A fiber-optic optical frequency transfer relay system and method based on spread spectrum technology

Abstract

The application discloses a kind of optical fiber optical frequency transmission relay system and method based on spread spectrum technology, the relay system includes receiving module, regenerative laser module and sending module;Receiving module is used to obtain demodulation optical signal by the spread spectrum optical signal demodulation according to second pseudo-random sequence;Spread spectrum optical signal is the signal that is spread spectrum to optical signal by front stage and is transmitted by optical fiber link;Regenerative laser module is used to lock the frequency of demodulation optical signal and generate regenerative optical signal, and its beam splitting obtains return optical signal and transmission optical signal;Receiving module is also modulated according to second pseudo-random sequence and is returned to front stage by return optical signal;Sending module is used to obtain demodulation return optical signal by the modulated return optical signal demodulation of rear stage, and transmission optical signal is transmitted to rear stage after noise suppression and spread spectrum by the demodulation return optical signal and third pseudo-random sequence.This application filters out second-order Rayleigh scattering signal by introducing spread spectrum technology, significantly improves the stability and reliability of system.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of optical fiber communication, and particularly relates to an optical fiber optical frequency transfer relay system and method based on a spread spectrum technology. BACKGROUND

[0002] With the rapid development of optical atomic clock technology, its stability has broken through 10 -19 orders of magnitude, and its performance is two orders of magnitude higher than that of a traditional microwave atomic clock. A high-performance optical clock network has a wide application prospect in the fields of time frequency metrology, radio astronomy observation, gravitational wave detection and fundamental physical law verification. However, the optical atomic clock itself has inherent limitations such as strong environmental sensitivity and large system volume, which makes it difficult to be directly applied to large-scale distributed application scenarios, thereby greatly limiting its practical process. Therefore, optical frequency transfer technology based on optical fiber emerges as the times require and becomes a core support means for promoting the practicality of optical clocks. The technology uses the low transmission loss and strong anti-electromagnetic interference of the optical fiber transmission medium, and can realize the sharing of the same high-performance frequency reference by users at different spatial positions.

[0003] In the process of optical fiber frequency transfer, the Doppler noise cancellation technology is usually used to accurately detect and effectively suppress the additional phase noise introduced by the optical fiber link, so as to ensure the signal transmission quality. However, various reflection points existing in the optical fiber link will induce the second-order Rayleigh scattering effect, which will introduce nonlinear interference and cause the transfer system to have a cycle slip or even lose lock, thereby seriously threatening the long-term stable operation of the link. On the other hand, with the increase of the transmission distance, the inherent attenuation of the optical fiber will cause the signal power to decrease significantly, so the industry generally uses bidirectional erbium-doped fiber amplifiers (Bi-EDFAs) to compensate for the power loss, but the introduction of the amplifiers will inevitably aggravate the second-order Rayleigh scattering effect. Therefore, the system cycle slip and loss lock problem caused by the second-order Rayleigh scattering is particularly prominent in long-distance optical fiber frequency transmission links, and has become a key technical bottleneck restricting long-distance optical fiber optical frequency transmission. SUMMARY

[0004] In order to solve the above problems existing in the prior art, the application provides an optical fiber optical frequency transfer relay system and method based on a spread spectrum technology.

[0005] The technical problem to be solved by the application is solved by the following technical scheme:

[0006] In a first aspect, the application provides an optical fiber optical frequency transfer relay system based on a spread spectrum technology, comprising a receiving module, a regenerative laser module and a sending module.

[0007] The receiving module is configured to receive a spread spectrum optical signal, demodulate the spread spectrum optical signal according to a second pseudo-random sequence to obtain a demodulated optical signal; the spread spectrum optical signal is a signal modulated by a front stage using a first pseudo-random sequence and transmitted through an optical fiber link; the second pseudo-random sequence is obtained by phase adjustment of the first pseudo-random sequence; the phase adjustment is matched with a delay between the front stage and the receiving module;

[0008] The regenerative laser module is configured to lock a frequency of the demodulated optical signal to generate a regenerated optical signal; one beam of the regenerated optical signal is transmitted to the receiving module as a return optical signal, and another beam is transmitted to the sending module as a transmission optical signal;

[0009] The receiving module is further configured to modulate the return optical signal according to the second pseudo-random sequence to obtain a modulated return optical signal; the modulated return optical signal is transmitted back to the front stage;

[0010] The sending module is configured to modulate the transmission optical signal using a third pseudo-random sequence to obtain a spread spectrum optical signal transmitted to a back stage, and demodulate a modulated return optical signal from the back stage using the third pseudo-random sequence to obtain a demodulated return optical signal, and use the demodulated return optical signal to realize noise suppression of the transmission optical signal.

