Time-frequency signal synchronization transmission method and system based on stable optical fiber link

Abstract

The application belongs to the technical field of microwave photonics, and relates to a time-frequency signal synchronous transmission method and system based on a stable optical fiber link. The synchronous transmission method comprises the following steps: obtaining a millimeter wave optical probe signal, perceiving existing optical fiber delay variation based on the millimeter wave optical probe signal; detecting phase variation of the millimeter wave optical probe signal to obtain an error signal of the optical fiber delay variation; compensating for optical fiber link delay jitter in real time based on the error signal to obtain a stable optical fiber link; modulating a time signal and a frequency signal onto optical carriers of different wavelengths respectively, coupling the time signal and the frequency signal with the millimeter wave optical probe signal, and transmitting the time signal and the frequency signal to a remote end through the stable optical fiber link; and demodulating the received signal at the remote end to realize synchronous transmission of the time signal and the frequency signal. The application can make the stability of the optical fiber link reach the order of tens of femtoseconds, and based on the stable link, the wavelength division multiplexing technology is used to simultaneously transmit the time signal and the frequency signal, so that the precise synchronization of the time signal and the frequency signal is realized.

Description

Technical Field

[0001] This invention relates to the field of microwave photonics technology, and in particular to a method and system for synchronous transmission of time and frequency signals based on a stable optical fiber link. Background Technology

[0002] High-precision time-frequency signal synchronization transmission systems have wide applications in distributed systems such as distributed antenna arrays, very long baseline interferometry, and X-ray free-electron lasers. The synchronization accuracy determines the system's performance, including signal-to-noise ratio and spatial resolution. To achieve coherent operation in these distributed systems, picosecond-level synchronization accuracy is required between the central node and all remote nodes over distances on the order of kilometers. The synchronization process requires the stable transmission of a second pulse time signal and a 10MHz frequency signal to provide a reference clock for the remote nodes. Optical fiber, with its advantages of low transmission loss, high bandwidth, and resistance to electromagnetic interference, has become an ideal medium for long-distance, high-precision signal transmission. However, the transmission delay in optical fiber varies with environmental factors such as temperature fluctuations and mechanical stress, worsening phase noise and leading to a decrease in the stability of the transmitted signal.

[0003] To address fiber optic delay jitter and achieve stable time and frequency transmission, three main transmission methods exist: First, assigning independent optical carriers to the time and frequency signals and locking them to their respective reference signals. This approach is relatively simple to implement and avoids mutual interference between the two signals, but independent locking can lead to misalignment between the time and frequency signals. Furthermore, when transmitting multiple signals, each signal requires an independent phase-locked loop system, significantly increasing system complexity. Second, focusing on stabilizing the fiber optic link itself: actively compensating for delay variations in the fiber optic link, the stabilized link can carry multiple time and frequency signals. Its core advantage is that once the link delay is stabilized, the transmission delay of the optical carrier within the fiber's transmission bandwidth also stabilizes, allowing the use of wavelength division multiplexing (WDM) technology to transmit multiple signals. However, the stability of the transmitted signals is still limited by the stability of the fiber optic link. The third method adds the time and frequency signals to the same optical carrier, ensuring they have completely identical transmission delays. Stabilizing the transmission delay of the optical carrier guarantees stable transmission of both time and frequency signals, effectively avoiding signal misalignment caused by fiber dispersion. However, this scheme has the problem of complex modulation and demodulation, and there is a risk of mutual interference between signals.

[0004] Therefore, there is an urgent need to propose a method for synchronous transmission of time and frequency signals that combines high efficiency and high stability. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies by proposing a method and system for time-frequency signal synchronization transmission based on a stable optical fiber link. This invention combines millimeter-wave optical probe signals with dual-optical mixing detection technology to achieve real-time compensation for optical fiber link delay jitter, resulting in link stability on the order of tens of femtoseconds. Based on this stable link, wavelength division multiplexing (WDM) technology is used to simultaneously transmit time and frequency signals, achieving precise synchronization of the time and frequency signals.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a method for time-frequency signal synchronization transmission based on a stable optical fiber link, comprising the following steps: S1. Acquire millimeter-wave optical probe signals and sense the delay changes of existing optical fibers based on the millimeter-wave optical probe signals; S2. Detect the phase change of the millimeter-wave optical probe signal to obtain the error signal of the fiber delay change; S3. Real-time compensation of fiber optic link delay jitter based on error signals to obtain a stable fiber optic link; S4. The time signal and frequency signal are modulated onto optical carriers of different wavelengths, coupled with the millimeter-wave optical probe signal, and then transmitted to the remote end via a stable optical fiber link; S5. Demodulate the received signal at the remote end to achieve synchronous transmission of time and frequency signals.

