An optical fiber optical frequency transfer system and method

Abstract

The present invention discloses an optical fiber optical frequency transfer system and method. The system includes a local end and a remote end. The local end includes: a laser, a first X-type coupler, a first Y-type coupler, a first acousto-optic modulator, and a second acousto-optic modulator. The remote end includes: a bidirectional erbium-doped fiber amplifier Bi-EDFA, a third acousto-optic modulator AOM3, and a second Y-type coupler OC3. The present invention uses the 0-level light of the first acousto-optic modulator with a negative frequency shift to pass back and forth through the optical fiber link, can detect the phase noise of the optical fiber accessories, and uses the radio frequency signal containing the optical link noise information to drive the first acousto-optic modulator, so as to eliminate the noise of the optical fiber link without a high-precision clock reference.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical fiber communication, and particularly relates to an optical fiber optical frequency transfer system and method. Background Art

[0002] With the rapid development of research in the field of time and frequency, the measurement accuracy of time and frequency has reached an unprecedented level. Internationally, the uncertainty and stability of optical clocks represented by strontium atomic optical frequency standards have reached or even exceeded the E -18 magnitude. With these high-precision optical clocks as tools, high-precision time and frequency can be used to carry out precision measurement research and applications, which play an important role in promoting the development of science and technology and engineering applications. Therefore, how to transmit frequency signals with high precision has become an urgent need in the fields of basic scientific research and engineering applications in China.

[0003] Currently, the highest precision frequency transfer can be achieved using optical fibers. When a laser transmits in an optical fiber, due to the existence of transmission delay, there is a phase difference in the laser output from the optical fiber. Under the influence of the external environment, the optical path of the transmission optical fiber changes, resulting in fluctuations in the phase of the transmitted optical field, which is equivalent to additional laser frequency noise. The goal of optical fiber optical frequency transfer technology is to: minimize the influence of various factors on the phase of the optical field, so that users can obtain an optical frequency signal with approximate stability and accuracy at the remote end of the optical fiber as that at the local end of the optical fiber, that is, to achieve the remote transfer of optical frequency signals. Therefore, it is necessary to actively suppress interference factors during the transmission process.

[0004] In the prior art, during the optical fiber optical frequency transfer process, an acousto-optic modulator is usually used to suppress the Doppler frequency shift introduced by the optical fiber link. However, the driving radio frequency source of the acousto-optic modulator and the reference radio frequency source for demodulation will be asynchronous due to different locations, so its own noise will affect the transfer stability performance. Summary of the Invention

[0005] In order to solve the above problems existing in the prior art, the present invention provides an optical fiber optical frequency transfer system and method. The technical problems to be solved by the present invention are achieved through the following technical solutions:

[0006] In a first aspect, the present invention provides an optical fiber optical frequency transfer system, including: a local end and a remote end, the local end communicates with the remote end through an optical fiber link; the local end includes: a laser, a first X-type coupler, a first Y-type coupler, a first acousto-optic modulator, and a second acousto-optic modulator, and the remote end includes: a bidirectional erbium-doped optical fiber amplifier, a third acousto-optic modulator, and a second Y-type coupler; wherein,

[0007] The laser is sequentially connected to a first X-type coupler, a first acousto-optic modulator, a first Y-type coupler, and a second acousto-optic modulator through an optical fiber. The laser is used to generate a laser beam. The first X-type coupler is used to split the laser beam to obtain a first sub-beam. The first acousto-optic modulator is used to perform frequency shifting on the first sub-beam to obtain a zero-order diffraction output and a -1-order diffraction output. The first Y-type coupler is used to couple the zero-order diffraction output and the -1-order diffraction output to obtain a second sub-beam. The second acousto-optic modulator is used to perform frequency shifting on the second sub-beam to obtain a third sub-beam;

[0008] The bidirectional erbium-doped fiber amplifier, the third acousto-optic modulator, and the second Y-type coupler are sequentially connected through an optical fiber. The bidirectional erbium-doped fiber amplifier is used to compensate the optical intensity of the third sub-beam transmitted through the optical fiber link. The third acousto-optic modulator is used to perform frequency shifting on the compensated third sub-beam to obtain a fourth sub-beam. The second Y-type coupler is used to split the fourth sub-beam to obtain an optical frequency signal sent to the user side.

