A signal purification method and device applied to an optical fiber microwave transmission system

Abstract

The application discloses a signal purification method and device applied to an optical fiber microwave transmission system, relates to the technical field of laser frequency stabilization, and comprises the following steps: a single-frequency laser module is used for acquiring a laser signal with improved stability; a conversion module under the optical frequency of a femtosecond optical comb is used for acquiring a femtosecond pulse laser signal; an optoelectronic conversion module is used for acquiring a microwave signal; a microwave frequency synthesis module is used for acquiring a microwave signal with improved stability; meanwhile, the microwave signal is compared with a preset reference signal; when the frequency of the microwave signal is deviated from the frequency of the preset reference signal, a frequency adjustment amount is determined to adjust the frequency of the laser signal with improved stability in real time; and when the frequency of the microwave signal is consistent with the frequency of the preset reference signal, a phase adjustment amount is determined to adjust the frequency of the laser signal with improved stability in real time. The application can meet the demand of remote transmission of a microwave atomic clock.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of laser frequency stabilization, and particularly relates to a signal purification method and device applied to a fiber microwave transmission system. BACKGROUND

[0002] With the increasing demand for frequency synchronization accuracy in the fields of high-precision atomic clock comparison, astronomical observation and deep space exploration, the traditional frequency transmission technology based on satellite link (with a long-term stability of only 10 -15 orders of magnitude) cannot meet the demand. The frequency transmission technology based on fiber link has achieved a breakthrough of 2-3 orders of magnitude in long-term stability, in which the fiber microwave frequency transmission technology can achieve a long-term stability of 10 -18 to 10 -19 orders of magnitude on a 100 km fiber link by means of an electrical phase compensation scheme.

[0003] In the existing fiber microwave frequency transmission system, signal purification is mostly dependent on a crystal oscillator or an atomic clock (such as a rubidium atomic clock), and based on the frequency stability and phase controllability, the frequency and phase of the output signal (such as 10 MHz) are calibrated in real time through precise feedback control. The specific process is as follows: the 10 MHz signal output by the atomic clock or the crystal oscillator and the reference signal are sent into a phase frequency detector (PFD), the PFD compares the phase difference and outputs an error voltage, a control voltage is generated after low-pass filtering, and digital phase fine tuning is performed through a digital control unit (such as FPGA or DSP); if the stability requirement of the reference signal is low, a crystal oscillator is used, otherwise a rubidium atomic clock, a hydrogen atomic clock or other higher-precision reference source is used, and the loop bandwidth (mHz-Hz order) is optimized to balance the tracking speed and noise suppression capability. The existing signal purification core component is a crystal oscillator or an atomic clock, and the short-term stability of the purified signal is limited by the device itself (the optimal second stability is only 10 -13 orders of magnitude), which does not match the fiber frequency transmission system index (better than 10 -14 orders of magnitude), and will seriously deteriorate the transmission stability of the system; in addition, in a high-precision fiber microwave frequency transmission network, the performance of the purification equipment needs to be higher than that of the system reference signal (microwave atomic clock index), and the traditional method is no longer applicable due to the insufficient accuracy of the local oscillator signal.

[0004] Therefore, it is urgent to provide a signal purification method and device applied to a fiber microwave transmission system to improve the defects in the prior art. SUMMARY

[0005] In order to solve the above problems in the prior art, the application provides a signal purification method and device applied to a fiber microwave transmission system. The technical problems to be solved by the application are solved by the following technical scheme:

[0006] The first aspect of the present application provides a signal purification device applied to an optical fiber microwave transmission system, comprising:

[0007] A single-frequency laser module, comprising a laser, a plurality of optical elements and an F-P cavity, the laser is used for emitting a laser signal, the plurality of optical elements are used for locking the frequency of the laser signal on the F-P cavity by using a quasi-synchronous digital hierarchy frequency locking scheme, so as to obtain a laser signal with improved stability;

[0008] A conversion module under the frequency of a femtosecond optical comb, which is used for transmitting the stability of the laser signal with improved stability to a microwave frequency band by using the frequency gear effect of the femtosecond optical comb, so as to obtain a femtosecond pulse laser signal;

[0009] An optoelectronic conversion module, which is used for converting the femtosecond pulse laser signal into a microwave signal;

[0010] A microwave frequency synthesis module, which is used for filtering and amplifying the microwave signal, so as to obtain a microwave signal with improved stability as an output signal after purification; meanwhile, the microwave frequency synthesis module is used for comparing the microwave signal with a preset reference signal, determining a frequency adjustment amount when the frequency of the microwave signal is deviated from the frequency of the preset reference signal, processing the frequency adjustment amount by using a preset filtering algorithm, and feeding back the processing result to the single-frequency laser module, so as to adjust the frequency of the laser signal with improved stability in real time; and determining a phase adjustment amount when the frequency of the microwave signal is consistent with the frequency of the preset reference signal, processing the phase adjustment amount by using the preset filtering algorithm, and feeding back the processing result to the single-frequency laser module, so as to adjust the frequency of the laser signal with improved stability in real time.

