Accurate phase synchronization system based on frequency division multiplexing

Abstract

A frequency signal accurate phase synchronization system based on frequency division multiplexing is composed of a transmitting system and a receiving system, the transmitting system and the receiving system are connected through a laser link, the transmitting system comprises a first microwave power divider, an even-number-multiple frequency divider, a second microwave power divider, a microwave beam combiner, a laser, an optical circulator, a bidirectional conjugate delay line and a first coupling device, and the receiving system comprises a second microwave power divider, a second microwave beam combiner and a second coupling device. The receiving system comprises a second coupling device, a 1 * 2 optical coupler, an optical reflector, a first photoelectric detector and a first microwave filter; the method is simple and convenient to manufacture, and accurate phase synchronization under different link types and link lengths can be accurately realized.

Description

Technical Field

[0001] The invention relates to a new precise phase synchronization system based on frequency division multiplexing, which is mainly used in the field of phase synchronization of high-precision frequency signal transmission, and can also be used in other fields of stable phase transmission. Background Art

[0002] Frequency standards are fundamental to various important technologies in modern society, such as navigation and positioning, internet communications, and the precision measurement of physical constants. High-precision time and frequency transfer technology is particularly important in coherent detection. For example, in the field of coherent detection arrays, its accuracy is limited by the consistency of clock signals within each node. In high-precision time and frequency transmission, PPS time signal synchronization generates or reproduces PPS time signals based on the frequency signals of each terminal. This avoids the accumulation of errors in transmission and measurement processes when directly synchronizing time signals. Noise compensation in classic time and frequency transfer systems ensures a constant real-time phase difference between the transmitted and transmitted signals, known as frequency synchronization, but cannot achieve real-time absolute phase stability of the frequency signal or a specific phase marker of the transmitted signal, known as phase synchronization. To improve the time synchronization accuracy of time and frequency transfer to picosecond levels, the frequency signal must be synchronized with phase synchronization in addition to frequency synchronization. Phase synchronization, in addition to frequency synchronization, addresses the issues of initial phase alignment, phase period ambiguity resolution, and repetition consistency of the transmitted signal. Therefore, designing a phase synchronization scheme for frequency signals is key to achieving higher-precision time synchronization in time and frequency transfer.

[0003] Several technical solutions have been proposed for the design of phase synchronization schemes used in high-precision time-frequency transmission.

[0004] Prior Art 1:

[0005] The patent of Wang Bo et al. from Tsinghua University, "A system and method for absolute phase synchronization based on optical fiber transmission" (patent number CN202010423599.5), uses phase conjugate mixing to automatically cancel out the phase delay of the frequency signal during transmission through the optical fiber link. The principle is simple. At the same time, the absolute phase synchronization effect exists between any two or all three of the phase distribution device, the phase reproduction device, and the intermediate download device. However, there are problems with this solution. First, the solution relies on a strict twice-frequency relationship between the reference signals, and additional frequency remote synchronization needs to be established when applied, which increases the complexity. Second, the solution is based on bidirectional same-wavelength laser transmission, and the backscattered and reflected signals in the optical fiber will seriously affect the frequency signal transmission performance.

[0006] Prior Art 2:

[0007] Wu Rui et al.'s patent "Fiber-optic phase synchronization system based on optical active compensation" (patent number CN202110245163.6) uses high-speed digital logic and optimized control based on zero-crossing detection to achieve absolute phase marking of the frequency signal, and transfers the excellent characteristics of the frequency signal phase to the marking signal, thereby obtaining an ultra-high stability time reference signal, and realizing the stability and consistency of phase synchronization in scenarios such as restart operations and link changes. However, the use of pulse measurement to achieve phase period detection and then resolve period ambiguity is more complicated.

