Optical fiber optical frequency transmission relay system and method based on spread spectrum technology

Abstract

The invention discloses an optical fiber optical frequency transmission relay system and method based on a spread spectrum technology. The relay system comprises a receiving module, a regenerative laser module and a sending module. The receiving module is used for demodulating the spread spectrum optical signal according to the second pseudo-random sequence to obtain a demodulated optical signal; the spread spectrum optical signal is a signal which is transmitted through an optical fiber link after a preceding stage performs spectrum spreading on the optical signal; the regeneration laser module is used for locking the frequency of the demodulation optical signal to generate a regeneration optical signal and splitting the regeneration optical signal to obtain a return optical signal and a transmission optical signal; the receiving module is also used for modulating the returned optical signal according to the second pseudo-random sequence and transmitting the modulated returned optical signal back to the preceding stage; and the sending module is used for demodulating the post-stage modulation return optical signal to obtain a demodulated return optical signal, carrying out noise suppression and spectrum spreading on the transmission optical signal according to the demodulated return optical signal and a third pseudo-random sequence, and then transmitting the transmission optical signal to the post-stage. By introducing the spread spectrum technology, the second-order Rayleigh scattering signals are filtered out, and the stability and reliability of the system are remarkably improved.

Description

Technical Field

[0001] This invention belongs to the field of optical fiber communication technology, specifically relating to an optical fiber frequency transmission relay system and method based on spread spectrum technology. Background Technology

[0002] With the rapid development of optical atomic clock technology, its stability has exceeded 10. -19 The performance of optical clocks is orders of magnitude higher than that of traditional microwave atomic clocks, representing an improvement of two orders of magnitude or even more. High-performance optical clock networks have shown broad application prospects in time and frequency metrology, radio astronomy observation, gravitational wave detection, and verification of fundamental physical laws. However, optical atomic clocks themselves have inherent limitations such as high environmental sensitivity and large system size, making them difficult to directly apply to large-scale distributed application scenarios, thus greatly limiting their practical application. Therefore, optical frequency transfer technology based on optical fibers has emerged, becoming a core supporting means to promote the practical application of optical clocks. This technology utilizes the excellent characteristics of optical fiber transmission media, such as low transmission loss and strong resistance to electromagnetic interference, to enable users in different spatial locations to share the same high-performance frequency reference.

[0003] In fiber optic frequency transmission, Doppler noise cancellation technology is typically used to accurately detect and effectively suppress additional phase noise introduced by the fiber optic link, thereby ensuring signal transmission quality. However, various reflection points in the fiber optic link can induce second-order Rayleigh scattering, which introduces nonlinear interference, causing cycle slips or even loss of lock in the transmission system, seriously threatening the long-term stable operation of the link. On the other hand, as the transmission distance increases, the inherent attenuation of the fiber leads to a significant decrease in signal power. Therefore, the industry commonly uses bidirectional erbium-doped fiber amplifiers (Bi-EDFAs) to compensate for power loss, but the introduction of these amplifiers inevitably exacerbates the second-order Rayleigh scattering effect. Therefore, the system cycle slip and loss of lock problems caused by second-order Rayleigh scattering are particularly prominent in long-distance fiber optic frequency transmission links, and have become a key technical bottleneck restricting long-distance fiber optic optical frequency transmission. Summary of the Invention

[0004] To address the aforementioned problems in the prior art, this invention provides a fiber optic frequency transmission relay system and method based on spread spectrum technology.

[0005] The technical problem to be solved by this invention is achieved through the following technical solution: In a first aspect, the present invention provides a fiber optic frequency transmission relay system based on spread spectrum technology, including a receiving module, a regenerating laser module and a transmitting module; The receiving module is used to receive a spread spectrum optical signal and 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 that has been modulated by a first pseudo-random sequence in the previous 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 the delay between the previous stage and the receiving module. The regenerating laser module is used to lock the frequency of the demodulated optical signal and generate a regenerated optical signal; one beam of the regenerated optical signal is transmitted to the receiving module as a return optical signal, and the other beam is transmitted to the transmitting module as a transmission optical signal. The receiving module is further configured to modulate the returned optical signal according to the second pseudo-random sequence to obtain a modulated returned optical signal; the modulated returned optical signal is transmitted back to the front-end; The transmitting module is used to modulate the transmitted optical signal with a third pseudo-random sequence to obtain a spread spectrum optical signal, which is then transmitted to the next stage. Simultaneously, it uses a third pseudo-random sequence to demodulate the modulated return optical signal from the next stage to obtain a demodulated return optical signal, and uses this demodulated return optical signal to suppress noise in the transmitted optical signal.

