Pure Rayleigh scattering signal acquisition method and system based on laser source

Abstract

The invention relates to a pure Rayleigh scattering signal acquisition method and system based on a laser source. The method comprises the following steps: performing beam splitting treatment on femtosecond pulses output by a laser source to obtain at least two beams of femtosecond pulses; inputting the first beam of femtosecond pulse into a nonlinear crystal for frequency mixing processing to obtain a preprocessed scattering signal; performing frequency shift processing on the second beam of femtosecond pulse by using an acousto-optic frequency shifter or performing phase lock processing on the second beam of femtosecond pulse by using an optical phase lock ring to obtain a coherent local oscillator signal; inputting the preprocessed scattering signal and the coherent local oscillator signal into a frequency mixer for interference processing to obtain a mixed difference frequency signal; and performing multi-stage signal processing on the mixed difference frequency signal by using a signal processor to obtain a target Rayleigh scattering signal. According to the invention, the Rayleigh scattering signal acquisition accuracy can be improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of ultra-short pulse laser Rayleigh scattering detection, in particular to a pure Rayleigh scattering signal acquisition method and system based on a laser source. BACKGROUND

[0002] In the field of ultra-short pulse laser Rayleigh scattering detection, the purity and accuracy of Rayleigh scattering signals directly determine the precision of medium physical parameter inversion. The time-domain amplitude evolution and frequency-domain spectral peak distribution contained in the signals are the core basis for calculating medium temperature, density and molecular polarizability.

[0003] If the signal purity is insufficient or the accuracy is deviated, the temperature measurement error will exceed 10%, the density inversion deviation will be greater than 15%, and even the key parameters such as molecular polarizability cannot be accurately extracted, which seriously restricts the application of ultra-short pulse laser in the fields of atmospheric remote sensing, material micro-characterization, and biological tissue non-destructive testing.

[0004] Therefore, there is an urgent need for a method that can efficiently acquire high-purity and high-accuracy Rayleigh scattering signals in the ultra-short pulse laser excitation scenario, providing a reliable data basis for subsequent accurate inversion of medium physical parameters, and meeting the technical needs of high-precision detection scenarios. SUMMARY

[0005] Therefore, there is an urgent need for a method that can efficiently acquire high-purity and high-accuracy Rayleigh scattering signals in the ultra-short pulse laser excitation scenario, providing a reliable data basis for subsequent accurate inversion of medium physical parameters, and meeting the technical needs of high-precision detection scenarios.

[0006] In a first aspect, the present application provides a pure Rayleigh scattering signal acquisition method based on a laser source, which comprises:

[0007] Splitting the femtosecond pulse output by the laser source to obtain at least two femtosecond pulses;

[0008] Inputting the first femtosecond pulse into a nonlinear crystal for mixing processing to obtain a pretreated scattering signal, and using an acousto-optic frequency shifter to perform frequency shift processing on the second femtosecond pulse or using an optical phase-locked loop to perform phase-locked processing on the second femtosecond pulse to obtain a coherent local signal;

[0009] Inputting the pretreated scattering signal and the coherent local signal into a mixer for interference processing to obtain a mixed difference frequency signal;

[0010] Using a signal processor to perform multi-stage signal processing on the mixed difference frequency signal to obtain a target Rayleigh scattering signal.

[0011] In a second aspect, the application also provides a laser source-based pure Rayleigh scattering signal acquisition system, which comprises, in sequence, a laser source, a nonlinear crystal, an acousto-optic frequency shifter or an optical phase-locked loop, a frequency mixer, and a signal processor.

[0012] The laser source is configured to output femtosecond pulses.

[0013] The nonlinear crystal is configured to receive the first beam of femtosecond pulses after the beam splitting and perform frequency mixing on the first beam of femtosecond pulses to obtain a preprocessed scattering signal.

[0014] The acousto-optic frequency shifter is configured to receive the second beam of femtosecond pulses after the beam splitting and perform frequency shifting on the second beam of femtosecond pulses to obtain a coherent local oscillator signal, or the optical phase-locked loop is configured to receive the second beam of femtosecond pulses after the beam splitting and perform phase locking on the second beam of femtosecond pulses to obtain a coherent local oscillator signal.

[0015] The frequency mixer is configured to perform interference processing on the preprocessed scattering signal and the coherent local oscillator signal to obtain a mixed difference frequency signal.

[0016] The signal processor is configured to perform multi-stage signal processing on the mixed difference frequency signal to obtain a target Rayleigh scattering signal.

[0017] The laser source-based pure Rayleigh scattering signal acquisition method and system can realize independent processing of scattering signal generation and coherent local oscillator control through beam splitting of femtosecond pulses output by the laser source, avoid signal crosstalk caused by single-pulse multi-function operation, and provide a basis for subsequent accurate processing. The first beam of pulses is mixed by the nonlinear crystal to amplify weak Rayleigh scattering signals, filter out part of the noise interference, reduce the masking of signals by inelastic scattering, and improve the purity of the initial signal. The second beam of pulses is processed by frequency shifting or phase locking to ensure that the coherent local oscillator signal is frequency-matched and phase-stable with the preprocessed scattering signal, thereby avoiding interference distortion caused by insufficient coherence. The two signals are interfered by the frequency mixer to generate a mixed difference frequency signal, which can convert weak optical signals into easily detectable radio frequency electrical signals, reduce noise interference during direct detection, and improve signal acquisition accuracy. Finally, the signal processor performs multi-stage processing to gradually remove high-frequency noise, periodic interference, and nonlinear components, completely restores the intrinsic time-domain and frequency-domain characteristics of the Rayleigh scattering signal, and finally significantly improves the accuracy of Rayleigh scattering signal acquisition, providing reliable data support for subsequent accurate inversion of medium temperature, density, and molecular polarizability. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 FIG. 1 is a flowchart of a laser source-based pure Rayleigh scattering signal acquisition method according to an embodiment of the application;

[0019] Figure 2 FIG. 2 is a flowchart of a laser source-based pure Rayleigh scattering signal acquisition method according to another embodiment of the application.

[0020] Figure 3 This is a flowchart illustrating a method for acquiring pure Rayleigh scattering signals based on a laser source in another embodiment;

[0021] Figure 4 This is a flowchart illustrating a method for acquiring pure Rayleigh scattering signals based on a laser source in another embodiment;

[0022] Figure 5 This is a flowchart illustrating a method for acquiring pure Rayleigh scattering signals based on a laser source in another embodiment;

[0023] Figure 6 This is a flowchart illustrating a method for acquiring pure Rayleigh scattering signals based on a laser source in another embodiment;

[0024] Figure 7 This is a flowchart illustrating a method for acquiring pure Rayleigh scattering signals based on a laser source in another embodiment;

[0025] Figure 8 This is a flowchart illustrating a method for acquiring pure Rayleigh scattering signals based on a laser source, as described in another embodiment. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0027] In one embodiment, such as Figure 1 As shown, a method for acquiring pure Rayleigh scattering signals based on a laser source is provided, including the following steps:

[0028] Step 101: Split the femtosecond pulses output from the laser source to obtain at least two femtosecond pulses.

[0029] Among them, femtosecond pulses refer to ultrashort laser pulses with a pulse duration in the femtosecond range. They have the characteristics of high peak power and high time resolution, and can accurately excite medium molecules to produce instantaneous Rayleigh scattering. They can also reduce thermal damage to the object being detected, making them the excitation source in ultrashort pulse laser Rayleigh scattering detection.

[0030] In this embodiment of the application, the femtosecond pulses output by the laser source are split into at least two femtosecond pulses.

[0031] In one specific embodiment, a femtosecond laser adapted for ultrashort pulse transmission is selected as the laser source. The output femtosecond pulse is first calibrated by a beam collimation component to ensure stable beam propagation direction, and then incident on a polarization-independent fiber beam splitter. Through the power distribution and optical path splitting functions of the beam splitter, a single femtosecond pulse is divided into at least two sub-pulses. One sub-pulse is used to generate a preprocessed scattered signal, and the other sub-pulse is used to prepare a coherent local oscillator signal. At the same time, the structural design of the beam splitter ensures the time synchronization of the two sub-pulses, meeting the timing matching requirements of the signal and the local oscillator light in the subsequent optical path.

[0032] Step 102: Input the first femtosecond pulse into a nonlinear crystal for mixing to obtain a preprocessed scattering signal; and use an acousto-optic frequency shifter to perform frequency shifting on the second femtosecond pulse or use an optical phase-locked loop to perform phase-locking on the second femtosecond pulse to obtain a coherent local oscillator signal.

[0033] Among them, nonlinear crystals are crystal materials with nonlinear optical effects (such as mixing, frequency doubling, parametric amplification, etc.) (such as periodically polarized LBO crystals, BBO crystals, etc.). In the embodiments of this application, they are used to perform mixing processing on the first femtosecond pulse. Through the nonlinear interaction inside the crystal, the Rayleigh scattering signal is amplified and some background noise is filtered out, thereby obtaining a pre-processed scattering signal with an initially improved signal-to-noise ratio.

[0034] Frequency mixing processing utilizes the nonlinear optical properties of nonlinear crystals to cause frequency mixing and energy exchange between the incident femtosecond pulse and the Rayleigh scattering signal generated by the medium inside the crystal. On the one hand, it enhances the amplitude of the weak Rayleigh scattering signal, and on the other hand, it suppresses interference signals of non-target frequencies (such as some stray light), thereby achieving pre-optimization of the scattered signal and providing a high-quality signal source for subsequent processing.

[0035] The preprocessed scattering signal is the scattering signal obtained after the first femtosecond pulse is mixed by a nonlinear crystal. Compared with the original Rayleigh scattering signal, it has a higher signal-to-noise ratio and fewer interference components. It has completed the initial signal purification and enhancement and can be directly used as the target signal source for subsequent interference with the coherent local oscillator signal.