[0008] Prior Art 3:

[0009] Zhao Kan et al.'s patent, "A Device and Method for Correcting the Phase of Frequency Signals in Fiber-Optic Time-Frequency Transmission" (Patent No. CN109379175A), uses time-frequency transmission technology to measure the time difference between a time signal and a time signal converted from a transmitted frequency signal. Phase shift correction is then applied to the transmitted frequency signal to achieve phase synchronization. However, this technology uses the time signal as a reference to improve the phase synchronization accuracy of the frequency signal, so the transmitted frequency signal cannot be used to improve the accuracy of the time signal. Summary of the Invention

[0010] In order to simplify the optical fiber phase synchronization method based on active compensation, an even-number frequency divider is used to achieve frequency synchronization of the divided frequency signal. Frequency division multiplexing is used to simultaneously transmit two frequency signals with the same phase on the optical carrier to achieve phase synchronization of the high-frequency frequency standard signal.

[0011] The technical solutions of the present invention are as follows:

[0012] A frequency signal precision phase synchronization system includes a transmitting system and a receiving system. The transmitting system includes a first microwave power splitter, a second microwave power splitter, an even-number frequency divider, a microwave beam combiner, a first laser, an optical circulator, a bidirectional conjugate delay line, a second photodetector, a second microwave filter, a microwave phase detector, a controller, and a first coupling device. The receiving system includes a 1×2 optical coupler, an optical reflector, a first photodetector, a first microwave filter, and a second coupling device.

[0013] The frequency signal is divided into two homologous frequency standard signals by the first microwave power divider, the first frequency standard signal enters the base frequency input port of the even-number frequency divider to which it belongs, and the second frequency standard signal enters the input port of the microwave combiner; the even-number frequency divider outputs the divided frequency signal; the divided frequency signal is divided into two frequency signals by the second microwave power divider, the first divided frequency signal enters the reference signal input port of the microwave phase detector, and the second divided frequency signal enters the input port of the microwave combiner; the combined complex microwave signal output by the microwave combiner enters the signal modulation port of the first laser;

[0014] The first laser modulates the input modulation signal to the laser amplitude and outputs a modulated optical signal; the modulated optical signal enters through the first port of the optical circulator, outputs from the second port of the optical circulator, then enters from the optical first port of the bidirectional conjugate delay line, outputs from the optical second port, then enters the first coupling device, and is coupled into the laser link; finally, it enters through the first port of the laser link, and the laser link transmits the optical signal from the transmitting system to the receiving system and outputs from the second port of the laser link; then the optical signal enters the second coupling device and is input into the single port of the 1×2 optical coupler; the dual ports of the 1×2 optical coupler output two optical signals; the first optical signal is input into the first photodetector, and the first photodetector demodulates the optical signal. frequency signal; the demodulated complex microwave frequency signal is filtered after passing through the first microwave filter to obtain a frequency standard signal with precise phase synchronization, which is supplied to the user of the receiving system; the second optical signal passes through the optical reflector back to the 1×2 optical coupler, and is output from the single port of the 1×2 optical coupler, and passes through the second coupling device, the laser link, the first coupling device, and the bidirectional conjugate delay line in sequence to reach the transmitting system; the looped optical signal is output from the first port of the bidirectional conjugate delay line and enters the second port of the optical circulator, and is output from the third port of the optical circulator and enters the second photodetector; the frequency signal demodulated by the second photodetector is filtered out by the second microwave filter to obtain the frequency signal after frequency division, and enters the signal to be measured port of the microwave phase detector;

[0015] The microwave phase detector measures the phase difference of the divided frequency signal after round-trip transmission relative to the reference frequency signal, and outputs the round-trip phase difference to the data input port of the controller; the controller obtains an error signal to ensure the one-way phase consistency of the frequency through calculation and processing, and converts it into a control signal of the bidirectional conjugate delay line and inputs it into its electrical control end, thereby stabilizing the transmission delay of the bidirectional conjugate delay line and the laser link, and realizing frequency synchronization of the divided frequency signal and phase synchronization of the frequency standard signal.

[0016] The signal functions involved in the frequency signal precise phase synchronization system are as follows:

[0017] 1) The complex microwave signal after the microwave beam combiner can be expressed as:

[0018]

[0019] Where ω1 is the frequency of the frequency standard signal, ω2 is the frequency of the frequency signal after even-number frequency division, is the out-of-loop phase delay inside the transmitter chassis, It is the initial phase difference between the frequency standard signal and the frequency signal after even-number frequency division.

