Low modulation depth laser modulation and demodulation system

Abstract

The invention relates to a low-modulation depth laser modulation and demodulation system, which comprises a low-modulation depth laser modulation and demodulation system A and a low-modulation depth laser modulation and demodulation system B. The low-modulation depth laser modulation and demodulation system A and the low-modulation depth laser modulation and demodulation system B are respectively configured in different satellites. According to the method, the two EOMs are adopted at the local end and the far end respectively to modulate the received ranging and communication signals, so that local oscillation optical signals at the receiving end do not contain pseudo codes and communication signals, aliasing of the pseudo codes in output signals of a photoelectric detector is avoided, the signal-to-noise ratio of pseudo code signals is increased, and then the ranging precision is improved.

Description

Technical Field

[0001] This invention relates to a low modulation depth laser modulation and demodulation system, belonging to the field of time and frequency. Background Technology

[0002] Space-based gravitational wave detection missions require the use of Time Delay Interferometer (TDI) technology to suppress laser frequency noise below shot noise. Achieving TDI technology necessitates accurate measurement of absolute distances between satellites and inter-satellite communication. Therefore, distance measurement and communication technologies are crucial components of space-based gravitational wave detection systems and are among the core technologies enabling such systems to conduct gravitational wave observations. Compared to common ranging systems, the signal waveform of a gravitational wave detection system includes an interferometric main carrier, a clock sideband signal, and a ranging signal. After mixed reception, these three signals overlap, resulting in differences from currently used inter-satellite / satellite-to-ground ranging techniques. This necessitates the research of novel laser modulation and demodulation systems and methods. Summary of the Invention

[0003] The technical problem solved by this invention is to overcome the shortcomings of the prior art and provide a low modulation depth laser modulation and demodulation system. Two EOMs are used at the local and remote ends to modulate the received ranging and communication signals, so that the local oscillator light signal at the receiving end does not contain pseudo-code and communication signals, avoiding the aliasing of pseudo-code in the output signal of the photodetector, improving the signal-to-noise ratio of the pseudo-code signal, and thus improving the ranging accuracy. The technical solution of this invention is: A low-modulation-depth laser modulation and demodulation system includes two main parts: a low-modulation-depth laser modulation and demodulation system A and a low-modulation-depth laser modulation and demodulation system B, which are respectively configured on different satellites; The low-modulation depth laser modulation and demodulation system A includes a first clock, a first frequency synthesizer, a first digital signal processing unit, a first photodetector, a first laser, a first beam splitter, a first EOM, a first beam combiner, a first combiner, a second EOM, a first circulator, and a first optical transceiver module. The low-modulation depth laser modulation and demodulation system B includes a second clock, a second frequency synthesizer, a second digital signal processing unit, a second photodetector, a second laser, a second beam splitter, a third EOM, a second beam combiner, a second combiner, a fourth EOM, a second circulator, a second optical transceiver module, and an optical phase-locked loop. In the low modulation depth laser modulation and demodulation system A, a first clock provides a time and frequency reference signal, a first frequency synthesizer receives the time and frequency reference signal generated by the first clock, and generates the radio frequency signal required by the second EOM and the first combiner, as well as the reference clock signal required by the first digital signal processing unit; a first laser provides an optical carrier signal, which is split into two paths by the first beam splitter and sent to the first EOM and the second EOM respectively. The first digital signal processing unit receives the reference clock signal generated by the first frequency synthesizer, generates pseudo-code and data signals, and sends the pseudo-code and data signals to the first combiner; and analyzes the output signal of the first photodetector to calculate the ranging value. The first combiner combines the radio frequency signal generated by the first frequency synthesizer and the pseudo-code and data signals generated by the first digital signal processing unit and sends them to the first EOM; the first EOM modulates the signal output from the first combiner onto the laser carrier transmitted by the first beam splitter and sends it to the first optical transceiver module through the first circulator; the first optical transceiver module sends the modulated laser carrier signal to the second optical transceiver module; simultaneously, the first optical transceiver module receives the modulated laser carrier signal from the low modulation depth laser modulation and demodulation system B sent by the second optical transceiver module and sends it to the first combiner through the first circulator; the modulated laser carrier signal from the low modulation depth laser modulation and demodulation system B is the laser modulation signal transmitted by the remote satellite; The second EOM modulates the radio frequency signal provided by the first frequency synthesizer onto the laser carrier provided by the first beam splitter, and then sends it to the first beam combiner; the first beam combiner combines the two signals and sends them to the first photodetector. The combined signal includes the pseudorange measurement signal and the clock sideband signal.