[0011] Optionally, the receiving module comprises a first acousto-optic modulator, a first band-pass filter, a first frequency mixer, a first digital frequency synthesizer, a first IQ modulation module, a first delay module and a first voltage-controlled oscillator.

[0012] The first IQ modulation module is configured to generate the first pseudo-random sequence, the first pseudo-random sequence is adjusted in phase by the first delay module to form the second pseudo-random sequence, and the second pseudo-random sequence is input to the first voltage-controlled oscillator; the first digital frequency synthesizer generates a first local oscillator signal; an output signal of the first voltage-controlled oscillator and the first local oscillator signal are mixed by the first frequency mixer; a mixed signal is filtered by the first band-pass filter to form a driving frequency of the first acousto-optic modulator; and the first acousto-optic modulator is connected to the front stage and the regenerative laser module.

[0013] Optionally, the regenerative laser module comprises a first beam splitter, a first Faraday rotating mirror, a first photodetector, a second band-pass filter, a first frequency division ratio phase module, a first proportional-integral controller, a relay laser and a second beam splitter.

[0014] The relay laser is used for generating a regenerated optical signal, a frequency of the regenerated optical signal changes according to a control voltage; the regenerated optical signal is divided into two beams by the second beam splitter, one beam enters the first beam splitter as a return optical signal, and the other beam is transmitted to the sending module as a transmission optical signal;

[0015] The return optical signal entering the first beam splitter is divided into two beams, one beam returns to the receiving module, and the other beam enters the first photodetector after being reflected by the first Faraday rotating mirror;

[0016] The demodulated optical signal output by the receiving module enters the first photodetector through the first beam splitter, the first photodetector beats the input demodulated optical signal and the return optical signal to obtain a beat signal, the beat signal enters the first frequency division and phase comparison module after being filtered by the second band-pass filter, the first frequency division and phase comparison module frequency-divides and phase-compares the filtered signal and a reference clock signal to obtain a first error voltage signal, and the first proportional integral controller generates the control voltage according to the first error voltage signal.

[0017] Optionally, the sending module comprises a third beam splitter, a second Faraday rotating mirror, a second photodetector, a servo control unit and a second acousto-optic modulator;

[0018] The transmission optical signal output by the regenerated laser module is divided into two beams by the third beam splitter, one beam is a reference optical signal, enters the second photodetector after being reflected by the second Faraday rotating mirror, and the other beam enters the second acousto-optic modulator; the second acousto-optic modulator is connected to a receiving module in a later stage through an optical fiber link;

[0019] The second acousto-optic modulator demodulates a modulated return optical signal from the later stage to obtain a demodulated return optical signal, and the demodulated return optical signal enters the second photodetector through the third beam splitter;

[0020] The second photodetector beats the input reference optical signal and the demodulated return optical signal to obtain an in-loop beat signal, and the servo control unit forms a drive frequency of the second acousto-optic modulator according to the in-loop beat signal and a third pseudo-random sequence.

[0021] Optionally, the servo control unit comprises a third band-pass filter, a second frequency division and phase comparison module, a second proportional integral controller, a second digital frequency synthesizer, a second IQ modulation module, a second voltage-controlled oscillator, a second frequency mixer and a fourth band-pass filter;

[0022] The third band-pass filter filters the ring-in beat frequency signal, the second frequency division ratio phase module frequency-divides and phase discriminates the filtered ring-in beat frequency signal and the reference clock signal to obtain a second error voltage signal, the second proportional integral controller generates a control signal of the second digital frequency synthesizer according to the second error voltage signal, so that the second digital frequency synthesizer generates a second local oscillator signal, and the second IQ modulation module generates the third pseudo-random sequence to the second voltage-controlled oscillator; the signal output by the second voltage-controlled oscillator is mixed with the second local oscillator signal by the second mixer, and the mixed signal is filtered by the fourth band-pass filter to form a driving frequency of the second acousto-optic modulator.

[0023] Optionally, the relay system further comprises a GPS servo rubidium clock.

[0024] The GPS servo rubidium clock is configured to generate a time synchronization signal and the reference clock signal; the first pseudo-random sequence and the third pseudo-random sequence are both generated based on the time synchronization signal.

[0025] In a second aspect, the application provides a method for optical fiber optical frequency transfer based on a spread spectrum technology, comprising: transferring an optical signal between a source sending end and a destination receiving end by using the above-mentioned relay system for optical fiber optical frequency transfer based on a spread spectrum technology; the sending module of the source sending end is the same as the sending module of the relay system; and the receiving module and the regenerative laser module of the destination receiving end are the same as the receiving module and the regenerative laser module of the relay system.

