A remote phase noise compensation optical frequency standard fiber precise transmission system and method

Abstract

The application discloses a kind of optical frequency standard optical fiber precision transmission systems and methods of remote phase noise compensation, the system includes laser light source, optical fiber, first acoustooptic modulator, Raman amplifier, laser light source is used to simulate optical frequency standard signal, optical fiber is used to transmit optical frequency standard signal;First acoustooptic modulator is connected with laser light source by first optical fiber coupler, first optical isolator, for frequency shift to laser;Raman amplifier is connected with optical fiber, optical frequency standard signal enters optical fiber after first acoustooptic modulator, and the power of optical signal is amplified using Raman amplifier.The application obtains optical fiber phase noise signal by the light heterodyne detection of the phase-locked loop at the far end of optical fiber, and the working frequency of acoustooptic modulator is controlled by negative feedback to compensate optical fiber phase noise in real time, reduce the influence of phase noise on optical frequency standard signal frequency stability, solve the limitation of transmission end position of traditional method, and be applicable to the transmission of optical frequency standard of multiport output mode.

Description

TECHNICAL FIELD

[0001] The present application relates to a kind of far-end phase noise compensation optical frequency standard optical fiber precision transmission system and method, belong to optical frequency standard transmission field. BACKGROUND

[0002] Optical frequency standard transmission has extensive demand in the development of space navigation, precise positioning, radio telescope array, basic physics research etc. applications.Optical fiber has high stability, low loss, convenient to use and other advantages, is the ideal medium for optical signal transmission.The global well-established optical fiber communication network also provides convenience for the research of optical frequency standard long-distance precision transmission.In the research of optical frequency standard long-distance transmission, mainly facing two problems:

[0003] First, the communication optical fiber laid in outdoor, will be affected by surrounding vibration and environmental temperature fluctuation etc. factors, cause the random fluctuation of optical fiber length.When optical frequency standard signal is transmitted in optical fiber, it is embodied as the jitter of transmission time, and it is observed in frequency domain as phase noise.The size of phase noise will be accumulated with the extension of transmission distance, leading to the broadening of optical frequency standard signal and the decline of frequency stability.Therefore, how to effectively suppress optical fiber phase noise becomes the key research work in the field of optical frequency standard transmission.

[0004] Near-end phase noise compensation is the most commonly used scheme in the current research of optical frequency standard transmission.With the development of noise compensation technology, transmission instability can be reduced to 10 -19 order of magnitude, meet the requirements of optical frequency standard transmission.In this scheme, transmission laser source and noise compensation system are set in near-end laboratory, have the advantage of convenient in actual operation and control.On the other hand, the structural characteristics of this near-end phase noise compensation scheme limit the location of transmission terminal, so it is only suitable for point-to-point optical frequency standard transmission.In the application based on time frequency standard, high-precision time synchronization is often needed between multiple points.

[0005] On the other hand, for the problem of optical power loss in the process of optical frequency standard transmission, at present, the commonly used methods are mainly divided into four kinds. Among them, the most commonly used way is to use bidirectional EDFA to directly amplify the optical signal. The advantage of this method is that it does not need to add additional control, which can reduce the maintenance cost of system operation. However, the power surge of the node position is easy to introduce strong scattering effect, which affects the stability of the optical frequency standard transmission; secondly, the signal light is regenerated and amplified by establishing a relay station, which is also a common method at present. However, the establishment and maintenance cost of the relay station is high, which is not conducive to popularization in practical application; in addition, Brillouin amplifier has also been applied more in the research work in recent years. Brillouin amplifier uses a whole section of optical fiber as the gain medium, which effectively avoids the power surge. However, its gain bandwidth is only tens of MHz, and for multi-port and multi-channel transmission, frequency-matched pump laser needs to be selected respectively, and frequency stabilization control is needed, so it is not suitable for optical frequency standard transmission in multi-port output mode. SUMMARY

[0006] In order to solve the above technical problems, the present application provides a kind of optical frequency standard fiber precision transmission system and method of remote phase noise compensation, realize the optical signal of ultra-high frequency stability is transmitted by optical fiber for long distance.This application obtains the optical fiber phase noise signal by the light heterodyne detection of the phase-locked loop at the far end of the optical fiber, and the working frequency of the acousto-optic modulator is controlled by negative feedback to compensate the optical fiber phase noise in real time, which reduces the influence of phase noise on the frequency stability of optical frequency standard signal. Since the phase noise compensation is completed at the far end of the optical fiber, there is no restriction on the transmission terminal, and multi-port output in optical frequency standard transmission can be realized. In addition, the Raman amplifier of the present application uses a whole section of optical fiber as the gain medium, and the gain bandwidth reaches THz level, without the need for frequency stabilization control of pump laser, which is suitable for optical frequency standard transmission in multi-port output mode.