[0009] In an embodiment of the present invention, the remote end further includes: a second Faraday mirror;

[0010] The second Faraday mirror is used to reflect a fifth sub-beam obtained by splitting the fourth sub-beam back to the second Y-type coupler, and after sequentially passing through the third acousto-optic modulator, the bidirectional erbium-doped fiber amplifier, the second acousto-optic modulator, the first Y-type coupler, the first acousto-optic modulator, and the first X-type coupler, inject it into a detector in the local end.

[0011] In an embodiment of the present invention, the local end further includes: a first Faraday mirror, a detector, a band-pass filter, and a frequency divider. Port a1 of the first X-type coupler is connected to the laser, port a2 is connected to the detector, port a3 is connected to the first Faraday mirror, and port a4 is connected to the first acousto-optic modulator; wherein,

[0012] The first X-type coupler is further used to split the laser beam to obtain a sixth sub-beam. The first Faraday mirror is used to reflect the sixth sub-beam to the detector, so that the sixth sub-beam and the fifth sub-beam obtain a beat signal after beating. The detector is used to detect the frequency of the beat signal. The band-pass filter is used to perform band-pass filtering on the beat signal to obtain a radio frequency signal containing optical fiber noise information. The frequency divider is used to perform frequency division by two on the radio frequency signal to obtain a driving signal of the first X-type coupler.

[0013] In an embodiment of the present invention, the driving frequency of the second acousto-optic modulator is ω l , and the driving frequency of the third acousto-optic modulator is ωr The driving frequency of the first acousto-optic modulator is ω l +ω r .

[0014] In an embodiment of the present invention, the splitting ratio of ports a3 and a4 in the first acousto-optic modulator is 90:10.

[0015] In an embodiment of the present invention, the linewidth of the laser is less than 5 Hz.

[0016] Second, the present invention provides an optical frequency transfer method for optical fiber, which is applied to the optical fiber optical frequency transfer system described in the first aspect, and includes:

[0017] Generate a laser beam at the local end;

[0018] After splitting the laser beam, obtain a first sub-beam and a sixth sub-beam, perform frequency shifting on the first sub-beam, and obtain 0th-order diffraction output and -1st-order diffraction output;

[0019] Couple the 0th-order diffraction output and the -1st-order diffraction output to obtain a second sub-beam;

[0020] Perform frequency shifting on the second sub-beam to obtain a third sub-beam, and transmit the third sub-beam to the remote end through an optical fiber link;

[0021] Compensate the optical intensity of the third sub-beam transmitted through the optical fiber link, and perform frequency shifting on the compensated third sub-beam to obtain a fourth sub-beam;

[0022] After splitting the fourth sub-beam, obtain an optical frequency signal sent to the user end and a fifth sub-beam, reflect the fifth sub-beam back to the second Y-type coupler, and make the fifth sub-beam pass through the third acousto-optic modulator, the bidirectional erbium-doped fiber amplifier, the second acousto-optic modulator, the first Y-type coupler, the first acousto-optic modulator, and the first X-type coupler in sequence, and then inject it into the detector in the local end;

[0023] Perform beat frequency between the fifth sub-beam and the sixth sub-beam to obtain a beat frequency signal;

[0024] Detect the frequency of the beat frequency signal, and perform band-pass filtering on the beat frequency signal to obtain a radio frequency signal containing optical fiber noise information;

[0025] Perform frequency division by two on the radio frequency signal to obtain a driving signal for the first X-type coupler.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0027] The present invention provides an optical fiber optical frequency transfer system and method. By using the 0th-order light of the first acousto-optic modulator with a negative frequency shift to travel back and forth through the optical fiber link, the phase noise of the optical fiber accessories can be detected, and the radio frequency signal containing the optical fiber link noise information is used to drive the first acousto-optic modulator, thereby eliminating the noise of the optical fiber link without the need for a high-precision clock reference.

[0028] The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. Description of the Drawings

[0029] Figure 1 is a schematic structural diagram of an optical fiber optical frequency transfer system provided by an embodiment of the present invention;

[0030] Figure 2 is a flowchart of an optical fiber optical frequency transfer method provided by an embodiment of the present invention. Detailed Embodiments

[0031] The present invention will be further described in detail below in conjunction with specific embodiments, but the embodiments of the present invention are not limited thereto.

