Signal generation device, optical frequency comb generation device, and signal generation method

Abstract

An optical coupler (3) multiplexes first and second laser beams (LCW1, LCW2), and branches into first and second multiplexed lights (LM1, LM2). A signal generation unit (10) outputs an output signal (OUT) based on the second multiplexed light (LM2). A signal source (4) outputs a signal of a predetermined frequency. A Michelson interferometer (20) delays the branched one light and the other light of the first multiplexed light (LM1) and outputs interference light (L) with light modulated according to a signal of a predetermined frequency. An optical signal separation unit (5) outputs first and second detection signals (D1, D2) of the light having first and second frequencies. A noise signal extraction unit (6) outputs first and second feedback signals (FB1, FB2) indicating the frequency noise of the first and second laser beams (LCW1, LCW2). First and second light sources (1, 2) suppress the frequency noise of the first and second laser beams (LCW1, LCW2) on the basis of the first and second feedback signals (FB1, FB2).

Description

Signal generator, optical frequency comb generator, and signal generation method

[0001] The present disclosure relates to a signal generating device, an optical frequency comb generating device, and a signal generating method.

[0002] The use of microwave and optical frequency combs is progressing in various fields. Microwave sources are widely used. Microwave and optical frequency combs with low phase noise are expected to dramatically improve the performance of various devices, such as information and communication equipment, particle accelerators, laser synchronization systems, electronic measuring instruments such as signal generators and signal source analyzers, and radar equipment.

[0003] As a method for generating microwaves with low phase noise, for example, a method using an ultra-stable resonator (Non-Patent Document 1) and a method using a cryogenic sapphire oscillator (Non-Patent Document 2) have been proposed.

[0004] Also, a method has been proposed in which microwaves are generated using two continuous wave laser beams and stabilized in an optical fiber delay line (Patent Document 1).

[0005] U.S. Pat. No. 5,687,261

[0006] TM Fortier et al. Sub-femtosecond absolute timing jitter with a 10 GHz hybrid photonic-microwave oscillator. Appl. Phys. Lett. 100, 231111 (2012). JG Hartnett, NR Nand, and C. Lu, “Ultra-low-phase-noise cryocooled microwave dielectric-sapphire-resonator oscillators,” Appl. Phys. Lett. AJ Metcalf, F. Quinlan, TM Fortier, SA Diddams, and AM Weiner, “Broadly tunable, low timing jitter, high repetition rate optoelectronic comb generator,” Electron. Lett. 51, 1596-1598 (2015).100, 183501 (2012).

[0007] As described above, the methods described in Non-Patent Documents 1 and 2, which use an ultra-stable resonator or a cryogenic sapphire oscillator to suppress microwave phase noise, have the drawback of being complicated in structure and expensive in components. Furthermore, the costs required for maintenance and repair are high, limiting their use.

[0008] Furthermore, the technique disclosed in Patent Document 1 has the drawback that noise characteristics stabilized from optical frequencies in the band of several hundred THz are included in the microwave signal, resulting in insufficient noise suppression.

[0009] A signal generating device according to one aspect of the present disclosure includes a first light source that outputs a first laser beam of a first frequency, a second light source that outputs a second laser beam of a second frequency, an optical multiplexing / demultiplexing unit that outputs first and second multiplexed beams obtained by multiplexing the first laser beam and the second laser beam, a signal generating unit that detects the second multiplexed beam and outputs an output signal that is a beat note signal of the first and second laser beams, and a first feedback loop unit that detects frequency noise of the first and second laser beams based on the first multiplexed beam and outputs first and second feedback signals based on the detection result, wherein the first feedback loop unit includes a first signal source that outputs at least a signal of a first predetermined frequency, and a first signal source that receives the first multiplexed beam, branches the first multiplexed beam, and outputs one of the branched beams as a single beam. a first interferometer that delays the other branched light by propagating it through a delay path of a predetermined length and causes light modulated in accordance with the signal of the first predetermined frequency to interfere with it, thereby outputting interference light; a first signal separation unit that outputs a first detection signal obtained by detecting light of the first frequency included in the interference light and a second detection signal obtained by detecting light of the second frequency; and a first noise signal extraction unit that outputs first and second feedback signals indicative of frequency noise of the first and second laser lights, obtained by removing a component of a frequency twice the first predetermined frequency from the first and second detection signals, to the first and second light sources, respectively, wherein the first and second light sources suppress the frequency noise of the first and second laser lights based on the first and second feedback signals.

[0010] A signal generating method according to one aspect of the present disclosure includes splitting a light beam obtained by combining a first laser beam having a first frequency output from a first light source and a second laser beam having a second frequency output from a second light source into first and second combined beams and outputting the split beams, outputting at least a signal having a first predetermined frequency, detecting the second combined beam, outputting an output signal that is a beat note signal of the first and second laser beams, splitting the first combined beam, delaying one of the split beams and the other of the split beams by propagating them through a delay path of a predetermined length, and outputting a light beam modulated in accordance with the signal of the first predetermined frequency. and outputting interference light, outputting a first detection signal obtained by detecting light of the first frequency included in the interference light and a second detection signal obtained by detecting light of the second frequency, and outputting first and second feedback signals indicating frequency noise of the first and second laser lights obtained by removing a frequency component that is twice the first predetermined frequency from the first and second detection signals to the first and second light sources, respectively, and the first and second light sources suppress the frequency noise of the first and second laser lights based on the first and second feedback signals.

[0011] According to the present disclosure, it is possible to provide a signal generator that generates signals in a wide frequency band while suitably suppressing phase noise, and an optical frequency comb generator using the same.

[0012] FIG. 1 is a diagram schematically illustrating a configuration of a signal generating apparatus according to a first embodiment. FIG. 2 is a diagram illustrating a configuration of a signal generating apparatus according to the first embodiment in more detail. FIG. 3 is a diagram schematically illustrating a configuration of a signal generating apparatus according to a second embodiment. FIG. 4 is a diagram schematically illustrating a configuration of a signal generating apparatus according to a third embodiment. FIG. 5 is a diagram schematically illustrating a configuration of a signal generating apparatus according to a fourth embodiment. FIG. 6 is a diagram schematically illustrating a configuration of an optical frequency comb generator according to a fifth embodiment. FIG. 7 is a diagram schematically illustrating a configuration example of an intensity phase adjuster. FIG. 8 is a diagram schematically illustrating a configuration example of an electro-optic modulator. FIG. 9 is a diagram schematically illustrating a configuration of a dual optical frequency comb generator according to a sixth embodiment. FIG. 10 is a diagram schematically illustrating a configuration of an intensity phase adjuster. FIG. 11 is a diagram schematically illustrating a configuration of an intensity phase adjuster. FIG. 12 is a diagram schematically illustrating a configuration of a power combiner. FIG. 13 is a diagram schematically illustrating a configuration of a signal generating apparatus according to a seventh embodiment.

[0013] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the drawings, the same elements are designated by the same reference numerals, and redundant explanations will be omitted as necessary.

[0014] First Embodiment A signal generating device 100 according to a first embodiment will be described. Fig. 1 is a diagram schematically illustrating the configuration of the signal generating device 100 according to the first embodiment. The signal generating device 100 includes laser light sources 1 and 2, an optical coupler 3, a signal source 4, an optical signal separating unit 5, a noise signal extracting unit 6, a signal generating unit 10, and a Michelson interferometer 20. In the signal generating device 100, the signal source 4, the optical signal separating unit 5, the noise signal extracting unit 6, and the Michelson interferometer 20 constitute a first feedback loop unit 101 that outputs a feedback signal for suppressing noise from the laser light sources 1 and 2.

[0015] The laser light source 1 emits a continuous wave laser light L having a frequency of ν1. ＣＷ１ The laser light source 1 is configured as a variable frequency laser device, and outputs the laser light L ＣＷ１ It is possible to adjust the frequency v1 of the laser light L. Hereinafter, the frequency v1 will also be referred to as the first frequency. ＣＷ１ is also referred to as the first laser beam.

[0016] The laser light source 2 emits a continuous wave laser light L having a frequency of ν2. ＣＷ２ to the optical coupler 3. The laser light source 2 is configured as a variable frequency laser device, and outputs the laser light L in response to a feedback signal FB2 output from a noise signal extraction unit 6, which will be described later, for example. ＣＷ２ It is possible to adjust the frequency v2 of the laser light L to a desired frequency. Hereinafter, the frequency v2 will also be referred to as the second frequency. ＣＷ２ is also referred to as the second laser beam.