[0007] The first object of the present application is to provide a kind of optical frequency standard fiber precision transmission system of remote phase noise compensation, comprising:

[0008] Laser light source for simulating optical frequency standard signal, its output end is connected with first optical isolator, for avoiding the light in optical path being reflected back to the laser light source;

[0009] Optical fiber: for transmitting optical frequency standard signal;

[0010] First acousto-optic modulator is connected to the near end of the optical fiber transmission, and is connected with laser light source through first optical fiber coupler and first optical isolator, for frequency shifting of laser;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 with a first Faraday mirror;

[0011] a Raman amplifier connected to the optical fiber, the optical frequency marker signal entering the optical fiber after passing through the first acousto-optic modulator, and the Raman amplifier being used to amplify the power of the optical signal;

[0012] a phase noise compensation unit comprising a second acousto-optic modulator connected to the far end of the optical fiber and a phase-locked loop, the second acousto-optic modulator being used to generate frequency shift of laser and compensate for the phase noise of the optical fiber; the output end of the second acousto-optic modulator is connected to a second optical fiber coupler, the output end of the second optical fiber coupler is connected to the phase-locked loop and a third optical fiber coupler respectively, the output end of the phase-locked loop is connected to the second acousto-optic modulator, which is used to detect and extract the phase noise signal of the optical fiber in real time, and to control the working frequency of the second acousto-optic modulator by negative feedback, so as to compensate for the phase noise; the output end of the third optical fiber coupler is connected to a second Faraday mirror and a third acousto-optic modulator respectively, 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 an embodiment of the present application, 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 connected to two optical attenuators respectively, the output end of the two optical attenuators is connected to two wavelength division multiplexers respectively, and the two wavelength division multiplexers are located on the optical fiber; the light emitted by the pump laser enters the fourth optical fiber coupler after passing through the second optical isolator, the fourth optical fiber coupler outputs two equal-power light paths which enter two optical attenuators respectively, and then enter the optical fiber through the wavelength division multiplexers.

[0014] In an embodiment of the present application, the working frequency of the third acousto-optic modulator is equal to zero plus the working frequency of the first acousto-optic modulator and the second acousto-optic modulator.

[0015] In an embodiment of the present application, the first Faraday mirror is located at the near end of the optical fiber, and the second Faraday mirror is located at the far end of the optical fiber, and 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 an embodiment of the present application, the laser light source adopts a narrow line width laser; and the optical fiber is a single mode optical fiber.

[0017] In an embodiment of the present application, 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 branching or combining of the optical signal in the optical fiber.

[0018] In one embodiment of the present application, the phase noise is obtained by optical heterodyne detection of the single-pass light and the three-pass light in the optical fiber, and the phase noise is compensated at the far end.

[0019] In one embodiment of the present application, the Raman amplifier can amplify the power of the light signals transmitted in the forward direction and the reverse direction.

[0020] A second object of the present application is to provide a method for precise transmission of an optical frequency standard fiber, using the system for precise transmission of an optical frequency standard fiber with remote phase noise compensation.

[0021] S1, the optical frequency standard signal with a frequency of ω0 is shifted by ω1 through a first acousto-optic modulator, and the optical frequency becomes ω0+ω1; the light is transmitted to a second acousto-optic modulator at the far end through a section of optical fiber, and the optical frequency is shifted by ω1 again, becoming ω0+ω1+ω2;

[0022] S2, the length fluctuation of the optical fiber causes the phase of the optical frequency standard signal output at the far end to fluctuate, i.e. the phase noise is φ1, and the total phase of the single-pass light in the optical fiber is (ω0+ω1+ω2)t+φ1;

[0023] S3, the single-pass light is reflected by the second Faraday mirror at the far end, returns to the optical fiber along the original path, and then reaches the near end after being shifted by ω2 through the second acousto-optic modulator and by ω1 through the first acousto-optic modulator, and the phase noise in the return transmission is φ2, and the total phase of the double-pass light is (ω0+2ω1+2ω2)t+φ1+φ2;