[0007] As one possible implementation, the millimeter-wave optical probe signal is obtained by modulating a radio frequency signal of a preset frequency onto an optical carrier and selecting the left and right first-order sidebands as the millimeter-wave optical probe signal.

[0008] As one possible implementation, the delay variation of existing optical fibers can be sensed based on millimeter-wave optical probe signals, specifically as follows: The millimeter-wave optical probe signal is divided into a reference signal and a probe signal according to a preset ratio. The reference signal remains locally, while the probe signal is transmitted to a remote location via an optical fiber link. After frequency shifting at the remote location, the probe signal is reflected back to the local location.

[0009] As one possible implementation, S2 includes the following sub-steps: S20. The reflected detection signal and the reference signal are interfered locally to filter out the left and right first-order sidebands respectively, and the beat frequency generates two intermediate frequency signals; S21. Zero-difference mixing of two intermediate frequency signals yields an error signal used to characterize the delay jitter of the fiber optic link, specifically: in, The angular frequency of the microwave signal. This indicates fiber optic link delay jitter.

[0010] As one possible implementation, the spacing between the left and right first-order sidebands is 70 GHz to 90 GHz.

[0011] As one possible implementation method, the preset ratio is 1:9 to 3:7.

[0012] In a second aspect, the present invention provides a time-frequency signal synchronization transmission system based on a stable optical fiber link, used to implement the time-frequency signal synchronization transmission method based on a stable optical fiber link provided in the first aspect, comprising: a millimeter-wave optical probe signal acquisition device, a time-frequency signal modulation device, an optical fiber link compensation device, a first dense wavelength division multiplexer, and a time-frequency signal demodulation device. The millimeter-wave optical probe signal acquisition device is used to acquire millimeter-wave optical probe signals and divides the millimeter-wave optical probe signals into reference signals and detection signals according to a preset ratio.

[0013] The time-frequency signal modulation device is used to generate optical time signals and frequency signals, and modulates the time signals and frequency signals onto optical carriers of different wavelengths to generate optical time signals and optical frequency signals. The fiber optic link compensation device includes a polarization beamsplitter, a mixing and phase-detection unit, a first phase-locked loop, an optical delay line, an optical fiber, an acousto-optic frequency shifter, and a Faraday rotator. The polarization beamsplitter distinguishes between forward-transmitting and reverse-transmitting probe signals. The forward-transmitting probe signal is transmitted to the far end through the optical fiber. After being frequency-shifted by the acousto-optic frequency shifter at the far end, it is reflected back to the local end by the Faraday rotator as a reverse-transmitting probe signal. The mixing and phase-detection unit interferes with the reflected probe signal and the reference signal locally, filtering out the left and right first-order sidebands respectively. After generating two intermediate frequency signals by beating, the two intermediate frequency signals are mixed with zero difference to obtain an error signal used to characterize the delay jitter of the fiber optic link. The first phase-locked loop generates a control signal based on the error signal to drive the optical delay line to compensate for the delay jitter of the optical fiber, thereby obtaining a stable fiber optic link. The first dense wavelength division multiplexer is used to locally couple the optical time signal, the optical frequency signal, and the millimeter-wave optical probe signal into a stable optical fiber link. The time-frequency signal demodulation device is used to demodulate the received signal, recover the time signal and frequency signal, and realize the synchronous transmission of the time signal and frequency signal.