[0017] The optical coupler 3 couples the laser light L ＣＷ１ and L ＣＷ１The optical coupler 3 is an optical multiplexer / demultiplexer that multiplexes and demultiplexes the laser beam L and outputs the first and second multiplexed beams. Here, an example will be described in which the optical coupler 3 is configured as a two-input, two-output optical coupler. ＣＷ１ is input to the other input, and laser light L ＣＷ２ The optical coupler 3 receives the laser light L ＣＷ１ and laser light L ＣＷ２ The combined light is then equally split into the signal generating unit 10 and the Michelson interferometer 20. Hereinafter, the combined light output from the optical multiplexing / demultiplexing unit to the Michelson interferometer of the first feedback loop unit is referred to as the first combined light L. Ｍ１ The multiplexed light output from the optical multiplexing / demultiplexing unit to the signal generating unit is called the second multiplexed light L Ｍ２ The optical coupler 3 is also referred to as a first optical coupler.

[0018] Next, a description will be given of the configurations of the signal source 4, the optical signal separation unit 5, the noise signal extraction unit 6, the signal generation unit 10, and the Michelson interferometer 20. Fig. 2 is a diagram showing in more detail the configuration of the signal generating device 100 according to the first embodiment. In Fig. 2, the signal source 4, the optical signal separation unit 5, the noise signal extraction unit 6, the isolator 7, and the Michelson interferometer 20 of the signal generating device 100 configure a first feedback loop unit 101.

[0019] The signal source 4 has an oscillator 4A, an amplifier 4B, and a frequency doubler 4C. The oscillator 4A outputs a signal S1 of frequency f. The amplifier 4B amplifies the signal S1 and outputs the amplified signal S1 to the Michelson interferometer 20. The frequency doubler 4C outputs a signal S2 of frequency 2f, which is twice the frequency f of the signal S1, to the noise signal extraction unit 6.

[0020] The signal generating unit 10 includes a light receiving element 11 and a TIA 12. The light receiving element 11 receives the second multiplexed light L from the optical coupler 3. Ｍ２ The second multiplexed light L output from the optical coupler 3 to the signal generating unit 10 is Ｍ２ is the laser light L of frequency ν1 ＣＷ１ and laser light L of frequency ν ＣＷ２ Since the light receiving element 11 detects the beat note signals of these two frequencies, it outputs the current signal C Ｄ Output as

[0021] The TIA 12 outputs a current signal C Ｄ is converted into a voltage signal of a desired level, the frequency f ＯＵＴ By adjusting the difference |ν1-ν2| between the frequencies v1 and v2, the signal generating unit 10 outputs an output signal OUT having a wide frequency band including microwaves, millimeter waves, and terahertz waves. Hereinafter, the TIA 12 is also referred to as a current-voltage converter.

[0022] The Michelson interferometer 20 includes an optical coupler 21, Faraday rotator mirrors (hereinafter referred to as FRMs) 22 and 23, an optical fiber delay line (also referred to as OFDL) 24, and an acousto-optic modulator (hereinafter referred to as AOM) 25.

[0023] As shown in FIG. 2, the optical coupler 21 receives the first multiplexed light L from the optical coupler 3 via the isolator 7, for example. Ｍ１ The optical coupler 21 receives the first multiplexed light L Ｍ１ The FRM 22 splits the first multiplexed light L Ｍ１ is reflected to the optical coupler 21.

[0024] The optical fiber delay line 24 is composed of an optical fiber of about several kilometers. Ｍ１ is delayed by propagating through the optical fiber delay line 24 and then enters the AOM 25. The AOM 25 receives the signal S1 of frequency f output from the signal source 4. The AOM 25 receives the first multiplexed light L Ｍ１ is frequency-modulated based on the signal S1. ＭＯＤ The FRM 23 outputs the modulated light L ＭＯＤ The modulated light L is reflected to the AOM 25. ＭＯＤ passes through the AOM 25 and the optical fiber delay line 24 and returns to the optical coupler 21 .

[0025] The first multiplexed light L returned from the FRM 22 Ｍ１ and modulated light L returned from the delay path consisting of the FRM 23, the optical fiber delay line 24, and the AOM 25. ＭＯＤ When these two light beams are combined by the optical coupler 21, they interfere with each other, generating interference light L. The interference light L becomes an optical signal having a carrier frequency of 2f, which is twice the modulation frequency f of the AOM 25. The optical coupler 21 outputs the interference light L generated by the interference to the optical signal separation unit 5.

[0026] For each frequency component, the phase difference between two laser beams returning from different paths is converted into light intensity information in the interference light L. This phase difference is proportional to the delay time caused by the difference in the lengths of the two paths in the Michelson interferometer multiplied by the frequency noise of the laser beam, so the longer the optical fiber that provides the delay is made, the longer the delay time is, and the more the sensitivity of measuring frequency noise can be improved.

[0027] In this configuration, the first multiplexed light L from the FRM 22 Ｍ１ and modulated light L from the delay path ＭＯＤ is reflected by the FRM and returned to the optical coupler 21, the polarization states of the two light beams always match. Therefore, the intensity of the interference light beam L can be easily maximized without using a polarization control means such as a polarization controller.

[0028] The optical signal separator 5 includes a circulator 5A, an optical fiber grating (hereinafter referred to as FBG: Fiber Bragg Grating) 5B, light receiving elements 5C and 5D, and transimpedance amplifiers 5E and 5F.

[0029] The interference light L output from the Michelson interferometer 20 is incident on the circulator 5A. The circulator 5A outputs the interference light L to the FBG 5B. The FBG 5B outputs the component L of the interference light L having the frequency ν1. １ is transmitted to the light receiving element 5C, and the component L of frequency ν2 ２ The circulator 5A reflects the component L of frequency ν2 reflected by the FBG 5B. ２ is output to the light receiving element 5D.

[0030] The light receiving element 5C detects the component L of frequency ν1 in the interference light L. １ The TIA 5E outputs a current signal C1, which is the detection result of the above, to the TIA 5E. The TIA 5E converts the current signal C1 into a detection signal D1, which is a voltage signal including a carrier of frequency 2f, and outputs the detection signal D1 to the noise signal extraction unit 6. Hereinafter, the detection signal D1 will also be referred to as a first detection signal.

[0031] The light receiving element 5D detects the component L of frequency ν2 in the interference light L. ２ The TIA 5F outputs a current signal C2, which is the detection result of the above, to the TIA 5F. The TIA 5F converts the current signal C2 into a detection signal D2, which is a voltage signal including a carrier of frequency 2f, and outputs the detection signal D2 to the noise signal extraction unit 6. Hereinafter, the detection signal D2 will also be referred to as a second detection signal.

[0032] The noise signal extraction unit 6 has frequency mixers 6A and 6B and loop filters 6C and 6D.

[0033] The frequency mixer 6A mixes the detection signal D1 with the signal S2 to remove the component of the carrier frequency 2f from the detection signal D1. As a result, the laser light L of frequency ν1 ＣＷ１ Only the frequency noise component of the frequency band N1 is obtained as a baseband signal. The frequency mixer 6A outputs the obtained signal to the loop filter 6C as a frequency noise signal N1. The loop filter 6C outputs a feedback signal FB1, which is a DC signal obtained by smoothing the frequency noise signal N1, to the laser light source 1. Hereinafter, the feedback signal FB1 will also be referred to as a first feedback signal.

[0034] The frequency mixer 6B mixes the detection signal D2 with the signal S2 to remove the component of the carrier frequency 2f from the detection signal D2. ＣＷ２ Only the frequency noise component of N1 is obtained as a baseband signal. The frequency mixer 6B outputs the obtained signal to the loop filter 6D as a frequency noise signal N2. The loop filter 6D outputs a feedback signal FB2, which is a DC signal obtained by smoothing the frequency noise signal N2, to the laser light source 2. Hereinafter, the feedback signal FB2 will also be referred to as a second feedback signal.

[0035] As described above, in this configuration, the drive current, piezoelectric element, and device temperature of the laser light source are feedback-controlled using a frequency noise signal extracted from the laser light output by the laser light source. This allows the laser light source to appropriately control the drive current, piezoelectric element, and device temperature to suppress frequency noise. This allows the frequency noise (δν) of the laser light to be stabilized to the length (L) of the optical fiber (δν / ν = δL / L).

[0036] As described above, according to this configuration, by adjusting the frequencies of the two laser beams, signals in a wide frequency band including microwaves, millimeter waves, and terahertz waves can be generated.