[0006] Optionally, the receiving module includes a first acousto-optic modulator, a first bandpass filter, a first 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 generates the first pseudo-random sequence. The first pseudo-random sequence is phase-adjusted by the first delay module to form the second pseudo-random sequence, which is then fed to the first voltage-controlled oscillator. The first digital frequency synthesizer generates the first local oscillator signal. The output signal of the first voltage-controlled oscillator and the first local oscillator signal are mixed by the first mixer. The mixed signal is filtered by the first bandpass filter to form the driving frequency of the first acousto-optic modulator. The first acousto-optic modulator is connected to the preamplifier and the regenerative laser module.

[0007] Optionally, the regenerated laser module includes a first beam splitter, a first Faraday rotator, a first photodetector, a second bandpass filter, a first frequency division and phase comparison module, a first proportional-integral controller, a relay laser, and a second beam splitter. The relay laser is used to generate a regenerated optical signal, the frequency of which changes according to the control voltage; the regenerated optical signal is split 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 transmitting module as a transmission optical signal. The return optical signal entering the first beam splitter is split 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 demodulated optical signal output by the receiving module passes through the first beam splitter and enters the first photodetector. The first photodetector performs beat frequency analysis on the input demodulated optical signal and the returned optical signal to obtain a beat frequency signal. The beat frequency signal is filtered by the second bandpass filter and then enters the first frequency division and phase comparison module. The first frequency division and phase comparison module performs frequency division and phase detection on the filtered signal and the reference clock signal to obtain a first error voltage signal. The first proportional-integral controller generates the control voltage based on the first error voltage signal.

[0008] Optionally, the transmitting module includes a third beam splitter, a second Faraday rotator, a second photodetector, a servo control unit, and a second acousto-optic modulator; The transmitted optical signal output by the regenerated laser module is split into two beams by the third beam splitter. One beam serves as a reference optical signal, which is reflected by the second Faraday rotating mirror and enters the second photodetector. The other beam enters the second acousto-optic modulator. The second acousto-optic modulator is connected to the subsequent receiving module via an optical fiber link. The second acousto-optic modulator demodulates the modulated return optical signal from the subsequent stage to obtain a demodulated return optical signal, which then enters the second photodetector via the third beam splitter. The second photodetector beats the input reference optical signal and the demodulated return optical signal to obtain an in-loop beat frequency signal. The servo control unit forms the driving frequency of the second acousto-optic modulator based on the in-loop beat frequency signal and the third pseudo-random sequence.

[0009] Optionally, the servo control unit includes a third bandpass filter, a second frequency divider and phase ratio module, a second proportional-integral controller, a second digital frequency synthesizer, a second IQ modulation module, a second voltage-controlled oscillator, a second mixer, and a fourth bandpass filter; The third bandpass filter filters the intra-loop beat frequency signal. The second frequency division and phase comparison module divides and phase-detects the filtered intra-loop beat frequency signal and the reference clock signal to obtain a second error voltage signal. The second proportional-integral controller generates a control signal for the second digital frequency synthesizer based on the second error voltage signal, causing the second digital frequency synthesizer to generate a second local oscillator signal. The second IQ modulation module generates the third pseudo-random sequence and sends it to the second voltage-controlled oscillator. The signal output by the second voltage-controlled oscillator and the second local oscillator signal are mixed by the second mixer. The mixed signal is filtered by the fourth bandpass filter to form the driving frequency of the second acousto-optic modulator.

[0010] Optionally, 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; both the first pseudo-random sequence and the third pseudo-random sequence are generated based on the time synchronization signal.

[0011] Secondly, the present invention provides a fiber optic frequency transmission method based on spread spectrum technology, comprising: transmitting an optical signal between a source transmitter and a destination receiver using the aforementioned fiber optic frequency transmission relay system based on spread spectrum technology; the transmitting module of the source transmitter being the same as the transmitting module of the relay system; and the receiving module and regenerated laser module of the destination receiver being the same as the receiving module and regenerated laser module of the relay system.

