An optical frequency comparison device and method based on time-delay interferometry

Abstract

An optical frequency comparison device and method based on time-delay interferometry are disclosed. The device comprises a local end, a first transmission link, a relay end, a second transmission link, and a remote end. This invention compares the optical frequency signals of the local and remote ends through relay segmentation and eliminates phase noise introduced in the two transmission links by delay matching, effectively suppressing phase noise caused by time delay and further improving the accuracy of optical frequency comparison.

Description

Technical Field

[0001] This invention relates to optical frequency comparison, and particularly to an optical frequency comparison device and method based on time-delay interference. Background Technology

[0002] High-precision time and frequency synchronization technology plays a vital role in time and frequency metrology, navigation and positioning, radio astronomy, deep space exploration, and fundamental physics research, and is an important means of developing scientific research and ensuring national defense. Although the optical frequency standard provided by optical atomic clocks is two orders of magnitude more unstable than that of conventional atomic clocks, it is still necessary to synchronize optical frequency standards at different geographical locations for practical applications. To this end, researchers at home and abroad have carried out research on related technologies such as optical frequency comparison. For example, in 2014, Calosso et al. reported an experimental scheme for two-way optical frequency comparison, which compares frequencies by independently acquiring data at two transmitting ends of the optical frequency [see C, E, Calosso, et al. Frequency transfer via a two-way optical phase comparison on a multiplexed fiber network[J]. Optics Letters, 2014.]. Furthermore, in 2017, Lee et al. achieved an experiment where optical frequency comparison was performed only at the local end, which effectively solved the problem of asynchronous data acquisition in bidirectional optical frequency comparison, but it also worsened residual phase noise [see WKLee, et al. Hybrid fber links for accurate optical frequency comparison[J]. Applied Physics B, 2017]. However, these optical frequency comparison methods are all limited by the residual phase noise introduced by delay in the fiber optic link, so the comparison accuracy of these methods decreases with the increase of transmission distance. Summary of the Invention

[0003] The purpose of this invention is to address the shortcomings of existing technologies and methods by providing an optical frequency comparison device and method based on time-delay interferometry. With relay assistance, the comparison link is segmented, and the phase noise introduced in each segment is detected separately. Then, through delay matching and data processing, the influence of phase noise caused by the delay is eliminated, thereby improving the instability of optical frequency transmission.

[0004] To achieve the above objectives, the technical solution of the present invention is as follows:

[0005] An optical frequency comparison device based on time-delay interferometry includes a local end, a first transmission link, a relay end, a second transmission link, and a remote end, characterized in that...

[0006] The local terminal consists of a first optical isolator, a first optical coupler, a first photodetector unit, a second optical coupler, a first Faraday rotator mirror, a first frequency shifter, and a first microwave source. The input terminal of the first optical isolator is the input terminal of the first optical frequency signal (E1) to be compared at the local terminal. The output terminal of the first optical isolator is connected to port 1 of the first optical coupler. Ports 2 and 3 of the first optical coupler are respectively connected to the input terminal of the first photodetector unit and port 1 of the second optical coupler. Ports 2 and 3 of the second optical coupler are respectively connected to the first Faraday rotator mirror and port 1 of the first frequency shifter. Ports 2 and 3 of the first frequency shifter are respectively connected to the output terminal of the first microwave source and the input terminal of the first transmission link.

[0007] The relay terminal comprises a second frequency shifter, a second microwave source, a second photodetector unit, a third optical coupler, a second Faraday rotator mirror, a fourth optical coupler, a second optical isolator, a laser, a third Faraday rotator mirror, a fifth optical coupler, a third photodetector, a third frequency shifter, and a third microwave source. Ports 1, 2, and 3 of the second frequency shifter are respectively connected to the output of the first transmission link, the output of the second microwave source, and port 1 of the third optical coupler. Ports 2, 3, and 4 of the third optical coupler are respectively connected to the second Faraday rotator mirror, the second photodetector unit, and the third microwave source. The input terminal of the element is connected to port 1 of the fourth optical coupler. Ports 2 and 3 of the fourth optical coupler are connected to the output terminal of the second optical isolator and port 4 of the fifth optical coupler, respectively. The input terminal of the second optical isolator is connected to the output terminal of the laser. Ports 1, 2, and 3 of the fifth optical coupler are connected to the input terminals of the third Faraday rotating mirror, the third photodetector, and port 1 of the third frequency shifter, respectively. Ports 2 and 3 of the third frequency shifter are connected to the output terminal of the third microwave source and the input terminal of the second transmission link, respectively.