[0004] Furthermore, the system composition and operation mode of the low modulation depth laser modulation and demodulation system B are the same as those of the low modulation depth laser modulation and demodulation system A.

[0005] Furthermore, the first laser is an ultra-stable laser, serving as the master laser in the entire modulation and demodulation system, while the second laser is a tunable laser, serving as a servo laser in the entire modulation and demodulation system.

[0006] Furthermore, by setting an optical phase-locked loop in the low modulation depth laser modulation and demodulation system B to adjust the frequency and phase of the second laser, the second laser is locked to the laser carrier signal sent by the low modulation depth laser modulation and demodulation system A, and thus indirectly locked to the first laser.

[0007] Furthermore, the first frequency synthesizer multiplies the frequency signal generated by the first clock to the GHz level. The first digital signal processing unit generates a baseband signal. The baseband signal and the radio frequency signal are combined by the first combiner, then pass through the first EOM and the first circulator, and finally sent to the second optical transceiver module through the first optical transceiver module. The signal received by the second optical transceiver module is sent to the second beam combiner through the second circulator. It is then combined with the signal generated by the second laser and arrives at the second beam combiner through the fourth EOM and enters the second photodetector. The output signal of the second photodetector enters the second digital signal processing unit. The second digital processing unit adjusts the second laser based on the phase difference between the beat frequency signals of the two lasers and the signal after mixing and filtering the local radio frequency signal of the second digital signal processing unit, so that the second laser is indirectly locked to the first laser. At the same time, the unidirectional time delay T from the low modulation depth laser modulation and demodulation system A to B is measured. AB And the one-way time delay measurement T from B to A BA Finally, the clock difference between the first clock and the second clock is obtained.

[0008] Furthermore, the generation of GHz-level radio frequency signals involves the following steps: 1) Calculate the amplitude of the radio frequency signal that needs to be generated: (1) Among them, P sb It is the power of the radio frequency signal, measured in dBm. It is a system of adjustment, V hw It is the half-wave voltage of EOM.

[0009] 2) The frequency signal generated by the first clock is multiplied to the GHz level using a frequency synthesizer; The specific steps for generating pseudocode and data signals are as follows: 1) Calculate the amplitude of the pseudocode to be generated, using the following formula: (2) Among them, P pn This is the power of the pseudocode, measured in dBm. It is a pseudo-code modulation system, V hw It is the half-wave voltage of EOM; 2) The pseudocode and data signals are generated by the digital signal processing unit, which are the baseband signals.

[0010] Furthermore, a second photodetector is used to receive the laser modulation signal transmitted from the remote end, specifically as follows: 1) After receiving the modulated signal from the remote end, the second optical transceiver module sends it to the second beam combiner via the second circulator; 2) Adjust the second laser by using an optical phase-locked loop to make the signal generated by the second laser heterodyne interfere with the main frequency of the signal received by the second optical transceiver module; the main frequency of the beat frequency signal should be as close as possible to the center frequency of the bandwidth of the second photodetector. 3) The second laser splits the laser signal into two beams via the second beam splitter. One beam of laser signal is sent as the local oscillator signal to the second photodetector via the fourth EOM and the second beam combiner. The other laser signal is sent as the signal light to the third EOM. 4) The combined signal output from the second beam combiner enters the second photodetector.

[0011] Furthermore, the output signal of the second photodetector is processed to calculate the distance, clock error, and data information. The specific steps are as follows: 1) The output signal of the second photodetector enters the second digital signal processing unit to measure the unidirectional time delay T from the low modulation depth laser modulation and demodulation system A to B. AB The data information is calculated, which includes the time delay value T measured by the remote satellite. BA Finally, the clock difference ΔT between the two clocks is obtained. 21 = (T AB -T BA ) / 2-ΔT L ; where ΔT L This is the sum of errors such as link non-reciprocity and device latency. 2) The second digital signal processing unit adjusts the second laser based on the phase difference between the beat frequency signals of the two lasers and the signal after mixing and filtering the local radio frequency signal of the second digital signal processing unit, so that the second laser is indirectly locked onto the first laser.

