Delay measurement and stabilization system and method for optical microwave transmission link

Abstract

The invention discloses a delay measurement and stabilization system and method for a microwave over fiber transmission link, and belongs to the technical field of microwave photonics, and the system comprises a delay measurement unit which is used for generating and processing a probe signal to obtain link delay information, and the delay measurement unit comprises a first laser, a first modulator, a frequency shift unit and a dual-light frequency mixing unit; the signal transmission unit is used for generating and transmitting to-be-stabilized microwave signals to be transmitted; the wave combining and splitting unit is used for multiplexing the probe signal and the microwave signal to be transmitted to the same optical fiber link for transmission and far-end separation; the signal processing unit is used for calculating link delay according to the intermediate frequency signal output by the delay measurement unit and generating a delay control signal; and the delay execution unit is connected to the optical fiber link and is used for adjusting the optical path of the link according to the delay control signal so as to stabilize the transmission delay of the microwave signal to be transmitted. According to the invention, the high-precision measurement capability of wide-range delay can be considered while the high-stable transmission of the optical microwave is realized.

Description

Technical Field

[0001] This invention belongs to the field of microwave photonics technology, and particularly relates to a delay measurement and stabilization system and method for optical microwave transmission links. Background Technology

[0002] The core functions of distributed interferometric antenna arrays include coherent synthesis of multi-node signals and target localization based on time difference of arrival (TDOA). However, optical microwave links are affected by factors such as ambient temperature fluctuations and mechanical disturbances during actual transmission, resulting in unstable delays [1], which reduces the signal-to-noise ratio of coherent synthesis and reduces the accuracy of target localization. Therefore, achieving highly stable optical microwave transmission with known absolute delay is crucial for ensuring system performance.

[0003] Currently, widely studied methods for measuring fiber optic link delay can be categorized into three types: time-domain measurement, frequency-domain measurement, and radio frequency (RF) phase-shift measurement. The basic principle of time-domain measurement is to directly detect the flight time of the probe signal, primarily using time interval counters, optical time-domain reflectometers, and chaotic light-based measurement methods. Time-domain delay measurement methods typically have a large measurement range, but their accuracy is usually only on the order of nanoseconds. Frequency-domain measurement converts delay measurement into frequency measurement, mainly including optical frequency-domain reflectometers, mode-locked laser repetition frequency measurement, and free-running laser mode interval measurement; its measurement accuracy is typically on the order of picoseconds. Due to limitations in measurement accuracy, the aforementioned time-domain and frequency-domain delay measurement methods cannot meet the high-precision delay compensation requirements of distributed systems. RF phase-shift measurement methods obtain the delay by measuring the phase shift after probe signal transmission. Using high-frequency probe signals can achieve higher delay detection accuracy, but due to limitations in sampling rate and receiving bandwidth, the phase of high-frequency probe signals is difficult to detect directly. In existing research, probe signal frequencies are still mainly concentrated in the range of a few GHz, failing to fully utilize the delay measurement accuracy advantages of high-frequency probes. Furthermore, methods for measuring delay using single-frequency signal phase shifts suffer from phase ambiguity, leading to a trade-off between measurement accuracy and range. Existing research primarily addresses phase integer ambiguity using swept-frequency signals or multi-frequency probe signals, thereby broadening the measurement range. However, these methods involve complex probe signal generation, computationally expensive phase ambiguity resolution algorithms, and direct probe phase detection limits the probe frequency. Moreover, current research methods largely focus on static link delay measurements, lacking active control and stabilization capabilities for transmission delays, making it difficult to meet the combined demands of distributed systems for high coherence and dynamic compensation. Summary of the Invention

[0004] In view of this, the main objective of the present invention is to provide a delay measurement and stabilization system and method for optical microwave transmission links, in order to at least partially solve the above-mentioned technical problems.

[0005] To achieve the above objectives, as a first aspect of the present invention, a delay measurement and stabilization system for an optical microwave transmission link is proposed, comprising: A delay measurement unit is used to generate and process probe signals to obtain link delay information. The delay measurement unit includes a first laser, a first modulator, a frequency shifting unit, and a dual-optical mixing unit. The signal transmission unit is used to generate and transmit the microwave signal to be stabilized. The multiplexing / demultiplexing unit is used to multiplex the probe signal and the microwave signal to be transmitted to the same optical fiber link for transmission and to separate them at a remote end. The signal processing unit is used to calculate the link delay based on the intermediate frequency signal output by the delay measurement unit and generate a delay control signal. A delay execution unit, connected to the optical fiber link, is used to adjust the optical path of the link according to the delay control signal in order to stabilize the transmission delay of the microwave signal to be transmitted.

