Optical phase-locked loop system and method based on linear feedback algorithm

Abstract

The invention provides an optical phase-locked loop system and method based on a linear feedback algorithm, and relates to the technical field of laser control. The system comprises an optical module, a frequency-locking and phase-locking module and a feedback control module. The optical module obtains a to-be-locked beat frequency electric signal, the frequency and phase detection module compares the to-be-locked beat frequency electric signal with a reference signal to obtain a control signal, the control signal is processed by the feedback control module to obtain a feedback signal to control the laser, and closed-loop locking of the optical phase-locked loop is formed. According to the optical phase-locked loop, the loop gain regulator is added in the feedback control module, when the frequency difference between the beat frequency electric signal and the reference signal is large, the loop gain is improved, and when the frequency difference is low, the loop gain is reduced, so that the optical phase-locked loop always works in a relatively adaptive environment, and the performance of the optical phase-locked loop is greatly improved.

Description

Technical Field

[0001] The present invention relates to the field of laser control technology, and in particular to an optical phase-locked loop system and method based on a linear feedback algorithm. Background Art

[0002] Atomic interferometer gravimeter is a gravity measuring instrument with highly sensitive inertial and rotation sensors, which plays an important role in various fields such as military and geophysics. The laser light source system is the core of the atomic interferometer gravimeter, which determines the accuracy of atomic interference fringes and gravity measurement. The system uses a pair of phase-locked Raman laser pulses with stable frequency difference, power and constant phase difference to achieve atomic internal state interference. At present, the main methods of Raman laser generation are modulation method and optical phase-locked loop method. Compared with the modulation method, the optical phase-locked loop method has high locking accuracy and large servo frequency bandwidth, and is the first choice for the laser control technology of atomic interferometer gravimeter. The frequency detection and phase detection process and feedback system of the traditional optical phase-locked loop are composed of analog circuits. Since the frequency difference required by the atomic interferometer is large, it is difficult to realize the analog circuit. With the emergence of integrated circuits such as digital phase detectors and phase-locked loop chips, optical phase-locked loops are increasingly using digital phase detection technology based on frequency detection and phase detection. The digital phase-locked loop not only solves the shortcomings of the analog phase-locked loop, such as DC zero drift, device saturation, and susceptibility to power supply and ambient temperature changes, but also has the ability to process discrete sample values ​​in real time, and has the advantages of good reliability, high precision, programmable loop bandwidth and center frequency, etc. However, the locking speed and stability of the existing digital optical phase-locked loop system still have room for improvement. In view of this, the present invention proposes to use a linear feedback algorithm to implement an optical phase-locked loop, which can improve the performance of the optical phase-locked loop. Summary of the invention

[0003] In view of the problems existing in the existing optical phase-locked loop, the purpose of the present invention is to provide an optical phase-locked loop system and method based on a linear feedback algorithm, by down-converting the beat frequency electrical signal to be locked into a local oscillator signal and comparing it with a reference signal to obtain a phase detection signal, and after loop filtering processing, a feedback signal is obtained to control the laser, thereby forming a closed-loop locking of the optical phase-locked loop.

[0004] An optical phase-locked loop system based on a linear feedback algorithm, the system comprising an optical module, a frequency-locked phase-locked module and a feedback control module;

[0005] The frequency-locked phase-locked module includes a frequency meter, an N frequency divider, a comparator, an FPGA chip, a first signal generator, and a second signal generator; the FPGA chip includes a frequency detector, a phase detector, a time-to-digital converter, and a loop filter;

[0006] The frequency meter and the N frequency divider respectively obtain the beat frequency electrical signal, extract the frequency value of the beat frequency electrical signal through the frequency meter and input it into the host computer, and then convert it into a square wave signal through the comparator after the frequency is divided by the N frequency divider;

[0007] The first signal generator and the second signal generator are used to provide a frequency and phase discrimination reference signal and a clock frequency to the frequency and phase discrimination detector and the time to digital converter respectively;

[0008] The frequency and phase detector extracts the frequency and phase reference signal and the pulse signal carrying the phase difference information and inputs them into the time-to-digital converter;

[0009] The time-to-digital converter converts the pulse signal carrying the phase difference information into a digital quantity and inputs it into a loop filter. After the high-frequency noise is filtered out by the loop filter, a control signal is generated and input into a feedback control module.

[0010] The feedback control module processes the control signal generated by the loop filter and acts on the piezoelectric ceramic of the slave laser; and changes the loop gain according to the difference between the beat frequency electrical signal frequency calculated by the host computer and the target frequency.