[0024] S4, the double-pass 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 being shifted by ω1 through the first acousto-optic modulator and by ω2 through the second acousto-optic modulator, and the phase noise in the third transmission is φ3, and the total phase of the three-pass light is (ω0+3ω1+3ω2)t+φ1+φ2+φ3;

[0025] S5, the single-pass light and the three-pass light are output from the optical fiber coupler, and the optical heterodyne detection is performed in a phase-locked loop, and the total phase of the beat signal is (2ω1+2ω2)t+φ2+φ3, and a radio frequency reference signal with a frequency of 2ω1+2ω2 is used to demodulate the beat signal, and the phase noise φ2+φ3 is obtained;

[0026] S6, assuming that the phase noise does not change much with time, then φ1≈φ2≈φ3, and according to the phase noise signal obtained in S5, a negative feedback signal -φ1 is output from the phase-locked loop and acts on the second acousto-optic modulator, to compensate for the phase noise of the single-pass light in the optical fiber, and the total phase of the single-pass light output after the second acousto-optic modulator becomes (ω0+ω1+ω2)t, i.e. the phase noise of the optical fiber does not affect the transmission of the optical frequency standard signal.

[0027] S7, the working frequency of the third acousto-optic modulator is set to -(ω1+ω2), so that the frequency of the final output optical frequency standard signal is restored to ω0, consistent with the frequency of the input optical frequency standard signal.

[0028] A third object of the present application is to provide an optical frequency standard fiber precision transmission method, which uses the optical frequency standard fiber precision transmission system for remote phase noise compensation, comprising the following steps:

[0029] Step 1, after the narrow linewidth laser passes through the isolator and the coupler and enters the first acousto-optic modulator for frequency shifting, it is transmitted in the optical fiber in the forward direction, and the remote user at any node in the optical fiber can extract the optical frequency standard signal; the optical signal is output after passing through the second acousto-optic modulator at the remote end; the near end and the remote end of the optical fiber are connected with Faraday mirrors, which 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 and the three-pass transmission light reflected by the remote end and the near end Faraday mirrors are output in the remote end coupler and sent to the phase-locked loop for detecting and extracting the optical fiber phase noise, and outputting the reverse compensation signal to control the working frequency of the second acousto-optic modulator, so as to achieve the purpose of suppressing the phase noise;

[0031] Step 3, after the optical frequency standard signal is compensated for phase noise, it enters the third acousto-optic modulator through the optical fiber coupler, and the third acousto-optic modulator is used for frequency shifting of the remote optical frequency standard signal, so that the sum of the working 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 input end and the output end of the optical fiber are consistent.

[0032] The present application has the following advantages:

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

[0034] 2. The present application introduces a Raman amplifier in the remote phase noise compensation system to realize 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 wide, it is not necessary to stabilize the frequency of the pump laser, which is suitable for optical frequency transmission in multiple user channels.

[0035] In summary, the application provides a kind of optical frequency standard optical fiber precision transmission system and method of remote phase noise compensation, realize the light signal of ultra-high frequency stability is transmitted by optical fiber at long distance.The application uses Raman amplifier as optical power gain device, and completes phase noise compensation at the far end of optical fiber, solves the phase noise problem of optical frequency standard signal in optical fiber transmission caused by environmental factors.The phase noise compensation unit of the application obtains optical fiber phase noise signal at the far end of optical fiber by optical heterodyne detection, and carries out negative feedback control to acoustooptic modulator, realizes real-time compensation of phase noise at the far end, reduces the influence of phase noise on the frequency stability of optical frequency standard signal.And since the phase noise compensation is completed at the far end of optical fiber, there is no limit to the transmission terminal, and the multi-port output mode of optical frequency standard transmission can be realized, thereby solving the limitation of the transmission terminal position of the traditional method.In addition, the application uses Raman amplifier for the gain of optical frequency standard signal, which uses a whole optical fiber as a gain medium, and the gain bandwidth reaches THz level, so it is not necessary to control the frequency of pump laser, and it is suitable for multi-port output mode of optical frequency standard transmission. BRIEF DESCRIPTION OF DRAWINGS

[0036] In order to more clearly illustrate the technical solutions in the embodiments of the application, the drawings needed in the embodiment description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the application, and other drawings can be obtained by those skilled in the art without creative labor.

[0037] Figure 1 The structure diagram of the optical frequency standard precision transmission system with remote phase noise compensation provided by the application is shown in the figure.

