A relay station for digitally locking and real-time monitoring of phase noise in an optical fiber link

Abstract

The present invention discloses a relay station for digitally locking and real-time monitoring phase noise of an optical fiber link, belonging to the technical field of relay stations for optical fiber optical frequency transfer, which includes a main control circuit, a link locking circuit and a laser locking circuit; the link locking circuit is bidirectionally connected to the main control circuit, the link locking circuit sends a link locking error signal to the main control circuit, and the main control circuit sends a link locking instruction to the link locking circuit; the laser locking circuit is bidirectionally connected to the main control circuit, the laser locking circuit sends a laser locking error signal to the main control circuit, and the main control circuit sends a laser locking instruction to the laser locking circuit; the link locking circuit includes an optical frequency digital phase-locked loop, and the optical frequency digital phase-locked loop uses a DDS to drive an acousto-optic modulator to adjust the frequency and phase of an optical frequency signal. The optical frequency digital phase-locked loop is used to measure the phase noise power spectrum in real time, which can not only achieve link locking, but also determine whether the power spectrum of the signal after locking reaches the theoretical noise suppression limit.

Description

Technical Field

[0001] The present invention belongs to the technical field of relay stations for fiber optic optical frequency transfer, and particularly relates to a relay station for digitally locking a fiber optic link and real-time monitoring of phase noise. Background Art

[0002] In the field of fiber optic optical frequency transfer, in order to achieve remote optical clock comparison, long-distance optical frequency transfer requires a relay station to improve the transfer accuracy. By phase-locking the regenerated light source to the input light, optical regeneration amplification is achieved, which can effectively filter out noise outside the amplification bandwidth and improve the signal-to-noise ratio.

[0003] The circuit of the relay station consists of three parts: a link phase error compensation part circuit, a laser locking part circuit, and a main control circuit. The relay station serves to achieve long-distance fiber optic link locking, and high-precision long-distance link locking is required to achieve long-distance optical clock comparison.

[0004] Existing relay stations cannot achieve long-term locking and will have problems of fiber optic link unlocking under low signal-to-noise ratio conditions. To determine the locking result, it is necessary to measure the phase noise power spectral density of the locked signal. Existing instruments for measuring phase noise power spectral density are large in volume and very heavy. This equipment is laid on a link of thousands of kilometers and requires fully automated monitoring. Large phase noise meters are not suitable for actual test requirements. The fiber optic link is laid along the highway, with a lot of noise, resulting in a low signal-to-noise ratio of the link error signal. As Figure 6 shown, the fiber optic link is prone to unlocking due to environmental impact interference, and the impact interference will directly damage the control bandwidth of the phase-locked loop, and the impact interference is very intense. Summary of the Invention

[0005] The purpose of the present invention is to overcome the problem that large phase noise meters are not suitable for long-distance fiber optic link test requirements, and propose a relay station for digitally locking a fiber optic link and real-time monitoring of phase noise.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] In a first aspect, the present invention provides a relay station for digitally locking a fiber optic link and real-time monitoring of phase noise, including a main control circuit, a link locking circuit, and a laser locking circuit;

[0008] The link locking circuit is bidirectionally connected to the main control circuit. The link locking circuit sends a link locking error signal to the main control circuit, and the main control circuit sends a link locking instruction to the link locking circuit;

[0009] The laser locking circuit is bidirectionally connected to the main control circuit. The laser locking circuit sends a laser locking error signal to the main control circuit, and the main control circuit sends a laser locking instruction to the laser locking circuit;

[0010] The link locking circuit includes an optical frequency digital phase-locked loop, and the optical frequency digital phase-locked loop uses a DDS to drive an acousto-optic modulator to adjust the frequency and phase of the optical frequency signal.

[0011] Further, the optical frequency digital phase-locked loop includes an ADC, an I / Q demodulator, a low-pass filter, a coordinate rotation digital computing device, a PID, a DDS, and an acousto-optic modulator;

[0012] The ADC converts the analog phase error signal into a digital signal, the I / Q demodulator decomposes the digital signal into I / Q components, the low-pass filter filters out the high-frequency noise of the I / Q components, the coordinate rotation digital computing device uses the CORDIC algorithm to calculate the phase error according to the I / Q components after filtering out the high-frequency noise, the PID adjusts the control parameters according to the phase error, the DDS generates a control signal according to the control parameters, and the acousto-optic modulator adjusts the frequency and phase of the optical frequency signal according to the control signal.