[0012] This invention provides a fiber optic frequency relay system based on spread spectrum technology, comprising a receiving module, a regenerating laser module, and a transmitting module. The receiving module uses a second pseudo-random sequence to demodulate the received spread spectrum optical signal to filter out second-order Rayleigh scattering signals, obtaining a demodulated optical signal. Simultaneously, the receiving module also uses the second pseudo-random sequence to modulate the return optical signal output from the regenerating laser module and transmits it back to the preceding stage for noise suppression and second-order Rayleigh scattering signal filtering. Here, the spread spectrum optical signal is the signal spread by the preceding stage and transmitted via the fiber optic link. The transmitting module uses a third pseudo-random sequence to demodulate the modulated return optical signal from the subsequent stage. Simultaneously, the transmitting module performs link noise suppression and spread spectrum on the transmitted optical signal based on the demodulated return optical signal and the third pseudo-random sequence before transmitting it to the subsequent stage. This invention modulates optical signals using pseudo-random sequences, giving them a unique, time-varying phase shift. Therefore, when demodulating with a pseudo-random sequence that has a matched time delay, based on the autocorrelation properties of the pseudo-random sequence, only optical signals with precisely matched time delays can be effectively recovered, while second-order Rayleigh scattering signals with mismatched time delays are filtered out. By introducing spread spectrum technology, this invention achieves the filtering out of second-order Rayleigh scattering signals, avoiding system cycle slips and lock-out problems caused by second-order Rayleigh scattering, thereby significantly improving the stability and reliability of the system.

[0013] The present invention will now be described in further detail with reference to the accompanying drawings. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the structure of a fiber optic frequency transmission relay system based on spread spectrum technology provided in an embodiment of the present invention; Figure 2 This is a schematic diagram of a fiber optic frequency transmission system. Figure 3This is a schematic diagram of the structure of a source transmitter adapted to the relay system provided in the embodiments of the present invention; Figure 4 This is a schematic diagram of the structure of a destination receiver adapted to the relay system provided in the embodiments of the present invention; Figure 5 This is a flowchart of the optical signal transmission process of the fiber optic frequency transmission system provided in an embodiment of the present invention. Detailed Implementation

[0015] The present invention will be further described in detail below with reference to specific embodiments, but the implementation of the present invention is not limited thereto.

[0016] To filter out second-order Rayleigh scattering signals during fiber optic frequency transmission and avoid system cycle slips and loss-of-lock problems caused by second-order Rayleigh scattering, thereby significantly improving the stability and reliability of fiber optic frequency transmission, this invention provides a fiber optic frequency transmission relay system and method based on spread spectrum technology.

[0017] First, the fiber optic frequency transfer relay system (also known as a relay station) based on spread spectrum technology provided in the embodiments of the present invention will be described in detail. See [link to relevant documentation]. Figure 1 The relay system includes a receiving module, a regenerating laser module, and a transmitting module.

[0018] The receiving module receives the spread spectrum optical signal and demodulates it according to a second pseudo-random sequence to obtain a demodulated optical signal. This spread spectrum optical signal is a signal modulated by a first pseudo-random sequence by the preceding stage (pre-stage repeater or source transmitter) and transmitted via an optical fiber link. The second pseudo-random sequence is obtained by phase adjustment of the first pseudo-random sequence, and this phase adjustment is matched to the delay between the preceding stage and the receiving module.

[0019] Specifically, the front-end uses a first pseudo-random sequence to spread the optical signal, converting it into a broadband signal, and injects it into the fiber optic link for transmission to the receiving module. The receiving module then receives the spread-spectrum optical signal, which contains noise signals such as second-order Rayleigh scattering due to transmission through the fiber optic link. Demodulating this spread-spectrum optical signal using a second pseudo-random sequence utilizes the autocorrelation properties of the pseudo-random sequence to filter out the second-order Rayleigh scattering signal, resulting in a demodulated optical signal.