[0032] Figure 1 is a schematic structural diagram of an optical fiber optical frequency transfer system provided by an embodiment of the present invention. Please refer to Figure 1 An optical fiber optical frequency transfer system provided by an embodiment of the present invention includes: a local site and a remote site, and the local site communicates with the remote site through an optical fiber link; the local site includes: a laser, a first X-type coupler OC1, a first Y-type coupler OC2, a first acousto-optic modulator AOM1, and a second acousto-optic modulator AOM2, and the remote site includes: a bidirectional erbium-doped fiber amplifier Bi-EDFA, a third acousto-optic modulator AOM3, and a second Y-type coupler OC3; wherein,

[0033] The laser is sequentially connected to the first X-type coupler OC1, the first acousto-optic modulator AOM1, the first Y-type coupler OC2, and the second acousto-optic modulator AOM2 through an optical fiber. The laser is used to generate a laser beam. The first X-type coupler OC1 is used to split the laser beam to obtain a first sub-beam. The first acousto-optic modulator AOM1 is used to perform frequency shift processing on the first sub-beam to obtain a 0th-order diffraction output and a -1st-order diffraction output. The first Y-type coupler OC2 is used to couple the 0th-order diffraction output and the -1st-order diffraction output to obtain a second sub-beam. The second acousto-optic modulator AOM2 is used to perform frequency shift processing on the second sub-beam to obtain a third sub-beam;

[0034] The bidirectional erbium-doped fiber amplifier Bi-EDFA, the third acousto-optic modulator AOM3, and the second Y-coupler OC3 are sequentially connected by optical fibers. The bidirectional erbium-doped fiber amplifier Bi-EDFA is used to compensate the optical intensity of the third sub-beam transmitted through the optical fiber link. The third acousto-optic modulator AOM3 is used to perform a frequency shift process on the compensated third sub-beam to obtain a fourth sub-beam. The second Y-coupler OC3 is used to split the fourth sub-beam to obtain an optical frequency signal transmitted to the user terminal.

[0035] In this embodiment, the above optical fiber optical frequency transmission system includes a local end and a remote end, which communicate through an optical fiber link. Specifically, the local end includes: a laser, a first X-coupler OC1, a first Y-coupler OC2, a first acousto-optic modulator AOM1, and a second acousto-optic modulator AOM2. Among them, the laser is sequentially connected to the first X-coupler OC1, the first acousto-optic modulator AOM1, the first Y-coupler OC2, and the second acousto-optic modulator AOM2 through optical fibers, as Figure 1 shown. First, the laser generates a laser beam. After the laser beam enters the first X-coupler OC1, it is divided into two beams. Among them, the first sub-beam is injected into the first acousto-optic modulator AOM1. The first acousto-optic modulator AOM1 performs a frequency shift process on the first sub-beam to obtain two diffraction outputs, namely, the 0th-order diffraction output and the -1st-order diffraction output. Among them, the frequency of the 0th-order diffraction output does not shift, while the frequency of the -1st-order diffraction output shifts. These two outputs are combined by the first Y-coupler OC2 to form a second sub-beam and injected into the second acousto-optic modulator AOM2 (+1st order). Further, the second acousto-optic modulator AOM2 performs a frequency shift process on the second sub-beam to obtain a third sub-beam. The third sub-beam is transmitted to the remote end through the optical fiber link.

[0036] Exemplarily, the driving frequency of the second acousto-optic modulator AOM2 is ω l , then after the second sub-beam is frequency-shifted by it, the frequency changes from ω l to ω0 - ω l .

[0037] On the other hand, the remote end includes a bidirectional erbium-doped fiber amplifier Bi-EDFA, a third acousto-optic modulator AOM3, and a second Y-coupler OC3, and the three are also sequentially connected by optical fibers; please continue to refer to Figure 1 , at the remote end, the bidirectional erbium-doped fiber amplifier Bi-EDFA first compensates the optical intensity of the third sub-beam transmitted through the optical fiber link, and then injects the compensated third sub-beam into the third acousto-optic modulator AOM3 for frequency shift processing. It should be noted that the driving frequency of the third acousto-optic modulator AOM3 is ω r , then after the frequency shift processing, the frequency of the compensated third sub-beam changes from ω0 - ω l to ω0 - ω l - ωr ; Then, the second Y-shaped coupler OC3 splits the fourth sub-beam to obtain an optical frequency signal transmitted to the user side.