[0013] Further, the laser locking circuit uses the PDH frequency stabilization method to lock the laser.

[0014] In a second aspect, the present invention provides a control method for a relay station that digitally locks an optical fiber link and real-time monitors phase noise. Using the relay station that digitally locks an optical fiber link and real-time monitors phase noise, the method includes the following steps:

[0015] When the optical fiber link is unlocked, the optical fiber link is automatically locked based on the link locking circuit using the link automatic locking algorithm;

[0016] After the optical fiber link is automatically locked, the phase noise power spectral density analysis method is used to check whether the signal noise of the optical fiber link after using the link automatic locking algorithm reaches the noise suppression limit.

[0017] Further, the phase noise power spectral density analysis method includes the following steps:

[0018] The link phase error signal obtained by phase discrimination of the coordinate rotation digital computing device;

[0019] Apply the Hamming window function to the link phase error signal to obtain the Hamming window function result of the link phase error signal;

[0020] After selecting the number of sampling points for the fast Fourier transform, calculate the fast Fourier transform of the Hamming window function result of the link phase error signal to obtain the fast Fourier transform result;

[0021] Calculate the power spectral density according to the power spectral density calculation formula. The power spectral density is a symmetric two-sided spectrum, and the power spectral density is corrected unilaterally to obtain the unilateral power spectral density.

[0022] Furthermore, the Hamming window function is as follows:

[0023]

[0024] where is the value of the Hamming window at the m-th sampling point, is a constant, , N is the window length, and π is a mathematical constant;

[0025] The result of the Hamming window function for the link phase error signal is:

[0026]

[0027] where is the result of the Hamming window function for the link phase error signal, is the link phase error signal, is the Hamming window function.

[0028] Furthermore, the number of sampling points for the fast Fourier transform is the recording length of the discrete-time signal.

[0029] Furthermore, the power spectral density calculation formula is:

[0030]

[0031] where is the power spectral density, is the result of the fast Fourier transform, f is the sampling frequency, n is the number of sampling points, and abs is the modulus.

[0032] Furthermore, the link automatic locking algorithm includes the following steps:

[0033] Step 1: Determine whether the link phase error signal is within the first locking interval, and the first locking interval is a closed interval. If so, proceed to Step 3; otherwise, proceed to Step 2.

[0034] Step 2: The main control circuit sends a command to control the opening of the tracking amplitude stabilization filter, and then determines whether the link phase error signal is within the first locking interval again. If so, proceed to Step 3.

[0035] Step 3: The main control circuit issues a link locking instruction to the link locking circuit, and writes the proportional parameter, integral parameter, and differential parameter into the optical frequency digital phase-locked loop.

[0036] Step 4: Determine whether the link phase error signal is within the second locking interval, and the second locking interval is a closed interval and within the first locking interval. If so, complete the link automatic locking.

[0037] Furthermore, the tracking and amplitude stabilizing filter described in step three is arranged between the main control circuit and the link locking circuit.

[0038] Compared with the prior art, the present invention has the following beneficial technical effects:

[0039] A relay station for digital locking of an optical fiber link and real-time monitoring of phase noise, which is used for optical frequency transfer, integrates digital phase-locked loop technology, laser locking technology and main control circuit technology to achieve high-precision compensation of link phase error signals, laser phase locking and comprehensive monitoring of the system. The optical frequency digital phase-locked loop is used to measure the phase noise power spectrum in real time, which can not only achieve link locking, but also determine whether the power spectrum of the signal after locking reaches the theoretical noise suppression limit. By realizing the automatic locking of the link phase error signal, the problem that the equipment cannot be manually operated when distributed on a long-distance link, such as a 3000-kilometer link, is solved, reducing human and material resources, contributing to the stable operation of the equipment and the system, and is particularly suitable for scenarios such as long-distance optical frequency transfer and optical fiber communication testing, with advantages such as high precision, high stability and high automation level. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The drawings described herein are for illustrative purposes only and are not intended to limit the scope of the disclosure of the present invention in any way. Additionally, the shapes and proportional dimensions of the components in the figures are only schematic and are used to assist in understanding the present invention, rather than specifically defining the shapes and proportional dimensions of the components of the present invention. In the drawings:

[0041] Figure 1 is a schematic diagram of the principle of a relay station for digital locking of an optical fiber link and real-time monitoring of phase noise.

[0042] Figure 2 is a flowchart of the operation of the optical frequency digital phase-locked loop.