[0006] In one possible implementation, the frequency shifting unit includes: An acousto-optic frequency shifter is used to apply a fixed frequency shift to probe signals transmitted to a remote location. A Faraday rotator is used to reflect optical signals and change their polarization state, so that the returned signal is shifted again by the acousto-optic frequency shifter.

[0007] In one possible implementation, the dual-optical mixing unit includes: An optical filter is used to separate the positive first-order sideband combination and the negative first-order sideband combination after mixing the returned probe signal with the local reference probe signal; and A balanced photodetector is used to convert the two separated sideband combinations into a first intermediate frequency signal and a second intermediate frequency signal, respectively.

[0008] In one possible implementation, the signal processing unit includes: The signal processing unit is used to collect the first intermediate frequency signal and the second intermediate frequency signal, and obtain the link delay value through phase calculation; A phase-locked loop control unit is used to generate the delay control signal based on the third intermediate frequency signal generated by the dual-optical mixer unit.

[0009] In one possible implementation, the first modulator is used to modulate a two-tone probe signal generated by a radio frequency signal generator onto an optical carrier emitted by the first laser, generating modulated light containing ±1st-order sidebands.

[0010] In one possible implementation, the delay execution unit includes at least one of a piezoelectric ceramic fiber stretcher and a motor optical delay line.

[0011] In one possible implementation, the first modulator and / or the second modulator in the signal transmission unit are connected to a bias point controller for stabilizing the bias point of the modulator at the carrier suppression point or the quadrature point.

[0012] In one possible implementation, the radio frequency signal generator is configured to generate multiple sets of dual-tone probe signals with different frequency intervals to eliminate phase ambiguity through hierarchical resolution, wherein the minimum frequency interval is used to determine the ambiguity-free range of the delay measurement, and the maximum frequency interval is used to determine the delay measurement accuracy.

[0013] As a second aspect of the present invention, a delay measurement method for an optical microwave transmission link is also proposed, comprising the following steps: S1: Generate a dual-tone probe signal and a microwave signal to be transmitted, and modulate them onto optical carriers of different wavelengths respectively; S2: The probe optical signal and the service optical signal are multiplexed into the same optical fiber link and transmitted to the remote end. After separation, the probe optical signal is frequency-shifted twice and reflected back. S3: The returned probe optical signal is mixed with the local reference probe optical signal, and the sidebands are separated and detected by a balanced photodetector to obtain a beat frequency signal that contains at least the first intermediate frequency signal, the second intermediate frequency signal and the third intermediate frequency signal. S4: Calculate the absolute transmission delay of the optical fiber link based on the phase information of the first intermediate frequency signal and the second intermediate frequency signal; S5: Generate an error signal based on the third intermediate frequency signal, and dynamically compensate for the delay jitter of the link by controlling the delay execution unit through feedback, so as to stabilize the transmission delay of the microwave signal to be transmitted.

[0014] In one possible implementation, the phase of the intermediate frequency signal output by the balanced photodetector is measured via a data acquisition card, and the link delay is determined.

[0015] In one possible implementation, the intermediate frequency signal is phase-determined by Fourier transform, and phase ambiguity is eliminated by adjusting the frequency interval of the two-tone signal.

[0016] Based on the above technical solution, it can be seen that the delay measurement and stabilization system and method for optical microwave transmission links of the present invention has at least one of the following beneficial effects compared with the prior art: 1. By designing a high-frequency dual-tone probe signal to accurately detect link delay, the measurement and stabilization of link delay are integrated; 2. By employing a high-frequency dual-tone probe signal combined with a dual-optical mixing structure, the high-frequency phase change is transferred to the intermediate frequency for detection, avoiding the limitations of direct high-frequency sampling; by setting multiple sets of signals with different frequency intervals, the phase ambiguity problem is effectively solved, achieving femtosecond-level measurement accuracy while ensuring a large absolute delay measurement range; 3. High-precision delay measurement is achieved by utilizing the high-frequency difference between ±1st order sideband dual-tone signals. The system integrates high-precision delay measurement and active stabilization control, and significantly improves the long-term stability of microwave signal transmission in fiber optic links through real-time feedback compensation via phase-locked loop. Attached Figure Description