[0008] The remote end consists of a fourth frequency shifter, a fourth microwave source, a sixth optical coupler, a fourth Faraday rotator, a seventh optical coupler, a fourth photodetector, and a third optical isolator. Port 1 of the fourth frequency shifter is connected to the output of the second transmission link. Ports 2 and 3 of the fourth frequency shifter are connected to the output of the fourth microwave source and the 2 port of the sixth optical coupler, respectively. Ports 1 and 3 of the sixth optical coupler are connected to the fourth Faraday rotator and the 3 port of the seventh optical coupler, respectively. Ports 1 and 2 of the seventh optical coupler are connected to the output of the third optical isolator and the input of the fourth photodetector, respectively. The input of the third optical isolator is the input of the second optical frequency signal (E2) to be compared at the remote end.

[0009] The first and second transmission links are optical fiber links or free space links.

[0010] The method for comparing optical frequency signals using the aforementioned time-delay interference-based optical frequency comparison device is characterized by the following specific steps:

[0011] 1) The optical frequency signal emitted by the laser in the relay terminal can be expressed as:

[0012]

[0013] Where, v0 and These represent the angular frequency and initial phase of the optical frequency signal emitted by the laser in the relay end, respectively. Signal E0, after passing through the second optical isolator and the fourth optical coupler, is divided into two parts: one part passes through the third optical coupler and is further divided into two parts; one part passes through the second frequency shifter and enters the first transmission link and the local end, then passes through the first frequency shifter, the second optical coupler, and the first Faraday rotator, is reflected by the first Faraday rotator, and returns along the original path to the relay end; after being reflected by the second Faraday rotator, it enters the second photodetector; the other part serves as a reference signal entering the second photodetector. The beat frequency of the two signals can be expressed as:

[0014]

[0015] Wherein, ω1 is the angular frequency of the microwave signal applied from the first microwave source to the first frequency shifter, and ω2 is the angular frequency of the microwave signal applied from the second microwave source to the second frequency shifter. The phase noise introduced in the first transmission link is τ1, and the time delay introduced in the first transmission link is τ1. Similarly, the other part is split into two parts again after passing through the fifth optical coupler. One part passes through the third frequency shifter and enters the second transmission link and the far end. After passing through the fourth frequency shifter, the sixth optical coupler, and the fourth Faraday rotator, it is reflected by the fourth Faraday rotator and returns to the relay end along the original path. After being reflected by the third Faraday rotator, it enters the third photodetector. The other part enters the third photodetector as a reference signal. The beat frequency signal of the two signals can be expressed as:

[0016]

[0017] Wherein, ω3 is the angular frequency of the microwave signal applied by the third microwave source to the third frequency shifter, and ω4 is the angular frequency of the microwave signal applied by the fourth microwave source to the fourth frequency shifter. τ2 is the phase noise introduced in the second transmission link, and τ2 is the time delay introduced in the second transmission link;

[0018] 3) The first optical frequency signal E1 to be compared in the local terminal passes through the first optical isolator and the first optical coupler before entering the first photodetector as a reference optical signal. The optical frequency signal E0 emitted by the laser in the relay terminal passes through the second optical isolator, the fourth optical coupler, the third optical coupler, the second frequency shifter, the first transmission link, the first frequency shifter, the second optical coupler, and the first optical coupler before entering the first photodetector. The beat frequency result with the reference optical signal can be written as:

[0019]

[0020] Among them, v1 and These are the angular frequency and initial phase of the first optical frequency signal to be compared in the local terminal.

