized light. The second laser light (H-polarized) that has passed through the second waveplate 15 passes through the second polarization beam splitter 14 and is incident on the first waveplate 13.

[0077] The first laser beam (V-polarized) incident on the first wave plate 13 becomes circularly polarized in the first rotation direction. The second laser beam (H-polarized) incident on the first wave plate 13 becomes circularly polarized in the second rotation direction, which is opposite to the first rotation direction.

[0078] The first laser beam (first circular polarization) and the second laser beam (second circular polarization) that have passed through the half-wave plate 12 are incident on the third polarizing beam splitter 18. Of the first laser beam (first circular polarization) and the second laser beam (second circular polarization), the V component is reflected by the third polarizing beam splitter 18 and incident on the first photodetector 21. The first photodetector 21 measures the intensity of the first interference light, which is the result of the V component of the first laser beam and the V component of the second laser beam interfering with each other. The first interference light is the result of the interference between the first laser beam that has passed through the resonator 16 from the first side and the second laser beam that has been reflected on the first side without passing through the resonator 16.

[0079] Of the first laser beam (first circular polarization) and the second laser beam (second circular polarization), the H component passes through the third polarization beam splitter 18 and is incident on the second photodetector 22. The second photodetector 22 measures the intensity of the second interference light, which is formed when the H component of the first laser beam and the H component of the second laser beam interfere with each other. The second interference light is formed when the first laser beam that has passed through the resonator 16 from the first side and the second laser beam that has been reflected on the first side without passing through the resonator 16 interfere with each other.

[0080] The second interference light is the light obtained when the first laser beam and the second laser beam interfere with each other with a different phase difference than the first interference light. The first laser beam (first circular polarization) and the second laser beam (second circular polarization) are circularly polarized in opposite directions. In the first interference light, the phase of the second laser beam relative to the first laser beam is π / 2, and in the second interference light, the phase of the second laser beam relative to the first laser beam is -π / 2.

[0081] The signal generation unit 23 obtains an error signal by subtracting the intensity signal output by the second photodetector 22 from the intensity signal output by the first photodetector 21. The error signal generation device 3a can also obtain an error signal having the same characteristics as the error signal S obtained by the error signal generation device 3 of Embodiment 1.

[0082] [Embodiment 3] Other embodiments of the present invention are described below. For the sake of clarity, components having the same function as those described in the above embodiments will be denoted by the same reference numerals, and their descriptions will not be repeated.

[0083] Figure 9 shows the configuration of the error signal generation device 3b of this embodiment. The error signal generation device 3b comprises a beam splitter 19, a first optical waveguide 24, a second optical waveguide 25, a resonator 16b, a first photodetector 21, a second photodetector 22, and a signal generation unit 23. The error signal generation device 3b is formed using an optical circuit board on which optical waveguides are formed. The optical elements are formed within the optical circuit board. The wiring connecting the first photodetector 21 and the second photodetector 22 to the signal generation unit 23 is formed in a layer different from the layer on which the optical waveguides are formed.

[0084] The beam splitter 19 (BS) splits the laser light (linearly polarized) from the incident laser light source into a fifth laser beam and a sixth laser beam.

[0085] The first optical waveguide 24 is an optical waveguide formed on the optical circuit board. The first optical waveguide 24 connects the beam splitter 19 and the first photodetector 21.

[0086] The second optical waveguide 25 is an optical waveguide formed on the optical circuit board. The second optical waveguide 25 connects the beam splitter 19 and the second photodetector 22.

[0087] The resonator 16b (Cavity) is a ring resonator. The resonator 16b is coupled to the first optical waveguide 24 on the first side and to the second optical waveguide 25 on the second side, which is different from the first side.

[0088] Laser light (linearly polarized) is incident from the laser light source onto the beam splitter 19. The fifth laser beam emitted from the beam splitter 19 travels through the first optical waveguide 24. The fifth laser beam is incident on the first side of the resonator 16b.

[0089] A portion of the fifth laser beam passes through the resonator 16b and enters the second optical waveguide 25. The portion of the fifth laser beam that passes through the resonator 16b is called the first laser beam. The first laser beam that enters the second optical waveguide 25 is incident on the second photodetector 22. Here, the laser beam that passes through the ring resonator refers to the laser beam that is emitted from one optical waveguide through the ring resonator and into another optical waveguide.