[0012] Furthermore, the output signal of the first photodetector is processed to calculate the distance, clock error, and data information. The specific steps are as follows: 1) The output signal of the first photodetector enters the first digital signal processing unit to measure the one-way time delay T from the low modulation depth laser modulation and demodulation system B to A. BA The data information is calculated, which includes the time delay value T measured by the remote satellite. AB ; 2) Calculate the clock difference ΔT between the two clocks. 12 = (T BA -T AB ) / 2+ΔT L ΔT L It is the sum of errors such as link non-reciprocity and device delay.

[0013] The advantages of this invention compared to the prior art are: The present invention proposes a low-modulation-depth laser modulation and demodulation system and method, which uses two EOMs at the local and remote ends to modulate and demodulate the received ranging and communication signals, so that the local oscillator optical signal at the receiving end does not contain pseudo-code and communication signals, avoids the aliasing of pseudo-code in the output signal of the photodetector, improves the signal-to-noise ratio of the pseudo-code signal, and thus improves the ranging accuracy. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of a low-modulation-depth laser modulation and demodulation system. Detailed Implementation

[0015] The specific embodiments of the present invention will now be described in further detail with reference to the accompanying drawings.

[0016] This invention provides a low modulation depth laser modulation and demodulation system, such as Figure 1 As shown, it includes: The system consists of two main parts: a low-modulation-depth laser modulation and demodulation system A and a high-modulation-depth laser modulation and demodulation system B. The low-modulation-depth laser modulation and demodulation system A and the low-modulation-depth laser modulation and demodulation system B are configured on different satellites.

[0017] The low-modulation depth laser modulation and demodulation system A includes a clock 1, a frequency synthesizer 1, a digital signal processing unit 1, a photodetector 1, a laser 1, a beam splitter 1, an EOM 1, a beam combiner 1, a combiner 1, an EOM 2, a circulator 1, and an optical transceiver module 1. It primarily performs signal modulation and demodulation, transmission and reception, and signal processing functions.

[0018] The low-modulation-depth laser modulation and demodulation system B includes a clock 2, a frequency synthesizer 2, a digital signal processing unit 2, a photodetector 2, a laser 2, a beam splitter 2, an EOM 3, a beam combiner 2, a combiner 2, an EOM 4, a circulator 2, an optical transceiver module 2, and an optical phase-locked loop. It primarily performs signal modulation and demodulation, transmission and reception, and signal processing functions.

[0019] The system composition and operation of the low-modulation-depth laser modulation and demodulation system B are basically the same as those of the low-modulation-depth laser modulation and demodulation system A. System B has one more optical phase-locked loop than system A. Laser 1 is an ultra-stable laser, serving as the master laser in the entire modulation and demodulation system, while laser 2 is a tunable laser, serving as the servo laser. By setting up an optical phase-locked loop in the low-modulation-depth laser modulation and demodulation system B to adjust the frequency and phase of laser 2, laser 2 is locked to the laser carrier signal sent by the low-modulation-depth laser modulation and demodulation system A, indirectly locking to laser 1.

[0020] Specifically as follows: In the low modulation depth laser modulation and demodulation system A, clock 1 provides a time and frequency reference signal, frequency synthesizer 1 receives the time and frequency reference signal generated by clock 1, and generates the radio frequency signal required by EOM2 and combiner 1, as well as the reference clock signal required by digital signal processing unit 1; laser 1 provides an optical carrier signal, which is split into two paths by beam splitter 1 and sent to EOM1 and EOM2 respectively. The digital signal processing unit 1 receives the reference clock signal generated by the frequency synthesizer 1, generates pseudo-code and data signals, and sends the pseudo-code and data signals to the combiner 1; and analyzes the output signal of the photodetector 1 to calculate the ranging value. The combiner 1 combines the radio frequency signal generated by the frequency synthesizer 1 and the pseudo-code and data signal generated by the digital signal processing unit and sends them to the EOM1; the EOM1 modulates the signal output from the combiner 1 onto the laser carrier transmitted by the beam splitter 1 and sends it to the optical transceiver module 1 through the circulator 1; the optical transceiver module 1 sends the modulated laser carrier signal to the optical transceiver module 2; at the same time, the optical transceiver module 1 receives the modulated laser carrier signal from the low modulation depth laser modulation and demodulation system B sent by the optical transceiver module 2 and sends it to the combiner 1 through the circulator 1; the modulated laser carrier signal from the low modulation depth laser modulation and demodulation system B is the laser modulation signal transmitted by the remote satellite; EOM2 modulates the radio frequency signal provided by frequency synthesizer 1 onto the laser carrier provided by beam splitter 1, and then sends it to beam combiner 1; beam combiner 1 combines the two signals and sends them to photodetector 1. The combined signal includes pseudorange measurement signal and clock sideband signal.

