Relay station for digitally locking optical fiber link and monitoring phase noise in real time

Abstract

The invention discloses a relay station for digitally locking an optical fiber link and monitoring phase noise in real time, which belongs to the technical field of relay stations for optical fiber optical frequency transmission and comprises a main control circuit, a link locking circuit and a laser locking circuit. The link locking circuit is bidirectionally connected with 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 with 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 comprises an optical frequency digital phase-locked loop; and the optical frequency digital phase-locked loop adopts a DDS (Direct Digital Synthesizer) to drive an acousto-optic modulator to adjust the frequency and the phase of an optical frequency signal. An optical frequency digital phase-locked loop is adopted to measure a phase noise power spectrum in real time, link locking can be achieved, and meanwhile whether the power spectrum of a locked signal reaches a theoretical noise suppression limit or not can be judged.

Description

Technical Field

[0001] The invention belongs to the technical field of relay stations for optical fiber optical frequency transmission, and in particular relates to a relay station for digitally locking an optical fiber link and monitoring phase noise in real time. Background Art

[0002] In the field of fiber-optic frequency transmission, in order to achieve remote optical clock comparison, long-distance optical frequency transmission requires relay stations to improve transmission accuracy. By locking the phase of the regenerated light source to the input light, optical regenerative 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: link phase error compensation circuit, laser locking circuit and main control circuit. The relay station is used to achieve long-distance optical fiber link locking. The 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 cause optical fiber link unlocking problems 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 and very heavy. This equipment is laid on a link of several thousand kilometers and requires fully automated monitoring. Large phase noise meters are not suitable for actual testing needs. Optical fiber links are laid along highways and are very noisy, resulting in a low signal-to-noise ratio for link error signals. Figure 6 As shown, the optical fiber link is easily lost due to environmental impact interference. The impact interference will directly destroy the control bandwidth of the phase-locked loop, and the impact interference is very severe. Summary of the invention

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

[0006] In order to achieve the above object, the present invention adopts the following technical scheme: In a first aspect, the present invention provides a relay station for digitally locking an optical fiber link and monitoring phase noise in real time, including 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 comprises an optical frequency digital phase locked loop, and the optical frequency digital phase locked loop adopts a DDS to drive an acousto-optic modulator to adjust the frequency and phase of the optical frequency signal.

[0007] Further, the optical frequency digital phase-locked loop includes an ADC, an I / Q demodulator, a low-pass filter, a coordinate rotation digital calculation 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 high-frequency noise of the I / Q components, the coordinate rotation digital calculation 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 parameter according to the phase error, the DDS generates a control signal according to the control parameter, and the acousto-optic modulator adjusts the frequency and phase of the optical frequency signal according to the control signal.

[0008] Furthermore, the laser locking circuit utilizes a PDH frequency stabilization method to lock the laser.

[0009] In a second aspect, the present invention provides a control method for a relay station for digitally locking an optical fiber link and monitoring phase noise in real time, using the relay station for digitally locking an optical fiber link and monitoring phase noise in real time, comprising the following steps: When the optical fiber link is unlocked, the optical fiber link is automatically locked using a link automatic locking algorithm based on the link locking circuit; After the optical fiber link is automatically locked, the phase noise power spectrum density analysis method is used to check whether the signal noise of the optical fiber link after the link automatic locking algorithm is used reaches the noise suppression limit.

[0010] Furthermore, the phase noise power spectrum density analysis method comprises the following steps: The coordinate rotation digital computing device is used to detect the phase and obtain the link phase error signal; Applying a Hamming window function to the link phase error signal to obtain a Hamming window function result of the link phase error signal; After selecting the number of sampling points for fast Fourier transform, the fast Fourier transform of the Hamming window function result of the link phase error signal is calculated to obtain a fast Fourier transform result; The power spectrum density is calculated according to the power spectrum density calculation formula. The power spectrum density is a symmetrical bilateral spectrum. The power spectrum density is corrected unilaterally to obtain the unilateral power spectrum density.

[0011] Furthermore, the Hamming window function is:

[0012] in, is the value of the Hamming window at the mth sampling point, is a constant, , N is the window length, π is a mathematical constant; The Hamming window function result of the link phase error signal is:

[0013] in, is the Hamming window function result of the link phase error signal, is the link phase error signal, is the Hamming window function.

[0014] Furthermore, the number of sampling points used for fast Fourier transform is the record length of the discrete-time signal.

[0015] Furthermore, the power spectrum density calculation formula is:

[0016] in, is the power spectral density, is the result of fast Fourier transform, f is the sampling frequency, n is the number of sampling points, and abs is the modulus length.