[0036] An optical frequency shifter is an optical frequency control device based on the acousto-optic effect. By applying a radio frequency driving signal to an acousto-optic medium (such as quartz or lithium niobate), the incident second femtosecond pulse undergoes acousto-optic diffraction, thereby precisely changing the frequency of the pulse (the frequency shift is usually in the MHz to GHz range). This ensures that the frequency of the output local oscillator signal and the pre-processed scattered signal meet the interference condition, making it one of the core devices for generating coherent local oscillator signals.

[0037] An optical phase-locked loop (PLL) is an optical control system used to achieve precise phase locking between two beams of light. It uses a phase detector to detect the phase difference between the second femtosecond pulse and the reference light (or a pre-processed scattered signal) in real time. Then, a feedback control module (such as a piezoelectric controller or an electro-optic modulator) adjusts the phase of the second femtosecond pulse to keep it stable and consistent with the target phase, ultimately generating a local oscillator signal with high phase coherence. It is suitable for interference scenarios with extremely high requirements for phase stability.

[0038] The coherent local oscillator signal is a laser signal obtained by frequency shifting (acousto-optic frequency shifter) or phase-locking (optical phase-locked loop) of the second femtosecond pulse. It has the characteristics of frequency controllability and phase stability, and meets the coherence condition (similar frequency and stable phase difference) with the pre-processed scattering signal. It can convert the weak scattering signal into a mixed difference frequency signal that is easy to detect through interference, which is the key to improving the detection sensitivity of Rayleigh scattering signal.

[0039] In this embodiment, a first femtosecond pulse is input into a nonlinear crystal for mixing to obtain a preprocessed scattering signal; and a second femtosecond pulse is frequency-shifted using an acousto-optic frequency shifter or phase-locked using an optical phase-locked loop to obtain a coherent local oscillator signal.

[0040] In one specific embodiment, during the preprocessing of the scattering signal generation process, a nonlinear crystal with efficient nonlinear mixing characteristics is selected. The first femtosecond pulse obtained by beam splitting is introduced into the nonlinear crystal, and the Rayleigh scattering light generated by the medium to be detected is also coupled into the nonlinear crystal. The nonlinear optical effect of the crystal is used to mix the femtosecond pulse and the Rayleigh scattering light, thereby enhancing and initially denoising the Rayleigh scattering signal, ultimately obtaining the preprocessed scattering signal. During the coherent local oscillator signal generation process, two processing methods can be adopted: one is to use an acousto-optic frequency shifter to adjust the frequency of the second femtosecond pulse so that the pulse frequency meets the requirements of subsequent interference; the other is to use an optical phase-locked loop to precisely control the phase of the second femtosecond pulse, maintaining a stable phase relationship with the preprocessed scattering signal. Both methods can obtain a coherent local oscillator signal that meets the interference conditions.

[0041] Step 103: Input the preprocessed scattering signal and the coherent local oscillator signal into the mixer for interference processing to obtain the mixed difference frequency signal.

[0042] Among them, a mixer refers to an optical device used to realize the interference of two coherent light beams (such as a 50:50 fiber beam splitter, polarizer combination, etc.). Its core function is to introduce the pre-processed scattered signal and the coherent local oscillator signal into the same optical path, so that the two beams of light are superimposed in space and interfere. By using the interference effect of light, the frequency difference between the two beams of light is converted into a mixed difference frequency signal whose intensity varies with time, thus completing the conversion from "optical signal to detectable difference frequency signal".

[0043] The mixed difference frequency signal is generated by interfering the preprocessed scattered signal and the coherent local oscillator signal through a mixer. Its frequency is equal to the frequency difference between the two incident beams, and the signal strength is directly related to the strength of the preprocessed scattered signal.

[0044] In this embodiment, the preprocessed scattering signal and the coherent local oscillator signal are input into a mixer for interference processing to obtain a mixed difference frequency signal.

[0045] In one specific embodiment, a mixer with signal combining and interference functions is selected. The pre-processed scattered signal and the coherent local oscillator signal obtained in the previous stage are respectively input to the corresponding signal input terminals of the mixer. Inside the mixer, the two signals are spatially superimposed through optical path coupling. Utilizing the interference properties of light, the two signals interact, converting the frequency difference between the two signals into a detectable intensity change signal. Finally, a mixed difference frequency signal is obtained through the output terminal of the mixer. This signal has initially integrated the effective information of the pre-processed scattered signal, laying the foundation for subsequent multi-stage processing.

[0046] Step 104: Use a signal processor to perform multi-stage signal processing on the mixed difference frequency signal to obtain the target Rayleigh scattering signal.

[0047] Among them, the signal processor is an electronic or optoelectronic integrated system used to perform a series of optimization processes on the mixed difference frequency signal. It typically includes a filtering module (such as a low-pass filter or a band-pass filter), an amplification module (such as a radio frequency low-noise amplifier), an analog-to-digital conversion module, a nonlinear stripping module, a time-frequency correction module, etc. It can gradually strip away interference components, restore the intrinsic characteristics of the Rayleigh scattering signal, and finally output a high-purity target Rayleigh scattering signal.

[0048] The target Rayleigh scattering signal is the Rayleigh scattering signal obtained after multi-stage signal processing. It has removed interference components such as inelastic scattering (Raman, Brillouin scattering), system noise (speckle, stray light), and nonlinear effects (Kerr effect). It has the characteristics of high purity and high accuracy and can be directly used for the accurate inversion of physical parameters such as medium temperature, density, and molecular polarizability.

[0049] In this embodiment, a signal processor is used to perform multi-level signal processing on the mixed difference frequency signal to obtain the target Rayleigh scattering signal.

[0050] In one specific embodiment, a signal processor integrating multiple signal processing functions is used. After the mixed difference frequency signal is input into the processor, it first passes through a filtering module to remove noise and interference components from the signal, and then passes through a signal conversion module to complete the signal format conversion to adapt it to the subsequent processing flow. Subsequently, the processor removes nonlinear interference from the signal through a nonlinear interference stripping module, and finally optimizes the time and frequency domain characteristics of the signal through a time-frequency correction module to restore the intrinsic characteristics of the Rayleigh scattering signal. After the above multi-stage processing, the target Rayleigh scattering signal is finally output.

[0051] In the aforementioned method for acquiring pure Rayleigh scattering signals based on a laser source, the femtosecond pulses output from the laser source are split into multiple beams, enabling independent processing of scattering signal generation and coherent local oscillator modulation. This avoids signal crosstalk caused by single-pulse multi-functional operation, providing a foundation for subsequent precise processing. The first pulse is mixed by a nonlinear crystal, amplifying the weak Rayleigh scattering signal and filtering out some clutter interference, reducing the masking effect of inelastic scattering on the signal and improving the purity of the initial signal. The second pulse is processed by frequency shifting or phase-locking to ensure frequency matching and phase stability between the coherent local oscillator signal and the preprocessed scattering signal. To avoid interference distortion caused by insufficient coherence, the two signals are interfered by a mixer to generate a mixed difference frequency signal, which can convert the weak optical signal into an easily detectable radio frequency electrical signal, reduce noise interference during direct detection, and improve signal acquisition accuracy. Finally, through multi-level processing by a signal processor, high-frequency noise, periodic interference, and nonlinear components are stripped away layer by layer to fully restore the intrinsic time-domain and frequency-domain characteristics of the Rayleigh scattering signal, ultimately significantly improving the accuracy of Rayleigh scattering signal acquisition and providing reliable data support for the accurate inversion of parameters such as medium temperature, density, and molecular polarizability.

[0052] In one embodiment, such as Figure 2 As shown, the above-mentioned "using a signal processor to perform multi-level signal processing on the mixed difference frequency signal to obtain the target Rayleigh scattering signal" includes:

[0053] Step 201: The mixed difference frequency signal is sequentially processed by photoelectric conversion and optical lock-in amplification using a signal processor to obtain a digital difference frequency signal.

[0054] Photoelectric conversion processing refers to the process of converting mixed difference-frequency signals (optical signals) into electrical signals, typically achieved through photodetectors (such as high-speed photodiodes, avalanche photodiodes, etc.). After receiving the mixed difference-frequency signals, the detector uses the photoelectric effect to convert the changes in light intensity of the optical signal into corresponding changes in current or voltage, outputting an analog electrical signal. This provides an operable signal form for subsequent electrical processing (such as lock-in amplification and analog-to-digital conversion), and is a crucial link connecting optical signal processing and electrical signal processing.

[0055] The digital difference frequency signal is a hybrid difference frequency signal converted into an analog electrical signal through photoelectric conversion, then amplified by optical lock-in amplification, and finally converted into a digital signal by an analog-to-digital converter. This signal retains the time-domain amplitude, phase, and frequency characteristics of the hybrid difference frequency signal, and the noise has been initially suppressed, so it can be directly used for subsequent digital domain signal extraction and optimization processing.

[0056] In this embodiment, the mixed difference frequency signal (optical signal) is input to the photoelectric conversion module of the signal processor. This module has a built-in adapted photoelectric detection element, which converts the light intensity change of the mixed difference frequency signal into a corresponding analog electrical signal through the photoelectric effect. The analog electrical signal is then transmitted to the optical lock-in amplifier module. The repetition frequency of the femtosecond pulse output by the laser source is used as the reference signal. The module uses synchronous detection technology to filter out the effective signal components that match the frequency and phase of the reference signal, suppress asynchronous background noise, and then converts the processed analog electrical signal into digital form through analog-to-digital conversion, finally obtaining the digital difference frequency signal.

[0057] Step 202: Extract and process the digital difference frequency signal to obtain a digital stream signal.

[0058] Digital stream signals are continuous digital signal streams formed by encoding and serializing signal feature parameters (such as time-domain envelope data, phase parameters, and frequency-domain eigenvalues) obtained from the extraction and processing of digital difference frequency signals according to a preset format (such as binary or hexadecimal). This signal stream has a regular structure, is easy to transmit and store, and contains the core information of the Rayleigh scattering signal, making it the direct input for subsequent filtering and nonlinear stripping processes.

