Space-time-frequency comparison system and method based on photo-generated millimeter waves

Abstract

A high-precision space-time-frequency comparison system and method based on photo-generated millimeter waves. A master end and a slave end each obtain, by means of heterodyne frequency beating, a millimeter-wave signal from an optical frequency locked to an optical reference consisting of a super-stable laser and an optical atomic clock; the master end and the slave end each send the millimeter-wave signal to the opposite end by means of a spatial link; the master end and the slave end each measure the time difference between a local millimeter-wave signal and the millimeter-wave signal that is received from the opposite end; the slave end can obtain the clock difference between the master end and the slave end by means of a bidirectional comparison principle, thereby implementing bidirectional time-frequency comparison; moreover, the slave end adjusts an optical reference frequency and a phase, such that time-frequency synchronization between the master end and the slave end can be realized. The present invention effectively alleviates the problems of inadequate precision in traditional microwave-based time-frequency comparison methods and relatively low continuous operation rates in traditional optical-carrier-based time-frequency comparison; and has the advantages of a low noise floor, simple structure and high reliability.

Description

Spatial-temporal-frequency alignment system and method based on photogenerated millimeter waves Technical Field

[0001] This invention relates to the field of optical time and frequency transfer, and is particularly applicable to high-precision spatial time and frequency comparison and transfer based on photogenerated millimeter waves. Background Technology

[0002] Time-frequency alignment and transmission enable remote frequency standard calibration and distribution, allowing users in different geographical locations to share a unified time-frequency reference source. To ensure the performance of the reference source, the accuracy of the time-frequency alignment must be at least an order of magnitude higher than the time-frequency signal being aligned. Based on the excellent characteristics of fiber optic transmission, significant progress has been made in large-scale, long-distance fiber optic time-frequency alignment and synchronization technology. However, fiber optic links have limitations in coverage, especially when dealing with mobile platforms (such as vehicle-to-ground, satellite-to-ground, inter-satellite, and sea-to-land connections), making it impossible to provide full-coverage, high-precision time-frequency alignment across the "air-space-land-sea" domain. Satellite-based space-based microwave time-frequency alignment systems offer advantages such as flexibility and lack of geographical limitations, but their accuracy is limited. In recent years, significant breakthroughs have been made in the spatial-temporal-frequency comparison and transfer of fundamental optical carriers. For example, NIST in the United States achieved 300km free-space time-frequency comparison and transfer based on optical frequency combs [Caldwell, ED, Deschenes, JD, Ellis, J., Swann, WC, Stuhl, BK, Bergeron, H., Newbury, NR and Sinclair, LC, 2023. Quantum-limited optical time transfer for future geosynchronous links. Nature, 618(7966), pp.721-726.], and the University of Science and Technology of China has also achieved 113km time-frequency comparison and transfer [Shen, Q., Guan, JY, Ren, JG, Zeng, T., Hou, L., Li, M., Cao, Y., Han, JJ, Lian, MZ, Chen, YW and Peng, XX, 2022. Free-space dissemination of time and frequency with 10-19 instability over [113km. Nature, 610(7933), pp.661-666.] The performance of time comparison and transmission can reach the femtosecond level, and the stability of frequency comparison and transmission can reach 1E-19. However, space optical links, especially near-ground links, are affected by weather, atmospheric turbulence and other factors, which can cause random signal fading or even loss. Therefore, the system can only operate normally under good weather conditions. Once it encounters rain, fog or snow, the system cannot operate normally. And the millimeter wave, especially the E-band millimeter wave (Mm-Wave), that is, the wireless signal with the frequency in the range of 71-76GHz and 81-86GHz, is considered to be very suitable for building long-distance wireless backhaul systems.Due to the E-band's ultra-wide bandwidth and relatively low atmospheric attenuation (less than 0.4 dB / km), and its ability to operate in adverse weather conditions, it offers significant advantages for long-distance transmission. Furthermore, the millimeter-wave band's shorter wavelength, on the order of 10 ps, ​​allows for phase measurement accuracy of 1 / 1000 and time difference measurement accuracy on the order of 10 fs, meeting the requirements for high-precision comparison and transmission.