[0020] Here, using a pseudo-random sequence to perform binary phase modulation / demodulation of the optical signal involves abstracting the pseudo-random sequence into a function. This transforms discrete binary code into continuous-domain operators. During modulation / demodulation, when the code element of the pseudo-random sequence is "0", the corresponding... At this time, the phase of the optical signal remains unchanged, while when the symbol of the pseudo-random sequence is "1", the corresponding... At this time, the phase of the optical signal is generated. The phase shift is measured in radians. Therefore, by using a pseudo-random sequence for phase modulation, a unique, time-varying phase shift can be assigned to the optical signal. When demodulating using a pseudo-random sequence, due to the autocorrelation property of the pseudo-random sequence, i.e. ,and It is still a random code, among which, Indicates the time delay of the optical signal. The time delay representing the noise signal. Therefore, if the time delay of the pseudo-random sequence is the same as the time delay of the optical signal... Exact matching (i.e., the time delay of the pseudo-random sequence is...) The delay is... The optical signal can be effectively recovered, while the second-order Rayleigh scattering signal with time delay mismatch is filtered out because its product is still a random code. Here, the code length of the selected pseudo-random sequence must ensure that there are enough chips to distinguish the signals on the modulation side (such as the preamplifier) ​​and the demodulation side (such as the receiver module), that is, the code sequence period needs to cover the link delay of the modulation side and the demodulation side.

[0021] The regenerating laser module is used to lock the frequency of the demodulated optical signal and generate a regenerated optical signal. One beam of this regenerated optical signal is transmitted to the receiving module as a return optical signal, and the other beam is transmitted to the transmitting module as a transmission optical signal.

[0022] Specifically, the regenerating laser module beats the demodulated optical signal and the returned optical signal to obtain a beat frequency signal. Based on this beat frequency signal, the repeater laser is adjusted so that its output frequency accurately tracks and locks onto the demodulated optical signal. It is understandable that if the preceding stage does not use the first pseudo-random sequence to modulate the optical signal, but instead directly outputs an unspread optical signal, the phase of the demodulated optical signal locked by the regenerating laser module is determined by the vector sum of the unspread optical signal and various scattering signals, such as second-order Rayleigh scattering. These signals are superimposed in both the time and frequency domains, making them difficult to separate and severely degrading the quality of the optical signal. However, when the preceding stage uses the first pseudo-random sequence to modulate the phase of the optical signal, different signals (the optical signal and second-order Rayleigh scattering signals generated at different locations) have different propagation delays in the fiber optic link, thus carrying unique time-varying phase offsets. Therefore, in the receiving module, demodulating the received spread-spectrum optical signal using a second pseudo-random sequence filters out the second-order Rayleigh scattering signals in the spread-spectrum optical signal, resulting in the demodulated optical signal. The second pseudo-random sequence has the same code pattern as the first pseudo-random sequence, and the delay of the second pseudo-random sequence matches the delay of the optical fiber link between the front-end and the receiving module.

[0023] 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, which is then transmitted back to the previous stage.

[0024] Understandably, the modulated return optical signal is transmitted back to the front end via the fiber optic link.

[0025] The transmitting module is used to modulate the transmitted optical signal with a third pseudo-random sequence to obtain a spread spectrum optical signal, which is then transmitted to the subsequent stage. At the same time, it uses the third pseudo-random sequence to demodulate the modulated return optical signal from the subsequent stage (the subsequent relay station or the destination receiver) to obtain a demodulated return optical signal, and uses this demodulated return optical signal to suppress noise in the transmitted optical signal.

[0026] Specifically, by using a third pseudo-random sequence to demodulate the modulated return optical signal, the second-order Rayleigh scattering signal introduced by the fiber optic link transmission can be filtered out, thus obtaining the demodulated return optical signal. This demodulated return optical signal is the spread spectrum optical signal emitted by the transmitting module, which travels through the fiber optic link to the subsequent stage and then returns to the transmitting module from the subsequent stage via the fiber optic link. Therefore, this demodulated return optical signal carries twice the link noise. The transmitting module beats the demodulated return optical signal with a reference optical signal (one beam of the transmitted optical signal) to obtain an in-loop beat frequency signal. The transmitting module then uses the third pseudo-random sequence to spread the transmitted optical signal and simultaneously performs link noise suppression on the transmitted optical signal based on this in-loop beat frequency signal. Here, the third pseudo-random sequence is preferably the same as the first pseudo-random sequence, but it is not limited to this. The third pseudo-random sequence is precisely time-delay matched with the second pseudo-random sequence of the subsequent receiving module, that is, the second pseudo-random sequence of the subsequent receiving module is obtained by phase adjustment of the third pseudo-random sequence, and this phase adjustment matches the delay between the transmitting module and the subsequent receiving module.