[0037] As mentioned above, the frequencies of the two laser beams are stabilized in a common optical fiber delay line, so the absolute phase noise of the generated output signal can suppress common mode noise by the ratio of the laser beam frequency (ν1) to the output signal frequency (ν1-ν2), achieving high absolute phase noise performance.

[0038] Assuming that a 10 GHz microwave is generated from a laser beam with a wavelength of 1550 nm, the microwave noise can be reduced by approximately 20 log {(v1 - v2) / v1} = -86 dB from the noise of the laser beam. For example, the linewidth of laser beam stabilized in an optical fiber delay line of approximately 2 km is approximately 1 Hz, and when divided by a microwave frequency of 10 GHz, the phase noise is suppressed by approximately 86 dB, resulting in a low noise level of approximately -170 dBc / Hz at a Fourier frequency of 1 kHz.

[0039] In the first embodiment, a signal generating device was described that outputs an output signal OUT with suppressed frequency noise by stabilizing the laser light to the optical fiber length of the delay path of the Michelson interferometer. However, although the length of the optical fiber of the delay path used for stabilization is stable in the short term (for example, for a period of 1 second or less), it is subject to gradual temperature fluctuations (10 －５ It is expected that the length will fluctuate due to factors such as the influence of the temperature (approximately m / m·K).

[0040] Therefore, in this embodiment, a signal generator capable of compensating for the effects of long-term fluctuations in the optical fiber of the delay path will be described. FIG. 3 is a diagram schematically illustrating the configuration of a signal generator 200 according to a second embodiment. Compared to the signal generator 100 according to the first embodiment, the signal generator 200 has a configuration in which the signal generating unit 10 and the Michelson interferometer 20 are replaced with a signal generating unit 30 and a Michelson interferometer 40, respectively. Note that, for the sake of simplicity, the isolator 7 shown in FIG. 2 will be omitted below. In the signal generator 200, the signal source 4, the optical signal separating unit 5, the noise signal extracting unit 6, and the Michelson interferometer 40 form a first feedback loop unit. For the sake of simplicity, the first feedback loop unit is omitted from FIG. 3.

[0041] The signal generating section 30 has a configuration in which a distributor 31, a frequency mixer 32, an oscillator 33, a frequency multiplier 34, a loop filter 35, and an amplifier 36 are added to the signal generating section 10.

[0042] The distributor 31 branches the output signal OUT from the TIA 12 into two, and outputs one to the outside of the signal generating device 200 and the other to the frequency mixer 32 .

[0043] The oscillator 33 is configured as an oscillator that ensures long-term stability of the oscillation frequency. For example, the oscillator 33 may be configured with a quartz crystal resonator that has excellent long-term stability of the oscillation frequency. The oscillator 33 generates a frequency f ＳＴＢ The signal S3 is output to the frequency multiplier 34.

[0044] The frequency multiplier 34 multiplies the signal S3 by a target frequency f ＴＲＧ Then, the frequency multiplier 34 up-converts the reference signal SR to a target frequency f ＴＲＧ The reference signal SR is output to the frequency mixer 32. Hereinafter, the oscillator 33 and the frequency multiplier 34 are also referred to as a reference signal generating unit.

[0045] The frequency mixer 32 mixes the output signal OUT with the reference signal SR to remove the component of the reference signal SR from the output signal OUT. As a result, the frequency mixer 32 outputs a phase fluctuation signal SP indicating the phase fluctuation between the output signal OUT and the reference signal SR to the loop filter 35. Hereinafter, the frequency mixer 32 is also referred to as a first frequency mixer.

[0046] The loop filter 35 outputs a feedback signal FB3, which is a DC signal obtained by smoothing the phase fluctuation signal SP, to the amplifier 36. For simplicity, the loop filter is represented as LF in FIG. 3 . The amplifier 36 amplifies the feedback signal FB3 and then outputs it to the Michelson interferometer 40. Hereinafter, the feedback signal FB3 will also be referred to as a third feedback signal. The loop filter 35 and the amplifier 36 will also be referred to as a signal output unit.

[0047] The Michelson interferometer 40 has a configuration in which a fiber stretcher 41 is added to the Michelson interferometer 20. In FIG. 3, for simplicity, the fiber stretcher is denoted as FS. The fiber stretcher 41 is, for example, a device in which an optical fiber is wound around a piezoelectric element, and expands or contracts the wound optical fiber in response to a signal input to the piezoelectric element. In this configuration, the fiber stretcher 41 is inserted between the optical fiber delay line 24 and the AOM 25, and expands or contracts the optical path length of the delay path of the laser light in response to a feedback signal FB3 from the amplifier 36. As a result, the modulated light L returning to the optical coupler 21 ＭＯＤ Hereinafter, the fiber stretcher 41 is also referred to as a delay path length compensator.

[0048] As described above, according to this configuration, long-term fluctuations in frequency noise of the output signal OUT are monitored, and the monitoring results are fed back to the Michelson interferometer 40. By adjusting the length of the delay path of the laser light in the Michelson interferometer 40 accordingly, it is possible to maintain the accuracy of detecting frequency noise. As a result, it is possible to suppress long-term fluctuations in frequency noise of the output signal OUT.

[0049] Third Embodiment In the above-described embodiment, a configuration has been described in which the frequency of a laser beam is stabilized by an optical fiber delay line. In this configuration, the frequency of the stabilized laser beam has a value that is an integer multiple of the reciprocal of the delay time caused by the optical fiber. Therefore, the adjustment resolution of the frequency of the laser beam is the reciprocal of the delay time caused by the optical fiber. In this case, the resolution of the output signal OUT, which is a beat signal of two laser beams, also has the same value as the adjustment resolution of the frequency of the laser beam.

[0050] For example, if the length of the optical fiber delay line is 2 km, the frequency adjustment resolution is 100 kHz, and in this case, the resolution of the output signal OUT is also the same.

[0051] In contrast, in this embodiment, a signal generating device that can adjust the frequency of the output signal OUT more precisely will be described. Fig. 4 is a diagram schematically showing the configuration of a signal generating device 300 according to a third embodiment. Compared to the signal generating device 200 according to the second embodiment, the signal generating device 300 has a configuration in which the signal generating section 30 is replaced with a signal generating section 50. In Fig. 4, as in Fig. 3, the first feedback loop section is not shown in order to simplify the drawing.

[0052] The signal generating section 50 has a configuration in which an oscillator 51, a frequency mixer 52, a filter 53, and an amplifier 54 are further provided in addition to the components of the signal generating section 30.

[0053] In this configuration, the signal input from the TIA 12 to the distributor 31 is referred to as signal S10. It goes without saying that the signal S10 here is the same signal as the output signal OUT output from the TIA 12 in the first and second embodiments.

[0054] The oscillator 51 is configured as a low-frequency signal source, and outputs a signal S4 having a frequency lower than that of the output signal OUT to the frequency mixer 52. Hereinafter, the oscillator 51 is also referred to as a signal generating unit.

[0055] The frequency mixer 52 mixes the output signal OUT and the signal S4, and outputs a signal S5 having a frequency equal to the difference between the frequency of the output signal OUT and the frequency of the signal S4 to the filter 53. Hereinafter, the frequency mixer 52 is also referred to as a second frequency mixer.

[0056] The filter 53 removes unnecessary high frequency components from the signal S5 and outputs the signal after removal as an output signal OUT to the amplifier 54. The amplifier 54 amplifies the output signal OUT and outputs the amplified output signal OUT.

[0057] As described above, according to this configuration, a signal obtained by adjusting the frequency of the signal S10 output from the TIA 12 by the frequency of the low-frequency signal S4 can be output as the output signal OUT. In this case, by setting the frequency of the signal S4 so that adjustment can be made at a smaller level than the adjustment resolution of the laser light frequency determined by the optical fiber delay line, the frequency of the output signal OUT can be adjusted more precisely than in the first and second embodiments.

[0058] In this configuration, a low-frequency signal S4 is used to adjust the frequency of the output signal OUT. However, the frequency of the signal S4 is significantly lower than the frequency of the output signal. Therefore, the phase noise added to the output signal OUT is negligibly small. Therefore, it is possible to maintain the signal quality of the output signal OUT favorably.

[0059] In the first and second embodiments, a configuration in which the frequency of a laser beam is stabilized by an optical fiber delay line has been described. In this configuration, the noise floor of the output signal OUT is limited by the shot noise of the photodiode used as the light receiving element and the feedback bandwidth of the laser stabilization. In particular, outside the feedback bandwidth, the common noise suppression effect achieved by frequency division of the laser beam when generating the output signal OUT cannot be obtained, and high-frequency phase noise remains unchanged.

