CN107727367A - A kind of laser frequency noise measuring method and system - Google Patents

Abstract

The invention discloses a laser frequency noise measurement method and system, belonging to the field of optical measurement. In this method, the laser to be tested is used as the light source of the optoelectronic hybrid oscillator constructed based on the optical comb modulator, and then the phase noise of the radio frequency oscillation signal in the optoelectronic hybrid oscillator is measured; the frequency noise of the laser to be tested is obtained according to the phase noise; the optoelectronic The hybrid oscillator is a photoelectric hybrid oscillator with a double ring structure. This scheme indirectly measures the frequency noise of the laser by measuring the phase noise of the radio frequency oscillation signal, so it can distinguish the frequency noise and intensity noise of the laser, and has extremely high measurement sensitivity.

Description

A laser frequency noise measurement method and system

technical field

The invention relates to a laser frequency noise measurement method and system. By constructing an optoelectronic hybrid oscillator based on an optical comb modulator, the laser to be tested is used as the light source of the optoelectronic oscillator. During the formation of oscillation, the frequency noise of the laser to be tested will be transferred to the radio frequency oscillation signal of the optoelectronic oscillator. The frequency noise of the laser to be tested can be obtained by measuring the phase noise of the radio frequency oscillation signal, which belongs to the field of optical measurement.

Background technique

Single-frequency, narrow-linewidth lasers are key components of many application systems, including lidar, coherent optical communication systems, high-precision optical sensing, and high-stability optical atomic clocks. In general, the linewidth of a laser is measured by measuring the power spectral density of its frequency noise. In recent years, with the development of narrow linewidth lasers, it is more and more important to measure the frequency noise of narrow linewidth lasers with high sensitivity. The traditional schemes for measuring laser frequency noise mainly include four categories: the first category is to use the time-delayed Selfie method; the second category is to transfer the frequency noise of the laser into laser power jitter based on the Mach-Zehnder modulator, so as to obtain the measured laser frequency noise. The third type is to use an optical resonator to convert the frequency noise of the laser into optical power jitter, and then measure the frequency noise of the laser; the fourth type is to use a narrow linewidth laser with extremely low frequency noise as a reference source. Beat the frequency of the laser under test and the reference laser, and then obtain the frequency noise of the laser under test.

The following are some existing laser frequency noise measurement techniques and schemes:

Scheme 1 is the measurement scheme described in the document D. Derickson, Fiber Optic Test and Measurement (Prentice-Hall, 1998). This scheme uses a delay interferometer to delay the signal of the laser to be tested. After the delay, the de-correlation operation of the delayed signal and the undelayed signal is achieved, and finally the optical signal directly output by the laser to be tested is photographed. Frequency, so as to achieve the purpose of measuring the laser frequency noise.

Scheme 2 is a public patent applied by Zhejiang University, and the publication number is CN 102183362. The scheme uses the Mach-Zehnder interferometer to complete the conversion of the frequency noise of the laser to the light intensity, and achieves the purpose of measuring the frequency noise of the laser.

Scheme 3 is a public patent applied by Zhejiang University, and the publication number is CN 102692314A. This scheme uses a fiber resonant cavity to convert the frequency noise of the laser into the amplitude fluctuation of the optical field, thus achieving the purpose of measuring the frequency noise of the laser.

Scheme 4 is a scheme proposed by the Paris Observatory in 2017 to characterize the frequency noise of narrow linewidth lasers (X.Xie, R.Bouchand, D.Nicolodi, M.Lours, C.Alexandre, and Y.L.Coq, "Phase noisecharacterization of sub- hertz linewidth lasers via digital crosscorrelation," Opt. Lett. 42(7), 1217-1220(2017).). This scheme uses two single-frequency lasers with extremely narrow linewidth and extremely low frequency noise as references, beats the laser to be tested with the two reference lasers, and measures the phase noise of the beat frequency electrical signal to achieve indirect measurement. The purpose of measuring the frequency noise of the laser.

The method of measuring laser frequency noise based on a delay interferometer requires an optical fiber with a length of several kilometers or even tens of kilometers as a delay line when measuring an extremely narrow linewidth laser. Due to the introduction of a long optical fiber, it causes a huge optical loss. And the introduction of long optical fiber brings inevitable scattering noise, which reduces the sensitivity of the frequency noise measurement system, and this method does not distinguish the frequency noise and intensity noise of the laser; the scheme based on the Mach-Zehnder modulator is limited by The low frequency noise to intensity noise conversion coefficient of the modulator makes it difficult to achieve higher sensitivity laser frequency noise measurement; the fiber resonator scheme is used, the measurement system is easily disturbed by the environment, which reduces the stability of laser frequency noise measurement ; The solution of using a reference laser with low frequency noise and high stability is expensive to implement, and it is difficult to measure the frequency noise of a laser with any wavelength.