[0020] 2) When the link delay is locked each time, the phase detector can drive the bidirectional conjugate delay line so that the phase of the signal entering the phase detector after returning is:

[0021]

[0022] in is the tunable set point of the phase detector, assuming that the set point is set to 0, and N is an arbitrary integer. Characterize the round-trip phase delay provided by a bidirectional conjugate delay line and laser link.

[0023] At this time, the controller controls One-way phase delay locking can be achieved.

[0024] 3) After delay locking, the demodulated frequency signal of the first photodetector of the receiving system can be expressed as:

[0025]

[0026] Since the two frequency signals are transmitted through the same link and wavelength, M is an even multiple of N, with a magnification of 2k. Therefore, for the frequency-divided signal, there is period ambiguity when N is odd or even, while for the standard frequency signal ω1, the transmission phase delay is an even multiple of its period M and a set phase There is no period ambiguity caused by parity, thus achieving long-distance and precise phase synchronization of standard frequency signals.

[0027] Compared with the prior art, the characteristics and advantages of the present invention are:

[0028] 1) The frequency signal precise phase synchronization scheme based on frequency division multiplexing proposed in the present invention provides a feasible solution for frequency phase synchronization technology. It eliminates the phase period ambiguity of the high-frequency standard signal through the frequency synchronization of the divided frequency signal, realizes its phase synchronization, improves the time-frequency transmission coherence, and adapts to more application requirements.

[0029] 2) Compared with existing technologies, the present invention's solution for achieving long-distance phase synchronization of frequency signals is simple and convenient. It eliminates the need for an additional time transmission signal to phase-mark the frequency signals. Simultaneously, the two frequency signals are modulated on the same carrier, saving channels. Other time-frequency simultaneous transmission technologies use time signals to achieve frequency phase synchronization, but their phase synchronization accuracy depends on the accuracy of time synchronization. Therefore, they cannot inherently improve time synchronization accuracy by utilizing frequency signal phase synchronization. However, the present technology possesses phase synchronization capability without the aid of a time signal. Therefore, with a simple extension, after performing time-frequency simultaneous transmission, it is possible to utilize frequency signal phase synchronization to improve time signal synchronization accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a block diagram of the system for realizing long-distance phase synchronization of frequency standard signals based on laser links of the present invention.

[0031] Figure 2 This is a system block diagram of the present invention that realizes phase synchronization of a frequency standard signal after simple expansion and time synchronization based on this. DETAILED DESCRIPTION

[0032] The present invention will be further described below with reference to the embodiments and drawings, but the scope of protection of the present invention should not be limited thereto.

[0033] See also Figure 1 , Figure 1 This is a block diagram of a system for realizing long-distance phase synchronization of frequency standard signals based on a laser link according to the present invention. As can be seen from the figure, the present invention includes a first microwave power divider 1, a second microwave power divider 3, an even-number frequency divider 2, a microwave combiner 4, a first laser 5, an optical circulator 6, a bidirectional conjugate delay line 7, a second photodetector 14, a second microwave filter 15, a microwave phase detector 16, a controller 17 and a first coupling device 8; a 1×2 optical coupler 10, an optical reflector 11, a first photodetector 12, a first microwave filter 13 and a second coupling device 9; the first coupling device and the second coupling device can be flanges connecting optical fibers or telescopes connecting optical fibers and space; and the laser link between the transmitting system and the receiving system can be an optical fiber link, a free space link or an underwater link.

[0034] Figure 1The intermediate frequency signal is divided into two frequency standard signals of the same source by the first microwave power divider 1. The first frequency standard signal enters the base frequency input port of the even-number frequency divider 2 to which it belongs, and the second frequency standard signal enters the input port of the microwave combiner 4. The even-number frequency divider 2 outputs the divided frequency signal. The divided frequency signal is divided into two frequency signals by the second microwave power divider 3. The first divided frequency signal enters the reference signal input port of the microwave phase detector 16, and the second divided frequency signal enters the input port of the microwave combiner 4. The combined complex microwave signal output by the microwave combiner 4 enters the signal modulation port of the first laser 5.