[0026] The application provides a fiber-optic optical frequency transfer relay system based on a spread spectrum technology, comprising a receiving module, a regenerative laser module and a sending module; wherein the receiving module demodulates a received spread spectrum optical signal by using a second pseudo-random sequence to realize filtering of a second-order Rayleigh scattering signal and obtain a demodulated optical signal, and simultaneously, the receiving module modulates a returned optical signal output by the regenerative laser module by using the second pseudo-random sequence and transmits it back to a previous stage to realize noise suppression and second-order Rayleigh scattering signal filtering in the previous stage. Here, the spread spectrum optical signal is a signal that is spread by the previous stage and transmitted through an optical fiber link; the sending module demodulates a modulated returned optical signal from a subsequent stage by using a third pseudo-random sequence, and simultaneously, the sending module transmits a transmission optical signal to the subsequent stage after link noise suppression and spread spectrum according to the demodulated returned optical signal and the third pseudo-random sequence. Wherein, the application modulates an optical signal by using a pseudo-random sequence, which can give the optical signal a unique time-varying phase offset, so that when demodulation is performed by using a pseudo-random sequence with a matching time delay, only the optical signal with an accurately matched time delay can be effectively recovered based on the autocorrelation characteristics of the pseudo-random sequence, and the second-order Rayleigh scattering signal with a non-matched time delay is filtered out. The application realizes filtering of the second-order Rayleigh scattering signal by introducing the spread spectrum technology, avoids system cycle slip and loss of lock caused by the second-order Rayleigh scattering, and thus significantly improves the stability and reliability of the system.

[0027] The application will be further described in detail below with reference to the drawings and the application. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 FIG. 1 is a structural schematic diagram of a fiber-optic optical frequency transfer relay system based on a spread spectrum technology provided by an embodiment of the application;

[0029] Figure 2 FIG. 2 is a structural schematic diagram of a fiber-optic optical frequency transfer system;

[0030] Figure 3 FIG. 3 is a structural schematic diagram of a source sending end adapted to the relay system provided by the embodiment of the application;

[0031] Figure 4 FIG. 4 is a structural schematic diagram of a destination receiving end adapted to the relay system provided by the embodiment of the application;

[0032] Figure 5 FIG. 5 is an optical signal transmission flowchart of the fiber-optic optical frequency transfer system provided by the embodiment of the application. DETAILED DESCRIPTION

[0033] The application will be further described in detail below with reference to the drawings and the application.

[0034] In order to filter out the second-order Rayleigh scattering signal in the optical fiber optical frequency transfer process, avoid the system cycle slip and loss of lock caused by the second-order Rayleigh scattering, and thus significantly improve the stability and reliability of the optical fiber optical frequency transfer. The embodiment of the present application provides a kind of relay system and method of optical fiber optical frequency transfer based on spread spectrum technology.

[0035] First, the relay system (also called relay station) of optical fiber optical frequency transfer based on spread spectrum technology provided by the embodiment of the present application is described in detail. Referring to Figure 1 , the relay system comprises: receiving module, regenerative laser module and sending module.

[0036] Among them, the receiving module is used to receive spread spectrum optical signal, and the spread spectrum optical signal is demodulated according to the second pseudo-random sequence to obtain demodulation optical signal. The spread spectrum optical signal is the signal modulated by the first pseudo-random sequence by the front stage (front stage relay station or source sending end) and transmitted through the optical fiber link. The second pseudo-random sequence is obtained by adjusting the phase of the first pseudo-random sequence, and the phase adjustment is matched with the delay between the front stage and the receiving module.

[0037] Specifically, the front stage uses the first pseudo-random sequence to spread the optical signal, which is converted into wide spectrum, and is injected into the optical fiber link to be transmitted to the receiving module. At this time, the receiving module receives the spread spectrum optical signal, which contains noise signals such as second-order Rayleigh scattering signals due to transmission through the optical fiber link, and the spread spectrum optical signal is demodulated by the second pseudo-random sequence. Then, by using the autocorrelation characteristics of the pseudo-random sequence, the second-order Rayleigh scattering signal in the spread spectrum optical signal can be filtered out to obtain the demodulation optical signal.

[0038] Here, the binary phase modulation / demodulation of the optical signal by the pseudo-random sequence is to abstract the pseudo-random sequence as a function , so as to convert the discrete binary code into continuous domain operation symbol. In the process of modulation / demodulation, when the code element of the pseudo-random sequence is "0", it corresponds to , at this time the phase of the optical signal remains unchanged, and when the code element of the pseudo-random sequence is "1", it corresponds to , at this time the phase of the optical signal produces radian shift. Therefore, by using the pseudo-random sequence for phase modulation, a unique, time-varying phase shift can be given to the optical signal. When demodulation is performed by using the pseudo-random sequence, since the pseudo-random sequence has autocorrelation characteristics, i.e. , and is still a random code, wherein represents the time delay of the optical signal, represents the time delay of the noise signal, . Therefore, if the time delay of the pseudo-random sequence is matched with the time delay of the optical signal precisely matches (i.e. the time delay of the pseudo-random sequence is ), the optical signal with a time delay of can be effectively recovered, while the second-order Rayleigh scattering signal with a time delay that does not match is filtered out because the product is still a random code. Here, the code length of the pseudo-random sequence used needs to ensure that there are enough chips to distinguish the signals on the modulation side (such as the previous stage) and the demodulation side (such as the receiving module), that is, the code sequence period needs to cover the link time delay between the modulation side and the demodulation side.