[0090] Of the fifth laser beam, the remainder does not pass through the resonator 16b and instead passes through the first optical waveguide 24 as the fourth laser beam, entering the first photodetector 21. If the frequency of the fifth laser beam is the resonant frequency of the resonator 16b, then almost all of it passes through the resonator 16b. If the frequency of the fifth laser beam deviates from the reference frequency, the proportion that passes through the resonator 16b decreases, and the proportion that does not pass through the resonator 16b increases. Laser beam that does not pass through the ring resonator refers to laser beam that travels through an optical waveguide coupled to the ring resonator, rather than traveling through the ring resonator to another optical waveguide. Laser beam that does not pass through the ring resonator can also be described as laser beam reflected from the ring resonator.

[0091] The sixth laser beam emitted from the beam splitter 19 travels through the second optical waveguide 25. The sixth laser beam is incident at the coupling between the resonator 16b and the second optical waveguide 25. A portion of the sixth laser beam passes through the resonator 16b and enters the first optical waveguide 24. The portion of the sixth laser beam that passes through the resonator 16b is designated as the third laser beam. The third laser beam passes through the resonator 16b in the opposite direction to the first laser beam. The third laser beam that enters the first optical waveguide 24 is incident on the first photodetector 21. The remainder of the sixth laser beam does not pass through the resonator 16b and continues through the second optical waveguide 25 as the second laser beam, incident on the second photodetector 22.

[0092] The second photodetector 22 measures the intensity of the first interference light, which is formed when the first laser beam and the second laser beam interfere with each other. The first interference light is formed when the first laser beam that has passed through the resonator 16b from the first side and the second laser beam that has not passed through the resonator 16b interfere with each other.

[0093] The first photodetector 21 measures the intensity of the second interference light, which is produced when the third laser beam and the fourth laser beam interfere with each other. The second interference light is produced when the third laser beam that has passed through the resonator 16b from the second side and the fourth laser beam that has not passed through the resonator 16b interfere with each other.

[0094] The second interference light is the light obtained when the transmitted light that has passed through the resonator 16b and the untransmitted light that has not passed through the resonator interfere with each other with a different phase difference than the first interference light. For example, by adjusting the optical path length in the first optical waveguide 24, the phase difference φ between the transmitted and untransmitted light can be arbitrarily adjusted (for example, to π / 2).

[0095] The signal generation unit 23 obtains an error signal by subtracting the intensity signal output by the second photodetector 22 from the intensity signal output by the first photodetector 21. The error signal generation device 3b can also obtain an error signal having the same characteristics as the error signal S obtained by the error signal generation device 3 of Embodiment 1.

[0096] [Embodiment 4] Other embodiments of the present invention are described below. For the sake of clarity, components having the same function as those described in the above embodiments will be denoted by the same reference numerals, and their descriptions will not be repeated.

[0097] Figure 10 shows the configuration of the error signal generation device 3c of this embodiment. The error signal generation device 3c comprises a beam splitter 19b, a first optical waveguide 24, a second optical waveguide 25, a third optical waveguide 26, a fourth optical waveguide 27, a resonator 16b, a first photodetector 21, a second photodetector 22, and a signal generation unit 23. In the error signal generation device 3c, the optical waveguides and wiring do not intersect in a plan view. Therefore, the wiring connecting the first photodetector 21 and the second photodetector 22 to the signal generation unit 23 can be formed in the same layer as the layer in which the optical waveguides are formed.

[0098] The first optical waveguide 24 is connected to the input port of the beam splitter 19b. The second optical waveguide 25 is connected to the input port of the beam splitter 19b. The third optical waveguide 26 connects the output port of the beam splitter 19b to the first photodetector 21. The fourth optical waveguide 27 connects the output port of the beam splitter 19b to the second photodetector 22.

[0099] The resonator 16b (Cavity) is a ring resonator. The resonator 16b is coupled to the first optical waveguide 24 on the first side and to the second optical waveguide 25 on the second side, which is different from the first side. The resonator 16b is located on the laser light source side of the beam splitter 19b.

[0100] The beam splitter 19b is a coupler having two input ports and two output ports. The beam splitter 19b splits the laser light input from the first optical waveguide 24 and outputs the split laser light to the third optical waveguide 26 and the fourth optical waveguide 27, respectively. The beam splitter 19b also splits the laser light input from the second optical waveguide 25 and outputs the split laser light to the third optical waveguide 26 and the fourth optical waveguide 27, respectively. The splitting ratio is 50:50.