[0021] 3) The second optical frequency signal E2 to be compared in the remote end passes through the third optical isolator and the seventh optical coupler before entering the fourth photodetector as a reference optical signal. The optical frequency signal E0 emitted by the laser in the relay end passes through the second optical isolator, the fourth optical coupler, the fifth optical coupler, the third frequency shifter, the second transmission link, the fourth frequency shifter, the sixth optical coupler, and the seventh optical coupler before entering the fourth photodetector. The beat frequency result with the reference optical signal can be written as:

[0022]

[0023] Among them, v2 and These are the angular frequency and initial phase of the second optical frequency signal to be compared at the far end, respectively.

[0024] 4) The microwave signals obtained from the first, second, third, and fourth photodetectors are E5, E3, E4, and E6, respectively. After appropriate data processing, the phase noise introduced by the delay mismatch in the fiber optic link can be eliminated. By introducing delays of 2τ2 and 2τ1 into E3 and E4 respectively and subtracting them, we can obtain:

[0025]

[0026] The signal obtained by dividing the radio frequency signal E7 by 2 can be represented as:

[0027]

[0028] By introducing delays of τ1+2τ2 and τ2+2τ1 into E5 and E6 respectively, and then subtracting them, we can obtain:

[0029]

[0030] Subtracting E8 from E9 yields:

[0031]

[0032] Therefore, this process eliminates phase noise introduced by time delay, and can effectively compare the first optical frequency signal to be transmitted with the second optical frequency signal to be transmitted.

[0033] Compared with the prior art, the beneficial effects of the present invention are:

[0034] 1) This invention utilizes the method of segmented extraction of link phase noise by laser relay stations to perform optical frequency comparison, which can increase the distance of the comparison system to a certain extent.

[0035] 2) The length symmetry of the two transmission links in this invention is not limited, making it more suitable for real-world fiber optic links with greater phase noise.

[0036] 3) This invention uses a simple data processing method to solve the problem of delay mismatch. The algorithm is simple and the system has no complicated radio frequency circuits. It can effectively suppress optical frequency instability caused by uncompensated phase noise due to delay, and break through the phase noise limitation of conventional bidirectional comparison systems. Attached Figure Description

[0037] Figure 1This is a schematic diagram of an embodiment of the optical frequency comparison device based on time-delay interference of the present invention.

[0038] Figure 2 This is an embodiment of the test terminal / data processing terminal of the present invention. Detailed Implementation

[0039] 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.

[0040] Figure 1 This is a schematic diagram of an embodiment of the optical frequency comparison device based on time-delay interferometry of the present invention. As can be seen from the figure, the optical frequency comparison device based on time-delay interferometry of the present invention includes a local end 1, a first transmission link 2, a relay end 3, a second transmission link 4, and a remote end 5.

[0041] The local terminal 1 is composed of a first optical isolator 101, a first optical coupler 102, a first photodetector unit 103, a second optical coupler 104, a first Faraday rotator mirror 105, a first frequency shifter 106, and a first microwave source 107. The input terminal of the first optical isolator 101 is the input terminal of the first optical frequency signal (E1) to be compared at the local terminal. The output terminal of the first optical isolator 101 is connected to port 1 of the first optical coupler 102. Ports 2 and 3 of the first optical coupler 102 are respectively connected to the input terminal 103 of the first photodetector unit and port 1 of the second optical coupler 104. Ports 2 and 3 of the second optical coupler 104 are respectively connected to the first Faraday rotator mirror 105 and port 1 of the first frequency shifter 106. Ports 2 and 3 of the first frequency shifter 106 are respectively connected to the output terminal of the first microwave source 107 and the input terminal of the first transmission link 2.

[0042] The relay terminal 3 is composed of a second frequency shifter 301, a second microwave source 302, a second photodetector unit 303, a third optical coupler 304, a second Faraday rotator mirror 305, a fourth optical coupler 306, a second optical isolator 307, a laser 308, a third Faraday rotator mirror 309, a fifth optical coupler 310, a third photodetector 311, a third frequency shifter 312, and a third microwave source 313. Ports 1, 2, and 3 of the second frequency shifter 301 are respectively connected to the output of the first transmission link 2, the output of the second microwave source 302, and port 1 of the third optical coupler 304. Ports 2, 3, and 4 of the third optical coupler 304 are respectively connected to the second Faraday rotator mirror 305. The input terminal of the second photodetector unit 303 is connected to port 1 of the fourth optical coupler 306. Ports 2 and 3 of the fourth optical coupler 306 are respectively connected to the output terminal of the second optical isolator 307 and port 4 of the fifth optical coupler 310. The input terminal of the second optical isolator 307 is connected to the output terminal of the laser 308. Ports 1, 2, and 3 of the fifth optical coupler 310 are respectively connected to the input terminals of the third Faraday rotator mirror 309, the third photodetector 311, and port 1 of the third frequency shifter 312. Ports 2 and 3 of the third frequency shifter 312 are respectively connected to the output terminal of the third microwave source 313 and the input terminal of the second transmission link 4.