[0011] The second aspect of the present application further provides a signal purification method applied to an optical fiber microwave transmission system, which is applied to the signal purification device applied to the optical fiber microwave transmission system provided in the above, comprising:

[0012] Obtaining a laser signal, locking the frequency of the laser signal, locking the frequency of the laser signal on the resonance peak of a preset F-P cavity, and obtaining a laser signal with improved stability;

[0013] Transmitting the stability of the laser signal with improved stability to a microwave frequency band by using the frequency gear effect of a femtosecond optical comb, and obtaining a femtosecond pulse laser signal;

[0014] Converting the femtosecond pulse laser signal into a microwave signal;

[0015] The microwave signal is filtered and amplified to obtain a more stable microwave signal, which serves as the purified output signal. Simultaneously, the microwave signal is compared with a preset reference signal. When the frequency of the microwave signal deviates from the preset reference signal's frequency, a frequency adjustment amount is determined, and a preset filtering algorithm is used to process the frequency adjustment amount. Based on the processing result, the frequency of the more stable laser signal is adjusted in real time. When the frequency of the microwave signal matches the preset reference signal's frequency, a phase adjustment amount is determined, and a preset filtering algorithm is used to process the phase adjustment amount. Based on the processing result, the frequency of the more stable laser signal is adjusted in real time.

[0016] The beneficial effects of this invention are:

[0017] This invention provides a signal purification method and apparatus for fiber optic microwave transmission systems. Through the collaborative design of an ultra-stable single-frequency laser module (PDH frequency-locked to an ultra-stable FP cavity) and a femtosecond optical comb frequency conversion module, it achieves efficient transfer of optical stability to the microwave frequency band; achieving a performance better than 10... -14 The invention achieves short-term stability on the order of seconds, solving the problem of insufficient accuracy of existing crystal oscillators or rubidium atomic clocks in fiber optic microwave frequency transmission systems. Furthermore, based on the external reference discipline mechanism of Kalman filtering, the invention achieves synchronization between the long-term stability of the optically generated microwave source and the external reference signal, taking into account both short-term and long-term performance. By synergistically achieving short-term and long-term stability, a high-precision frequency transmission system can be constructed to meet the requirements for remote transmission of microwave atomic clocks.

[0018] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of a signal purification device applied to an optical fiber microwave transmission system provided in an embodiment of the present invention;

[0020] Figure 2 This is another schematic diagram of a signal purification device for fiber optic microwave transmission systems provided in an embodiment of the present invention;

[0021] Figure 3 This is a schematic diagram of a microwave frequency integration module provided in an embodiment of the present invention;

[0022] Figure 4 This is a flowchart of a signal purification method for fiber optic microwave transmission systems provided in an embodiment of the present invention;

[0023] Figure 5 This is a schematic diagram of the Kalman filter state update process provided in an embodiment of the present invention. Detailed Implementation

[0024] The present invention will be further described in detail below with reference to specific embodiments, but the implementation of the present invention is not limited thereto.

[0025] Please see Figure 1 and Figure 2 , Figure 1 This is a schematic diagram of a signal purification device applied to an optical fiber microwave transmission system provided in an embodiment of the present invention. Figure 2 This is another schematic diagram of a signal purification device for an optical fiber microwave transmission system provided in an embodiment of the present invention. The signal purification device for an optical fiber microwave transmission system provided by the present invention includes:

[0026] The single-frequency laser module 10 includes a laser 11, multiple optical elements 12, and a FP cavity 13. The laser 11 is used to emit laser signals. Optionally, the laser 11 is a 1.5μm laser. The multiple optical elements 12 are used to lock the frequency of the laser signal onto the FP cavity 13 using a quasi-synchronous digital architecture frequency locking scheme to obtain a laser signal with improved stability. Optionally, the laser signal with improved stability is a laser signal with improved frequency stability, which is an ultra-high frequency stability laser signal with a relative frequency change of less than 10. -14 per second; the single-frequency laser module 10 is an ultra-stable single-frequency laser module.

[0027] In this scheme, according to the quasi-synchronous digital hierarchy (PDH) frequency locking scheme, a high-precision FP cavity (such as a quartz cavity or a silicon cavity) is used as the frequency reference. After the laser signal is modulated and demodulated by multiple optical elements 12, the error signal drives the PID controller to lock the frequency of the laser signal to the resonance peak of the FP cavity 13.

[0028] The femtosecond optical comb frequency conversion module 20 is used to transfer the stability of the laser signal, which improves stability, to the microwave frequency band by using the frequency gearing action of the femtosecond optical comb, so as to obtain a femtosecond pulse laser signal.