[0003] Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by providing a high-precision spatial time-frequency comparison system and method based on optically generated millimeter waves. This system can achieve time-frequency comparison and synchronization based on highly stable millimeter waves. By adopting this structure, the system's continuous operation rate can be significantly improved while ensuring time-frequency comparison and transmission. This overcomes the problems of insufficient continuous operation rate of existing optical carrier-based time-frequency comparison and transmission systems and insufficient accuracy of microwave-based time-frequency comparison and transmission systems, thus meeting the requirements for high-precision, uninterrupted time-frequency comparison and transmission in free-space links.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows:

[0006] On the one hand, the present invention provides a spatial-temporal-frequency comparison method based on photogenerated millimeter waves, characterized by the following steps:

[0007] The master end sends the millimeter-wave signal locked to the optical reference to the slave end through the free-space millimeter-wave link, while the slave end sends the millimeter-wave signal locked to the optical reference to the master end through the millimeter-wave link.

[0008] The time difference T between the millimeter-wave signal locally locked to the optical reference measured at the master end and the millimeter-wave signal locked to the optical reference received at the slave end. BA The signal is transmitted to the slave end via a free-space microwave link. Simultaneously, the slave end measures the time difference T between the millimeter-wave signal locally locked to the optical reference and the millimeter-wave signal received by the master end. AB ;

[0009] The slave end calculates the clock difference ΔT between the master and slave ends based on the bidirectional comparison principle, ΔT = (T AB -T BA ) / 2, and adjust the optical reference frequency and phase of the slave end to make ΔT→0, thereby achieving time and frequency synchronization between the master and slave ends.

[0010] Furthermore, the millimeter-wave signal locked to the optical reference by the master or slave end specifically means:

[0011] - Obtain an optical frequency comb locked to a master or slave optical reference, wherein the master or slave optical frequency comb is locked to the master or slave optical reference, the master or slave optical reference comprising an ultrastable laser and an optical atomic clock; the optical frequency comb has N frequencies f i,m Each laser is locked by N consecutive lasers, where N = 2 or 3, and i = 0, ..., N;

[0012] - A millimeter-wave signal is obtained by heterodyne beat frequency from a continuous laser locked to an optical frequency comb.

[0013] Furthermore, the optical frequency is used to obtain a millimeter-wave signal via heterodyne beat frequency, specifically:

[0014] After the output light of the nth continuous laser is coupled to that of the (n+1)th continuous laser, a millimeter-wave signal f is obtained by beat frequency measurement on a photodetector. mmi+1,m , used to send to the other end, and / or, used locally to beat the received millimeter wave.

[0015] Furthermore, the time difference T between the millimeter-wave signal locally locked to the optical reference measured by the master end and the millimeter-wave signal locked to the optical reference received by the slave end is... BA Alternatively, the time difference T between the millimeter-wave signal locally locked to the optical reference measured at the slave end and the millimeter-wave signal locked to the optical reference received at the master end. AB Specifically:

[0016] The main terminal up-frequency acquires a millimeter-wave signal f from the main photodetector. mm1,m The signal is transmitted to the slave end via a free-space millimeter-wave link. The output light from two slave continuous lasers is coupled and beats on the slave photodetector to obtain the millimeter-wave signal f. mm2,s Millimeter-wave signals f are obtained from time measurement and demodulation. mm1,m and millimeter wave signal f mm2,s Time difference T after mixing AB ;

[0017] The slave device will obtain the millimeter-wave signal f from the photodetector via beat frequency. mm1,s The signal is transmitted to the main terminal via a free-space millimeter-wave link. The two main continuous lasers at the main terminal are coupled together, and the millimeter-wave signal f is obtained by beat frequency on the main photodetector. mm2,m Millimeter-wave signal f is obtained using master time measurement and modulator. mm2,s and millimeter wave signal f mmi+1,m Time difference T BA .

[0018] On the other hand, the present invention also provides a high-precision spatial time-frequency comparison system based on optically generated millimeter waves, including a master end, a slave end, and a free space link connecting the master end and the slave end, characterized in that the free space link includes a free space millimeter wave link and a free space microwave link.