[0017] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a schematic diagram of the delay measurement and stabilization system for the optical microwave transmission link of the present invention; Figure 2 This is a schematic diagram of the motor delay line measurement results of the delay measurement and stabilization system of the optical microwave transmission link of the present invention; Figure 3 This is a schematic diagram of the delay measurement results of the back-to-back delay stabilization link without fiber optic connection in the delay measurement and stabilization system of the optical microwave transmission link of the present invention. Figure 4 This is a schematic diagram of the delay measurement and stabilization system for optical microwave transmission links of the present invention, showing the transmission delay stability measurement of a 45-kilometer link. Figure 5 This is a schematic diagram of the overlap Allen deviation of the delay measurement and stabilization system of the optical microwave transmission link of the present invention; Figure 6 This is a schematic diagram showing the relationship between the delay measurement accuracy and the frequency interval of the two-tone signal in the delay measurement and stabilization system of the optical microwave transmission link of the present invention. Figure 7 This is a schematic diagram of the absolute delay measurement of the delay stabilization link in the delay measurement and stabilization system of the optical microwave transmission link of the present invention. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0020] The terminology used in this invention is for the purpose of describing particular embodiments only and is not intended to limit the embodiments of the invention. The singular forms “a,” “the,” and “the” as used in the embodiments of the invention and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0021] Existing technologies mostly focus on static measurement of link delay, lacking active control and stabilization capabilities for transmission delay, making it difficult to meet the comprehensive requirements of distributed systems for high coherence and dynamic compensation. Through in-depth research, it was discovered that by employing a dual-tone probe signal combined with a dual-optical mixing structure, the phase change of the dual-tone probe signal is converted to an intermediate frequency signal, avoiding direct detection of the high-frequency phase. This achieves high-precision detection of delay changes while expanding the unambiguous range of delay measurement. Utilizing a self-designed zero-difference phase-locked loop for feedback compensation of delay changes, the integration of link transmission delay measurement and stabilization is realized.

[0022] Therefore, as Figure 1 As shown, the inventors have proposed a delay measurement and stabilization system for optical microwave transmission links, comprising: The delay measurement unit is used to generate and process probe signals to obtain link delay information. The delay measurement unit includes a first laser, a first modulator, a frequency shifting unit, and a dual-optical mixing unit. To address the difficulty of direct phase detection of high-frequency probes, the phase change of the high-frequency signal is transferred to the intermediate-frequency signal by modulating the dual-tone signal and using a dual-optical mixing structure, without the need for a high-bandwidth photodetector and a high sampling rate.

[0023] The signal transmission unit is used to generate and transmit the microwave signal to be stabilized. The multiplexing / demultiplexing unit is used to multiplex the probe signal and the microwave signal to be transmitted to the same optical fiber link for transmission and to separate them at a remote end. The signal processing unit is used to calculate the link delay based on the intermediate frequency signal output by the delay measurement unit and generate a delay control signal. A delay execution unit, connected to the optical fiber link, is used to adjust the optical path of the link according to the delay control signal in order to stabilize the transmission delay of the microwave signal to be transmitted.

[0024] In one possible implementation, the frequency shifting unit includes: An acousto-optic frequency shifter is used to apply a fixed frequency shift to probe signals transmitted to a remote location. A Faraday rotator is used to reflect optical signals and change their polarization state, so that the returned signal is shifted again by the acousto-optic frequency shifter.

[0025] In one possible implementation, the dual-optical mixing unit includes: An optical filter is used to separate the positive first-order sideband combination and the negative first-order sideband combination after mixing the returned probe signal with the local reference probe signal; and A balanced photodetector is used to convert the two separated sideband combinations into a first intermediate frequency signal and a second intermediate frequency signal, respectively.

[0026] In one possible implementation, the signal processing unit includes: The signal processing unit is used to collect the first intermediate frequency signal and the second intermediate frequency signal, and obtain the link delay value through phase calculation; A phase-locked loop control unit is used to generate the delay control signal based on the third intermediate frequency signal generated by the dual-optical mixer unit.

[0027] In one possible implementation, the first modulator is used to modulate a two-tone probe signal generated by a radio frequency signal generator onto an optical carrier emitted by the first laser, generating modulated light containing ±1st-order sidebands.