[0029] Photoelectric conversion module 30; used to convert femtosecond pulse laser signals into microwave signals; optionally, photoelectric conversion module 30 is a low-noise photoelectric conversion module 30, and the microwave signal can be a 3.6GHz microwave signal or a 9GHz microwave signal.

[0030] The microwave frequency synthesis module 40 is used to filter and amplify the microwave signal to obtain a microwave signal with improved stability, which serves as the purified output signal. Simultaneously, it compares the microwave signal with a preset reference signal. When there is a deviation between the microwave signal frequency and the preset reference signal frequency, it determines the frequency adjustment amount, processes the frequency adjustment amount using a preset filtering algorithm, and feeds the processing result back to the single-frequency laser module 10 to adjust the frequency of the laser signal with improved stability in real time. When the microwave signal frequency matches the preset reference signal frequency, it determines the phase adjustment amount, processes the phase adjustment amount using a preset filtering algorithm, and feeds the processing result back to the single-frequency laser module 10 to adjust the frequency of the laser signal with improved stability in real time.

[0031] Optionally, both the microwave signal and the preset reference signal are either 3.6 GHz or 9 GHz signals.

[0032] It should be noted that the fiber optic microwave transmission system receives a 10MHz clock signal.

[0033] For details, please continue to see Figure 1 and Figure 2 This embodiment employs optically generated microwave technology to construct the local oscillator. Through the collaborative design of the ultra-stable single-frequency laser module 10 (PDH frequency locking to ultra-stable FP cavity 13) and the femtosecond optical comb frequency conversion module 20 (comb tooth locking and zero-frequency control), the efficient transfer of optical stability to the microwave frequency band is achieved; achieving better than 10 -14 The short-term stability on the order of seconds solves the problem of insufficient accuracy of existing crystal oscillators or rubidium atomic clocks in fiber optic microwave frequency transmission systems. In addition, this embodiment uses an external reference discipline mechanism based on Kalman filtering to achieve synchronization between the long-term stability of the optically generated microwave source and the external reference signal, taking into account both short-term and long-term performance. By achieving short-term and long-term stability in a coordinated manner, a high-precision frequency transmission system can be constructed to meet the requirements of remote transmission of microwave atomic clocks.

[0034] In an optional embodiment of the present invention, it further includes: a data acquisition module 50, used to acquire in real time the laser signal with improved stability output by the single-frequency laser module 10, the femtosecond pulse laser signal output by the femtosecond optical comb conversion module 20, the microwave signal output by the photoelectric conversion module 30, and the purified output signal output by the microwave frequency integration module 40.

[0035] In an optional embodiment of the present invention, the plurality of optical elements 12 include an acousto-optic modulator 61, a half-wave plate 62, a first polarization beam splitter 63, a first optoelectronic modulator 64, a second optoelectronic modulator 65, a second polarization beam splitter 66, a quarter-wave plate 67, a photodetector 68, a first mixer 69, a low-pass filter 70, a controller 71, a first DDS chip 72, and a second DDS chip 73; wherein;

[0036] An acousto-optic modulator 61 receives a laser signal and adjusts its parameters. A half-wave plate 62 receives the signal processed by the acousto-optic modulator 61 and adjusts its polarization state. A first polarization beamsplitter 63 receives the signal processed by the half-wave plate 62 and processes signals with different polarization states. A first opto-modulator 64 receives the signal processed by the first polarization beamsplitter 63 and adjusts its parameters. A second opto-modulator 65 receives the signal processed by the first opto-modulator 64 and adjusts its parameters. A second polarization beamsplitter 66 receives the signal processed by the second opto-modulator 65 and processes signals with different polarization states. A quarter-wave plate 67 receives the signal processed by the second polarization beamsplitter 66 and adjusts its polarization state. The quarter-wave plate 67 also receives the signal reflected from the FP cavity 13 and adjusts its polarization state. The second polarization beamsplitter 66 also receives the signal processed by the quarter-wave plate 67 and processes signals with different polarization states. The photodetector 68 is used to receive the signal processed by the second polarization beam splitter 66 and convert the optical signal into an electrical signal. The first DDS chip 72 is used to receive the signal processed by the photodetector 68 and adjust the frequency of the signal. The second photoelectric modulator 65 is also used to receive the signal processed by the first DDS chip 72 and adjust the parameters of the signal. The first mixer 69 is used to receive the signal processed by the first DDS chip 72 and the signal processed by the photodetector 68 and perform mixing processing. The low-pass filter 70 is used to receive the signal processed by the first mixer 69 and perform filtering processing. The controller 71 is used to receive the signal processed by the low-pass filter 70 and generate frequency and power control quantities to adjust the frequency and power of the laser 11. The second DDS chip 73 is used to receive the signal processed by the microwave frequency synthesis module 40 and adjust the frequency of the signal. The first photoelectric modulator 64 is also used to receive the signal processed by the second DDS chip 73 to adjust the frequency of the laser signal with improved stability in real time.

