CN107727367B - Laser frequency noise measurement method and system - Google Patents
Laser frequency noise measurement method and system Download PDFInfo
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Abstract
The invention discloses a method and a system for measuring frequency noise of a laser, and belongs to the field of optical measurement. The method comprises the steps of taking a laser to be measured as a light source of a photoelectric hybrid oscillator constructed based on an optical comb modulator, and then measuring phase noise of a radio frequency oscillation signal in the photoelectric hybrid oscillator; obtaining the frequency noise of the laser to be tested according to the phase noise; the opto-electric hybrid oscillator is a dual-ring opto-electric hybrid oscillator. The scheme indirectly measures the frequency noise of the laser by measuring the phase noise of the radio frequency oscillation signal, so that the frequency noise and the intensity noise of the laser can be distinguished, and the method has extremely high measurement sensitivity.
Description
Technical Field
The invention relates to a method and a system for measuring frequency noise of a laser. The method comprises the steps of constructing an optical comb modulator-based photoelectric hybrid oscillator, taking a laser to be measured as a light source of the photoelectric oscillator, transferring frequency noise of the laser to be measured to phase noise of a radio frequency oscillation signal in the photoelectric oscillator in the oscillation forming process, and obtaining the frequency noise of the laser to be measured by measuring the phase noise of the radio frequency oscillation signal, and belongs to the field of optical measurement.
background
Single-frequency, narrow linewidth lasers are key components of many application systems, including laser radars, coherent optical communication systems, high-precision optical sensing, and high-stability optical atomic clocks. In general, the line width 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, frequency noise measurement of high-sensitivity narrow linewidth lasers becomes more and more important. The conventional schemes for measuring the frequency noise of the laser mainly include four types: the first type is a time-delay self-beat method; the second type is that the frequency noise of the laser is transferred to laser power jitter based on a Mach-Zehnder modulator, so that the frequency noise of the laser to be measured is obtained; the third type is that the frequency noise of the laser is converted into the jitter of the optical power by adopting an optical resonant cavity, and then the frequency noise of the laser is obtained by measurement; and the fourth type is that a narrow linewidth laser with extremely low frequency noise is used as a reference source, the laser to be detected and the reference laser are subjected to beat frequency, and then the frequency noise of the laser to be detected is obtained.
The following are some existing techniques and schemes for measuring laser frequency noise:
Scheme 1 is a Measurement scheme described in document d.derickson, Fiber optical Test and Measurement (precision-Hall, 1998). The scheme utilizes a delay interferometer to achieve the purpose of measuring the frequency noise of the laser by delaying the signal of the laser to be measured, performing decorrelation operation of a delayed signal and a non-delayed signal after the delay, and finally performing beat frequency on the delayed signal and the optical signal directly output by the laser to be measured.
Scheme 2 is a published patent application at Zhejiang university with publication number CN 102183362. According to the scheme, the Mach-Zehnder interferometer is adopted to complete conversion from frequency noise of the laser to light intensity, and the purpose of measuring the frequency noise of the laser is achieved.
scheme 3 is a published patent application at Zhejiang university with publication number CN 102692314A. The scheme utilizes an optical fiber resonant cavity to convert the frequency noise of the laser into the amplitude fluctuation of an optical field, thereby achieving the purpose of measuring the frequency noise of the laser.
Scheme 4 is a scheme proposed by paris astrodourf, france in 2017 to characterize narrow-linewidth laser frequency noise (x.xie, r.bouchand, d.nicolodi, m.lours, c.alexandre, and y.l.coq. "Phase noise characterization of sub-hertz linewidth laser video digital correlation," opt.lett.42(7),1217 + 1220 (2017)). The scheme adopts two single-frequency lasers with extremely narrow line width and extremely low frequency noise as references, the laser to be measured and the two reference lasers are respectively subjected to beat frequency, and the aim of indirectly measuring the frequency noise of the laser to be measured is achieved by measuring the phase noise of beat frequency electric signals.