[0043] The remote end 5 consists of a fourth frequency shifter 501, a fourth microwave source 502, a sixth optical coupler 503, a fourth Faraday rotator 504, a seventh optical coupler 505, a fourth photodetector 506, and a third optical isolator 507. Port 1 of the fourth frequency shifter 501 is connected to the output of the second transmission link 4. Ports 2 and 3 of the fourth frequency shifter 501 are connected to the output of the fourth microwave source 502 and the second port of the sixth optical coupler 503, respectively. Ports 1 and 3 of the sixth optical coupler 503 are connected to the fourth Faraday rotator 504 and the third port of the seventh optical coupler 505, respectively. Ports 1 and 2 of the seventh optical coupler 505 are connected to the output of the third optical isolator 507 and the input of the fourth photodetector 506, respectively. The input of the third optical isolator 507 is the input of the second optical frequency signal (E2) to be compared at the remote end.

[0044] In this embodiment, the first transmission link 2 and the second transmission link 4 are composed of optical fiber links. The local end 1 is located at one end of the first transmission link 2, the relay end 3 is located at the other end of the first transmission link 2, the remote end 5 is located at one end of the second transmission link 4, and the relay end 3 is located at the other end of the second transmission link 2.

[0045] The specific steps of the optical frequency signal comparison method using the aforementioned time-delay interferometry-based optical frequency comparison device are as follows:

[0046] 1) The optical frequency signal emitted by the laser in relay terminal 3 can be expressed as:

[0047]

[0048] Where, v0 and These represent the angular frequency and initial phase of the optical frequency signal emitted by the laser in relay 3, respectively. Signal E0 is split into two parts after passing through the second optical isolator 307 and the fourth optical coupler 306: one part is further split into two parts after passing through the third optical coupler 304; one part passes through the second acousto-optic frequency shifter 301 and enters the first transmission link 2 and then the local terminal 1. After passing through the first acousto-optic frequency shifter 106, the second optical coupler 104, and the first Faraday rotator 105, it is reflected by the first Faraday rotator 105 and returns along the original path to relay 3. After being reflected by the second Faraday rotator 305, it enters the second photodetector 303. The other part, as a reference signal, is also represented by E0 and directly enters the second photodetector 303. The beat frequency signals of the two signals can be expressed as:

[0049]

[0050] Wherein, ω1 is the angular frequency of the microwave signal applied by the first microwave source 107 to the first acousto-optic frequency shifter 106, and ω2 is the angular frequency of the microwave signal applied by the second microwave source 302 to the second acousto-optic frequency shifter 301. The phase noise introduced in the first transmission link 2 is represented by τ1, which is the time delay introduced in the first transmission link 2. Similarly, the other part is split into two parts again after passing through the fifth optical coupler 310. One part passes through the third acousto-optic frequency shifter 312 and enters the second transmission link 4 and the far end 5. After passing through the fourth acousto-optic frequency shifter 501, the sixth optical coupler 503, and the fourth Faraday rotator 504, it is reflected by the fourth Faraday rotator 504 and returns to the relay end 3 along the original path. After being reflected by the third Faraday rotator 309, it enters the third photodetector 311. The other part, as a reference signal, is also represented by E0 and directly enters the third photodetector 311. The beat frequency signal of the two signals can be expressed as:

[0051]

[0052] Wherein, ω3 is the angular frequency of the microwave signal applied by the third microwave source 313 to the third acousto-optic frequency shifter 312, and ω4 is the angular frequency of the microwave signal applied by the fourth microwave source 502 to the fourth acousto-optic frequency shifter 501. τ2 is the phase noise introduced in the second transmission link 4, and τ2 is the time delay introduced in the second transmission link 4.

