Optical frequency standard optical fiber precision transmission system and method for far-end phase noise compensation

Abstract

The invention discloses an optical frequency standard optical fiber precision transmission system and method for far-end phase noise compensation, the system comprises a laser light source, an optical fiber, a first acousto-optic modulator and a Raman amplifier, the laser light source is used for simulating an optical frequency standard signal, and the optical fiber is used for transmitting the optical frequency standard signal; the first acousto-optic modulator is connected with the laser light source through the first optical fiber coupler and the first optical isolator and is used for generating frequency shift for laser; the Raman amplifier is connected with the optical fiber, an optical frequency standard signal enters the optical fiber after passing through the first acousto-optic modulator, and the Raman amplifier is used for amplifying the power of the optical signal. The optical fiber phase noise signal is obtained through optical heterodyne detection of the phase-locked loop at the far end of the optical fiber, real-time compensation of the working frequency of the acoustic optical modulator on the optical fiber phase noise is controlled in a negative feedback mode, the influence of the phase noise on the frequency stability of the optical frequency standard signal is reduced, and the limitation of a traditional method on the transmission end point position is solved; the method is suitable for optical frequency standard transmission in a multi-port output mode.

Description

Technical Field

[0001] The invention relates to an optical frequency standard optical fiber precision transmission system and method for far-end phase noise compensation, belonging to the field of optical frequency standard transmission. Background Art

[0002] Optical frequency standard transmission is widely needed in the development of space navigation, precise positioning, radio telescope arrays, basic physics research and other applications. Optical fiber has the advantages of high stability, low loss, and convenient use, and is an ideal medium for optical signal transmission. The perfect optical fiber communication network established around the world also provides convenience for the research of remote and precise transmission of optical frequency standards. In the research of remote transmission of optical frequency standards, there are two main problems:

[0003] First, the communication optical fiber laid outdoors will be affected by factors such as surrounding vibration and ambient temperature fluctuations, causing random fluctuations in the length of the optical fiber. When the optical frequency standard signal is transmitted in the optical fiber, it manifests itself as jitter in the transmission time, which is observed as phase noise in the frequency domain. The magnitude of the phase noise will continue to accumulate as the transmission distance increases, resulting in the broadening of the optical frequency standard signal and a decrease in frequency stability. Therefore, how to effectively suppress optical fiber phase noise has become a key research task in the field of optical frequency standard transmission.

[0004] Near-end phase noise compensation is the most commonly used solution in current research on optical frequency standard transmission. With the development of noise compensation technology, the transmission instability can be reduced to 10 -19 The phase noise compensation scheme is below the order of magnitude, which meets the requirements of optical frequency standard transmission. In this scheme, the transmission laser light source and the noise compensation system are both located in the near-end laboratory, which has the advantage of convenience in actual operation and control. On the other hand, the structural characteristics of this near-end phase noise compensation scheme limit the location of the transmission end point, so it is only suitable for point-to-point optical frequency standard transmission. In applications based on time and frequency standards, high-precision time synchronization is often required between multiple points.

[0005] On the other hand, for the problem of optical power loss during the transmission of optical frequency standards, there are currently four commonly used methods. Among them, the most commonly used method is to use a bidirectional EDFA to directly amplify the optical signal. The advantage of this method is that no additional control is required, which can reduce the maintenance cost of system operation. However, the power surge at the node position is likely to introduce a strong scattering effect, affecting the stability of the optical frequency standard transmission; secondly, the establishment of a relay station to achieve regenerative amplification of the signal light is also a common method at present. However, the establishment and maintenance costs of the relay station are high, which is not conducive to promotion in practical applications; in addition, the Brillouin amplifier has also been widely used in research work in recent years. The Brillouin amplifier uses the entire section of optical fiber as the gain medium, which effectively avoids power surges. However, its gain bandwidth is only tens of MHz. For multi-port and multi-channel transmission, it is necessary to select frequency-matched pump lasers separately and perform frequency stabilization control. Therefore, it is not suitable for optical frequency standard transmission in multi-port output mode. Summary of the invention

[0006] In order to solve the above technical problems, the present invention provides a system and method for precise transmission of optical frequency standard optical fiber with remote phase noise compensation, which realizes the long-distance transmission of optical signals with ultra-high frequency stability via optical fiber. The present invention obtains the optical fiber phase noise signal through optical heterodyne detection of the phase-locked loop at the remote end of the optical fiber, and negatively feedback controls the working frequency of the acousto-optic modulator to compensate for the optical fiber phase noise in real time, thereby reducing the influence of phase noise on the frequency stability of the optical frequency standard signal. Since the phase noise compensation is completed at the remote end of the optical fiber, there is no restriction on the transmission end point, and multi-port output in the optical frequency standard transmission can be realized. In addition, the Raman amplifier of the present invention uses the entire section of optical fiber as the gain medium, and the gain bandwidth reaches the THz level. There is no need to perform frequency stabilization control on the pump laser, and it is suitable for optical frequency standard transmission in multi-port output mode.