when an extremely narrow linewidth laser is measured, an optical fiber with the length of several kilometers or even dozens of kilometers is needed as a delay line, great optical loss is caused due to the introduction of the long optical fiber, inevitable scattering noise is brought by the introduction of the long optical fiber, the sensitivity of a frequency noise measurement system is reduced, and the frequency noise and the intensity noise of the laser are not distinguished by the method; the scheme based on the Mach-Zehnder modulator is limited by the limitation that the conversion coefficient from the frequency noise to the intensity noise of the modulator is low, and the frequency noise measurement of a laser with higher sensitivity is difficult to realize; by adopting the scheme of the optical fiber resonant cavity, the measuring system is easily interfered by the environment, and the stability of the frequency noise measurement of the laser is reduced; the scheme of the reference laser with low frequency noise and high stability is adopted, so that the realization cost is high, and the frequency noise of the laser with any wavelength is difficult to measure.
Disclosure of Invention
Aiming at the technical problems in the prior art, the invention aims to provide a method and a system for measuring frequency noise of a laser. The method is suitable for measuring the frequency noise of the laser with any wavelength, narrow line width and extremely low frequency noise, and is particularly suitable for measuring the frequency noise of the laser with the narrow line width.
The technical scheme of the invention is as follows:
A laser frequency noise measurement method is characterized in that a laser to be measured is used as a light source of a photoelectric hybrid oscillator constructed based on an optical comb modulator, and then phase noise of a radio frequency oscillation signal in the photoelectric hybrid oscillator is measured; and obtaining the frequency noise of the laser to be tested according to the phase noise.
Further, the optoelectronic hybrid oscillator is a dual-ring structure optoelectronic hybrid oscillator, and includes an optical comb modulator, an output end of the optical comb modulator is connected with an optical coupler through an optical fiber amplifier, an output end of the optical coupler is connected with a section of first single mode fiber, and an output signal of the first single mode fiber is converted into an electrical signal through a first photodetector and is input into a microwave power synthesizer; the other output end of the optical coupler is connected with a section of second single-mode fiber, and an output signal of the second single-mode fiber is converted into an electric signal through a second photoelectric detector and is input into the microwave power combiner; the output end of the microwave power coupler sequentially passes through a microwave phase shifter, a first low-phase noise amplifier and an electric band-pass filter and then is input into the microwave directional coupler; one output end of the microwave directional coupler is used for connecting a phase noise tester, and the other output end of the microwave directional coupler is connected with an alternating current voltage input port of a direct current 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 arranged between the electric band-pass filter and the microwave directional coupler; the optical fiber length of the first single mode optical fiber is more than 10 times of the optical fiber length of the second single mode optical fiber.
Further, the phase noise obtains the frequency noise of the laser to be testedWherein beta is the modulation index of the optical comb modulator, FSR is the free spectral range of a Fabry-Perot cavity in the optical comb modulator,H (f) is the transfer function of the opto-electric hybrid oscillator,Additional phase noise introduced by various photoelectric devices in the photoelectric mixed oscillator and additional phase noise introduced by frequency noise of the laser to be tested to oscillation signals of the photoelectric mixed oscillator
Furthermore, the hybrid optoelectronic oscillator is a hybrid optoelectronic oscillator with a polarization dual-ring structure, and includes an optical comb modulator, an output end of the optical comb modulator is connected with a polarization beam splitter through an optical fiber amplifier, an output end of the polarization beam splitter is connected with a first polarization maintaining fiber, and an output signal of the first polarization maintaining fiber is converted into an electrical signal through a first photodetector and is input into the polarization beam combiner; the other output end of the polarization beam splitter is connected with a section of second polarization-maintaining optical fiber, and an output signal of the second polarization-maintaining optical fiber is converted into an electric signal through a second photoelectric detector and is input into the polarization beam combiner; the output end of the polarization beam combiner is sequentially input into a microwave directional coupler after passing through a microwave phase shifter, a first low-phase noise amplifier and an electric band-pass filter; one output end of the microwave directional coupler is used for connecting a phase noise tester, and the other output end of the microwave directional coupler is connected with an alternating current voltage input port of a direct current biaser; the output port of the DC biaser is connected with the drive port of the optical comb modulator.
furthermore, a second low-phase noise amplifier and a third low-phase noise amplifier are arranged between the electric band-pass filter and the microwave directional coupler.