[0059] In this embodiment, the digital difference frequency signal is input into the extraction and processing module of the signal processor. The module first performs time-domain analysis on the digital difference frequency signal to capture the amplitude variation law and phase fluctuation characteristics of the signal. Then, through frequency domain transformation operation, it sorts out the frequency domain distribution of the signal. Subsequently, based on the inherent characteristics of Rayleigh scattering signal (such as specific time-domain envelope and frequency domain spectral peak range), it selects key parameters related to the target signal. These parameters are integrated into a continuous digital sequence according to preset encoding rules to complete the extraction and processing of the digital difference frequency signal and obtain a digital stream signal.

[0060] Step 203: The digital stream signal is sequentially filtered and nonlinear stripped to obtain the target denoised signal.

[0061] In this embodiment of the application, the digital stream signal is sequentially filtered and nonlinear stripped to obtain the target denoised signal.

[0062] In one specific embodiment, the digital stream signal is first input to the filtering module of the signal processor. This module removes the superimposed high-frequency random noise and periodic interference signals from the digital stream signal through a preset filtering algorithm, retaining the effective components that conform to the Rayleigh scattering signal frequency band. Then, the pre-filtered signal is transmitted to the nonlinear stripping module. The module identifies and separates the nonlinear interference components in the signal based on a pre-constructed nonlinear interference model (such as the signal characteristic model of Raman scattering and Kerr effect). After two processing steps, a target denoised signal with significantly reduced noise and nonlinear interference is obtained.

[0063] Step 204: Perform time-domain and frequency-domain correction processing on the target denoised signal to obtain the target Rayleigh scattering signal.

[0064] In this embodiment of the application, the target denoised signal is subjected to time-domain and frequency-domain correction processing to obtain the target Rayleigh scattering signal.

[0065] In a specific embodiment, the target denoised signal is input into the time-frequency correction module of the signal processor. In the time-domain correction stage, the module adjusts the time-domain parameters of the signal by comparing the actual time delay and amplitude attenuation of the signal with theoretical reference values ​​to correct time-domain distortion. In the frequency-domain correction stage, the module analyzes the frequency-domain spectral peak position, bandwidth and other characteristics of the signal, and combines them with the system frequency response characteristics to correct the frequency-domain offset and bandwidth broadening problems through compensation algorithms, so that the signal restores the intrinsic time-frequency characteristics of Rayleigh scattering, and finally obtains the target Rayleigh scattering signal.

[0066] In one embodiment, such as Figure 3 As shown, the signal processor includes a high-bandwidth photodetector, an optical lock-in amplifier, and an analog-to-digital converter. Based on this, the aforementioned "performing photoelectric conversion and optical lock-in amplification sequentially on the mixed difference frequency signal to obtain a digital difference frequency signal" includes:

[0067] Step 301: Input the mixed difference frequency signal into a high bandwidth photodetector for photoelectric conversion processing to obtain an analog electrical signal.

[0068] In this embodiment, the mixed difference frequency signal is input into a wide bandwidth photodetector for photoelectric conversion processing to obtain an analog electrical signal.

[0069] In one specific embodiment, the mixed difference frequency signal (optical signal) obtained through mixer interference is guided to the detection window of a high-bandwidth photodetector, ensuring that the signal is incident at a preset angle and covers the effective photosensitive area of ​​the detector. The high-bandwidth photodetector uses its internal photoelectric conversion element to convert the light intensity change of the mixed difference frequency signal into a corresponding current change. Then, the signal conditioning circuit built into the detector converts the current signal into a voltage signal, and finally outputs an analog electrical signal that meets the requirements of subsequent processing. This signal completely preserves the time and frequency domain characteristics of the mixed difference frequency signal, only realizing the conversion of the signal form from optical to electrical.

[0070] Step 302: The analog electrical signal is input to the optical lock-in amplifier. The optical lock-in amplifier performs synchronous detection processing on the analog electrical signal based on the frequency of the femtosecond pulse to obtain the synchronously detected analog electrical signal.

[0071] In this embodiment, an analog electrical signal is input to an optical lock-in amplifier, which performs synchronous detection processing on the analog electrical signal based on the frequency of the femtosecond pulse to obtain a synchronously detected analog electrical signal.

[0072] In one specific embodiment, the analog electrical signal obtained from photoelectric conversion is input to the signal input terminal of an optical lock-in amplifier, while the frequency signal of the femtosecond pulse output from the laser source is used as a reference signal input to the reference terminal of the optical lock-in amplifier. Internally, the optical lock-in amplifier uses a signal comparison module to compare the frequency and phase of the analog electrical signal with the reference signal, filtering out only signal components with the same frequency and phase correlation as the reference signal. Irrelevant background noise (such as electrical signals generated by ambient stray light, circuit noise, etc.) is suppressed. After synchronous detection processing, the output is a synchronously detected analog electrical signal with significantly reduced noise and prominent effective signal components.

[0073] Step 303: Input the analog electrical signal after synchronous detection into the analog-to-digital converter for analog-to-digital conversion processing to obtain the digital difference frequency signal.

[0074] In this embodiment of the application, the analog electrical signal after synchronous detection is input into an analog-to-digital converter for analog-to-digital conversion processing to obtain a digital difference frequency signal.

[0075] In one specific embodiment, the analog electrical signal after synchronous detection is transmitted to the signal input interface of the analog-to-digital converter. The analog-to-digital converter first preprocesses the analog electrical signal (such as signal amplification and filtering to eliminate minor interference), and then discretizes the continuously changing analog electrical signal into a series of digital quantities according to the preset sampling rules. Finally, the encoding module converts these digital quantities into binary digital sequences that conform to the digital signal processing standard, and finally obtains a digital difference frequency signal that can be processed by the subsequent digital signal extraction module.

[0076] In one embodiment, such as Figure 4 As shown, the above-mentioned "extracting and processing the digital difference frequency signal to obtain a digital stream signal" includes:

[0077] Step 401: Perform Fast Fourier Transform and Peak Detection on the digital difference frequency signal in sequence to obtain the processed digital difference frequency signal.

[0078] In this embodiment of the application, the digital difference frequency signal is sequentially subjected to fast Fourier transform processing and peak detection processing to obtain the processed digital difference frequency signal.

[0079] In one specific embodiment, the digital difference frequency signal is input to the transformation processing module of the signal processor. The module first performs a fast Fourier transform on the digital difference frequency signal to convert the signal from the time domain to the frequency domain, clearly presenting the frequency domain distribution of the signal and distinguishing between the effective signal frequency band and the noise frequency band. Subsequently, the peak detection module identifies the signal peaks related to Rayleigh scattering in the frequency domain based on preset threshold conditions, eliminates false peaks caused by noise interference, and retains key information such as the frequency and amplitude corresponding to the peaks, ultimately obtaining a processed digital difference frequency signal with more prominent effective signal characteristics.

[0080] Step 402: Extract the time-domain envelope, phase information, and frequency-domain features of the processed digital difference frequency signal to obtain signal feature parameters.

[0081] In this embodiment of the application, the time-domain envelope, phase information and frequency-domain features of the processed digital difference frequency signal are extracted to obtain signal feature parameters.

[0082] In a specific embodiment, for the processed digital difference frequency signal, multi-dimensional information extraction is performed by the feature extraction module of the signal processor: in the time domain dimension, the module captures the change law of signal amplitude over time through the envelope detection algorithm to obtain the time domain envelope; in the phase dimension, phase unwrapping technology is used to eliminate phase ambiguity and obtain continuous phase information of the signal; in the frequency domain dimension, combined with the previous fast Fourier transform results, frequency domain features such as spectral peak position, bandwidth, and spectral line intensity of the signal are extracted, and the information extracted from these different dimensions is integrated into complete signal feature parameters.

[0083] Step 403: Encode the signal characteristic parameters to obtain a digital stream signal.

[0084] In the embodiments of this application, the signal characteristic parameters are encoded to obtain a digital stream signal.

[0085] In a specific embodiment, the extracted signal feature parameters (time domain envelope, phase information, and frequency domain features) are input into the encoding module of the signal processor. The module converts different types of feature parameters into digital codes of a unified format according to preset encoding rules. For example, the amplitude data of the time domain envelope, the angle data of the phase information, and the frequency data of the frequency domain features are serialized and encoded respectively. Then, these encoded digital segments are combined into a continuous digital sequence in a fixed order, and finally a well-structured digital stream signal that is easy to process later is formed.

[0086] In one embodiment, such as Figure 5 As shown, the signal processor also includes a low-pass filter, a band-pass filter, a notch filter, and a dual-channel differential model. Based on this, the aforementioned "performing sequential filtering and nonlinear stripping of the digital stream signal to obtain the target denoised signal" includes:

[0087] Step 501: Input the digital stream signal to a low-pass filter to filter out high-frequency random noise in the digital stream signal and obtain the initial signal.

[0088] In this embodiment, the digital stream signal is input to a low-pass filter, and the high-frequency random noise in the digital stream signal is filtered out by the low-pass filter to obtain the initial signal.

[0089] In one specific embodiment, the digital stream signal is connected to the signal input of a low-pass filter. The low-pass filter suppresses high-frequency random noise in the digital stream signal that has a frequency higher than the preset cutoff frequency. During processing, the filter uses an internal signal attenuation mechanism to allow only effective signal components with frequencies lower than the cutoff frequency to pass through, while significantly reducing the intensity of high-frequency noise to prevent it from interfering with subsequent signal processing. The final output is an initial signal with high-frequency random noise removed and a more stable signal baseline.

[0090] Step 502: Input the initial signal into a bandpass filter. The bandpass filter retains the Rayleigh scattering radio frequency band signal in the initial signal to obtain the intermediate signal.

