Temperature-controlled optical fiber delay line-based optical microwave stable transmission system

Abstract

The application provides a temperature-controlled optical fiber delay line-based optical microwave stable transmission system, wherein a delay jitter change is accurately reversely compensated by a delay compensation module, excellent delay consistency is shown in ultra-wideband signal transmission, and the demand of simultaneously meeting large range, high precision, fast response and wide spectrum compensation capacity of traditional piezoelectric ceramic fiber stretcher, optical phase shifter, cascaded multi-stage optical switch and motor space optical delay line compensation devices cannot be met; that is, the link delay compensation scheme in the transmission system of the application has the advantages of large range, high precision, fast response and wide spectrum compensation capacity, and is particularly suitable for optical fiber transmission links with high stability and consistency requirements for ultra-wideband signal transmission.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of communication, and particularly relates to a stable transmission system of optical microwave based on temperature control optical fiber delay line BACKGROUND

[0002] Taking advantage of the low loss and large bandwidth of the photonic system, the frequency, amplitude and phase of the microwave signal can be regulated in the optical domain, which can realize the functions and tasks that the traditional microwave system cannot realize. Therefore, the stable transmission of the optical microwave signal in the optical fiber has a wide range of application requirements in the fields of interferometric antenna array, distributed phase coherent radar and large antenna surface deformation correction. For example, in the large antenna surface deformation correction, China is planning to build a large-scale millimeter wave astronomical telescope with a 60m aperture, which requires the root mean square error of the antenna reflector surface shape to be less than 30um. In recent years, the millimeter wave adaptive optical measurement method has developed, which provides an effective solution for real-time high-precision detection and correction of large antenna surface shape. The key of the millimeter wave adaptive optical measurement method is to deploy a micro antenna on the antenna panel unit, and use the antenna to emit mutually highly synchronized microwave signals to monitor and correct the antenna surface distortion in real time. This requires the length of the transmission link to be stable in the micron level, or the transmission delay stability to reach the femtosecond level, so as to meet the <30um millimeter wave antenna surface error requirement. However, the fundamental problem is that the transmission delay of the optical fiber is unstable due to environmental temperature, vibration and other factors. Considering the dispersion effect in the optical fiber, the transmission delay of different wavelength signals also varies. In the case of harsh external environment changes and long-distance optical fiber stable transmission, the difference of the instability is more significant. This puts forward very high requirements for the detection and compensation of the link delay variation.

[0003] An ideal delay compensation device needs to have the ability of large range, high precision, fast response and wide spectrum compensation. The existing delay line includes a piezoelectric ceramic optical fiber stretcher, an optical phase shifter, a cascaded multi-stage optical switch and a motor space optical delay line. The delay compensation scheme composed of the above delay compensation devices cannot completely meet the requirements of the delay compensation device which should have the ability of large range, high precision, fast response and wide spectrum compensation. The compensation precision is not high, the continuous compensation ability is lacked, or it is difficult to compensate the dispersion inhomogeneity caused by temperature change when transmitting wideband signals.

[0004] Due to the different application scenarios of microwave signal transmission, the existing delay line composed of the delay compensation module cannot meet the requirements of high precision, large range and wide spectrum compensation ability in the optical fiber stable transmission link. The transmission ability of wideband microwave signals in long-distance optical fiber link and in harsh environment is limited. Compared with the existing delay line, the temperature control optical fiber delay line can realize large range delay adjustment and has the ability of dispersion compensation. However, it has the disadvantages of low compensation precision and slow response speed. SUMMARY

[0005] To solve the above problems, the application provides a stable transmission system of optical microwave based on temperature-controlled optical fiber delay line, which has the ability of large range, high precision, fast response and wide spectrum compensation, so as to realize multi-wavelength compensation and stable transmission.

[0006] The stable transmission system of optical microwave based on temperature-controlled optical fiber delay line comprises a first laser module, a first photodetector, a third photodetector, a first modulator, a first wavelength division multiplexer, a second wavelength division multiplexer, an optical fiber coupler, a polarization beam splitter, a delay compensation module, a single-mode optical fiber with a length of at least 10 km, a vector network analyzer, a radio frequency signal generator, a frequency mixer and a phase-locked loop.

[0007] The first laser emitted by the first laser module is modulated by a probe signal output by the radio frequency signal generator and a to-be-transmitted signal output by the vector network analyzer at the first modulator, and a first modulated signal obtained is sequentially transmitted to the optical fiber coupler via the polarization beam splitter, the first wavelength division multiplexer, the delay compensation module, the single-mode optical fiber and the second wavelength division multiplexer.