[0007] The first object of the present invention is to provide an optical frequency standard optical fiber precision transmission system for far-end phase noise compensation, comprising:

[0008] A laser light source, used for simulating an optical frequency standard signal, wherein an output end of the laser light source is connected to a first optical isolator to prevent light in the optical path from being reflected back to the laser light source;

[0009] Optical fiber: used to transmit optical frequency standard signals;

[0010] A first acousto-optic modulator is connected to the proximal end of the optical fiber transmission, and is connected to the laser light source through a first optical fiber coupler and a first optical isolator, and is used to shift the laser frequency; the light output by the laser light source enters the first acousto-optic modulator through the first optical isolator and the first optical fiber coupler; the other end of the first optical fiber coupler is connected to a first Faraday mirror;

[0011] A Raman amplifier is connected to the optical fiber, and the optical frequency standard signal enters the optical fiber after passing through the first acousto-optic modulator, and the Raman amplifier is used to amplify the optical signal power;

[0012] The phase noise compensation unit comprises a second acousto-optic modulator connected to the far end of the optical fiber transmission, and a phase-locked loop, wherein the second acousto-optic modulator is used to generate frequency shift for laser and compensate for optical fiber phase noise; the output end of the second acousto-optic modulator is connected to a second optical fiber coupler, the output ends of the second optical fiber coupler are respectively connected to the phase-locked loop and a third optical fiber coupler, the output end of the phase-locked loop is connected to the second acousto-optic modulator, and is used to detect and extract the optical fiber phase noise signal in real time, and negative feedback control is performed on the working frequency of the second acousto-optic modulator to realize compensation for phase noise; the output ends of the third optical fiber coupler are respectively connected to a second Faraday mirror and a third acousto-optic modulator, and the third acousto-optic modulator is used to compensate for the frequency shift of the first acousto-optic modulator and the second acousto-optic modulator.

[0013] In one embodiment of the present invention, the Raman amplifier includes a pump laser, a second optical isolator, a fourth optical fiber coupler, an optical attenuator and a wavelength division multiplexer. The output end of the pump laser is connected to the second optical isolator, the output end of the second optical isolator is connected to the fourth optical fiber coupler, the output end of the fourth optical fiber coupler is respectively connected to two optical attenuators, the output ends of the two optical attenuators are respectively connected to two wavelength division multiplexers, and the two wavelength division multiplexers are located on the optical fiber; the light emitted by the pump laser passes through the second optical isolator and enters the fourth optical fiber coupler, and the fourth optical fiber coupler outputs two equal-power lights that enter the two optical attenuators and then enter the optical fiber through the wavelength division multiplexer.

[0014] In one embodiment of the present invention, the sum of the operating frequency of the third AOM and the operating frequencies of the first AOM and the second AOM is equal to zero.

[0015] In one embodiment of the present invention, the first Faraday mirror is located at the proximal end of the optical fiber, and the second Faraday mirror is located at the distal end of the optical fiber. The first Faraday mirror and the second Faraday mirror are used to reflect the optical signal and rotate the polarization direction of the light passing back and forth by 90°.

[0016] In one embodiment of the present invention, the laser light source is a narrow linewidth laser; and the optical fiber is a single-mode optical fiber.

[0017] In one embodiment of the present invention, the first optical fiber coupler, the second optical fiber coupler, the third optical fiber coupler, and the fourth optical fiber coupler are used to realize the branching or combining of optical signals in the optical fiber.

[0018] In one embodiment of the present invention, the phase noise is obtained by optical heterodyne detection of single-transmission light and triple-transmission light in an optical fiber, and the phase noise is compensated at a remote end.

[0019] In one embodiment of the present invention, the Raman amplifier can amplify the power of optical signals transmitted in both forward and reverse directions.