[0017] Furthermore, the link automatic locking algorithm comprises the following steps: Step 1: Determine whether the link phase error signal is in the first locking interval, the first locking interval is a closed interval, if yes, proceed to step 3, if not, proceed to step 2; Step 2: The main control circuit sends a command to control the start of the tracking and amplitude stabilization filter, and again determines whether the link phase error signal is in the first locking interval. If so, proceed to step 3; Step 3: The main control circuit sends 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; Step 4: Determine whether the link phase error signal is in 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, the link is automatically locked.

[0018] Furthermore, the tracking and amplitude stabilization filter in step three is arranged between the main control circuit and the link locking circuit.

[0019] Compared with the prior art, the present invention has the following beneficial technical effects: The present invention proposes a relay station for digitally locking an optical fiber link and monitoring phase noise in real time, which is used for optical frequency transmission and integrates digital phase-locked loop technology, laser locking technology and main control circuit technology to achieve high-precision link phase error signal compensation, 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 locked signal reaches the theoretical noise suppression limit. By realizing the automatic locking of the link phase error signal, the problem of equipment being dispersed and arranged on a long-distance link, such as a 3,000-kilometer link, and being unable to be manually operated is solved, which reduces human and material resources, and is conducive to the stable operation of equipment and systems. It is particularly suitable for scenarios such as long-distance optical frequency transmission and optical fiber communication testing, and has the advantages of high precision, high stability and high automation level. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The drawings described herein are for explanation purposes only and are not intended to limit the scope of the present invention in any way. In addition, the shapes and proportional dimensions of the components in the drawings are only for illustration purposes to help understand the present invention, and are not intended to specifically limit the shapes and proportional dimensions of the components of the present invention. In the drawings: Figure 1 A schematic diagram of a repeater station that digitally locks an optical fiber link and monitors phase noise in real time.

[0021] Figure 2 This is the working flow chart of the optical frequency digital phase-locked loop.

[0022] Figure 3 Schematic diagram of phase noise calculation process.

[0023] Figure 4 This is a schematic diagram of the phase noise display interface.

[0024] Figure 5 Figure 2 is a flowchart of the link automatic locking algorithm.

[0025] Figure 6 Schematic diagram of link unlocking and maintaining lock status. DETAILED DESCRIPTION

[0026] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0027] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which the present invention belongs. The terms used herein in the specification of the present invention 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.

[0028] 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 are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in a sequence other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0029] Embodiment 1 This embodiment provides a relay station for digitally locking an optical fiber link and monitoring phase noise in real time, such as Figure 1 As shown, the principle block diagram of the relay station 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, see Figure 2 , the digital phase-locked loop was developed based on FPGA (Field-Programmable Gate Array) and improved into a phase-locked loop suitable for optical frequency transmission system.

[0030] Link noise compensation is performed based on digital phase-locked loop technology. The optical frequency digital phase-locked loop consists of ADC (Analog-to-Digital Converter), I / Q (In-phase / Quadrature) demodulation, low-pass filter, coordinate rotation digital calculation device, PID (Proportional-Integral-Derivative) controller, and DDS (Direct Digital Synthesis).

[0031] ADC is used to convert analog phase error signals into digital signals, laying the foundation for subsequent digital processing; I / Q demodulator decomposes digital signals into I / Q components, providing key data for subsequent phase error calculation; low-pass filter effectively filters out high-frequency noise and improves the accuracy of phase error measurement, thereby ensuring that subsequent PID controller and DDS can generate more accurate control signals; coordinate rotation digital calculation device uses CORDIC algorithm to calculate phase error based on I / Q components and provide input for PID controller; PID adjusts control parameters according to phase error signal to ensure the stability and accuracy of phase-locked loop. The goal of PID controller is to minimize phase error so that the output frequency and phase of the system are stable at the desired values; the adjusted control parameters are sent to DDS, which generates accurate control signals according to the control parameters to drive acousto-optic modulator to achieve fine phase adjustment.

[0032] The general phase-locked loop is output from the DAC. The phase-locked loop suitable for optical frequency transmission in this embodiment changes this part to DDS, and integrates the DDS circuit onto the circuit board. DDS uses the RF signal to drive the acousto-optic modulator. The acousto-optic modulator is located in the optical path of the optical frequency transmission system. Strictly speaking, it is an optical device. Its function is to shift the frequency of light, such as from frequency f1 to f1+f2, where f2 is the frequency of DDS. As an actuator, the acousto-optic modulator adjusts the frequency and phase of the optical frequency signal according to the control signal.

[0033] The link locking circuit can realize the normal operation of the coordinate rotation digital calculation device under the condition of low signal-to-noise ratio, and the link will not be unlocked due to abnormal operation of the coordinate rotation digital calculation device.