[0043] Figure 3 is a schematic diagram of the phase noise calculation process.

[0044] Figure 4 is a schematic diagram of the phase noise display interface.

[0045] Figure 5 is a schematic diagram of the link automatic locking algorithm process.

[0046] Figure 6 is a schematic diagram of link unlocking and maintaining the locked state. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0047] To enable those skilled in the art to better understand the solution of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

[0048] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used in the specification of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0049] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects and do not necessarily need to describe a specific order or sequence. It should be understood that such data can be interchanged under appropriate circumstances so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "comprising" and "having" and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device comprising a series of steps or units does not necessarily have to be limited to those steps or units clearly listed, but may include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0050] Embodiment 1

[0051] This embodiment provides a relay station for digitally locking and real-time monitoring the phase noise of an optical fiber link, as Figure 1 shown in the principle block diagram of the relay station, which includes a main control circuit, a link locking circuit, and a laser locking circuit; the link locking circuit is bidirectionally connected to the main control circuit, the link locking circuit sends a link locking error signal to the main control circuit, and the main control circuit sends a link locking instruction to the link locking circuit; the laser locking circuit is bidirectionally connected to the main control circuit, the laser locking circuit sends a laser locking error signal to the main control circuit, and the main control circuit sends a laser locking instruction to the laser locking circuit;

[0052] The link locking circuit includes an optical frequency digital phase-locked loop. Refer to Figure 2 , and improve the digital phase-locked loop developed based on FPGA (Field-Programmable Gate Array) to a phase-locked loop suitable for an optical frequency transfer system.

[0053] Link noise compensation is carried out based on digital phase-locked loop technology. The optical frequency digital phase-locked loop consists of an ADC (Analog-to-Digital Converter), I / Q (In-phase / Quadrature) demodulation, a low-pass filter, a coordinate rotation digital computing device, a PID (Proportional-Integral-Derivative) controller, and a DDS (Direct Digital Synthesis).

[0054] The ADC is used to convert the analog phase error signal into a digital signal, laying the foundation for subsequent digital processing; the I / Q demodulator decomposes the digital signal into I / Q components, providing key data for subsequent phase error calculation; the low-pass filter effectively filters out high-frequency noise, improving the accuracy of phase error measurement, thereby ensuring that the subsequent PID controller and DDS can generate more accurate control signals; the coordinate rotation digital computing device uses the CORDIC algorithm to calculate the phase error based on the I / Q components, providing an input for the PID controller; the PID adjusts the control parameters according to the phase error signal to ensure the stability and accuracy of the phase-locked loop. The goal of the PID controller is to minimize the phase error, so that the output frequency and phase of the system are stabilized at the desired values; the adjusted control parameters are sent to the DDS, and the DDS generates precise control signals according to the control parameters to drive the acousto-optic modulator to achieve fine adjustment of the phase.

[0055] In a general phase-locked loop, the output is from the DAC. In this embodiment, for the phase-locked loop applicable to optical frequency transfer, this part is changed to a DDS, and the DDS circuit is integrated onto the circuit board. The DDS uses the radio frequency signal to drive the acousto-optic modulator. The acousto-optic modulator is located in the optical path part of the optical frequency transfer system. Strictly speaking, it is an optical device. Its function is to shift the frequency of the light, for example, from frequency f1 to f1 + f2, where f2 is the frequency of the DDS. As an actuator, the acousto-optic modulator adjusts the frequency and phase of the optical frequency signal according to the control signal.

[0056] The link locking circuit can enable the coordinate rotation digital computing device to work properly under low signal-to-noise ratio conditions, and the link will not lose lock due to abnormal operation of the coordinate rotation digital computing device.

[0057] Suppress according to the phase error signal obtained by phase discrimination of the coordinate rotation digital computing device, and perform fast Fourier analysis on the phase error signal to obtain the phase noise power spectral density.

[0058] During the process of link noise suppression, it is possible to judge whether the noise has been suppressed to the theoretical noise suppression limit according to the magnitude of the phase noise power spectral density.

[0059] The meaning of laser locking is to phase-lock the regenerated light source phase of the laser on the input light phase, and then use the regenerated light as an amplified technology for the output light. Within a certain bandwidth, the output light phase is consistent with the input light phase, and the output light power is several orders of magnitude greater than the input light power, thereby realizing the regeneration and amplification of optical signals. The laser uses the PDH (Pound-Drever-Hall, the principle of laser frequency stabilization named after three developers) frequency stabilization principle for locking.