[0020] The second object of the present invention is to provide a method for precise transmission of optical frequency standard optical fiber with far-end phase noise compensation, which uses the above-mentioned precise transmission system of optical frequency standard optical fiber with far-end phase noise compensation, and comprises the following steps:

[0021] S1, frequency is ω 0 The optical frequency standard signal is shifted by the first acousto-optic modulator 1 After that, the optical frequency becomes ω 0 +ω 1 ; It is transmitted through a section of optical fiber to the second acousto-optic modulator at the far end, and then frequency-shifted ω 1 After that, the optical frequency becomes ω 0 +ω 1 +ω 2 ;

[0022] S2. The length fluctuation of the optical fiber causes the phase of the optical frequency standard signal output from the far end to jitter, which is the phase noise φ 1 , then the total phase of the one-way transmission light in the optical fiber is (ω 0 +ω 1 +ω 2 )t+φ 1 ;

[0023] S3, the one-way transmission light is reflected by the second Faraday mirror at the far end, returns to the optical fiber along the original path, and then passes through the second acousto-optic modulator to shift the frequency ω 2 、The first acousto-optic modulator shifts frequency ω 1 After returning to the near end, the phase noise in the return transmission is φ 2 , then the total phase of the two-way transmission light is (ω 0 +2ω 1 +2ω 2 )t+φ 1 +φ 2 ;

[0024] S4, the two-way transmission light is reflected by the first Faraday mirror at the near end, returns to the optical fiber again, and then passes through the first acousto-optic modulator to shift the frequency ω 1 、The second AOM frequency shift ω 2 After reaching the far end, the phase noise in the third-pass transmission is φ 3 , then the total phase of the three-pass transmission light is (ω 0 +3ω 1 +3ω2 )t+φ 1 +φ 2 +φ 3 ;

[0025] S5, the one-way transmission light and the three-way transmission light are output from the fiber coupler and optical heterodyne detection is performed in the phase-locked loop, and the total phase of the beat frequency signal is (2ω 1 +2ω 2 )t+φ 2 +φ 3 , the frequency of use is 2ω 1 +2ω 2 The beat frequency signal is demodulated by the RF reference signal to obtain the phase noise φ 2 +φ 3 ;

[0026] S6. Assuming that the phase noise does not change much over time, we have φ 1 ≈φ 2 ≈φ 3 According to the phase noise signal obtained in S5, a negative feedback signal -φ is output through the phase-locked loop 1 Acting on the second AOM, it is used to compensate for the fiber phase noise in the one-way transmission light. Therefore, the total phase of the one-way transmission light output after the second AOM becomes (ω 0 +ω 1 +ω 2 )t, that is, the optical fiber phase noise has no effect on the transmitted optical frequency standard signal;

[0027] S7, set the operating frequency of the third acousto-optic modulator to -(ω 1 +ω 2 ), so the frequency of the final output optical frequency standard signal is restored to ω 0 , which is consistent with the frequency of the input optical frequency standard signal.

[0028] The third object of the present invention is to provide a method for precise transmission of optical frequency standard optical fiber, which uses the optical frequency standard optical fiber precise transmission system with far-end phase noise compensation, and comprises the following steps:

[0029] Step 1: After passing through an isolator and a coupler, the narrow linewidth laser enters the first acousto-optic modulator for frequency shifting, and then enters the optical fiber for forward transmission. A remote user at any node in the optical fiber can extract the optical frequency standard signal; the optical signal is output at the remote end after passing through the second acousto-optic modulator; the near end and the remote end of the optical fiber are both connected to a Faraday mirror to return the end face optical signal to the original path and rotate the vibration direction of the light by 90°;

[0030] Step 2: The single-pass transmission light in the optical fiber and the three-pass transmission light reflected by the far-end and near-end Faraday mirrors are output in the far-end coupler and sent to the phase-locked loop for detecting and extracting the optical fiber phase noise, and outputting a reverse compensation signal to control the operating frequency of the second acousto-optic modulator to achieve the purpose of suppressing phase noise;

[0031] Step 3: The optical frequency standard signal after phase noise compensation enters the third acousto-optic modulator through the optical fiber coupler. The third acousto-optic modulator is used to shift the frequency of the remote optical frequency standard signal so that the sum of the operating frequencies of the first acousto-optic modulator, the second acousto-optic modulator and the third acousto-optic modulator is 0, and the optical signal frequencies at the optical fiber input and output ends remain consistent.

[0032] The present invention has the following advantages:

[0033] 1. The present invention extracts the optical fiber phase noise signal at the far end of the optical fiber transmission; compared with the conventional near-end optical heterodyne detection, the phase noise extraction and compensation work of the present invention are completed at the far end of the optical fiber transmission, thereby eliminating the restriction on the transmission end position and realizing the optical frequency standard transmission of the multi-port output mode.