Contents of the invention

In view of the technical problems existing in the prior art solutions, the object of the present invention is to provide a method and system for measuring laser frequency noise. The invention is applicable to frequency noise measurement of lasers with arbitrary wavelength, narrow line width and extremely low frequency noise, and is especially suitable for measuring frequency noise of lasers with narrow line width.

Technical scheme of the present invention is:

A laser frequency noise measurement method is characterized in that, the laser to be tested is used as the light source of the optoelectronic hybrid oscillator built based on the optical comb modulator, and then the phase noise of the radio frequency oscillation signal in the optoelectronic hybrid oscillator is measured; according to the phase Noise is the frequency noise of the laser under test.

Further, the optoelectronic hybrid oscillator is a double-ring structure optoelectronic hybrid oscillator, which includes an optical comb modulator, the output end of the optical comb modulator is connected to an optical coupler through a fiber amplifier, and the optical coupler One output end of the optical coupler is connected to a section of the first single-mode optical fiber, the output signal of the first single-mode optical fiber is converted into an electrical signal by a first photodetector and input to the microwave power combiner; the other output end of the optical coupler is connected to a section of the first Two single-mode optical fibers, the output signal of the second single-mode optical fiber is converted into an electrical signal by a second photodetector and input to the microwave power combiner; the output end of the microwave power coupler is sequentially passed through the microwave phase shifter, the first low The phase noise amplifier and the electric bandpass filter are input into the microwave directional coupler; one output end of the microwave directional coupler is used to connect the phase noise tester, and the other output end is connected to the AC voltage input port of the DC bias device; the DC The output port of the bias is connected with the driving port of the optical comb modulator.

Further, a second low phase noise amplifier and a third low phase noise amplifier are provided between the electrical bandpass filter and the microwave directional coupler; the fiber length of the first single-mode optical fiber is the second Single-mode fiber is more than 10 times the fiber length.

Further, the phase noise gets the frequency noise of the laser to be tested Among them, β is the modulation index of the optical comb modulator, FSR is the free spectral range of the Fabry-Perot cavity in the optical comb modulator, H(f) is the transfer function of the optoelectronic hybrid oscillator, The additional phase noise introduced by various optoelectronic devices in the optoelectronic hybrid oscillator, the additional phase noise introduced by the frequency noise of the laser to be tested on the oscillation signal of the optoelectronic hybrid oscillator

Further, the photoelectric hybrid oscillator is a polarized double-ring structure photoelectric hybrid oscillator, which includes an optical comb modulator, the output end of the optical comb modulator is connected to a polarization beam splitter through a fiber amplifier, and the polarization An output end of the beam splitter is connected to a section of the first polarization-maintaining optical fiber, and the output signal of the first polarization-maintaining optical fiber is converted into an electrical signal by a first photodetector and input into the polarization beam combiner; the other output of the polarization beam splitter The end is connected to a second polarization-maintaining optical fiber, and the output signal of the second polarization-maintaining optical fiber is converted into an electrical signal by a second photodetector and input to the polarization beam combiner; the output end of the polarization beam combiner is sequentially passed through a microwave phase shifter , the first low-phase-noise amplifier, the electric band-pass filter, and then enter the microwave directional coupler; one output end of the microwave directional coupler is used to connect the phase noise tester, and the other output end is connected to the AC voltage input of the DC bias device port; the output port of the DC bias is connected to the drive port of the optical comb modulator.

Further, a second low phase noise amplifier and a third low phase noise amplifier are provided between the electrical bandpass filter and the microwave directional coupler.

Further, the fiber length of the first polarization-maintaining fiber is more than 10 times the fiber length of the second polarization-maintaining fiber.