[0039] The regenerative laser module is used to lock the frequency of the demodulated optical signal and generate a regenerated optical signal. One of the regenerated optical signals is transmitted to the receiving module as a return optical signal, and the other is transmitted to the sending module as a transmission optical signal.

[0040] Specifically, the regenerative laser module obtains a beat signal by beating the demodulated optical signal and the return optical signal, and adjusts the relay laser based on the beat signal, so that the output frequency of the relay laser accurately tracks and locks the demodulated optical signal. It can be understood that if the previous stage does not modulate the optical signal with the first pseudo-random sequence, but directly outputs the optical signal without spread spectrum, the phase of the demodulated optical signal locked by the regenerative laser module is determined by the vector sum of the optical signal without spread spectrum and various scattering signals such as second-order Rayleigh scattering. These signals superimpose on each other in time and frequency domains, and are difficult to separate, which seriously degrades the quality of the optical signal. When the previous stage modulates the optical signal with the first pseudo-random sequence, different signals (optical signal and second-order Rayleigh scattering signals generated at different positions) have different time delays in the optical fiber link, so different signals carry unique time-varying phase offsets. Therefore, in the receiving module, the second pseudo-random sequence is used to demodulate the received spread spectrum optical signal, so that the second-order Rayleigh scattering signal in the spread spectrum optical signal is filtered out, and the demodulated optical signal is obtained. The second pseudo-random sequence has the same code type as the first pseudo-random sequence, and the time delay of the second pseudo-random sequence matches the delay of the optical fiber link between the previous stage and the receiving module.

[0041] The receiving module is also used to modulate the return optical signal according to the second pseudo-random sequence to obtain a modulated return optical signal, and the modulated return optical signal is transmitted back to the previous stage.

[0042] It can be understood that the modulated return optical signal is transmitted back to the previous stage through the optical fiber link.

[0043] The sending module is used to modulate the transmission optical signal with a third pseudo-random sequence to obtain a spread spectrum optical signal and transmit the spread spectrum optical signal to the subsequent stage, and demodulate the modulated return optical signal from the subsequent stage (the subsequent relay station or the destination receiving end) with the third pseudo-random sequence to obtain a demodulated return optical signal, and use the demodulated return optical signal to realize noise suppression of the transmission optical signal.

[0044] Specifically, by using the third pseudo-random sequence to demodulate the modulated return light signal, the second-order Rayleigh scattering signal introduced by the optical fiber link transmission can be filtered out, so as to obtain the demodulated return light signal. The demodulated return light signal is the spread spectrum light signal emitted by the sending module, which reaches the later stage through the optical fiber link, and then returns to the sending module from the later stage through the optical fiber link, so that the demodulated return light signal carries the double-link noise. The sending module mixes the demodulated return light signal with the reference light signal (one of the transmitted light signals) to obtain an intra-loop mixing signal, and the sending module uses the third pseudo-random sequence to spread the transmitted light signal, and at the same time, the transmitted light signal is subjected to link noise suppression based on the intra-loop mixing signal. Here, the third pseudo-random sequence is preferably the same as the first pseudo-random sequence, but is not limited thereto. The third pseudo-random sequence is accurately matched in time delay with the second pseudo-random sequence of the receiving module in the later stage, that is, the second pseudo-random sequence of the receiving module in the later stage is obtained by phase adjustment of the third pseudo-random sequence, and the phase adjustment is matched with the delay between the sending module and the receiving module in the later stage.

[0045] The optical fiber optical frequency transfer relay system based on the spread spectrum technology provided by the application can modulate the optical signal by the pseudo-random sequence, so as to give the optical signal a unique time-varying phase offset. Therefore, when the pseudo-random sequence with a matched time delay is used for demodulation, only the optical signal with an accurately matched time delay can be effectively recovered based on the autocorrelation characteristics of the pseudo-random sequence, and the second-order Rayleigh scattering signal with an unmatched time delay is filtered out. By introducing the spread spectrum technology, the second-order Rayleigh scattering signal is filtered out, the problems of system cycle slip and loss of lock caused by the second-order Rayleigh scattering are avoided, and the stability and reliability of the system are significantly improved.