[0060] The main control circuit realizes the transmission and communication of control signals. Both the link locking signal and the laser locking signal need to be controlled through this main control circuit. The main control circuit will issue laser locking instructions and link locking instructions, and the error signal will be uploaded to the main control circuit for monitoring the locking state. In addition, the main control circuit will also monitor the temperature inside the chassis and the working state of the devices.

[0061] Embodiment 2

[0062] This embodiment provides a control method for a relay station that digitally locks an optical fiber link and real-time monitors phase noise, including the following steps:

[0063] When the optical fiber link is unlocked, the optical fiber link is automatically locked based on the link locking circuit using the link automatic locking algorithm;

[0064] After the optical fiber link is automatically locked, use the phase noise analysis and calculation method to check whether the signal noise of the optical fiber link after using the link automatic locking algorithm reaches the noise suppression limit.

[0065] The principle of suppressing the phase noise of the optical fiber link is that the reference arm of the local signal transmitter and the transmission link form an unequal-arm Michelson interferometer. In this interferometer, the optical signal (frequency f) is divided into two paths: one path is transmitted to the receiver through the optical fiber link (frequency f1), and the other path remains at the transmitter as the reference light.

[0066] The optical signal f is transmitted from the transmitter to the receiver through the optical fiber link f1. At the receiver, part of the transmitted light is reflected by the Faraday mirror and returns to the transmitter along the original optical fiber line. This returned optical signal has experienced two transmissions of the optical fiber link, so its frequency becomes f + 2×f1, where 2×f1 is the phase noise introduced by the optical fiber link.

[0067] At the transmitter, the returned optical signal (frequency f + 2×f1) and the reference light (frequency f) are subjected to beat frequency detection. This beat frequency process generates a signal with a frequency of 2×f1, which represents the phase noise on the double-pass optical fiber link. To suppress this noise, the transmitter uses an acousto-optic phase modulator to generate a modulation signal with a phase opposite to the noise (frequency -2×f1).

[0068] By controlling the acousto-optic phase modulator, the modulation signal generated by it is made to completely cancel the phase noise on the optical fiber link. In this way, the optical signal obtained at the receiving end only has the original frequency f and no longer contains the phase noise introduced by the optical fiber link.

[0069] The core of the whole principle lies in ensuring that after the signal light is transmitted through a hundred-kilometer optical fiber, its line width and phase will not be lost, realizing high-precision transmission.

[0070] As Figure 3 shown, the method for analyzing the phase noise power spectrum density includes the following steps:

[0071] Store the phase error signal obtained by phase discrimination of the coordinate rotation digital computing device in the variable ;

[0072] Phase discrimination uses the CORDIC (Coordinate Rotation Digital Computer) algorithm to identify the phase value of a signal at each moment. The principle of CORDIC is to make the accumulated rotated angle infinitely close to the target angle through an iterative method. It is a method of numerical calculation approximation. Thus, the phase angle value can be obtained, which is the phase error signal.

[0073] Apply the Hamming window function to prevent spectral leakage. By introducing smooth transitions at the beginning and end of the signal, discontinuities are reduced. Applying the Hamming window function to the link phase error signal can reduce the sidelobes in the FFT (Fast Fourier Transform) result, thereby improving the accuracy of spectral estimation.

[0074] The Hamming window function is:

[0075]

[0076] where is the value of the Hamming window at the m-th sampling point, is a constant, , N is the window length, and π is a mathematical constant;

[0077] The result of the Hamming window function of the variable is:

[0078]

[0079] where is the result of the Hamming window function of the variable, is the variable, is the Hamming window function.

[0080] Select the number of sampling points \(n\) for FFT. \(n\) is the recording length of the discrete-time signal. Here, \(n\) is a power of 2 and \(n = 1024\).

[0081] Calculate the fast Fourier transform of, and obtain ;

[0082] Calculate the power spectral density:

[0083]

[0084] where is the result of the fast Fourier transform, \(f\) is the sampling frequency, \(n\) is the number of sampling points, \(n = 1024\), is the symmetric two-sided spectrum, and abs is the modulus length;

[0085] Subsequently, perform unilateral correction to obtain , that is, only display the unilateral power spectral density;

[0086] As Figure 4 shown, display the result on the host computer interface according to the frequency range and the amplitude range of the power spectral density. Figure 4 In, PSD represents Power Spectral Density, that is, power spectral density, Frequency represents frequency, the \(e\) in \(1e + 01\), \(1e + 02\), \(1e + 03\) represents the base of the natural logarithm, dBc / Hz represents the ratio of the phase noise within each hertz bandwidth to the carrier power, dBc represents the ratio of the power at this frequency in dB to the total power (or carrier power), and Hz represents the frequency unit.