[0021] Frequency synthesizer 1 multiplies the frequency signal generated by clock 1 to the GHz level. Digital signal processing unit 1 generates a baseband signal. The baseband signal and the radio frequency signal are combined by combiner 1, then pass through EOM1 and circulator 1, and finally sent to optical transceiver module 2 through optical transceiver module 1. The signal received by optical transceiver module 2 is sent to beam combiner 2 through circulator 2. It is combined with the signal generated by laser 2 that reaches beam combiner 2 through EOM4 and then enters photodetector 2. The output signal of photodetector 2 enters digital signal processing unit 2. Digital processing unit 2 adjusts laser 2 according to the phase difference between the beat frequency signals of the two lasers and the signal after mixing and filtering the local radio frequency signal of digital signal processing unit 2, so that laser 2 is indirectly locked to laser 1. At the same time, the one-way time delay T from low modulation depth laser modulation and demodulation system A to B is measured. AB And the one-way time delay measurement T from B to A BA Finally, the clock difference between clocks 1 and 2 is obtained.

[0022] (1) Generate GHz-level radio frequency signals. The specific steps are as follows: 1) Based on formula (1) and considering the attenuation of the filter combiner, calculate the amplitude of the required RF signal.

[0023] (1) Among them, P sb It is the power of the radio frequency signal, measured in dBm. It is a system of adjustment, V hw It is the half-wave voltage of EOM.

[0024] 2) The frequency signal generated by clock 1 is multiplied to the GHz level by a frequency synthesizer.

[0025] 3) The frequency signals generated by the two frequency synthesizers A and B in the low modulation depth laser modulation and demodulation system differ by 1MHz-10MHz. The specific frequency point needs to be combined with the requirements of ranging accuracy on code rate, with the criterion of not affecting pseudocode measurement.

[0026] (2) Generate pseudocode and data signals. The specific steps are as follows: 1) Based on the modulation index formula, and considering the attenuation of the filter combiner, calculate the amplitude of the pseudocode to be generated. The modulation index formula is as follows: (2) Among them, P pn This is the power of the pseudocode, measured in dBm. It is a pseudo-code modulation system, V hw It is the half-wave voltage of EOM.

[0027] 2) The pseudocode and data signals, referred to as baseband signals, are generated by the digital signal processing unit. The code rate of this baseband signal needs to be considered in combination with the ranging accuracy of the pseudocode and the bandwidth of the detector.

[0028] (3) The low-modulation-depth laser modulation and demodulation system A combines the pseudo-code and data signals with the radio frequency signals in combiner 1 and then sends them to EOM1. The specific steps are as follows: 1) The pseudocode and data signals are sent to the combiner 1 after passing through the low-pass filter inside the digital signal processing unit 1; 2) The radio frequency signal generated by frequency synthesizer 1 is sent to combiner 1 after passing through a bandpass filter; 3) The pseudo code and data signal are combined with the radio frequency signal in combiner 1 and then sent to EOM1.

[0029] (4) The modulated signal output from EOM1 is transmitted via the optical transceiver module. The specific steps are as follows: 1) The modulated signal output by EOM1 is sent to circulator 1, and the output signal of circulator 1 is sent to optical transceiver module 1; 2) The output signal of optical transceiver module 1 is sent to the remote optical transceiver module 2.

[0030] (5) Use photodetector 2 to receive the laser modulation signal sent from the remote end.