[0019] Both the master and slave ends include an optical frequency reference, an optical frequency comb, N consecutive lasers (N=2 or 3), a first optical coupler, a first photodetector, a second optical coupler, a second photodetector, a millimeter-wave polarization multiplexer, a millimeter-wave antenna, a millimeter-wave mixer, a time measurement and modulator, and a microwave antenna.

[0020] The optical frequency reference includes at least an ultrastable laser and an optical atomic clock. The optical frequency comb is locked to the optical frequency reference. The N consecutive lasers are respectively locked to the N frequencies f of the main optical frequency comb through optical phase-locked loops or injection locking. i,m Above, i=0,...,N;

[0021] After the output light of the first continuous laser is coupled to the output light of the second continuous laser, a first millimeter-wave signal is obtained by frequency beat on the first photodetector, which is used to transmit to the other end; after the output light of the second continuous laser is coupled to the output light of the third continuous laser, a second millimeter-wave signal is obtained by frequency beat on the second photodetector, which is used to beat with the millimeter-wave signal received from the other end.

[0022] The first millimeter-wave signal passes sequentially through the millimeter-wave polarization multiplexer, millimeter-wave antenna, and free-space millimeter-wave link to reach the other end. The output light from the millimeter-wave antenna and millimeter-wave polarization multiplexer at the other end, along with the output light from the second and third continuous lasers at the other end, passes through the second optical coupler and is beat-frequencyd on the second photodetector to obtain the second millimeter-wave signal, which then reaches the two input terminals of the millimeter-wave mixer. The time difference of the output signal of the millimeter-wave mixer is measured using a time measurement and demodulation unit, i.e., the time difference of the millimeter-wave signal is measured at the master end and the slave end respectively.

[0023] The master end transmits the time difference of the millimeter-wave signal measured by the master end to the slave end through the free space microwave link, and the slave end demodulates to obtain the time difference of the millimeter-wave signal measured by the master end.

[0024] The slave end adjusts its optical reference frequency and phase based on the clock difference between the time difference of the millimeter-wave signal measured by the demodulation master end and the time difference of the millimeter-wave signal measured by the local slave end, so that the clock difference is infinitely close to 0, thereby achieving time and frequency synchronization between the master and slave ends.

[0025] Compared with the prior art, the beneficial effects of the present invention are:

[0026] This invention improves the continuous operation rate of the system by using high-precision spatial time-frequency comparison and transmission based on optically generated millimeter waves. This invention overcomes the shortcomings of existing microwave-based and optical carrier-based time-frequency comparison methods, which have insufficient time difference measurement range. In particular, existing optical carrier-based time-frequency comparison methods suffer from significant losses during spatial transmission, and are completely unable to work under weather conditions such as clouds, fog, rain, and snow. This invention effectively improves the problems of insufficient accuracy of traditional microwave-based time-frequency comparison methods and low continuous operation rate of optical carrier-based time-frequency comparison methods. The system has the advantages of low noise, simple structure, and high reliability. Attached Figure Description

[0027] Figure 1 is a schematic diagram of Embodiment 1 of the high-precision spatial-temporal-frequency comparison system based on bidirectional photogenerated millimeter waves of the same frequency according to the present invention.

[0028] Figure 2 is a schematic diagram of Embodiment 2 of the high-precision spatial-temporal-frequency comparison system based on bidirectional photogenerated millimeter waves of different frequencies according to the present invention. Detailed Implementation

[0029] The present invention will be further described below with reference to the embodiments and accompanying drawings. The embodiments are implemented based on the technical solutions of the present invention, and detailed implementation methods and specific workflows are given. However, the scope of protection of the present invention is not limited to the following embodiments.

[0030] Please refer to Figure 1. Figure 1 is a schematic diagram of Embodiment 1 of the high-precision space-time-frequency comparison system based on bidirectional optically generated millimeter waves of the same frequency according to the present invention. As shown in the figure, it includes a master end 1 and a slave end 2, as well as a free space link 3 connecting the master end 1 and the slave end 2.

[0031] Free space link 3 consists of free space millimeter wave link 301 and free space microwave link 302.

[0032] The master and slave ends respectively obtain millimeter-wave signals by heterodyne beat frequency from the optical frequencies locked to the optical reference. These millimeter-wave signals have the same frequency stability as the optical reference.