A laser frequency noise measurement system is characterized in that it includes an optical comb modulator, the optical comb modulator is used to modulate the laser output from the laser to be tested, and the output end of the optical comb modulator is coupled with an optical fiber amplifier through an optical fiber amplifier The optical coupler is connected, and an output end of the optical coupler is connected to a section of the first single-mode optical fiber, and the output signal of the first single-mode optical fiber is converted into an electrical signal by a first photodetector and input to the microwave power combiner; the optical coupler's The other output end is connected to a section of second single-mode optical fiber, the output signal of the second single-mode optical fiber is converted into an electrical signal by a second photodetector and input to the microwave power combiner; the output end of the microwave power coupler is sequentially passed through the microwave The phase shifter, the first low phase noise amplifier, and the electric bandpass filter are input to the microwave directional coupler; one output end of the microwave directional coupler is used to connect the phase noise tester, and the other output end is connected to the DC bias device. an AC voltage input port; the output port of the DC biaser is connected to the drive port of the optical comb modulator.

A laser frequency noise measurement system is characterized in that it includes an optical comb modulator, the optical comb modulator is used to modulate the laser output from the laser to be measured, and the output end of the optical comb modulator is connected to a polarization splitter via an optical fiber amplifier. The beam splitter is connected, and an output end of the polarization beam splitter is connected to a section of the first polarization-maintaining fiber, and the output signal of the first polarization-maintaining fiber is converted into an electrical signal by a first photodetector and input into the polarization beam combiner; the polarization splitter The other output end of the beamer is connected to a second polarization-maintaining optical fiber, and the output signal of the second polarization-maintaining optical fiber is converted into an electrical signal by a second photodetector and input to the polarization beam combiner; the output end of the polarization beam combiner After passing through the microwave phase shifter, the first low phase noise amplifier, and the electric bandpass filter in sequence, it is input into the microwave directional coupler; one output end of the microwave directional coupler is used to connect the phase noise tester, and the other output end is connected to the DC bias The AC voltage input port of the biaser; the output port of the DC biaser is connected with the drive port of the optical comb modulator.

Further, a second low phase noise amplifier and a third low phase noise amplifier are provided between the electrical bandpass filter and the microwave directional coupler.

This scheme is based on the photoelectric hybrid oscillator of the optical comb modulator to realize the frequency noise measurement method of the laser to be tested. The invention transfers the frequency noise of the laser to be measured to the phase noise of the radio frequency oscillation signal in the optoelectronic hybrid oscillator by using the laser to be tested as the light source of the optoelectronic hybrid oscillator constructed based on the optical comb modulator, and then measures the frequency noise of the radio frequency oscillation The phase noise of the signal can be obtained from the frequency noise of the laser under test. The measurement sensitivity of the system is only limited by the shot noise of the photodetector in the photoelectric hybrid oscillator, the spontaneous emission noise of the erbium-doped fiber amplifier, and the thermal noise and flicker noise of the electrical amplifier, etc., which can achieve extremely high laser frequency noise measurement sensitivity.

Compared with prior art, positive effect of the present invention:

1. This solution uses an optoelectronic hybrid oscillator based on an optical comb modulator with a high-precision Fabry-Perot cavity to realize the conversion of the frequency noise of the narrow linewidth laser to be measured to the phase noise of the oscillation signal. Compared with the method of delay interferometer, it does not need a longer optical fiber, avoids the problem of decreased sensitivity of frequency noise measurement caused by long optical fiber transmission loss, scattering noise, etc., and can measure the frequency noise of narrower linewidth lasers. In addition, this scheme indirectly measures the frequency noise of the laser by measuring the phase noise of the radio frequency oscillation signal, so it can distinguish the frequency noise and intensity noise of the laser.

2. This solution uses a high-precision Fabry-Perot cavity optical comb modulator to realize the conversion of the frequency noise of the laser to the phase noise of the radio frequency oscillation signal in the photoelectric hybrid oscillator. The measurement sensitivity of the system is only limited by The device noise in the photoelectric oscillation loop has extremely high measurement sensitivity.

3. This solution does not need to use a laser with extremely narrow line width and extremely low frequency noise as a reference source, which reduces the complexity and cost of the system. At the same time, the method can measure the frequency noise of lasers with arbitrary wavelengths.

Description of drawings

Fig. 1 is the scheme schematic diagram of the present invention;

Fig. 2 is the experimental result figure of the scheme of the present invention;

(a) is A point spectrum result figure among Fig. 1;

(b) is the electric spectrum result figure of point B in Fig. 1;

(c) is the phase noise result figure of point B in Fig. 1;

(d) is a comparison chart of the laser frequency noise measurement results of this scheme and the commercial frequency noise tester.

detailed description

The solutions of the present invention will be further described in detail below in conjunction with the accompanying drawings.