[0091] In this embodiment, the initial signal is input to a bandpass filter, and the Rayleigh scattering radio frequency band signal in the initial signal is retained by the bandpass filter to obtain an intermediate signal.

[0092] In one specific embodiment, the initial signal obtained by low-pass filtering is transmitted to the input interface of a band-pass filter. The band-pass filter is pre-set with a passband frequency range that matches the characteristics of the Rayleigh scattering signal. During processing, the filter retains only the Rayleigh scattering frequency band signal in the initial signal that falls within this passband range, while blocking and attenuating other frequency band interference signals outside the passband range (such as frequency band components of non-target scattering signals). This ensures that the output signal contains only the core frequency band components related to Rayleigh scattering, resulting in an intermediate signal with more accurate frequency bands and further reduced interference.

[0093] Step 503: Input the intermediate signal to the notch filter, and suppress the periodic interference in the intermediate signal to obtain the preliminary signal.

[0094] In this embodiment, the intermediate signal is input to a notch filter, and the periodic interference in the intermediate signal is suppressed by the notch filter to obtain a preliminary signal.

[0095] In one specific embodiment, the intermediate signal after bandpass filtering is input to a notch filter. The notch filter pre-sets a stopband frequency consistent with the frequency of periodic interference present in the intermediate signal (such as fixed-frequency interference caused by equipment vibration, power fluctuations, etc.). During processing, the filter deeply suppresses the periodic interference signal at the stopband frequency, significantly reducing its amplitude, while showing no significant attenuation of the effective signal components at other frequencies. Finally, it outputs a preliminary signal with suppressed periodic interference and improved signal stability.

[0096] Step 504: Input the preliminary signal into the dual-channel differential model, and perform nonlinear stripping processing on the preliminary signal through the dual-channel differential model to obtain the target denoised signal.

[0097] In this embodiment, the initial signal is input into a dual-channel differential model, and the initial signal is subjected to nonlinear stripping processing by the dual-channel differential model to obtain the target denoised signal.

[0098] In one specific embodiment, the preliminary signal obtained by notch filtering is input to the signal input of a dual-channel differential model. This model includes a reference channel and a signal channel: the reference channel receives a pre-acquired reference signal containing only nonlinear interference (such as Raman scattering and Kerr effect interference), while the signal channel receives the preliminary signal. By comparing the characteristic differences between the two channels, the model identifies the nonlinear interference components in the preliminary signal. Then, through differential operations, these nonlinear interference components are removed from the preliminary signal, while the effective Rayleigh scattering signal is fully preserved. Finally, the model outputs a target denoised signal with significantly reduced nonlinear interference.

[0099] In one embodiment, such as Figure 6As shown, the above-mentioned "performing time-domain and frequency-domain correction processing on the target denoised signal to obtain the target Rayleigh scattering signal" includes:

[0100] Step 601: Perform a fast Fourier transform on the target denoised signal to obtain the frequency domain signal of the target denoised signal.

[0101] In this embodiment of the application, a fast Fourier transform is performed on the target denoised signal to obtain the frequency domain signal of the target denoised signal.

[0102] In a specific embodiment, the target denoised signal is input into the Fourier transform module of the signal processor. The module analyzes and transforms the time-domain characteristics of the target denoised signal according to the preset transformation rules, transforming the signal amplitude and phase information that originally changed with time into frequency domain distribution information with frequency as the coordinate, clearly presenting the intensity and phase characteristics of the signal at different frequencies, and finally obtaining a frequency domain signal that can intuitively reflect the frequency domain attributes of the target denoised signal.

[0103] Step 602: The frequency domain signal of the target denoised signal is corrected by using the deconvolution compensation algorithm to obtain the corrected frequency domain signal.

[0104] In this embodiment, the frequency domain signal of the target denoised signal is corrected using a deconvolution compensation algorithm to obtain the corrected frequency domain signal.

[0105] In a specific embodiment, after obtaining the frequency domain signal of the target denoised signal, the preset deconvolution compensation algorithm in the signal processor is called. The algorithm first constructs a system distortion model based on the frequency response characteristics (such as frequency attenuation and phase shift during signal transmission) calibrated in the early stage of the system; then, through deconvolution operation, it compensates and corrects the problems such as spectral peak shift, bandwidth broadening, and amplitude attenuation caused by system distortion in the frequency domain signal, so that the frequency domain signal can restore the true frequency domain characteristics of the Rayleigh scattering signal, and finally obtains the corrected frequency domain signal.

[0106] Step 603: Perform an inverse fast Fourier transform on the corrected frequency domain signal to obtain the target Rayleigh scattering signal.

[0107] In this embodiment of the application, the corrected frequency domain signal is subjected to inverse fast Fourier transform to obtain the target Rayleigh scattering signal.

[0108] In one specific embodiment, the corrected frequency domain signal is transmitted to the inverse Fourier transform module of the signal processor. The module performs inverse transformation processing on the frequency, amplitude, phase and other information of the frequency domain signal, and converts the corrected frequency domain distribution information back into a time domain signal that varies with time. This time domain signal completely preserves the intrinsic time domain characteristics of the Rayleigh scattering signal and has eliminated the distortion and interference remaining in the previous processing, and finally obtains the target Rayleigh scattering signal that meets the detection requirements.

[0109] In one embodiment, such as Figure 7 As shown, the above-mentioned "inputting the preprocessed scattering signal and the coherent local oscillator signal into a mixer for interference processing to obtain a mixed difference frequency signal" includes:

[0110] Step 701: Perform optical path slow drift compensation processing on the coherent local oscillator signal to obtain a coherent local oscillator signal with long-term optical path drift compensation.

[0111] In the embodiments of this application, the coherent local oscillator signal is subjected to optical path slow drift compensation processing to obtain a coherent local oscillator signal with long-term optical path drift compensation.

[0112] In one specific embodiment, the coherent local oscillator signal is introduced into a Brillouin sensing unit. This unit collects optical path drift correlation information (such as optical path deviation trend and drift rate) caused by factors such as changes in ambient temperature and optical path vibration during the transmission of the coherent local oscillator signal in real time, and transmits this information to a PID controller. The PID controller analyzes the optical path drift correlation information based on a preset optical path compensation algorithm, generates corresponding adjustment commands, and drives the optical path adjustment components in the optical path (such as a movable mirror or a piezoelectric ceramic displacement stage) to adjust the transmission optical path of the coherent local oscillator signal, thereby offsetting the optical path deviation caused by the slow drift, and finally obtaining a coherent local oscillator signal with long-term drift compensation.

[0113] Step 702: Perform phase high-frequency jitter compensation processing on the coherent local oscillator signal with long-term optical path drift compensation to obtain a coherent local oscillator signal with phase high-frequency jitter compensation.

[0114] In the embodiments of this application, the coherent local oscillator signal with long-term optical path drift compensation is subjected to phase high-frequency jitter compensation processing to obtain a coherent local oscillator signal with phase high-frequency jitter compensation.

[0115] In one specific embodiment, the coherent local oscillator signal with long-term optical path drift compensation and a portion of the pre-processed scattered signal are input together into an interference / beat frequency detection module. This module captures the phase difference data of the two signals in real time through a phase detection element and transmits the phase difference data to a phase-locked loop (PLL). The PLL performs rapid analysis on the phase difference data, identifies high-frequency phase jitter components in the coherent local oscillator signal (such as phase fluctuations caused by the laser source's own instability or high-frequency electromagnetic interference), and then outputs a corresponding phase control signal. This signal is used by an electro-optic modulator or an acousto-optic modulator to adjust the phase of the coherent local oscillator signal with long-term optical path drift compensation, thus counteracting the influence of high-frequency jitter on the phase and obtaining a coherent local oscillator signal with phase compensation for high-frequency jitter.

[0116] Step 703: Input the coherent local oscillator signal with phase compensation via high frequency jitter and the preprocessed scattered signal into the mixer. The mixer performs interference processing on the coherent local oscillator signal with phase compensation via high frequency jitter and the preprocessed scattered signal to obtain a mixed difference frequency signal.

[0117] In this embodiment, the coherent local oscillator signal with phase compensated by high frequency jitter and the preprocessed scattered signal are input into a mixer. The mixer performs interference processing on the coherent local oscillator signal with phase compensated by high frequency jitter and the preprocessed scattered signal to obtain a mixed difference frequency signal.

[0118] In one specific embodiment, the coherent local oscillator signal with phase compensated for high-frequency jitter and the preprocessed scattered signal are respectively input to the two signal input terminals of the mixer through an adapted optical path coupling component. The mixer, through its internal optical path integration structure, superimposes the two signals in the same spatial region. Utilizing the interference characteristics of light, the two signals interact, converting the frequency difference between the two beams into a signal whose intensity changes periodically with time. Simultaneously, the mixer's signal filtering mechanism suppresses interference from non-interference components, ultimately outputting a mixed difference frequency signal containing the core information of the preprocessed scattered signal.

[0119] In one embodiment, such as Figure 8 As shown, the above method also includes:

[0120] Step 801: Input the femtosecond pulse into the signal output model to obtain the simulated Rayleigh scattering signal output by the signal output model.

[0121] In this embodiment of the application, a femtosecond pulse is input into the signal output model to obtain a simulated Rayleigh scattering signal output by the signal output model.

[0122] In one specific embodiment, the femtosecond pulses output from the laser source and pre-processed (including key parameters such as pulse width, repetition frequency, and light intensity) are parameterized according to the input format requirements of the signal output model. Then, the parameterized femtosecond pulses are input into the signal output model through the model's data interface. This signal output model incorporates a Rayleigh scattering physics simulation algorithm, capable of simulating the Rayleigh scattering process generated by the interaction of femtosecond pulses with medium molecules. It performs calculations based on preset medium characteristic parameters (such as molecular density and temperature), ultimately outputting a simulated Rayleigh scattering signal that reflects the characteristics of the Rayleigh scattering signal. This signal includes characteristic information corresponding to the actual Rayleigh scattering signal, such as time-domain amplitude and frequency-domain spectral peaks.