[0033] The master and slave ends transmit the millimeter-wave signal locked to the optical reference to the slave and master ends respectively through the millimeter-wave link 301.

[0034] The main terminal 1 includes a main optical frequency reference, a main optical frequency comb 103, a second main continuous laser 104, a third main continuous laser 105, a fourth main continuous laser 106, a first main optical coupler 107, a first main photodetector 108, a second main optical coupler 109, a second main photodetector 110, a main millimeter-wave polarization multiplexer 111, a main millimeter-wave antenna 112, a main millimeter-wave mixer 113, a main time measurement and modulation unit 114, and a main microwave antenna 115.

[0035] The main optical frequency reference includes a first main continuous laser 101, a main optical reference cavity 102, and a main optical atomic clock. Ports 1, 2, 3, and 4 of the first main optical coupler 107 are connected to the output port of the second main continuous laser 104, the first output port of the third main continuous laser 105, and ports 1 and 2 of the first main photodetector 108, respectively. Ports 1, 2, 3, and 4 of the second main optical coupler 109 are connected to the second output port of the third main continuous laser 105, the output port of the fourth main continuous laser 106, and ports 1 and 2 of the second main photodetector 110, respectively. The first, second, and third ports of the main millimeter-wave polarization multiplexer 111 are respectively connected to the output port of the first main photodetector 108, the first port of the main millimeter-wave mixer 113, and the input port of the main millimeter-wave antenna 112. The second and third ports of the main millimeter-wave mixer 113 are respectively connected to the output port of the second main photodetector 110 and the input port of the main time measurement and modulator 114. The output port of the main time measurement and modulator 114 is connected to the input port of the main microwave antenna 115.

[0036] The master end is locked to the millimeter-wave signal of the optical reference, specifically:

[0037] The primary optical frequency comb 103 is locked to the primary optical reference;

[0038] The second main continuous laser 104, the third main continuous laser 105, and the fourth main continuous laser 106 respectively lock the f of the main optical frequency comb through an optical phase-locked loop or injection locking method. 0,m ,f 1,m f 2,m In terms of frequency, the second master continuous laser 104 and the third master continuous laser 105, after passing through the first master optical coupler 107, beat on the first master photodetector 108 to obtain a millimeter-wave signal f. mm1,m The signal is sent to the slave end; simultaneously, the third master continuous laser 105 and the fourth master continuous laser 106, after passing through the second master optical coupler 109, beat on the second master photodetector 110 to obtain a millimeter-wave signal f. mm2,m It is used for local beat frequencies with millimeter waves received.

[0039] The slave end 2 includes a slave optical frequency reference, a master optical frequency comb 203, a second slave continuous laser 204, a third slave continuous laser 205, a fourth slave continuous laser 206, a first slave optical coupler 207, a first slave photodetector 208, a second slave optical coupler 209, a second slave photodetector 210, a slave millimeter-wave polarization multiplexer 211, a slave millimeter-wave antenna 212, a slave millimeter-wave mixer 213, a slave time measurement and demodulation unit 214, and a slave microwave antenna 215.

[0040] Among them, the optical frequency reference includes a first continuous laser 201, an optical reference cavity 202, and an optical atomic clock.

[0041] The optical frequency comb 203 is locked to the optical frequency reference, and the second continuous laser 204, the third continuous laser 205, and the fourth continuous laser 206 are locked to the f-frequency of the intermediate optical frequency comb via optical phase-locked loops or injection locking methods, respectively. 0,m ,f 1,m f 2,m In terms of frequency, ports 1, 2, 3, and 4 of the first slave optical coupler 207 are respectively connected to the output port of the second slave continuous laser 204, the first output port of the third slave continuous laser 205, and ports 1 and 2 of the first slave photodetector 208; and ports 1, 2, 3, and 4 of the second slave optical coupler 209 are respectively connected to the second output port of the third slave continuous laser 205, the output port of the fourth slave continuous laser 206, and ports 1 and 2 of the second slave photodetector 210. The first, second, and third ports of the millimeter-wave polarization multiplexer 211 are respectively connected to the output port of the first photodetector 208, the first port of the millimeter-wave mixer 213, and the input port of the millimeter-wave antenna 112. The second and third ports of the millimeter-wave mixer 213 are respectively connected to the output port of the second photodetector 210 and the input port of the time measurement and demodulator 214. The output port of the time measurement and demodulator 214 is connected to the output port of the microwave antenna 215.