Further, the optical fiber length of the first polarization maintaining optical fiber is more than 10 times of the optical fiber length of the second polarization maintaining optical fiber.
A laser frequency noise measurement system is characterized by comprising an optical comb modulator, wherein the optical comb modulator is used for modulating laser output by a laser to be measured, the output end of the optical comb modulator is connected with an optical coupler through an optical fiber amplifier, one output end of the optical coupler is connected with a section of first single-mode optical fiber, and an output signal of the first single-mode optical fiber is converted into an electric signal through a first photoelectric detector and then is input into a microwave power synthesizer; the other output end of the optical coupler is connected with a section of second single-mode fiber, and an output signal of the second single-mode fiber is converted into an electric signal through a second photoelectric detector and is input into the microwave power combiner; the output end of the microwave power coupler sequentially passes through a microwave phase shifter, a first low-phase noise amplifier and an electric band-pass filter and then is input into the microwave directional coupler; one output end of the microwave directional coupler is used for connecting a phase noise tester, and the other output end of the microwave directional coupler is connected with an alternating current voltage input port of a direct current biaser; the output port of the DC biaser is connected with the drive port of the optical comb modulator.
a laser frequency noise measurement system is characterized by comprising an optical comb modulator, wherein the optical comb modulator is used for modulating laser output by a laser to be measured, the output end of the optical comb modulator is connected with a polarization beam splitter through an optical fiber amplifier, one output end of the polarization beam splitter is connected with a first polarization maintaining fiber, and an output signal of the first polarization maintaining fiber is converted into an electric signal through a first photoelectric detector and then is input into a polarization beam combiner; the other output end of the polarization beam splitter is connected with a section of second polarization-maintaining optical fiber, and an output signal of the second polarization-maintaining optical fiber is converted into an electric signal through a second photoelectric detector and is input into the polarization beam combiner; the output end of the polarization beam combiner is sequentially input into a microwave directional coupler after passing through a microwave phase shifter, a first low-phase noise amplifier and an electric band-pass filter; one output end of the microwave directional coupler is used for connecting a phase noise tester, and the other output end of the microwave directional coupler is connected with an alternating current voltage input port of a direct current biaser; the output port of the DC biaser is connected with the drive port of the optical comb modulator.
Furthermore, a second low-phase noise amplifier and a third low-phase noise amplifier are arranged between the electric band-pass filter and the microwave directional coupler.
The scheme is a method for realizing frequency noise measurement of a laser to be measured based on a photoelectric mixed oscillator of an optical comb modulator. The laser to be measured is used as a light source of the photoelectric mixed oscillator constructed based on the optical comb modulator, the frequency noise of the laser to be measured is transferred to the phase noise of the radio frequency oscillation signal in the photoelectric mixed oscillator, and the frequency noise of the laser to be measured can be obtained by measuring the phase noise of the radio frequency oscillation signal. The system measurement sensitivity is only limited by shot noise of a photoelectric detector in the photoelectric mixed oscillator, spontaneous radiation noise of the erbium-doped fiber amplifier, thermal noise and flicker noise of the electric amplifier and the like, and extremely high laser frequency noise measurement sensitivity can be realized.