[0053] 2) The first optical frequency signal E1 to be compared in the local terminal 1 passes through the first optical isolator 101 and the first optical coupler 102 and then enters the first photodetector 103 as a reference optical signal. The optical frequency signal E0 emitted by the laser in the relay terminal 3 passes through the second optical isolator 307, the fourth optical coupler 306, the third optical coupler 304, the second acousto-optic frequency shifter 301, the first transmission link 2, the first acousto-optic frequency shifter 106, the second optical coupler 104, and the first optical coupler 102 and then enters the first photodetector 103. The beat frequency result with the reference optical signal can be written as:

[0054]

[0055] Among them, v1 and These are the angular frequency and initial phase of the first optical frequency signal to be compared in local terminal 1, respectively.

[0056] 3) The second optical frequency signal O2 to be compared in the remote end 5 passes through the third optical isolator 507 and the seventh optical coupler 505 and then enters the fourth photodetector 506 as a reference optical signal. The optical frequency signal E0 emitted by the laser in the relay end 3 passes through the second optical isolator 307, the fourth optical coupler 306, the fifth optical coupler 310, the third acousto-optic frequency shifter 312, the second transmission link 4, the fourth acousto-optic frequency shifter 501, the sixth optical coupler 503, and the seventh optical coupler 505 and then enters the fourth photodetector 506. The beat frequency result with the reference optical signal can be written as:

[0057]

[0058] Among them, v2 and These are the angular frequency and initial phase of the second optical frequency signal to be compared in the far end 5, respectively.

[0059] 4) The microwave signals obtained from the first photodetector 103, the second photodetector 302, the third photodetector 311, and the fourth photodetector 506 are E5, E3, E4, and E6, respectively. These four microwave signals, after appropriate data processing, can eliminate phase noise introduced by the delay mismatch in the fiber optic link, such as... Figure 2 The diagram illustrates an embodiment of the data processing method of the present invention. After introducing delays of 2τ1 and 2τ2 into E4 and E3 respectively through the first electrical delay line 601 and the second electrical delay line 602, the signals are mixed by the first mixer 603. The lower sideband is filtered out by the first bandpass filter 604, resulting in:

[0060]

[0061] The signal after dividing the radio frequency signal E7 by two using the frequency divider 605 can be expressed as:

[0062]

[0063] After introducing delays of τ2+2τ1 and τ1+2τ2 to E6 and E5 respectively through the third electrical delay line 607 and the fourth electrical delay line 608, the frequencies are mixed by the second mixer 608. The lower sideband is then filtered out by the second bandpass filter 609, resulting in:

[0064]

[0065] By mixing E9 and E8 together through the third mixer 610 and filtering out the lower sideband through the third bandpass filter 611, we can obtain:

[0066]

[0067] Therefore, this process eliminates phase noise introduced by time delay, and can effectively compare the first optical frequency signal to be transmitted with the second optical frequency signal to be transmitted.

