Phase-locked loop circuit for optical fiber noise suppression and application

Abstract

The invention discloses a phase-locked loop circuit for optical fiber noise suppression and further discloses application of the phase-locked loop circuit for optical fiber noise suppression, the phase-locked loop circuit comprises a phase-locked loop, and the phase-locked loop comprises a filtering amplitude limiting circuit, a frequency divider, a phase frequency detector, a loop filter and a voltage-controlled oscillator circuit which are connected in sequence. The optical fiber noise suppression method can meet the actual demand of optical fiber noise suppression in the atomic optical frequency standard, has a good suppression effect on noise introduced by optical fiber transmission, and can effectively reduce the laser frequency instability introduced by the optical fiber noise; compared with a frequency mixer, a filter and a PID servo controller in a traditional technical scheme, a larger locking range can be achieved, automatic relocking can be achieved after unlocking, and miniaturization and integration of a circuit system can be achieved easily.

Description

Technical Field

[0001] The present invention belongs to the technical field of phase-locked loops, and in particular relates to a phase-locked loop circuit for optical fiber noise suppression, and also relates to an application of the phase-locked loop circuit for optical fiber noise suppression, which is suitable for suppressing phase noise generated in an optical fiber. Background Art

[0002] In atomic optical frequency standards, the detection of cold atomic systems using ultra-narrow linewidth clock lasers and the measurement of clock laser frequencies using femtosecond optical frequency combs are both inseparable from optical fiber transmission. In the above process, it is usually expected that optical fiber transmission will not introduce additional phase noise, so as not to affect the detection of clock transition spectral lines and the frequency measurement of clock lasers.

[0003] During the use of optical fiber, mechanical stretching, temperature fluctuations, and pressure changes will cause changes in its length and refractive index. During the transmission of clock laser in optical fiber, its phase will change randomly, resulting in the widening of the laser line width. For high-precision optical frequency standards, this widening cannot be ignored. Therefore, the noise introduced by optical fiber transmission must be suppressed.

[0004] The general scheme for fiber noise suppression in the experiment is: the beat frequency signal output by the photodetector PD is mixed with a stable reference signal (referenced to the hydrogen clock), and the error signal is obtained after low-pass filtering. The error signal is then input into the PID servo controller to generate a control signal to change the output frequency of the voltage-controlled oscillator VCO, and finally the acousto-optic modulator AOM is controlled to shift the frequency to achieve frequency locking between the beat frequency signal and the reference signal. This technical scheme has the following main disadvantages: (1) The locking range is small. The lock can only be established when the beat frequency signal and the reference signal frequency are very close, and the conditions for establishing the lock are relatively harsh; (2) After unlocking, it is impossible to automatically re-lock, and manual intervention is required, and the anti-interference ability is poor; (3) There are many devices and equipment used, and the overall power consumption is large, which is not conducive to the miniaturization and integration of the circuit system. Summary of the invention

[0005] The purpose of the present invention is to provide a phase-locked loop circuit for optical fiber noise suppression and an application of the phase-locked loop circuit for optical fiber noise suppression in view of the above problems existing in the prior art.

[0006] The above-mentioned purpose of the present invention is achieved by the following technical means:

[0007] A phase-locked loop circuit for optical fiber noise suppression comprises a phase-locked loop, wherein the phase-locked loop comprises a filter and limiter circuit, a frequency divider, a frequency detector, a loop filter, and a voltage-controlled oscillator circuit, wherein the output end of the filter and limiter circuit is connected to the input end of the frequency divider, the output end of the frequency divider is connected to the input end of the frequency detector, the output end of the frequency detector is connected to the input end of the loop filter, the output end of the loop filter is connected to the input end of the voltage-controlled oscillator circuit, and the frequency detector further comprises a reference input end;

[0008] The input end of the filter and limiter circuit is the signal input end of the phase-locked loop, the output end of the voltage-controlled oscillator circuit is the signal output end of the phase-locked loop, and the reference input end of the frequency detector and phase detector is connected to the external reference voltage signal.

[0009] As described above, the filtering and limiting circuit includes a bandpass filter and a limiting amplifier, the input end of the bandpass filter is the signal input end of the phase-locked loop, the output end of the bandpass filter is connected to the input end of the limiting amplifier, and the output end of the limiting amplifier is connected to the input end of the frequency divider.