[0011] The present invention also provides an optical phase-locked loop method based on a linear feedback algorithm, which is implemented by the following steps:

[0012] Step S1, the optical module is used to obtain a pair of lasers with a frequency difference of 6.834 GHz, convert the beat frequency signal of the laser into a beat frequency electrical signal and transmit it to the frequency locking and phase locking module;

[0013] Step S2, the frequency-locked and phase-locked module converts the pulse signal carrying the phase difference information in the beat frequency electrical signal into a control voltage and inputs the control voltage into the feedback control module;

[0014] Step S3, the feedback control module processes the control signal generated by the frequency-locked phase-locked module and applies it to the piezoelectric ceramic of the slave laser; and changes the gain of the loop gain regulator in the feedback control module according to the difference between the beat frequency electrical signal frequency calculated by the host computer and the target frequency.

[0015] Beneficial effects of the present invention: The optical phase-locked loop system described in the present invention is applied to cold atom gravimeters. Under the condition that the frequency of the non-stable laser and the frequency of the stable laser keep the frequency difference locked, a Raman laser pair is generated in the measurement process of the cold atom gravimeter. The laser generated by the master and slave lasers is split by a half-wave plate and a polarizer, and converged in an optical mixer for mixing. The mixed laser enters the photodetector and is converted into a beat frequency electrical signal, which carries the phase difference information of the two laser beams. The frequency of the beat frequency electrical signal is relatively large, and it is first divided by an N divider to reduce the frequency. At the same time, the beat frequency electrical signal is connected to a frequency meter to extract the frequency of the beat frequency electrical signal. The divided signal is converted into a square wave signal, and then enters a frequency detector to extract the phase difference information, and then enters a time-to-digital converter to convert it into a digital quantity, and then enters a loop filter to filter out the high-frequency component and enter the feedback control module. After being processed by the feedback control module, it is input to the modulation port of the piezoelectric ceramic to control the locking of the master and slave lasers. It has the following advantages:

[0016] 1. The optical phase-locked loop system of the present invention adopts digital frequency division instead of analog frequency mixing, which can reduce the system size, facilitate integration in the atom interferometer, and reduce costs.

[0017] 2. The frequency detector, time-to-digital converter and loop filter described in the present invention are all implemented digitally in FPGA. The digital phase-locked loop not only solves the shortcomings of the analog phase-locked loop, such as DC zero drift, device saturation and susceptibility to power supply and ambient temperature changes, but also has the ability to process discrete sample values ​​in real time, and has the advantages of good reliability, high accuracy, and programmable loop bandwidth and center frequency.

[0018] 3. The frequency and phase detector described in the present invention adopts a dual D flip-flop type, which can detect frequency difference and phase difference information at the same time, and a delay unit is added to prevent the occurrence of phase detection dead zone phenomenon.

[0019] 4. The optical phase-locked loop system of the present invention is provided with an improved structure based on a linear feedback algorithm. Changing the loop gain in real time can improve the frequency locking speed when the frequency difference between the two signals is large and the system is in the frequency locking stage, and can also improve the system phase locking stability when the frequencies are basically the same and the system is in the phase locking stage. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a principle diagram of an optical phase-locked loop system based on a linear feedback algorithm according to the present invention;

[0021] Figure 2 A schematic diagram of an optical module in an optical phase-locked loop system based on a linear feedback algorithm according to the present invention;

[0022] Figure 3It is a schematic diagram of a frequency-locking and phase-locking module in an optical phase-locked loop system based on a linear feedback algorithm according to the present invention;

[0023] Figure 4 It is a schematic diagram of a dual D-flip-flop type frequency and phase detector in an optical phase-locked loop system based on a linear feedback algorithm according to the present invention;

[0024] Figure 5 This is a schematic diagram of a feedback control module in an optical phase-locked loop system based on a linear feedback algorithm described in the present invention.