[0123] Step 802: Correct the simulated Rayleigh scattering signal based on the target Rayleigh scattering signal to obtain the correction parameters.

[0124] In this embodiment of the application, the simulated Rayleigh scattering signal is corrected based on the target Rayleigh scattering signal to obtain the correction parameters.

[0125] In a specific embodiment, the actual detected target Rayleigh scattering signal and the simulated Rayleigh scattering signal output by the signal output model are first imported into the signal comparison and analysis module. This module compares the time-domain waveform, frequency-domain distribution, and phase characteristics of the two signals dimension by dimension to identify the differences between the simulated signal and the target signal (such as amplitude deviation, spectral peak position shift, phase lag, etc.). Then, a parameter optimization algorithm is invoked to iteratively adjust the key influencing parameters of the simulated Rayleigh scattering signal (such as the molecular polarizability coefficient and scattering cross-section parameters in the model) with the goal of reducing the differences between the two signals, until the consistency between the simulated signal and the target signal reaches a preset standard. At this point, the adjusted parameter values ​​are recorded, which are the correction parameters used to correct the signal output model.

[0126] Step 803: The signal output model is corrected according to the correction parameters to obtain the corrected signal output model; the corrected signal output model is used to generate the corrected simulated Rayleigh scattering signal based on the femtosecond pulse.

[0127] In this embodiment, the signal output model is corrected according to the correction parameters to obtain the corrected signal output model; the corrected signal output model is used to generate the corrected simulated Rayleigh scattering signal based on the femtosecond pulse.

[0128] In one specific embodiment, the acquired correction parameters are converted to a new format according to the parameter configuration rules of the signal output model. Then, the correction parameters are input into the signal output model through the model's parameter configuration interface. Upon receiving the correction parameters, the signal output model automatically updates its core parameters related to Rayleigh scattering simulation (such as replacing the original molecular polarizability coefficient and adjusting the scattering cross-section calculation parameters), and optimizes the model's computational logic to ensure that the corrected model can accurately simulate the Rayleigh scattering process based on the new parameters. After the update is complete, the model is validated by inputting a new set of femtosecond pulse parameters. The deviation between the corrected simulated Rayleigh scattering signal output by the model and the target Rayleigh scattering signal is confirmed to be within acceptable limits. Finally, a corrected signal output model that can be used to generate accurate simulated Rayleigh scattering signals is obtained.

[0129] In one embodiment, the above method further includes:

[0130] Step 1: Split the femtosecond pulses output from the laser source to obtain at least two femtosecond pulses.

[0131] Step 2: The first femtosecond pulse is input into a nonlinear crystal for mixing to obtain a preprocessed scattering signal. Then, the second femtosecond pulse is frequency-shifted using an acousto-optic frequency shifter or phase-locked using an optical phase-locked loop to obtain a coherent local oscillator signal.

[0132] Step 3: Perform optical path slow drift compensation processing on the coherent local oscillator signal to obtain a coherent local oscillator signal with long-term optical path drift compensation.

[0133] Step 4: Perform phase high-frequency jitter compensation processing on the coherent local oscillator signal with long-term optical path drift compensation to obtain a coherent local oscillator signal with phase high-frequency jitter compensation.

[0134] Step 5: Input the coherent local oscillator signal with high-frequency jitter compensation and the preprocessed scattered signal into a mixer. The mixer performs interference processing on the coherent local oscillator signal with high-frequency jitter compensation and the preprocessed scattered signal to obtain the mixed difference frequency signal.

[0135] Step 6: Input the mixed difference frequency signal into the wide bandwidth photodetector for photoelectric conversion processing to obtain an analog electrical signal.

[0136] Step 7: Input the analog electrical signal into the optical lock-in amplifier, and use the optical lock-in amplifier to perform synchronous detection processing on the analog electrical signal based on the frequency of the femtosecond pulse to obtain the synchronously detected analog electrical signal.

[0137] Step 8: Input the analog electrical signal after synchronous detection into the analog-to-digital converter for analog-to-digital conversion processing to obtain the digital difference frequency signal.

[0138] Step 9: Perform Fast Fourier Transform and Peak Detection processing on the digital difference frequency signal in sequence to obtain the processed digital difference frequency signal.

[0139] Step 10: Extract the time-domain envelope, phase information, and frequency-domain features of the processed digital difference frequency signal to obtain signal feature parameters.

[0140] Step 11: Encode the signal feature parameters to obtain the digital stream signal.

[0141] Step 12: Input the digital stream signal to the low-pass filter, and filter out the high-frequency random noise in the digital stream signal to obtain the initial signal.

[0142] Step 13: Input the initial signal into the bandpass filter, and retain the Rayleigh scattering radio frequency band signal in the initial signal through the bandpass filter to obtain the intermediate signal.

[0143] Step 14: Input the intermediate signal to the notch filter, and suppress the periodic interference in the intermediate signal through the notch filter to obtain the preliminary signal.

[0144] Step 15: Input the preliminary signal into the dual-channel differential model, and perform nonlinear stripping processing on the preliminary signal through the dual-channel differential model to obtain the target denoised signal.

[0145] Step 16: Perform a Fast Fourier Transform on the target denoised signal to obtain the frequency domain signal of the target denoised signal.

[0146] Step 17: Use the deconvolution compensation algorithm to correct the frequency domain signal of the target denoised signal to obtain the corrected frequency domain signal.

[0147] Step 18: Perform an inverse fast Fourier transform on the corrected frequency domain signal to obtain the target Rayleigh scattering signal.

[0148] Step 19: Input the femtosecond pulse into the signal output model to obtain the simulated Rayleigh scattering signal output by the signal output model.

[0149] Step 20: Correct the simulated Rayleigh scattering signal based on the target Rayleigh scattering signal to obtain correction parameters.

[0150] Step 21: The signal output model is calibrated according to the calibration parameters to obtain a calibrated signal output model. The calibrated signal output model is used to generate a calibrated simulated Rayleigh scattering signal based on the femtosecond pulse.

[0151] In one embodiment, the method further includes:

[0152] I. Physical Principles and Implementation Steps of OPA Preamplifier

[0153] 1. Physics and Mathematics Foundations

[0154] Three-wave mixing process: in a nonlinear crystal (such as...) In LBO (or LBO), a strong pump light (frequency) is used. ) and weak scattered signal light (frequency) Secondary optical parametric amplification (OPA) occurs, generating an amplified signal. ) and newborn idle frequency (idler, frequency) ).

[0155] Coupling equation: Under the approximation of neglecting dissipation, the signal field With idle frequency field The asymptotic amplification can be described by a simplified coupled differential equation:

[0156]

[0157] Where κ is the nonlinear coupling coefficient, which is related to the second-order nonlinear polarizability of the crystal. and pump light intensity Proportional; This is a phase mismatch term; is the complex conjugate of the idler frequency optical field amplitude, used in complex number operations to describe the effect of the optical field phase relationship on energy coupling; z is the distance the light travels in the nonlinear crystal, a variable describing spatial position.

[0158] Gain and phase matching: in Under the condition of ≈0 (quasi-phase matching, QPM), the signal strength increases exponentially:

[0159]

[0160] in, For effective nonlinear coefficients, For the corresponding refractive index; Let Z be the intensity of the signal light propagating a distance z in the crystal; The initial intensity of the signal light at the incident end of the crystal (z=0); It is the gain coefficient, which is equal to the nonlinear coupling coefficient κ, and determines the rate of increase in signal light intensity; This represents the distance light travels in a nonlinear crystal. The effective nonlinear coefficient reflects the strength of the nonlinear optical effect of the crystal under a specific propagation direction and polarization state; The wavelength of the signal light is a physical quantity that describes the spatial periodicity of light and determines the propagation characteristics of light in a crystal. Pump light intensity is the energy source in the optical parametric amplification process and determines the magnitude of the gain.

[0161] 2. System Construction and Steps

[0162] Nonlinear crystal and pump source selection

[0163] A periodically polarized LBO crystal is selected to achieve broadband QPM;

[0164] The pump laser uses a branch of the same source main laser, which is frequency doubled to output ~400 nm, with a pulse width <100 fs and a peak power of tens of MW.

[0165] 3. Optical path design and alignment

[0166] Signal channel: Rayleigh scattered light is collected by the objective lens, spatially filtered and collimated, and then sent to the signal input terminal of the OPA crystal;

[0167] Pump channel: The pump light power and polarization are adjusted using a half-wave plate and polarization beam splitter (PBS) to ensure optimal phase matching;

[0168] Spatiotemporal coincidence: The pump and signal are precisely aligned in the time domain within the crystal by using a variable delay line (step accuracy ≤ 50fs);

[0169] 4. Gain, bandwidth, and noise control

[0170] By fine-tuning the QPM slope through adjusting the crystal temperature (±0.1°C control), high gain coverage of different signal wavelengths (620–680nm range) can be achieved.

[0171] The pump intensity is kept below the threshold that does not induce photorefractive, while an optical isolator is used to prevent idler feedback;

[0172] 5. Signal Extraction and Output

[0173] The amplified signal passes through a high-pass filter (HR at the pump wavelength) and a band-pass filter (only allowing the signal band), effectively suppressing pump and idler remnants;

[0174] The final signal is received by a high-bandwidth (≥100GHz) photodiode or ultrafast photodetector, converted into an electrical signal, and sent to the subsequent processing stage.

[0175] II. Principles and Implementation Steps of Heterodyne Detection Technology

[0176] 1. Basic Principles and Mathematical Description

[0177] Signal superposition with local oscillator: superimposing the scattered signal to be measured With coherent local oscillator The two beams are synchronized in an optical mixer (50:50 beam splitter) to cause interference between the two light fields.