[0038] Optionally, the above remote end further includes: a second Faraday mirror;

[0039] The second Faraday mirror is used to reflect the fifth sub-beam obtained by splitting the fourth sub-beam back to the second Y-shaped coupler OC3, and after sequentially passing through the third acousto-optic modulator AOM3, bidirectional erbium-doped fiber amplifier Bi-EDFA, second acousto-optic modulator AOM2, first Y-shaped coupler OC2, first acousto-optic modulator AOM1, and first X-shaped coupler OC1, it is injected into the detector in the local end.

[0040] Optionally, the local end further includes: a first Faraday mirror, a detector, a band-pass filter, and a frequency divider. Port a1 of the first X-shaped coupler OC1 is connected to the laser, port a2 is connected to the detector, port a3 is connected to the first Faraday mirror, and port a4 is connected to the first acousto-optic modulator AOM1; wherein,

[0041] The first X-shaped coupler OC1 is also used to split the laser beam to obtain a sixth sub-beam. The first Faraday mirror is used to reflect the sixth sub-beam to the detector, so that the sixth sub-beam and the fifth sub-beam obtain a beat signal after beating. The detector is used to detect the frequency of the beat signal. The band-pass filter is used to perform band-pass filtering on the beat signal to obtain a radio frequency signal containing fiber noise information. The frequency divider is used to perform frequency division by two on the radio frequency signal to obtain a drive signal for the first X-shaped coupler OC1.

[0042] In this embodiment, since port a3 of the first X-shaped coupler OC1 is connected to the first Faraday mirror and port a4 is connected to the first acousto-optic modulator AOM1, after the first X-shaped coupler OC1 splits the laser beam into a first sub-beam and a second sub-beam, the first sub-beam is injected into the first acousto-optic modulator AOM1, and the second sub-beam is connected to the first Faraday mirror. The Faraday mirror can rotate the polarization state of the second sub-beam by 45 degrees and reflect it. When the second Faraday mirror at the remote end reflects the fifth sub-beam to the local end, the second sub-beam can beat with the fifth sub-beam, and then the detector detects the frequency of the beat signal. Obviously, if the driving frequency of the third acousto-optic modulator AOM3 is ω r and the driving frequency of the second acousto-optic modulator AOM2 is ω l , then the frequency of the beat signal is 2(ω l + ω r ).

[0043] Furthermore, use a frequency divider by two to divide the detected beat signal, and then drive the first acousto-optic modulator AOM1. The output frequency shift of the first acousto-optic modulator AOM1 is -(ωl +ω r )。

[0044] Optionally, the splitting ratio of ports a3 and a4 in the first X-type coupler OC1 is 90:10; that is, 10% of the light in the first sub-beam enters the first Faraday mirror FM1 (i.e., the sixth sub-beam), and 90% of the light (i.e., the first sub-beam) enters the first acousto-optic modulator AOM1.

[0045] In addition, the above-mentioned fiber optic optical frequency transfer system uses a narrow linewidth laser. For example, the linewidth of this laser needs to be less than 5 Hz.

[0046] The fiber optic optical frequency transfer system provided by the present invention utilizes the 0th-order light of the first acousto-optic modulator AOM1 with negative frequency shift to pass back and forth through the fiber optic link, and can detect the phase noise of the fiber optic accessories, and uses the radio frequency signal containing the noise information of the optical fiber link to drive the first acousto-optic modulator AOM1, thereby eliminating the noise of the fiber optic link. Exemplarily, if the fiber optic link drifts forward by 1 HZ, the driving signal can cause the first acousto-optic modulator AOM1 to shift frequency backward by 1 HZ, so as to achieve noise suppression and zero frequency drift. Furthermore, the frequency shift obtained by the remote end can be zero, the noise of the fiber optic link is eliminated, and it is not affected by local radio frequency synchronization.