[0035] Figure 1 The first laser 5 modulates the input modulation signal to the laser amplitude and outputs a modulated optical signal; the modulated optical signal enters through the first port 61 of the optical circulator 6, is output from the second port 62 of the optical circulator 6, is then input from the optical first port of the bidirectional conjugate delay line 7, is output from the optical second port, then enters the first coupling device 8, is coupled into the first port of the laser link and is output from the second port of the laser link. The laser link transmits the optical signal from the transmitting system to the receiving system; then the optical signal output from the second port of the laser link enters the second coupling device 9, and then enters the single port of the 1×2 optical coupler 10; the dual ports of the 1×2 optical coupler 10 output two optical signals; the first optical signal is input into the first photodetector 12, and the first photodetector 12 demodulates the frequency rate signal; the demodulated frequency signal is filtered after passing through the first microwave filter 13 to obtain a frequency standard signal with precise phase synchronization, which is supplied to the user of the receiving system; the second optical signal passes through the optical reflector 11 and returns to the 1×2 optical coupler 10, and is output from the single port of the 1×2 optical coupler 10, and passes through the second coupling device 9, the laser link, the first coupling device 8, and the bidirectional conjugate delay line 7 in sequence to reach the transmitting system; the looped optical signal is output from the first port of the bidirectional conjugate delay line 7 and enters the second port 62 of the optical circulator 6, and is output from the third port 63 of the optical circulator 6 and enters the second photodetector 14; the frequency signal demodulated by the second photodetector 14 is filtered out by the second microwave filter 15 to obtain the frequency signal after frequency division, and enters the signal to be measured port of the microwave phase detector 16;

[0036] Figure 1The microwave phase detector 16 measures the phase difference of the divided frequency signal after round-trip transmission relative to the reference frequency signal, and outputs the round-trip phase difference to the data input port of the controller 17; the controller 17 obtains an error signal to ensure the one-way phase consistency of the frequency after calculation and processing, and converts it into a control signal of the bidirectional conjugate delay line 7 and inputs it into its electrical control end, thereby stabilizing the transmission delay of the bidirectional conjugate delay line 7 and the laser link, and realizing the frequency synchronization of the divided frequency signal and the phase synchronization of the frequency standard signal.

[0037] Example 1: Long-distance, precise phase synchronization of a 12 GHz frequency standard signal

[0038] In the first embodiment, Figure 1 As shown, the output wavelength of the first laser 5 is 1550.12 nm, and the even-number frequency divider has a 2-fold ratio of 12; the bandwidth of the second photodetector 14 is 2 GHz, and is used to detect the 1 GHz frequency division signal; the bandwidth of the first photodetector 12 is 15 GHz, and is used to detect the 12 GHz frequency standard signal;

[0039] Figure 1 The microwave phase detector 16 measures the phase difference of the divided 1 GHz frequency signal relative to the reference 1 GHz frequency signal after round-trip transmission, and outputs the round-trip phase difference to the controller 17, obtains the error signal and drives the bidirectional conjugate delay line 7, thereby achieving stable transmission delay and realizing frequency synchronization of the divided 1 GHz frequency signal and phase synchronization of the 12 GHz frequency standard signal.

[0040] Example 2: Achieving ps-level time synchronization using precise phase synchronization of frequency standard signals

[0041] In the second embodiment, Figure 2 As shown, the system is expanded by adding a first time synchronization module 24, a second laser 25, a fourth photodetector 23 and a first wavelength division multiplexer 18 at the transmitting end; and adding a second time synchronization module 21, a third laser 22, a third photodetector 20 and a second wavelength division multiplexer 19 at the receiving end. After achieving precise phase synchronization of the frequency standard signal, the illustrated system uses the signal as a reference to generate and reproduce the time standard signal through the first time synchronization module 24 for transmission and the second time synchronization module 21 for the receiving terminal. The time standard signal is then modulated onto optical signals of different wavelengths through the second laser 25 and the third laser 22. These optical signals are compounded and demodulated with the optical signal carrying the frequency standard signal information through the first wavelength division multiplexer 18 and the second wavelength division multiplexer 19 to achieve more precise time synchronization.