[0027] The fiber optic frequency relay system based on spread spectrum technology provided by this invention modulates the optical signal using a pseudo-random sequence, which imparts a unique, time-varying phase shift to the optical signal. Therefore, when demodulated using a pseudo-random sequence with matched time delays, based on the autocorrelation characteristics of the pseudo-random sequence, only optical signals with precisely matched time delays can be effectively recovered, while second-order Rayleigh scattering signals with mismatched time delays are filtered out. By introducing spread spectrum technology, the filtering of second-order Rayleigh scattering signals is achieved, avoiding system cycle slip and lock-out problems caused by second-order Rayleigh scattering, thereby significantly improving the stability and reliability of the system.

[0028] In one embodiment, such as Figure 1 As shown, the receiving module includes a first acousto-optic modulator AOM1, a first bandpass 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.

[0029] The first IQ modulation module generates a first pseudo-random sequence, which is then phase-adjusted by the first delay module to form a second pseudo-random sequence, which is then fed 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 signal is mixed by the first mixer, and the mixed signal is filtered by the first bandpass filter BPF1 to form the driving frequency of the first acousto-optic modulator AOM1. The first acousto-optic modulator AOM1 is connected to the preamplifier and the regenerating laser module. Here, the delay of the first delay module is matched with the delay between the preamplifier 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 the demodulated optical signal, and outputs it to the regenerating laser module. At the same time, the first acousto-optic modulator AOM1 modulates the return optical signal generated by the regenerating laser module and transmits it back to the preamplifier.

[0030] In one embodiment, such as Figure 1 As shown, the regenerated laser module includes a first beam splitter OC1, a first Faraday rotator FM1, a first photodetector PD1, a second bandpass filter BPF2, a first frequency divider / phase divider module, a first proportional-integral controller PID1, a relay laser, and a second beam splitter OC2.

[0031] The relay laser generates a regenerated optical signal whose frequency varies according to a control voltage. This regenerated optical signal is split into two beams by a second beam splitter OC2. One beam serves as a return optical signal and enters the first beam splitter OC1, while the other beam serves as a transmission optical signal and is transmitted to the transmitting module.

[0032] The return optical signal entering the first beam splitter OC1 is split into two beams. One beam returns to the receiving module, and the other beam is reflected by the first Faraday rotating mirror FM1 and enters the first photodetector PD1.

[0033] The demodulated optical signal output by the receiving module passes through the first beam splitter OC1 and enters the first photodetector PD1. The first photodetector PD1 beats the input demodulated optical signal and the returned optical signal to obtain a beat frequency signal. This beat frequency signal is filtered by the second bandpass filter BPF2 and then enters the first frequency divider and phase comparison module. The first frequency divider and phase comparison module divides the filtered signal and the reference clock signal (such as a 10MHz clock signal) and performs frequency division and phase detection to obtain the first error voltage signal. The first proportional-integral controller generates the control voltage of the repeater laser based on the first error voltage signal.

[0034] In one embodiment, such as Figure 1As shown, the transmitting module includes a third beam splitter OC3, a second Faraday rotator FM2, a second photodetector PD2, a servo control unit, and a second acoustic-optical modulator AOM2.

[0035] The transmitted optical signal output by the regenerated laser module is split into two beams by the third beam splitter OC3. One beam serves as a reference optical signal, which is reflected by the second Faraday rotator FM2 and enters the second photodetector PD2. The other beam enters the second acousto-optic modulator AOM2. The second acousto-optic modulator AOM2 is connected to the receiving module of the subsequent stage through an optical fiber link. It can demodulate the modulated return optical signal from the subsequent stage, thereby filtering out the second-order Rayleigh scattering signal in the modulated return optical signal and obtaining a demodulated return optical signal carrying twice the link noise. This demodulated return optical signal enters the second photodetector PD2 through the third beam splitter OC3.

[0036] The second photodetector PD2 beats the input reference optical signal and the demodulated return optical signal to obtain an in-loop beat frequency signal. The servo control unit uses this in-loop beat frequency signal and a third pseudo-random sequence to form the driving frequency of the second acousto-optic modulator AOM2, thereby driving AOM2 to perform link noise suppression and spread spectrum on the transmitted optical signal, and outputting the result to the subsequent receiving module. Here, AOM2 performs frequency shifting on the transmitted optical signal to achieve link noise suppression.