[0037] In an optional embodiment of the present invention, the photoelectric conversion module 30 includes a fast photodetector for converting femtosecond pulsed laser signals into microwave signals.

[0038] In an optional embodiment of the present invention, please refer to Figure 3 , Figure 3This is a schematic diagram of a microwave frequency synthesis module provided in an embodiment of the present invention. The microwave frequency synthesis module 40 includes a phase shifter 74, a second mixer 75, an analog-to-digital converter 76, and a digital processing module 77; wherein,

[0039] Phase shifter 74 is used to receive a preset reference signal and adjust the phase of the signal. Second mixer 75 is used to receive the signal processed by phase shifter 74 and microwave signal and perform mixing processing. Analog-to-digital converter 76 is used to receive the signal processed by second mixer 75 and convert analog signal into digital signal. Digital processing module 77 is used to receive the signal processed by analog-to-digital converter 76 and estimate the frequency deviation and frequency drift of the signal. The signal processed by digital processing module is fed back to single-frequency laser module 10.

[0040] It should be noted that the signal output by the second mixer 75 is a microwave signal with improved stability, which serves as the purified output signal.

[0041] Based on the same inventive concept, please refer to Figure 4 , Figure 4 This is a flowchart of a signal purification method for an optical fiber microwave transmission system provided in an embodiment of the present invention. The present invention also provides a signal purification method for an optical fiber microwave transmission system, which is applied to the signal purification device for an optical fiber microwave transmission system provided in the above embodiments of the present invention. Embodiments of the device can be referred to the above, and will not be repeated here. The method includes:

[0042] S101. Acquire the laser signal, lock the frequency of the laser signal to the preset resonance peak of FP cavity 13, and obtain a laser signal with improved stability.

[0043] S102. The frequency gearing action of the femtosecond optical comb is used to transfer the stability of the laser signal, which improves stability, to the microwave frequency band to obtain a femtosecond pulse laser signal.

[0044] Specifically, in this embodiment, the femtosecond optical comb's first... Frequency of each comb tooth Beat frequency locking with the laser signal to improve stability, and locking to zero frequency. Among them, the femtosecond optical comb's first... The frequency of each comb tooth satisfies , Indicates the repetition frequency.

[0045] S103. Convert the femtosecond pulse laser signal into a microwave signal.

[0046] S104. The microwave signal is filtered and amplified to obtain a microwave signal with improved stability, which serves as the purified output signal. Simultaneously, the microwave signal is compared with a preset reference signal. When there is a deviation between the frequency of the microwave signal and the frequency of the preset reference signal, the frequency adjustment amount is determined, and a preset filtering algorithm is used to process the frequency adjustment amount. The frequency of the laser signal with improved stability is adjusted in real time based on the processing result. When the frequency of the microwave signal matches the frequency of the preset reference signal, the phase adjustment amount is determined, and a preset filtering algorithm is used to process the phase adjustment amount. The frequency of the laser signal with improved stability is adjusted in real time based on the processing result.

[0047] Specifically, in this embodiment, when there is a deviation between the frequency of the microwave signal and the frequency of the preset reference signal, a mixing signal between the microwave signal and the preset reference signal is acquired. The low-frequency band of the mixing signal exhibits a sinusoidal characteristic, and the frequency of the sinusoidal curve represents the frequency difference between the microwave signal and the preset reference signal. The voltage of the mixing signal has a trigonometric function relationship with the phase difference between the two signals, and the mixing signal is represented as follows:

[0048] ;

[0049] in, Represents microwave signals. This indicates the preset reference signal. Indicates the amplitude of the microwave signal. This indicates the amplitude of the preset reference signal. This represents the angular frequency of a microwave signal. This indicates the angular frequency of the preset reference signal. Indicates the phase of the microwave signal. This indicates the preset reference signal phase. Represents a time variable;

[0050] Adjust the frequency of the microwave signal or the frequency of the preset reference signal to obtain the frequency difference between the adjusted microwave signal and the preset reference signal. Based on the change of the frequency difference, the positive or negative relationship between the two frequency differences can be determined from the increase or decrease of the frequency difference, thus realizing frequency discrimination. The DC voltage signal of the frequency discrimination result is converted into the first digital signal.

[0051] The first digital signal is processed using a preset Kalman filter algorithm. Through recursive prediction and observation updates, the frequency deviation and frequency drift rate are estimated in real time, and the frequency of the laser signal with improved stability is adjusted in real time so that the laser signal with improved stability is in sync with the preset reference signal.