[0025] In the figure: 1. master laser, 2. slave laser, 3. first half-wave plate, 4. first optical polarizer, 5. second half-wave plate, 6. second optical polarizer, 7. optical mixer, 8. photodetector, 9. frequency meter, 10. N divider, 11. comparator, 12. frequency detector and phase detector, 13. time-to-digital converter, 14. loop filter, 15. first signal generator, 16. second signal generator, 17. FPGA chip, 18. first D / A converter, 19. second D / A converter, 20. voltage-controlled resistor, 21. voltage amplifier, 22. loop gain regulator, 23. piezoelectric ceramics, 24. host computer. DETAILED DESCRIPTION

[0026] Specific implementation method 1. Combination Figures 1 to 4 This embodiment describes an optical phase-locked loop system based on a linear feedback algorithm, the system comprising an optical module, a frequency-locked phase-locked module and a feedback control module;

[0027] The optical module is used to obtain a pair of lasers with a certain frequency difference (6.834 GHz), and convert the beat frequency signals of the pair of lasers into beat frequency electrical signals and transmit them to the frequency locking and phase locking module;

[0028] like Figure 2 As shown, the optical module in this embodiment includes a master laser 1, a slave laser 2, a first half-wave plate 3, a first polarizer 4, a second half-wave plate 5, a second polarizer 6, an optical mixer 7 and a photodetector 8;

[0029] The output port of the master laser 1 is connected to the host computer 24 and the first half-wave plate 3, the first half-wave plate 3 is connected to the first polarizer 4, the first polarizer 4 is connected to the optical mixer 7. The output port of the slave laser is connected to the host computer 24 and the second half-wave plate 5, the second half-wave plate 5 is connected to the second polarizer 6, the second polarizer 6 is connected to the optical mixer 7, the optical mixer 7 is connected to the photodetector 8;

[0030] The master laser 1 is used to emit a frequency-stabilized laser, and the slave laser 2 is used to emit a laser to be locked. The output of the two lasers is controlled by the host computer 24; the laser emitted by the master laser 1 is divided into a main transmitted light and a main reflected light by the first half-wave plate 3 and the first polarizer 4; the laser emitted by the slave laser 2 is divided into a slave transmitted light and a slave reflected light by the second half-wave plate 5 and the second polarizer 6; the main transmitted light and the slave transmitted light are used as the final output results; the main reflected light and the slave reflected light are combined and mixed by the optical mixer 7, and finally received by the photoelectric detector 8 and converted into a beat frequency electrical signal and input into the frequency locking and phase locking module;

[0031] The host computer 24 is used to extract the real-time frequency difference between the two lasers and use it for processing of the feedback control module.

[0032] In this embodiment, the master laser 1 and the slave laser 2 are both semiconductor lasers, with an output frequency of about 384THz and a frequency difference of 6.834GHz, so the frequency of the electrical signal after the beat is also 6.834GHz. The power of the laser output by the master laser 1 is 2mW, and the power of the laser output by the slave laser 2 is 2mW; the models used by the master and slave lasers are both stable frequency cat's eye lasers produced by Zhongke Kuyuan Technology. After mixing, the laser output by the master laser and the slave laser is about 6.834GHz, which is exactly the laser frequency difference required by the Raman laser pair. The first half-wave plate 3 and the second half-wave plate 5 are both 1 / 2 wave plates, and the splitting ratio can be adjusted by rotating the mirror seat and the polarizer; the optical mixer 7 selects a 180° optical mixer; the photodetector 8 selects a balanced optical detector. The model used by the optical mixer is COH24-X, and its function is to convert the laser signal of the frequency THz level that cannot be directly processed into a processable 6.834GHz signal. The photodetector used is model PDB415A.

[0033] The frequency-locked and phase-locked module is used to convert the phase difference information included in the beat frequency electrical signal into a control voltage and input it into the feedback control module;

[0034] like Figure 3 As shown, in this embodiment, the frequency-locked phase-locked module includes a frequency meter 9, an N frequency divider 10, a comparator 11, a first signal generator 15, a second signal generator 16 and an FPGA chip 17; the FPGA chip 17 includes a frequency detector 12, a time-to-digital converter 13 and a loop filter 14;

[0035] The frequency meter 9 extracts the beat frequency electrical signal and inputs it to the host computer 24, and then waits for the feedback control module to process it. The type of the frequency meter 9 is a direct frequency measurement counter, which has high accuracy and high speed. The host computer 24 controls the frequency locking and phase locking module through serial communication, performs Verilog programming in the computer, and burns the program into the FPGA through serial communication. The operator changes the parameters by re-burning.

[0036] The beat frequency electrical signal is simultaneously converted into a low frequency signal through an N frequency divider 10, wherein the N value is adjustable and the N value is set to 1000. Therefore, the frequency of the low frequency signal after frequency division is about 6.834 MHz, which is suitable for subsequent processing. The sine wave signal after frequency division is input into a comparator 11 and converted into a square wave signal (a pulse signal carrying phase difference information), and the pulse signal carrying phase difference information is input into an FPGA chip 17 for programming design.