[0014] As one possible implementation, the millimeter-wave optical probe signal acquisition device includes a low phase noise radio frequency source, an intensity modulator, and a narrow linewidth single-frequency laser. The low phase noise radio frequency source generates a radio frequency signal, and the intensity modulator modulates the radio frequency signal onto the optical carrier output by the narrow linewidth single-frequency laser to generate left and right first-order sidebands, which are the millimeter-wave optical probe signals. The time-frequency signal modulation device includes a cesium atomic clock, a time signal direct-modulation laser, and a frequency signal direct-modulation laser. The cesium atomic clock is used to generate time signals and frequency signals. The time signals and frequency signals drive the time signal direct-modulation laser and the frequency signal direct-modulation laser, respectively, to modulate the time signals and frequency signals onto optical carriers of different wavelengths to generate optical time signals and optical frequency signals. The time-frequency signal demodulation device includes a second dense wavelength division multiplexer, a time signal detector, and a frequency signal demodulation unit. The second dense wavelength division multiplexer is used to demodulate the received signal according to wavelength. The frequency signal demodulation unit includes a radio frequency signal detector, a second phase-locked loop, and a voltage-controlled oscillator. The second phase-locked loop is used to lock the phase of the radio frequency signal output by the voltage-controlled oscillator to the phase of the frequency signal received by the radio frequency signal detector in order to suppress high-frequency noise.

[0015] As one possible implementation, the mixing phase detector unit includes a 2×2 polarization-maintaining coupler, two filter detection branches, and a mixer. Each filter detection branch includes an optical filter and a photodetector connected in sequence. A 2×2 polarization-maintaining coupler is used to locally interfere the reflected probe signal and the reference signal; The two filtering detection branches filter out the left and right first-order sidebands respectively, and the beat frequency generates two intermediate frequency signals; A mixer is used to mix two intermediate frequency signals at zero difference to obtain an error signal that characterizes the delay jitter of an optical fiber link.

[0016] As one possible implementation, the optical filters in the two filtering detection branches have different wavelengths, with a wavelength difference of 0.8 nm.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The present invention proposes a time-frequency signal synchronization transmission method and system based on a stable optical fiber link. In view of the fact that the stability of existing signal transmission is limited by the instability of the optical fiber link, the present invention combines millimeter-wave optical probe signal with dual-optical mixing detection technology to achieve real-time compensation for optical fiber link delay jitter, so that the stability of the optical fiber link reaches the tens of femtosecond level.

[0018] 2. The time-frequency signal synchronization transmission method and system based on a stable optical fiber link proposed in this invention utilizes wavelength division multiplexing technology to transmit time and frequency signals simultaneously on the basis of a stable optical fiber link. This effectively avoids interference between signals, ensures transmission stability, and achieves precise synchronization of time and frequency signals.

[0019] 3. The time-frequency signal synchronization transmission method and system based on a stable optical fiber link proposed in this invention achieves a stable optical fiber link with a standard deviation of 22 fs for delay jitter. Experiments show that both stations achieve 10 [unclear - possibly a specific value or parameter] over an average time of 1000 seconds.-15 Allen bias of magnitude and 10 -12 The time deviation of orders of magnitude provides a precise synchronization solution for distributed antenna arrays. Attached Figure Description

[0020] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this invention, illustrate exemplary embodiments of the invention and are used to explain the invention, but do not constitute an undue limitation of the invention. In the drawings: Figure 1 , Figure 2 , Figure 3 A schematic diagram of a time-frequency signal synchronization transmission system based on a stable optical fiber link, provided for an embodiment of the present invention; Figure 4 A flowchart of a time-frequency signal synchronization transmission method based on a stable optical fiber link provided in an embodiment of the present invention; Figure 5 The figure shows the experimental results of evaluating fiber optic link delay jitter using a 27.4 GHz microwave signal in an embodiment of the present invention. Figure 6 This is a diagram showing the experimental results of evaluating the stability of a transmitted 100 MHz signal using a phase noise analyzer at the remote end in an embodiment of the present invention. Figure 7 This is a diagram showing the experimental results of measuring the time interval between the remote and local 1PPS time signals using a time interval analyzer in an embodiment of the present invention.

[0021] Figure Labels 1-Millimeter-wave optical probe signal acquisition device, 10-Low phase noise radio frequency source, 11-Intensity modulator, 12-Narrow linewidth single-frequency laser; 2-Time and frequency signal modulation device; 20-Cesium atomic clock; 21-Time signal direct-modulation laser; 22-Frequency signal direct-modulation laser; 3-Fiber optic link compensation device, 30-Polarization beam splitter, 31-Mixer phase detector unit, 310-2×2 polarization maintaining coupler, 311-Mixer, 312-First optical filter, 313-First photodetector, 314-Second optical filter, 315-Second photodetector, 32-First phase-locked loop, 33-Optical delay line, 34-Fiber optic cable, 35-Acousto-optic frequency shifter, 36-Faraday rotator; 4-First Dense Wavelength Division Multiplexer; 5-Time and frequency signal demodulation device, 50-Second dense wavelength division multiplexer, 51-Frequency signal demodulation unit, 510-RF signal detector, 511-Second phase-locked loop, 512-Voltage controlled oscillator, 52-Time signal detector. Detailed Implementation