[0052] Furthermore, in this embodiment, when the frequency of the microwave signal matches the frequency of a preset reference signal, a mixing signal between the microwave signal and the preset reference signal is acquired. The low-frequency band of the mixing signal is displayed as a DC component, representing the phase difference between the microwave signal and the preset reference signal; wherein, the mixing signal is represented as:

[0053] ;

[0054] in, Represents microwave signals. This indicates the preset reference signal. Indicates the amplitude of the microwave signal. This indicates the amplitude of the preset reference signal. This represents the angular frequency of a microwave signal. This indicates the angular frequency of the preset reference signal. Indicates the phase of the microwave signal. This indicates the preset reference signal phase. Represents a time variable;

[0055] The phase of the preset reference signal is adjusted, the phase difference between the adjusted preset reference signal and the microwave signal is obtained, the relationship between the phase difference change and the mixing signal is established, phase detection is realized, and the DC voltage signal of the phase detection result is converted into a second digital signal.

[0056] The second digital signal is processed using a pre-defined Kalman filter algorithm. Through recursive prediction and observation updates, the frequency deviation and frequency drift rate are estimated in real time, and the frequency of the laser signal is adjusted in real time to improve stability.

[0057] It should be noted that the mixed signal after frequency and phase discrimination is mixed with environmental noise (such as temperature drift and circuit thermal noise) and low-frequency disturbances of the system. Traditional filtering methods (such as FIR / IIR) are difficult to dynamically separate the actual drift from the noise. In view of this, this embodiment adopts a Kalman filter algorithm based on state-space modeling, which estimates the frequency deviation and frequency drift rate of the photogenerated microwave source in real time through recursive prediction and observation update, thereby achieving optimal noise suppression.

[0058] Furthermore, a preset filtering algorithm is used to process the frequency adjustment amount, including:

[0059] A pre-defined Kalman filter algorithm is used to establish the state equation of a linear system. Based on the state equation of the linear system, the observed data are input and output. Based on the minimum mean square error criterion, the state of the linear system is optimally estimated. Since the calculation process of the Kalman filter algorithm is a time-domain recursive form, the estimation result can be obtained by inputting the observed values, without the need to store a large amount of data, which is suitable for real-time processing. The estimated state of the linear system represents the frequency deviation and frequency drift rate to be adjusted.

[0060] The phase adjustment amount is processed using a preset filtering algorithm, including:

[0061] A pre-defined Kalman filter algorithm is used to establish the state equation of the linear system. Based on the input and output observation data of the state equation of the linear system, the optimal estimate of the state of the linear system is performed based on the minimum mean square error criterion. The estimated state of the linear system represents the frequency deviation and frequency drift rate to be adjusted.

[0062] Assuming in The frequency difference measured at time is Then the system state description equation and observation equation can be expressed in the following forms:

[0063] ;

[0064] ;

[0065] in, The describing equations representing the state of a linear system. The observation equations representing the state of a linear system. and The system state transition matrix represents the state transition of the system from... Time's up The relationship between states at different times. This represents the known input of the system. Represents the process noise vector. Represents a linear connection matrix. Indicates observation noise; optional. With a mean of 0 and a covariance of Gaussian white noise sequence, With a mean of 0 and a covariance of The Gaussian white noise sequence. The Kalman filter iterative formula is as follows, and the corresponding state update process is as follows. Figure 5 As shown.

[0066] Update of the describing equations representing the state of a linear system;

[0067] Updates to the observation equations representing the state of a linear system;

[0068] This represents the covariance of the estimation error;

[0069] This indicates that the estimated value is updated based on the observations;

[0070] This indicates the calculation of the Kalman filter gain;

[0071] This represents the update error covariance.

[0072] The state of the next signal can be predicted from the current iteration relationship, thus returning a control input and outputting a control signal to precisely adjust the microwave signal of the photoelectric microwave transmission system. Furthermore, since the Kalman filtering process is updated in real time, the entire control process is continuous and dynamic, ensuring that the microwave signal output by the photoelectric microwave transmission system closely follows the changes in the reference source, thereby maintaining extremely high accuracy and stability. In terms of performance indicators, the output signal maintains extremely high short-term stability while its long-term stability closely follows the reference source's performance.

[0073] In summary, the signal purification method and system for fiber optic microwave transmission systems provided by this invention have the following beneficial effects:

[0074] First, the signal purification method for fiber optic microwave transmission systems proposed in this invention employs photogenerated microwave technology, achieving a short-term stability of 10 for the device's output signal. -14 Achieving short-term stability exceeding 1 second, surpassing existing crystal oscillators or rubidium atomic clocks (10... -13 (On the order of magnitude) to match the accuracy requirements of fiber optic frequency transmission systems.