[0038] Figure 2 The structure diagram of the Raman amplifier provided by the application is shown in the figure.

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

[0040] With reference to the drawings of the embodiments of the present application, the technical solutions in the embodiments of the present application will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments of the present application, all the other embodiments obtained by a person of ordinary skill in the art without creative effort belong to the scope of the present application.

[0041] In the present application, unless specifically defined and limited otherwise, the terms "connected", "connected", "fixed" should be understood broadly, for example, it can be fixed connection, or detachable connection, or integrated; it can be mechanical connection, or electrical connection; it can be directly connected, or indirectly connected through intermediate medium, or the internal communication of two elements or the interaction relationship between two elements. For a person of ordinary skill in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0042] In the present application, unless specifically defined and limited otherwise, the first feature "on" or "under" the second feature can include that the first and second features are in direct contact, or that the first and second features are not in direct contact but are in contact through another feature between them. Moreover, the first feature "on", "above" and "above" the second feature includes that the first feature is directly above and obliquely above the second feature, or only indicates that the horizontal height of the first feature is higher than that of the second feature. The first feature "under", "below" and "below" the second feature includes that the first feature is directly below and obliquely below the second feature, or only indicates that the horizontal height of the first feature is less than that of the second feature.

[0043] As shown in Figure 1 and Figure 2 The present application provides a remote phase noise compensation optical frequency standard optical fiber precision transmission system, comprising:

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

[0045] An optical fiber 7 is used to transmit the optical frequency standard signal, and the length of the optical fiber 7 is adjusted according to the transmission requirement;

[0046] A first acousto-optic modulator 5 is connected to the near end of the optical fiber 7 transmission, and is connected to the laser light source 1 through a first optical fiber coupler 4 and a first optical isolator 2, and is used to produce frequency shift for the laser; 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 with a first Faraday mirror 3;

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

[0048] A phase noise compensation unit includes a second acousto-optic modulator 8 connected to the far end of the optical fiber 7, and a phase-locked loop 11, the second acousto-optic modulator 8 is used to generate frequency shift of laser and compensate for the phase noise of the optical fiber; the output end of the second acousto-optic modulator 8 is connected to the second optical fiber coupler 9, and the output end of the second optical fiber coupler 9 is connected to the phase-locked loop 11 and the third optical fiber coupler 10 respectively, the output end of the phase-locked loop 11 is connected to the second acousto-optic modulator 8, which is used to detect and extract the phase noise signal of the optical fiber in real time, and to control the working frequency of the second acousto-optic modulator 8 by negative feedback, so as to compensate for the phase noise; the output end of the third optical fiber coupler 10 is respectively connected with 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 realize the optical fiber remote precise transmission of the optical frequency standard signal.

[0049] In the embodiment, the far end of the single-mode optical fiber 7 is connected to the second acousto-optic modulator 8, which is used to generate frequency shift of laser and compensate for the phase noise of the optical fiber. The output light of the second acousto-optic modulator 8 is sequentially connected to the second optical fiber coupler 9 and the third optical fiber coupler 10 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 frequency-doubled and demodulated in the phase-locked loop 11, which is used to detect and extract the phase noise signal of the optical fiber in real time, and the phase-locked loop 11 outputs a negative feedback control signal to control the working frequency of the second acousto-optic modulator 8, so as to compensate for the phase noise of the optical fiber. Among them, the phase noise is obtained by heterodyne detection of single-pass transmission light and three-pass transmission light in the optical fiber, and the phase noise is compensated at the far end.

[0050] Further, 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 connected to two optical attenuators 17 respectively, the output ends of the two optical attenuators 17 are connected to two wavelength division multiplexers 18 respectively, and the two wavelength division multiplexers 18 are located on the optical fiber 7; the light emitted by the pump laser 14 enters the fourth optical fiber coupler 16 after passing through the second optical isolator 15, the fourth optical fiber coupler 16 outputs two equal-power light paths which enter two optical attenuators 17 respectively, and then enter the optical fiber 7 through the wavelength division multiplexer 18.

[0051] In the embodiment, the Raman amplifier 6 firstly performs depolarization on the pump laser 14, and the pump laser 14 after the second optical isolator 15 enters the fourth optical fiber coupler 16 to be divided into two paths, and then the fourth optical fiber coupler 16 is used to adjust the optical power in the two optical fibers; the two paths of light are coupled into the optical fiber 7 by using the wavelength division multiplexer 18 for forward pumping and backward pumping. The Raman amplifier 6 can amplify the optical signal power transmitted in the forward direction and the backward direction.