[0178] Photoelectric signal: detector output current

[0179]

[0180] Among them, the cross term uses the difference frequency This format allows for high-precision sampling in the radio frequency range (MHz–GHz); The current output by the detector at time t reflects the change in the intensity of the optical signal received by the detector over time. The square of the amplitude of the superimposed signal light field and local oscillator light field. According to optical principles, light intensity is proportional to the square of the light field amplitude. Here, it represents the total light intensity after superposition. The square of the amplitude of the signal light field corresponds to the light intensity of the signal light; The square of the amplitude of the local oscillator light field corresponds to the light intensity of the local oscillator light. : The angular frequency of the signal light; The angular frequency of a local oscillator light is related to the frequency f by ω = 2πf, which describes the time periodicity of light. The phase of the signal light reflects its phase state during propagation and affects the interference effect.

[0181] 2. Methods and Steps

[0182] Local oscillator light source preparation

[0183] The light from the main laser branch is extracted, filtered and attenuated in an adjustable manner, and then used as the local oscillator to ensure its frequency. Adjacent to the scattering band;

[0184] By finely adjusting the time delay of the local oscillator and the signal using an optical fiber delay line, the two can be phase-matched at the detection surface.

[0185] Optical Hybridization and Polarization Control

[0186] Using polarization control components (waveplate, PBS) ensures that the signal is polarized with the local oscillator, thereby improving interference efficiency;

[0187] An adjustable attenuator is added to the mixer output to balance the power of the two paths and obtain the best interference contrast.

[0188] RF filtering and amplification

[0189] The detector current output is passed through an RF bandpass filter to retain only the difference frequency signal near Ω, removing DC and high-frequency noise.

[0190] Amplification is performed using a radio frequency low-noise amplifier (LNA) to ensure that the difference frequency signal amplitude is suitable for the subsequent ADC (≥12bit, sampling rate ≥10GS / s).

[0191] Digitalization and Time / Frequency Domain Analysis

[0192] The high-speed ADC digitizes the radio frequency difference signal and sends it to the FPGA or data acquisition card.

[0193] The FPGA incorporates a digital lock-in amplifier algorithm to demodulate and obtain the envelope of the scattered signal. and phase Synchronous output;

[0194] The broadband scattering spectrum distribution is calculated in real time using the FFT module. And it is synchronously associated with the time-domain response.

[0195] 3. Technological advantages

[0196] High time resolution + wide bandwidth: The bandwidth of the difference frequency signal can be controlled by the local oscillator and the signal frequency difference, easily covering the DC–5GHz range, while retaining the original femtosecond-level time domain information;

[0197] Balancing gain and noise suppression: The local oscillator strength significantly improves the cross-term signal, with a gain of 20–30 dB compared to direct detection SNR;

[0198] Phase-sensitive detection: It can simultaneously acquire the amplitude and phase information of the scattered signal, providing complete data for subsequent molecular polarization dynamics modeling.

[0199] By combining the above two parts, the system can achieve signal pre-amplification of tens to hundreds of times at the OPA level, and can also achieve high-sensitivity and high-resolution joint time and frequency domain measurements in the radio frequency domain by using different frequency demodulation, fundamentally breaking through the contradiction between traditional detection bandwidth and sensitivity.

[0200] III. Optical Lock-in Amplifier and Synchronous Detection

[0201] 1. Principles and Mathematical Model

[0202] The electronic pulse signal (repetition rate) output from the laser oscillator cavity The branch, after being buffered and shaped, serves as the reference input for the lock-in amplifier.

[0203] Ensure that the phase of the reference signal is strictly locked to the moment when the light pulse arrives at the detector.

[0204] Signal modulation and demodulation

[0205] Rayleigh scattering electrical signals obtained from OPA or heterogeneous frequency detection The output from the photodetector is amplified and then multiplied by the reference signal.

[0206]

[0207] in, The modulated mixed signal is an intermediate signal in the lock-in amplification process; The Rayleigh scattering electrical signal obtained by OPA or heterogeneous frequency detection is the raw signal that needs to be modulated. The repetition frequency of the electronic pulse signal output by the laser oscillator cavity is the reference frequency for the lock-in amplification and determines the frequency of the modulation signal. The phase of the reference signal ensures that the modulated signal and the original signal have a specific phase relationship in order to achieve effective signal demodulation.

[0208] Ideally, the scattered signal contains... The same-frequency component (i.e., the linear Rayleigh scattering triggered by each pulse) is down-converted to DC or low-frequency components; all asynchronous noise (broadband white noise, Raman scattering and other non-same-frequency components) is shifted to the high-frequency region.

[0209] Mixing signal With bandwidth much lower After a low-pass filter (cutoff frequency of approximately a few kHz–a few MHz), the DC / low-frequency components are preserved. :

[0210]

[0211] in, Low-pass filter is an abbreviation for a low-pass filter, which means that a mixed input signal is filtered in a low-pass manner, allowing only signals below the cutoff frequency to pass through. The modulated mixed signal is the input signal of the low-pass filter.

[0212] It is proportional to the original Rayleigh scattering intensity, but effectively eliminates all noise components that are out of sync with the pulse.

[0213] 2. Implementation Steps

[0214] 2.1 Reference Path

[0215] TTL synchronization pulses are extracted from the laser main control electronic module and fed into the Ref input of the lock-in amplifier;

[0216] By fine-tuning the electronic delay (resolution ≤10ps), the arrival time of the reference pulse and the scattered signal at the detector is ensured to be aligned.

[0217] 2.2 Signal pathway

[0218] The Rayleigh scattering signal is then passed through an OPA / heterogeneous frequency detector and enters a high-bandwidth preamplifier (adjustable gain, noise figure <2dB).

[0219] The amplified analog signal is fed into the Signal input of the lock-in amplifier.

[0220] 2.3 Lock-in Amplifier Settings

[0221] Set internal reference frequency ;

[0222] Choose an appropriate time constant Balance response time with noise suppression, along with the filter slope (e.g., 24dB / oct).

[0223] 2.5 Output and Digitization

[0224] Lock-in amplifier output (DC or low-frequency voltage) is digitized by a 16-bit ADC (sampling rate 500kS / s) and then fed into the FPGA.

[0225] IV. Digital Filtering and Adaptive Noise Reduction (FPGA Real-Time)

[0226] 1. Digital Filter Design

[0227] 1.1 Multi-stage FIR filter

[0228] Design a set of cascaded finite impulse response (FIR) filters:

[0229] Low-pass filter: cutoff frequency Below the Raman dispersion frequency band and speckle noise band;

[0230] Band-stop filters: For known Raman dispersion radio frequency bands (offset frequencies of several THz, but mapped to the MHz level after mixing, corresponding band-stop filters can be designed);

[0231] Notch filter: Deeply suppresses laser howling harmonics and chassis power supply interference (50 / 60Hz and its harmonics).

[0232] The coefficients of each filter stage are calculated offline using the Kaiser window method and downloaded to the coefficient storage unit of the FPGA.

[0233] 1.2 Filter Cascade and Fixed-Point Implementation

[0234] FIR filtering is implemented on the FPGA using fixed-point arithmetic (16-bit), and the pipeline structure ensures that multiple levels of filtering operations can be completed in each sampling cycle.

[0235] By synchronizing the hardware clock domain (50MHz processing clock) with the signal sampling (5MHz), we ensure that real-time processing is not lost.

[0236] 2. Adaptive noise reduction algorithm

[0237] 2.1 Noise Estimation

[0238] Using a real-time sliding window (duration) For example, low-order statistics (minimum, median) within 100 μs are used to estimate the background noise baseline. ;

[0239] Simultaneously monitor high-frequency energy changes to identify sudden speckle noise pulses.

[0240] 2.2 Adaptive Weight Update

[0241] Based on the LMS (LeastMeanSquares) algorithm, for each sampling point With the estimated noise model Provide error feedback:

[0242]

[0243] in For step size parameters, For reference noise input (such as high-frequency channel residuals);

[0244] error It also serves as the output signal after noise reduction.

[0245] 3. Implementation process

[0246] 3.1 Sampling: Lock-in Amplifier Output Converted to digital stream via ADC;

[0247] 3.2 FIR pre-filtering: Removes known fixed-spectrum noise and relatively wideband interference;

[0248] 3.3 Noise Baseline Estimation: Statistical Window Update ;

[0249] 3.3LMS Adaptive Filtering: Real-time adjustment of the noise model and generation of the net signal ;

[0250] 3.4 Output buffer: Writes the net signal into the FIFO for the host computer or subsequent processing unit to read.

[0251] 4. Comparison of effects and advantages, see Table 1.

[0252] Table 1

[0253]

[0254] By employing the aforementioned two-stage high signal-to-noise ratio extraction scheme—optical lock-in amplification combined with FPGA real-time digital filtering and adaptive noise reduction—this system can effectively eliminate background, speckle noise, and inelastic Raman components, achieving high-fidelity capture of weak Rayleigh scattering signals under ultrashort pulse lasers.

[0255] 5. High-precision pumping – detection synchronization and delay control

[0256] 1. Optical Delay Line: Total Internal Reflection Mirror Platform + Picosecond-Level Piezoelectric Drive

[0257] Optical path difference With time delay Basic relationships:

[0258]

[0259] Wherein, Δt: time delay, which is the time difference caused by light propagating at different optical paths, is a physical quantity that needs to be precisely controlled in pump-probe synchronization control; ΔL: optical path difference, which refers to the difference in the optical path traversed by two beams of light during propagation, is the cause of time delay; c: speed of light in vacuum, approximately 3×10⁸ m / s, is a fundamental physical constant that determines the speed of light propagation in vacuum; n: refractive index of the working medium (air or optical fiber) in the delay line, reflects the medium's ability to refract light, and affects the speed of light propagation in the medium.

[0260] 2. Platform Composition and Driving Solution

[0261] 2.1 Total Internal Reflection Mirror Moving Platform

[0262] A superplane mirror is mounted on a low-inertia optical pusher, and the pusher is provided with linear displacement by a piezoelectric actuator (PZT).