[0101] Laser light (linearly polarized) is incident from the laser light source into the first optical waveguide 24. The laser light is incident at the coupling between the first optical waveguide 24 and the resonator 16b. A portion of the laser light from the laser light source passes through the resonator 16b and enters the second optical waveguide 25. The laser light from the laser light source that has passed through the resonator 16b is called the first laser light. The first laser light that has entered the second optical waveguide 25 is incident on the beam splitter 19b. The remaining laser light from the laser light source does not pass through the resonator 16b, but passes through the first optical waveguide 24 and is incident on the beam splitter 19b as the second laser light.

[0102] A portion of the first laser beam incident on the beam splitter 19b is emitted into the third optical waveguide 26. The remaining portion of the first laser beam incident on the beam splitter 19b is emitted into the fourth optical waveguide 27. A portion of the second laser beam incident on the beam splitter 19b is emitted into the third optical waveguide 26. The remaining portion of the second laser beam incident on the beam splitter 19b is emitted into the fourth optical waveguide 27.

[0103] Specifically, the beam splitter 19b emits first interference light, which is the result of the interference of the first laser beam and the second laser beam, into the third optical waveguide 26. The beam splitter 19b also emits second interference light, which is the result of the interference of the first laser beam and the second laser beam, into the fourth optical waveguide 27. The first interference light incident on the third optical waveguide 26 is incident on the first photodetector 21. The second interference light incident on the fourth optical waveguide 27 is incident on the second photodetector 22.

[0104] The first photodetector 21 measures the intensity of the first interference light. The first interference light is the result of interference between the first laser light that passed through the resonator 16b from the first side and the second laser light that did not pass through the resonator 16b.

[0105] The second photodetector 22 measures the intensity of the second interference light. The second interference light is the result of the interference between the first laser light that passed through the resonator 16b from the first side and the second laser light that did not pass through the resonator 16b.

[0106] The phase difference φ between the transmitted and untransmitted light can be arbitrarily set by the beam splitter 19b used. For example, it can be set to φ = π / 2. Therefore, the second interference light is the result of the interference of the first laser beam and the second laser beam with a different phase difference φ than the first interference light.

[0107] The signal generation unit 23 obtains an error signal by subtracting the intensity signal output by the second photodetector 22 from the intensity signal output by the first photodetector 21. The error signal generation device 3c can also obtain an error signal having the same characteristics as the error signal S obtained by the error signal generation device 3 of Embodiment 1.

[0108] [Embodiment 5] Other embodiments of the present invention are described below. For the sake of clarity, components having the same function as those described in the above embodiments will be denoted by the same reference numerals, and their descriptions will not be repeated.

[0109] Figure 11 shows the configuration of the error signal generation device 3d of this embodiment. The error signal generation device 3d comprises a beam splitter 19, a first optical waveguide 24, a second optical waveguide 25, a third optical waveguide 26, a fourth optical waveguide 27, a fifth optical waveguide 28, a first circulator 31, a second circulator 32, a resonator 16d, a first photodetector 21, a second photodetector 22, and a signal generation unit 23.

[0110] The beam splitter 19 (BS) splits the laser light (linearly polarized) from the incident laser light source into a fifth laser beam and a sixth laser beam.

[0111] The first optical waveguide 24 connects the beam splitter 19 to the first circulator 31. The second optical waveguide 25 connects the beam splitter 19 to the second circulator 32. The third optical waveguide 26 connects the first circulator 31 to the first photodetector 21. The fourth optical waveguide 27 connects the second circulator 32 to the second photodetector 22. The fifth optical waveguide 28 connects the first circulator 31 to the second circulator 32.

[0112] The resonator 16d is located in the middle of the fifth optical waveguide 28. The resonator 16d has two gratings (FBG) spaced apart from each other. The resonator length of the resonator 16d is the length over which the reference frequency laser light resonates.

[0113] The first circulator 31 emits laser light incident from the first optical waveguide 24 to one end of the fifth optical waveguide 28. The first circulator 31 emits laser light incident from the fifth optical waveguide 28 to the third optical waveguide 26.