[0042] The millimeter-wave signal, from end-locked to the optical reference, is specifically:

[0043] From the optical frequency comb 203 locked to the optical reference, the second continuous laser 204, the third continuous laser 205, and the fourth continuous laser 206 are respectively locked to the f-axis of the optical frequency comb via optical phase-locked loops or injection locking methods. 0,s ,f 1,s f 2,sIn terms of frequency, the second continuous laser 204 and the third continuous laser 105, after passing through the first optical coupler 207, beat on the first photodetector 208 to obtain the millimeter-wave signal f. mm1,s The signal is sent to the master terminal; simultaneously, the third slave continuous laser 205 and the fourth slave continuous laser 206, after passing through the second slave optical coupler 209, beat on the second slave photodetector 210 to obtain a millimeter-wave signal f. mm2,s It is used for local beat frequencies with millimeter waves received.

[0044] The main terminal 1 transmits the second main continuous laser 104 and the third main continuous laser 105 through the first main optical coupler 107, and then beats the signal f on the first main photodetector 108 to obtain a millimeter-wave signal f. mm1,m millimeter wave signal f mm1,m The millimeter-wave signal reaches the slave end 2 via the main millimeter-wave polarization multiplexer 111, the main millimeter-wave antenna 112, and the free millimeter-wave space link 301. The millimeter-wave signal received at the slave end passes through the slave millimeter-wave antenna 212, the slave millimeter-wave polarization multiplexer 211, the third slave continuous laser 205, and the fourth slave continuous laser 206, and then through the second slave optical coupler 209 before being beat on the second slave photodetector 210 to obtain the millimeter-wave signal f. mm2,s The time difference T of the output signal from the millimeter-wave mixer 213 is measured by the time measurement and demodulation unit 214 at the two input terminals of the millimeter-wave mixer 213. AB .

[0045] Similarly, the slave end 2 transmits the second slave continuous laser 204 and the third slave continuous laser 205 through the first slave optical coupler 207 to the first slave photodetector 208 to obtain the millimeter-wave signal f. mm1,s millimeter wave signal f mm1,s The millimeter-wave signal, after passing through the millimeter-wave polarization multiplexer 211, the millimeter-wave antenna 212, and the free-space millimeter-wave link 301, reaches the main terminal 1. The millimeter-wave signal received at the main terminal passes through the main millimeter-wave antenna 112, the main millimeter-wave polarization multiplexer 111, the third main continuous laser 105, and the fourth main continuous laser 106, and then through the second main optical coupler 109, before being beat on the second main photodetector 110 to obtain the millimeter-wave signal f. mm2,m The main millimeter-wave mixer 113 has two input terminals respectively, and the main time measurement and modulator 114 measures the time difference T of the output signal of the main millimeter-wave mixer 113. BA At the same time, the measured TAB The time is transmitted to the slave end 2 via the master time measurement and modulator 114, the master microwave antenna 115, and the free space microwave link 302. The slave end 2 demodulates the T measured by the master end through the slave microwave antenna 215 and the slave time measurement and demodulator 214. BA .

[0046] The slave end 2 calculates the clock difference ΔT between the master and slave ends based on the bidirectional comparison principle, where ΔT = (T AB - T BA ) / 2, and adjust the frequency and phase of the optical reference (i.e., the first continuous laser 201, the optical reference cavity 202, and the optical atomic clock) to make ΔT→0, thereby achieving time and frequency synchronization between the master and slave ends.

[0047] Example 2:

[0048] Please refer to Figure 2, which is a schematic diagram of Embodiment 2 of the high-precision spatial-temporal-frequency comparison system based on bidirectional optically generated millimeter waves of different frequencies according to the present invention. Unlike Embodiment 1, in Embodiment 2, only two continuous lasers at the master end 1 and the slave end 2 are locked to an optical frequency comb via injection locking or an optical phase-locked loop. The two continuous lasers at the master end 1 and the two continuous lasers at the slave end 2 generate millimeter-wave signals of different frequencies on a photodetector. The master end 1 and the slave end 2 can directly mix the signals on a millimeter-wave mixer to generate an output signal for time difference measurement.