[0008] The optical fiber coupler divides the first modulated signal into two sub-paths, a first sub-path enters the first photodetector, and a second sub-path is reflected by a Faraday rotator and sequentially transmitted to the third photodetector via the optical fiber coupler, the second wavelength division multiplexer, the single-mode optical fiber, the delay compensation module, the first wavelength division multiplexer and the polarization beam splitter.

[0009] The first photodetector performs beat frequency on the first sub-path to obtain a first beat frequency signal, and the third photodetector performs beat frequency on the second sub-path to obtain a second beat frequency signal, wherein the first beat frequency signal is input into the vector network analyzer, and the second beat frequency signal is input into the frequency mixer.

[0010] The vector network analyzer judges the stability of the system in transmitting the to-be-transmitted signal according to phase jitter of the first beat frequency signal.

[0011] The frequency mixer performs phase discrimination on the second beat frequency signal and the probe signal output by the radio frequency signal generator to obtain a direct current error signal.

[0012] An output voltage generated after the direct current error signal is processed by the phase-locked loop is used to control the delay of the delay compensation module, so as to compensate for the delay jitter of the to-be-transmitted signal caused by the delay change of the single-mode optical fiber.

[0013] Further, the delay compensation module comprises a piezoelectric ceramic fiber stretcher and a temperature-controlled optical fiber delay line, and the method for compensating for the delay jitter of the to-be-transmitted signal caused by the delay change of the single-mode optical fiber by the delay compensation module is as follows:

[0014] The output voltage generated by the direct current error signal after the phase-locked loop processing is directly loaded to the piezoelectric ceramic fiber stretcher, so as to change the deformation of the piezoelectric ceramic fiber stretcher, thereby changing the length of the total fiber link composed of the delay compensation module and the single-mode optical fiber, wherein when the delay caused by the single-mode optical fiber is shortened to cause the to-be-transmitted signal to be ahead of schedule, the deformation of the piezoelectric ceramic fiber stretcher is increased, and when the delay caused by the single-mode optical fiber is increased to cause the to-be-transmitted signal to be behind schedule, the deformation of the piezoelectric ceramic fiber stretcher is reduced.

[0015] Meanwhile, the phase-locked loop monitors and adjusts the voltage of the piezoelectric ceramic fiber stretcher in real time, so as to ensure that the working voltage range of the piezoelectric ceramic fiber stretcher is always maintained between -Vmax and Vmax, wherein Vmax is the upper limit of the working voltage of the piezoelectric ceramic fiber stretcher; when it is monitored that the voltage of the piezoelectric ceramic fiber stretcher approaches -Vmax, a temperature control control signal is generated for the temperature control fiber delay line according to the direct current error signal, so as to reduce the temperature of the temperature control fiber delay line, thereby changing the delay of the total fiber link and making the voltage of the piezoelectric ceramic fiber stretcher return to the initial state close to 0V; when the voltage of the piezoelectric ceramic fiber stretcher approaches Vmax, a temperature control control signal is generated for the temperature control fiber delay line according to the direct current error signal, so as to increase the temperature of the temperature control fiber delay line, thereby changing the delay of the total fiber link and making the voltage of the piezoelectric ceramic fiber stretcher return to the initial state close to 0V.

[0016] Further, the temperature control fiber delay line comprises a temperature control element and an optical fiber, wherein the temperature control element is a semiconductor refrigerator, and the length of the optical fiber is 1.5km.

[0017] Further, the first laser module comprises a first laser and a first bias controller.

[0018] The first bias controller is used to adjust the working point of the first modulator, so that the first modulator modulates the probe signal and the to-be-transmitted signal onto the first laser generated by the first laser.

[0019] Further, a second laser module, a second modulator and a second photodetector are further included.

[0020] The second laser emitted by the second laser module is modulated by the to-be-transmitted signal at the second modulator, and the obtained second modulation signal enters the first wavelength division multiplexer; wherein the wavelength of the second laser is different from the wavelength of the first laser.

[0021] The first modulation signal and the second modulation signal are synthesized into a total modulation signal at the first wavelength division multiplexer, and the total modulation signal passes through the delay compensation module and the single-mode optical fiber in sequence to reach the second wavelength division multiplexer.

[0022] The second wave division multiplexer divides the first modulation signal and the second modulation signal from the total modulation signal, the first modulation signal enters the optical fiber coupler, and the first modulation signal is divided into two sub-paths by the optical fiber coupler to perform subsequent operations, and the second modulation signal enters the second photoelectric detector;

[0023] The second photoelectric detector performs beat frequency on the second modulation signal to obtain a third beat frequency signal, and the third beat frequency signal is input into the vector network analyzer;

[0024] The vector network analyzer determines the stability of the system in transmitting the to-be-transmitted signal at different wavelengths according to the phase jitter of the first beat frequency signal and the third beat frequency signal.