[0087] As Figure 5 shown, the link automatic locking algorithm includes the following steps:

[0088] 1. Observe the link phase error signal on the control panel of the main control circuit and determine whether the phase error signal is within the first locking interval \([-a, a]\). If it is within the interval, it indicates that the phase error is small and perhaps no additional adjustment is needed, then proceed to step 3; if it is not within the interval, it indicates that the phase error is large and measures need to be taken to reduce the error, then proceed to step 2;

[0089] 2. The main control circuit sends a command to control the activation of the tracking amplitude stabilization filter. After the filter is activated, determine again whether the link phase error signal is within the first locking interval \([-a, a]\). If it is within the interval, it indicates that the filter is effective, then proceed to step 3;

[0090] The tracking amplitude stabilization filter is arranged between the main control circuit and the link locking circuit and is used to preprocess the signal to reduce the phase error;

[0091] 3. Send commands from the main control circuit to write the proportional parameter P = 100, integral parameter I = 700, and derivative parameter D = 10 into the link locking circuit; these parameters are the parameters of a proportional-integral-derivative (PID) controller, which are used to adjust the behavior of the link locking circuit to further reduce the phase error.

[0092] 4. Determine again whether the link phase error signal is within the second locking interval [-b, b] (b < a). At this time, the error signal appears as a straight line (or very close to a straight line), indicating that the phase error has been stabilized within a very small range. If the phase error signal is within the second locking interval, it means that the link has been locked and the algorithm terminates.

[0093] The link automatic locking algorithm can automatically adjust the PID parameters according to the phase error signal to achieve fast and stable locking of the link. In the case of link unlocking, the link automatic locking algorithm can achieve automatic locking of the link, which is of great significance for long-term testing.

[0094] Upon reading the above description, many embodiments and many applications beyond the provided examples will be obvious to those skilled in the art. Therefore, the scope of this teaching should not be determined with reference to the above description, but rather should be determined with reference to the full scope of the foregoing claims and the equivalents thereof. For the sake of completeness, all articles and references, including patent applications and published announcements, are incorporated herein by reference. The omission of any aspect of the subject matter disclosed herein in the foregoing claims is not intended to abandon such subject matter, nor should the applicant be regarded as not having considered such subject matter as part of the disclosed inventive subject matter.

[0095] The above content is a further detailed description of the present invention. It cannot be determined that the specific implementation of the present invention is limited thereto. For those of ordinary skill in the technical field to which the present invention pertains, without departing from the concept of the present invention, several simple deductions or substitutions can be made, and all should be regarded as belonging to the scope determined by the present invention for protection.

Claims

1. A relay station for digitally locking and real-time monitoring of phase noise in an optical fiber link, characterized in that It includes a main control circuit, a link locking circuit and a laser locking circuit; The link locking circuit is bidirectionally connected to the main control circuit. The link locking circuit sends a link locking error signal to the main control circuit, and the main control circuit sends a link locking instruction to the link locking circuit; The laser locking circuit is bidirectionally connected to the main control circuit. The laser locking circuit sends a laser locking error signal to the main control circuit, and the main control circuit sends a laser locking instruction to the laser locking circuit; The link locking circuit includes an optical frequency digital phase-locked loop, and the optical frequency digital phase-locked loop uses a DDS to drive an acousto-optic modulator to adjust the frequency and phase of the optical frequency signal; When the optical fiber link loses lock, the optical fiber link is automatically locked based on the link locking circuit using a link automatic locking algorithm; After the optical fiber link is automatically locked, a phase noise power spectral density analysis method is used to check whether the signal noise of the optical fiber link after using the link automatic locking algorithm reaches the noise suppression limit.