[0037] The first signal generator 15 and the second signal generator 16 are connected to the input end of the FPGA chip 17, and provide a frequency detection phase detection reference signal and a clock frequency to the frequency detection phase detector 12 and the time digital converter 13. The frequency detection phase detection reference signal is about 6.834MHz, and the clock frequency is about 100MHz.

[0038] The phase frequency detector 12 extracts the phase frequency detector reference signal and the pulse signal carrying the phase difference information and inputs them into the time digital converter 13; Figure 4 As shown, the frequency detector 12 is a dual D flip-flop structure, consisting of two D flip-flops and a delay unit. The frequency detector reference signal and the pulse signal carrying the phase difference information are used as the clock signal of the D flip-flop, and the input of the D flip-flop is kept at 1. When the outputs of the two D flip-flops are 1 at the same time, the reset terminal of the D flip-flop is started, and the final output is set to 0. In order to maintain a specific pulse width at the output under the condition of zero input phase difference, a delay unit is added. The pulse signal width output by the frequency detector 12 is determined by the phase difference between the pulse signal carrying the phase difference information and the frequency detector reference signal. When the frequencies of the two signals are different, the output is a pulse signal of irregular width. When the frequencies of the two signals are the same, the output pulse signal width is determined by the phase difference size according to whether the phase is advanced.

[0039] The pulse signal carrying the phase difference information is converted into a digital quantity by the time-to-digital converter 13 and input into the loop filter 14 .

[0040] In this embodiment, the time-to-digital converter 13 adopts a pulse counting method, which is controlled by the clock frequency generated by the second signal generator 16. Whenever the clock frequency is at a rising edge, if the pulse signal is also at a rising edge, the count value is increased by 1, and finally the effect of converting the pulse signal carrying the phase difference information into a digital quantity is achieved. The loop filter 14 is a passive proportional-integral filter that filters out high-frequency noise and generates a control signal to be processed by the subsequent feedback control module.

[0041] The feedback control module is used to process the control signal generated by the loop filter 14 and finally act on the piezoelectric ceramic 23; and the loop gain is changed according to the difference between the beat frequency electrical signal frequency calculated by the host computer and the target frequency, so as to improve the locking speed in the frequency locking link and improve the system stability in the phase locking link.

[0042] In this embodiment, the frequency meter 9 adopts HP5347A, and its maximum measurement frequency can reach 20GHz. The N divider 10 adopts ADF4106 frequency synthesizer design. The divider reduces the frequency of the beat frequency electrical signal of the GHz level to the MHz level. The FPGA chip 17 adopts the XCZU15EG model chip of Xilinx. The first signal generator 15 adopts DG1000 produced by Puyuan Company with a maximum output frequency of 25MHz. The second signal generator 16 adopts DG4000 produced by Puyuan Company with a maximum output frequency of 200MHz.

[0043] like Figure 5 As shown, in this embodiment, the control signal generated by the frequency-locked phase-locked module is subsequently input into the feedback control module. The feedback control module includes a first D / A converter 18, a second D / A converter 19, a loop gain regulator 22 and a piezoelectric ceramic 23; the main structure of the loop gain regulator 22 adopts an amplifier circuit composed of a voltage-controlled resistor 20 and a voltage amplifier 21.

[0044] Connect the host computer 24 to the control end of the voltage-controlled resistor 20, connect the output end of the FPGA chip 17 to the input end of the loop gain regulator 22, and connect the feedback control module to the modulation port of the piezoelectric ceramic 23;

[0045] The control signal output by the loop filter 14 in the FPGA chip 17 is first input into the first D / A converter 18 for D / A conversion and then input into the loop gain regulator 22. The control signal is amplified by the amplifier circuit of the loop gain regulator 22 to change the voltage value input into the piezoelectric ceramic 23. After the control voltage is output from the loop gain regulator 22, it is input into the modulation port of the piezoelectric ceramic 23 to modulate the output frequency of the slave laser to lock the frequency phase of the master and slave lasers. The loop gain regulator 22 can achieve a linear change of the loop gain with the beat frequency voltage.