[0263] By utilizing a dual-mirror symmetrical folding design, the unidirectional displacement is amplified by twice the equivalent optical path difference, thereby increasing the adjustment range.

[0264] 2.2 Piezoelectric Drive and Stepper Control

[0265] The PZT model uses a stacked piezoelectric actuator with closed-loop displacement feedback, and the bare die resolution is ≤10nm.

[0266] because ,when When, corresponding (n≈1 in air), which meets the stepping accuracy requirements;

[0267] The driver is controlled by a high-precision digital-to-analog converter (DAC), with each voltage step corresponding to an equivalent time step increment of ≤10fs.

[0268] 3. Stability monitoring: Fiber optic Brillouin sensing

[0269] 3.1 Sensing Principle: Utilizing the characteristic that the phonon frequency shift in the fiber optic Brillouin scattering effect changes with strain / temperature, the minute displacements and temperature drift of the PZT pusher and the mirror frame are monitored online.

[0270] 3.2 Feedback Loop:

[0271] A small number of fiber optic sensing segments were attached to the side wall of the pusher; the Brillouin sensor demodulator was used to measure the radio frequency shift. Converted into equivalent strain ; Controller according to Feedback adjustment of the PZT drive voltage suppresses optical path drift caused by temperature or mechanical jitter, ensuring time jitter. fs.

[0272] V. Electronic Synchronization: Phase-Locked Loop (PLL) + Ultra-Low Jitter Clock Source

[0273] 1. Synchronization requirements and jitter analysis

[0274] To ensure that the pump-probe pulses arrive at the sample "at the same time" and synchronously trigger the detector, the total system jitter... It must be controlled at the tens of fs level:

[0275]

[0276] in, Total system jitter reflects the instability of the entire pump-detector system in time synchronization, measured in femtoseconds (fs), and is a key indicator for evaluating system synchronization performance. The square of the laser source jitter is used in the total jitter calculation. Each component of the jitter needs to be squared first and then summed. The square of the clock source jitter; fs: the square of the phase-locked loop jitter; fs: the symbol for the unit of femtosecond.

[0277] 2. Phase-locked loop design

[0278] Fundamental frequency signal of laser cavity sampling cavity mirror electro-optic conversion The reference signal serves as the reference source for the PLL; after low-noise amplification and limiting, it enters the reference input terminal of the PLL; the VCO output is used to drive the ADC sampling clock and detector trigger signal, and is maintained at the same level through feedback. Synchronization; VCO noise floor (Phase Noise) should be lower than -140dBc / Hz@1kHz offset to ensure time-domain jitter <30fs. A second-order low-pass loop filter is selected, with a loop bandwidth of [missing information]. Adjusted to approximately (tens of kHz to hundreds of kHz), achieving a balance between accelerating locking speed and suppressing high-frequency jitter; optimizing phase margin and bandwidth through simulation and actual measurement to ensure stable locking even under ambient temperature changes of ±1°C.

[0279] 3. Ultra-low jitter clock source

[0280] Crystal selection: Use a temperature-controlled crystal oscillator (OCXO) or a temperature-controlled surface acoustic wave oscillator (TC-SAW) with a basic jitter ≤100fsrms; the OCXO output is distributed to multiple trigger and sampling clocks through a low-noise buffer amplifier; a low-pass filter is added during the distribution process to further remove high-frequency spurious noise and power switching noise.

[0281] 4. Use the cross-correlation method to verify pump-probe synchronization jitter and ensure... ;

[0282] The critical circuits were placed inside a metal shielding box, and the power supply adopted a low-noise linear regulator. The entire system was tested in a temperature control chamber to verify its stability against jitter and lock-up during long-term operation.

[0283] Through the aforementioned optical and electronic methods, this scheme achieves high-precision synchronization and delay control of pump-probe on a femtosecond timescale, providing a solid time-domain foundation for transient measurement of Rayleigh scattering under ultrashort pulse lasers.

[0284] VI. Nonlinear Effect Stripping and Correction

[0285] Channel A (Full Signal Channel): Directly collects all scattered light within the pump-probe overlapping region, including elastic Rayleigh scattering and all nonlinear signals (Kerr self-focusing, Raman scattering, plasma emission, etc.).

[0286] Channel B (Nonlinear Reference Channel): A thin-film interferometer filter is inserted into the optical path to pump the dominant wavelength. Offset After sampling (e.g., using a grating or tunable cavity), only the inelastic scattering components located in the Raman or Kerr shift regions are sampled.

[0287] Mathematical difference model: Let the time-domain voltage signal of channel A be...

[0288]

[0289] in For pure Rayleigh scattering components, This contributes to the total nonlinearity.

[0290] The output of channel B contains only nonlinear signals:

[0291]

[0292] In the gain factor The difference in response from the two-channel detector and the beam splitting ratio of the optical path can be measured through calibration.

[0293] Differential stripping:

[0294]

[0295] This operation allows for the acquisition of Rayleigh scattering signals free from nonlinear interference, either in real-time or during post-processing. .

[0296] In the absence of Rayleigh scattering samples (such as vacuum or pure nitrogen), only the outputs of nonlinear channel B and channel A are recorded to calculate... ;

[0297] Perform a rapid calibration sequence and dynamically update it before actual measurements. To counteract drift.

[0298] VII. Time-Domain-Frequency Domain Joint Nonlinear Correction Algorithm

[0299] 1. Kerr effect: Instantaneous change in the refractive index of a medium This causes phase modulation of the Rayleigh signal. The corresponding phase response function can be expressed as:

[0300]

[0301] in The pump pulse intensity envelope; The Dirac function, which has filtering properties, is used in convolution operations to represent the instantaneous response; here it represents the instantaneous phase modulation effect of the Kerr effect. The nonlinear refractive index coefficient of a medium reflects the degree of refractive index response of the medium to changes in the intensity of a light field and is an important parameter for describing the strength of the Kerr effect. The pump pulse intensity envelope at time t describes the variation of the pump pulse intensity over time, and its variation will cause a corresponding change in the phase modulation of the Kerr effect.

[0302] Raman scattering: Response function of medium vibrational modes It is usually approximated by multiple exponential decay:

[0303]

[0304] Among them, the amplitude of each mode With decay time constant It can be obtained through independent Raman spectroscopy experiments.

[0305] 2. Joint Convolution Model

[0306] Nonlinear phase and amplitude distortion can be represented as a convolution with a pure Rayleigh signal:

[0307]

[0308] Under weakly nonlinear conditions, it can be linearized and expanded as follows:

[0309]

[0310] in, The pure Rayleigh scattering component at time t is the main term in the formula; The cross term caused by the Kerr effect, where i is the imaginary unit, and the convolution of R(t) and hKerr(t) represents the time-domain extension of the phase modulation of the Rayleigh scattering signal by the Kerr effect. This cross term reflects the contribution of the Kerr effect to the total signal. The cross term caused by Raman scattering, the convolution of R(t) and hRaman(t), represents the time-domain extension of the amplitude modulation of the Rayleigh scattering signal by Raman scattering. This cross term reflects the contribution of Raman scattering to the total signal. Under weakly nonlinear conditions, higher-order small terms are ignored, and the total signal can be approximately represented as the sum of these three terms.

[0311] FFT transform: for the stripped signal and the original full signal Perform a Fast Fourier Transform (FFT) to obtain the frequency domain representation. .

[0312] Response function spectrum: obtained through experiments or simulations , .

[0313] 3. Deconvolution compensation:

[0314]

[0315] in, : The frequency domain representation of the Rayleigh scattering signal after frequency domain compensation is the result of correcting the frequency domain characteristics of the real Rayleigh scattering signal; ω: angular frequency variable, describing the distribution characteristics of the signal in the frequency domain; Frequency domain representation of the Kerr effect phase response function after FFT; The frequency domain representation of the Raman scattering response function after FFT reflects the influence of Raman scattering on the signal in the frequency domain. This formula allows for the removal of nonlinear effects such as the Kerr effect and Raman scattering in the frequency domain, yielding the corrected frequency domain representation of the Rayleigh scattering signal.

[0316] IFFT restoration: Performing an inverse FFT yields a time-domain Rayleigh signal with both amplitude and phase corrected. .

[0317] After the initial compensation, With stripping signal Compare and calculate residual error;

[0318] Update based on error and Parameters, iterate 2–3 times until convergence (error <1%);

[0319] By combining dual-channel differential stripping with time-frequency domain joint correction, this scheme can efficiently strip and compensate for various nonlinear effects caused by the high intensity of ultrashort pulses without sacrificing time resolution, and obtain high-fidelity pure Rayleigh scattering transient signals.

[0320] VIII. Comprehensive Theoretical Model and Numerical Simulation

[0321] 1. Establishment of the improved Maxwell–Bloch equation system

[0322] Scattering particles in a medium are considered as a two-level molecular system, and their polarization can be divided into linear and nonlinear components;

[0323] The pulse width of a femtosecond laser is much smaller than the relaxation time of the medium, so the immediate effect of vibrational dissipation on the polarization transient can be ignored.

[0324] Dispersion and nonlinearity are considered in spatial propagation, while transient molecular polarization and self-interaction are considered in time-domain response.

[0325] Maxwell's equations (simplified one-dimensional propulsion model)

[0326] )

[0327] in For electric field, For polarization density, is the linear refractive index; z: spatial position variable, describing the position of light in the direction of propagation in the medium; c is the speed of light in a vacuum; Vacuum permeability; Polarization intensity at spatial position z and time t describes the degree of polarization of a medium under the action of an electric field, and is the result of the interaction between the electric field and the medium.