[0114] The second circulator 32 emits the laser light incident from the second optical waveguide 25 to the other end of the fifth optical waveguide 28. The second circulator 32 emits the laser light incident from the fifth optical waveguide 28 to the fourth optical waveguide 27.

[0115] Laser light (linearly polarized) is incident from the laser light source onto the beam splitter 19. The fifth laser beam emitted from the beam splitter 19 travels through the first optical waveguide 24. The fifth laser beam is introduced into the fifth optical waveguide 28 by the first circulator 31 and incident on the first side of the resonator 16d.

[0116] A portion of the fifth laser beam passes through the resonator 16d and enters the second circulator 32. The portion of the fifth laser beam that passes through the resonator 16d is called the first laser beam. The first laser beam that enters the second circulator 32 is introduced into the fourth optical waveguide 27 and enters the second photodetector 22.

[0117] Of the fifth laser beam, the remainder is reflected by the resonator 16d and incident on the first circulator 31. The portion of the fifth laser beam reflected by the resonator 16d is designated as the fourth laser beam. The fourth laser beam is introduced into the third optical waveguide 26 by the first circulator 31 and incident on the first photodetector 21. If the frequency of the fifth laser beam is the resonant frequency of the resonator 16d, almost all of it passes through the resonator 16d. If the frequency of the fifth laser beam deviates from the reference frequency, the proportion that passes through the resonator 16d decreases, and the proportion that does not pass through the resonator 16d increases.

[0118] The sixth laser beam emitted from the beam splitter 19 travels through the second optical waveguide 25. The sixth laser beam is introduced into the fifth optical waveguide 28 by the second circulator 32 and incident on the second side of the resonator 16d.

[0119] A portion of the sixth laser beam passes through the resonator 16d and enters the first circulator 31. The portion of the sixth laser beam that passes through the resonator 16d is designated as the third laser beam. The third laser beam that enters the first circulator 31 is introduced into the third optical waveguide 26 and enters the first photodetector 21.

[0120] Of the sixth laser beam, the remainder is reflected by the resonator 16d and incident on the second circulator 32. The portion of the sixth laser beam reflected by the resonator 16d is called the second laser beam. The second laser beam is introduced into the fourth optical waveguide 27 by the second circulator 32 and incident on the second photodetector 22.

[0121] The second photodetector 22 measures the intensity of the first interference light, which is formed when the first laser beam and the second laser beam interfere with each other. The first interference light is formed when the first laser beam transmitted through the resonator 16d from the first side and the second laser beam reflected from the second side of the resonator 16d interfere with each other.

[0122] The first photodetector 21 measures the intensity of the second interference light, which is produced when the third laser beam and the fourth laser beam interfere with each other. The second interference light is produced when the third laser beam transmitted through the second side of the resonator 16d and the fourth laser beam reflected from the first side of the resonator 16b interfere with each other.

[0123] The second interference light is the light obtained when the transmitted light that has passed through the resonator 16d and the untransmitted light that has not passed through it interfere with each other with a different phase difference than the first interference light. For example, by adjusting the optical path length in the first optical waveguide 24, the phase difference φ between the transmitted and untransmitted light can be arbitrarily adjusted (for example, to π / 2).

[0124] The signal generation unit 23 obtains an error signal by subtracting the intensity signal output by the second photodetector 22 from the intensity signal output by the first photodetector 21. The error signal generation device 3d can also obtain an error signal having the same characteristics as the error signal S obtained by the error signal generation device 3 of Embodiment 1.

[0125] [Embodiment 6] Other embodiments of the present invention are described below. For the sake of clarity, components having the same function as those described in the above embodiments will be denoted by the same reference numerals, and their descriptions will not be repeated.

[0126] Figure 12 shows the configuration of the error signal generation device 3e of this embodiment. The error signal generation device 3e comprises a beam splitter 19b, a first optical waveguide 24, a second optical waveguide 25, a third optical waveguide 26, a fourth optical waveguide 27, a first circulator 31, a resonator 16d, a first photodetector 21, a second photodetector 22, and a signal generation unit 23.

[0127] The first optical waveguide 24 connects the first circulator 31 to the input port of the beam splitter 19b. The second optical waveguide 25 connects the first circulator 31 to the input port of the beam splitter 19b. The third optical waveguide 26 connects the output port of the beam splitter 19b to the first photodetector 21. The fourth optical waveguide 27 connects the output port of the beam splitter 19b to the second photodetector 22. The resonator 16d is located in the middle of the first optical waveguide 24.