[0049] Specifically,

[0050] The main terminal 1 includes a main optical frequency reference, a main optical frequency comb 103, a second main continuous laser 104, a third main continuous laser 105, a main optical coupler 106, a main photodetector 107, a main millimeter-wave power divider 108, a main millimeter-wave polarization multiplexer 109, a main millimeter-wave antenna 110, a main millimeter-wave mixer 111, a main time measurement and modulation unit 112, and a main microwave antenna 113.

[0051] The main optical frequency reference includes a first main continuous laser 101, a main optical reference cavity 102, and a main optical atomic clock. The first, second, third, and fourth ports of the main optical coupler 106 are connected to the output port of the second main continuous laser 104, the first output port of the third main continuous laser 105, and the first and second ports of the main photodetector 107, respectively. The input port and the first and second output ports of the main millimeter-wave power divider 108 are connected to the input port of the main photodetector 107, the first port of the main millimeter-wave polarization multiplexer 109, and the second input port of the main millimeter-wave mixer 111, respectively. The second and third ports of the main millimeter-wave polarization multiplexer 109 are connected to the second port of the main millimeter-wave mixer 111 and the input terminal of the main millimeter-wave antenna 112, respectively. The output port of the main millimeter-wave mixer 111 is connected to the input port of the main time measurement and modulator 112, and the output port of the main time measurement and modulator 112 is connected to the main microwave antenna 113.

[0052] The master end is locked to the millimeter-wave signal of the optical reference, specifically:

[0053] The primary optical frequency comb 103 is locked to the primary optical reference; the second primary continuous laser 104 and the third primary continuous laser 105 are respectively locked to the f-frequency of the primary optical frequency comb via optical phase-locked loops or injection locking. 0,m ,f 1,m In terms of frequency, the second master continuous laser 104 and the third master continuous laser 105, after passing through the master optical coupler 106, beat on the master photodetector 107 to obtain a millimeter-wave signal f. mm,m , used to send to the slave end.

[0054] The slave end 2 includes a slave optical frequency reference, a master optical frequency comb 203, a second slave continuous laser 204, a third slave continuous laser 205, a slave optical coupler 206, a slave photodetector 207, a slave millimeter-wave power divider 208, a slave millimeter-wave polarization multiplexer 209, a slave millimeter-wave antenna 210, a slave millimeter-wave mixer 211, a slave time measurement and demodulation unit 212, and a slave microwave antenna 213.

[0055] Among them, the optical frequency reference includes a first continuous laser 201, an optical reference cavity 202, and an optical atomic clock.

[0056] The main optical frequency reference includes a first slave continuous laser 201, a slave optical reference cavity 202, and a main optical atomic clock. The first, second, third, and fourth ports of the optical coupler 206 are respectively connected to the output port of the second continuous laser 204, the first output port of the third continuous laser 205, and the first and second ports of the photodetector 207. The input port and the first and second output ports of the millimeter-wave power divider 208 are respectively connected to the input port of the photodetector 207, the first port of the millimeter-wave polarization multiplexer 209, and the second input port of the millimeter-wave mixer 211. The second and third ports of the millimeter-wave polarization multiplexer 209 are respectively connected to the second port of the millimeter-wave mixer 211 and the input terminal of the millimeter-wave antenna 212. The output port of the millimeter-wave mixer 211 is connected to the input port of the time measurement and demodulator 212. The output port of the master time measurement and demodulator 112 is connected to the microwave antenna 213.

[0057] The millimeter-wave signal, from end-locked to the optical reference, is specifically:

[0058] From the optical frequency comb 203 locked to the optical reference, the second continuous laser 204 and the third continuous laser 205 are locked to the frequency f from the optical frequency comb via optical phase-locked loops or injection locking methods, respectively. 0,s ,f 1,s In terms of frequency, the second continuous laser 204 and the third continuous laser 205, after passing through the optical coupler 206, beat on the photodetector 207 to obtain the millimeter-wave signal f. mm,s , used to send to the master.