[0028] In this embodiment of the application, the structure of the transmission delay high-precision stabilization and measurement system is as follows: Figure 1As shown, two-tone (DT) signals of 37 GHz and 38 GHz are generated using a first microwave source and a second microwave source, respectively, as probes. These signals are modulated onto a 1550.12 nm optical carrier emitted by a first laser using a first modulator to detect link delay. A 25 GHz microwave signal to be transmitted is generated using a vector network analyzer (VNA) and modulated onto a 1549.44 nm optical carrier emitted by a second laser using a second modulator. A bias point controller is used to stabilize the bias point of the first modulator in a carrier-suppressed state and the bias point of the second modulator in a quadrature state. The DT probe signal modulated by the first modulator is split into two parts by a 30:70 bias-maintaining coupler: 30% is used as a reference signal, and the reference signal is further split into two paths by another 30:70 bias-maintaining coupler. 70% of the reference light is used for mixing with the return signal light, and the remaining 30% of the reference light is photoelectrically converted by a first photodetector and then used by a data acquisition card to detect the initial phase difference of the DT signal. 70% of the signal is split by a polarization beamsplitter, then multiplexed into the same fiber optic link by a wavelength division multiplexer and transmitted to the remote end. At the remote end, the optical signal is frequency-shifted by 40 MHz by an acousto-optic frequency shifter and then split by the wavelength division multiplexer. The transmitted signal is demodulated by a second photodetector and then used in a vector network analyzer to measure the transmission delay. The probe signal is reflected back to the local end by a Faraday rotator, mixed with the reference signal, and then filtered out using optical filters to extract the positive and negative first-order sidebands. These are then detected by third and fourth photodetectors to obtain the intermediate frequency (IF) signals IF1 and IF2 used for delay measurement, and the IF3 signal used to stabilize the link delay. IF1 and IF2 are acquired by a data acquisition card, their phases are detected, and the link delay is measured. The two IF3 signals filtered out by the bandpass filter are subjected to zero-difference mixing, and the resulting error signal is input into a phase-locked loop (PLL) for feedback control of the piezoelectric ceramic fiber stretcher and the motor optical delay line, compensating for link delay variations.

[0029] In one possible implementation, the delay execution unit includes at least one of a piezoelectric ceramic fiber stretcher and a motor optical delay line.

[0030] In this embodiment of the application, the mechanism is physically connected to the optical fiber link, receives the delay control signal from the phase-locked loop control unit, and compensates for transmission delay jitter in real time and accurately by changing the optical path of the link.

[0031] In one possible implementation, the first modulator and / or the second modulator in the signal transmission unit are connected to a bias point controller for stabilizing the bias point of the modulator at the carrier suppression point or quadrature point. Both the first modulator and the second modulator in the signal transmission unit are connected to a bias point controller (BPC) for real-time monitoring and automatic adjustment of the modulator's DC bias voltage to stabilize its operating point in the specific state required by the system.

[0032] In this embodiment, for the first modulator used for the probe signal, its bias point controller is configured to perform carrier suppression point locking. The controller monitors the output optical power of the first modulator and applies a low-frequency, small-amplitude sinusoidal jitter signal (typically a few kHz to tens of kHz) to the bias voltage. By detecting the correlation (i.e., error signal) between the output optical power change and the jitter signal through phase-locked loop detection, the controller drives the bias voltage to minimize the output optical power. At this time, the modulator is in a carrier suppression state, the optical carrier is greatly suppressed, the modulation efficiency is the highest, which is beneficial for generating pure ±1st order sidebands. For the second modulator used for the microwave signal to be transmitted, its bias point controller is configured to perform quadrature point locking. The control principle is similar, but the goal is to adjust the bias voltage to the linear midpoint of the transmission curve (cosine square function) of the output optical power changing with the bias voltage. At this operating point, the modulator has the best linear response, which can minimize the nonlinear distortion generated by the microwave signal to be transmitted during modulation. By employing the aforementioned bias point controller, the system can overcome the modulator operating point drift caused by factors such as changes in ambient temperature and device aging, ensuring that the first modulator always outputs a high-quality carrier-suppressed double-sideband signal for high-precision delay measurement, while ensuring that the second modulator always operates in the optimal linear region to achieve low-distortion transmission of the microwave signal to be transmitted.

[0033] In one possible implementation, the radio frequency signal generator is configured to generate multiple sets of dual-tone probe signals with different frequency intervals to eliminate phase ambiguity through hierarchical resolution, wherein the minimum frequency interval is used to determine the ambiguity-free range of the delay measurement, and the maximum frequency interval is used to determine the delay measurement accuracy.

[0034] In this embodiment of the application, to address the problem of the mutual constraint between measurement range and measurement accuracy, a larger delay measurement range is achieved while maintaining high accuracy by setting different frequency intervals of dual-tone signals.

[0035] As a second aspect of the present invention, a method for delay measurement and stabilization of optical microwave transmission links is also proposed, comprising the following steps: S1: Generate a dual-tone probe signal and a microwave signal to be transmitted, and modulate them onto optical carriers of different wavelengths respectively; S2: The probe optical signal and the service optical signal are multiplexed into the same optical fiber link and transmitted to the remote end. After separation, the probe optical signal is frequency-shifted twice and reflected back. S3: The returned probe optical signal is mixed with the local reference probe optical signal, and the sidebands are separated and detected by a balanced photodetector to obtain a beat frequency signal that contains at least the first intermediate frequency signal, the second intermediate frequency signal and the third intermediate frequency signal. S4: Calculate the absolute transmission delay of the optical fiber link based on the phase information of the first intermediate frequency signal and the second intermediate frequency signal; S5: Generate an error signal based on the third intermediate frequency signal, and dynamically compensate for the delay jitter of the link by controlling the delay execution unit through feedback, so as to stabilize the transmission delay of the microwave signal to be transmitted.