[0025] Further, the second laser module comprises a second laser and a second bias controller;

[0026] The second bias controller is used to adjust the operating point of the second modulator, so that the second modulator modulates the to-be-transmitted signal onto the second laser generated by the second laser.

[0027] Beneficial effects:

[0028] 1. The application provides a temperature-controlled optical fiber delay line-based optical microwave stable transmission system, which accurately reversely compensates delay jitter changes through a delay compensation module, exhibits excellent delay consistency in ultra-wideband signal transmission, and overcomes the inability of traditional piezoelectric ceramic optical fiber stretchers, optical phase shifters, cascaded multi-stage optical switches and motor space optical delay lines and other compensation devices to simultaneously meet the requirements of a large range, high precision and fast response; that is, the link delay compensation scheme in the transmission system of the application has the capabilities of a large range, high precision, fast response and wide spectrum compensation, and is particularly suitable for optical fiber transmission links with high stability and consistency requirements for ultra-wideband signal transmission.

[0029] 2. The application provides a temperature-controlled optical fiber delay line-based optical microwave stable transmission system, which adopts a piezoelectric ceramic optical fiber stretcher as a high-precision small-range delay compensation device and a temperature-controlled optical fiber delay line as a coarse adjustment large-range delay compensation device, and the two are cascaded to form a delay compensation module; through the cascaded operation of the piezoelectric ceramic optical fiber stretcher and the temperature-controlled optical fiber delay line, real-time dynamic balance of the link delay is realized, ensuring the delay stability of the transmission link; at the same time, the application realizes the effects of high precision, large range and fast compensation in long-distance optical fiber transmission link compensation by designing and manufacturing a temperature-controlled optical fiber delay line and reasonably controlling the temperature of the temperature-controlled optical fiber delay line, while avoiding the problem of delay inconsistency in ultra-wideband signal transmission.

[0030] 3. The application provides a temperature-controlled optical fiber delay line-based optical microwave stable transmission system, which is used for solving the problem of uneven delay caused by different wavelengths of optical fiber dispersion in broadband signal transmission, adopts a dual-wavelength laser, and designs a dynamic delay compensation method to verify the stability of the system in transmitting the to-be-transmitted signal at different wavelengths. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 A temperature-controlled optical fiber delay line-based optical microwave stable transmission system is provided in the application.

[0032] Figure 2 A temperature-controlled optical fiber delay line-based optical microwave stable transmission system is provided in the application.

[0033] Fig. 3(a) is a schematic diagram of delay variation under locked and unlocked conditions according to the application.

[0034] Fig. 3(b) is a schematic diagram of overlapping Allan deviation of transmission signals under locked and unlocked conditions according to the application.

[0035] Fig. 4(a) is a schematic diagram of absolute and relative delay variation of two channels of 1546nm and 1554nm obtained under a motor space optical delay line control module according to the application.

[0036] Fig. 4(b) is a schematic diagram of absolute and relative delay variation of two channels of 1546nm and 1554nm obtained under a temperature-controlled optical fiber delay line control module according to the application. DETAILED DESCRIPTION

[0037] In order to enable personnel in the art to better understand the present application, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the present application.

[0038] The experimental system of the application is composed of a local end and a remote end, and to-be-transmitted data is transmitted between the local end and the remote end. The local end and the remote end are connected by a single-mode optical fiber with a length of at least 10km. When the to-be-transmitted signal is transmitted in the 10km single-mode optical fiber, noise information will be introduced due to changes in the external environment, and the optical fiber transmission delay will also be affected by temperature and stress fluctuations, thereby causing large delay jitter of the to-be-transmitted signal.

[0039] The temperature-controlled optical fiber delay line-based optical microwave stable transmission system of the application is as follows: Figure 1As shown, it comprises a first laser module, a second laser module, a first photodetector, a second photodetector, a third photodetector, a first modulator, a second modulator, a first wavelength division multiplexer, a second wavelength division multiplexer, a fiber coupler, a polarization beam splitter, a delay compensation module, a single-mode fiber with a length of at least 10 km, a vector network analyzer, a radio frequency signal generator, a mixer, and a phase-locked loop; wherein the first laser module comprises a first laser and a first bias controller; the second laser module comprises a second laser and a second bias controller; the delay compensation module comprises a piezoelectric ceramic fiber stretcher and a temperature-controlled fiber delay line;

[0040] The first laser emitted by the first laser module is modulated by the 9.9 GHz probe signal output by the radio frequency signal generator and the 10 GHz to-be-transmitted signal output by the vector network analyzer at the first modulator, and the obtained first modulation signal passes through the polarization beam splitter, the first wavelength division multiplexer, the delay compensation module, the single-mode fiber, and the second wavelength division multiplexer in sequence to reach the fiber coupler; wherein the first bias controller is used to adjust the operating point of the first modulator, so that the first modulator modulates the probe signal and the to-be-transmitted signal onto the first laser generated by the first laser.