[0022] To facilitate a clear description of the technical solutions in the embodiments of the present invention, the terms "first" and "second" are used to distinguish identical or similar items with essentially the same function and effect. For example, the first threshold and the second threshold are merely used to distinguish different thresholds and do not limit their order. Those skilled in the art will understand that the terms "first" and "second" do not limit the quantity or execution order, and that the terms "first" and "second" are not necessarily different.

[0023] It should be noted that in this invention, the terms "exemplary" or "for example" are used to indicate examples, illustrations, or descriptions. Any embodiment or design described as "exemplary" or "for example" in this invention should not be construed as being more preferred or advantageous than other embodiments or designs. Specifically, the use of terms such as "exemplary" or "for example" is intended to present the relevant concepts in a concrete manner.

[0024] In this invention, "at least one" refers to one or more, and "more than one" refers to two or more. "And / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A alone, A and B simultaneously, or B alone, where A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects are in an "or" relationship. "At least one" or similar expressions refer to any combination of these items, including any combination of singular or plural items. For example, "at least one of a, b, or c" can represent: a, b, c, a combination of a and b, a combination of a and c, a combination of b and c, or a, b, and c, where a, b, and c can be single or multiple.

[0025] This invention aims to provide a method and system for time-frequency signal synchronization transmission based on a stable optical fiber link, addressing issues such as high system complexity, time-frequency signal misalignment, insufficient optical fiber link stability, and complex modulation / demodulation in existing technologies. This invention combines millimeter-wave optical probe signals with dual-optical mixing detection technology to achieve real-time compensation for optical fiber link delay jitter, resulting in optical fiber link stability on the order of tens of femtoseconds. Based on this stable link, wavelength division multiplexing (WDM) technology is used to simultaneously transmit time and frequency signals, achieving precise synchronization of the time and frequency signals.

[0026] In a first aspect, embodiments of the present invention provide a time-frequency signal synchronization transmission system based on a stable optical fiber link, see [link to previous document]. Figure 1 It includes: a millimeter-wave optical probe signal acquisition device 1, a time-frequency signal modulation device 2, an optical fiber link compensation device 3, a first dense wavelength division multiplexer 4, and a time-frequency signal demodulation device 5. Among them, the millimeter-wave optical probe signal acquisition device 1 is used to acquire the millimeter-wave optical probe signal; and divides the millimeter-wave optical probe signal into a reference signal and a detection signal according to a preset ratio; The time-frequency signal modulation device 2 is used to generate optical time signals and frequency signals, and modulates the time signals and frequency signals onto optical carriers of different wavelengths to generate optical time signals and optical frequency signals. The fiber optic link compensation device 3 includes a polarization beamsplitter 30, a mixing and phase-detection unit 31, a first phase-locked loop 32, an optical delay line 33, an optical fiber 34, an acousto-optic frequency shifter 35, and a Faraday rotator 36. The polarization beamsplitter 30 is used to distinguish between the forward-transmitting probe signal and the reverse-transmitting probe signal. The forward-transmitting probe signal is transmitted to the far end through the optical fiber 34. After being frequency-shifted by the acousto-optic frequency shifter 35 at the far end, it is reflected by the Faraday rotator 36 as the reverse-transmitting probe signal and returns to the local end. The mixing and phase-detection unit 31 interferes with the reflected probe signal and the reference signal locally, filters out the left and right first-order sidebands respectively, generates two intermediate frequency signals by beat frequency, and then mixes the two intermediate frequency signals with zero difference to obtain an error signal used to characterize the delay jitter of the fiber optic link. The first phase-locked loop 32 generates a control signal based on the error signal to drive the optical delay line 33 to compensate for the delay jitter of the optical fiber 34, thereby obtaining a stable fiber optic link. The first dense wavelength division multiplexer 4 is used to locally couple the optical time signal, the optical frequency signal and the millimeter-wave optical probe signal into a stable optical fiber link; The time and frequency signal demodulation device 5 is used to demodulate the received signal, recover the time signal and frequency signal, and realize the synchronous transmission of the time signal and frequency signal.