[0031] 1) After receiving the modulated signal from the remote end, the optical transceiver module 2 sends it to the beam combiner 2 via the optical circulator 2; 2) Adjust laser 2 using an optical phase-locked loop to make the signal generated by laser 2 heterodyne-interfere with the main frequency of the signal received by optical transceiver module 2. The main frequency of the beat frequency signal should be as close as possible to the center frequency of the detector bandwidth; 3) The laser 2 splits the signal into two beams via the beam splitter 2. One beam is sent as the local oscillator signal to the photodetector 2 via the EOM4 and the beam combiner 2, and the other beam is sent as the signal light to the EOM3. 4) The output signal of the beam combiner 2 enters the photodetector 2.

[0032] (6) Process the output signal of photodetector 2 to calculate the distance, clock error, and data information. The specific steps are as follows: 1) The output signal of photodetector 2 enters digital signal processing unit 2 to measure the unidirectional time delay T from low modulation depth laser modulation and demodulation system A to B. AB The data information is calculated, which includes the time delay value T measured by the remote satellite. BA After deducting errors such as link non-reciprocity error and device delay error, denoted here as ΔT, further subtract these errors. L， Finally, the clock difference ΔT between the two clocks, 2 and 1, is obtained. 21 = (T AB -T BA ) / 2-ΔT L ; 2) The digital signal processing unit adjusts laser 2 based on the phase difference between the beat frequency signals of the two lasers and the signal after mixing and filtering the local radio frequency signal of the digital signal processing unit, so that the laser is indirectly locked onto laser 1.

[0033] (7) The low-modulation-depth laser modulation and demodulation system B combines the pseudo-code and data signals with the radio frequency signals in combiner 2 and then sends them to EOM3. The specific steps are as follows: 1) The pseudocode and data signals are sent to the combiner 2 after passing through the low-pass filter inside the digital signal processing unit 2; 2) The radio frequency signal generated by frequency synthesizer 2 is sent to combiner 2 after passing through a bandpass filter; 3) The pseudo code and data signals are combined with the radio frequency signals in combiner 2 and then sent to EOM3.

[0034] (8) The modulated signal output from EOM3 is transmitted via optical transceiver module 2. The specific steps are as follows: 1) The modulated signal output by EOM3 is sent to circulator 2, and the output signal of circulator 2 is sent to optical transceiver module 2; 2) The output signal of optical transceiver module 2 is sent to the remote optical transceiver module 1.

[0035] (9) Use photodetector 1 to receive the laser modulation signal transmitted from the remote end. The specific steps are as follows: 1) After receiving the modulated signal from the remote end, the optical transceiver module 1 sends it to the beam combiner 1 via the optical circulator 1; 2) The laser 1 splits the signal into two beams via the beam splitter 1. One beam is sent as the local oscillator signal to the photodetector 1 via the EOM2 and the beam combiner 1, and the other beam is sent as the signal light to the EOM1. 3) The output signal of the beam combiner 1 enters the photodetector 1.

[0036] (10) Process the output signal of photodetector 1 to calculate the distance, clock error, and data information. The specific steps are as follows: 1) The output signal of photodetector 1 enters digital signal processing unit 1 to measure the one-way time delay T from low modulation depth laser modulation and demodulation system B to A. BA The data information is calculated, which includes the time delay value T measured by the remote satellite. AB ; 2) Further deduct errors such as link non-reciprocity error and device delay error, denoted here as ΔT L， Finally, the clock difference ΔT between the two atomic clocks, 1 and 2, is obtained. 12 = (T BA -T AB ) / 2+ΔT L .

[0037] Example: (1) Generate GHz-level radio frequency signals. The specific steps are as follows: 1) According to formula (1), with a signal modulation of 15% and a half-wave voltage of EOM of 6V, P is calculated. sb ≈10.6dBm; 2) The 10MHz signal generated by clock 1 is multiplied to 2.4GHz by frequency synthesizer 1; 3) The 10MHz signal generated by clock 2 is multiplied to 2.401GHz by frequency synthesizer 2.

[0038] (2) Generate pseudocode and data signals. The specific steps are as follows: 1) According to formula (2), with a pseudo-code modulation degree of 1.5% and a half-wave voltage of EOM of 6V, P is calculated to obtain pn ≈3.4dBm; 2) The digital signal processing unit generates pseudo-random codes and data signals, referred to as baseband signals. The pseudo-random code in this baseband signal is a Gold sequence with a code rate of 0.5 Mbps. 2048 modulation bits are used to encode one communication bit, resulting in a communication rate of 24.4 kbps.