[0041] The fiber coupler divides the first modulation signal into two sub-paths, the first sub-path enters the first photodetector, and the second sub-path is reflected by a Faraday rotator and passes through the fiber coupler, the second wavelength division multiplexer, the single-mode fiber, the delay compensation module, the first wavelength division multiplexer, and the polarization beam splitter in sequence to reach the third photodetector;

[0042] The first photodetector performs beat frequency on the first sub-path to obtain a first beat frequency signal, and the third photodetector performs beat frequency on the second sub-path to obtain a second beat frequency signal, wherein the first beat frequency signal is input into the vector network analyzer, and the second beat frequency signal is input into the mixer;

[0043] The vector network analyzer judges the stability of the system in transmitting the to-be-transmitted signal according to the phase jitter of the first beat frequency signal;

[0044] The mixer performs phase discrimination on the second beat frequency signal and the probe signal output by the radio frequency signal generator to obtain a direct current error signal;

[0045] The output voltage generated after the direct current error signal is processed by the phase-locked loop is used to control the delay time of the delay compensation module, so as to compensate for the delay jitter of the to-be-transmitted signal caused by the delay time variation of the single-mode fiber.

[0046] It should be noted that the delay compensation module composed of the piezoelectric ceramic fiber stretcher (high-precision small-range delay compensation device) and the temperature-controlled fiber delay line (coarse adjustment large-range delay compensation device) is cascaded, and its compensation principle and process are as follows:

[0047] The far-end backhaul optical signal is beat by a photoelectric detector to obtain a 9.9GHz radio frequency probe signal, and the radio frequency probe signal is phase demodulated with a 9.9GHz radio frequency probe signal at a local end in a mixer to obtain a zero frequency intermediate frequency signal (error signal). The signal carries noise information introduced in the round-trip link due to changes in the external environment, and the level thereof reflects the noise level in the link. The application controls a delay compensation module to perform reverse delay compensation, so that the level of the error signal is restored to zero as much as possible, thereby realizing real-time compensation of the link noise.

[0048] Specifically, the method for compensating the delay jitter of the to-be-transmitted signal caused by the delay change of the single-mode optical fiber by the delay compensation module is as follows:

[0049] The output voltage generated after the direct current error signal is processed by the phase-locked loop is directly loaded to the piezoelectric ceramic fiber stretcher, so as to change the deformation of the piezoelectric ceramic fiber stretcher, thereby changing the length of the total optical fiber link composed of the delay compensation module and the single-mode optical fiber. When the delay caused by the single-mode optical fiber is shortened and the to-be-transmitted signal leads, the deformation of the piezoelectric ceramic fiber stretcher is increased; when the delay caused by the single-mode optical fiber is increased and the to-be-transmitted signal lags, the deformation of the piezoelectric ceramic fiber stretcher is reduced.

[0050] Meanwhile, the phase-locked loop monitors and adjusts the voltage of the piezoelectric ceramic fiber stretcher in real time, so as to ensure that the working voltage range of the piezoelectric ceramic fiber stretcher is always maintained between -Vmax and Vmax, wherein Vmax is the upper limit of the working voltage of the piezoelectric ceramic fiber stretcher. When it is monitored that the voltage of the piezoelectric ceramic fiber stretcher approaches -Vmax, a temperature control control signal is generated for the temperature control fiber delay line according to the direct current error signal, so as to reduce the temperature of the temperature control fiber delay line, thereby changing the delay of the total optical fiber link and making the voltage of the piezoelectric ceramic fiber stretcher return to the initial state close to 0V. When the voltage of the piezoelectric ceramic fiber stretcher approaches Vmax, a temperature control control signal is generated for the temperature control fiber delay line according to the direct current error signal, so as to increase the temperature of the temperature control fiber delay line, thereby changing the delay of the total optical fiber link and making the voltage of the piezoelectric ceramic fiber stretcher return to the initial state close to 0V.

[0051] As can be seen, the application realizes real-time dynamic balance of the link delay by the cascade operation of the piezoelectric ceramic fiber stretcher and the temperature control fiber delay line, and ensures stable locking of the transmission link.