[0027] See Figure 1 and 2 As one possible implementation, the millimeter-wave optical probe signal acquisition device 1 includes a low phase noise radio frequency source 10, an intensity modulator 11, and a narrow linewidth single-frequency laser 12; the low phase noise radio frequency source 10 generates a radio frequency signal, and the intensity modulator 11 modulates the radio frequency signal onto the optical carrier output by the narrow linewidth single-frequency laser 12 to generate left and right first-order sidebands, which are the millimeter-wave optical probe signals. See Figure 2 The time and frequency signal modulation device 2 includes a cesium atomic clock 20, a time signal direct-modulation laser 21, and a frequency signal direct-modulation laser 22. The cesium atomic clock 20 is used to generate time signals and frequency signals. The time signals and frequency signals drive the time signal direct-modulation laser 21 and the frequency signal direct-modulation laser 22 to modulate the time signals and frequency signals onto optical carriers of different wavelengths to generate optical time signals and optical frequency signals. See Figure 1 and 2The time-frequency signal demodulation device 5 includes a second dense wavelength division multiplexer 50, a frequency signal demodulation unit 51, and a time signal detector 52. The second dense wavelength division multiplexer 50 is used to demodulate the received signal. The frequency signal demodulation unit 51 includes a radio frequency signal detector 510, a second phase-locked loop 511, and a voltage-controlled oscillator 512. The second phase-locked loop 511 is used to lock the phase of the radio frequency signal output by the voltage-controlled oscillator 512 to the phase of the frequency signal received by the radio frequency signal detector 510 in order to suppress high-frequency noise.

[0028] See Figure 3 As one possible implementation, the mixing phase detector unit 31 includes a 2×2 polarization-maintaining coupler 310, two filtering and detection branches, and a mixer 311. The first filtering and detection branches each include a first optical filter 312 and a first photodetector 313 connected in sequence, and the second filtering and detection branches each include a second optical filter 314 and a second photodetector 315 connected in sequence. Among them, the 2×2 polarization-maintaining coupler 310 is used to locally interfere the reflected probe signal and the reference signal; The two filtering detection branches filter out the left and right first-order sidebands respectively, and the beat frequency generates two intermediate frequency signals; Mixer 311 is used to mix two intermediate frequency signals at zero difference to obtain an error signal that characterizes the delay jitter of the optical fiber link.

[0029] See Figure 3 As one possible implementation, the first optical filter 312 and the second optical filter 314 have different wavelengths, with a wavelength difference of 0.8 nm.

[0030] Secondly, embodiments of the present invention provide a method for time-frequency signal synchronization transmission based on a stable optical fiber link, see [link to relevant documentation]. Figure 4 It includes the following steps: S1. Acquire millimeter-wave optical probe signals and sense the delay changes of existing optical fibers based on the millimeter-wave optical probe signals; As one possible implementation, the millimeter-wave optical probe signal is obtained by modulating a radio frequency signal of a preset frequency onto an optical carrier and selecting the left and right first-order sidebands as the millimeter-wave optical probe signal.

[0031] As one possible implementation, the spacing between the left and right first-order sidebands is 70 GHz to 90 GHz, for example, 70 GHz, 75 GHz, 80 GHz, 85 GHz, and 90 GHz.

[0032] As one possible implementation, the delay variation of existing optical fibers is sensed based on millimeter-wave optical probe signals, specifically as follows: The millimeter-wave optical probe signal is divided into a reference signal and a probe signal according to a preset ratio. The reference signal remains locally, while the probe signal is transmitted to a remote location via an optical fiber link. After frequency shifting at the remote location, the probe signal is reflected back to the local location.

[0033] As one possible implementation, the preset ratio is 1:9 to 3:7, for example, 1:9, 2:8, 3:7.

[0034] In practical implementation, to obtain the maximum signal-to-noise ratio, the preset ratio is 3:7. This ratio has been calculated and verified by simulation, and is the optimal preset ratio for this scheme.