[0010] As mentioned above, the phase-locked loop also includes a voltage stabilizing circuit, the input end of the voltage stabilizing circuit is connected to an external power supply, and the output end of the voltage stabilizing circuit is connected to the filter limiting circuit, the frequency divider, the frequency detector, the phase detector, the loop filter, and the power supply end of the voltage-controlled oscillator circuit.

[0011] As described above, the limiting amplifier also includes a polarity control port, and the output end of the limiting amplifier includes a positive phase output end and a negative phase output end; the input end of the frequency divider includes a positive phase input end and a negative phase input end, the positive phase input end of the frequency divider is connected to the negative phase output end of the limiting amplifier, the negative phase input end of the frequency divider is connected to the positive phase output end of the limiting amplifier, and the output end of the frequency divider includes a positive phase output end and a negative phase output end; the input end of the frequency detector includes a positive phase input end and a negative phase input end, the positive phase input end of the frequency detector is connected to the negative phase output end of the frequency divider, the negative phase input end of the frequency detector is connected to the positive phase output end of the frequency divider, and the frequency detector includes a NU output end and a ND output end; the input end of the loop filter includes a positive phase input end and a negative phase input end, the positive phase input end of the loop filter is connected to the NU output end of the frequency detector, and the negative phase input end of the loop filter is connected to the ND output end of the frequency detector.

[0012] It also includes a first buffer and a first LED status indicator light. The input end of the first buffer is connected to the LOS output end of the limiting amplifier, and the LOS signal output end of the first buffer is connected to the first LED status indicator light.

[0013] It also includes a second buffer and a second LED status indicator light. The frequency detector and phase detector also include an LD signal output end. The LD signal output end of the frequency detector and phase detector is connected to the input end of the second buffer. The output end of the second buffer is connected to the second LED status indicator light.

[0014] The voltage-controlled oscillator circuit includes a locking switch and a voltage-controlled oscillator. The input end of the locking switch is connected to the output end of the loop filter, the output end of the locking switch is connected to the input end of the voltage-controlled oscillator, and the output end of the voltage-controlled oscillator is the output end of the voltage-controlled oscillator circuit.

[0015] An application of a phase-locked loop circuit for optical fiber noise suppression, using the phase-locked loop circuit for optical fiber noise suppression as described above, including an optical fiber noise suppression optical path and a power amplifier, wherein the optical fiber noise suppression optical path includes a polarization beam splitter prism, a photodetector, a Faraday rotator, an acousto-optic modulator, an optical fiber, a total reflection mirror, and a quarter wave plate, wherein an incident laser is input into the polarization beam splitter prism, the polarization beam splitter prism divides the incident light into a signal light and a reference light, the reference light passes through the quarter wave plate and the total reflection mirror and is input into the photodetector, and the signal light passes through the Faraday rotator and the acousto-optic modulator in sequence. The optical modulator enters the optical fiber, and the optical fiber outlet end face reflects the signal light. The reflected signal light then passes through the optical fiber, the acousto-optic modulator, the Faraday rotator, and the polarization beam splitter in sequence and is input into the photodetector. The photodetector beats the reference light and the reflected signal light, and the photodetector outputs the beat frequency electrical signal to the bandpass filter of the phase-locked loop. Finally, the voltage-controlled oscillator of the phase-locked loop outputs the frequency-modulated signal, which is input into the electrical signal input end of the acousto-optic modulator after passing through the power amplifier. The acousto-optic modulator shifts the frequency of the signal light according to the amplified frequency-modulated signal to lock the frequency of the signal light.

[0016] As mentioned above, the signal light frequency is locked to f l +(N / 2)*f Ref , where f l is the frequency of the incident laser before entering the polarization beam splitter, N is the frequency division coefficient of the frequency divider, * is the multiplication sign, f Ref is the frequency of the external reference voltage signal.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] (1) The present invention can meet the practical needs of optical fiber noise suppression in atomic optical frequency standards, has a good suppression effect on the noise introduced by optical fiber transmission, and can effectively reduce the laser frequency instability introduced by optical fiber noise.