Compared with the prior art, the invention has the following positive effects:
1. The scheme adopts the photoelectric hybrid oscillator of the optical comb modulator based on the Fabry-Perot cavity with high fineness, and realizes the conversion from the frequency noise of the narrow linewidth laser to be measured to the phase noise of the oscillation signal. Compared with a method of a delay interferometer, the method does not need longer optical fibers, avoids the problem that the measurement sensitivity of frequency noise is reduced due to transmission loss, scattering noise and the like of the long optical fibers, and can measure the frequency noise of the laser with narrower line width. In addition, the scheme indirectly measures the frequency noise of the laser by measuring the phase noise of the radio frequency oscillation signal, so that the frequency noise and the intensity noise of the laser can be distinguished.
2. the scheme adopts the optical comb modulator with the Fabry-Perot cavity with high fineness to realize the conversion from the frequency noise of the laser to the phase noise of the radio frequency oscillation signal in the photoelectric hybrid oscillator, and the system measurement sensitivity is only limited by the device noise in the photoelectric oscillation loop, so that the system has extremely high measurement sensitivity.
3. The scheme does not need to adopt a laser with extremely narrow line width and extremely low frequency noise as a reference source, thereby reducing the complexity and the cost of the system. Meanwhile, the method can measure the frequency noise of the laser with any wavelength.
Drawings
FIG. 1 is a schematic diagram of the inventive arrangement;
FIG. 2 is a graph showing the results of an experiment according to the embodiment of the present invention;
(a) is a graph of the spectrum result of point A in FIG. 1;
(b) Is a graph of the spectrum result at point B in FIG. 1;
(c) is a graph of the phase noise results at point B in fig. 1;
(d) the comparison graph of the laser frequency noise measurement result and the measurement result of the commercial frequency noise tester is shown.
Detailed Description
The embodiments of the present invention will be described in further detail with reference to the accompanying drawings.
The principle of the solution of the invention is shown in fig. 1. The laser to be tested outputs continuous wave laser and injects the laser into an optical comb modulator, the optical comb modulator is formed by plating high reflection films at two ends of a high-bandwidth electro-optic phase modulator, and the high reflection films at the two ends form a high-fineness Fabry-Perot cavity. Because the optical comb modulator has a certain optical loss, the erbium-doped fiber amplifier is required 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, the two beams of light are sent into standard single-mode optical fibers with different lengths, two sections of standard single-mode optical fibers are adopted to form a photoelectric mixed oscillation loop with a double-loop structure, so that an auxiliary oscillation mode of the oscillation signal in the photoelectric mixed oscillator can be inhibited, and the larger the length difference of the two sections of optical fibers is, the auxiliary oscillation mode can be more effectively inhibited. The length of the longest optical fiber section does not exceed 10km generally, and the difference of the lengths of the two optical fibers is more than 10 times generally. The two photoelectric detectors respectively convert optical signals transmitted by the standard single-mode optical fiber into electric signals, and the electric signals output by the two photoelectric detectors are subjected to power synthesis through a 50% microwave power synthesizer. Besides a double-ring structure of the Optoelectronic hybrid Oscillator, which is formed by dividing two beams of light by An Optical fiber coupler, a polarization double-ring structure is also adopted, that is, An Optical fiber polarization beam splitter is adopted to divide incident light into two polarization directions, the light in the two polarization directions passes through two sections of polarization-maintaining Optical fibers with different lengths, then passes through a polarization beam combiner to synthesize one Optical signal, and two signals can be simultaneously detected by a photodetector, so that the Optoelectronic hybrid Oscillator with the double-ring structure is realized. The microwave phase shifter at the rear end of the 50% microwave power synthesizer can shift the phase of the microwave signal output by the 50% microwave power synthesizer. Because the power of the electric signal output by the photoelectric detector is small, three low-phase noise amplifiers are adopted for amplifying the electric signal in a cascade mode. The low phase noise amplifier has extremely low flicker noise and can reduce the influence of the electric amplifier on the measurement sensitivity of the frequency noise of the laser. Because a plurality of transmission peaks exist in a Fabry-Perot cavity in the optical comb modulator, oscillation of a plurality of modes in an opto-electric hybrid oscillation loop can be formed, and a narrow-band electric band-pass filter is added in the loop to suppress other oscillation modes. The output signal of the low phase noise amplifier 3 enters a microwave directional coupler, which has two output ports, wherein most of the microwave power is used for the alternating current input of the direct current biaser, a small part of the microwave power is separated out by the microwave directional coupler and used for measuring the phase noise of the microwave directional coupler, and the phase noise can be measured by a commercial phase noise tester. The DC biaser is provided with two input ports and an output port, wherein the input ports comprise an AC input port and a DC voltage port, and the output port of the DC biaser is connected with a driving port of the optical comb modulator. The DC bias voltage provided by the DC biaser is used as the bias voltage of an electro-optic modulator in the optical comb modulator and is provided by an externally added adjustable DC voltage stabilizing source.