[0052] Further, in order to compare the stability of the system in transmitting the to-be-transmitted signal at different wavelengths, the application adopts a second laser module, a second modulator and a second photoelectric detector to form another link branch, and the specific process is as follows:

[0053] The second laser emitted by the second laser module is modulated by the to-be-transmitted signal at the second modulator, and the obtained second modulated signal enters the first wavelength division multiplexer; wherein, the wavelength of the second laser is different from the wavelength of the first laser; the second bias controller is used to adjust the operating point of the second modulator, so that the second modulator modulates the to-be-transmitted signal onto the second laser generated by the second laser.

[0054] The first modulated signal and the second modulated signal are synthesized into one total modulated signal at the first wavelength division multiplexer, and the total modulated signal sequentially passes through the delay compensation module and the single-mode optical fiber to reach the second wavelength division multiplexer;

[0055] The second wavelength division multiplexer divides the first modulated signal and the second modulated signal from the total modulated signal, the first modulated signal enters the fiber coupler, and the fiber coupler divides the first modulated signal into two sub-paths to perform subsequent operations, while the second modulated signal enters the second photoelectric detector;

[0056] The second photoelectric detector performs beat frequency on the second modulated signal to obtain a third beat frequency signal, and the third beat frequency signal is input into the vector network analyzer;

[0057] The vector network analyzer determines the stability of the system in transmitting the to-be-transmitted signal at different wavelengths according to the phase jitter of the first beat frequency signal and the third beat frequency signal.

[0058] Based on this, as shown in a kind of stable transmission system of optical microwave based on temperature control optical fiber delay line, Figure 1 The functions of each device in the system can be summarized as follows:

[0059] Two lasers are used to generate two single-frequency optical carriers with a 8nm interval; a bias controller is used to effectively adjust the working point of the Mach-Zehnder modulator to improve the working stability of the modulator; the Mach-Zehnder modulator is used to modulate the radio frequency microwave signal to the single-frequency optical carrier generated by the laser; the polarization beam splitter is used to separate the modulated optical carrier according to the two polarization directions of the vertical and horizontal directions to realize the polarization separation of the optical wave; the wavelength division multiplexer is used to synthesize or separate different wavelengths; the tunable optical delay line is used to compensate the link delay jitter and dispersion variation caused by the external environment; the single-mode optical fiber is used to transmit the optical carrier to the remote end; the optical fiber coupler is used to separate or synthesize the optical wave; the radio frequency signal generator is used to generate the radio frequency signal for locking the optical fiber link; the mixer is used to zero difference mix the radio frequency signal generated by the radio frequency signal generator with the radio frequency signal output by the photoelectric detector after being returned to the local end from the remote end through the optical fiber link to obtain a direct current error signal; the phase-locked loop is used to integrate and amplify the direct current error signal output by the mixer to control the adjustable optical delay line to compensate the delay jitter of the optical fiber link and realize the locking of the optical fiber link; the photoelectric detector is used to convert the optical signal into an electrical signal; the power amplifier is used to amplify the power of the optical modulation microwave signal to ensure that the signal has sufficient strength and stability in the optical fiber transmission link; the power divider is used to uniformly or proportionally divide the input microwave radio frequency signal to multiple output ends to support multi-channel signal processing; the Faraday rotator is used to rotate the polarization state of the optical signal by 90° and reflect the optical path by the Faraday effect, which is used to eliminate polarization-related interference, improve the system stability, and realize the link delay jitter compensation at the local end; the data sending / receiving acquisition processing system, i.e. the vector network analyzer, is used to send data and accept the data collected from the remote end and perform data processing.

[0060] The piezoelectric ceramic optical fiber stretcher is combined with two delay control modules respectively composed of a temperature-controlled optical fiber delay line and a motor space optical delay line to compare the delay characteristics of the two modules, and the specific process is as follows:

[0061] The 1546nm laser light emitted at the local end is modulated by the 10GHz microwave signal output by the vector network analyzer and the 9.9GHz microwave signal output by the radio frequency signal source, and then enters the link delay control module and the 10km optical fiber link to the remote end in sequence after passing through the polarization beam splitter and the wavelength division multiplexer 1. The 9.9GHz signal is used as a probe to sense the link delay variation, and the 10GHz signal is a signal to be transmitted.

[0062] The 30% optical signal after passing through the 70:30 optical coupler is detected by the photodetector 1, and a 10GHz microwave signal is obtained, which enters the vector network analyzer to measure the delay jitter of the transmission signal. The 70% optical signal is reflected by the Faraday rotator and returned to the local end, and after passing through the polarization beam splitter, it enters the photodetector 3 to obtain a 9.9GHz microwave signal. The 9.9GHz reference signal in the local end is phase demodulated in the mixer to obtain a direct current error signal, which enters the phase-locked loop, and the phase-locked loop drives the delay compensation module to eliminate the problems of temperature and stress fluctuation affecting the fiber transmission delay in long-time measurement, and the dispersion jitter caused by different frequency optical signals in the fiber link, thereby reducing the delay jitter and realizing stable transmission of the microwave signal.