[0034] 2. The present invention introduces a Raman amplifier into the far-end phase noise compensation system to achieve optical power amplification. The Raman amplifier uses the transmission optical fiber as the gain medium, which can avoid the surge of optical power. At the same time, since the gain bandwidth of the Raman amplifier is relatively wide, there is no need to perform frequency stabilization control on the pump laser, which is suitable for optical frequency transmission of multi-user channels.

[0035] In summary, the present invention provides a far-end phase noise compensated optical frequency standard optical fiber precision transmission system and method, which realizes the long-distance transmission of ultra-high frequency stability optical signals through optical fibers. The present invention solves the phase noise problem caused by environmental factors in optical frequency standard signal transmission in optical fiber by using Raman amplifier as an optical power gain device and completing phase noise compensation at the far end of the optical fiber. Among them, the phase noise compensation unit of the present invention obtains the optical fiber phase noise signal at the far end of the optical fiber through optical heterodyne detection, and performs negative feedback control on the acousto-optic modulator, realizes real-time compensation of phase noise at the far end, and reduces the influence of phase noise on the frequency stability of the optical frequency standard signal. In addition, since the phase noise compensation is completed at the far end of the optical fiber, there is no restriction on the transmission end point, and the optical frequency standard transmission in a multi-port output mode can be realized, thereby solving the limitation of the transmission end point position by the traditional method. In addition, the present invention uses a Raman amplifier for the gain of the optical frequency standard signal, which uses the entire section of optical fiber as the gain medium, and the gain bandwidth reaches the THz level, so there is no need to perform frequency stabilization control on the pump laser, which is suitable for optical frequency standard transmission in a multi-port output mode. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0037] Figure 1 A schematic diagram of the structure of an optical frequency standard precision transmission system for far-end phase noise compensation provided by the present invention;

[0038] Figure 2 This is a schematic structural diagram of the Raman amplifier provided by the present invention.

[0039] In the figure: 1. laser light source; 2. first optical isolator; 3. first Faraday mirror; 4. first fiber coupler; 5. first acousto-optic modulator; 6. Raman amplifier; 7. optical fiber; 8. second acousto-optic modulator; 9. second fiber coupler; 10. third fiber coupler; 11. phase-locked loop; 12. second Faraday mirror; 13. third acousto-optic modulator; 14. pump laser; 15. second optical isolator; 16. fourth fiber coupler; 17. optical attenuator; 18. wavelength division multiplexer. DETAILED DESCRIPTION

[0040] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0041] In the present invention, unless otherwise clearly specified and limited, the terms "connected", "connected", and "fixed" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0042] In the present invention, unless otherwise clearly specified and limited, a first feature being "above" or "below" a second feature may include that the first and second features are in direct contact, or may include that the first and second features are not in direct contact but are in contact through another feature between them. Moreover, a first feature being "above", "above" and "above" a second feature includes that the first feature is directly above and obliquely above the second feature, or simply indicates that the first feature is higher in level than the second feature. A first feature being "below", "below" and "below" a second feature includes that the first feature is directly below and obliquely below the second feature, or simply indicates that the first feature is lower in level than the second feature.

[0043] like Figure 1 and Figure 2 As shown, the present invention provides an optical frequency standard optical fiber precision transmission system for remote phase noise compensation, comprising:

[0044] A laser light source 1, used for simulating an optical frequency standard signal, whose output end is connected to a first optical isolator 2, used for preventing the light in the optical path from being reflected back to the laser light source 1;

[0045] Optical fiber 7: used to transmit the optical frequency standard signal. The length of the optical fiber 7 is adjusted according to the transmission requirements.

[0046] A first acousto-optic modulator 5 is connected to the proximal end of the optical fiber 7 transmission, and is connected to the laser light source 1 through the first optical fiber coupler 4 and the first optical isolator 2, and is used to shift the laser frequency; the light output by the laser light source 1 enters the first acousto-optic modulator 5 through the first optical isolator 2 and the first optical fiber coupler 4; the other end of the first optical fiber coupler 4 is connected to the first Faraday mirror 3;

[0047] A Raman amplifier 6 is connected to the optical fiber 7. The optical frequency standard signal enters the optical fiber 7 after passing through the first acousto-optic modulator 5. The Raman amplifier 6 is used to amplify the optical signal power.