[0036] In this embodiment, the phase change measurement of the high-frequency signal is transformed into the relative phase change measurement of the low-frequency interval by using a dual-tone signal and a dual-optical mixing structure, which improves the phase detection accuracy and realizes high-precision delay measurement.

[0037] In one possible implementation, the phase of the intermediate frequency signal output by the balanced photodetector is measured via a data acquisition card, and the link delay is determined.

[0038] In one possible implementation, the intermediate frequency signal is phase-determined by Fourier transform, and phase ambiguity is eliminated by adjusting the frequency interval of the two-tone signal.

[0039] In this embodiment, the optical carrier is intensity modulated using a two-tone signal and a high-frequency two-tone probe signal. The normalized expression for the ±1st order sideband amplitude of the modulated optical signal is as follows: MERGEFORMAT (1) in, It is the frequency of the optical carrier. and It is the frequency component of a two-tone signal. and This is the initial phase of the two-tone signal. The modulated optical signal is divided into a reference signal and a probe signal. A portion of the reference signal is further separated, photoelectrically converted, and input to the acquisition card to detect the initial phase difference of the two-tone signal. The probe signal is transmitted to the remote end via an optical fiber link, undergoes an up-shift, is reflected, and then up-shifted again before being transmitted back to the local end; considering the frequency shift amount of 2... The positive and negative first-order sidebands of the return signal, which are much smaller than the optical frequency and contain link delay jitter information, can be represented as follows: MERGEFORMAT (2) MERGEFORMAT (3) in To determine the link transmission delay, ±1st order sidebands are selected from the mixed optical signal, and after photoelectric conversion, a beat frequency signal is obtained. The beat frequency signal contains IF1 and IF2 components used for delay measurement, and an IF3 component used for stabilizing the link delay. To represent the frequency interval of a two-tone signal, the frequencies of IF1 and IF2 can be expressed as: and The frequency of IF3 is 2. The expressions for each frequency component of the ±1st order sideband beat frequency signal are: MERGEFORMAT (4) MERGEFORMAT (5) MERGEFORMAT (6) As can be seen from the expressions of the three IF signals, by modulating the dual-tone probe signal and combining it with the dual-optical mixing structure, the phase shift of the high-frequency signal is transferred to the intermediate-frequency signal.

[0040] Adding the phases of the IF1 and IF2 signals generated by the beat frequencies of the ±1st order sidebands and subtracting the initial phase difference of the two-tone signal, the phase shift of the two-tone signal after passing through the link is obtained as follows: MERGEFORMAT (7) exist Given the known information, the delay can be calculated based on the measured phase. From equation MERGEFORMAT(7), it can be seen that the delay measurement range is determined by the frequency interval of the two-tone probe signal. When a larger range of delay needs to be measured, the problem of phase integer period ambiguity can be solved by setting two-tone signals with different frequency intervals and measuring the corresponding phase shifts. The phase shifts of different frequency intervals can be expressed as: MERGEFORMAT (8) MERGEFORMAT (9) in, This indicates the frequency interval of the i-th group of dual-tone signals. The phase shift is measured in the range of (-π, π). The frequency interval of the first set of dual-tone signals determines the delay measurement range. The integer period ambiguity of the remaining frequency intervals can be calculated by formula MERGEFORMAT (9) to obtain a coarse measurement result of the transmission delay. The accuracy of the coarse delay measurement result is determined by the frequency interval of the last set of dual-tone signals. Further subtracting the phase terms of the intermediate frequency signals of the ±1st order sidebands, the phase shift of the signal with a larger frequency interval between the ±1st order sidebands after transmission through the link can be obtained as follows: MERGEFORMAT (10) The measured value of the phase shift of the large frequency interval signal is in the range of (-π, π). From formula \MERGEFORMAT(10), it can be seen that the frequency of the dual-tone probe signal determines the accuracy of the delay measurement. Combined with the coarse delay measurement results, the integer period ambiguity of the phase shift corresponding to the large frequency interval can be calculated as: MERGEFORMAT (11) The precise measurement result of the delay can be expressed as: MERGEFORMAT (12) The system's high-frequency dual-tone probe signal design integrates delay measurement and stabilization capabilities, enabling stable link delay control while measuring delay. It selects two beat frequency signals from two channels. The components undergo zero-difference mixing, and the resulting error signal is used to compensate for delay variations in the link. The DC error signal obtained after zero-difference mixing can be expressed as: MERGEFORMAT (13) in, This indicates link delay jitter. The DC signal acts as an error signal, driving the optical delay line and closing the control loop. When the DC error signal remains at zero, A fixed value indicates that the delay variation of the fiber optic link is fully compensated. From equations \MERGEFORMAT (12) and \MERGEFORMAT (13), it can be seen that when the phase detection accuracy is constant, increasing the frequency of the probe signal can improve the delay detection accuracy, thereby improving the stability of the link transmission delay. It is worth noting that in practical applications, the link delay can remain stable throughout system operation. The link delay range can be pre-measured using other auxiliary means, and then high-precision measurement and stability of the link delay can be achieved simply by setting a two-tone signal with a fixed frequency interval.