[0059] Specific workflow:

[0060] The main terminal 1 transmits the second main continuous laser 104 and the third main continuous laser 105 through the main optical coupler 106 to the main photodetector 107 to obtain a millimeter-wave signal f. mm,m The millimeter-wave signal passes through the main millimeter-wave power divider 108, the main millimeter-wave polarization multiplexer 111, the main millimeter-wave antenna 112, and the free-space millimeter-wave link 301 to reach the slave end 2. The millimeter-wave signal received at the slave end passes through the slave millimeter-wave antenna 212, the slave millimeter-wave polarization multiplexer 211, the second slave continuous laser 204, and the third slave continuous laser 205, and then through the slave optical coupler 206 to beat on the slave photodetector 207 to obtain the millimeter-wave signal f. mm,sThe time difference T between the output signals of the millimeter-wave mixer 211 and the time measurement and demodulation unit 212 is respectively reached at the two input terminals of the millimeter-wave mixer 211. AB .

[0061] Similarly, the slave end 2 transmits the second slave continuous laser 204 and the third slave continuous laser 205 through the slave optical coupler 206 and then beats the signal on the slave photodetector 207 to obtain the millimeter-wave signal f. mm,s The millimeter-wave signal passes through the millimeter-wave power divider 208, the millimeter-wave polarization multiplexer 211, the millimeter-wave antenna 212, and the O1 millimeter-wave link 3 to reach the main terminal 1. The millimeter-wave signal received at the main terminal passes through the main millimeter-wave antenna 112, the main millimeter-wave polarization multiplexer 111, the second main continuous laser 104, and the third main continuous laser 105, and then through the main optical coupler 106 before being beat on the main photodetector 107 to obtain the millimeter-wave signal f. mm,m The main millimeter-wave mixer 111's two input terminals are respectively connected to the main millimeter-wave mixer 111, and the main time measurement and modulator 112 measures the time difference T of the output signal of the main millimeter-wave mixer 111. BA At the same time, the measured T BA The time is transmitted to the slave end 2 via the master time measurement and modulator 112, the master microwave antenna 113, and the free space microwave link 302. The slave end 2 demodulates the T measured by the master end through the slave microwave antenna 215 and the slave time measurement and demodulator 214. BA .

[0062] The slave end 2 calculates the clock difference ΔT between the master and slave ends based on the bidirectional comparison principle, where ΔT = (T AB -T BA ) / 2, and adjust the optical reference frequency and phase of the slave end to make ΔT→0, thereby achieving time and frequency synchronization between the master and slave ends.

Claims

1. A spatial-temporal-frequency comparison method based on photogenerated millimeter waves, characterized in that, The steps include the following: The master end sends the millimeter-wave signal locked to the optical reference to the slave end through the free-space millimeter-wave link, while the slave end sends the millimeter-wave signal locked to the optical reference to the master end through the millimeter-wave link. The time difference T between the millimeter-wave signal locally locked to the optical reference measured at the master end and the millimeter-wave signal locked to the optical reference received at the slave end. BA The signal is transmitted to the slave end via a free-space microwave link. Simultaneously, the slave end measures the time difference T between the millimeter-wave signal locally locked to the optical reference and the millimeter-wave signal received by the master end. AB ; The slave end calculates the clock difference ΔT between the master and slave ends based on the bidirectional comparison principle, ΔT = (T AB -T BA ) / 2, and adjust the optical reference frequency and phase of the slave end to make ΔT→0, thereby achieving time and frequency synchronization between the master and slave ends.

2. The high-precision spatial-temporal-frequency comparison method based on photogenerated millimeter waves according to claim 1, characterized in that, The millimeter-wave signal locked to the optical reference at the master or slave end is specifically: - Obtain an optical frequency comb locked to a master or slave optical reference, wherein the master or slave optical frequency comb is locked to the master or slave optical reference, the master or slave optical reference comprising an ultrastable laser and an optical atomic clock; the optical frequency comb has N frequencies f i,m Each laser is locked by N consecutive lasers, where N = 2 or 3, and i = 0, ..., N; - A millimeter-wave signal is obtained by heterodyne beat frequency from a continuous laser locked to an optical frequency comb.