[0048] The phase noise compensation unit comprises a second acousto-optic modulator 8 connected to the far end of the optical fiber 7 transmission, and a phase-locked loop 11, wherein the second acousto-optic modulator 8 is used to generate frequency shift for the laser and compensate for the optical fiber phase noise; the output end of the second acousto-optic modulator 8 is connected to the second optical fiber coupler 9, the output ends of the second optical fiber coupler 9 are respectively connected to the phase-locked loop 11 and the third optical fiber coupler 10, the output end of the phase-locked loop 11 is connected to the second acousto-optic modulator 8, and is used to detect and extract the optical fiber phase noise signal in real time, and to perform negative feedback control on the working frequency of the second acousto-optic modulator 8 to achieve compensation for the phase noise; the output end of the third optical fiber coupler 10 is respectively connected to the second Faraday mirror 12 and the third acousto-optic modulator 13, and the third acousto-optic modulator 13 is used to compensate for the frequency shift of the first acousto-optic modulator 5 and the second acousto-optic modulator 8, restore the frequency of the transmitted optical frequency standard signal, and achieve optical fiber remote precision transmission of the optical frequency standard signal.

[0049] In this embodiment, the remote end of the single-mode optical fiber 7 is connected to the second acousto-optic modulator 8, which is used to generate frequency shift for the laser and compensate for the optical fiber phase noise. The output light of the second acousto-optic modulator 8 is connected to the second optical fiber coupler 9 and the third optical fiber coupler 10 in sequence and then output. The other end of the second optical fiber coupler 9 is connected to the phase-locked loop 11, and one of the output ends of the third optical fiber coupler 10 is connected to the second Faraday mirror 12. The single-pass transmission light and the three-pass transmission light are beat and demodulated in the phase-locked loop 11 for real-time detection and extraction of the optical fiber phase noise signal. The phase-locked loop 11 outputs a negative feedback control signal to control the operating frequency of the second acousto-optic modulator 8 to achieve compensation for the optical fiber phase noise. Among them, the phase noise is obtained by optical heterodyne detection of the single-pass transmission light and the three-pass transmission light in the optical fiber, and the phase noise is compensated at the remote end.

[0050] Furthermore, the Raman amplifier 6 includes a pump laser 14, a second optical isolator 15, a fourth optical fiber coupler 16, an optical attenuator 17 and a wavelength division multiplexer 18, the output end of the pump laser 14 is connected to the second optical isolator 15, the output end of the second optical isolator 15 is connected to the fourth optical fiber coupler 16, the output end of the fourth optical fiber coupler 16 is respectively connected to two optical attenuators 17, the output ends of the two optical attenuators 17 are respectively connected to two wavelength division multiplexers 18, and the two wavelength division multiplexers 18 are located on the optical fiber 7; the light emitted by the pump laser 14 passes through the second optical isolator 15 and enters the fourth optical fiber coupler 16, the fourth optical fiber coupler 16 outputs two equal-power lights that enter the two optical attenuators 17 respectively, and then enter the optical fiber 7 after passing through the wavelength division multiplexer 18.

[0051] In this embodiment, the Raman amplifier 6 first depolarizes the pump laser 14, and then enters the 50:50 fourth fiber coupler 16 to split it into two paths after passing through the second optical isolator 15, and then uses the fourth fiber coupler 16 to adjust the optical power in the two optical fibers; and uses the wavelength division multiplexer 18 to couple the two paths of light into the optical fiber 7 for forward pumping and reverse pumping. Among them, the Raman amplifier 6 can amplify the power of both forward and reverse transmitted optical signals.

[0052] Furthermore, the sum of the operating frequency of the third AOM 13 and the operating frequencies of the first AOM 5 and the second AOM 8 is equal to zero.

[0053] Furthermore, the first Faraday mirror 3 is located at the proximal end of the optical fiber 7, and the second Faraday mirror 12 is located at the distal end of the optical fiber 7. The first Faraday mirror 3 and the second Faraday mirror 12 are used to reflect optical signals and rotate the polarization direction of the light passing back and forth by 90°.

[0054] Optionally, the laser light source 1 adopts a narrow linewidth laser.

[0055] Optionally, the optical fiber 7 is a single-mode optical fiber.

[0056] Optionally, the first optical fiber coupler 4, the second optical fiber coupler 9, the third optical fiber coupler 10, and the fourth optical fiber coupler 16 are used to realize the branching or combining of optical signals in the optical fiber.