[0060] In contrast, in this embodiment, a signal generating device that suppresses phase noise outside the feedback bandwidth will be described. Fig. 5 is a diagram schematically showing the configuration of a signal generating device 400 according to a fourth embodiment. Compared to the signal generating device 100 according to the first embodiment, the signal generating device 400 has a configuration in which the signal generating section 10 is replaced with a signal generating section 60. In Fig. 5, the first feedback loop section 101 is not shown to simplify the drawing.

[0061] The signal generating section 60 has a configuration in which a frequency mixer 61 , a loop filter 62 , an oscillator 63 and a branching filter 64 are further provided in addition to the components of the signal generating section 10 .

[0062] The oscillator 63 is configured as a signal source with a low noise floor at high Fourier frequencies. The oscillator 63 can be a relatively inexpensive microwave signal source that has a low noise floor at high Fourier frequencies but high overall phase noise at low Fourier frequencies. The oscillator 63 outputs a signal of a predetermined frequency to the branching filter 64 as an output signal OUT. Hereinafter, the oscillator 63 will also be referred to as a signal generating unit.

[0063] The branching filter 64 branches the output signal OUT and outputs it to an output target outside the signal generating device 400 and to the frequency mixer 61. Hereinafter, the branching filter 64 is also referred to as a distributor.

[0064] The frequency mixer 61 mixes the output signal OUT with the signal S10 from the TIA 12, and outputs a phase noise signal SN, obtained as a signal having a frequency equal to the difference between the frequency of the output signal OUT and the frequency of the signal S10, to the loop filter 62. In this configuration, the signal input from the TIA 12 to the distributor 31 is referred to as signal S10. Needless to say, the signal S10 referred to here is the same signal as the output signal OUT output from the TIA 12 in the first and second embodiments. Hereinafter, the frequency mixer 61 will also be referred to as a third frequency mixer.

[0065] The loop filter 62 outputs a feedback signal FB4, which is a DC signal obtained by smoothing the phase noise signal SN, to the oscillator 63. Hereinafter, the loop filter 62 will also be referred to as a signal output unit. The feedback signal FB4 will also be referred to as a fourth feedback signal. This makes it possible to feed back a signal indicating the phase noise of the output signal OUT to the oscillator 63 over a wide band.

[0066] As described above, in this configuration, the phase noise of the oscillator 63, which is an external signal source, tracks the low phase noise of the signal generated based on the stabilized laser light at low Fourier frequencies, while at high Fourier frequencies outside the feedback bandwidth, the phase noise of the oscillator 63 maintains its inherently low noise floor.

[0067] In this way, with this configuration, it is possible to effectively suppress phase noise in the output signal over a wider band.

[0068] Fifth Embodiment This embodiment describes an optical frequency comb generator using the signal generator described in the above-described embodiments. Electro-optic modulation frequency combs have wide frequency spacing between modes, making frequency adjustment easy, and are expected to be applied to communication devices such as 5G and 6G. It is known that the phase noise of microwave-based electro-optic modulation frequency combs is determined by the phase noise of the continuous wave laser driving the electro-optic modulator and the microwave signal (Non-Patent Document 3). Therefore, by using signals in bands including microwaves, submillimeter waves, and terahertz waves, which have low phase noise, as in the signal generator described in the above-described embodiment, it is expected that the phase noise of the optical frequency comb will be reduced.

[0069] 6 is a diagram schematically illustrating the configuration of an optical frequency comb generator 1000 according to a fifth embodiment. The optical frequency comb generator 1000 includes the signal generating device 100 according to the first embodiment, an optical coupler 8, and a comb generating unit 70. In FIG. 6, the first feedback loop unit 101 is not shown, as in FIG. 5, for simplicity of illustration.

[0070] The optical coupler 8 is inserted between the laser light source 1 and the optical coupler 3, and the laser light L ＣＷ１is branched toward the optical coupler 3 and the comb generator 70. Hereinafter, the optical coupler 8 is also referred to as a sixth optical coupler.

[0071] The comb generating section 70 includes a distributor 71 , an intensity / phase adjusting section 72 , an electro-optic modulator (EOM) 73 , and a bias generating section 74 .

[0072] The distributor 71 branches the output signal OUT from the signal generating device 100 and outputs it to an output target outside the signal generating device 100 and to the comb generating section 70 .

[0073] The intensity and phase adjustment unit 72 outputs a control signal CON to the EOM 73 based on the output signal OUT. The bias generation unit 74 outputs a desired bias voltage VB to the EOM 73. The EOM 73 adjusts the intensity and phase of the laser light L input from the laser light source 1. ＣＷ１ The optical frequency comb FC1 is generated and output from the control signal CON and the bias voltage VB.

[0074] 7 is a diagram schematically illustrating an example of the configuration of the intensity phase adjustment unit 72. The intensity phase adjustment unit 72 includes a distributor 72A, an attenuator 72B, an amplifier 72C, and a phase shifter PS １ ~PS Ｎ and amplifier A １ ~A Ｎ It has.

[0075] The distributor 72A divides the output signal OUT into an attenuator 72B and a phase shifter PS １ ~PS Ｎ The output signal OUT is adjusted to a predetermined power by an attenuator 72B and an amplifier 72C, and then output to an EOM 73 as an intensity-modulated signal INT.

[0076] Phase shifter PS １ ~PS Ｎ , respectively, shift the phase of the distributed output signal OUT by a predetermined amount, and the phase-modulated signal P １ ~P Ｎ amplifier A １ ~A Ｎ Amplifier A １ ~A Ｎ is the phase modulation signal P １ ~PＮ is amplified to a predetermined power and output to the EOM 73.

[0077] 8 is a diagram showing a schematic configuration example of the EOM 73. The EOM 73 includes an intensity modulation section 73A and one or more phase modulation sections PM. １ ~ PM Ｎ and,

[0078] The intensity modulation unit 73A receives the laser light L from the laser light source 1. ＣＷ１ In addition, a bias voltage VB is applied to the intensity modulation unit 73A. The intensity modulation unit 73A modulates the laser light L in response to the intensity modulation signal INT and the bias voltage VB. ＣＷ１ and modulates the modulated light through a cascade-arranged phase modulation unit PM. １ ~ PM Ｎ Output to.

[0079] Phase modulation unit PM １ ~ PM Ｎ are the phase modulation signals P １ ~P Ｎ The modulated light is phase-modulated in accordance with the phase modulation, and an optical frequency comb FC1 obtained by the phase modulation is output.

[0080] In this configuration, the frequency noise of any mode of the generated optical frequency comb is δν ｎ = δν + n × δF ｒｅｐ Here, δν is the frequency noise of the continuous wave laser light, δF ｒｅｐ is the frequency noise of the output signal OUT, and n is a number indicating the nth mode farthest from the center frequency of the continuous wave laser light.

[0081] In this configuration, the frequency of the output signal OUT can be adjusted by adjusting the frequency difference between the two continuous-wave laser beams, which in turn adjusts the mode spacing, i.e., the repetition rate, of the optical frequency comb FC1.

[0082] In this configuration, in order to prevent the bias point of the intensity modulator from drifting over time, the ratio between the input and output of the intensity modulator may be measured in real time, and the bias generating unit 74 may be feedback-controlled so that this ratio always remains constant.

[0083] Sixth Embodiment In the fifth embodiment, an optical frequency comb generator that generates a single optical frequency comb using a signal generator was described. In contrast, in the present embodiment, an optical frequency comb generator that generates a dual optical frequency comb using a signal generator is described. FIG. 9 is a diagram schematically illustrating the configuration of an optical frequency comb generator 2000 that generates a dual optical frequency comb according to the sixth embodiment. Compared to the optical frequency comb generator 1000 according to the fifth embodiment, the optical frequency comb generator 2000 has a configuration in which the comb generator 70 is replaced with a comb generator 80.

[0084] The comb generating section 80 includes a divider 81 , a frequency mixer 82 , an oscillator 83 , a filter 84 , intensity and phase adjusting sections 85 and 86 , a power combiner 87 , an EOM 88 , and a bias generating section 89 .

[0085] The divider 81 divides the output signal OUT input from the signal generator 100 to the frequency mixer 82 and the intensity phase adjuster 85. The oscillator 83 generates a frequency f Ａ The signal S6 is output to the frequency mixer 82.