[0063] In order to study the transmission delay difference of different wavelengths, a 1554nm laser 2 is used as the wavelength of the signal to be transmitted, and the emitted optical signal is modulated by the 10GHz microwave signal output by the vector network analyzer through the Mach-Zehnder modulator. After being combined with the first laser with a wavelength of λ1 through the wavelength division multiplexer 1, it successively passes through the link delay control module and the 10km optical fiber link and is transmitted to the remote end, and then is separated by the wavelength division multiplexer 2 and detected by the photodetector 2 to obtain a 10GHz microwave signal, which enters the vector network analyzer to measure the delay jitter of the out-of-loop signal at the remote end.

[0064] The temperature-controlled optical fiber delay line designed and prepared in the application uses a semiconductor cooler (TEC) as a temperature control element, and 1.5km of optical fiber is wound around it. In order to enhance the thermal conductivity between the temperature control element and the optical fiber, a heat-conducting metal is filled between the outer shell and the inner wall. In addition, in order to reduce the heat exchange between the optical fiber and the external environment, the entire device is subjected to heat preservation treatment, which can make the internal temperature change about 40℃. In the experiment, the temperature of the optical fiber is controlled to change in the range of 11℃ to 52℃, and the time delay at different temperatures is measured every 1℃ using a vector network analyzer. The measurement results are shown in Figure 2 The time delay increment and the temperature change show a good linear relationship. The rate of change of time delay with temperature, i.e. the compensation accuracy, is about 69.13ps / ℃. This experimental result shows that the application using the self-made temperature-controlled optical fiber delay line can realize the corresponding time delay change through accurate temperature control.

[0065] It should be noted that the phase-locked loop detection and compensation accuracy of the probe signal determines the transmission stability of the fiber link. Therefore, in order to evaluate the locking effect of the fiber link using the temperature-controlled fiber delay line as a delay compensation module, the present application locks the fiber link based on the probe signal, and measures the delay variation of the transmission signal under the conditions of phase-locked and not phase-locked at the far end, respectively. The measurement results are shown in Fig. 3(a). The 10 GHz microwave signal generated by the vector network analyzer is transmitted through the fiber link, and then enters the vector network analyzer after being frequency-mixed by the photoelectric detector at the far end, so as to calculate the delay variation. The receiving bandwidth of the vector network analyzer is set to 2 Hz. The experimental results show that for the unlocked fiber link, the delay variation caused by the change of the external environment within 5000 seconds is 693 ps. This is because the temperature in the laboratory is in a continuous rising stage. For the locked fiber link, the standard deviation of the delay variation is 27.8 fs. At the same time, the present application calculates the overlapping Allan deviation using the measured phase data, and the results are shown in Fig. 3(b). When the transmission delay changes with the environment, the frequency stability is about 10 -13 When the link is locked, the overlapping Allan deviation decreases linearly with the increase of the average time. The overlapping Allan deviation reaches 4.35 x 10 -17 within an average time of 1000 seconds, which shows excellent long-term frequency stability. The stability of the delay variation of the fiber link in the locked state demonstrates the ability of high-precision phase locking. It shows that the temperature-controlled fiber delay line designed and made by the present application can meet the compensation requirements of high-precision fiber link stable transmission.

[0066] For multi-wavelength signal transmission, the chromatic dispersion change caused by the temperature change of the fiber will cause the transmission delay of the optical microwave signal on different optical frequencies to be non-uniform. In order to verify the compensation effect of the temperature-controlled fiber delay line on such delay non-uniformity, the present application first closes the optical switch at position B, uses the motor spatial light delay compensation module to compensate for the delay variation of the fiber. At the far end, the present application measures the phase variation of the 10 GHz microwave signal transmitted by two light sources with a distance of 8 nm, and calculates the delay variation, as shown in Fig. 4(a). Within 4000 seconds, the temperature change of the 10 km fiber is 3.5℃. The delay variation of the λ1 locking path remains stable, while the delay variation of the λ2 transmission path increases obviously due to the influence of chromatic dispersion. The relative delay variation difference obtained by subtracting the delay variation of the two paths is shown in the inset of Fig. 4(a). Within 4000 seconds, the cumulative relative delay difference of the two paths is 577 femtoseconds.