[0047] Figure 2 is a flowchart of a fiber optic optical frequency transfer method provided by an embodiment of the present invention. Please refer to Figure 2 , an embodiment of the present invention provides a fiber optic optical frequency transfer method, which is applied to the above-mentioned fiber optic optical frequency transfer system, and includes:

[0048] S1. Generate a laser beam at the local end;

[0049] S2. Split the laser beam to obtain a first sub-beam and a sixth sub-beam, perform frequency shift processing on the first sub-beam, and obtain 0th-order diffraction output and -1st-order diffraction output;

[0050] S3. Couple the 0th-order diffraction output and the -1st-order diffraction output to obtain a second sub-beam;

[0051] S4. Perform frequency shift processing on the second sub-beam to obtain a third sub-beam, and transmit the third sub-beam to the remote end through the fiber optic link;

[0052] S5. Compensate the light intensity of the third sub-beam transmitted through the fiber optic link, and perform frequency shift processing on the compensated third sub-beam to obtain a fourth sub-beam;

[0053] S6. After splitting the fourth sub-beam, an optical frequency signal sent to the user side and a fifth sub-beam are obtained. The fifth sub-beam is reflected back to the second Y-branch coupler, and after the fifth sub-beam passes through the third acousto-optic modulator, bidirectional erbium-doped fiber amplifier, second acousto-optic modulator, first Y-branch coupler, first acousto-optic modulator and first X-branch coupler in sequence, it is injected into the detector in the local side;

[0054] S7. Beat the fifth sub-beam and the sixth sub-beam to obtain a beat signal;

[0055] S8. Detect the frequency of the beat signal and perform band-pass filtering on the beat signal to obtain a radio frequency signal containing fiber noise information;

[0056] S9. Divide the radio frequency signal by two to obtain the drive signal of the first X-branch coupler OC1.

[0057] As can be seen from the above embodiments, the beneficial effects of the present invention are as follows:

[0058] The present invention provides an optical fiber optical frequency transfer system and method. By using the 0-level light of the first acousto-optic modulator with negative frequency shift to travel back and forth through the fiber link, the phase noise of the fiber attachment can be detected, and a radio frequency signal containing the fiber link noise information is used to drive the first acousto-optic modulator, so as to eliminate the noise of the fiber link without a high-precision clock reference.

[0059] In the description of the present invention, the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of such features. In the description of the present invention, "a plurality of" means two or more, unless otherwise specifically defined.

[0060] In the present invention, unless otherwise clearly specified and limited, the first feature being "above" or "below" the second feature may include the first and second features being in direct contact, or may include the first and second features not being in direct contact but being in contact through other features therebetween. Moreover, the first feature being "above", "over" and "on" the second feature includes the first feature being directly above and obliquely above the second feature, or merely indicating that the first feature has a higher horizontal height than the second feature. The first feature being "below", "under" and "beneath" the second feature includes the first feature being directly below and obliquely below the second feature, or merely indicating that the first feature has a lower horizontal height than the second feature.

[0061] In the description of this specification, the descriptions referring to terms such as "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. mean that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in a suitable manner in any one or more embodiments or examples. In addition, those skilled in the art can combine and combine the different embodiments or examples described in this specification.

[0062] Although the present application has been described herein in connection with various embodiments, however, in the process of implementing the claimed present application, those skilled in the art can understand and achieve other variations of the disclosed embodiments by viewing the accompanying drawings, the disclosure, and the appended claims. Certain measures are recited in mutually different dependent claims, but this does not mean that these measures cannot be combined to produce good effects.

[0063] The above content is a further detailed description of the present invention in connection with specific preferred embodiments, and it cannot be determined that the specific implementation of the present invention is only limited to these descriptions. For those of ordinary skill in the technical field to which the present invention pertains, without departing from the concept of the present invention, several simple deductions or substitutions can still be made, and all should be regarded as belonging to the protection scope of the present invention.