[0052] Further, the sum of the working frequency of the third acousto-optic modulator 13 and the working frequencies of the first acousto-optic modulator 5 and the second acousto-optic modulator 8 is equal to zero.

[0053] Further, 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, and the first Faraday mirror 3 and the second Faraday mirror 12 are used to reflect the optical signal and rotate the polarization direction of the light passing back and forth by 90°.

[0054] Optionally, the laser light source 1 adopts a narrow line width 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 the optical signal in the optical fiber.

[0057] The application further provides an optical frequency standard fiber precision transmission method for remote phase noise compensation, which uses the optical frequency standard fiber precision transmission system for remote phase noise compensation.

[0058] S1, the optical frequency standard signal with a frequency of ω0 is frequency-shifted by ω1 through the first acousto-optic modulator, and the optical frequency becomes ω0+ω1; the optical frequency standard signal is transmitted to the second acousto-optic modulator at the distal end through a section of optical fiber, and the optical frequency becomes ω0+ω1+ω2 after frequency-shifted by ω1 again;

[0059] S2, the length fluctuation of the optical fiber causes the phase of the optical frequency standard signal output at the distal end to jitter, that is, the phase noise φ1, and the total phase of the single-pass transmission light in the optical fiber is (ω0+ω1+ω2)t+φ1;

[0060] S3, the single-pass transmission light is reflected by the second Faraday mirror at the distal end, returns to the optical fiber through the original path, and then returns to the proximal end after frequency-shifted by ω2 through the second acousto-optic modulator and frequency-shifted by ω1 through the first acousto-optic modulator, and the phase noise φ2 in the return transmission, and the total phase of the double-pass transmission light is (ω0+2ω1+2ω2)t+φ1+φ2;

[0061] S4, the double transmission light is reflected by the first Faraday mirror near the end, and then returns to the optical fiber, and then sequentially passes through the first acousto-optic modulator for frequency shift ω1, the second acousto-optic modulator for frequency shift ω2, and reaches the far end, and the phase noise φ3 in the third transmission, so the total phase of the three transmission lights is (ω0+3ω1+3ω2)t+φ1+φ2+φ3;

[0062] S5, the single transmission light and the three transmission lights are output from the optical fiber coupler, and are subjected to optical heterodyne detection in the phase-locked loop, so that the total phase of the beat frequency signal is (2ω1+2ω2)t+φ2+φ3, and the phase noise φ2+φ3 is obtained by using the radio frequency reference signal with the frequency of 2ω1+2ω2 to demodulate the beat frequency signal;

[0063] S6, assuming that the phase noise does not change much with time, so φ1≈φ2≈φ3, according to the phase noise signal obtained in S5, the negative feedback signal-φ1 is output by the phase-locked loop and acts on the second acousto-optic modulator, so as to compensate the optical fiber phase noise in the single transmission light, and the total phase of the single transmission light output after passing through the second acousto-optic modulator becomes (ω0+ω1+ω2)t, that is, the optical fiber phase noise does not affect the transmission of the optical frequency standard signal;

[0064] S7, the working frequency of the third acousto-optic modulator is set to-(ω1+ω2), so that the frequency of the finally output optical frequency standard signal is restored to ω0, which is consistent with the frequency of the input optical frequency standard signal.

[0065] The application also provides an optical frequency standard fiber precision transmission method, which uses the optical frequency standard fiber precision transmission system for far-end phase noise compensation.

[0066] Step 1, after the narrow linewidth laser passes through the isolator, the coupler, enters the first acousto-optic modulator for frequency shift, and then enters the optical fiber for forward transmission, the far-end user at any node in the optical fiber can extract the optical frequency standard signal; the optical signal is output after passing through the second acousto-optic modulator at the far end; the near end and the far end of the optical fiber are connected with Faraday mirrors, which return the end surface optical signal to the original path and rotate the vibration direction of the light by 90°;

[0067] Step 2, the single transmission light and the three transmission lights reflected by the far-end and near-end Faraday mirrors are output from the coupler at the far end and sent to the phase-locked loop for detection and extraction of the optical fiber phase noise, and an inverse compensation signal is output to control the working frequency of the second acousto-optic modulator, so as to suppress the phase noise;

[0068] Step 3, the optical frequency standard signal after phase noise compensation enters a third acousto-optic modulator through a fiber coupler, and the third acousto-optic modulator is used for frequency shifting of the remote optical frequency standard signal, so that the sum of the working 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 of the input end and the output end of the optical fiber remain consistent.