[0075] Secondly, the signal purification method proposed in this invention for fiber optic microwave transmission systems achieves long-term stability synchronization with an external high-stability reference signal (such as an atomic clock) through an external reference discipline mechanism, thereby realizing coordinated optimization of short-term, medium-term, and long-term stability.

[0076] Third, the signal purification method proposed in this invention for fiber optic microwave transmission systems provides key technical support for these systems, enabling system stability to reach 10%. -14 per second and 10 -17 Every day, it meets the time and frequency synchronization needs of fields such as high-precision atomic clock comparison and deep space exploration.

[0077] In an optional embodiment of the present invention, the effectiveness of the signal purification method for fiber optic microwave transmission systems provided in the above embodiments is verified by simulation experiments, specifically as follows:

[0078] For fiber optic microwave transmission systems within 50km, the signal purification equipment is configured as follows: an ultra-stable single-frequency laser module with a laser linewidth ≤0.5Hz and a second-level frequency stability better than 3×10⁻⁶. -15 The repetition frequency control bandwidth of the conversion module under the femtosecond optical comb optical frequency is 50kHz~1MHz, and the frequency control stability is better than 5×10⁻⁶. -17 per second; the stability of the low-noise photoelectric conversion module is better than 3×10 -15The adjustable low-noise microwave frequency synthesis module outputs 10MHz, 100MHz, and specified GHz frequency signals per second, with a stability of 5×10⁻⁶. -15 Every second.

[0079] Workflow: After the ultra-stable laser is locked into the ultra-stable FP cavity, a specific tooth of the femtosecond optical comb is locked to the ultra-stable laser. The photoelectric conversion module converts the third harmonic (4GHz) of the optical comb repetition frequency into an electrical signal (microwave signal), which is then filtered and amplified by the microwave frequency synthesis module before being output. By comparing the preset reference signal (10MHz) with the output signal through mixing, the ultra-stable laser frequency is dynamically adjusted using a Kalman filter algorithm to ensure that the short-term stability of the output signal is ≤10. -14 Every second, the long-term stability is synchronized with the reference signal.

[0080] In an optional embodiment of the present invention, the effectiveness of the signal purification method for fiber optic microwave transmission systems provided in the above embodiments is verified by simulation experiments, specifically as follows:

[0081] For a kilometer-scale frequency cascade transmission system, the signal purification equipment needs to suppress near-sideband oscillation peaks caused by accumulated noise. The configuration is as follows: an ultra-stable single-frequency laser module with a laser linewidth ≤ 1Hz and a second-level frequency stability better than 5×10⁻⁶. -15 The repetition rate and zero-frequency control bandwidth of the conversion module under the femtosecond optical comb optical frequency are 50kHz~1.6MHz, and the frequency control stability is better than 1×10⁻⁶. -16 per second; low-noise photoelectric conversion module stability better than 5×10 -15 The adjustable low-noise microwave frequency synthesis module outputs 10MHz, 100MHz, and specified GHz frequency signals per second, with a stability of 1×10⁻⁶. -14 Every second.

[0082] Workflow: The ultra-stable laser is locked into an ultra-stable optical cavity. A femtosecond optical comb transfers optical stability to the microwave band via frequency down-conversion. The photoelectric conversion module extracts the high-order harmonics of the repetition frequency and converts them into electrical signals. After filtering and synthesizing the signals, the microwave synthesis module uses one path for cascade transmission and the other path for mixing with the atomic clock reference signal. Error signals are processed by Kalman filtering, and the photogenerated microwave source is adjusted in real time to achieve a system transmission stability of 10. -14 per second and 10 -17 Every day, it meets the needs of long-distance cascading transmission.

[0083] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations are intended to cover non-exclusive inclusion, such that an article or device comprising a list of elements includes not only those elements but also other elements not expressly listed. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the article or device comprising said element. Terms such as "connected" or "linked" are not limited to physical or mechanical connections but can include electrical connections, whether direct or indirect. The orientations or positional relationships indicated by terms such as "upper," "lower," "left," and "right" are based on the orientations or positional relationships shown in the accompanying drawings and are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as limiting the invention.

[0084] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Furthermore, those skilled in the art can combine and integrate the different embodiments or examples described in this specification.

[0085] The above description, in conjunction with specific preferred embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such modifications and substitutions should be considered within the scope of protection of the present invention.