[0086] The frequency mixer 82 mixes the output signal OUT with the signal S6, and the frequency f ＯＵＴ and the frequency f of the signal S6 Ａ The frequency f ＯＵＴ +f Ａ The filter 84 outputs the signal S7 of frequency f ＯＵＴ +f Ａ After removing components other than the frequency f of the output signal OUT, the frequency mixer 82, the oscillator 83, and the filter 84 output the signal S7 to the intensity / phase adjuster 86. ＯＵＴ , frequency f ＯＵＴ +f Ａ The frequency changer outputs a signal S7 changed to the frequency change signal S4.

[0087] The intensity phase adjustment unit 85 will be described. Fig. 10 is a diagram schematically showing the configuration of the intensity phase adjustment unit 85. The intensity phase adjustment unit 85 has the same configuration as the intensity phase adjustment unit 72. The intensity phase adjustment unit 85 includes a divider 851, an attenuator 852, an amplifier 853, a phase shifter PSａ１ ~PS ａＮ and amplifier A ａ１ ~A ａＮ It has.

[0088] The distributor 851 divides the output signal OUT into an attenuator 852 and a phase shifter PS ａ１ ~PS ａＮ The output signal OUT is adjusted to a predetermined power by an attenuator 852 and an amplifier 853, and then output as an intensity-modulated signal INTa to an EOM 88. Hereinafter, the intensity-modulated signal INTa will also be referred to as a first intensity-modulated signal.

[0089] Phase shifter PS ａ１ ~PS ａＮ , respectively, shift the phase of the distributed output signal OUT by a predetermined amount, and the phase-modulated signal P ａ１ ~P ａＮ amplifier A ａ１ ~A ａＮ Amplifier A ａ１ ~A ａＮ is the phase modulation signal P ａ１ ~P ａＮ is amplified to a predetermined power and output to the EOM 88. ａ１ ~P ａＮ Each of these signals is also referred to as a first phase-modulated signal.

[0090] The intensity phase adjustment unit 86 will now be described. Fig. 11 is a diagram schematically illustrating the configuration of the intensity phase adjustment unit 86. The intensity phase adjustment unit 86 has a configuration in which a phase shifter is further provided in addition to the intensity phase adjustment unit 85. The intensity phase adjustment unit 86 includes a divider 861, an attenuator 862, an amplifier 863, a phase shifter 864, and a phase shifter PS ｂ１ ~PS ｂＮ and amplifier A ｂ１ ~A ｂＮ It has.

[0091] The phase shifter 864 outputs a signal S8 obtained by shifting the phase of the output signal OUT by a predetermined amount to the distributor 861 .

[0092] The distributor 861 divides the signal S8 into an attenuator 862 and a phase shifter PS ｂ１ ~PS ｂＮThe signal S8 is adjusted to a predetermined power by an attenuator 862 and an amplifier 863, and then output as an amplified intensity-modulated signal INTb to the EOM 88. Hereinafter, the intensity-modulated signal INTb will also be referred to as a second intensity-modulated signal.

[0093] Phase shifter PS ｂ１ ~PS ｂＮ , respectively, shift the phase of the distributed signal S8 by a predetermined amount, and generate the phase-modulated signal P ｂ１ ~P ｂＮ amplifier A ｂ１ ~A ｂＮ Amplifier A ｂ１ ~A ｂＮ is the phase modulation signal P ｂ１ ~P ｂＮ is amplified to a predetermined power and output to the EOM 88. ｂ１ ~P ｂＮ Each of these signals is also referred to as a second phase-modulated signal.

[0094] 12 is a diagram showing a schematic configuration of the power combiner 87. The power combiner 87 combines the intensity-modulated signal INTa and the intensity-modulated signal INTb, and outputs the combined signal to the EOM 88 as the intensity-modulated signal INT. The power combiner 87 also combines the phase-modulated signal P ａｉ and the phase modulation signal P ｂｉ By combining these, the phase control P ｉ That is, the phase modulation signal P ａ１ ~P ａＮ are phase modulated signals P ｂ１ ~P ｂＮ , and the combined phase-modulated signal P １ ~P Ｎ is output to the EOM 88.

[0095] The configurations and operations of the EOM 88 and bias generation unit 89 are similar to those of the EOM 73 and bias generation unit 74, respectively, and therefore redundant explanations will be omitted.

[0096] In this configuration, the intensity modulation signal INT given to the EOM 88 is a signal obtained by combining two intensity modulation signals generated from two signals of different frequencies.１ ~P Ｎ is a signal obtained by combining two phase-modulated signals generated from two signals of different frequencies. That is, the dual optical frequency comb FC2 is a signal obtained by combining an intensity-modulated signal INTa and a phase-modulated signal P ａ１ ~P ａＮ and a first optical frequency comb generated based on the intensity modulation signal INTb and the phase modulation signal P ｂ１ ~P ｂＮ and a second frequency comb generated based on the second frequency comb FC2, which allows the EOM 88 to output a dual optical frequency comb FC2.

[0097] Therefore, in this configuration, the mode spacing, i.e., the repetition frequency, of the dual optical frequency comb FC2 can be adjusted by adjusting the frequency difference between the two continuous wave laser beams.

[0098] In addition, when adjusting the difference in repetition frequency of the generated dual optical frequency comb, it is only necessary to adjust the frequency of the separate signal source used to generate the frequency difference, that is, the frequency of the oscillator 83 .

[0099] Seventh Embodiment In the above-described embodiments, a signal generating device has been described in which the first feedback loop unit including a Michelson interferometer configured as a fiber delay line interferometer suppresses frequency noise in the laser light output from the laser light sources 1 and 2. However, with this configuration, the noise that can be suppressed may be limited depending on the specifications of the laser light source.

[0100] For example, frequency noise can be suppressed by feeding back a feedback signal to the current driving the piezoelectric element or laser provided in the laser light source. In this case, frequency noise can only be suppressed within the bandwidth of the piezoelectric element or current modulation. However, the modulation bandwidth of the laser light source may be as narrow as about 10 kHz, and the dynamic range may also be limited. In this way, the frequency noise suppression effect is limited by the specifications of the laser light source.

[0101] Due to such limitations on frequency noise suppression, it is conceivable that frequency noise may not be sufficiently suppressed even if a feedback signal is provided to the laser light source by the first feedback loop. In contrast, in this embodiment, a signal generating device is described that further suppresses frequency noise by providing a feedback signal from a further feedback loop to the noise suppression means for laser light whose frequency noise has been suppressed by the laser light source.

[0102] 13 is a diagram schematically illustrating the configuration of a signal generating device according to a seventh embodiment. The signal generating device 700 according to the seventh embodiment further includes a second feedback loop unit including a Michelson interferometer in addition to the components of the signal generating device 100 according to the first embodiment. The signal generating device 700 has a first feedback loop unit 701 similar to the first feedback loop unit 101 of the signal generating device 100.

[0103] The signal generating device 700 is configured such that, compared to the signal generating device 100, a second feedback loop unit 702, optical couplers 711 to 714, and AOMs 715 and 716 are further provided.

[0104] In the signal generating device 700, the laser light whose frequency noise has been suppressed in the laser light sources 1 and 2 is frequency modulated by the AOMs 715 and 716 to which a feedback signal is given from the second feedback loop unit 702. This further suppresses the frequency noise (hereinafter also referred to as residual frequency noise) remaining in the laser light after the frequency noise has been suppressed in the laser light sources 1 and 2.

[0105] In the following description, the optical couplers 711 to 714 are also referred to as second to fifth optical couplers, respectively. The AOMs 715 and 716 are also referred to as first and second frequency shifters, respectively. ＣＷ１ and L ＣＷ２ By multiplexing / demultiplexing the first to third multiplexed lights L Ｍ１ ~L Ｍ３is referred to as an optical multiplexing / demultiplexing unit 720. The optical multiplexing / demultiplexing unit 720 in the signal generating device 700 replaces the optical coupler 3 in the signal generating device 100.

[0106] In the above embodiment, the first feedback loop unit 701 controls the laser light L output from the laser light sources 1 and 2. ＣＷ１ and L ＣＷ２ However, it is expected that frequency noise will remain in the laser light output from the laser light source. Therefore, in this configuration, as described above, the second feedback loop unit 702 further suppresses the residual frequency noise that remains in the laser light after frequency noise suppression. The configuration and operation of the signal generating device 700 will be specifically described below.

[0107] The optical couplers 711 and 712 are configured as one-input, two-output optical couplers. The optical coupler 713 is configured as a two-input, one-output optical coupler. The optical coupler 714 is configured as a two-input, two-output optical coupler.