2. The relay station for digitizing and locking and real-time monitoring of phase noise of an optical fiber link according to claim 1, wherein The optical frequency digital phase-locked loop includes an ADC, an I / Q demodulator, a low-pass filter, a coordinate rotation digital computing device, a PID, a DDS and an acousto-optic modulator; The ADC converts the analog phase error signal into a digital signal. The I / Q demodulator decomposes the digital signal into I / Q components. The low-pass filter filters out the high-frequency noise of the I / Q components. The coordinate rotation digital computing device uses the CORDIC algorithm to calculate the phase error according to the I / Q components after filtering out the high-frequency noise. The PID adjusts the control parameters according to the phase error. The DDS generates a control signal according to the control parameters. The acousto-optic modulator adjusts the frequency and phase of the optical frequency signal according to the control signal.

3. A control method for a relay station that digitally locks and real-time monitors the phase noise of an optical fiber link, characterized in that, A relay station for digital locking of an optical fiber link and real-time monitoring of phase noise includes a main control circuit, a link locking circuit and a laser locking circuit; The link locking circuit is bidirectionally connected to the main control circuit. The link locking circuit sends a link locking error signal to the main control circuit, and the main control circuit sends a link locking instruction to the link locking circuit; The laser locking circuit is bidirectionally connected to the main control circuit. The laser locking circuit sends a laser locking error signal to the main control circuit, and the main control circuit sends a laser locking instruction to the laser locking circuit; The link locking circuit includes an optical frequency digital phase-locked loop, and the optical frequency digital phase-locked loop uses a DDS to drive an acousto-optic modulator to adjust the frequency and phase of the optical frequency signal; A control method for a relay station for digital locking of an optical fiber link and real-time monitoring of phase noise includes the following steps: When the optical fiber link loses lock, the optical fiber link is automatically locked based on the link locking circuit using a link automatic locking algorithm; After the optical fiber link is automatically locked, a phase noise power spectral density analysis method is used to check whether the signal noise of the optical fiber link after using the link automatic locking algorithm reaches the noise suppression limit.

4. The control method of a relay station for digitizing and locking an optical fiber link and real-time monitoring phase noise according to claim 3, characterized in that The phase noise power spectral density analysis method includes the following steps: Using a coordinate rotation digital computing device to identify the phase, and obtaining a link phase error signal; Applying a Hamming window function to the link phase error signal to obtain the Hamming window function result of the link phase error signal; After selecting the number of sampling points for the fast Fourier transform, calculate the fast Fourier transform of the Hamming window function result of the link phase error signal to obtain the fast Fourier transform result; Calculate the power spectral density according to the power spectral density calculation formula. The power spectral density is a symmetric two-sided spectrum, and perform unilateral correction on the power spectral density to obtain the unilateral power spectral density.

5. The control method of a relay station for digitizing and locking an optical fiber link and real-time monitoring phase noise according to claim 4, characterized in that, The Hamming window function is: Among them, is the value of the Hamming window at the m-th sampling point, is a constant, , N is the window length, and π is a mathematical constant; The Hamming window function result of the link phase error signal is: Among them, is the Hamming window function result of the link phase error signal, is the link phase error signal, is the Hamming window function.

6. The control method of a relay station for digitizing and locking and real-time monitoring of phase noise of an optical fiber link according to claim 4, characterized in that, The number of sampling points for the fast Fourier transform is the record length of the discrete-time signal.

7. The control method of a relay station for digitizing and locking and real-time monitoring of phase noise in an optical fiber link according to claim 4, characterized in that, The power spectral density calculation formula is: wherein, is the power spectral density, is the fast Fourier transform result, f is the sampling frequency, n is the number of sampling points, and abs is the modulus length.

8. The control method of a relay station for digitally locking and real-time monitoring phase noise of an optical fiber link according to claim 3, characterized in that The link automatic locking algorithm includes the following steps: Step 1: Determine whether the link phase error signal is within the first locking interval. The first locking interval is a closed interval. If so, proceed to Step 3; otherwise, proceed to Step 2; Step 2: The master control circuit sends a command to control the activation of the tracking amplitude-stabilizing filter, and then determine again whether the link phase error signal is within the first locking interval. If so, proceed to Step 3; Step 3: The master control circuit issues a link locking instruction to the link locking circuit, and write the proportional parameter, integral parameter, and differential parameter into the optical frequency digital phase-locked loop; Step 4: Determine whether the link phase error signal is within the second locking interval. The second locking interval is a closed interval, and the second locking interval is within the first locking interval. If so, complete the link automatic locking.

9. The control method of a relay station for digitally locking and real-time monitoring phase noise of an optical fiber link according to claim 8, characterized in that, The tracking amplitude-stabilizing filter is arranged between the master control circuit and the link locking circuit.

Images