[0067] Then the light switch is closed in position A, the application replaces the motor space optical delay compensation module with a temperature-controlled optical fiber delay compensation module, and the measurement results are shown in Figure 4(b). Within 4000 seconds, the temperature of the 10-kilometer optical fiber in the temperature-controlled box changes by 1.7℃. Although the corresponding dispersion change leads to a relative delay change difference of 280fs, the delay changes of both the λ1 locking path and the λ2 transmission path remain a smooth trend. The relative delay change difference obtained by differencing the relative delay changes of the two paths is shown in the inset of Figure 4(b), and the standard deviation of the cumulative relative delay change difference of the two paths is 18.5 femtoseconds. It is verified that the application can eliminate the phenomenon of transmission delay unevenness of different wavelength optical signals caused by chromatic dispersion by using the self-made temperature-controlled optical fiber delay line to compensate the change of the optical fiber temperature in the opposite direction.

[0068] So far, the application demonstrates a multi-wavelength compensation and stable transmission system based on a temperature-controlled optical fiber delay line. The link delay compensation scheme in the system has a wide range, high precision, fast response, and wide spectrum compensation capability. For the transmission of a single-frequency signal in a 10-kilometer optical fiber link, the system realizes a delay jitter standard deviation of only 27.8 femtoseconds within 5000 seconds, showing excellent long-term stability. Compared with conventional delay compensation schemes, the scheme uses the self-made temperature-controlled optical fiber delay line to accurately compensate the temperature change in the opposite direction, and shows excellent delay consistency in ultra-wideband signal transmission. This delay compensation scheme is particularly suitable for optical fiber transmission links that require high stability and consistency for ultra-wideband signal transmission.

[0069] In summary, the application has the following advantages over the prior art:

[0070] 1. Multi-wavelength compensation system based on temperature-controlled optical fiber delay line:

[0071] A link delay compensation scheme combining a temperature-controlled optical fiber delay line and a piezoelectric ceramic optical fiber stretcher is proposed, which realizes wide spectrum, high precision, large range, and fast response delay compensation capability.

[0072] 2. Dispersion compensation capability:

[0073] The temperature change of the optical fiber is compensated in the opposite direction through the temperature-controlled optical fiber delay line, which eliminates the unevenness of transmission delay of different frequency components caused by external temperature changes and ensures the delay uniformity of ultra-wideband signals. This makes it possible to transmit wideband signals over long-distance optical fiber links in harsh environmental conditions.

[0074] 3. Optimization design of optical fiber link delay compensation module:

[0075] The performance differences between the temperature-controlled optical fiber delay line and the motor space optical delay line are compared through experimental design, which verifies the superiority of the former in delay compensation accuracy and wideband signal transmission uniformity.

[0076] 4. The delay jitter is extremely low:

[0077] In the experiment, the standard deviation of the transmission delay jitter through a 10km optical fiber link is only 27.8fs, showing ultra-high transmission stability.

[0078] 5. Multi-module coordinated optical fiber link stable transmission:

[0079] The system combines a polarization beam splitter, a wavelength division multiplexer, a frequency mixer, a phase-locked loop and other multi-modules to form a synergistic effect, ensuring the stability of the microwave signal in the optical fiber link.

[0080] 6. Wide application potential:

[0081] The system has good adaptability in the scene of severe changes in external environment and long-distance ultra-wideband signal transmission, and is particularly suitable for fields such as interferometric antenna arrays and distributed phase coherent radars that require high-stability signal transmission or high-stability wideband signal transmission.

[0082] Of course, the present application can also have other various embodiments, and those skilled in the art can certainly make various corresponding changes and modifications according to the present application without departing from the spirit and essence of the present application. However, these corresponding changes and modifications should all belong to the protection scope of the claims attached to the present application.