After the optical-electrical hybrid oscillation loop forms oscillation, the single-sideband phase noise power spectrum of the radio frequency signal at point B in fig. 1 can be represented as:
Wherein H (f) is the transfer function of the Dual-ring structure Optoelectronic hybrid Oscillator, and the specific expression form can be referred to in the documents 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 Lighting Technology,33(13), 2707-.AndThe frequency noise of the laser and the additional phase noise introduced by various photoelectric devices in the photoelectric hybrid oscillation loop are respectively. If the phase noise introduced by the optical amplifier is not taken into account,The calculation formula of (a) is as follows:
wherein F is the sum of the noise coefficients of the three low phase noise amplifiers, and k is the Boltzmann constantT is room temperature, e is electronic charge, IphFor the photocurrent output from the photodetector, Z is the impedance of the amplifier, NRINIntensity noise of the laser under test, b-1Is the sum of the flicker noise coefficients of the amplifier and photodetector, and f is the frequency of the deviating oscillator signal carrier.
The additional phase noise introduced by the frequency noise of the laser to the oscillation signal of the opto-electronic hybrid oscillator can be expressed as:
WhereinIs the modulation index, V, of the optical comb modulator0Is the drive voltage of the optical comb modulator, which is determined according to the specific experimental implementation, VπIs the half-wave voltage of the optical comb modulator, and the FSR is the free spectral range of the Fabry-Perot cavity in the optical comb modulator. The optical comb modulator adopted in the scheme is an OptoComb WTEC-01-25 of OptoComb of Japan company, according to an official data manual, when the frequency is modulated at 10GHz, the modulated half-wave voltage is 20V, and the free spectral range FSR of a Fabry-Perot cavity in optical comb modulation is 2.5 GHz. Sv(f) the frequency noise of the narrow linewidth laser to be measured. When the phase noise of the radio frequency oscillation signal formed by the optoelectronic oscillator is dominated by the frequency noise of the laser to be tested, the frequency noise of the laser to be tested can be reversely deduced as follows:
in order to verify the effectiveness of the scheme, the frequency noise measurement result of the same laser to be measured by the scheme and a commercial laser frequency noise measurement instrument is compared through experiments. In the experiment, the wavelength of the laser to be measured is 1550nm, the power is 17dBm, the radio frequency driving power of the optical comb modulator is 17dBm, the lengths of the adopted optical fibers are 500 meters and 2000 meters respectively, the center frequency of the electric band-pass filter is 10GHz, the bandwidth is 1GHz, and the gains of the three low-phase noise amplifiers are 15 dB. The results of the experimental tests are shown in fig. 2. Fig. 2(a) shows the spectrum of point a in structure fig. 1 with symmetric optical sidebands centered about the laser wavelength under test. FIG. 2(B) shows an electric spectrum of point B in the structure of FIG. 1, which has a center frequency of 10 GHz. Fig. 2(c) shows the phase noise result of the B-point rf oscillating signal in fig. 1, and the measured frequency offset ranges from 100Hz to 10MHz, and the shot noise floor of the photodetector, the thermal noise floor of the low phase noise amplifier, and the additional phase noise floor of the low phase noise amplifier are shown. These noise floors will limit the measurement sensitivity of the measurement scheme. The broken line in fig. 2(d) represents the frequency noise measurement result of the laser under test derived by the phase noise inversion measured in fig. 2(c), while the curve shown by the solid line represents the frequency noise measurement result of the laser under test obtained using the laser frequency noise measurement instrument commercially available from SYCAUTS corporation, japan. Comparing the frequency noise measurement results of the scheme and the commercial measuring instrument, the method can realize effective measurement of the frequency noise of the laser.