[0108] The optical coupler 711 couples the laser light L output from the laser light source 1 ＣＷ１ The branched light L output to the AOM 715 Ｂ１ and the branched light L output to the optical coupler 713 Ｂ２ The optical coupler 712 splits the laser light L output from the laser light source 2 equally. ＣＷ２ The branched light L output to the AOM 716 Ｂ３ and the branched light L output to the optical coupler 713 Ｂ４ The branched light L Ｂ１ ~L Ｂ４ are also referred to as the first to fourth branched lights, respectively.

[0109] One input of the optical coupler 713 receives the branched light L from the optical coupler 711. Ｂ２ is input to the other input, and branched light L from the optical coupler 712 is input to the other input. Ｂ４ The optical coupler 713 receives the branched light L Ｂ２ and branched light L Ｂ４ The first multiplexed light L Ｍ１is output to the Michelson interferometer 20 of the first feedback loop unit 701.

[0110] The AOM 715 outputs the branched light L based on the feedback signal FB5 output from the second feedback loop unit 702. Ｂ１ In order to suppress the residual frequency noise of Ｂ１ Then, the AOM 715 shifts the frequency of the laser light L after frequency shifting. Ｓ１ is output to the optical coupler 714.

[0111] The AOM 716 outputs the branched light L based on the feedback signal FB6 output from the second feedback loop unit 702. Ｂ３ In order to suppress the residual frequency noise of Ｂ３ Then, the AOM 716 shifts the frequency of the laser light L after frequency shifting. Ｓ２ is output to the optical coupler 714.

[0112] The optical coupler 714 receives the laser light L from the AOM 715 at one input. Ｓ１ is input, and the other input receives laser light L from AOM716. Ｓ２ The optical coupler 714 receives the laser light L Ｓ１ and laser light L Ｓ２ The combined light is then converted into second combined light L Ｍ２ and the third combined light L Ｍ３ The optical coupler 714 splits the second combined light L Ｍ２ to the signal generating unit 10, and the third multiplexed light L Ｍ３ to the second feedback loop unit 702.

[0113] Next, the second feedback loop unit 702 will be described. The second feedback loop unit 702 has a similar configuration to the first feedback loop unit 701, and receives the third multiplexed light L Ｍ３ The noise components are extracted from the feedback signals FB5 and FB6 generated based on the extraction results and output to the AOMs 715 and 716, respectively.

[0114] The second feedback loop unit 702 has a Michelson interferometer 90, a signal source 91, an optical signal separation unit 92, a noise signal extraction unit 93, and an isolator 94. The signal source 91, the optical signal separation unit 92, the noise signal extraction unit 93, and the isolator 94 have the same configurations as the signal source 4, the optical signal separation unit 5, the noise signal extraction unit 6, and the isolator 7, respectively.

[0115] The Michelson interferometer 90 has the same configuration as the Michelson interferometer 20. That is, the optical coupler 901, FRMs 902 and 903, OFDL 904, and AOM 905 of the Michelson interferometer 90 correspond to the optical coupler 21, FRMs 22 and 23, OFDL 24, and AOM 25 of the Michelson interferometer 20, respectively.

[0116] The optical coupler 901 receives the third multiplexed light L via an isolator 94. Ｍ３ The optical coupler 901 receives the third multiplexed light L Ｍ３ The FRM 902 splits the incident third multiplexed light L Ｍ３ is reflected to the optical coupler 901.

[0117] The third combined light L incident on the OFLD 904 Ｍ３ is delayed and then enters the AOM 905. The AOM 905 receives the third multiplexed light L Ｍ３ modulated light L based on the signal S91 ＭＯＤ２ The FRM 903 outputs the modulated light L ＭＯＤ２ The modulated light L is reflected to the AOM 905. ＭＯＤ２ passes through the AOM 905 and OFLD 904 and returns to the optical coupler 901 .

[0118] The third multiplexed light L returned from the FRM 902 Ｍ３ and modulated light L returned from the delay path consisting of FRM 903, OFDL 904, and AOM 905. ＭＯＤ２ When these light beams are combined by the optical coupler 901, they interfere with each other to generate interference light LL. The interference light LL becomes an optical signal having a carrier frequency that is twice the modulation frequency of the AOM 905. The optical coupler 901 outputs the interference light LL generated by the interference to the optical signal separation unit 92.

[0119] The signal source 91 has the same configuration as the signal source 4. That is, the oscillator 91A, amplifier 91B, and frequency doubler 91C of the signal source 91 correspond to the oscillator 4A, amplifier 4B, and frequency doubler 4C of the signal source 4, respectively. Furthermore, the signal S91 output by the oscillator 91A and the signal S92 output by the frequency doubler 91C correspond to the signal S1 output by the oscillator 4A and the signal S2 output by the frequency doubler 4C, respectively. The operation of the signal source 91 is the same as that of the signal source 4, so a duplicated description will be omitted.

[0120] The optical signal separating unit 92 has the same configuration as the optical signal separating unit 5. That is, the circulator 92A, the FBG 92B, the light receiving elements 92C and 92D, and the TIAs 92E and 92F of the optical signal separating unit 92 correspond to the circulator 5A, the FBG 5B, the light receiving elements 5C and 5D, and the TIAs 5E and 5F of the optical signal separating unit 5, respectively. ９１ and L ９２ , current signals C91 and C92, and detection signals D91 and D92 correspond to components L1 and L2, current signals C1 and C2, and detection signals D1 and D2, respectively. The operation of the optical signal separation unit 92 is similar to that of the optical signal separation unit 5, so a duplicated explanation will be omitted.

[0121] The noise signal extraction unit 93 has a configuration in which two voltage controlled oscillators (VCOs) and two amplifiers are added to the noise signal extraction unit 6. Frequency mixers 93A and 93B and loop filters 93C and 93D of the noise signal extraction unit 93 correspond to the frequency mixers 6A and 6B and loop filters 6C and 6D of the noise signal extraction unit 6, respectively. For simplicity of the drawing, the loop filters are denoted as LF in FIG. 13 .

[0122] The frequency mixer 93A outputs the resulting signal to the loop filter 93C as a residual frequency noise signal RN1. The loop filter 93C smooths the residual frequency noise signal RN1 to generate a residual frequency noise signal RN10, which is a DC signal, and outputs the resulting signal to the VCO 93E. The VCO 93E outputs a feedback signal FB5 having a frequency corresponding to the voltage value of the residual frequency noise signal RN10 to the amplifier 93G. The amplifier 93G amplifies the feedback signal FB5 to a predetermined intensity and outputs the amplified feedback signal FB5 to the AOM 715.

[0123] The frequency mixer 93B outputs the resulting signal to the loop filter 93D as the residual frequency noise signal RN2. The loop filter 93D smooths the residual frequency noise signal RN2 to generate a residual frequency noise signal RN20, which is a DC signal, and outputs the resulting signal to the VCO 93F. The VCO 93F outputs a feedback signal FB6 having a frequency corresponding to the voltage value of the residual frequency noise signal RN20 to the amplifier 93H. The amplifier 93H amplifies the feedback signal FB6 to a predetermined intensity and outputs the amplified feedback signal FB6 to the AOM 716.

[0124] The isolator 94 is similar to the isolator 7, and therefore a duplicated description will be omitted.

[0125] As described above, in this configuration, the second feedback loop unit 702 controls the laser light L after frequency noise suppression. ＣＷ１ and L ＣＷ２ The AOMs 715 and 716 detect the residual frequency noise of the branched light L after frequency noise suppression in response to the feedback signals FB5 and FB6. Ｂ１ and L Ｂ３ (i.e., laser light L ＣＷ１ and L ＣＷ２ ) is shifted in frequency. As a result, the frequency-shifted laser light L Ｓ１ and L Ｓ２ (i.e., laser light L ＣＷ１ and L ＣＷ２ ) becomes laser light in which residual frequency noise that could not be sufficiently suppressed by the first feedback loop unit 701 is also suppressed.

[0126] This configuration also makes it possible to adjust the noise bandwidth that can be suppressed. Therefore, by appropriately setting the bandwidth of the noise suppressed by the AOMs 715 and 716, the residual frequency noise of the laser light can be effectively suppressed. For example, even if the bandwidth of the frequency noise suppression by the laser light sources 1 and 2 is about 10 kHz, the bandwidth of the frequency noise suppression by the AOMs 715 and 716 can be set to, for example, about 500 kHz. In this case, the residual frequency noise of the laser light after the frequency noise suppression by the laser light sources 1 and 2 can be suppressed over a wide bandwidth. Furthermore, the AOMs 715 and 716 can also accommodate a dynamic range that cannot be handled by the laser light sources 1 and 2.