[0046] In one embodiment, as shown in FIG. 1, Figure 1 The receiving module includes a first acousto-optic modulator AOM1, a first band-pass filter BPF1, a first mixer, a first digital frequency synthesizer DDS1, a first IQ modulation module, a first delay module and a first voltage-controlled oscillator VCO1.

[0047] The first IQ modulation module is configured to generate a first pseudo-random sequence, which is phase-adjusted by the first delay module to form a second pseudo-random sequence and then provided to the first voltage-controlled oscillator VCO1. The first digital frequency synthesizer DDS1 generates a first local oscillator signal The output signal of the first voltage-controlled oscillator VCO1 and the first local oscillator signal The mixed signal is filtered by a first band-pass filter BPF1 to form a driving frequency of a first acousto-optic modulator AOM1, and the first acousto-optic modulator AOM1 is connected to the front-end and the regenerative laser module. Here, the delay of the first delay module matches the delay between the front-end and the receiving module. The first acousto-optic modulator AOM1 demodulates the received spread spectrum optical signal, filters out the second-order Rayleigh scattering signal in the spread spectrum optical signal, obtains a demodulated optical signal and outputs the demodulated optical signal to the regenerative laser module, and at the same time, the first acousto-optic modulator AOM1 modulates the return optical signal formed by the regenerative laser module and transmits the return optical signal to the front-end.

[0048] In one embodiment, as shown in FIG. 1, the regenerative laser module includes a first beam splitter OC1, a first Faraday rotating mirror FM1, a first photodetector PD1, a second band-pass filter BPF2, a first frequency division phase comparison module, a first proportional-integral controller PID1, a relay laser, and a second beam splitter OC2. Figure 1

[0049] The relay laser is used to generate a regenerated optical signal, and the frequency of the regenerated optical signal changes according to a control voltage. The regenerated optical signal is divided into two beams by the second beam splitter OC2, one of which enters the first beam splitter OC1 as a return optical signal, and the other of which transmits to the transmitting module as a transmission optical signal.

[0050] The return optical signal entering the first beam splitter OC1 is divided into two beams, one of which returns to the receiving module, and the other of which enters the first photodetector PD1 after being reflected by the first Faraday rotating mirror FM1.

[0051] The demodulated optical signal output by the receiving module enters the first photodetector PD1 through the first beam splitter OC1, the first photodetector PD1 beats the input demodulated optical signal and the return optical signal to obtain a beat signal, the beat signal is filtered by the second band-pass filter BPF2 and enters the first frequency division phase comparison module, the first frequency division phase comparison module divides and phase-compares the filtered signal and a reference clock signal (such as a 10 MHz clock signal) to obtain a first error voltage signal, and the first proportional-integral controller generates a control voltage of the relay laser according to the first error voltage signal.

[0052] In one embodiment, as shown in FIG. 1, the transmitting module includes a third beam splitter OC3, a second Faraday rotating mirror FM2, a second photodetector PD2, a servo control unit, and a second acousto-optic modulator AOM2. Figure 1

[0053] ​​The transmission light signal output by the regenerative laser module is split into two beams by the third beam splitter OC3, one of which is used as a reference light signal, enters the second photodetector PD2 after being reflected by the second Faraday mirror FM2, and the other enters the second acousto-optic modulator AOM2; the second acousto-optic modulator AOM2 is connected to the receiving module in the subsequent stage through an optical fiber link, can demodulate the modulated return light signal from the subsequent stage, realize filtering of the second-order Rayleigh scattering signal in the modulated return light signal, and obtain the demodulated return light signal carrying double-link noise, which enters the second photodetector PD2 through the third beam splitter OC3.

[0054] The second photodetector PD2 beats the input reference light signal and demodulated return light signal to obtain an intra-loop beat signal, and the servo control unit forms the drive frequency of the second acousto-optic modulator AOM2 according to the intra-loop beat signal and the third pseudo-random sequence, so as to drive the second acousto-optic modulator AOM2 to suppress and spread the link noise of the transmission light signal and output it to the receiving module in the subsequent stage. Here, the second acousto-optic modulator AOM2 performs frequency shift processing on the transmission light signal, thereby realizing link noise suppression.

[0055] In one embodiment, referring to Figure 1 The servo control unit includes a third band-pass filter BPF3, a second frequency division and phase comparison module, a second proportional-integral controller PID2, a second digital frequency synthesizer DDS2, a second IQ modulation module, a second voltage-controlled oscillator VCO2, a second mixer, and a fourth band-pass filter BPF4.