[0041] To verify the accuracy of the system's delay measurement, a step delay measurement experiment was designed. With the system disconnected from the optical fiber, the delay was increased in 40 fs steps by controlling the motor's optical delay line. The two-tone probe signal interval was set to 1 GHz to measure the link delay variation. The sampling rate of the acquisition card was set to 2.5 GS / s. A VNA was used to generate a 25 GHz signal to simultaneously measure the remote transmission delay variation, with the receiving bandwidth set to 10 Hz. Figure 2 The measurement results shown indicate that the difference between the proposed delay measurement method and the VNA measurement results is within ±10 fs. This error includes delay jitter introduced by differences in pigtails and RF devices, demonstrating that the system can accurately measure link transmission delay variations.

[0042] The system's 37 GHz and 38 GHz dual-tone probe signals can accurately detect transmission delay variations. Combined with a dual-optical mixing structure, the delay information is transferred to an 80 MHz intermediate frequency (IF) signal. The IF signals of the positive and negative first-order sidebands are mixed and phase-detected, and the resulting DC error signal is input into a zero-difference phase-locked loop (PLL) for precise feedback control of the link delay, thus achieving transmission delay stability. The delay measurement accuracy and system stability were further verified on a back-to-back, fiber-free, delay-stabilized link. Figure 3 The measurement results show that the std value of the delay jitter is 1.99 fs within a measurement time of 3600 s. This proves that the proposed system can achieve stable delay control while measuring link delay with high precision, and has the capability of integrated measurement and stability control.

[0043] To further verify the stable transmission performance of the system, a long-distance optical microwave transmission experiment was designed. A 25 GHz signal was transmitted using a VNA, and after passing through a 45 km single-mode fiber optic link to the remote end, the signal delay was measured using the VNA. The results are as follows: Figure 4 As shown in Figure 4, when the link delay was unstable, the transmission delay varied by more than 1.5 ns within 3600 s during changes in laboratory ambient temperature. When the link delay stabilized, thanks to the use of a high-frequency dual-tone probe signal and a dual-optical mixing structure, and utilizing a self-designed zero-difference phase-locked loop (PLL), high-precision detection and compensation of the link delay were achieved, stabilizing the std value of the link transmission delay jitter at 15.68 fs. The inset in Figure 4 shows the DC error signal spectrum measured using a spectrum analyzer. The resolution bandwidth of the spectrum analyzer was set to 1 Hz. When the link was locked, significant noise suppression was achieved within a loop bandwidth of approximately 150 Hz. The suppression of low-frequency noise verified the effectiveness of the PLL, indicating that the transmission link exhibits excellent performance in terms of long-term stability.

[0044] To further evaluate the system's noise characteristics and transmission stability, the overlap Allen bias was calculated based on the transmission delay jitter measurement results, as shown below. Figure 5As shown, in the unlocked state, the slope of the overlap Allen deviation curve changes from positive to negative due to variations in ambient temperature. This indicates the presence of random walk noise dependent on environmental changes, resulting in poor frequency stability, typically on the order of 10⁻¹⁴. Compared to the unlocked transmission system, the overlap Allen deviation decreases after link locking, especially over a longer averaging period. The overlap Allen deviation of the locked transmission link is 2.75 × 10⁻¹⁷ over a 1000 s averaging period, representing an improvement of more than two orders of magnitude in system frequency stability. This demonstrates that the system can maintain the accuracy of link delay measurement and compensation over a large delay range, and the long-term stability of transmission delay is significantly improved.