Claims

1. A signal purification device for use in fiber optic microwave transmission systems, characterized in that, include: A single-frequency laser module includes a laser, multiple optical elements, and a FP cavity. The laser is used to emit a laser signal, and the multiple optical elements are used to lock the frequency of the laser signal onto the FP cavity using a quasi-synchronous digital architecture frequency locking scheme to obtain a laser signal with improved stability. The femtosecond optical comb frequency conversion module is used to transfer the stability of the improved laser signal to the microwave frequency band by using the frequency gearing action of the femtosecond optical comb, so as to obtain a femtosecond pulse laser signal; Photoelectric conversion module; used to convert the femtosecond pulsed laser signal into a microwave signal; A microwave frequency synthesis module is used to filter and amplify the microwave signal to obtain a microwave signal with improved stability, which serves as the purified output signal. Simultaneously, it compares the microwave signal with a preset reference signal. When the frequency of the microwave signal deviates from the frequency of the preset reference signal, it determines a frequency adjustment amount, processes the frequency adjustment amount using a preset filtering algorithm, and feeds the processing result back to the single-frequency laser module to adjust the frequency of the improved laser signal in real time. When the frequency of the microwave signal matches the frequency of the preset reference signal, it determines a phase adjustment amount, processes the phase adjustment amount using a preset filtering algorithm, and feeds the processing result back to the single-frequency laser module. A single-frequency laser module is provided to adjust the frequency of the laser signal to improve stability in real time. The microwave frequency synthesis module includes a phase shifter, a second mixer, an analog-to-digital converter, and a digital processing module. The phase shifter receives a preset reference signal and adjusts its phase. The second mixer receives the signal processed by the phase shifter and the microwave signal, and performs mixing. The analog-to-digital converter receives the signal processed by the second mixer and converts the analog signal into a digital signal. The digital processing module receives the signal processed by the analog-to-digital converter and estimates the frequency deviation and frequency drift rate of the signal. The signal processed by the digital processing module is fed back to the single-frequency laser module. The plurality of optical elements include a plurality of DDS chips, some of which respond to the signals fed back by the microwave frequency synthesis module and adjust the frequency of the signals to adjust the frequency of the laser signals in real time to improve stability.

2. The signal purification device for fiber optic microwave transmission systems according to claim 1, characterized in that, Also includes: The data acquisition module is used to acquire in real time the laser signal with improved stability output by the single-frequency laser module, the femtosecond pulse laser signal output by the femtosecond optical comb optical frequency conversion module, the microwave signal output by the photoelectric conversion module, and the purified output signal output by the microwave frequency integration module.

3. The signal purification device for fiber optic microwave transmission systems according to claim 1, characterized in that, The plurality of optical elements include an acousto-optic modulator, a half-wave plate, a first polarization beam splitter, a first optoelectronic modulator, a second optoelectronic modulator, a second polarization beam splitter, a quarter-wave plate, a photodetector, a first mixer, a low-pass filter, a controller, a first DDS chip, and a second DDS chip. in ； The acousto-optic modulator receives the laser signal and adjusts its parameters. The half-wave plate receives the signal processed by the acousto-optic modulator and adjusts its polarization state. The first polarization beam splitter receives the signal processed by the half-wave plate and processes signals with different polarization states. The first opto-modulator receives the signal processed by the first polarization beam splitter and adjusts its parameters. The second opto-modulator receives the signal processed by the first opto-modulator and adjusts its parameters. The second polarization beam splitter receives the signal processed by the second opto-modulator and processes signals with different polarization states. The quarter-wave plate receives the signal processed by the second polarization beam splitter and adjusts its polarization state type. The quarter-wave plate also receives the signal reflected from the FP cavity and adjusts its polarization state type. The second polarization beam splitter also receives the signal processed by the quarter-wave plate and processes signals with different polarization states. The photodetector is used to receive the signal processed by the second polarization beam splitter and convert the optical signal into an electrical signal. The first DDS chip is used to receive the signal processed by the photodetector and adjust the frequency of the signal. The second photoelectric modulator is also used to receive the signal processed by the first DDS chip and adjust the parameters of the signal. The first mixer is used to receive the signal processed by the first DDS chip and the signal processed by the photodetector and perform mixing processing. The low-pass filter is used to receive the signal processed by the first mixer and perform filtering processing. The controller is used to receive the signal processed by the low-pass filter and generate frequency and power control quantities to adjust the frequency and power of the laser. The second DDS chip is used to receive the signal processed by the microwave frequency synthesis module and adjust the frequency of the signal. The first photoelectric modulator is also used to receive the signal processed by the second DDS chip to adjust the frequency of the laser signal with improved stability in real time.

4. The signal purification device for fiber optic microwave transmission systems according to claim 1, characterized in that, The photoelectric conversion module includes a fast photodetector for converting the femtosecond pulsed laser signal into a microwave signal.