[0056] Here, the third band-pass filter BPF3 filters the intra-loop beat signal, the second frequency division and phase comparison module divides and compares the phase of the filtered intra-loop beat signal and a reference clock signal (such as a 10MHz clock signal) to obtain a second error voltage signal, the second proportional-integral controller PID2 generates a control signal of the second digital frequency synthesizer DDS2 according to the second error voltage signal, so that the second digital frequency synthesizer DDS2 generates a second local oscillator signal The second IQ modulation module generates a third pseudo-random sequence to the second voltage-controlled oscillator VCO2. The signal output by the second voltage-controlled oscillator VCO2 is mixed with the second local oscillator signal by the second mixer, and the mixed signal is filtered by the fourth band-pass filter BPF4 to form the drive frequency of the second acousto-optic modulator AOM2.

[0057] In one embodiment, the relay system further includes a GPS servo rubidium clock. The GPS servo rubidium clock is used to generate a time synchronization signal and a reference clock signal.

[0058] Here, the reference clock signal is used by both the first frequency division phase comparison module in the regenerative laser module to generate the first error voltage signal and the second frequency division phase comparison module in the servo control unit to generate the second error voltage signal. The time synchronization signal is the basis for generating the pseudo-random sequence, i.e., the first pseudo-random sequence and the third pseudo-random sequence are both generated based on the time synchronization signal. Specifically, in the receiving module, the first IQ modulation module generates the first pseudo-random sequence based on the time synchronization signal; in the sending module, the second IQ modulation module generates the third pseudo-random sequence based on the time synchronization signal. For example, the time synchronization signal is a 1PPS (One Pulse Per Second) signal, and the reference clock signal is a 10MHz signal. It can be understood that the first pseudo-random sequence is also generated based on the time synchronization signal in the previous stage.

[0059] The application provides a fiber-optical frequency transfer relay system based on a spread spectrum technology, comprising a receiving module, a regenerative laser module and a sending module; wherein the receiving module demodulates a received spread spectrum optical signal by using a second pseudo-random sequence to realize filtering of a second-order Rayleigh scattering signal and obtain a demodulated optical signal, and the receiving module also modulates a returned optical signal output by the regenerative laser module by using the second pseudo-random sequence and transmits the returned optical signal back to the previous stage to realize noise suppression and second-order Rayleigh scattering signal filtering in the previous stage. Here, the spread spectrum optical signal is a signal that is spread by the previous stage and transmitted through an optical fiber link; the sending module demodulates a modulated returned optical signal from the next stage by using a third pseudo-random sequence, and the sending module transmits a transmission optical signal to the next stage after link noise suppression and spread spectrum according to the demodulated returned optical signal and the third pseudo-random sequence. In the application, the optical signal is modulated by using the pseudo-random sequence, so that the optical signal is given a unique time-varying phase offset; therefore, when the optical signal is demodulated by using the pseudo-random sequence with a matched time delay, only the optical signal with a time delay that is accurately matched can be effectively recovered, and the second-order Rayleigh scattering signal with a time delay that is not matched is filtered out. The application realizes filtering of the second-order Rayleigh scattering signal by introducing the spread spectrum technology, avoids system cycle slip and loss of lock caused by the second-order Rayleigh scattering, and significantly improves the stability and reliability of the system.

[0060] Based on the same inventive concept, the application also provides a fiber-optical frequency transfer method based on a spread spectrum technology, comprising: transmitting an optical signal between a source sending end and a destination receiving end by using the above-mentioned fiber-optical frequency transfer relay system based on a spread spectrum technology. Moreover, the sending module of the source sending end is the same as the sending module of the relay system, and the receiving module and the regenerative laser module of the destination receiving end are the same as the sending module and the regenerative laser module of the relay system.

[0061] Specifically, Figure 2A structural schematic diagram of a fiber-optical frequency delivery system is shown, wherein a transmitting module of a source transmitting end is as shown in Figure 3 A receiving module of a destination receiving end is as shown in Figure 4 It can be seen that the transmitting module of the source transmitting end is basically similar to the transmitting module in the relay system proposed above in structure and function, and is used to transmit the light beam generated by the cavity-stabilized laser as an optical signal, and to perform link noise suppression and modulation on the optical signal and then deliver it to the first-stage relay system. The receiving module and the regenerative laser module of the destination receiving end are basically similar to the receiving module and the regenerative laser module in the relay system proposed above in structure and function, and the difference lies in that the delivery optical signal output by the regenerative laser module of the destination receiving end is output to a user end for use.