[0039] (3) The low-modulation-depth laser modulation and demodulation system A combines the pseudo-code and data signals with the radio frequency signals in combiner 1 and then sends them to EOM1. The specific steps are as follows: 1) The baseband signal generated by the digital signal processing unit 1 is sent to the combiner 1 after passing through the low-pass filter inside the digital signal processing unit 1; 2) The radio frequency signal generated by frequency synthesizer 1 is sent to combiner 1 after passing through a bandpass filter; 3) The baseband signal and the radio frequency signal are combined in combiner 1 and then sent to EOM1.

[0040] (4) The modulated signal output from EOM1 is transmitted via the optical transceiver module. The specific steps are as follows: 1) The modulated signal output by EOM1 is sent to circulator 1, and the output signal of circulator 1 is sent to optical transceiver module 1.

[0041] 2) Optical transceiver module 1 sends the signal to the remote optical transceiver module 2.

[0042] 5) Fifth step: Use photodetector 2 to receive the laser modulation signal transmitted from the remote end. The specific steps are as follows: 1) After receiving the modulation signal from the optical transceiver module 1, the optical transceiver module 2 sends the signal to the beam combiner 2 via the optical circulator 2; 2) The laser 2 is adjusted via an optical phase-locked loop to perform heterodyne interference between the main frequency of the signal generated by the laser 2 and the main frequency of the signal received by the optical transceiver module 2. The center frequency of the detector is 10MHz, and the bandwidth is 20MHz. 3) The laser 2 splits the signal into two beams via the beam splitter 2. One beam is sent as the local oscillator signal to the photodetector 2 via the EOM4 and the beam combiner 2, and the other beam is sent as the signal light to the EOM3.

[0043] 6) Process the output signal of photodetector 2 to calculate the distance, clock error, and data information. The specific steps are as follows: 1) The output signal of photodetector 2 enters digital signal processing unit 2. After processing, digital signal processing unit 2 calculates the one-way time delay measurement value T from low modulation depth laser modulation and demodulation system A to B.AB Simultaneously, the data information is calculated, which includes the time delay value T measured by low-modulation-depth laser modulation and demodulation system A from low-modulation-depth laser modulation and demodulation system B to A. BA After deducting errors such as link non-reciprocity error and device delay error, denoted here as ΔT, further subtract these errors. L Finally, the clock difference ΔT between clocks 2 and 1 is obtained. 21 = (T AB -T BA ) / 2-ΔT L ; 2) The digital signal processing unit 2 adjusts the laser 2 according to the phase difference between the beat frequency signals of the two lasers and the local radio frequency signal generated by the digital signal processing unit 2 after mixing and filtering, so that the laser 2 is indirectly locked to the laser 1.

[0044] 7) The low-modulation-depth laser modulation and demodulation system B combines the pseudo-code and data signals with the RF signal at combiner 2 and then sends them to EOM3. The specific steps are as follows: 1) The baseband signal generated by the digital signal processing unit 2 is sent to the combiner 2 after passing through the low-pass filter inside the digital signal processing unit 2; 2) The radio frequency signal generated by frequency synthesizer 2 is sent to combiner 2 after passing through a bandpass filter; 3) The baseband signal and the radio frequency signal are combined in combiner 2 and then sent to EOM3.

[0045] 8) The modulated signal output from EOM3 is transmitted via optical transceiver module 2. The specific steps are as follows: 1) The modulated signal output by EOM3 is sent to circulator 2, and the output signal of circulator 2 is sent to optical transceiver module 2; 2) Optical transceiver module 2 sends the signal to the remote optical transceiver module 1.

[0046] 9) Use photodetector 1 to receive the laser modulation signal transmitted from the remote end. The specific steps are as follows: 1) After receiving the modulation signal from the remote optical transceiver module 2, the optical transceiver module 1 sends it to the beam combiner 1 via the optical circulator 1; 2) The laser 1 splits the signal into two beams via the beam splitter 1. One beam is sent as the local oscillator signal to the photodetector 1 via the EOM2 and the beam combiner 1, and the other beam is sent as the signal light to the EOM1. 3) The output signal of the beam combiner 1 enters the photodetector 1.