[0057] The present invention also provides a method for precise transmission of optical frequency standard optical fiber with far-end phase noise compensation, which uses the above-mentioned precise transmission system of optical frequency standard optical fiber with far-end phase noise compensation, and comprises the following steps:

[0058] S1, frequency is ω 0 The optical frequency standard signal is shifted by the first acousto-optic modulator 1 After that, the optical frequency becomes ω 0 +ω 1 ; It is transmitted through a section of optical fiber to the second acousto-optic modulator at the far end, and then frequency-shifted ω 1 After that, the optical frequency becomes ω 0 +ω 1 +ω 2 ;

[0059] S2. The length fluctuation of the optical fiber causes the phase of the optical frequency standard signal output from the far end to jitter, which is the phase noise φ 1 , then the total phase of the one-way transmission light in the optical fiber is (ω 0 +ω 1 +ω 2 )t+φ 1 ;

[0060] S3, the one-way transmission light is reflected by the second Faraday mirror at the far end, returns to the optical fiber along the original path, and then passes through the second acousto-optic modulator to shift the frequency ω 2 、The first acousto-optic modulator shifts frequency ω 1 After returning to the near end, the phase noise in the return transmission is φ 2 , then the total phase of the two-way transmission light is (ω 0 +2ω 1 +2ω 2 )t+φ 1 +φ 2 ;

[0061] S4, the two-way transmission light is reflected by the first Faraday mirror at the near end, returns to the optical fiber again, and then passes through the first acousto-optic modulator to shift the frequency ω 1 、The second AOM frequency shift ω 2 After reaching the far end, the phase noise in the third-pass transmission is φ 3 , then the total phase of the three-pass transmission light is (ω 0 +3ω 1 +3ω 2 )t+φ 1 +φ 2 +φ 3 ;

[0062] S5, the one-way transmission light and the three-way transmission light are output from the fiber coupler and optical heterodyne detection is performed in the phase-locked loop, and the total phase of the beat frequency signal is (2ω 1 +2ω 2 )t+φ 2 +φ 3 , the frequency of use is 2ω 1 +2ω 2 The beat frequency signal is demodulated by the RF reference signal to obtain the phase noise φ 2 +φ 3 ;

[0063] S6. Assuming that the phase noise does not change much over time, we have φ 1 ≈φ 2 ≈φ 3 According to the phase noise signal obtained in S5, a negative feedback signal -φ is output through the phase-locked loop 1 Acting on the second AOM, it is used to compensate for the fiber phase noise in the one-way transmission light. Therefore, the total phase of the one-way transmission light output after the second AOM becomes (ω 0 +ω 1 +ω 2 )t, that is, the optical fiber phase noise has no effect on the transmitted optical frequency standard signal;

[0064] S7, set the operating frequency of the third acousto-optic modulator to -(ω 1 +ω2 ), so the frequency of the final output optical frequency standard signal is restored to ω 0 , which is consistent with the frequency of the input optical frequency standard signal.

[0065] The present invention also provides an optical frequency standard optical fiber precision transmission method, which uses the optical frequency standard optical fiber precision transmission system with far-end phase noise compensation, and includes the following steps:

[0066] Step 1: After passing through an isolator and a coupler, the narrow linewidth laser enters the first acousto-optic modulator for frequency shifting, and then enters the optical fiber for forward transmission. A remote user at any node in the optical fiber can extract the optical frequency standard signal; the optical signal is output at the remote end after passing through the second acousto-optic modulator; the near end and the remote end of the optical fiber are both connected to a Faraday mirror to return the end face optical signal to the original path and rotate the vibration direction of the light by 90°;

[0067] Step 2: The single-pass transmission light in the optical fiber and the three-pass transmission light reflected by the far-end and near-end Faraday mirrors are output in the far-end coupler and sent to the phase-locked loop for detecting and extracting the optical fiber phase noise, and outputting a reverse compensation signal to control the operating frequency of the second acousto-optic modulator to achieve the purpose of suppressing phase noise;

[0068] Step 3: The optical frequency standard signal after phase noise compensation enters the third acousto-optic modulator through the optical fiber coupler. The third acousto-optic modulator is used to shift the frequency of the remote optical frequency standard signal so that the sum of the operating frequencies of the first acousto-optic modulator, the second acousto-optic modulator and the third acousto-optic modulator is 0, and the optical signal frequencies at the optical fiber input and output ends remain consistent.