[0019] (2) The present invention can achieve a larger locking range and can automatically relock after unlocking, which is also conducive to miniaturization and integration of the circuit system. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic diagram of the structure of the phase-locked loop and the connection between the phase-locked loop and the optical fiber noise suppression optical path according to the second embodiment of the present invention;

[0021] Figure 2 It is a schematic diagram of the structure of the optical fiber noise suppression optical path of Example 2 of the present invention;

[0022] Figure 3 This is a diagram showing the effect of optical fiber noise suppression according to Embodiment 2 of the present invention;

[0023] Reference numerals and corresponding component names:

[0024] 1—filter and limiter circuit; 2—frequency divider; 3—frequency detector and phase detector; 4—loop filter; 5—voltage-controlled oscillator circuit; 6—voltage stabilization circuit; 7—optical fiber noise suppression optical path; 8—power amplifier; 9—phase-locked loop. DETAILED DESCRIPTION

[0025] In order to facilitate those skilled in the art to understand and implement the present invention, the present invention is further described in detail below in conjunction with embodiments. The embodiments described herein are only used to illustrate and explain the present invention and are not intended to limit the present invention.

[0026] Embodiment 1:

[0027] A phase-locked loop circuit for optical fiber noise suppression comprises a phase-locked loop 9, wherein the phase-locked loop 9 comprises a filter and limiter circuit 1, a frequency divider 2, a frequency detector 3, a loop filter 4, and a voltage-controlled oscillator circuit 5, wherein the output end of the filter and limiter circuit 1 is connected to the input end of the frequency divider 2, the output end of the frequency divider 2 is connected to the input end of the frequency detector 3, the output end of the frequency detector 3 is connected to the input end of the loop filter 4, and the output end of the loop filter 4 is connected to the input end of the voltage-controlled oscillator circuit 5; the input end of the filter and limiter circuit 1 is the signal input end of the phase-locked loop 9, the output end of the voltage-controlled oscillator circuit 5 is the signal output end of the phase-locked loop 9, the frequency detector 3 further comprises a reference input end, and the reference input end of the frequency detector 3 is connected to an external reference voltage signal (Ref In).

[0028] The phase-locked loop 9 also includes a voltage stabilizing circuit 6, an input end of the voltage stabilizing circuit 6 is connected to an external power supply, an output end of the voltage stabilizing circuit 6 is connected to the power supply end of the filter limiting circuit 1, the frequency divider 2, the frequency detector 3, the loop filter 4, and the voltage controlled oscillator circuit 5, and the voltage stabilizing circuit 6 is used to provide a stable operating voltage for other modules of the phase-locked loop 9. In this embodiment, the voltage stabilizing circuit 6 outputs a +5V voltage and a +3.3V voltage.

[0029] As an implementable embodiment, the voltage stabilizing circuit 6 includes a switching circuit, a +5V power supply circuit and a +3.3V power supply circuit. The input end of the switching circuit is connected to an external power supply, and the output end of the switching circuit is connected to the input end of the +5V power supply circuit and the input end of the +3.3V power supply circuit. Both the +5V power supply circuit and the +3.3V power supply circuit use a DC voltage stabilizing chip. The external power supply supplies power for the +5V power supply circuit and the +3.3V power supply circuit. The +5V power supply circuit and the +3.3V power supply circuit output a stepped-down +5V voltage and a +3.3V voltage, respectively.

[0030] The +5V power supply circuit can use the ADM7150-5.0 chip, and the +3.3V power supply circuit can use the ADM7150-3.3 chip.

[0031] The filtering and limiting circuit 1 includes a bandpass filter (BPF) and a limiting amplifier. The input end of the bandpass filter is the signal input end of the phase-locked loop 9, the output end of the bandpass filter is connected to the input end of the limiting amplifier, and the output end of the limiting amplifier is connected to the input end of the frequency divider 2.

[0032] The bandpass filter uses a chip of the SXBP-161R5+ model; the limiting amplifier can use a chip of the ONET4201PA model; the power supply end of the limiting amplifier is connected to the +3.3V voltage output by the voltage stabilizing circuit 6; the output end of the limiting amplifier includes a positive phase output end and an inverting phase output end, and the limiting amplifier also includes a polarity control port, and the polarity of the signal output by the limiting amplifier can be adjusted by setting the level of the polarity control port. In this embodiment, when the polarity control port is set to a high level (High) or remains open (Open), the limiting amplifier outputs an output signal of normal polarity (Normal) through the positive phase output end, and when the polarity control port is set to a low level (Low) or grounded, the limiting amplifier outputs an output signal of inverted polarity (Inverted polarity) through the inverting output end.