The scheme principle of the present invention is shown in Figure 1. The laser to be tested outputs continuous wave laser and injects it into the optical comb modulator. The optical comb modulator is composed of high-bandwidth electro-optical phase modulator coated with high-reflection film at both ends. The high-reflection film at both ends forms a high-definition degree Fabry-Perot cavity. Since the optical comb modulator has a certain optical loss, it is necessary to use an erbium-doped fiber amplifier to amplify the optical signal output by the optical comb modulator. The amplified optical signal is divided into two beams by a 50% optical coupler, and the two beams are sent into standard single-mode optical fibers of different lengths. Two sections of standard single-mode optical fibers are used to form an optical-electrical hybrid oscillation ring with a double-ring structure. In order to suppress the sub-oscillation mode of the oscillating signal in the photoelectric hybrid oscillator, the greater the length difference between the two sections of optical fiber, the more effective the sub-oscillation mode can be suppressed. Generally, the length of the longest section of optical fiber does not exceed 10 km, and the difference in the length of the two sections of optical fiber is generally more than 10 times. The two photodetectors respectively convert the optical signal transmitted by the standard single-mode optical fiber into an electrical signal, and the electrical signals output by the two photodetectors are combined by a 50% microwave power combiner. In addition to using a fiber coupler to split the light into two beams to form a double-ring structure of an optoelectronic hybrid oscillator, there is also a polarization double-ring structure, that is, using a fiber optic polarization beam splitter to split the incident light into two polarization directions, and the light in the two polarization directions After passing through two sections of polarization-maintaining optical fiber with different lengths, one optical signal is synthesized by a polarization beam combiner, and two signals can be detected simultaneously by a photodetector, thereby realizing a double-ring structure photoelectric hybrid oscillator. The structure can be seen Literature J.Wang, Y.Long, W.Tian, Z.Tai, and Y.Ze, "An Optical Domain Combined Dual-Loop Optoelectronic Oscillator," IEEE Photonics Technology Letters, 19(11), 807-809, (2007) . The microwave phase shifter at the back end of the 50% microwave power combiner can phase-shift the microwave signal output by the 50% microwave power combiner. Since the power of the electric signal output by the photodetector is small, three low phase noise amplifiers are cascaded to amplify it. The low phase noise amplifier has extremely low flicker noise, which can reduce the influence of the electric amplifier on the measurement sensitivity of laser frequency noise. Since the Fabry-Perot cavity in the optical comb modulator has multiple transmission peaks, which will form multiple modes of oscillation in the photoelectric hybrid oscillation loop, a narrow-band electric bandpass filter is added in the loop Suppression of other oscillation modes. The output signal of the low phase noise amplifier 3 enters a microwave directional coupler, and the microwave directional coupler has two output ports, wherein most of the microwave power is used for the AC input of the DC bias device, and the microwave directional coupler splits a A small part of the microwave power is used to measure its phase noise, and the measurement of phase noise can be realized by a commercial phase noise tester. The DC bias has two input ports and an output port, wherein the input port includes an AC input port and a DC voltage port, and the output port of the DC bias is connected with the drive port of the optical comb modulator. The DC bias voltage provided by the DC bias device is used as the bias voltage of the electro-optical modulator in the optical comb modulator, and is provided by adding an adjustable DC voltage source.

After the photoelectric hybrid oscillation loop forms an oscillation, the SSB phase noise power spectrum of the RF signal at point B in Figure 1 can be expressed as:

Among them, H(f) is the transfer function of the double-ring structure optoelectronic hybrid oscillator, the specific expression form can refer to the literature H.Peng, C.Zhang, X.Xie, T.Sun, P.Guo, X.Zhu, L.Zhu , W.Hu, and Z.Chen, “Tunable DC-60GHz RF Generation Utilizing a Dual-Loop Optoelectronic Oscillator Based on Stimulated Brillouin Scattering,” Journal of Lightwave Technology, 33(13), 2707-2715(2015). with They are the frequency noise of the laser and the additional phase noise introduced by various optoelectronic devices in the optoelectronic hybrid oscillation loop. If the phase noise introduced by the optical amplifier is not considered, The calculation formula of is expressed as follows:

where F is the sum of the noise figures of the three low phase noise amplifiers, k is the Boltzmann constant, T is the room temperature, e is the electron charge, _Iph is the photocurrent output by the photodetector, Z is the impedance of the amplifier, and N _RIN is the intensity noise of the laser to be tested, b _-1 is the sum of the flicker noise figures of the amplifier and the photodetector, and f is the frequency away from the carrier of the oscillating signal.