[0045] In addition to achieving high-precision delay measurement and stable transmission, the proposed system can also achieve wide-range absolute delay measurement by adjusting the frequency interval of the dual-tone probe signals. To investigate the relationship between delay measurement accuracy and the frequency interval of the dual-tone probes, the delay measurement accuracy of dual-tone signals with different frequency intervals was measured in a back-to-back configuration without an optical fiber link. The results are shown in Figure 6. The measurement time for each frequency interval was 400 s, and the std value of the delay jitter was used to measure the measurement accuracy. Within the frequency interval range of 2 kHz to 1 GHz, the delay measurement accuracy showed a gradual increasing trend. Figure 6 The magnified image shows the delay jitter measurement results at three frequency intervals: 100kHz, 1MHz, and 100MHz. In large-range delay measurements, the setting of the two-tone signal frequency intervals should follow these principles: First, the first set of frequency intervals should be set according to the delay measurement range to ensure that there is no integer ambiguity in the phase shift. Second, the measurement accuracies of different frequency intervals need to be consistent to ensure accurate calculation of integer period ambiguity. Finally, the accuracy of the coarse delay measurement results is determined by the frequency intervals of the last set of two-tone signals; the coarse delay measurement accuracy should be included within half of the period corresponding to the high-frequency interval between ±1st-order sidebands. Therefore, selecting an appropriate two-tone signal frequency interval can eliminate phase integer period ambiguity, balancing the delay measurement range and accuracy.

[0046] To verify the system's measurement accuracy over a wide range of delays, an absolute delay measurement experiment was designed. The frequency interval of the two-tone signal was set to 2 kHz, 1 MHz, 100 MHz, and 1 GHz, respectively. The phase shift of the two-tone probes after passing through the link was measured at different intervals to resolve phase ambiguity issues, thus obtaining coarse delay measurement results. Then, the frequency interval of the two-tone signal was fixed at 1 GHz, and the delay variation of a 45 km fiber optic link was precisely measured over 3600 seconds. Figure 7As shown, under link delay locking, the peak-to-peak delay jitter of the coarse measurement result within a 3600 s measurement time is 5.21 ps, which does not exceed half of the period corresponding to the 75 GHz frequency interval between ±1st order sidebands, and can be consistent with the fine measurement result. The std value of the delay measurement result shown by the blue curve is 9.41 fs, indicating that the system can still achieve a delay measurement accuracy within 20 fs in a wide range of delay measurements. At the same time, the remote transmission delay was measured using a VNA to compare with the method proposed in this paper. Since the lowest frequency of the VNA output signal is 100 kHz, the absolute delay of the link transmission cannot be obtained directly by measuring the phase shift of the RF signal. First, the link delay was coarsely measured using an optical time domain reflectometer, and then 1MHz, 10MHz, 100MHz, 1GHz, 10GHz and 25GHz signals were sent using a VNA, the phase shift of their transmission through the link was measured, and the absolute transmission delay was calculated. The results are shown by the red curve in Fig. 8. The trend of the remote delay measurement results is consistent with that of the delay measurement results of this system. The difference in absolute delay between the two is mainly introduced by the differences in fiber optic pigtail length and RF devices. This indicates that the optical microwave link transmission delay measurement and stabilization system based on dual-tone probe signals has achieved highly stable optical microwave transmission with high-precision absolute delay knowledge.

[0047] This invention proposes a system for measuring and stabilizing transmission delay in optical microwave links based on dual-tone probe signals. By employing dual-tone probe signals combined with a dual-optical mixing structure, the phase changes of the dual-tone probe signals are converted into intermediate frequency signals, avoiding direct detection of high-frequency phase. This achieves high-precision detection of delay changes while expanding the unambiguous range of delay measurement. A self-designed zero-difference phase-locked loop is used to compensate for delay changes, integrating link transmission delay measurement and stabilization. Experimental results show that the designed system achieves a delay measurement accuracy within ±10 fs. Regarding transmission delay stabilization, the std value of the transmission delay jitter within 3600 s for a 45 km fiber optic link reaches 15.68 fs. The system's high-precision measurement capability over a wide range of sub-millisecond delays is also confirmed. Therefore, the proposed method can achieve highly stable optical microwave transmission with high-precision absolute delay knowledge, providing an effective solution for coherent synthesis and target localization of distributed interferometric antenna arrays, and has significant engineering practicality and application prospects.

[0048] The foregoing has described specific embodiments of the present invention. Other embodiments are within the scope of the appended claims. In some cases, the actions or steps described in the claims may be performed in a different order than that shown in the embodiments and may still achieve the desired results. Furthermore, the processes depicted in the drawings do not necessarily require the specific or sequential order shown to achieve the desired results. In some embodiments, multitasking and parallel processing are also possible or may be advantageous.

[0049] In the description of the embodiments of the present invention, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the present invention. In the embodiments of the present invention, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in a suitable manner in any one or more embodiments or examples. Furthermore, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in the embodiments of the present invention, as well as the features of the different embodiments or examples.

[0050] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of embodiments of the present invention, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0051] Any process or method description in the flowchart or otherwise herein can be understood as representing a module, segment, or portion of code comprising one or more executable instructions for implementing custom logic functions or processes, and the scope of preferred embodiments of the invention includes additional implementations in which functions may be performed not in the order shown or discussed, including substantially simultaneously or in reverse order depending on the functions involved, as should be understood by those skilled in the art to which embodiments of the invention pertain.