[0062] For example, when the fiber-optical frequency delivery system includes only one relay station, the optical signal delivery process thereof is as shown in Figure 5 and includes:

[0063] S10, generating an optical signal by a cavity-stabilized laser and spreading the optical signal by a first pseudo-random sequence to obtain a spread spectrum optical signal;

[0064] S20, demodulating the spread spectrum optical signal by a second pseudo-random sequence by a first acousto-optic modulator to obtain a demodulated optical signal;

[0065] S30, generating a regenerative optical signal by a relay laser and splitting it into two beams by a second beam splitter, one of which is transmitted to a first photodetector and a first acousto-optic modulator as a return optical signal, and the other of which is transmitted to a third beam splitter as a delivery optical signal; the first photodetector performs beat frequency on the demodulated optical signal and the return optical signal, and frequency-locks the regenerative optical signal on the demodulated optical signal based on the beat frequency signal;

[0066] S40, modulating the return optical signal by a second pseudo-random sequence by the first acousto-optic modulator to obtain a modulated return optical signal, which is returned to the source transmitting end;

[0067] S50, splitting the delivery optical signal into two beams by the third beam splitter, one of which enters a second photodetector as a reference optical signal, and the other of which enters a second acousto-optic modulator and is spread by a third pseudo-random sequence before being transmitted to the destination receiving end; at the same time, the modulated return optical signal returned by the destination receiving end is demodulated by the second acousto-optic modulator to obtain a demodulated return optical signal; the second photodetector performs beat frequency on the demodulated return optical signal and the reference optical signal, and performs noise suppression on the delivery optical signal according to the beat frequency signal;

[0068] S60, the spread spectrum optical signal entering the destination receiving end is demodulated by a second pseudo-random sequence to obtain a demodulated optical signal; the frequency of the demodulated optical signal is locked and a regenerated optical signal is generated by a relay laser; the regenerated optical signal is divided into two beams, one of which is modulated by a second pseudo-random sequence to obtain a modulated return optical signal and is transmitted back to the second acousto-optic modulator, and the other is transmitted to the user end for use.

[0069] It should be noted that, for the method embodiment, since its main implementation is based on the structure implementation of the relay system as shown in Figure 1 It should be noted that the description of the method embodiment is relatively simple because its main implementation is based on the structure implementation of the relay system as shown in

[0070] It should be noted that the terms "first", "second", and the like are used to distinguish similar objects, and do not necessarily have to be used to describe a particular order or sequence. It should be understood that the data used in this way can be interchanged under appropriate circumstances, so that the embodiments of the application described herein can be implemented in an order other than those illustrated or described herein. The implementation described in the following exemplary embodiments does not represent all implementations consistent with the present application. Rather, they are merely examples of devices and methods consistent with some aspects of the present application.

[0071] In the description of the present specification, the description of the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" and the like means that the specific features or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine different embodiments or examples described in the present specification.

[0072] Although the present application is described herein in conjunction with various embodiments, other variations of the disclosed embodiments can be understood and implemented by those skilled in the art with reference to the drawings and the disclosure. In the description of the present application, the word "comprising" does not exclude other components or steps, "one" or "an" does not exclude a plurality, and "plurality" means two or more, unless otherwise explicitly specified. In addition, some measures are described in different embodiments, but this does not mean that these measures cannot be combined to produce good results.

[0073] The above is further detailed description of the present application in combination with specific preferred embodiments, and cannot be deemed as limitation of the specific implementation of the present application to these descriptions. For those skilled in the art to which the present application belongs, without departing from the concept of the present application, a number of simple deductions or substitutions can be made, and all of them shall be deemed as falling within the protection scope of the present application.

Claims

1. A fiber-optic optical frequency transfer relay system based on spread-spectrum techniques, characterized in that, The receiving module, the regenerative laser module and the sending module are included. The receiving module is used for receiving a spread spectrum optical signal, demodulating the spread spectrum optical signal according to a second pseudo-random sequence to obtain a demodulated optical signal; the spread spectrum optical signal is a signal modulated by a first pseudo-random sequence in a front stage and transmitted through an optical fiber link; The second pseudo-random sequence is obtained by phase adjustment of the first pseudo-random sequence; the phase adjustment is matched with a delay between the front stage and the receiving module; The regenerative laser module is used for locking the frequency of the demodulated optical signal to generate a regenerated optical signal; one beam of the regenerated optical signal is transmitted to the receiving module as a return optical signal, and another beam is transmitted to the sending module as a transmission optical signal; The receiving module is further used for modulating the return optical signal according to the second pseudo-random sequence to obtain a modulated return optical signal; the modulated return optical signal is transmitted back to the front stage; The sending module is used for modulating the transmission optical signal by a third pseudo-random sequence to obtain a spread spectrum optical signal and transmitting the spread spectrum optical signal to a rear stage, and demodulating a modulated return optical signal from the rear stage by the third pseudo-random sequence to obtain a demodulated return optical signal, and using the demodulated return optical signal to realize noise suppression of the transmission optical signal.