Claims

1. An optical fiber optical frequency transfer system, characterized in that, Including: A local end and a remote end, where the local end communicates with the remote end through an optical fiber link; The local end includes: a laser, a first X-type coupler, a first Y-type coupler, a first acousto-optic modulator, and a second acousto-optic modulator, and the remote end includes: a bidirectional erbium-doped fiber amplifier, a third acousto-optic modulator, and a second Y-type coupler; where The laser is sequentially connected to the first X-type coupler, the first acousto-optic modulator, the first Y-type coupler, and the second acousto-optic modulator through an optical fiber. The laser is used to generate a laser beam. The first X-type coupler is used to split the laser beam to obtain a first sub-beam. The first acousto-optic modulator is used to perform frequency shift processing on the first sub-beam to obtain a zero-order diffraction output and a -1st order diffraction output. The first Y-type coupler is used to couple the zero-order diffraction output and the -1st order diffraction output to obtain a second sub-beam. The second acousto-optic modulator is used to perform frequency shift processing on the second sub-beam to obtain a third sub-beam; The bidirectional erbium-doped fiber amplifier, the third acousto-optic modulator, and the second Y-type coupler are sequentially connected through an optical fiber. The bidirectional erbium-doped fiber amplifier is used to compensate the optical intensity of the third sub-beam transmitted through the optical fiber link. The third acousto-optic modulator is used to perform frequency shift processing on the compensated third sub-beam to obtain a fourth sub-beam. The second Y-type coupler is used to split the fourth sub-beam to obtain an optical frequency signal sent to the user end.

2. The optical fiber optical frequency transfer system according to claim 1, wherein The remote end further includes: a second Faraday mirror; The second Faraday mirror is used to reflect a fifth sub-beam obtained by splitting the fourth sub-beam back to the second Y-type coupler, and after sequentially passing through the third acousto-optic modulator, the bidirectional erbium-doped fiber amplifier, the second acousto-optic modulator, the first Y-type coupler, the first acousto-optic modulator, and the first X-type coupler, it is injected into a detector in the local end.

3. The optical fiber optical frequency transfer system according to claim 2, wherein, The local end further includes: a first Faraday mirror, a detector, a band-pass filter, and a frequency divider. Port a1 of the first X-type coupler is connected to the laser, port a2 is connected to the detector, port a3 is connected to the first Faraday mirror, and port a4 is connected to the first acousto-optic modulator; where The first X-type coupler is further used to split the laser beam to obtain a sixth sub-beam. The first Faraday mirror is used to reflect the sixth sub-beam to the detector so that the sixth sub-beam and the fifth sub-beam obtain a beat signal after beating. The detector is used to detect the frequency of the beat signal. The band-pass filter is used to perform band-pass filtering on the beat signal to obtain a radio frequency signal containing optical fiber noise information. The frequency divider is used to perform frequency division by two on the radio frequency signal to obtain a driving signal of the first X-type coupler.

4. The optical fiber optical frequency transfer system according to claim 1, characterized in that, The driving frequency of the second acousto-optic modulator is ω l , the driving frequency of the third acousto-optic modulator is ω r , the driving frequency of the first acousto-optic modulator is ω l +ω r .

5. The optical fiber optical frequency transfer system according to claim 1, wherein The splitting ratio of ports a3 and a4 in the first X-type coupler is 90:

10.

6. The optical fiber optical frequency transfer system according to claim 1, wherein The line width of the laser is less than 5 Hz.

7. A method for optical frequency transfer of optical fiber, characterized in that, Applied to the optical fiber optical frequency transfer system according to any one of claims 1-6, including: Generating a laser beam at the local end; After splitting the laser beam, a first sub-beam and a sixth sub-beam are obtained. The first sub-beam is frequency-shifted to obtain a zero-order diffraction output and a -1-order diffraction output; The zero-order diffraction output and the -1-order diffraction output are coupled to obtain a second sub-beam; The second sub-beam is frequency-shifted to obtain a third sub-beam, and the third sub-beam is transmitted to the remote end through an optical fiber link; The optical intensity of the third sub-beam transmitted through the optical fiber link is compensated, and the compensated third sub-beam is frequency-shifted to obtain a fourth sub-beam; After splitting the fourth sub-beam, an optical frequency signal sent to the user end and a fifth sub-beam are obtained. The fifth sub-beam is reflected back to the second Y-type coupler, and the fifth sub-beam sequentially passes through the third acousto-optic modulator, the bidirectional erbium-doped fiber amplifier, the second acousto-optic modulator, the first Y-type coupler, the first acousto-optic modulator, and the first X-type coupler, and then is injected into the detector in the local end; The fifth sub-beam and the sixth sub-beam are beat to obtain a beat signal; The frequency of the beat signal is detected, and the beat signal is band-pass filtered to obtain a radio frequency signal containing fiber noise information; The radio frequency signal is divided by two to obtain a driving signal for the first X-type coupler.

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