Claims

1. An optical frequency comparison device based on time-delay interference, comprising a local end (1), a remote end (5), and a transmission link connecting the local end (1) and the remote end (5), characterized in that, A relay terminal (3) is set in the transmission link to form a first transmission link (2) connecting the local end (1) and the relay terminal (3) and a second transmission link (4) connecting the remote end (5) and the relay terminal (3). The relay terminal (3) includes at least a laser and a beam splitter module, wherein the laser is used to output an optical frequency signal. The beam splitter module is used to split the optical frequency signal. The signal is divided into two paths. One path is transmitted to the local end (1) via the first transmission link (2), and then returns to the relay end (3) via the first transmission link (2) again, along with the signal used as the reference light. Beat frequency, forming the first beat frequency signal Another path is transmitted to the remote end (5) via the second transmission link (4), and then returns to the relay end (3) via the second transmission link (4) again, along with the signal used as the reference light. Beat frequency, forming a second beat frequency signal ; The local terminal (1) is used to receive the first optical frequency signal to be compared. and the signal received by the first transmission link (2). Beat frequency, forming a third beat frequency signal ; The remote end (5) is used to receive the second optical frequency signal to be compared. and receive signals transmitted by the second transmission link (3). Beat frequency, forming the fourth beat frequency signal ; By analyzing the first beat frequency signal Second beat frequency signal Delaying and subtracting or mixing to remove sidebands, by adjusting the third beat frequency signal and the fourth beat frequency signal By performing a delay and subtraction or mixing to remove the sideband, the time delay and phase noise introduced by the first and second transmission links are eliminated, and the comparison between the first optical frequency signal to be transmitted and the second optical frequency signal to be transmitted is realized. The local terminal (1) consists of a first optical isolator (101), a first optical coupler (102), a first photodetector unit (103), a second optical coupler (104), a first Faraday rotator (105), a first frequency shifter (106), and a first microwave source (107). The input terminal of the first optical isolator (101) is the first optical frequency signal to be compared. At the local input end, the output end of the first optical isolator (101) is connected to port 1 of the first optical coupler (102), the ports 2 and 3 of the first optical coupler (102) are connected to the input end (103) of the first photodetector unit and the port 1 of the second optical coupler (104) respectively, the ports 2 and 3 of the second optical coupler (104) are connected to the ports 1 of the first Faraday rotator (105) and the first frequency shifter (106) respectively, and the ports 2 and 3 of the first frequency shifter (106) are connected to the output end of the first microwave source (107) and the input end of the first transmission link (2) respectively. The relay terminal (3) consists of a second frequency shifter (301), a second microwave source (302), a second photodetector unit (303), a third optical coupler (304), a second Faraday rotator (305), a fourth optical coupler (306), a second optical isolator (307), a laser (308), a third Faraday rotator (309), a fifth optical coupler (310), a third photodetector (311), a third frequency shifter (312), and a third microwave source (313). Ports 1, 2, and 3 of the second frequency shifter (301) are respectively connected to the output of the first transmission link (2), the output of the second microwave source (302), and port 1 of the third optical coupler (304). Ports 2, 3, and 4 of the third optical coupler (304) are respectively connected to the second Faraday rotator unit. The input terminals of the mirror (305) and the second photodetector (303) are connected to port 1 of the fourth optical coupler (306). Ports 2 and 3 of the fourth optical coupler (306) are connected to the output terminal of the second optical isolator (307) and port 4 of the fifth optical coupler (310), respectively. The input terminal of the second optical isolator (307) is connected to the output terminal of the laser (308). Ports 1, 2 and 3 of the fifth optical coupler (310) are connected to the input terminals of the third Faraday rotating mirror (309), the third photodetector (311) and port 1 of the third frequency shifter (312), respectively. Ports 2 and 3 of the third frequency shifter (312) are connected to the output terminal of the third microwave source (313) and the input terminal of the second transmission link (4), respectively. The remote end (5) consists of a fourth frequency shifter (501), a fourth microwave source (502), a sixth optical coupler (503), a fourth Faraday rotator (504), a seventh optical coupler (505), a fourth photodetector (506), and a third optical isolator (507). Port 1 of the fourth frequency shifter (501) is connected to the output of the second transmission link (4), and ports 2 and 3 of the fourth frequency shifter (501) are respectively connected to the output of the fourth microwave source (502). The output terminal is connected to port 2 of the sixth optical coupler (503). Ports 1 and 3 of the sixth optical coupler (503) are respectively connected to the fourth Faraday rotating mirror (504) and port 3 of the seventh optical coupler (505). Ports 1 and 2 of the seventh optical coupler (505) are respectively connected to the output terminal of the third optical isolator (507) and the input terminal of the fourth photodetector (506). The input terminal of the third optical isolator (507) is the second optical frequency signal to be compared. (At the remote input end.) 2. The optical frequency comparison device based on time-delay interferometry according to claim 1, characterized in that, The first transmission link (2) and the second transmission link (4) are optical fiber links or free space links.