The additional phase noise introduced by the frequency noise of the laser to the oscillation signal of the optoelectronic hybrid oscillator can be expressed as:

in is the modulation index of the optical comb modulator, V ₀ is the driving voltage of the optical comb modulator, and this value is determined according to the implementation of specific experiments, V _π is the half-wave voltage of the optical comb modulator, and FSR is the middle law of the optical comb modulator The free spectral range of a Brie-Perot cavity. The optical comb modulator used in this solution is the OptoComb WTEC-01-25 of the Japanese company OptoComb. According to the official data sheet, when the modulation frequency is 10GHz, the modulated half-wave voltage is 20V. The free spectral range FSR of Luo cavity is 2.5GHz. S _v (f) is the frequency noise of the narrow linewidth laser to be tested. When the phase noise of the radio frequency oscillation signal formed by the photoelectric oscillator is dominated by the frequency noise of the laser to be tested, the frequency noise of the laser to be tested can be reversed as:

In order to verify the effectiveness of the scheme, the experiment compares the frequency noise measurement results of the same laser under test measured by the scheme and the commercial laser frequency noise measurement instrument. In the experiment, the wavelength of the laser to be tested is 1550nm, the power is 17dBm, the RF driving power of the optical comb modulator is 17dBm, the lengths of the optical fibers used are 500 meters and 2000 meters respectively, and the center frequency of the electric bandpass filter is 10GHz. The bandwidth is 1GHz, and the gain of the three low phase noise amplifiers is 15dB. The experimental test results are shown in Figure 2. Fig. 2(a) shows the spectrum at point A in the structure Fig. 1, and the spectrum has symmetrical optical sidebands centered on the wavelength of the laser to be tested. Fig. 2(b) shows the electric spectrum at point B in the structure Fig. 1, and the center frequency of the electric spectrum is 10GHz. Fig. 2(c) shows the phase noise result of the radio frequency oscillation signal at point B in the structure Fig. 1. The measured frequency offset ranges from 100 Hz to 10 MHz. At the same time, the shot noise floor of the photodetector is shown in the figure. The thermal noise floor of a phase noise amplifier and the additive phase noise floor of a low phase noise amplifier. These noise floors will limit the measurement sensitivity of this measurement scheme. The dotted line in Fig. 2(d) represents the frequency noise measurement result of the laser under test deduced from the phase noise measured in Fig. 2(c), while the curve shown in the solid line represents the laser used commercially by SYCAUTS, Japan The frequency noise measurement result of the laser under test obtained by the frequency noise measuring instrument. Comparing the frequency noise measurement results of this scheme and commercial measuring instruments, it can be found that this scheme can realize effective measurement of laser frequency noise.

The above implementation is only used to illustrate the technical solution of the present invention and not to limit it. Those skilled in the art can modify or equivalently replace the technical solution of the present invention without departing from the spirit and scope of the present invention. Protection of the present invention The scope should be defined by the claims.

Claims (10)