[0046] The voltage-controlled resistor 20 is connected to the second D / A converter 19 and is controlled by the host computer 24. The host computer 24 records the frequency of the beat frequency electrical signal, calculates the difference with the target frequency after removing the peak noise by the moving median filter method, and converts it into a digital quantity through the second D / A converter 19, and finally inputs it into the loop gain regulator 22 to change the resistance value of the voltage-controlled resistor 20, and finally changes the entire loop gain. The loop gain regulator enables the system to adapt to the two situations of large frequency difference in the frequency locking link and small frequency difference in the phase locking link, and completes the frequency phase locking of the master laser and the slave laser.

[0047] In this embodiment, the amplification factor of the amplifier circuit is determined by the resistance value of the voltage-controlled resistor 20. By designing the parameters of the amplifier circuit, the loop gain changes linearly with the frequency error. The resistance value of the voltage-controlled resistor 20 is determined by the frequency of the beat frequency electrical signal. The voltage-controlled resistor 20 uses 07D220K, and the voltage amplifier 21 uses an operational amplifier OP27.

[0048] Specific implementation method 2: This implementation method is a phase-locking method implemented by an optical phase-locked loop system based on a linear feedback algorithm described in specific implementation method 1. The method is implemented by the following steps:

[0049] S1: The host computer 24 controls the start-up of the master laser 1 and the slave laser 2, which are mixed after light splitting and combining, and are converted into beat frequency electrical signals by the photodetector 8;

[0050] S2: The frequency meter 9 extracts the frequency of the beat frequency electrical signal and inputs it to the host computer 24 for subsequent processing;

[0051] S3: Start the frequency-locked phase-locked module, set the N value of the frequency divider 10, perform Verilog programming using the host computer 24, and burn it into the FPGA chip 17 using the serial communication function;

[0052] In this embodiment, the N value of the frequency divider 10 is set to 1000, the first signal generator 15 and the second signal generator 16 are connected to the FPGA chip 17, and the two signal generators are started, wherein the frequency of the first signal generator 15 is 6.834MHz, and the frequency of the second signal generator 16 is 100MHz. Then the Verilog program is burned into the FPGA chip 17 using serial communication, and the output of the FPGA chip 17 is the control signal;

[0053] S4: The difference between the beat frequency signal and the target frequency calculated by the host computer 24 is converted into a digital value by the second D / A converter 19, and finally input into the loop gain regulator 22 to change the resistance value of the voltage-controlled resistor 20, and finally change the entire loop gain;

[0054] S5: The control signal output by the FPGA chip 17 is amplified by the voltage amplifier 21 in the loop gain regulator 22, and the voltage value input to the piezoelectric ceramic 23 is changed, and finally the output frequency phase of the slave laser is changed, thereby completing the frequency phase locking between the master laser and the slave laser.

[0055] In this embodiment, all components should be powered on and preheated for a period of time before starting, and the main laser used should be locked by saturated spectrum modulation. It has good anti-interference ability in the face of small disturbances generated during the frequency and phase locking process. When disturbances occur, the feedback control module can automatically identify and remove spike noise, and can optimize control parameters so that the system always maintains optimal operation.

[0056] The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above-mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0057] The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. An optical phase-locked loop system based on a linear feedback algorithm, characterized in that: The system includes a frequency-locking and phase-locking module and a feedback control module; The frequency-locked and phase-locked module comprises a frequency meter (9), an N frequency divider (10), a comparator (11), an FPGA chip (17), a first signal generator (15) and a second signal generator (16); the FPGA chip (17) comprises a frequency detector and phase detector (12), a time-to-digital converter (13) and a loop filter (14); The frequency meter (9) and the N frequency divider (10) respectively obtain the beat frequency electrical signal, extract the frequency value of the beat frequency electrical signal through the frequency meter (9) and input it into the host computer (24), and then convert it into a square wave signal through the comparator (11) after dividing the frequency through the N frequency divider (20), and the square wave signal is a pulse signal carrying phase difference information; The first signal generator (15) and the second signal generator (16) are used to provide a frequency detection and phase detection reference signal and a clock frequency to the frequency detection and phase detection device (12) and the time digital converter (13), respectively; The frequency and phase detector (12) extracts the frequency and phase detection reference signal and the pulse signal carrying the phase difference information and inputs them into the time-to-digital converter (13); The time-to-digital converter (13) converts the pulse signal carrying the phase difference information into a digital quantity and inputs it into a loop filter (14). After filtering out high-frequency noise through the loop filter (14), a control signal is generated and input into a feedback control module. The feedback control module processes the control signal generated by the loop filter (14) and acts on the piezoelectric ceramic (23) of the slave laser; and changes the loop gain according to the difference between the beat frequency electrical signal frequency calculated by the host computer (24) and the target frequency.