3. A method for comparing optical frequency signals using the optical frequency comparison device based on time-delay interferometry as described in claim 1, characterized in that, The specific steps of this method are as follows: 1) Optical frequency signal emitted by the laser in the relay terminal (3) Represented as: in, and These are the angular frequency and initial phase of the optical frequency signal emitted by the laser in relay terminal (3), respectively; Signal After passing through the second optical isolator (307) and the fourth optical coupler (306), it is divided into two parts: A portion of the signal is split into two parts after passing through the third optical coupler (304). One part passes through the second frequency shifter (301) and enters the first transmission link (2) and the local end (1). After passing through the first frequency shifter (106), the second optical coupler (104), and the first Faraday rotator (105), it is reflected by the first Faraday rotator (105) and returns to the relay end (3) along the original path. After being reflected by the second Faraday rotator (305), it enters the second photodetector (303). The other part serves as a reference signal and is also represented as... Directly entering the second photodetector (303), the beat frequency signals of the two signals are expressed as follows: in, The angular frequency of the microwave signal loaded onto the first frequency shifter (106) by the first microwave source (107) is given by the microwave source (107). The angular frequency of the microwave signal loaded onto the second frequency shifter (301) by the second microwave source (302) is... The phase noise introduced in the first transmission link (2), The delay introduced in the first transmission link (2); Another part, after passing through the fifth optical coupler (310), is divided into two parts again. One part passes through the third frequency shifter (312) and enters the second transmission link (4) and the far end (5). After passing through the fourth frequency shifter (501), the sixth optical coupler (503), and the fourth Faraday rotator (504), it is reflected by the fourth Faraday rotator (504) and returns to the relay end (3) along the original path. After being reflected by the third Faraday rotator (309), it enters the third photodetector (311). The other part, as a reference signal, is also represented as... Directly entering the third photodetector (311), the beat frequency signals of the two signals are expressed as follows: in, The angular frequency of the microwave signal loaded onto the third frequency shifter (312) by the third microwave source (313) is... The angular frequency of the microwave signal applied to the fourth frequency shifter (501) by the fourth microwave source (502) is... The phase noise introduced in the second transmission link (4), The delay introduced in the second transmission link (4); 2) The first optical frequency signal to be compared in the local terminal (1) After passing through the first optical isolator (101) and the first optical coupler (102), the optical signal enters the first photodetector (103) as a reference optical signal. The optical frequency signal emitted by the laser in the relay terminal (3) is also a reference optical signal. After passing through the second optical isolator (307), the fourth optical coupler (306), the third optical coupler (304), the second frequency shifter (301), the first transmission link (2), the first frequency shifter (106), the second optical coupler (104), and the first optical coupler (102), the light enters the first photodetector (103) and interacts with the reference optical signal. Beat frequency, expressed as: in, and These are the angular frequency and initial phase of the first optical frequency signal to be compared in the local terminal (1), respectively; 3) The second optical frequency signal to be compared in the far end (5) After passing through the third optical isolator (507) and the seventh optical coupler (505), the optical signal enters the fourth photodetector (506) as a reference optical signal. The optical frequency signal emitted by the laser in the relay terminal (3) is also a reference optical signal. After passing through the second optical isolator (307), the fourth optical coupler (306), the fifth optical coupler (310), the third frequency shifter (312), the second transmission link (4), the fourth frequency shifter (501), the sixth optical coupler (503), and the seventh optical coupler (505), the light enters the fourth photodetector (506) and interacts with the reference optical signal. Beat frequency, expressed as: in, and The angular frequency and initial phase of the second optical frequency signal to be compared in the far end (5) are respectively; 4) The microwave signals obtained from the first photodetector (103), the second photodetector (303), the third photodetector (311), and the fourth photodetector (506) are respectively , , and ; 5) Perform data processing on the four microwave signals to eliminate phase noise introduced by the delay mismatch in the fiber optic link, specifically including: Through data processing methods, the signal With signal Introduced separately and After delaying and subtracting, the radio frequency signal is obtained. , means as follows: radio frequency signals The signal after dividing by 2 is represented as: Through data processing methods and Introduced separately and After delaying and subtracting, we get: Will and Subtraction yields: Therefore, this process eliminates phase noise introduced by time delay, and can effectively compare the first optical frequency signal to be transmitted with the second optical frequency signal to be transmitted.

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