[0035] As an example, using the time-frequency signal synchronization transmission system based on a stable fiber optic link provided in the first aspect, a low-phase-noise RF source generates a 40 GHz RF signal. An intensity modulator modulates the 40 GHz RF signal onto an optical carrier with a wavelength of 1550.12 nm, generating left and right first-order sidebands spaced 80 GHz apart, which serve as the millimeter-wave optical probe signal. A polarization beamsplitter splits the millimeter-wave optical probe signal at a 2:8 ratio, with 20% remaining locally as a reference signal. The reference signal is represented as follows: in, Indicates the optical carrier angular frequency. Indicates the angular frequency of the microwave signal. Indicates time.

[0036] The remaining 80% is the probe signal, which is transmitted to the remote end via a dense wavelength division multiplexer, an optical delay line, and 10km of optical fiber. At the remote end, it undergoes a 40MHz frequency shift via an acousto-optic frequency shifter and is then reflected back to the local end. The reflected probe signal is represented as follows: in, Indicates the frequency shift. This indicates the round-trip delay.

[0037] S2. Detect the phase change of the millimeter-wave optical probe signal to obtain the error signal of the fiber delay change; As one possible implementation, S2 includes the following sub-steps: S20. The reflected detection signal and the reference signal are interfered locally to filter out the left and right first-order sidebands respectively, and the beat frequency generates two intermediate frequency signals; The reference signal in the above example and the reflected detection signal This step generates two 80MHz signals.

[0038] S21. Two intermediate frequency signals are mixed at zero difference to obtain an error signal used to characterize the delay jitter of the fiber optic link, specifically: in, Right now , The angular frequency of the round-trip microwave signal. This refers to the jitter in the fiber optic link during signal round trip.

[0039] S3. Real-time compensation for fiber optic link jitter based on error signals to obtain a stable fiber optic link; S4. The time signal and frequency signal are modulated onto optical carriers of different wavelengths, coupled with the millimeter-wave optical probe signal, and then transmitted to the remote end via a stable optical fiber link; S5. Demodulate the received signal at the remote end to achieve synchronous transmission of time and frequency signals.

[0040] As an example, after the fiber optic link stabilizes, a cesium atomic clock generates a 1PPS time signal and a 100 MHz frequency signal. A direct-modulation laser for the time signal modulates the 1PPS time signal onto a 1551.72 nm optical carrier, and a direct-modulation laser for the frequency signal modulates the 100 MHz frequency signal onto a 1548.51 nm optical carrier. A dense wavelength division multiplexer with a bandwidth of 200 GHz is used to couple the optical time signal, optical frequency signal, and millimeter-wave optical probe signal locally within the stable fiber optic link. At the remote end, they are demultiplexed using a dense wavelength division multiplexer with the same bandwidth. The 1PPS time signal is recovered by a low-speed time signal detector, and the 100 MHz frequency signal is used as a phase-locked reference to lock a voltage-controlled oscillator to suppress high-frequency phase noise.

[0041] Next, specific experiments will be conducted to evaluate the compensation effect of fiber optic link delay jitter.

[0042] A 27.4 GHz microwave signal generated by a Vector Network Analyzer (VNA) was modulated onto a local 1553.51 nm laser using an intensity modulator. The remote site recovered the 27.4 GHz signal and sent it back to the VNA to evaluate fiber optic link delay jitter. Experimental results are as follows: Figure 5 As shown in the figure, the standard deviation of delay jitter within one hour is 22 fs, and the delay jitter is within 100 fs, indicating that the fiber optic link has excellent long-term stability and ensures the transmission accuracy of time and frequency signals.

[0043] The time-frequency signal transmitted via this stable fiber optic link also exhibits high stability: the stability of the transmitted 100 MHz frequency signal was evaluated at the remote end using a phase noise analyzer, and its Allen deviation was as follows: Figure 6 As shown, the Allen deviations of Link 1 and Link 2 over a 1000-second average time reach 8.6 × 10⁻⁶. -15 and 6.1×10-15 The difference in short-term stability between a stable fiber link and a free-running state due to Allen deviation is negligible because the recovered 100 MHz frequency signal originates from the voltage-controlled oscillator (VCO). This structure suppresses short-term noise caused by detector noise and rapid fiber oscillations, while long-term stability follows the stability of the fiber link. Therefore, short-term performance is primarily limited by the VCO phase noise, while long-term stability is determined by the stability of the stable fiber link.