1. a laser frequency noise measurement method is characterized in that, the laser to be measured is used as the light source of the optoelectronic hybrid oscillator built based on the optical comb modulator, and then measures the phase noise of the radio frequency oscillation signal in the optoelectronic hybrid oscillator; according to This phase noise yields the frequency noise of the laser under test. 2. The method according to claim 1, wherein the optical-electric hybrid oscillator is a double-ring structure optical-electric hybrid oscillator, which includes an optical comb modulator, and the output end of the optical comb modulator is passed through an optical fiber The amplifier is connected to an optical coupler, an output end of the optical coupler is connected to a section of first single-mode optical fiber, and the output signal of the first single-mode optical fiber is converted into an electrical signal by a first photodetector and input to a microwave power combiner; The other output end of the optical coupler is connected to a second single-mode optical fiber, and the output signal of the second single-mode optical fiber is converted into an electrical signal by a second photodetector and input to the microwave power combiner; The output end passes through the microwave phase shifter, the first low phase noise amplifier, and the electric band-pass filter in turn, and then enters the microwave directional coupler; one output end of the microwave directional coupler is used to connect the phase noise tester, and the other output end is connected to the The AC voltage input port of the DC biaser; the output port of the DC biaser is connected with the driving port of the optical comb modulator. 3. method as claimed in claim 2, is characterized in that, be provided with the second low phase noise amplifier, the 3rd low phase noise amplifier between described electric bandpass filter and described microwave directional coupler; The fiber length of a single-mode fiber is more than 10 times the fiber length of the second single-mode fiber. 4. The method according to claim 1, wherein the phase noise obtains the frequency noise of the laser to be measured Among them, β is the modulation index of the optical comb modulator, FSR is the free spectral range of the Fabry-Perot cavity in the optical comb modulator, H(f) is the transfer function of the optoelectronic hybrid oscillator, The additional phase noise introduced by various optoelectronic devices in the optoelectronic hybrid oscillator, the additional phase noise introduced by the frequency noise of the laser to be tested on the oscillation signal of the optoelectronic hybrid oscillator 5. method as claimed in claim 1, is characterized in that, described optical-electrical hybrid oscillator is the optical-electrical hybrid oscillator of a polarization double ring structure, and it comprises an optical comb modulator, and the output end of this optical comb modulator is passed through a The fiber amplifier is connected with a polarization beam splitter, and an output end of the polarization beam splitter is connected with a section of the first polarization-maintaining optical fiber, and the output signal of the first polarization-maintaining optical fiber is converted into an electrical signal by a first photodetector and input polarization combining beam splitter; the other output end of the polarization beam splitter is connected to a second polarization-maintaining optical fiber, and the output signal of the second polarization-maintaining optical fiber is converted into an electrical signal by a second photodetector and input to the polarization beam combiner; the polarization The output end of the beam combiner is input into the microwave directional coupler after successively passing through the microwave phase shifter, the first low phase noise amplifier and the electric bandpass filter; one output end of the microwave directional coupler is used to connect the phase noise tester, and the other An output terminal is connected with the AC voltage input port of the DC bias device; the output port of the DC bias device is connected with the driving port of the optical comb modulator. 6. The method according to claim 5, characterized in that a second low phase noise amplifier and a third low phase noise amplifier are arranged between the electric bandpass filter and the microwave directional coupler. 7. The method according to claim 5 or 6, wherein the fiber length of the first polarization-maintaining fiber is more than 10 times the fiber length of the second polarization-maintaining fiber. 8. A laser frequency noise measurement system is characterized in that, comprising an optical comb modulator, the optical comb modulator is used to modulate the laser output of the laser to be measured, and the output end of the optical comb modulator is through a fiber amplifier and a The optical coupler is connected, and an output end of the optical coupler is connected to a section of the first single-mode optical fiber, and the output signal of the first single-mode optical fiber is converted into an electrical signal by a first photodetector and input into the microwave power combiner; the optical coupling The other output end of the device is connected to a second single-mode optical fiber, and the output signal of the second single-mode optical fiber is converted into an electrical signal by a second photodetector and input to the microwave power combiner; the output end of the microwave power coupler is sequentially After the microwave phase shifter, the first low phase noise amplifier, and the electric bandpass filter, it is input into the microwave directional coupler; one output end of the microwave directional coupler is used to connect the phase noise tester, and the other output end is connected to the DC bias The AC voltage input port of the device; the output port of the DC bias device is connected with the driving port of the optical comb modulator. 9. A laser frequency noise measurement system, characterized in that it comprises an optical comb modulator, the optical comb modulator is used to modulate the laser output of the laser to be measured, and the output end of the optical comb modulator is connected to an optical fiber amplifier and a A polarization beam splitter is connected, an output end of the polarization beam splitter is connected to a section of the first polarization-maintaining optical fiber, the output signal of the first polarization-maintaining optical fiber is converted into an electrical signal by a first photodetector and input into the polarization beam combiner; The other output end of the polarization beam splitter is connected to a second polarization-maintaining optical fiber, and the output signal of the second polarization-maintaining optical fiber is converted into an electrical signal by a second photodetector and input to the polarization beam combiner; The output end passes through the microwave phase shifter, the first low phase noise amplifier, and the electric band-pass filter in turn, and then enters the microwave directional coupler; one output end of the microwave directional coupler is used to connect the phase noise tester, and the other output end is connected to the The AC voltage input port of the DC biaser; the output port of the DC biaser is connected with the drive port of the optical comb modulator. 10. The system according to claim 8 or 9, characterized in that a second low phase noise amplifier and a third low phase noise amplifier are arranged between the electric bandpass filter and the microwave directional coupler.