[0033] The filtering and limiting circuit 1 also includes a first buffer and a first LED status indicator. The input end of the first buffer is connected to the LOS output end of the limiting amplifier, and the LOS signal output end of the first buffer is connected to the first LED status indicator. The first buffer uses a chip of the SN74LVC1G17 model. The first LED status indicator is used as a LOS level status indicator. When the input signal of the limiting amplifier is lower than the set threshold, the LOS output end of the limiting amplifier outputs a high level, which is output to the first LED status indicator after passing through the first buffer, indicating that the signal is lost;

[0034] The frequency divider 2 can use a chip of the HMC394LP4 (E) type, and the power supply end of the frequency divider 2 is connected to the +5V voltage output by the voltage stabilizing circuit 6; the input end of the frequency divider 2 includes a positive input end and a negative input end, the positive input end of the frequency divider 2 is connected to the negative output end of the limiting amplifier, and the negative input end of the frequency divider 2 is connected to the positive output end of the limiting amplifier. The output end of the frequency divider 2 includes a positive output end and a negative output end, the positive output end of the frequency divider 2 outputs a signal with the same polarity as the input signal, and the negative output end of the frequency divider 2 outputs a signal with the opposite polarity to the input signal; the frequency divider 2 also includes a frequency division coefficient selection end, and the frequency division coefficient can be selected by setting different high and low level combinations;

[0035] An example truth table of the frequency division coefficient selection terminal of divider 2: when 00000 is input to the frequency division coefficient selection terminal, no division is performed; when 10000 is input to the frequency division coefficient selection terminal, the frequency division coefficient is 2, and divider 2 divides the input signal by 2; when 01000 is input to the frequency division coefficient selection terminal, the frequency division coefficient is 3, and divider 2 divides the input signal by 3, and so on, when 11000 is input to the frequency division coefficient selection terminal, the frequency division coefficient is 4, and divider 2 divides the input signal by 4; when 11100 is input to the frequency division coefficient selection terminal, the frequency division coefficient is 8, and divider 2 divides the input signal by 8; when 11110 is input to the frequency division coefficient selection terminal, the frequency division coefficient is 16, and divider 2 divides the input signal by 16; when 11111 is input to the frequency division coefficient selection terminal, the frequency division coefficient is 32, and divider 2 divides the input signal by 32.

[0036] The phase detector 3 can detect the phase difference and frequency difference between the external reference voltage signal and the output voltage signal of the frequency divider 2, and output the UP voltage signal and the DOWN voltage signal; the phase detector 3 can select the HMC3716LP4E chip, and the power supply end of the phase detector 3 is connected to the +5V voltage output by the voltage stabilizing circuit 6; the input end of the phase detector 3 includes a first differential input end (i.e., a positive input end) and a second differential input end (inverting input end), the first differential input end of the phase detector 3 is connected to the inverting output end of the frequency divider 2, the second differential input end of the phase detector 3 is connected to the positive output end of the frequency divider 2, and the phase detector 3 includes a NU output end and an ND output end, which respectively output the UP voltage signal and the DOWN voltage signal;

[0037] The phase frequency detector 3 also includes an LD (Locked) signal output terminal, the LD signal output terminal of the phase frequency detector 3 is connected to the input terminal of the second buffer, and the output terminal of the second buffer is connected to the second LED status indicator. When the input signal and the output signal of the phase frequency detector 3 are of the same frequency and phase, the LD signal output terminal of the phase frequency detector 3 outputs a high level and is input to the second LED status indicator through the second buffer, and the phase frequency detector 3 is in a locked state;

[0038] The second buffer may use a chip of the SN74LVC1G17 model, and the power supply end of the second buffer is connected to the +5V voltage output by the voltage regulator circuit 6.