[0052] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A delay measurement and stabilization system for an optical microwave transmission link, characterized in that, include: A delay measurement unit is used to generate and process probe signals to obtain link delay information. The delay measurement unit includes a first laser, a first modulator, a frequency shifting unit, and a dual-optical mixing unit. The signal transmission unit is used to generate and transmit the microwave signal to be stabilized. The multiplexing / demultiplexing unit is used to multiplex the probe signal and the microwave signal to be transmitted to the same optical fiber link for transmission and to separate them at a remote end. The signal processing unit is used to calculate the link delay based on the intermediate frequency signal output by the delay measurement unit and generate a delay control signal. A delay execution unit, connected to the optical fiber link, is used to adjust the optical path of the link according to the delay control signal in order to stabilize the transmission delay of the microwave signal to be transmitted.

2. The delay measurement and stabilization system for optical microwave transmission links according to claim 1, characterized in that, The frequency shifting unit includes: An acousto-optic frequency shifter is used to apply a fixed frequency shift to probe signals transmitted to a remote location. A Faraday rotator is used to reflect optical signals and change their polarization state, so that the returned signal is shifted again by the acousto-optic frequency shifter.

3. The delay measurement and stabilization system for optical microwave transmission links according to claim 1, characterized in that, The dual-optical mixing unit includes: An optical filter is used to mix the returned probe signal with the local reference probe signal, and then separate the positive first-order sideband combination and the negative first-order sideband combination; and A balanced photodetector is used to convert the two separated sideband combinations into a first intermediate frequency signal and a second intermediate frequency signal, respectively.

4. The delay measurement and stabilization system for optical microwave transmission links according to claim 3, characterized in that, The signal processing unit includes: The signal processing unit is used to collect the first intermediate frequency signal and the second intermediate frequency signal, and obtain the link delay value through phase calculation; A phase-locked loop control unit is used to generate the delay control signal based on the third intermediate frequency signal generated by the dual-optical mixer unit.

5. The delay measurement and stabilization system for optical microwave transmission links according to claim 1, characterized in that, The first modulator is used to modulate the dual-tone probe signal generated by the radio frequency signal generator onto the optical carrier emitted by the first laser to generate modulated light containing ±1st order sidebands.

6. The delay measurement and stabilization system for optical microwave transmission links according to claim 1, characterized in that, The first modulator and / or the second modulator in the signal transmission unit are connected to a bias point controller for stabilizing the bias point of the modulator at the carrier suppression point or the quadrature point.

7. The delay measurement and stabilization system for optical microwave transmission links according to claim 5, characterized in that, The radio frequency signal generator is configured to generate multiple sets of dual-tone probe signals with different frequency intervals to eliminate phase ambiguity through hierarchical resolution, wherein the minimum frequency interval is used to determine the ambiguity-free range of the delay measurement, and the maximum frequency interval is used to determine the delay measurement accuracy.

8. A method for delay measurement and stabilization of an optical microwave transmission link, comprising the delay measurement and stabilization system for an optical microwave transmission link as described in any one of claims 1-7, characterized in that, Includes the following steps: S1: Generate a dual-tone probe signal and a microwave signal to be transmitted, and modulate them onto optical carriers of different wavelengths respectively; S2: The probe optical signal and the service optical signal are multiplexed into the same optical fiber link and transmitted to the remote end. After separation, the probe optical signal is frequency-shifted twice and reflected back. S3: The returned probe optical signal is mixed with the local reference probe optical signal, and the sidebands are separated and detected by a balanced photodetector to obtain a beat frequency signal that contains at least the first intermediate frequency signal, the second intermediate frequency signal and the third intermediate frequency signal. S4: Calculate the absolute transmission delay of the optical fiber link based on the phase information of the first intermediate frequency signal and the second intermediate frequency signal; S5: Generate an error signal based on the third intermediate frequency signal, and dynamically compensate for the delay jitter of the link by controlling the delay execution unit through feedback, so as to stabilize the transmission delay of the microwave signal to be transmitted.

9. The delay measurement and stabilization method for optical microwave transmission links according to claim 8, characterized in that, The intermediate frequency (IF) signal output by the balanced photodetector is used to measure the phase of the IF signal via a data acquisition card, and the link delay is determined.

10. The delay measurement and stabilization method for optical microwave transmission links according to claim 8, characterized in that, The intermediate frequency signal is phase-determined by Fourier transform, and phase ambiguity is eliminated by adjusting the frequency interval of the two-tone signal.

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