[0034] The phase error signal obtained by phase detection of the coordinate rotation digital computing device is suppressed, and the phase noise power spectrum density is obtained by fast Fourier analysis based on the phase error signal.

[0035] In the process of link noise suppression, whether the noise is suppressed to the theoretical noise suppression limit can be judged based on the size of the phase noise power spectrum density.

[0036] Laser locking means locking the phase of the laser regeneration light source to the input light phase, and then using the regenerated light as an amplification 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 achieving regeneration and amplification of the optical signal. The laser is locked using the PDH (Pound-Drever-Hall, the principle of laser frequency stabilization named after three developers) frequency stabilization principle.

[0037] The main control circuit realizes the transmission and communication of control signals. Both the link lock signal and the laser lock signal need to be controlled by this main control circuit. The main control circuit will issue the laser lock command and the link lock command, and the error signal will be uploaded to the main control circuit to monitor the lock status. In addition, the main control circuit will also monitor the temperature inside the chassis and the working status of the device.

[0038] Embodiment 2 This embodiment provides a control method for a relay station for digitally locking an optical fiber link and monitoring phase noise in real time, comprising the following steps: When the optical fiber link is unlocked, the optical fiber link is automatically locked using a link automatic locking algorithm based on the link locking circuit; After the optical fiber link is automatically locked, the phase noise analysis and calculation method is used to check whether the signal noise of the optical fiber link after the link automatic locking algorithm is used reaches the noise suppression limit.

[0039] The principle of suppressing the phase noise of the optical fiber link is that the reference arm and the transmission link of the local signal sending end form an unequal-arm Michelson interferometer. In this interferometer, the optical signal (frequency is f) is divided into two paths: one is transmitted to the receiving end through the optical fiber link (frequency is f1), and the other is left at the sending end as a reference light.

[0040] 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 undergoes two transmissions of the optical fiber link, so its frequency becomes f+2×f1, where 2×f1 is due to the phase noise introduced by the optical fiber link.

[0041] At the transmitter, the returning optical signal (frequency f+2×f1) is detected by beating with the reference light (frequency f). This beating process generates a signal with a frequency of 2×f1, which represents the phase noise on the two-way optical fiber link. In order to suppress this noise, the transmitter uses an acousto-optic phase modulator to generate a modulated signal with a phase opposite to the noise (frequency -2×f1).

[0042] By controlling the acousto-optic phase modulator, the modulated signal generated by it completely cancels out the phase noise on the optical fiber link. In this way, the optical signal obtained at the receiving end only has the original f frequency, and no longer contains the phase noise introduced by the optical fiber link.

[0043] The core of the whole principle is to ensure that the line width and phase of the signal light will not be lost after being transmitted through hundreds of kilometers of optical fiber, thereby achieving high-precision transmission.

[0044] like Figure 3 As shown, the phase noise power spectrum density analysis method includes the following steps: The phase error signal obtained by the coordinate rotation digital calculation device phase detection Stored in variables middle; Phase detection uses the CORDIC (Coordinate Rotation Digital Computer) algorithm to identify the phase value of a signal at every moment. The principle of CORDIC is to make the accumulated rotation angle infinitely close to the target angle through an iterative method. It is a method of numerical calculation approximation. The phase angle value can be obtained, which is the phase error signal.

[0045] Apply the Hamming window function to prevent spectrum leakage. Reduce discontinuities by introducing smooth transitions at the beginning and end of the signal. Applying the Hamming window function to the link phase error signal can reduce the side lobes in the FFT (Fast Fourier Transform) results, thereby improving the accuracy of spectrum estimation.

[0046] The Hamming window function is:

[0047] in, is the value of the Hamming window at the mth sampling point, is a constant, , N is the window length, π is a mathematical constant; The Hamming window function result of the variable is:

[0048] in, is the result of the Hamming window function of the variable, is a variable, is the Hamming window function.

[0049] Select the number of sampling points n for FFT, where n is the record length of the discrete-time signal, where n is a power of 2, n=1024; calculate The fast Fourier transform of ; Calculate the power spectral density:

[0050] in, is the result of fast Fourier transform, f is the sampling frequency, n is the number of sampling points, n=1024, is a symmetrical double-sided spectrum, abs is the modulus length; Then a unilateral correction is performed to obtain , that is, only one-sided power spectrum density is displayed; As Figure 4 shown, the results are displayed 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, 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.