[0039] The loop filter 4 adopts a differential operational amplifier structure, which is used to integrate the UP pulse output from the NU output terminal of the phase frequency detector 3 or the DOWN pulse output from the ND output terminal into a usable tuning voltage to adjust the output frequency of the voltage controlled oscillator circuit 5;

[0040] The loop filter 4 can use a chip of the HMC394LP4 (E) model, and the power supply end of the loop filter 4 is connected to the +5V voltage output by the voltage regulator circuit 6; the input end of the loop filter 4 includes a positive input end and a negative input end, the positive input end of the loop filter 4 is connected to the NU output end of the frequency detector 3, and the negative input end of the loop filter 4 is connected to the ND output end of the frequency detector 3;

[0041] The voltage-controlled oscillator circuit 5 can use a chip of model CVSS-945-80.000; the voltage-controlled oscillator circuit 5 includes a lock switch (Lock SW) and a voltage-controlled oscillator, the input end of the lock switch is connected to the output end of the loop filter 4, the output end of the lock switch is connected to the input end of the voltage-controlled oscillator, the output end of the voltage-controlled oscillator is the output end of the voltage-controlled oscillator circuit 5, and is also the signal output end of the phase-locked loop 9, and the power supply end of the voltage-controlled oscillator is connected to the +5V voltage output by the voltage stabilizing circuit 6.

[0042] The present invention adopts a digital frequency detector and phase detector 3, which can provide a larger locking range and can achieve automatic relocking after unlocking; the filter limiter circuit 1, the divider 2 circuit and the loop filter 4 circuit replace the mixer, the filter, and the PID servo controller in the traditional technical solution, which is conducive to the miniaturization and integration of the circuit system.

[0043] Embodiment 2:

[0044] An application of a phase-locked loop circuit for optical fiber noise suppression also includes an optical fiber noise suppression optical path 7 and a power amplifier 8. The optical fiber noise suppression optical path 7 includes a polarization beam splitter prism, a photodetector, a Faraday rotator, an acousto-optic modulator, an optical fiber, a total reflector, and a quarter-wave plate. The incident laser is split into two beams after passing through the polarization beam splitter prism (PBS), and the transmitted light is used as the reference light and the reflected light is used as the signal light. The reference light is reflected to the photodetector (PD) via a 0° total reflection mirror, a quarter wave plate (λ / 4), and a polarization beam splitter prism; the signal light is polarized by 45° after passing through a Faraday rotator, and then enters the optical fiber through an acousto-optic modulator (AOM). The optical fiber outlet end face (FC / PC) will reflect part of the light back, and the reflected laser will pass through the optical fiber, the acousto-optic modulator (AOM), and the Faraday rotator again. The polarization of the reflected laser changes by 45° again, and just passes through the polarization beam splitter prism to the photodetector. The photodetector beats the reference light and the reflected signal light, and the photodetector outputs a beat signal to the bandpass filter of the phase-locked loop 9. Finally, the voltage-controlled oscillator of the phase-locked loop 9 outputs a frequency-modulated signal, and the frequency-modulated signal passes through a power amplifier 8 (Amplifier) ​​and is input to the electrical signal input end of the acousto-optic modulator. The acousto-optic modulator shifts the frequency of the signal light according to the amplified frequency-modulated signal to lock the frequency of the signal light.

[0045] The working principle of the present invention is: Assuming that the frequency of the incident laser before entering the polarization beam splitter is f l The signal light passes through the acousto-optic modulator (driving frequency is f AOM ) after the positive first-order diffraction light frequency is f l +f AOM , assuming the frequency noise introduced by the optical fiber is f n , then the frequency of light out of the optical fiber is f l +f AOM +f n , the laser is reflected by the end face of the optical fiber outlet and passes through the optical fiber and the acousto-optic modulator again. Since the time for the signal light to pass through the optical fiber twice is extremely short, it can be considered that the noise introduced by the two passes through the optical fiber is the same. Therefore, the frequency of the reflected laser finally reaching the photodetector is f l +2f AOM +2f n , since the reference light frequency is f l , then the frequency of the two-way optical beat signal detected by the photodetector is 2f AOM +2f n , the beat frequency electrical signal is input into the phase-locked loop 9, and the external reference voltage signal frequency f Ref =20MHz, so that f Ref =(2f AOM +2f n) / N (N=8), the frequency modulation signal output by the phase-locked loop 9 is amplified by the power amplifier 8 (Amplifier) ​​and controls the frequency shift of the acousto-optic modulator, so that the laser frequency out of the optical fiber is locked to f l +(N / 2)*f Ref .