[0037] In one embodiment, see Figure 1 As shown, the servo control unit includes a third bandpass filter BPF3, a second frequency divider and phase ratio 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 bandpass filter BPF4.

[0038] Here, the third bandpass filter BPF3 filters the beat frequency signal within the loop. The second frequency divider and phase comparison module divides and phase-detects the filtered beat frequency signal within the loop and the reference clock signal (such as a 10MHz clock signal) to obtain the second error voltage signal. The second proportional-integral controller PID2 generates a control signal for the second digital frequency synthesizer DDS2 based on the second error voltage signal, causing the second digital frequency synthesizer DDS2 to generate the second local oscillator signal. The second IQ modulation module generates a third pseudo-random sequence, which is then fed to the second voltage-controlled oscillator (VCO2). The signal output from the second VCO2 is compared with the second local oscillator signal. The signal is mixed by the second mixer, and the mixed signal is filtered by the fourth bandpass filter BPF4 to form the driving frequency of the second acousto-optic modulator AOM2.

[0039] In one embodiment, the relay system further includes a GPS servo rubidium clock. This GPS servo rubidium clock is used to generate time synchronization signals and reference clock signals.

[0040] Here, the reference clock signal is both the reference clock signal used by the first frequency divider / phase comparison module in the regenerated laser module to generate the first error voltage signal, and the reference clock signal used by the second frequency divider / phase comparison module in the servo control unit to generate the second error voltage signal. The time synchronization signal serves as the reference for generating the pseudo-random sequence; that is, both the first and third pseudo-random sequences are generated based on this time synchronization signal. Specifically, in the receiving module, the first IQ modulation module generates the first pseudo-random sequence based on this time synchronization signal; in the transmitting module, the second IQ modulation module generates the third pseudo-random sequence based on this 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 preceding stages.

[0041] This invention provides a fiber optic frequency relay system based on spread spectrum technology, comprising a receiving module, a regenerating laser module, and a transmitting module. The receiving module uses a second pseudo-random sequence to demodulate the received spread spectrum optical signal to filter out second-order Rayleigh scattering signals, obtaining a demodulated optical signal. Simultaneously, the receiving module also uses the second pseudo-random sequence to modulate the return optical signal output from the regenerating laser module and transmits it back to the preceding stage for noise suppression and second-order Rayleigh scattering signal filtering. Here, the spread spectrum optical signal is the signal spread by the preceding stage and transmitted via the fiber optic link. The transmitting module uses a third pseudo-random sequence to demodulate the modulated return optical signal from the subsequent stage. Simultaneously, the transmitting module performs link noise suppression and spread spectrum on the transmitted optical signal based on the demodulated return optical signal and the third pseudo-random sequence before transmitting it to the subsequent stage. This invention modulates optical signals using pseudo-random sequences, giving them a unique, time-varying phase shift. Therefore, when demodulating with a pseudo-random sequence that has a matched time delay, based on the autocorrelation properties of the pseudo-random sequence, only optical signals with precisely matched time delays can be effectively recovered, while second-order Rayleigh scattering signals with mismatched time delays are filtered out. By introducing spread spectrum technology, this invention achieves the filtering out of second-order Rayleigh scattering signals, avoiding system cycle slips and lock-out problems caused by second-order Rayleigh scattering, thereby significantly improving the stability and reliability of the system.

[0042] Based on the same inventive concept, this invention also provides a fiber optic frequency transmission method based on spread spectrum technology, comprising: transmitting an optical signal between a source transmitter and a destination receiver using the aforementioned fiber optic frequency transmission relay system based on spread spectrum technology. Furthermore, the transmitting module at the source transmitter is the same as the transmitting module of the relay system, and the receiving module and regenerated laser module at the destination receiver are the same as the transmitting module and regenerated laser module of the relay system.

[0043] Specifically, Figure 2 A schematic diagram of a fiber optic frequency transfer system is shown, wherein the transmitting module at the source transmitting end is as follows: Figure 3 As shown, the receiving module of the destination receiver is as follows: Figure 4 As shown, the transmitting module at the source transmitter is structurally and functionally similar to the transmitting module in the relay system proposed above. It is used to transmit the beam generated by the cavity-stabilized laser as an optical signal, and after performing link noise suppression and modulation on the optical signal, it is transmitted to the first-level relay system. The receiving module and regenerated laser module at the destination receiver are structurally and functionally similar to the receiving module and regenerated laser module in the relay system proposed above. The difference is that in the destination receiver, the transmitted optical signal output by its regenerated laser module is output to the user end for use.