3. The high-precision spatial-temporal-frequency comparison method based on photogenerated millimeter waves according to claim 2, characterized in that, The optical frequency is used to obtain a millimeter-wave signal via heterodyne beat frequency, specifically: After the output light of the nth continuous laser is coupled to the output light of the (n+1)th continuous laser, a millimeter-wave signal is obtained by beating the photodetector, which is used to transmit to the other end and / or to beat the received millimeter wave locally.

4. The high-precision spatial-temporal-frequency comparison method based on photogenerated millimeter waves according to claim 3, characterized in that, The time difference T between the millimeter-wave signal locally locked to the optical reference measured by the master end and the millimeter-wave signal locked to the optical reference received by the slave end is measured. BA Alternatively, the time difference T between the millimeter-wave signal locally locked to the optical reference measured at the slave end and the millimeter-wave signal locked to the optical reference received at the master end. AB Specifically: The main terminal up-frequency acquires a millimeter-wave signal f from the main photodetector. mm1,m The signal is transmitted to the slave end via a free-space millimeter-wave link. The output light from two slave continuous lasers is coupled and beats on the slave photodetector to obtain the millimeter-wave signal f. mm2,s Millimeter-wave signals f are obtained from time measurement and demodulation. mm1,m and millimeter wave signal f mm2,s Time difference T after mixing AB ; The slave device will obtain the millimeter-wave signal f from the photodetector via beat frequency. mm1,s The signal is transmitted to the main terminal via a free-space millimeter-wave link. The two main continuous lasers at the main terminal are coupled together, and the millimeter-wave signal f is obtained by beat frequency on the main photodetector. mm2,m Millimeter-wave signal f is obtained using master time measurement and modulator. mm2,s and millimeter wave signal f mmi+1,m Time difference T after mixing BA .

5. A high-precision spatial-temporal-frequency comparison system based on photogenerated millimeter waves, comprising a master end, a slave end, and a free-space link connecting the master end and the slave end, characterized in that, The free-space links include free-space millimeter-wave links and free-space microwave links; Both the master and slave ends include an optical frequency reference, an optical frequency comb, N consecutive lasers (N=2 or 3), a first optical coupler, a first photodetector, a second optical coupler, a second photodetector, a millimeter-wave polarization multiplexer, a millimeter-wave antenna, a millimeter-wave mixer, a time measurement and modulator, and a microwave antenna. The optical frequency reference includes at least an ultrastable laser and an optical atomic clock. The optical frequency comb is locked to the optical frequency reference. The N consecutive lasers are respectively locked to the N frequencies f of the main optical frequency comb through optical phase-locked loops or injection locking. i,m Above, i=0,...,N; After the output light of the first continuous laser is coupled to the output light of the second continuous laser, a first millimeter-wave signal is obtained by frequency beat on the first photodetector, which is used to transmit to the other end; after the output light of the second continuous laser is coupled to the output light of the third continuous laser, a second millimeter-wave signal is obtained by frequency beat on the second photodetector, which is used to beat with the millimeter-wave signal received from the other end. The first millimeter-wave signal passes sequentially through the millimeter-wave polarization multiplexer, millimeter-wave antenna, and free-space millimeter-wave link to reach the other end. The output light from the millimeter-wave antenna and millimeter-wave polarization multiplexer at the other end, along with the output light from the second and third continuous lasers at the other end, passes through the second optical coupler and is beat-frequencyd on the second photodetector to obtain the second millimeter-wave signal, which then reaches the two input terminals of the millimeter-wave mixer. The time difference of the output signal of the millimeter-wave mixer is measured using a time measurement and demodulation unit, i.e., the time difference of the millimeter-wave signal is measured at the master end and the slave end respectively. The master end transmits the time difference of the millimeter-wave signal measured by the master end to the slave end through the free space microwave link, and the slave end demodulates to obtain the time difference of the millimeter-wave signal measured by the master end. The slave end adjusts its optical reference frequency and phase based on the clock difference between the time difference of the millimeter-wave signal measured by the demodulation master end and the time difference of the millimeter-wave signal measured by the local slave end, so that the clock difference is infinitely close to 0, thereby achieving time and frequency synchronization between the master and slave ends.

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