the above embodiments are only for illustrating the technical solution of the present invention and not for limiting the same, and a person skilled in the art can make modifications or equivalent substitutions to the technical solution of the present invention without departing from the spirit and scope of the present invention, and the scope of the present invention should be determined by the claims.
Claims (9)
1. A laser frequency noise measurement method is characterized in that a laser to be measured is used as a light source of a photoelectric hybrid oscillator constructed based on an optical comb modulator, and then phase noise of a radio frequency oscillation signal in the photoelectric hybrid oscillator is measured; obtaining the frequency noise of the laser to be tested according to the phase noise; the optical comb modulator is formed by plating high-reflection films at two ends of an electro-optic phase modulator, and the high-reflection films at the two ends form a Fabry-Perot cavity with high fineness; the photoelectric hybrid oscillator is a double-ring photoelectric hybrid oscillator and comprises an optical comb modulator, wherein the output end of the optical comb modulator is connected with an optical coupler through an optical fiber amplifier, one output end of the optical coupler is connected with a section of first single-mode optical fiber, and an output signal of the first single-mode optical fiber is converted into an electric signal through a first photoelectric detector and is input into a microwave power synthesizer; the other output end of the optical coupler is connected with a section of second single-mode fiber, and an output signal of the second single-mode fiber is converted into an electric signal through a second photoelectric detector and is input into the microwave power combiner; the output end of the microwave power coupler sequentially passes through a microwave phase shifter, a first low-phase noise amplifier and an electric band-pass filter and then is input into the microwave directional coupler; one output end of the microwave directional coupler is used for connecting a phase noise tester, and the other output end of the microwave directional coupler is connected with an alternating current voltage input port of a direct current biaser; the output port of the DC biaser is connected with the drive port of the optical comb modulator.
2. The method of claim 1, wherein a second low phase noise amplifier, a third low phase noise amplifier are provided between the electric band pass filter and the microwave directional coupler; the optical fiber length of the first single mode optical fiber is more than 10 times of the optical fiber length of the second single mode optical fiber.
3. The method of claim 1, wherein the frequency noise of the laser under test is derived from the phase noise when the phase noise of the rf oscillating signal formed by the opto-electronic oscillator is dominated by the frequency noise of the laser under testWherein beta is the modulation index of the optical comb modulator, FSR is the free spectral range of a Fabry-Perot cavity in the optical comb modulator,H (f) is the transfer function of the opto-electric hybrid oscillator,is a photoelectric hybrid oscillatoradditional phase noise introduced by various photoelectric devices, and additional phase noise introduced by frequency noise of laser to be tested to oscillation signal of photoelectric hybrid oscillator
4. A laser frequency noise measurement method is characterized in that a laser to be measured is used as a light source of a photoelectric hybrid oscillator constructed based on an optical comb modulator, and then phase noise of a radio frequency oscillation signal in the photoelectric hybrid oscillator is measured; obtaining the frequency noise of the laser to be tested according to the phase noise; the optical comb modulator is formed by plating high-reflection films at two ends of an electro-optic phase modulator, and the high-reflection films at the two ends form a Fabry-Perot cavity with high fineness; the photoelectric hybrid oscillator is a polarized double-ring structure photoelectric hybrid oscillator and comprises an optical comb modulator, the output end of the optical comb modulator is connected with a polarization beam splitter through an optical fiber amplifier, one output end of the polarization beam splitter is connected with a section of first polarization maintaining fiber, and an output signal of the first polarization maintaining fiber is converted into an electric signal through a first photoelectric detector and is input into the polarization beam combiner; the other output end of the polarization beam splitter is connected with a section of second polarization-maintaining optical fiber, and an output signal of the second polarization-maintaining optical fiber is converted into an electric signal through a second photoelectric detector and is input into the polarization beam combiner; the output end of the polarization beam combiner is sequentially input into a microwave directional coupler after passing through a microwave phase shifter, a first low-phase noise amplifier and an electric band-pass filter; one output end of the microwave directional coupler is used for connecting a phase noise tester, and the other output end of the microwave directional coupler is connected with an alternating current voltage input port of a direct current biaser; the output port of the DC biaser is connected with the drive port of the optical comb modulator.