[0127] Generally, according to feedback control theory, the amount of noise that can be suppressed corresponds to the feedback loop gain. For example, DFB (Distributed Feedback) laser diodes are known to have a large amount of frequency noise. Therefore, when using a DFB laser diode, a large feedback loop gain is required. In contrast, with this configuration, two feedback loops are used to achieve a large feedback loop gain. Therefore, even when a DFB laser diode is used as the laser light source, frequency noise can be suppressed to a desired level.

[0128] Therefore, according to this configuration, the laser light L with the residual frequency noise suppressed can be obtained. Ｓ１ and L Ｓ２ (i.e., laser light L ＣＷ１ and L ＣＷ２ ) into a second combined light L Ｍ２ to the signal generating section 10, the signal generating section 10 can output an output signal OUT of higher quality than in the above-described embodiment.

[0129] Other Embodiments The present disclosure has been described above with reference to the embodiments, but the present disclosure is not limited to the above-described embodiments. Various modifications that can be understood by those skilled in the art can be made to the configuration and details of the present disclosure within the scope of the present disclosure. Furthermore, each embodiment can be combined with other embodiments as appropriate.

[0130] In the above description, the laser light output from the laser light sources 1 and 2 is input to the optical coupler 3, but this is merely an example. For example, an optical coupler may be provided on one or both of the paths between the laser light source 1 and the optical coupler 3 and between the laser light source 2 and the optical coupler 3, and the laser light L ＣＷ１ and L ＣＷ２ In this way, the signal generator can supply not only the output signal OUT, which is a signal, but also continuous wave laser light. Also, the optical frequency comb generator can supply not only the output signal OUT and the optical frequency comb, which are signals, but also continuous wave laser light.

[0131] Although the signal generating device according to the fourth embodiment has been described as a modified example of the signal generating device 100 according to the first embodiment, this is merely an example. For example, the signal generating devices according to the second and third embodiments may be configured to suppress phase noise of the output signal over a wide band by providing a frequency mixer 61, a loop filter 62, an oscillator 63, and a branching filter 64 in the output path of the output signal OUT, as in the fourth embodiment.

[0132] The optical frequency comb generators according to the fifth and sixth embodiments have been described as using the signal generator 100 according to the first embodiment, but this is merely an example. For example, instead of the signal generator 100, an optical frequency comb generator may be configured using a signal generator other than the signal generator 100 described in the above-mentioned embodiments. In addition, the comb generator may include a laser beam L branched by the optical coupler 8. ＣＷ１ The comb generator receives the laser light L branched by an optical coupler inserted between the laser light source 2 and the optical coupler 3. ＣＷ２ is input, and the comb generator generates laser light LＣＷ２ may be used to generate an optical frequency comb.

[0133] Although the signal generating device 700 according to the seventh embodiment has been described as a modified example of the signal generating device 100 according to the first embodiment, this is merely an example. Similarly to the signal generating device 700, the signal generating devices according to the second to sixth embodiments may also be provided with a second feedback loop unit to suppress residual frequency noise.

[0134] In the signal generating device 700 according to the seventh embodiment, the signal generating unit 10 may be replaced with the signal generating unit 30, and a fiber stretcher similar to the fiber stretcher 41 of the Michelson interferometer 40 may be provided in the Michelson interferometer 90. In this way, in the signal generating device according to the seventh embodiment, the detection accuracy of the residual frequency noise can be maintained by adjusting the length of the delay path of the laser light in the Michelson interferometer of the second feedback loop unit.

[0135] In the signal generating device 700 according to the seventh embodiment, the signal generating unit 10 may be replaced with a signal generating unit 50, and a fiber stretcher similar to the fiber stretcher 41 of the Michelson interferometer 40 may be provided in the Michelson interferometer 90. As a result, in the signal generating device according to the seventh embodiment, as in the signal generating device 300, the frequency of the output signal can be precisely adjusted with an accuracy higher than the adjustment resolution of the frequency of the laser light determined by the optical fiber delay line.

[0136] In the signal generating device 700 according to the seventh embodiment, the signal generating section 10 may be replaced with a signal generating section 60. As a result, the signal generating device according to the seventh embodiment can also effectively suppress phase noise in the output signal over a wide band, similar to the signal generating device 400.

[0137] An optical frequency comb generator may be configured using the signal generating device 700 according to the seventh embodiment, as in the fifth embodiment. In this case, an optical coupler 8 may be inserted between the AOM 715 and the optical coupler 714. This suppresses residual frequency noise, thereby realizing an optical frequency comb generator that outputs an optical frequency comb with higher precision.

[0138] The signal generating device 700 according to the seventh embodiment may be used to configure an optical frequency comb generator that generates a dual optical frequency comb, as in the sixth embodiment. In this case, an optical coupler 8 may be inserted between the AOM 715 and the optical coupler 714. This suppresses residual frequency noise, thereby realizing an optical frequency comb generator that outputs a higher-precision dual optical frequency comb.

[0139] In the above-described embodiment, an example has been described in which an optical coupler is used as the optical multiplexing / demultiplexing means, but this is merely an example. For example, other optical elements such as a Y-branch or a directional coupler may be used as the optical multiplexing / demultiplexing means. Furthermore, the optical multiplexing / demultiplexing means may be configured by combining the same or multiple types of optical elements.

[0140] Each drawing is merely an example for describing one or more embodiments. Each drawing may not relate to only one particular embodiment, but may also relate to one or more other embodiments. As will be understood by those skilled in the art, various features or steps described with reference to any one drawing can be combined with features or steps shown in one or more other drawings to create, for example, an embodiment not explicitly shown or described. Not all features or steps shown in any one drawing are necessary to describe an exemplary embodiment, and some features or steps may be omitted. The order of steps described in any drawing may be changed as appropriate.

[0141] This application claims priority based on Japanese Patent Application No. 2023-221357, filed December 27, 2023, the disclosure of which is incorporated herein by reference in its entirety.

[0142] 1, 2 Laser light source 3, 8, 21, 711 to 714, 901 Optical coupler 4, 91 Signal source 4A, 33, 51, 63, 83, 91A Oscillator 4B, 36, 54, 91B, 72C, 853, 863 Amplifier 4C, 91C Frequency doubler 5, 92 Optical signal separation unit 5A, 92A Circulator 5B, 92B Fiber optic grating (FBG) 5C, 5D, 92C, 92D Photodetector 5E, 5F, 12, 92E, 92F Transimpedance amplifier (TIA) 6, 93 Noise signal extraction unit 6A, 6B, 93A, 93B Frequency mixer 6C, 6D, 35, 62, 93C, 93D Loop filter 7, 94 Isolator 10, 30, 50, 60 Signal generating section 11 Light receiving element 20, 40, 90 Michelson interferometer 22, 23, 902, 903 Faraday rotator mirror (FRM) 24, 904 Optical fiber delay line (OFDL) 25, 715, 716, 905 Acousto-optic modulator (AOM) 31, 71, 81, 72A, 851, 861 Distributor 32, 52, 61, 82 Frequency mixer 34 Frequency multiplier 41 Fiber stretcher 53, 84 Filter 64 Demultiplexer 70, 80 Comb generating section 72, 85, 86 Intensity phase adjusting section 73, 88 EOM 74, 89 Bias generating section 87 Power combiner 93E, 93F VCO 93G, 94H Amplifier 100, 200, 300, 400, 700 Signal generator 101, 701 First feedback loop unit 702 Second feedback loop unit 720 Optical multiplexer / demultiplexer unit 72B, 852, 862 Attenuator 73A Intensity modulation unit 864 Phase shifter 1000, 2000 Optical frequency comb generator C1, C2, C91, C92 Current signal C Ｄ Current signals D1, D2, D91, D92 Detection signals FB1 to FB6 Feedback signals L, LL Interference light L ＣＷ１ , L ＣＷ２、 L Ｓ１ , L Ｓ２ Laser light L Ｍ１ ~L Ｍ３ 1st to 3rd combined light L ＭＯＤ , L ＭＯＤ２Modulated light N1, N2 Frequency noise signal OUT Output signal RN1, RN2, RN10, RN20 Residual noise signal S1 to S8, S10, S91, S92 Signal SN Phase noise signal SP Phase fluctuation signal SR Reference signal VB Bias voltage

Claims

1. A signal generation device comprising: a first light source that outputs first laser light of a first frequency; a second light source that outputs second laser light of a second frequency; a wavelength division multiplexer that outputs first and second combined light beams obtained by multiplexing the first laser light and the second laser light; a signal generation unit that detects the second combined light beam and outputs an output signal that is a beat note signal of the first and second laser lights; and a first feedback loop unit that detects frequency noise of the first and second laser lights based on the first combined light beam and outputs first and second feedback signals based on the detection result. The first feedback loop unit includes: a first signal source that outputs at least a signal of a first predetermined frequency; a first interferometer into which the first combined light beam is input, which branches the first combined light beam, propagates one of the branched lights and the other of the branched lights through a delay path of a predetermined length to delay them, and interferes the light modulated according to the signal of the first predetermined frequency to output interference light; a first signal separation unit that outputs a first detection signal obtained by detecting the light of the first frequency included in the interference light and a second detection signal obtained by detecting the light of the second frequency; and a first noise signal extraction unit that outputs first and second feedback signals indicating the frequency noise of the first and second laser lights, obtained by removing a component of a frequency twice the first predetermined frequency from the first and second detection signals, to the first and second light sources respectively. The first and second light sources suppress the frequency noise of the first and second laser lights based on the first and second feedback signals.