Claims

1. A long-distance frequency signal precision phase synchronization system, characterized in that: Contains the transmitting system, receiving system and laser link, The transmitting system comprises a first microwave power divider (1), an even-number frequency divider (2), a second microwave power divider (3), a microwave beam combiner (4), a first laser (5), an optical circulator (6), a bidirectional conjugate delay line (7), a second photodetector (14), a second microwave filter (15), a microwave phase detector (16), a controller (17), and a first coupling device (8); The receiving system comprises a second coupling device (9), a 1×2 optical coupler (10), an optical reflector (11), a first photodetector (12), and a first microwave filter (13); The laser link transmits the optical signal output from the transmitting system to the receiving system; The frequency signal is divided into two frequency standard signals of the same source through a first microwave power divider (1), the first frequency standard signal enters an even-number frequency divider (2), and the second frequency standard signal enters a microwave beam combiner (4); the even-number frequency divider (2) outputs a divided frequency signal; the divided frequency signal is divided into two divided frequency signals through a second microwave power divider (3), the first divided frequency signal enters a microwave phase detector (16), and the second divided frequency signal enters a microwave beam combiner (4); the microwave beam combiner (4) outputs a combined complex microwave signal, which enters a first laser (5); The first laser (5) modulates the input beam-combined complex microwave signal to the laser amplitude and outputs a modulated optical signal; The modulated optical signal passes through an optical circulator (6), a bidirectional conjugate delay line (7), and then enters a first coupling device (8) to be coupled into a laser link; The laser link transmits the modulated optical signal to the second coupling device (9), and then enters the 1×2 optical coupler (10) to output two modulated optical signals; The first modulated optical signal is input into the first photodetector (12) to demodulate a frequency signal; the demodulated frequency signal is filtered through the first microwave filter (13) to obtain a frequency standard signal with precise phase synchronization, which is provided to the user of the receiving system; The second modulated optical signal outputted by the 1×2 optical coupler (10) returns to the 1×2 optical coupler (10) through the optical reflector (11), and then returns to the second coupling device (9), the laser link, and the return transmission system in sequence to form a loopback modulated optical signal; The loopback modulated optical signal passes through a first coupling device (8) and a bidirectional conjugate delay line (7), enters an optical circulator (6), and is output by the optical circulator (6) and enters a second photodetector (14); after the frequency signal is demodulated by the second photodetector (14), the frequency signal after frequency division is filtered out by a second microwave filter (15), and enters a microwave phase detector (16); In the microwave phase detector (16), a phase difference between the first frequency-divided frequency signal and the frequency signal after frequency division of the loopback modulated optical signal is obtained. The phase difference is processed by the controller (17) to obtain an error signal for ensuring the frequency one-way phase consistency, and is converted into a control signal of the bidirectional conjugate delay line (7) and input into its electrical control end to stabilize the transmission delay of the bidirectional conjugate delay line (7) and the laser link, thereby achieving frequency synchronization of the frequency signal after frequency division and phase synchronization of the frequency standard signal.

2. The long-distance frequency signal precise phase synchronization system according to claim 1, characterized in that: The laser link is an optical fiber link, a free space link or an underwater link.

3. The long-distance frequency signal precise phase synchronization system according to claim 1, characterized in that: The transmitting system also includes a first time synchronization module (24), a second laser (25), a fourth photodetector (23) and a first wavelength division multiplexer (18); The receiving system also includes a second time synchronization module (21), a third laser (22), a third photodetector (20) and a second wavelength division multiplexer (19); After achieving phase synchronization of the frequency standard signal, the frequency standard signal is used as a reference to generate and reproduce the time standard signal through a first time synchronization module (24) and a second time synchronization module (21); then, the time standard signal is modulated onto optical signals of different wavelengths through a second laser (25) and a third laser (22); these optical signals are combined and demodulated with the modulated optical signal carrying the frequency standard signal information through a first wavelength division multiplexer (18) and a second wavelength division multiplexer (19) to achieve time synchronization.

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