2. The optical phase-locked loop system based on the linear feedback algorithm according to claim 1, characterized in that: The frequency and phase detector (12) adopts a dual D flip-flop structure and is equipped with a delay unit; The pulse width output by the frequency and phase detector (12) is determined by the phase difference between the beat frequency electrical signal and the frequency and phase detector reference signal; when the two signals have different frequencies, the output is a pulse signal with irregular width; when the two signals have the same frequency, the output pulse signal width is determined by the phase difference depending on whether the phase is advanced.

3. The optical phase-locked loop system based on the linear feedback algorithm according to claim 2, characterized in that: The time-to-digital converter (13) adopts a direct counting structure and is controlled by the clock frequency generated by the second signal generator (16). When the clock frequency is at a rising edge, if the pulse signal is also at a rising edge, the count value is increased by 1, thereby finally converting the phase difference information into a digital quantity.

4. The optical phase-locked loop system based on a linear feedback algorithm according to claim 1, characterized in that: The feedback control module comprises a first D / A converter (18), a second D / A converter (19), a loop gain regulator (22) and a piezoelectric ceramic (23); The control signal is D / A converted by a first DA converter (18) and then outputs a control voltage to a loop gain adjustment circuit (22); The loop gain adjustment circuit (22) comprises a voltage-controlled resistor (20) and a voltage amplifier (21); The frequency value of the beat frequency electrical signal received by the host computer (24) is D / A converted by the second D / A converter (19) and then input into the loop gain adjustment circuit (22); The control voltage output by the loop gain adjustment circuit (22) acts on the piezoelectric ceramic to modulate the output frequency of the slave laser to lock the frequency phase of the master and slave lasers.

5. The optical phase-locked loop system based on the linear feedback algorithm according to claim 4, characterized in that: The loop gain adjustment circuit (22) adopts a voltage amplification circuit composed of a voltage-controlled resistor (20) and a piezoelectric amplifier (21); The amplification factor of the voltage amplifier circuit is determined by the resistance value of the voltage-controlled resistor (20), and the resistance value of the voltage-controlled resistor (20) is determined by the control voltage, and the control voltage is determined by the difference in optical frequency output from the master laser and the slave laser extracted by the host computer (24).

6. The optical phase-locked loop system based on a linear feedback algorithm according to claim 1, characterized in that: The invention also comprises an optical module, wherein the optical module comprises a master laser (1), a slave laser (2), a first half-wave plate (3), a first polarizer (4), a second half-wave plate (5), a second polarizer (6), an optical mixer (7) and a photodetector (8); The master laser (1) emits a frequency-stabilized laser, and the slave laser (2) emits a laser to be locked; the laser emitted by the master laser (1) is divided into a main transmitted light and a main reflected light via a first half-wave plate (3) and a first polarizing plate (4); the laser emitted by the slave laser (2) is divided into a slave transmitted light and a slave reflected light via a second half-wave plate (5) and a second polarizing plate (6); the main reflected light and the slave reflected light are combined and mixed by an optical mixer (7), and finally received by a photoelectric detector (8) and converted into a beat frequency electrical signal, which is input into a frequency-locking and phase-locking module.

7. The optical phase-locked loop system based on the linear feedback algorithm according to claim 6, characterized in that: The optical frequency difference between the outputs of the master laser (1) and the slave laser (2) is 6.834 GHz; the frequency divider is used to down-convert the frequency of the beat frequency electrical signal of the GHz level to the MHz level; the N value of the N frequency divider is adjustable, the reference signal frequency is 6.834 MHz; and the clock signal frequency is 100 MHz.

8. A phase locking method for an optical phase locked loop system based on a linear feedback algorithm according to any one of claims 1 to 7, characterized in that: The method is implemented by the following steps: Step S1, the optical module is used to obtain a pair of lasers with a frequency difference of 6.834 GHz, convert the beat frequency signal of the laser into a beat frequency electrical signal and transmit it to the frequency locking and phase locking module; Step S2, the frequency-locked and phase-locked module converts the pulse signal carrying the phase difference information in the beat frequency electrical signal into a control voltage and inputs the control voltage into the feedback control module; Step S3, the feedback control module processes the control signal generated by the frequency-locked and phase-locked module and applies it to the piezoelectric ceramic of the slave laser; and changes the loop gain according to the difference between the beat frequency electrical signal frequency calculated by the host computer and the target frequency.

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