[0044] For example, when a fiber optic frequency transfer system includes only one repeater station, its optical signal transmission process is as follows: Figure 5 As shown, it includes: S10. A cavity-stabilized laser generates an optical signal and spreads the optical signal using a first pseudo-random sequence to obtain a spread-spectrum optical signal. S20. The spread spectrum optical signal is demodulated by the first acousto-optic modulator using the second pseudo-random sequence to obtain the demodulated optical signal; S30. A regenerated optical signal is generated using a relay laser and split into two beams by a second beam splitter. One beam is transmitted as a return optical signal to the first photodetector and the first acousto-optic modulator; the other beam is transmitted as a transmission optical signal to the third beam splitter. The first photodetector beats the demodulated optical signal and the return optical signal, and locks the frequency of the regenerated optical signal onto the demodulated optical signal based on the beat frequency signal. S40. The return optical signal is modulated by the first acousto-optic modulator using the second pseudo-random sequence to obtain a modulated return optical signal, which is then returned to the source transmitter. S50. The transmitted optical signal is split into two beams by the third beam splitter. One beam serves as a reference optical signal and enters the second photodetector. The other beam enters the second acousto-optic modulator and is spread using a third pseudo-random sequence before being transmitted to the destination receiver. Simultaneously, the modulated return optical signal returned from the destination receiver is demodulated by the second acousto-optic modulator to obtain the demodulated return optical signal. The second photodetector performs beat frequency analysis on the demodulated return optical signal and the reference optical signal, and performs noise suppression on the transmitted optical signal based on the beat frequency signal. S60. The spread spectrum optical signal entering the destination receiver is demodulated by the 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 the relay laser. The regenerated optical signal is divided into two beams. One beam is used as a return optical signal and modulated by the second pseudo-random sequence to obtain a modulated return optical signal and is transmitted back to the second acousto-optic modulator. The other beam is transmitted to the user end for use.

[0045] It should be noted that, for the method implementation examples, since their main implementation is based on Figure 1 The relay system shown is implemented in a simple way, so the description is relatively straightforward. For relevant details, please refer to the description of the relay system embodiment.

[0046] It should be noted that the terms "first," "second," etc., are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present invention. Rather, they are merely examples of apparatuses and methods consistent with some aspects of the invention.

[0047] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Furthermore, those skilled in the art can combine and integrate the different embodiments or examples described in this specification.

[0048] Although the invention has been described herein in conjunction with various embodiments, those skilled in the art will understand and implement other variations of the disclosed embodiments by reviewing the accompanying drawings and the disclosure in carrying out the claimed invention. In the description of the invention, the word "comprising" does not exclude other components or steps, "a" or "an" does not exclude a plurality, and "a plurality" means two or more, unless otherwise explicitly specified. Furthermore, while different embodiments may describe certain measures, this does not mean that these measures cannot be combined to produce good results.

[0049] The above description, in conjunction with specific preferred embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such modifications and substitutions should be considered within the scope of protection of the present invention.

Claims

1. A fiber optic frequency transfer relay system based on spread spectrum technology, characterized in that, It includes a receiving module, a regenerating laser module, and a transmitting module; The receiving module is used to receive the spread spectrum optical signal and demodulate the spread spectrum optical signal according to the second pseudo-random sequence to obtain the demodulated optical signal; the spread spectrum optical signal is a signal that has been modulated by the first pseudo-random sequence in the previous stage and transmitted through the 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 the delay between the front-end and the receiving module; The regenerating laser module is used to lock the frequency of the demodulated optical signal and generate a regenerated optical signal; one beam of the regenerated optical signal is transmitted to the receiving module as a return optical signal, and the other beam is transmitted to the transmitting module as a transmission optical signal. The receiving module is further configured to modulate the returned optical signal according to the second pseudo-random sequence to obtain a modulated returned optical signal; the modulated returned optical signal is transmitted back to the front-end; The transmitting module is used to modulate the transmitted optical signal with a third pseudo-random sequence to obtain a spread spectrum optical signal, which is then transmitted to the next stage. Simultaneously, it uses a third pseudo-random sequence to demodulate the modulated return optical signal from the next stage to obtain a demodulated return optical signal, and uses this demodulated return optical signal to suppress noise in the transmitted optical signal.