[0044] For a 1PPS time signal, a time interval analyzer was used to measure the time interval between the remote and local ends. The results are as follows: Figure 7 As shown, the time deviation between Link 1 and Link 2 reaches 1.5 × 10⁻⁶ over an average time of 1000 seconds. -12 and 0.72×10 -12 In the short term, the performance during free operation is similar to that after stabilization, with the main limitation stemming from detector noise rather than fiber-induced jitter. Long-term phase drift is effectively compensated for by the stable fiber link.

[0045] Time-frequency signals are transmitted through an optical fiber link using two carrier wavelengths. Due to fiber dispersion, a difference in transmission delay occurs. The centers of the two carrier wavelengths are located at 1551.72 nm and 1548.51 nm, respectively, with a wavelength difference of approximately 3.2 nm. For a 10 km optical fiber link, the fixed delay difference of the time-frequency signal is approximately 550 ps. Assuming a dispersion coefficient temperature drift of 1.5 fs / km / nm / K, the relative transmission delay of the time-frequency signal is approximately 48 fs / K.

[0046] The experimental results above demonstrate that this invention achieves a stable fiber optic link with a delay jitter standard deviation of 22 fs. Both sites achieved 10 [unclear - possibly a specific value or parameter] over an average time of 1000 seconds. -15 Allen bias of magnitude and 10 -12 The time deviation of orders of magnitude provides a precise synchronization solution for distributed antenna arrays.

[0047] 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, the disclosure, and the description of the drawings, in carrying out the claimed invention. In this specification, the word "comprising" does not exclude other components or steps, and "a" or "an" does not exclude multiple components. A single processor or other unit can implement several of the functions listed in the specification. While certain measures are described in different embodiments, this does not mean that these measures cannot be combined to produce good results.

[0048] Although the invention has been described in conjunction with specific features and embodiments, it is obvious that various modifications and combinations can be made therein without departing from the spirit and scope of the invention. Accordingly, this specification and drawings are merely illustrative of the invention and are considered to cover any and all modifications, variations, combinations, or equivalents within the scope of the invention. Clearly, those skilled in the art can make various alterations and modifications to the invention without departing from its spirit and scope. Thus, if such modifications and modifications fall within the scope of the invention and its equivalents, the invention is also intended to include such modifications and modifications.

Claims

1. A method for time-frequency signal synchronization transmission based on a stable optical fiber link, characterized in that, Includes the following steps: S1. Acquire millimeter-wave optical probe signal and sense the existing optical fiber delay change based on the millimeter-wave optical probe signal; specifically: divide the millimeter-wave optical probe signal into a reference signal and a probe signal according to a preset ratio, the reference signal is kept locally, and the probe signal is transmitted to the remote end through the optical fiber link, and after frequency shift is generated at the remote end, it is reflected back to the local end; S2. Detect the phase change of the millimeter-wave optical probe signal to obtain the error signal of the fiber delay change; specifically: S20. The reflected detection signal and the reference signal are interfered locally to filter out the left and right first-order sidebands respectively, and the beat frequency generates two intermediate frequency signals; S21. Zero-difference mixing of two intermediate frequency signals yields an error signal used to characterize the delay jitter of the fiber optic link, specifically: in, The angular frequency of the microwave signal. Indicates fiber optic link delay jitter; S3. Real-time compensation of fiber optic link delay jitter based on error signals to obtain a stable fiber optic link; S4. The time signal and frequency signal are modulated onto optical carriers of different wavelengths, coupled with the millimeter-wave optical probe signal, and then transmitted to the remote end via a stable optical fiber link; S5. Demodulate the received signal at the remote end to recover the time and frequency signals, and realize the synchronous transmission of the time and frequency signals.

2. The time-frequency signal synchronization transmission method based on a stable optical fiber link according to claim 1, characterized in that, The millimeter-wave optical probe signal is obtained by modulating a radio frequency signal of a preset frequency onto an optical carrier and selecting the left and right first-order sidebands as the millimeter-wave optical probe signal.

3. The time-frequency signal synchronization transmission method based on a stable optical fiber link according to claim 1, characterized in that, The spacing between the left and right first-order sidebands is 70 GHz to 90 GHz.

4. The time-frequency signal synchronization transmission method based on a stable optical fiber link according to claim 1, characterized in that, The preset ratio is 1:9 to 3:

7.