Claims

1. A stable transmission system of microwaves over a temperature-controlled optical fiber delay line, characterized in that: The system comprises a first laser module, a first photodetector, a third photodetector, a first modulator, a first wavelength division multiplexer, a second wavelength division multiplexer, a fiber coupler, a polarization beam splitter, a delay compensation module, a single-mode optical fiber with a length of at least 10 km, a vector network analyzer, a radio frequency signal generator, a mixer, and a phase-locked loop; The first laser emitted by the first laser module is modulated at the first modulator by the probe signal output by the radio frequency signal generator and the signal to be transmitted output by the vector network analyzer. The obtained first modulated signal passes through the polarization beam splitter, the first wavelength division multiplexer, the delay compensation module, the single-mode optical fiber, and the second wavelength division multiplexer in sequence to reach the optical fiber coupler; The fiber coupler divides the first modulated signal into two sub-paths. The first sub-path enters the first photodetector, and the second sub-path is reflected by the Faraday rotator and passes through the fiber coupler, the second wavelength division multiplexer, the single-mode optical fiber, the delay compensation module, the first wavelength division multiplexer, and the polarization beam splitter to reach the third photodetector. The first photodetector beats the first sub-path to obtain a first beat frequency signal, and the third photodetector beats the second sub-path to obtain a second beat frequency signal, wherein the first beat frequency signal is input into the vector network analyzer, and the second beat frequency signal is input into the mixer; The vector network analyzer determines the stability of the system in transmitting the signal to be transmitted based on the phase jitter of the first beat frequency signal; The mixer performs phase detection on the second beat frequency signal and the probe signal output by the RF signal generator to obtain a DC error signal; The output voltage generated by the DC error signal after being processed by the phase-locked loop is used to control the delay of the delay compensation module, thereby compensating for the delay jitter caused by the delay variation of the single-mode optical fiber to the signal to be transmitted; The delay compensation module includes a piezoelectric ceramic optical fiber stretcher and a temperature-controlled optical fiber delay line. The method by which the delay compensation module compensates for the delay jitter caused by the delay variation of the single-mode optical fiber to the signal to be transmitted is as follows: The output voltage generated by the DC error signal after being processed by the phase-locked loop is directly applied to the piezoelectric ceramic fiber stretcher, thereby changing the deformation of the piezoelectric ceramic fiber stretcher, thereby changing the length of the total optical fiber link composed of the delay compensation module and the single-mode optical fiber. Specifically, when the delay caused by the single-mode optical fiber is shortened, causing the signal to be transmitted to advance, the deformation of the piezoelectric ceramic fiber stretcher is increased. When the delay caused by the single-mode optical fiber is increased, causing the signal to be transmitted to lag, the deformation of the piezoelectric ceramic fiber stretcher is reduced. At the same time, the phase-locked loop monitors and adjusts the voltage of the piezoelectric ceramic fiber stretcher in real time to ensure that the operating voltage range of the piezoelectric ceramic fiber stretcher is always maintained between -Vmax and Vmax, where Vmax is the upper limit of the operating voltage of the piezoelectric ceramic fiber stretcher; when the voltage of the piezoelectric ceramic fiber stretcher is monitored to be -Vmax, a temperature control signal is generated for the temperature-controlled optical fiber delay line according to the DC error signal, thereby reducing the temperature of the temperature-controlled optical fiber delay line, thereby changing the delay of the total optical fiber link, and returning the voltage of the piezoelectric ceramic fiber stretcher to its initial state of 0V; when the voltage of the piezoelectric ceramic fiber stretcher is Vmax, a temperature control signal is generated for the temperature-controlled optical fiber delay line according to the DC error signal, thereby increasing the temperature of the temperature-controlled optical fiber delay line, thereby changing the delay of the total optical fiber link, and returning the voltage of the piezoelectric ceramic fiber stretcher to its initial state of 0V.

2. The stable transmission system of microwaves over a temperature-controlled optical fiber delay line according to claim 1, characterized in that: The temperature-controlled optical fiber delay line includes a temperature-controlled element and an optical fiber, wherein the temperature-controlled element is a semiconductor refrigerator and the length of the optical fiber is 1.5 km.

3. The stable transmission system of microwaves over a temperature-controlled optical fiber delay line according to claim 1, characterized in that: The first laser module includes a first laser and a first bias controller; The first bias controller is used to adjust the operating point of the first modulator so that the first modulator modulates the probe signal and the signal to be transmitted onto the first laser generated by the first laser.

4. The stable transmission system of microwaves over a temperature-controlled optical fiber delay line according to claim 1, characterized in that: Also includes a second laser module, a second modulator, and a second photodetector; The second laser light emitted by the second laser module is modulated by the signal to be transmitted at the second modulator, and the obtained second modulated signal enters the first wavelength division multiplexer; wherein the wavelength of the second laser light is different from the wavelength of the first laser light; The first modulated signal and the second modulated signal are combined into a total modulated signal at the first wavelength division multiplexer, and the total modulated signal passes through the delay compensation module and the single-mode optical fiber in sequence to reach the second wavelength division multiplexer; The second wavelength division multiplexer divides the total modulated signal into a first modulated signal and a second modulated signal. The first modulated signal enters the optical fiber coupler, and the optical fiber coupler divides the first modulated signal into two sub-paths for subsequent operations, while the second modulated signal enters the second photodetector. The second photoelectric detector performs beat frequency processing on the second modulated signal to obtain a third beat frequency signal, and the third beat frequency signal is then input into the vector network analyzer; The vector network analyzer determines the stability of the system in transmitting the signal to be transmitted at different wavelengths according to the phase jitter of the first beat frequency signal and the third beat frequency signal.

5. The stable transmission system of microwaves over a temperature-controlled optical fiber delay line according to claim 4, characterized in that: The second laser module includes a second laser and a second bias controller; The second bias controller is used to adjust the operating point of the second modulator so that the second modulator modulates the signal to be transmitted onto the second laser generated by the second laser.

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