[0047] 10) Process the output signal of photodetector 1 to calculate the distance, clock error, and data information. The specific steps are as follows: 1) The output signal of photodetector 1 enters digital signal processing unit 1. After processing, digital processing unit 1 calculates the one-way time delay measurement value T from low modulation depth laser modulation and demodulation system B to A. BA Simultaneously, it calculates data information, which includes the time delay value T from low modulation depth laser modulation and demodulation system A to B, measured by low modulation depth laser modulation and demodulation system B. AB ; 2) Further deduct errors such as link non-reciprocity error and device delay error, denoted here as ΔT L Finally, the clock difference ΔT between the two atomic clocks, 1 and 2, was obtained. 12 = (T BA -T AB ) / 2+ΔT L .

[0048] The parts of this invention not described in detail are common knowledge to those skilled in the art.

Claims

1. A low modulation depth laser modulation and demodulation system, characterized in that, include: The system consists of two main parts: a low-modulation-depth laser modulation and demodulation system A and a low-modulation-depth laser modulation and demodulation system B. The low-modulation-depth laser modulation and demodulation system A and the low-modulation-depth laser modulation and demodulation system B are configured on different satellites. The low-modulation depth laser modulation and demodulation system A includes a first clock, a first frequency synthesizer, a first digital signal processing unit, a first photodetector, a first laser, a first beam splitter, a first EOM, a first beam combiner, a first combiner, a second EOM, a first circulator, and a first optical transceiver module. The low-modulation depth laser modulation and demodulation system B includes a second clock, a second frequency synthesizer, a second digital signal processing unit, a second photodetector, a second laser, a second beam splitter, a third EOM, a second beam combiner, a second combiner, a fourth EOM, a second circulator, a second optical transceiver module, and an optical phase-locked loop. In the low modulation depth laser modulation and demodulation system A, a first clock provides a time and frequency reference signal, a first frequency synthesizer receives the time and frequency reference signal generated by the first clock, and generates the radio frequency signal required by the second EOM and the first combiner, as well as the reference clock signal required by the first digital signal processing unit; a first laser provides an optical carrier signal, which is split into two paths by the first beam splitter and sent to the first EOM and the second EOM respectively. The first digital signal processing unit receives the reference clock signal generated by the first frequency synthesizer, generates pseudo-code and data signals, and sends the pseudo-code and data signals to the first combiner; and analyzes the output signal of the first photodetector to calculate the ranging value. The first combiner combines the radio frequency signal generated by the first frequency synthesizer and the pseudo-code and data signals generated by the first digital signal processing unit and sends them to the first EOM; the first EOM modulates the signal output from the first combiner onto the laser carrier transmitted by the first beam splitter and sends it to the first optical transceiver module through the first circulator; the first optical transceiver module sends the modulated laser carrier signal to the second optical transceiver module; simultaneously, the first optical transceiver module receives the modulated laser carrier signal from the low modulation depth laser modulation and demodulation system B sent by the second optical transceiver module and sends it to the first combiner through the first circulator; the modulated laser carrier signal from the low modulation depth laser modulation and demodulation system B is the laser modulation signal transmitted by the remote satellite; The second EOM modulates the radio frequency signal provided by the first frequency synthesizer onto the laser carrier provided by the first beam splitter, and then sends it to the first beam combiner; the first beam combiner combines the two signals and sends them to the first photodetector. The combined signal includes the pseudorange measurement signal and the clock sideband signal.

2. A low modulation depth laser modem system as claimed in claim 1, characterized in that: The system composition and operation mode of the low modulation depth laser modulation and demodulation system B are the same as those of the low modulation depth laser modulation and demodulation system A.

3. A low modulation depth laser modem system as claimed in claim 1, wherein: The first laser is an ultra-stable laser, serving as the master laser in the entire modulation and demodulation system. The second laser is a tunable laser, serving as a servo laser in the entire modulation and demodulation system.

4. A low modulation depth laser modem system as claimed in claim 3, characterized in that: By setting an optical phase-locked loop in the low modulation depth laser modulation and demodulation system B to adjust the frequency and phase of the second laser, the second laser is locked to the laser carrier signal sent by the low modulation depth laser modulation and demodulation system A, and is indirectly locked to the first laser.