[0046] As an implementation method, the external reference voltage signal frequency f Ref =20MHz, the frequency of the phase detector 3 is f PFD =20MHz and the voltage controlled oscillator frequency f VCO =80MHz, tuning sensitivity K vco =2kHz / V, modulation bandwidth Modulation BW=10kHz, loop bandwidth Loop BW of loop filter 4=5kHz, phase margin PhaseMargin=45deg; optical fiber adopts 5m polarization-maintaining optical fiber (in this embodiment, P5-488PM-FC-5 optical fiber of Thorlabs Company is adopted), one end of the optical fiber is an 8° bevel (FC / APC), the other end of the optical fiber is a plane (FC / PC), the entrance end face of the incident laser selects the optical fiber end face of FC / APC, which can prevent the light reflected from the entrance end face from interfering with the extraction of optical fiber noise, and the exit end is the optical fiber end face of FC / PC; the incident laser is 578nm laser, the laser power is 3mW, and the driving frequency of the acousto-optic modulator is about 80MHz;

[0047] In order to evaluate the fiber noise suppression effect of the phase-locked loop 9 of the present invention, the Allan variance is usually used to characterize the relative instability of the laser frequency caused by the fiber noise. The counter used in this example is the KEYSIGHT 53230A model frequency counter. The beat frequency signal after down-conversion is measured in the dead zone free measurement mode (total measurement time: 2h, measurement interval: 1s). Then, the Allan variance is calculated using the Stable32 software and its variation curve is plotted. The obtained noise suppression effect evaluation results are shown in Figure 3 As shown in the figure, the relative instability of the laser frequency caused by the residual fiber noise is 2.59×10 -18 (1s averaging time).

[0048] The frequency of the beat frequency electrical signal output by the photodetector is 160MHz. Since it is difficult to directly measure high-frequency signals, it is considered to down-convert the beat frequency electrical signal. The specific method of down-conversion is: select a signal with a frequency of 160.1MHz (referenced to the hydrogen clock) and mix it with the original beat frequency electrical signal and perform low-pass filtering to obtain a down-converted beat frequency signal with a frequency of 100kHz. The down-converted beat frequency signal retains the frequency stability of the original beat frequency electrical signal.

[0049] It should be noted that the embodiments described in the present invention are merely examples of the spirit of the present invention. Those skilled in the art may make various modifications or additions to the described embodiments or replace them in similar ways, but they will not deviate from the spirit of the present invention or exceed the scope defined by the attached claims.

Claims

1. A phase-locked loop circuit for optical fiber noise suppression, comprising a phase-locked loop (9), characterized in that: The phase-locked loop (9) comprises a filtering and limiting circuit (1), a frequency divider (2), a frequency detector (3), a loop filter (4), and a voltage-controlled oscillator circuit (5); the output end of the filtering and limiting circuit (1) is connected to the input end of the frequency divider (2); the output end of the frequency divider (2) is connected to the input end of the frequency detector (3); the output end of the frequency detector (3) is connected to the input end of the loop filter (4); the output end of the loop filter (4) is connected to the input end of the voltage-controlled oscillator circuit (5); and the frequency detector (3) further comprises a reference input end; The input end of the filtering and limiting circuit (1) is the signal input end of the phase-locked loop (9), the output end of the voltage-controlled oscillator circuit (5) is the signal output end of the phase-locked loop (9), and the reference input end of the frequency detector (3) is connected to an external reference voltage signal.

2. A phase-locked loop circuit for optical fiber noise suppression according to claim 1, characterized in that: The filtering and limiting circuit (1) comprises a bandpass filter and a limiting amplifier, the input end of the bandpass filter is the signal input end of the phase-locked loop (9), the output end of the bandpass filter is connected to the input end of the limiting amplifier, and the output end of the limiting amplifier is connected to the input end of the frequency divider (2).

3. A phase-locked loop circuit for optical fiber noise suppression according to claim 2, characterized in that: The phase-locked loop (9) further comprises a voltage stabilizing circuit (6), the input end of the voltage stabilizing circuit (6) being connected to an external power supply, and the output end of the voltage stabilizing circuit (6) being connected to a filtering and limiting circuit (1), a frequency divider (2), a frequency detector and a phase detector (3), a loop filter (4), and a power supply end of a voltage-controlled oscillator circuit (5).