5. The method of claim 4, wherein a second low phase noise amplifier and a third low phase noise amplifier are disposed between the electric bandpass filter and the microwave directional coupler.
6. the method of claim 4 or 5, wherein the first polarization maintaining fiber has a fiber length that is more than 10 times the fiber length of the second polarization maintaining fiber.
7. A laser frequency noise measurement system is characterized by comprising an optical comb modulator, wherein the optical comb modulator is used for modulating laser output by a laser to be measured, the output end of the optical comb modulator is connected with an optical coupler through an optical fiber amplifier, one output end of the optical coupler is connected with a section of first single-mode optical fiber, and an output signal of the first single-mode optical fiber is converted into an electric signal through a first photoelectric detector and then is input into a microwave power synthesizer; the other output end of the optical coupler is connected with a section of second single-mode fiber, and an output signal of the second single-mode fiber is converted into an electric signal through a second photoelectric detector and is input into the microwave power combiner; the output end of the microwave power coupler sequentially passes through a microwave phase shifter, a first low-phase noise amplifier and an electric band-pass filter and then is input into the microwave directional coupler; one output end of the microwave directional coupler is used for connecting a phase noise tester, and the other output end of the microwave directional coupler is connected with an alternating current voltage input port of a direct current biaser; the output port of the direct current biaser is connected with the driving port of the optical comb modulator; the optical comb modulator is formed by plating high-reflection films at two ends of an electro-optic phase modulator, and the high-reflection films at the two ends form a Fabry-Perot cavity with high fineness.
8. A laser frequency noise measurement system is characterized by comprising an optical comb modulator, wherein the optical comb modulator is used for modulating laser output by a laser to be measured, the output end of the optical comb modulator is connected with a polarization beam splitter through an optical fiber amplifier, one output end of the polarization beam splitter is connected with a first polarization maintaining fiber, and an output signal of the first polarization maintaining fiber is converted into an electric signal through a first photoelectric detector and then is input into a polarization beam combiner; the other output end of the polarization beam splitter is connected with a section of second polarization-maintaining optical fiber, and an output signal of the second polarization-maintaining optical fiber is converted into an electric signal through a second photoelectric detector and is input into the polarization beam combiner; the output end of the polarization beam combiner is sequentially input into a microwave directional coupler after passing through a microwave phase shifter, a first low-phase noise amplifier and an electric band-pass filter; one output end of the microwave directional coupler is used for connecting a phase noise tester, and the other output end of the microwave directional coupler is connected with an alternating current voltage input port of a direct current biaser; the output port of the direct current biaser is connected with the driving port of the optical comb modulator; the optical comb modulator is formed by plating high-reflection films at two ends of an electro-optic phase modulator, and the high-reflection films at the two ends form a Fabry-Perot cavity with high fineness.
9. The system according to claim 7 or 8, wherein a second low phase noise amplifier and a third low phase noise amplifier are arranged between the electric band-pass filter and the microwave directional coupler.
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