2. The signal generation device according to claim 1, wherein the signal generation unit includes: a light receiving element that receives the second combined light beam and outputs a current signal; and a current-voltage converter that converts the current signal into a voltage signal and outputs the voltage signal as the output signal.

3. The first interferometer includes a delay path length compensation means for adjusting the length of the delay path according to a third feedback signal, which is provided to compensate for the change in the length of the delay path over time. The signal generation unit includes: a reference signal generation means for generating a reference signal having a target frequency of the output signal; a first frequency mixer for mixing the output signal and the reference signal and outputting a phase fluctuation signal indicating the phase fluctuation of the output signal with the component of the reference signal removed from the output signal; and a signal output means for outputting the third feedback signal corresponding to the phase fluctuation signal to the delay path length compensation means. The signal generation device according to claim 2.

4. The first interferometer includes a delay path length compensation means for adjusting the length of the delay path according to a third feedback signal, which is provided to compensate for the change in the length of the delay path over time. The signal generation unit includes: a light receiving element for receiving the second combined light and outputting a current signal; a current-voltage converter for converting the current signal into a voltage signal and outputting the voltage signal; a reference signal generation means for generating a reference signal having a target frequency of the output signal; a first frequency mixer for mixing the voltage signal and the reference signal and outputting a phase fluctuation signal indicating the phase fluctuation of the voltage signal with the component of the reference signal removed from the voltage signal; a signal output means for outputting the third feedback signal corresponding to the phase fluctuation signal to the delay path length compensation means; a signal generation means for generating a signal having a lower frequency than the voltage signal; and a second frequency mixer for mixing the signal generated by the signal generation means and the voltage signal and outputting, as the output signal, a signal having a frequency equal to the difference between the frequency of the signal generated by the signal generation means and the frequency of the voltage signal. The signal generation device according to claim 1.

5. The signal generation unit includes: a light receiving element that receives the second combined optical signal and outputs a current signal; a current-voltage converter that converts the current signal into a voltage signal and outputs the voltage signal; a third frequency mixer that mixes the voltage signal and the output signal and outputs a phase noise signal having a frequency equal to the difference in frequency between the voltage signal and the output signal; signal output means that outputs a fourth feedback signal according to the phase noise signal; signal generation means that outputs, as the output signal, a signal with phase noise suppressed according to the fourth feedback signal; and a distributor that distributes the output signal to the output target of the output signal and the third frequency mixer. The signal generation apparatus according to claim 1.

6. The optical multiplexer / demultiplexer is configured as a first optical coupler that branches the combined optical signal obtained by multiplexing the first laser beam and the second laser beam into the first and second combined optical signals, outputs the first combined optical signal to the first feedback loop unit, and outputs the second combined optical signal to the signal generation unit. The signal generation apparatus according to claim 1 or 2.

7. The optical multiplexing and demultiplexing unit includes: A second optical coupler that branches the first laser light output from the first light source into first and second branched lights; A third optical coupler that branches the second laser light output from the second light source into third and fourth branched lights; A fourth optical coupler that outputs the first combined light obtained by combining the second branched light and the fourth branched light to the first feedback loop unit; A first frequency shifter that shifts the frequency of the first branched light according to a fifth feedback signal and outputs the first branched light after frequency shifting; A second frequency shifter that shifts the frequency of the third branched light according to a sixth feedback signal and outputs the third branched light after frequency shifting; A fifth optical coupler that branches the combined light obtained by combining the first branched light output from the first frequency shifter and the third branched light output from the second frequency shifter into second combined light and third combined light; The signal generator further includes a second feedback loop unit that detects the noise of the first branched light output from the first frequency shifter and the noise of the third branched light output from the second frequency shifter based on the third combined light, and outputs the fifth and sixth feedback signals based on the detection result. The second feedback loop unit includes: A second signal source that outputs at least a signal of a second predetermined frequency; A second interferometer that receives the third combined light, branches the third combined light, propagates one of the branched lights and the other branched light through a delay path of a predetermined length to delay them, and interferes the light modulated according to the signal of the second predetermined frequency to output interference light; A second signal separation unit that outputs a third detection signal obtained by detecting the light of the first frequency included in the interference light and a fourth detection signal obtained by detecting the light of the second frequency; A second noise signal extraction unit that outputs a fifth feedback signal indicating the noise of the light output from the first frequency shifter and a sixth feedback signal indicating the noise of the light output from the second frequency shifter, which are obtained by removing a component of a frequency twice the second predetermined frequency from the third and fourth detection signals, to the first and second frequency shifters respectively. The signal generator according to claim 1 or 2.

8. A signal generation device according to claim 1 or 2, a sixth optical coupler that branches and outputs a part of the first laser light from the first light source or a part of the second laser light from the second light source, a bias generation unit that outputs a bias voltage, an intensity-phase adjustment unit that outputs an intensity-modulated signal obtained by adjusting the intensity of the output signal and a plurality of phase-modulated signals generated by giving different phase shift amounts to the output signal, and an electro-optic modulator that intensity-modulates the first laser light or the second laser light branched by the sixth optical coupler according to the bias voltage and the intensity-modulated signal, and outputs an optical frequency comb generated by phase-modulating the intensity-modulated light according to the plurality of phase-modulated signals. An optical frequency comb generation device.

9. A signal generation device according to claim 1 or 2, a sixth optical coupler that branches and outputs a part of the first laser light from the first light source or a part of the second laser light from the second light source, a bias generation unit that outputs a bias voltage, a frequency change unit that outputs a signal obtained by shifting the frequency of the output signal by a predetermined frequency, an intensity-phase adjustment unit that outputs a first intensity-modulated signal obtained by adjusting the intensity of the output signal, a plurality of first phase-modulated signals generated by giving different phase shift amounts to the output signal, a second intensity-modulated signal obtained by adjusting the intensity of the signal after giving a predetermined phase shift amount to the signal obtained by shifting the frequency of the output signal by a predetermined frequency, and a plurality of second phase-modulated signals generated by giving different phase shift amounts to the signal after giving the predetermined phase shift amount to the signal obtained by shifting the frequency of the output signal by a predetermined frequency, and an electro-optic modulator that intensity-modulates the first laser light or the second laser light branched by the sixth optical coupler according to the bias voltage and the first intensity-modulated signal, and outputs a first optical frequency comb generated by phase-modulating the intensity-modulated light according to the plurality of first phase-modulated signals, and intensity-modulates the first laser light or the second laser light branched by the sixth optical coupler according to the bias voltage and the second intensity-modulated signal, and outputs a second optical frequency comb generated by phase-modulating the intensity-modulated light according to the plurality of second phase-modulated signals. An optical frequency comb generation device.

10. A signal generation method, which combines the first laser beam of the first frequency output from the first light source and the second laser beam of the second frequency output from the second light source, branches the combined light into first and second combined light beams and outputs them, outputs at least a signal of a first predetermined frequency, detects the second combined light beam, outputs an output signal which is a beat note signal of the first and second laser beams, branches the first combined light beam, delays one of the branched lights and the other of the branched lights by propagating them through a delay path of a predetermined length, and interferes the light modulated according to the signal of the first predetermined frequency to output interference light, outputs a first detection signal obtained by detecting the light of the first frequency included in the interference light and a second detection signal obtained by detecting the light of the second frequency, outputs first and second feedback signals indicating the frequency noises of the first and second laser beams, which are obtained by removing a component of a frequency twice the first predetermined frequency from the first and second detection signals, to the first and second light sources respectively, and the first and second light sources suppress the frequency noises of the first and second laser beams based on the first and second feedback signals.

Images