5. A low modulation depth laser modem system as claimed in claim 1, wherein: The first frequency synthesizer multiplies the frequency signal generated by the first clock to the GHz level. The first digital signal processing unit generates a baseband signal. The baseband signal and the radio frequency signal are combined by the first combiner and then pass through the first EOM and the first circulator. Finally, they are sent to the second optical transceiver module through the first optical transceiver module. The signal received by the second optical transceiver module is sent to the second beam combiner through the second circulator. It is then combined with the signal generated by the second laser, which arrives at the second beam combiner via the fourth EOM, and enters the second photodetector. The output signal of the second photodetector enters the second digital signal processing unit. Based on the phase difference between the beat frequency signals of the two lasers and the signal after mixing and filtering the local RF signal of the second digital signal processing unit, the second digital processing unit adjusts the second laser to indirectly lock onto the first laser. Simultaneously, it measures the unidirectional time delay T from the low modulation depth laser modulation and demodulation system A to B. AB And the one-way time delay measurement T from B to A BA Finally, the clock difference between the first clock and the second clock is obtained.

6. A low modulation depth laser modem system as claimed in claim 5, characterized in that: The specific steps for generating GHz-level radio frequency signals are as follows: 1) Calculate the amplitude of the radio frequency signal that needs to be generated: （1） where P sb is the power of the radio frequency signal in dBm, is the modulation index, V hw is the half-wave voltage of the EOM. 2) The frequency signal generated by the first clock is multiplied to the GHz level using a frequency synthesizer; The specific steps for generating pseudocode and data signals are as follows: 1) Calculate the amplitude of the pseudocode to be generated, using the following formula: （2） Among them, P pn This is the power of the pseudocode, measured in dBm. It is a pseudo-code modulation system, V hw It is the half-wave voltage of EOM; 2) The pseudocode and data signals are generated by the digital signal processing unit, which are the baseband signals.

7. A low modulation depth laser modem system as claimed in claim 5, wherein: The second photodetector is used to receive the laser modulation signal transmitted from the remote end, specifically as follows: 1) After receiving the modulated signal from the remote end, the second optical transceiver module sends it to the second beam combiner via the second circulator; 2) Adjust the second laser by using an optical phase-locked loop to make the signal generated by the second laser heterodyne interfere with the main frequency of the signal received by the second optical transceiver module; the main frequency of the beat frequency signal should be as close as possible to the center frequency of the bandwidth of the second photodetector. 3) The second laser splits the laser signal into two beams via the second beam splitter. One beam of laser signal is sent as the local oscillator signal to the second photodetector via the fourth EOM and the second beam combiner. The other laser signal is sent as the signal light to the third EOM. 4) The beam combining signal output by the second beam combiner enters the second photodetector.

8. A low modulation depth laser modem system as claimed in claim 5, wherein: The output signal of the second photodetector is processed to calculate the distance, clock error, and data information. The specific steps are as follows: 1) The output signal of the second photodetector enters the second digital signal processing unit to measure the unidirectional time delay T from the low modulation depth laser modulation and demodulation system A to B. AB The data information is calculated, which includes the time delay value T measured by the remote satellite. BA Finally, the clock difference ΔT between the two clocks is obtained. 21 = (T AB -T BA ) / 2-ΔT L ; where ΔT L This is the sum of link non-reciprocity and device delay error; 2) The second digital signal processing unit adjusts the second laser based on the phase difference between the beat frequency signals of the two lasers and the signal after mixing and filtering the local radio frequency signal of the second digital signal processing unit, so that the second laser is indirectly locked onto the first laser.

9. A low modulation depth laser modem system as claimed in claim 5, wherein: The output signal of the first photodetector is processed to calculate the distance, clock error, and data information. The specific steps are as follows: 1) The output signal of the first photodetector enters the first digital signal processing unit to measure the one-way time delay T from the low modulation depth laser modulation and demodulation system B to A. BA The data information is calculated, which includes the time delay value T measured by the remote satellite. AB ; 2) Calculate the clock difference ΔT between the two clocks. 12 = (T BA -T AB ) / 2+ΔT L ΔT L This is the sum of link non-reciprocity and device delay error.

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