5. A signal purification method for use in an optical fiber microwave transmission system, comprising the signal purification device for use in an optical fiber microwave transmission system as described in any one of claims 1 to 4, characterized in that, include: Acquire a laser signal, lock the frequency of the laser signal to the resonance peak of a preset FP cavity, and obtain a laser signal with improved stability. The stability of the laser signal, which has been improved by using the frequency gearing action of a femtosecond optical comb, is transferred to the microwave frequency band to obtain a femtosecond pulsed laser signal. The femtosecond pulsed laser signal is converted into a microwave signal; The microwave signal is filtered and amplified to obtain a microwave signal with improved stability, which serves as the purified output signal. Simultaneously, the microwave signal is compared with a preset reference signal. When the frequency of the microwave signal deviates from the frequency of the preset reference signal, a frequency adjustment amount is determined, and a preset filtering algorithm is used to process the frequency adjustment amount. The frequency of the improved laser signal is then adjusted in real time based on the processing result. When the frequency of the microwave signal matches the frequency of the preset reference signal, a phase adjustment amount is determined, and a preset filtering algorithm is used to process the phase adjustment amount. The frequency of the improved laser signal is then adjusted in real time based on the processing result.

6. The signal purification method for fiber optic microwave transmission systems according to claim 5, characterized in that, The stability of the laser signal, improved by using a femtosecond optical comb, is transferred to the microwave frequency band to obtain a femtosecond pulsed laser signal, including: The femtosecond optical comb Frequency of each comb tooth The beat frequency is locked to the laser signal that improves stability, and locked to zero frequency. ; wherein, the femtosecond optical comb's first The frequency of each comb tooth satisfies , Indicates the repetition frequency.

7. The signal purification method for fiber optic microwave transmission systems according to claim 5, characterized in that, When the frequency of the microwave signal deviates from the frequency of the preset reference signal, a frequency adjustment amount is determined, a preset filtering algorithm is used to process the frequency adjustment amount, and the frequency of the laser signal for improved stability is adjusted in real time based on the processing result, including: A mixed signal is obtained between the microwave signal and the preset reference signal. The low-frequency band of the mixed signal exhibits a sinusoidal characteristic, and the frequency of the sinusoidal curve represents the frequency difference between the microwave signal and the preset reference signal. The mixed signal is represented as follows: ； in, Represents microwave signals. This indicates the preset reference signal. Indicates the amplitude of the microwave signal. This indicates the amplitude of the preset reference signal. This represents the angular frequency of a microwave signal. This indicates the angular frequency of the preset reference signal. Indicates the phase of the microwave signal. This indicates the preset reference signal phase. Represents a time variable; Adjust the frequency of the microwave signal or the frequency of the preset reference signal to obtain the frequency difference between the adjusted microwave signal and the preset reference signal. Based on the change in the frequency difference, frequency discrimination is achieved, and the DC voltage signal of the frequency discrimination result is converted into a first digital signal. The first digital signal is processed using a preset Kalman filter algorithm. Through recursive prediction and observation updates, the frequency deviation and frequency drift rate are estimated in real time, and the frequency of the laser signal with improved stability is adjusted in real time.

8. The signal purification method for fiber optic microwave transmission systems according to claim 5, characterized in that, When the frequency of the microwave signal matches the frequency of the preset reference signal, a phase adjustment amount is determined, a preset filtering algorithm is used to process the phase adjustment amount, and the frequency of the laser signal for improved stability is adjusted in real time based on the processing result, including: A mixed signal between the microwave signal and the preset reference signal is obtained, wherein the low-frequency band of the mixed signal is displayed as a DC component, representing the phase difference between the microwave signal and the preset reference signal; wherein the mixed signal is represented as: ； in, Represents microwave signals. This indicates the preset reference signal. Indicates the amplitude of the microwave signal. This indicates the amplitude of the preset reference signal. This represents the angular frequency of a microwave signal. This indicates the angular frequency of the preset reference signal. Indicates the phase of the microwave signal. This indicates the preset reference signal phase. Represents a time variable; The phase of the preset reference signal is adjusted, the phase difference between the adjusted preset reference signal and the microwave signal is obtained, the relationship between the phase difference change and the mixing signal is established, phase detection is realized, and the DC voltage signal of the phase detection result is converted into a second digital signal. The second digital signal is processed using a preset Kalman filter algorithm. Through recursive prediction and observation updates, the frequency deviation and frequency drift rate are estimated in real time, and the frequency of the laser signal with improved stability is adjusted in real time.

9. The signal purification method for fiber optic microwave transmission systems according to claim 5, characterized in that, The frequency adjustment amount is processed using a preset filtering algorithm, including: A preset Kalman filter algorithm is used to establish the state equation of a linear system. Based on the state equation of the linear system, the observed data are input and output. Based on the minimum mean square error criterion, the state of the linear system is optimally estimated. The estimated state of the linear system represents the frequency deviation and frequency drift rate to be adjusted. The phase adjustment amount is processed using a preset filtering algorithm, including: A preset Kalman filter algorithm is used to establish the state equation of a linear system. Based on the state equation of the linear system, the observed data are input and output. Based on the minimum mean square error criterion, the state of the linear system is optimally estimated. The estimated state of the linear system represents the frequency deviation and frequency drift rate to be adjusted.

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