5. A time-frequency signal synchronization transmission system based on a stable optical fiber link, used to implement the time-frequency signal synchronization transmission method based on a stable optical fiber link as described in any one of claims 1 to 4, characterized in that, include: Millimeter-wave optical probe signal acquisition device, time-frequency signal modulation device, optical fiber link compensation device, first dense wavelength division multiplexer, and time-frequency signal demodulation device; Among them, the millimeter-wave optical probe signal acquisition device is used to acquire millimeter-wave optical probe signals; and divides the millimeter-wave optical probe signals into reference signals and detection signals according to a preset ratio; The time-frequency signal modulation device is used to generate optical time signals and frequency signals, and modulates the time signals and frequency signals onto optical carriers of different wavelengths to generate optical time signals and optical frequency signals. The fiber optic link compensation device includes a polarization beamsplitter, a mixing and phase-detection unit, a first phase-locked loop, an optical delay line, an optical fiber, an acousto-optic frequency shifter, and a Faraday rotator. The polarization beamsplitter distinguishes between forward-transmitting and reverse-transmitting probe signals. The forward-transmitting probe signal is transmitted to the far end through the optical fiber. After being frequency-shifted by the acousto-optic frequency shifter at the far end, it is reflected back to the local end by the Faraday rotator as a reverse-transmitting probe signal. The mixing and phase-detection unit interferes with the reflected probe signal and the reference signal locally, filtering out the left and right first-order sidebands respectively. After generating two intermediate frequency signals by beating, the two intermediate frequency signals are mixed with zero difference to obtain an error signal used to characterize the delay jitter of the fiber optic link. The first phase-locked loop generates a control signal based on the error signal to drive the optical delay line to compensate for the delay jitter of the optical fiber, thereby obtaining a stable fiber optic link. The first dense wavelength division multiplexer is used to locally couple the optical time signal, the optical frequency signal, and the millimeter-wave optical probe signal into a stable optical fiber link. The time and frequency signal demodulation device is used to demodulate the received signal, recover the time signal and frequency signal, and realize the synchronous transmission of the time signal and frequency signal.

6. The time-frequency signal synchronization transmission system based on a stable optical fiber link according to claim 5, characterized in that, The millimeter-wave optical probe signal acquisition device includes a low phase noise radio frequency source, an intensity modulator, and a narrow linewidth single-frequency laser. The low phase noise radio frequency source generates a radio frequency signal, and the intensity modulator modulates the radio frequency signal onto the optical carrier output by the narrow linewidth single-frequency laser to generate left and right first-order sidebands, which are the millimeter-wave optical probe signals. The time-frequency signal modulation device includes a cesium atomic clock, a time signal direct-modulation laser, and a frequency signal direct-modulation laser. The cesium atomic clock is used to generate time signals and frequency signals. The time signals and frequency signals drive the time signal direct-modulation laser and the frequency signal direct-modulation laser, respectively, to modulate the time signals and frequency signals onto optical carriers of different wavelengths to generate optical time signals and optical frequency signals. The time-frequency signal demodulation device includes a second dense wavelength division multiplexer, a time signal detector, and a frequency signal demodulation unit. The second dense wavelength division multiplexer is used to demodulate the received signal according to wavelength. The frequency signal demodulation unit includes a radio frequency signal detector, a second phase-locked loop, and a voltage-controlled oscillator. The second phase-locked loop is used to lock the phase of the radio frequency signal output by the voltage-controlled oscillator to the phase of the frequency signal received by the radio frequency signal detector in order to suppress high-frequency noise.

7. The time-frequency signal synchronization transmission system based on a stable optical fiber link according to claim 5, characterized in that, The mixing phase detection unit includes a 2×2 polarization-maintaining coupler, two filter detection branches, and a mixer. Each filter detection branch includes an optical filter and a photodetector connected in sequence. A 2×2 polarization-maintaining coupler is used to locally interfere the reflected probe signal and the reference signal; The two filtering detection branches filter out the left and right first-order sidebands respectively, and the beat frequency generates two intermediate frequency signals; A mixer is used to mix two intermediate frequency signals at zero difference to obtain an error signal that characterizes the delay jitter of an optical fiber link.

8. The time-frequency signal synchronization transmission system based on a stable optical fiber link according to claim 7, characterized in that, The optical filters in the two filtering detection branches have different wavelengths, with a wavelength difference of 0.8 nm.

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