[0051] As Figure 5 shown, the link automatic locking algorithm includes the following steps: 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; 2. Send a command from the main control circuit 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; 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; 3. Send instructions from the main control circuit to write the proportional parameter P = 100, the integral parameter I = 700, and the derivative parameter D = 10 into the link locking circuit; these parameters are the parameters of the proportional-integral-derivative (PID) controller and are used to adjust the behavior of the link locking circuit to further reduce the phase error; 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 presents 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 indicates that the link has been locked and the algorithm terminates.

[0052] 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.

[0053] Many embodiments and many applications beyond the examples provided will be apparent to those skilled in the art upon reading the above description. Therefore, the scope of the present teachings should not be determined with reference to the above description, but rather with reference to the preceding claims and the full scope of equivalents to which such claims are entitled. For the purpose of comprehensiveness, all articles and references, including disclosures of patent applications and publications, are incorporated herein by reference. The omission of any aspect of the subject matter disclosed herein in the preceding claims is not intended to be a disclaimer of such subject matter, nor should it be considered that the applicants did not consider such subject matter to be part of the disclosed inventive subject matter.

[0054] The above content is a further detailed description of the present invention. It cannot be determined that the specific implementation methods of the present invention are limited to these. For ordinary technicians in the technical field to which the present invention belongs, they can make several simple deductions or substitutions without departing from the concept of the present invention, which should be regarded as belonging to the scope of protection submitted for the present invention.

Claims

1. A relay station for digitally locking an optical fiber link and monitoring phase noise in real time, 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 comprises an optical frequency digital phase locked loop, and the optical frequency digital phase locked loop adopts a DDS to drive an acousto-optic modulator to adjust the frequency and phase of the optical frequency signal.

2. A relay station for digitally locking an optical fiber link and monitoring phase noise in real time according to claim 1, characterized in that: The optical frequency digital phase-locked loop includes an ADC, an I / Q demodulator, a low-pass filter, a coordinate rotation digital calculation 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 high-frequency noise of the I / Q components, the coordinate rotation digital calculation 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 parameter according to the phase error, the DDS generates a control signal according to the control parameter, and the acousto-optic modulator adjusts the frequency and phase of the optical frequency signal according to the control signal.

3. A relay station for digitally locking an optical fiber link and monitoring phase noise in real time according to claim 1, characterized in that: The laser locking circuit uses a PDH frequency stabilization method to lock the laser.

4. A control method for a relay station for digitally locking an optical fiber link and monitoring phase noise in real time, using a relay station for digitally locking an optical fiber link and monitoring phase noise in real time according to claims 1-3, characterized in that: The steps include: When the optical fiber link is unlocked, the optical fiber link is automatically locked using a link automatic locking algorithm based on the link locking circuit; After the optical fiber link is automatically locked, the phase noise power spectrum density analysis method is used to check whether the signal noise of the optical fiber link after the link automatic locking algorithm is used reaches the noise suppression limit.

5. The control method of a relay station for digitally locking an optical fiber link and monitoring phase noise in real time according to claim 4, characterized in that: The phase noise power spectrum density analysis method comprises the following steps: Using a coordinate rotation digital computing device to detect phase, a link phase error signal is obtained; Applying a Hamming window function to the link phase error signal to obtain a Hamming window function result of the link phase error signal; After selecting the number of sampling points for fast Fourier transform, the fast Fourier transform of the Hamming window function result of the link phase error signal is calculated to obtain a fast Fourier transform result; The power spectrum density is calculated according to the power spectrum density calculation formula. The power spectrum density is a symmetrical bilateral spectrum. The power spectrum density is corrected unilaterally to obtain the unilateral power spectrum density.

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

7. The control method of a relay station for digitally locking an optical fiber link and monitoring phase noise in real time according to claim 5, characterized in that: The number of sampling points used for fast Fourier transform is the record length of the discrete time signal.

8. The control method of a relay station for digitally locking an optical fiber link and monitoring phase noise in real time according to claim 5, characterized in that: The power spectrum density calculation formula is: in, is the power spectral density, is the result of fast Fourier transform, f is the sampling frequency, n is the number of sampling points, and abs is the modulus length.

9. The control method of a relay station for digitally locking an optical fiber link and monitoring phase noise in real time according to claim 4, characterized in that: The link automatic locking algorithm comprises the following steps: Step 1: Determine whether the link phase error signal is in the first locking interval, the first locking interval is a closed interval, if yes, proceed to step 3, if not, proceed to step 2; Step 2: The main control circuit sends a command to control the start of the tracking and amplitude stabilization filter, and again determines whether the link phase error signal is in the first locking interval. If so, proceed to step 3; Step 3: The main control circuit sends 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; Step 4: Determine whether the link phase error signal is in 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, the link is automatically locked.

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

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