[0069] In conclusion, the present application provides a kind of remote phase noise compensation optical frequency standard optical fiber precision transmission system and method, realizes the optical signal of ultra-high frequency stability is transmitted by optical fiber over long distances.The present application uses Raman amplifier as optical power gain device, and completes phase noise compensation at the remote end of optical fiber, solves the phase noise problem of optical frequency standard signal in optical fiber transmission caused by environmental factors.The phase noise compensation unit of the present application obtains optical fiber phase noise signal at the remote end of optical fiber by optical heterodyne detection, and carries out negative feedback control to acousto-optic modulator, realizes real-time compensation of phase noise at the remote end, reduces the influence of phase noise on the frequency stability of optical frequency standard signal.And since phase noise compensation is completed at the remote end of optical fiber, there is no limit to the transmission terminal, and the multi-port output mode of optical frequency standard transmission can be realized, so as to solve the limitation of transmission terminal position in traditional method.In addition, the present application uses Raman amplifier for gain of optical frequency standard signal, which uses a whole optical fiber as gain medium, and the gain bandwidth reaches THz level, so there is no need to stabilize the frequency of pump laser, and it is suitable for multi-port output mode of optical frequency standard transmission.

[0070] Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art can modify the technical solutions recorded in the foregoing embodiments, or replace some of the technical features with equivalent ones, any modification, equivalent replacement, improvement, etc. within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. An optical frequency standard fiber precise transmission system with remote phase noise compensation, characterized in that, The application relates to a laser frequency standard signal transmission device. The device comprises a laser light source for simulating a laser frequency standard signal, the output end of the laser light source being connected with a first optical isolator for avoiding the reflection of light in an optical path back to the laser light source; an optical fiber for transmitting the laser frequency standard signal; a first acousto-optic modulator connected to the near end of the optical fiber transmission, the first acousto-optic modulator being connected with the laser light source through a first optical fiber coupler and a first optical isolator, and being used for frequency shifting of laser light; the laser 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 with a first Faraday mirror; a Raman amplifier connected with the optical fiber, the laser frequency standard signal entering the optical fiber after passing through the first acousto-optic modulator, and the Raman amplifier being used for amplifying the power of the optical signal; a phase noise compensation unit comprising a second acousto-optic modulator connected to the far end of the optical fiber transmission and a phase-locked loop, the second acousto-optic modulator being used for frequency shifting of laser light and compensation of optical fiber phase noise; the output end of the second acousto-optic modulator is connected with a second optical fiber coupler, the output end of the second optical fiber coupler is connected with the phase-locked loop and a third optical fiber coupler respectively, the output end of the phase-locked loop is connected with the second acousto-optic modulator, the phase-locked loop being used for real-time detection and extraction of an optical fiber phase noise signal, and negative feedback control of the working frequency of the second acousto-optic modulator, so as to realize the compensation of the phase noise; the output end of the third optical fiber coupler is connected with a second Faraday mirror and a third acousto-optic modulator respectively, the third acousto-optic modulator being used for compensating the frequency shift amount of the first acousto-optic modulator and the second acousto-optic modulator.

2. The optical frequency standard fiber precise transmission system with remote phase noise compensation according to claim 1, 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 with the second optical isolator, the output end of the second optical isolator is connected with the fourth optical fiber coupler, the output end of the fourth optical fiber coupler is connected with two optical attenuators respectively, the output ends of the two optical attenuators are connected with two wavelength division multiplexers respectively, and the two wavelength division multiplexers are located on the optical fiber; the light emitted by the pump laser enters the fourth optical fiber coupler after passing through the second optical isolator, the fourth optical fiber coupler outputs two equal-power light paths which enter two optical attenuators respectively, and then enter the optical fiber through the wavelength division multiplexers.

3. The optical frequency standard fiber precise transmission system with remote phase noise compensation according to claim 2, characterized in that, The sum of the working frequencies of the first acousto-optic modulator and the second acousto-optic modulator is equal to zero.

4. The optical frequency standard fiber precise transmission system with remote phase noise compensation according to claim 3, characterized in that, The first Faraday mirror is located at the near end of the optical fiber, and the second Faraday mirror is located at the far end of the optical fiber; the first Faraday mirror and the second Faraday mirror are used for reflecting the optical signal and rotating the polarization direction of the light passing back and forth by 90 degrees.