4. A phase-locked loop circuit for optical fiber noise suppression according to claim 3, characterized in that: The limiting amplifier further comprises a polarity control port, the output end of the limiting amplifier comprises a positive phase output end and a negative phase output end; the input end of the frequency divider (2) comprises a positive phase input end and a negative phase input end, the positive phase input end of the frequency divider (2) is connected to the negative phase output end of the limiting amplifier, the negative phase input end of the frequency divider (2) is connected to the positive phase output end of the limiting amplifier, and the output end of the frequency divider (2) comprises a positive phase output end and a negative phase output end; the input end of the frequency detector (3) comprises a positive phase input end and a negative phase input end, the frequency detector (3) comprises a positive phase input end and a negative phase input end, The positive phase input end of the phase detector (3) is connected to the negative phase output end of the frequency divider (2), the negative phase input end of the frequency detector (3) is connected to the positive phase output end of the frequency divider (2), and the frequency detector (3) includes a NU output end and an ND output end; the input end of the loop filter (4) includes a positive phase input end and a negative phase input end, the positive phase input end of the loop filter (4) is connected to the NU output end of the frequency detector (3), and the negative phase input end of the loop filter (4) is connected to the ND output end of the frequency detector (3).

5. A phase-locked loop circuit for optical fiber noise suppression according to claim 4, characterized in that: It also includes a first buffer and a first LED status indicator light. The input end of the first buffer is connected to the LOS output end of the limiting amplifier, and the LOS signal output end of the first buffer is connected to the first LED status indicator light.

6. A phase-locked loop circuit for optical fiber noise suppression according to claim 5, characterized in that: It also includes a second buffer and a second LED status indicator light. The frequency detector (3) also includes an LD signal output end. The LD signal output end of the frequency detector (3) is connected to the input end of the second buffer, and the output end of the second buffer is connected to the second LED status indicator light.

7. A phase-locked loop circuit for optical fiber noise suppression according to claim 6, characterized in that: The voltage-controlled oscillator circuit (5) comprises a locking switch and a voltage-controlled oscillator, wherein the input end of the locking switch is connected to the output end of the loop filter (4), the output end of the locking switch is connected to the input end of the voltage-controlled oscillator, and the output end of the voltage-controlled oscillator is the output end of the voltage-controlled oscillator circuit (5).

8. An application of a phase-locked loop circuit for optical fiber noise suppression, using the phase-locked loop circuit for optical fiber noise suppression according to claim 7, characterized in that: The invention comprises an optical fiber noise suppression optical path (7) and a power amplifier (8), wherein the optical fiber noise suppression optical path (7) comprises a polarization beam splitter prism, a photoelectric detector, a Faraday rotator, an acousto-optic modulator, an optical fiber, a total reflection mirror, and a quarter wave plate, wherein an incident laser is input into the polarization beam splitter prism, the polarization beam splitter prism divides the incident light into a signal light and a reference light, the reference light passes through the quarter wave plate and the total reflection mirror and is input into the photoelectric detector, the signal light passes through the Faraday rotator and the acousto-optic modulator in sequence and enters the optical fiber, and the optical fiber outlet end face reflects the signal light. The reflected signal light is then sequentially transmitted through an optical fiber, an acousto-optic modulator, a Faraday rotator, and a polarization beam splitter and then input into a photodetector. The photodetector beats the reference light and the reflected signal light, and then outputs a beat frequency electrical signal to a bandpass filter of a phase-locked loop (9). Finally, a voltage-controlled oscillator of the phase-locked loop (9) outputs a frequency-modulated signal, and the frequency-modulated signal is input into an electrical signal input end of the acousto-optic modulator after passing through a power amplifier (8). The acousto-optic modulator shifts the frequency of the signal light according to the amplified frequency-modulated signal, and locks the frequency of the signal light.

9. The application of a phase-locked loop circuit for optical fiber noise suppression according to claim 8, characterized in that: The signal light frequency is locked to f l +(N / 2)*f Ref , where f l is the frequency of the incident laser before entering the polarization beam splitter, N is the frequency division coefficient of the frequency divider (2), * is the multiplication sign, f Ref is the frequency of the external reference voltage signal.

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