2. The fiber optic frequency transfer relay system based on spread spectrum technology according to claim 1, characterized in that, The receiving module includes a first acousto-optic modulator, a first bandpass filter, a first 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 generates the first pseudo-random sequence. The first pseudo-random sequence is phase-adjusted by the first delay module to form the second pseudo-random sequence, which is then fed to the first voltage-controlled oscillator. The first digital frequency synthesizer generates the first local oscillator signal. The output signal of the first voltage-controlled oscillator and the first local oscillator signal are mixed by the first mixer. The mixed signal is filtered by the first bandpass filter to form the driving frequency of the first acousto-optic modulator. The first acousto-optic modulator is connected to the preamplifier and the regenerative laser module.

3. The fiber optic frequency transfer relay system based on spread spectrum technology according to claim 1, characterized in that, The regenerated laser module includes a first beam splitter, a first Faraday rotator, a first photodetector, a second bandpass filter, a first frequency division and phase comparison module, a first proportional-integral controller, a relay laser, and a second beam splitter. The relay laser is used to generate a regenerated optical signal, the frequency of which changes according to the control voltage; the regenerated optical signal is split 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 transmitting module as a transmission optical signal. The return optical signal entering the first beam splitter is split 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 demodulated optical signal output by the receiving module passes through the first beam splitter and enters the first photodetector. The first photodetector performs beat frequency analysis on the input demodulated optical signal and the returned optical signal to obtain a beat frequency signal. The beat frequency signal is filtered by the second bandpass filter and then enters the first frequency division and phase comparison module. The first frequency division and phase comparison module performs frequency division and phase detection on the filtered signal and the reference clock signal to obtain a first error voltage signal. The first proportional-integral controller generates the control voltage based on the first error voltage signal.

4. The fiber optic frequency transfer relay system based on spread spectrum technology according to claim 1, characterized in that, The transmitting module includes a third beam splitter, a second Faraday rotator, a second photodetector, a servo control unit, and a second acousto-optic modulator; The transmitted optical signal output by the regenerated laser module is split into two beams by the third beam splitter. One beam serves as a reference optical signal, which is reflected by the second Faraday rotating mirror and enters the second photodetector. The other beam enters the second acousto-optic modulator. The second acousto-optic modulator is connected to the subsequent receiving module via an optical fiber link. The second acousto-optic modulator demodulates the modulated return optical signal from the subsequent stage to obtain a demodulated return optical signal, which then enters the second photodetector via the third beam splitter. The second photodetector beats the input reference optical signal and the demodulated return optical signal to obtain an in-loop beat frequency signal. The servo control unit forms the driving frequency of the second acousto-optic modulator based on the in-loop beat frequency signal and the third pseudo-random sequence.

5. The fiber optic frequency transfer relay system based on spread spectrum technology according to claim 4, characterized in that, The servo control unit includes a third bandpass filter, a second frequency divider and phase ratio module, a second proportional-integral controller, a second digital frequency synthesizer, a second IQ modulation module, a second voltage-controlled oscillator, a second mixer, and a fourth bandpass filter; The third bandpass filter filters the intra-loop beat frequency signal. The second frequency division and phase comparison module divides and phase-detects the filtered intra-loop beat frequency signal and the reference clock signal to obtain a second error voltage signal. The second proportional-integral controller generates a control signal for the second digital frequency synthesizer based on the second error voltage signal, causing the second digital frequency synthesizer to generate a second local oscillator signal. The second IQ modulation module generates the third pseudo-random sequence and sends it to the second voltage-controlled oscillator. The signal output by the second voltage-controlled oscillator and the second local oscillator signal are mixed by the second mixer. The mixed signal is filtered by the fourth bandpass filter to form the driving frequency of the second acousto-optic modulator.

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

7. A fiber optic frequency transfer method based on spread spectrum technology, characterized in that, include: An optical signal is transmitted between a source transmitter and a destination receiver using a fiber optic frequency transmission relay system based on spread spectrum technology as described in any one of claims 1 to 6; the transmitting module of the source transmitter is the same as the transmitting module of the relay system; the receiving module and regenerated laser module of the destination receiver are the same as the receiving module and regenerated laser module of the relay system.

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