2. The spread-spectrum technology based fiber-optic optical frequency transfer relay system of claim 1, wherein, The receiving module includes a first acousto-optic modulator, a first band-pass filter, a first frequency mixer, a first digital frequency synthesizer, a first IQ modulation module, a first delay module and a first voltage-controlled oscillator. The first IQ modulation module is used for generating the first pseudo-random sequence, the first pseudo-random sequence is adjusted in phase by the first delay module to form the second pseudo-random sequence, the second pseudo-random sequence is input to the first voltage-controlled oscillator, the first digital frequency synthesizer generates a first local oscillator signal, the output signal of the first voltage-controlled oscillator and the first local oscillator signal are mixed by the first frequency mixer, the mixed signal is filtered by the first band-pass filter to form the driving frequency of the first acousto-optic modulator, and the first acousto-optic modulator is connected to the front stage and the regenerative laser module.

3. The spread-spectrum technology based fiber-optic optical frequency transfer relay system of claim 1, wherein, The regenerative laser module includes a first beam splitter, a first Faraday rotating mirror, a first photodetector, a second band-pass filter, a first frequency division ratio phase module, a first proportional-integral controller, a relay laser and a second beam splitter. The relay laser is used for generating a regenerated optical signal, the frequency of the regenerated optical signal changes according to a control voltage; the regenerated optical signal is divided into two beams by the second beam splitter, one beam enters the first beam splitter as a return optical signal, and the other beam is transmitted to the sending module as a transmission optical signal; The return optical signal entering the first beam splitter is divided into two beams, one beam returns to the receiving module, and the other beam enters the first photodetector after being reflected by the first Faraday rotating mirror; The return optical signal entering the first beam splitter is divided into two beams, one beam returns to the receiving module, and the other beam enters the first photodetector after being reflected by the first Faraday rotating mirror; The demodulation light signal output by the receiving module enters the first photodetector through the first beam splitter, the first photodetector beats the input demodulation light signal and the return light signal to obtain a beat signal, the beat signal is filtered by the second band-pass filter and then enters the first frequency division phase comparison module, the first frequency division phase comparison module divides and phase-compares the filtered signal and the reference clock signal to obtain a first error voltage signal, and the first proportional integral controller generates the control voltage according to the first error voltage signal.

4. The spread-spectrum-based optical fiber-optical frequency transfer relay system of claim 1, wherein, The sending module comprises a third beam splitter, a second Faraday rotating mirror, a second photodetector, a servo control unit and a second acousto-optic modulator. The transfer light signal output by the regenerative laser module is split into two beams by the third beam splitter, one of which is used as a reference light signal, enters the second photodetector after being reflected by the second Faraday rotating mirror, and the other enters the second acousto-optic modulator; the second acousto-optic modulator is connected to the receiving module in the rear stage through an optical fiber link; The second acousto-optic modulator demodulates the modulated return light signal from the rear stage to obtain a demodulated return light signal, and the demodulated return light signal enters the second photodetector through the third beam splitter; The second photodetector beats the input reference light signal and the demodulated return light signal to obtain an intra-loop beat signal, and the servo control unit forms the driving frequency of the second acousto-optic modulator according to the intra-loop beat signal and the third pseudo-random sequence.

5. The spread-spectrum technology based fiber-optic optical frequency transfer relay system of claim 4, wherein, The servo control unit comprises a third band-pass filter, a second frequency division phase comparison module, a second proportional integral controller, a second digital frequency synthesizer, a second IQ modulation module, a second voltage-controlled oscillator, a second frequency mixer and a fourth band-pass filter; The third band-pass filter filters the intra-loop beat signal, the second frequency division phase comparison module divides and phase-compares the filtered intra-loop beat signal and the reference clock signal to obtain a second error voltage signal, the second proportional integral controller generates a control signal of the second digital frequency synthesizer according to the second error voltage signal, so that the second digital frequency synthesizer generates a second local oscillator signal, the second IQ modulation module generates the third pseudo-random sequence and feeds it to the second voltage-controlled oscillator; the signals output by the second voltage-controlled oscillator and the second local oscillator signal are mixed by the second frequency mixer, and the mixed signal is filtered by the fourth band-pass filter to form the driving frequency of the second acousto-optic modulator.

6. The spread-spectrum technology based fiber-optic optical frequency transfer relay system according to claim 3 or 5, characterized in that, The relay system further comprises a GPS servo rubidium clock; The GPS servo rubidium clock is used to generate a time synchronization signal and the reference clock signal; the first pseudo-random sequence and the third pseudo-random sequence are both generated based on the time synchronization signal.

7. A method of optical fiber optical frequency transfer based on spread spectrum technique, characterized in that, It comprises: An optical signal is transmitted between a source sending end and a destination receiving end by using the optical fiber optical frequency transfer relay system based on spread spectrum technology in any one of claims 1-6; the sending module of the source sending end is the same as the sending module of the relay system; the receiving module and the regenerative laser module of the destination receiving end are the same as the receiving module and the regenerative laser module of the relay system.

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