[0069] In summary, the present invention provides a far-end phase noise compensated optical frequency standard optical fiber precision transmission system and method, which realizes the long-distance transmission of ultra-high frequency stability optical signals through optical fibers. The present invention solves the phase noise problem caused by environmental factors in optical frequency standard signal transmission in optical fiber by using Raman amplifier as an optical power gain device and completing phase noise compensation at the far end of the optical fiber. Among them, the phase noise compensation unit of the present invention obtains the optical fiber phase noise signal at the far end of the optical fiber through optical heterodyne detection, and performs negative feedback control on the acousto-optic modulator, realizes real-time compensation of phase noise at the far end, and reduces the influence of phase noise on the frequency stability of the optical frequency standard signal. In addition, since the phase noise compensation is completed at the far end of the optical fiber, there is no restriction on the transmission end point, and the optical frequency standard transmission in a multi-port output mode can be realized, thereby solving the limitation of the transmission end point position by the traditional method. In addition, the present invention uses a Raman amplifier for the gain of the optical frequency standard signal, which uses the entire section of optical fiber as the gain medium, and the gain bandwidth reaches the THz level, so there is no need to perform frequency stabilization control on the pump laser, which is suitable for optical frequency standard transmission in a multi-port output mode.

[0070] Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments, or to make equivalent replacements for some of the technical features therein. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. An optical frequency standard optical fiber precision transmission system with far-end phase noise compensation, characterized in that: include: A laser light source, used for simulating an optical frequency standard signal, wherein an output end of the laser light source is connected to a first optical isolator to prevent light in the optical path from being reflected back to the laser light source; Optical fiber: used to transmit optical frequency standard signals; A first acousto-optic modulator is connected to the proximal end of the optical fiber transmission, and is connected to the laser light source through a first optical fiber coupler and a first optical isolator, and is used to shift the laser frequency; the light output by the laser light source enters the first acousto-optic modulator through the first optical isolator and the first optical fiber coupler; the other end of the first optical fiber coupler is connected to a first Faraday mirror; A Raman amplifier is connected to the optical fiber, and the optical frequency standard signal enters the optical fiber after passing through the first acousto-optic modulator, and the Raman amplifier is used to amplify the optical signal power; The phase noise compensation unit comprises a second acousto-optic modulator connected to the far end of the optical fiber transmission, and a phase-locked loop, wherein the second acousto-optic modulator is used to generate frequency shift for laser and compensate for optical fiber phase noise; the output end of the second acousto-optic modulator is connected to a second optical fiber coupler, the output ends of the second optical fiber coupler are respectively connected to the phase-locked loop and a third optical fiber coupler, the output end of the phase-locked loop is connected to the second acousto-optic modulator, and is used to detect and extract the optical fiber phase noise signal in real time, and negative feedback control is performed on the working frequency of the second acousto-optic modulator to realize compensation for phase noise; the output ends of the third optical fiber coupler are respectively connected to a second Faraday mirror and a third acousto-optic modulator, and the third acousto-optic modulator is used to compensate for the frequency shift of the first acousto-optic modulator and the second acousto-optic modulator.

2. The optical frequency standard optical fiber precision transmission system for far-end phase noise compensation according to claim 1 is characterized in that: The Raman amplifier comprises a pump laser, a second optical isolator, a fourth optical fiber coupler, an optical attenuator and a wavelength division multiplexer. The output end of the pump laser is connected to the second optical isolator, the output end of the second optical isolator is connected to the fourth optical fiber coupler, the output end of the fourth optical fiber coupler is respectively connected to two optical attenuators, the output ends of the two optical attenuators are respectively connected to two wavelength division multiplexers, and the two wavelength division multiplexers are located on the optical fiber; the light emitted by the pump laser passes through the second optical isolator and enters the fourth optical fiber coupler, the fourth optical fiber coupler outputs two paths of light with equal power and enters the two paths of light attenuators respectively, and then enters the optical fiber after passing through the wavelength division multiplexer.

3. The optical frequency standard optical fiber precision transmission system for far-end phase noise compensation according to claim 2 is characterized in that: The sum of the operating frequency of the third AOM and the operating frequencies of the first AOM and the second AOM is equal to zero.

4. The optical frequency standard optical fiber precision transmission system for far-end phase noise compensation according to claim 3 is characterized in that: The first Faraday mirror is located at the proximal end of the optical fiber, and the second Faraday mirror is located at the distal end of the optical fiber. The first Faraday mirror and the second Faraday mirror are used to reflect optical signals and rotate the polarization direction of the light passing back and forth by 90°.

5. The optical frequency standard optical fiber precision transmission system for far-end phase noise compensation according to claim 1, characterized in that: The laser light source adopts a narrow line width laser; the optical fiber is a single mode optical fiber.

6. The optical frequency standard optical fiber precision transmission system for far-end phase noise compensation according to claim 3 is characterized in that: The first optical fiber coupler, the second optical fiber coupler, the third optical fiber coupler, and the fourth optical fiber coupler are used to realize the branching or combining of optical signals in the optical fiber.