[0128] The first circulator 31 emits laser light incident from the laser light source into the first optical waveguide 24. The first circulator 31 emits laser light incident from the first optical waveguide 24 into the second optical waveguide 25.

[0129] Laser light (linearly polarized) is incident on the first circulator 31 from the laser light source. The laser light from the laser light source is introduced into the first optical waveguide 24 by the first circulator 31 and incident on the first side of the resonator 16d.

[0130] Of the laser light incident on the resonator 16d, a portion passes through the resonator 16d and is incident on the beam splitter 19b. The portion of the laser light incident on the resonator 16d that passes through the resonator 16d is called the first laser beam.

[0131] Of the laser light incident on the resonator 16d, the remainder is reflected by the resonator 16d and incident on the first circulator 31. Of the laser light incident on the resonator 16d, the portion reflected by the resonator 16d is called the second laser light. The second laser light is introduced into the second optical waveguide 25 by the first circulator 31 and incident on the beam splitter 19b.

[0132] Next, similar to Embodiment 4, the beam splitter 19b separates the first laser beam and the second laser beam, and combines them with different phase differences. This yields the first interference beam and the second interference beam.

[0133] The first photodetector 21 measures the intensity of the first interference light. The first interference light is the result of interference between the first laser light that passed through the resonator 16d from the first side and the second laser light that did not pass through the resonator 16d.

[0134] The second photodetector 22 measures the intensity of the second interference light. The second interference light is the result of the interference between the first laser light that passed through the resonator 16d from the first side and the second laser light that did not pass through the resonator 16d.

[0135] The signal generation unit 23 obtains an error signal by subtracting the intensity signal output by the second photodetector 22 from the intensity signal output by the first photodetector 21. The error signal generation device 3e can also obtain an error signal having the same characteristics as the error signal S obtained by the error signal generation device 3 of Embodiment 1.

[0136] (modified version) The resonator 16d may have a configuration that includes two gratings provided in the optical waveguide of the optical circuit board. Furthermore, the optical waveguides of the error signal generation devices 3b to 3e may be formed using optical fibers. The resonator 16d may also have a configuration that includes two fiber Bragg gratings provided in the optical fiber.

[0137] In the error signal generation device 3b, instead of the resonator 16b, a combination of two ring resonators may be used. In this case, by reversing the direction in which the first optical waveguide 24 is coupled to the resonator 16b, the optical waveguide and wiring can be prevented from intersecting in a plan view.

[0138] In the error signal generation devices 3 and 3a, the half-wave plate 12 may be omitted.

[0139] In the error signal generation devices 3 and 3a, the first waveplate 13 may convert linearly polarized light to elliptically polarized light.

[0140] In the error signal generation devices 3d and 3e, a beam splitter may be used instead of the first circulator 31 and the second circulator 32. The beam splitter emits at least a portion of the light from the laser light source side to the resonator 16d, and emits at least a portion of the light from the resonator 16d to the first photodetector 21 side or the second photodetector 22 side. For example, a 2-input × 2-output beam splitter may be used. In this case, half of the energy of the laser light will be wasted.

[0141] The signal generation unit 23 may generate an error signal based solely on the output of the first photodetector 21. For example, the signal generation unit 23 may use the output of the first photodetector 21 minus a predetermined value as the error signal. The predetermined value may be the output of the first photodetector 21 when the laser light is at a reference frequency. This can be done when the intensity of the laser light from the laser light source is constant. In this case, the second photodetector 22 can be omitted.

[0142] The present invention is not limited to the embodiments described above, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention. [Explanation of symbols]

[0143] 1, 1a Laser oscillation frequency control device 2. Laser light source 3, 3a, 3b, 3c, 3d, 3e error signal generator 4. Frequency Control Unit 5 Adjustment elements 11. First Polarizing Beam Splitter 12 1 / 2 wave plate 13 1st wavelength plate (1 / 4 wavelength plate) 14. Second Polarizing Beam Splitter 15 Second wave plate (1 / 4 wave plate) 16, 16b, 16d resonator 17 Third wave plate (1 / 4 wave plate) 18. Third Polarizing Beam Splitter 19, 19b Beam Splitter 21 First photodetector 22. Second photodetector 23 Signal generation unit 31. First Circulator 32. Second Circulator

Claims

1. A resonator into which laser light emitted from a laser light source is incident, A first photodetector measures the first interference light obtained when the first laser light that has passed through the resonator and the second laser light that has not passed through the resonator interfere with each other. An error signal generating device comprising: a signal generating unit that generates an error signal based on the output of the first photodetector.