5. The optical frequency standard fiber precise transmission system with remote phase noise compensation according to claim 1, characterized in that, The laser light source adopts a narrow line width laser; and the optical fiber is a single-mode optical fiber.

6. The optical frequency standard fiber precise transmission system with remote phase noise compensation according to claim 3, 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 for realizing the branching or combining of the optical signal in the optical fiber.

7. The optical frequency standard fiber precise transmission system with remote phase noise compensation according to claim 3, characterized in that, The phase noise is obtained through the heterodyne detection of single transmission light and three times transmission light in the optical fiber, and the phase noise is compensated at the far end.

8. The optical frequency standard fiber precise transmission system with remote phase noise compensation according to claim 3, characterized in that, The Raman amplifier can amplify the power of the optical signal transmitted in the forward direction and the reverse direction.

9. A method for precise optical frequency standard fiber transmission with remote phase noise compensation, characterized in that, The application relates to a remote phase noise compensation optical frequency standard fiber precision transmission system, which comprises the following steps: S1, an optical frequency standard signal with a frequency of omega0 is frequency-shifted by omega1 through a first acousto-optic modulator, and the optical frequency becomes omega0+omega1; The single-path transmission light is reflected by a second Faraday mirror at the remote end, returns to the fiber, and is frequency-shifted by omega2 through the second acousto-optic modulator and by omega1 through the first acousto-optic modulator to return to the near end, and the phase noise phi2 in the return transmission is obtained, so that the total phase of the double-path transmission light is (omega0+2*omega1+2*omega2)*t+phi1+phi2; S4, the double-path transmission light is reflected by the first Faraday mirror at the near end, returns to the fiber again, and is frequency-shifted by omega1 through the first acousto-optic modulator and by omega2 through the second acousto-optic modulator to reach the remote end, and the phase noise phi3 in the third transmission is obtained, so that the total phase of the three-path transmission light is (omega0+3*omega1+3*omega2)*t+phi1+phi2+phi3; S5, the single-path transmission light and the three-path transmission light are output from the fiber coupler and are optically heterodyne detected in a phase-locked loop, the total phase of the beat signal is (2*omega1+2*omega2)*t+phi2+phi3, a radio frequency reference signal with a frequency of 2*omega1+2*omega2 is used to demodulate the beat signal, and the phase noise phi2+phi3 is obtained; S6, assuming that the phase noise does not change much with time, so phi1=phi2=phi3, according to the phase noise signal obtained in S5, a negative feedback signal-phi1 is output by the phase-locked loop and is used to compensate the fiber phase noise in the single-path transmission light, so that the total phase of the single-path transmission light output after the second acousto-optic modulator becomes (omega0+omega1+omega2)*t, that is, the fiber phase noise does not affect the transmission of the optical frequency standard signal; S7, the working frequency of the third acousto-optic modulator is set as -(omega1+omega2), so that the frequency of the finally output optical frequency standard signal is restored to omega0, which is consistent with the frequency of the input optical frequency standard signal. The application relates to a remote phase noise compensation optical frequency standard fiber precision transmission system, which comprises the following steps: Step 1, a narrow-line-width laser passes through an isolator and a coupler, enters a first acousto-optic modulator to be frequency-shifted, and is transmitted in the fiber in a forward direction, and a remote user at any node in the fiber can extract the optical frequency standard signal; the optical signal is output after passing through a second acousto-optic modulator at the remote end; the near end and the remote end of the fiber are connected with Faraday mirrors, the end surface optical signal returns to the original path and the vibration direction of the light is rotated by 90 degrees.

10. An optical frequency standard optical fiber precision transmission method, characterized by, ​ ​ Step 2, the single-pass light in the optical fiber and the three-pass light after reflecting by the far-end and near-end Faraday mirrors are output in the coupler at the far end and sent to the phase-locked loop for detecting and extracting the phase noise of the optical fiber, and an inverse compensation signal is output to control the operating frequency of the second acousto-optic modulator, so as to achieve the purpose of suppressing the phase noise; Step 3, the optical frequency standard signal after phase noise compensation enters the third acousto-optic modulator through the optical fiber coupler, and the third acousto-optic modulator is used for frequency shifting of the far-end 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 input end and the output end of the optical fiber remain consistent.

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