7. The optical frequency standard optical fiber precision transmission system for far-end phase noise compensation according to claim 3 is characterized in that: The phase noise is obtained by optical heterodyne detection of single-transmission light and triple-transmission light in an optical fiber, and the phase noise is compensated at a remote end.

8. The optical frequency standard optical fiber precision transmission system for far-end phase noise compensation according to claim 3 is characterized in that: The Raman amplifier can amplify the power of optical signals transmitted in both forward and reverse directions.

9. A method for precise transmission of optical frequency standard optical fiber with far-end phase noise compensation, characterized in that: An optical frequency standard optical fiber precision transmission system using a far-end phase noise compensation as claimed in any one of claims 1 to 8 comprises the following steps: S1, the optical frequency standard signal with a frequency of ω0, after being shifted by ω1 by the first acousto-optic modulator, the optical frequency becomes ω0+ω1; After being transmitted through a section of optical fiber to the second acousto-optic modulator at the far end, the optical frequency becomes ω0+ω1+ω2 after being shifted by ω1. S2. The length fluctuation of the optical fiber causes the phase of the optical frequency standard signal output at the far end to jitter, which is the phase noise φ1. Then the total phase of the one-way transmission light in the optical fiber is (ω0+ω1+ω2)t+φ1. S3, the one-way transmission light is reflected by the second Faraday mirror at the far end, returns to the optical fiber along the original path, and then passes through the second acousto-optic modulator with a frequency shift of ω2 and the first acousto-optic modulator with a frequency shift of ω1 to return to the near end. The phase noise in the return transmission is φ2, so the total phase of the two-way transmission light is (ω0+2ω1+2ω2)t+φ1+φ2; S4, the two-way transmission light is reflected by the first Faraday mirror at the near end, returns to the optical fiber again, and then reaches the far end after the first acousto-optic modulator shifts the frequency by ω1 and the second acousto-optic modulator shifts the frequency by ω2. The phase noise in the third-way transmission is φ3, so the total phase of the three-way transmission light is (ω0+3ω1+3ω2)t+φ1+φ2+φ3; S5, the one-way transmission light and the three-way transmission light are output from the optical fiber coupler, and optical heterodyne detection is performed in a phase-locked loop, and the total phase of the beat frequency signal is obtained to be (2ω1+2ω2)t+φ2+φ3, and the beat frequency signal is demodulated using a radio frequency reference signal with a frequency of 2ω1+2ω2 to obtain a phase noise φ2+φ3; S6. Assuming that the phase noise does not change much over time, then φ1≈φ2≈φ3. According to the phase noise signal obtained in S5, the negative feedback signal -φ1 is output through the phase-locked loop to act on the second acousto-optic modulator to compensate for the fiber phase noise in the one-way transmission light. Therefore, the total phase of the one-way transmission light output after the second acousto-optic modulator becomes (ω0+ω1+ω2)t, that is, the fiber phase noise has no effect on the transmitted optical frequency standard signal. S7, setting the operating frequency of the third acousto-optic modulator to -(ω1+ω2), so that the frequency of the optical frequency standard signal finally output is restored to ω0, which is consistent with the frequency of the input optical frequency standard signal.

10. A method for precise transmission of optical frequency standard optical fiber, characterized in that: An optical frequency standard optical fiber precision transmission system using a far-end phase noise compensation as claimed in any one of claims 1 to 8 comprises the following steps: Step 1: After passing through an isolator and a coupler, the narrow linewidth laser enters the first acousto-optic modulator for frequency shifting, and then enters the optical fiber for forward transmission. A remote user at any node in the optical fiber can extract the optical frequency standard signal; the optical signal is output at the remote end after passing through the second acousto-optic modulator; the near end and the remote end of the optical fiber are both connected to a Faraday mirror to return the end face optical signal to the original path and rotate the vibration direction of the light by 90°; Step 2: The single-pass transmission light in the optical fiber and the three-pass transmission light reflected by the far-end and near-end Faraday mirrors are output in the far-end coupler and sent to the phase-locked loop for detecting and extracting the optical fiber phase noise, and outputting a reverse compensation signal to control the operating frequency of the second acousto-optic modulator to achieve the purpose of suppressing phase noise; Step 3: The optical frequency standard signal after phase noise compensation enters the third acousto-optic modulator through the optical fiber coupler. The third acousto-optic modulator is used to shift the frequency of the remote optical frequency standard signal so that the sum of the operating frequencies of the first acousto-optic modulator, the second acousto-optic modulator and the third acousto-optic modulator is 0, and the optical signal frequencies at the optical fiber input and output ends remain consistent.

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