2. The system includes a second photodetector that measures a second interference light obtained when the first laser light and the second laser light interfere with each other with a phase difference different from that of the first interference light. The error signal generating device according to claim 1, wherein the signal generating unit generates the error signal based on the difference between the output of the first photodetector and the output of the second photodetector.

3. The error signal generation apparatus according to claim 2, comprising a first beam splitter into which the first laser beam and the second laser beam are incident and which emits the first interference light and the second interference light.

4. The resonator is equipped with a second photodetector that measures a second interference light obtained when a third laser beam transmitted through the resonator in the opposite direction to the first laser beam and a fourth laser beam that did not pass through the resonator interfere with each other. The error signal generating device according to claim 1, wherein the signal generating unit generates the error signal based on the difference between the output of the first photodetector and the output of the second photodetector.

5. The laser light emitted from the aforementioned laser light source is divided into a fifth laser beam and a sixth laser beam by a first beam splitter, Of the fifth laser beam incident on the resonator from the first side, the portion that passes through the resonator is the first laser beam, and the portion that does not pass through the resonator is the fourth laser beam. The error signal generating apparatus according to claim 4, wherein of the sixth laser light incident on the resonator from a second side different from the first side, the portion that passes through the resonator is the third laser light, and the portion that does not pass through the resonator is the second laser light.

6. The error signal generating device according to any one of claims 1 to 5, wherein the resonator is a ring resonator.

7. The error signal generating apparatus according to any one of claims 1 to 5, wherein the resonator has two gratings provided in an optical fiber or optical waveguide.

8. The resonator is connected to an optical circulator, The error signal generation device according to claim 7, wherein the optical circulator emits light from the laser light source side to the resonator and emits light from the resonator to the first photodetector side.

9. The resonator is connected to a second beam splitter, The error signal generation apparatus according to claim 7, wherein the second beam splitter emits at least a portion of the light from the laser light source to the resonator and emits at least a portion of the light from the resonator to the first photodetector.

10. Polarizing beam splitter and A first wave plate, which is a quarter-wave plate, is disposed between the polarizing beam splitter and the first side of the resonator. The error signal generating apparatus according to claim 1, comprising: a second wave plate which is a quarter-wave plate disposed between the polarizing beam splitter and a second side of the resonator different from the first side.

11. The polarization beam splitter divides the laser light from the laser light source into a first polarization laser beam and a second polarization laser beam having different polarization directions. Of the first polarized laser light, the portion that passes through the resonator is the first laser light. Of the second polarized laser light, the portion reflected by the resonator is the second laser light. Of the second polarized laser light, the portion that passes through the resonator is the third laser light. Of the first polarized laser light, the portion reflected by the resonator is the fourth laser light. The resonator is equipped with a second photodetector that measures a second interference light obtained when the third laser beam transmitted through the resonator in the opposite direction to the first laser beam and the fourth laser beam that did not pass through the resonator interfere with each other. The error signal generating device according to claim 10, wherein the signal generating unit generates the error signal based on the difference between the output of the first photodetector and the output of the second photodetector.

12. The error signal generation apparatus according to claim 10 or 11, further comprising a third wave plate which is a quarter-wave plate that emits the laser light from the laser light source to the polarizing beam splitter in an elliptical polarized state.

13. A second photodetector measures a second interference light obtained when the first laser light and the second laser light interfere with each other with a phase difference different from the first interference light. The system comprises a polarization beam splitter and a third wave plate, which is a quarter-wave plate into which the first laser light and the second laser light are incident, positioned between the first and second photodetectors. The error signal generating device according to claim 10, wherein the signal generating unit generates the error signal based on the difference between the output of the first photodetector and the output of the second photodetector.

14. A step of generating a first laser beam that has passed through the resonator and a second laser beam that has not passed through the resonator from laser light emitted from a laser light source, using a resonator, A measurement step of measuring the first interference light obtained when the first laser light and the second laser light interfere with each other, An error signal generation method comprising: a signal generation step of generating an error signal based on the intensity of the first interference light.

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