[0328] Bloch equations (molecular polarization dynamics)

[0329]

[0330] in The difference in the number of particles between the two energy levels. , These are relaxation time and decoherence time, respectively. It is a dipole moment element; Reduced Planck constant; The number density of particles at a higher energy level (second energy level) at spatial position z and time t; The number density of particles at a low energy level (first energy level) at spatial position z and time t; The electric field intensity at spatial position z and time t is the external field source that drives the molecules to generate polarization.

[0331] Considering the Kerr effect induced by high-intensity pulses, a third-order polarization term is added to the Maxwell equations. ;

[0332] The final merged result is:

[0333]

[0334] Molecular polarization term It is given by the Bloch equation.

[0335] Derivation of Rayleigh scattering response

[0336] In the initial stage of pulse irradiation (weak field approximation), The Bloch equation can be linearized to obtain the transient polarization response function. ;

[0337] Scattering intensity After integration and phase matching, the time-domain response can be derived:

[0338]

[0339] in, Transient polarization response function, describing the polarization response of a molecule at time t after it is subjected to an electric field at time τ. Indicates time delay, reflecting the time lag characteristic of the response.

[0340] 2. Numerical simulation method

[0341] Divide the propagation distance into small steps. Each step is divided into two half-steps:

[0342] Nonlinear stage: The field strength is updated in the time domain, including the Kerr third-order response and Bloch polarization coupling;

[0343] Dispersion stage: The field spectrum is converted into the frequency domain using FFT and multiplied by the dispersion operator. Then IFFT back to the time domain.

[0344] Iterate until the desired propagation distance is reached, then output. and .

[0345] Finite Difference Time Domain Method (FDTD)

[0346] Gridded space and time ( Satisfying the Courant condition );

[0347] The Maxwell–Bloch equations are directly discretized to update the electric field and polarization.

[0348] 3. GPU parallel implementation

[0349] Use CUDA or OpenCL to build kernel functions, kernelizing Split-StepFFT, nonlinear driving, and FDTD updates; each thread processes one time-domain grid point or frequency-domain sampling point to maximize hardware throughput.

[0350] Time-domain and frequency-domain data are stored in global video memory respectively, and adjacent time-domain segments are cached using shared memory; streams and asynchronous copying are used to achieve overlap between data transmission and computation, thereby reducing latency.

[0351] On an NVIDIA H100 GPU, the round-trip propagation time for a 2mm optical path (4096×4096 grid) is simulated to be less than 100ms; compared to the CPU serial implementation, the speedup is >100x.

[0352] 4. Parameter scanning and model calibration

[0353] 4.1 Parameter Combination

[0354] Pulse width: 50fs, 100fs, 200fs;

[0355] Peak power: 10GW / cm², 50GW / cm², 100GW / cm²;

[0356] Particle diameters: 0.1µm, 0.5µm, 1µm.

[0357] 4.2 Simulation Output

[0358] Time-domain scattering response curve ;

[0359] Frequency domain scattering spectrum .

[0360] 4.3 Comparison with experimental data

[0361] In the optimization model using the least squares method Undetermined parameters;

[0362] Convergence is achieved within an error range of <5% after ≤10 iterations.

[0363] 5. Final Model Validation

[0364] Repeated verification under independent experimental conditions ensures the universality and prediction accuracy of the numerical simulation model.

[0365] Through the above comprehensive theory and numerical simulation, we can not only gain a deep understanding of the transient mechanism of Rayleigh scattering under ultrashort pulse lasers, but also provide reliable computational prediction tools for subsequent experimental design and parameter optimization.

[0366] In one embodiment, a pure Rayleigh scattering signal acquisition system based on a laser source is provided. The system includes a laser source, a nonlinear crystal, an acousto-optic frequency shifter or an optical phase-locked loop, a mixer, and a signal processor connected in sequence. The laser source outputs femtosecond pulses. The nonlinear crystal receives a first femtosecond pulse after beam splitting and performs frequency mixing on the first femtosecond pulse to obtain a preprocessed scattering signal. The acousto-optic frequency shifter receives a second femtosecond pulse after beam splitting and performs frequency shifting on the second femtosecond pulse to obtain a coherent local oscillator signal. Alternatively, the optical phase-locked loop receives the second femtosecond pulse after beam splitting and performs phase-locking on the second femtosecond pulse to obtain a coherent local oscillator signal. The mixer performs interference processing on the preprocessed scattering signal and the coherent local oscillator signal to obtain a mixed difference frequency signal. The signal processor performs multi-stage signal processing on the mixed difference frequency signal to obtain the target Rayleigh scattering signal.

[0367] In one embodiment, the signal processor includes a high-bandwidth photodetector, an optical lock-in amplifier, an analog-to-digital converter, a low-pass filter, a band-pass filter, a notch filter, and a dual-channel differential model connected in sequence. The high-bandwidth photodetector is used to perform photoelectric conversion processing on the mixed difference frequency signal to obtain an analog electrical signal. The optical lock-in amplifier is used to perform synchronous detection processing on the analog electrical signal based on the frequency of a femtosecond pulse to obtain a synchronously detected analog electrical signal. The analog-to-digital converter is used to perform analog-to-digital conversion processing on the synchronously detected analog electrical signal to obtain a digital difference frequency signal. The digital difference frequency signal is sequentially subjected to fast Fourier transform processing and peak detection processing to obtain a processed digital difference frequency signal. The time-domain envelope, phase information, and frequency domain characteristics of the processed digital difference frequency signal are extracted. The process involves several steps: first, identifying signal characteristic parameters; second, encoding these parameters to obtain a digital stream signal; third, using a low-pass filter to remove high-frequency random noise from the digital stream signal to obtain an initial signal; fourth, using a band-pass filter to retain the Rayleigh scattering frequency band signal in the initial signal to obtain an intermediate signal; fifth, using a notch filter to suppress periodic interference in the intermediate signal to obtain a preliminary signal; sixth, using a dual-channel differential model to perform nonlinear stripping on the preliminary signal to obtain a denoised target signal; seventh, performing a Fast Fourier Transform (FFT) on the denoised target signal to obtain its frequency domain signal; eighth, using a deconvolution compensation algorithm to correct the frequency domain signal of the denoised target signal to obtain a corrected frequency domain signal; and finally, performing an Inverse Fast Fourier Transform (IFFT) on the corrected frequency domain signal to obtain the target Rayleigh scattering signal.

[0368] It should be understood that although the steps in the flowcharts of the embodiments described above are shown sequentially according to the arrows, these steps are not necessarily executed in the order indicated by the arrows. Unless explicitly stated herein, there is no strict order restriction on the execution of these steps, and they can be executed in other orders. Moreover, at least some steps in the flowcharts of the embodiments described above may include multiple steps or multiple stages. These steps or stages are not necessarily completed at the same time, but can be executed at different times. The execution order of these steps or stages is not necessarily sequential, but can be performed alternately or in turn with other steps or at least some of the steps or stages of other steps.

[0369] Those skilled in the art will understand that all or part of the processes in the above embodiments can be implemented by a computer program instructing related hardware. The computer program can be stored in a non-volatile computer-readable storage medium. When executed, the computer program can include the processes of the embodiments described above. Any references to memory, databases, or other media used in the embodiments provided in this application can include at least one of non-volatile and volatile memory. Non-volatile memory can include read-only memory (ROM), magnetic tape, floppy disk, flash memory, optical memory, high-density embedded non-volatile memory, resistive random access memory (ReRAM), magnetic random access memory (MRAM), ferroelectric random access memory (FRAM), phase change memory (PCM), graphene memory, etc. Volatile memory can include random access memory (RAM) or external cache memory, etc. By way of illustration and not limitation, RAM can take many forms, such as Static Random Access Memory (SRAM) or Dynamic Random Access Memory (DRAM). The databases involved in the embodiments provided in this application may include at least one type of relational database and non-relational database. Non-relational databases may include, but are not limited to, blockchain-based distributed databases. The processors involved in the embodiments provided in this application may be general-purpose processors, central processing units, graphics processing units, digital signal processors, programmable logic devices, quantum computing-based data processing logic devices, etc., and are not limited to these.

[0370] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0371] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of this patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

Claims

1. A method for acquiring pure Rayleigh scattering signals based on a laser source, characterized in that, The method includes: The femtosecond pulses output from the laser source are split into at least two femtosecond pulses. The first femtosecond pulse is input into a nonlinear crystal for frequency mixing to obtain a preprocessed scattering signal; and the second femtosecond pulse is frequency-shifted using an acousto-optic frequency shifter or phase-locked using an optical phase-locked loop to obtain a coherent local oscillator signal. The preprocessed scattering signal and the coherent local oscillator signal are input into a mixer for interference processing to obtain a mixed difference frequency signal; The mixed difference frequency signal is processed in multiple stages using a signal processor to obtain the target Rayleigh scattering signal.

2. The method according to claim 1, characterized in that, The step of using a signal processor to perform multi-level signal processing on the mixed difference frequency signal to obtain the target Rayleigh scattering signal includes: The signal processor is used to sequentially perform photoelectric conversion and optical lock-in amplification on the mixed difference frequency signal to obtain a digital difference frequency signal. The digital difference frequency signal is extracted and processed to obtain a digital stream signal; The digital stream signal is sequentially filtered and nonlinear stripped to obtain the target denoised signal; The target denoised signal is subjected to time-domain and frequency-domain correction processing to obtain the target Rayleigh scattering signal.

3. The method according to claim 2, characterized in that, The signal processor includes a high-bandwidth photodetector, an optical lock-in amplifier, and an analog-to-digital converter. The process of sequentially performing photoelectric conversion and optical lock-in amplification on the mixed difference frequency signal to obtain a digital difference frequency signal includes: The hybrid difference frequency signal is input into the wide bandwidth photodetector for photoelectric conversion processing to obtain an analog electrical signal; The analog electrical signal is input to the optical lock-in amplifier, and the optical lock-in amplifier performs synchronous detection processing on the analog electrical signal based on the frequency of the femtosecond pulse to obtain the synchronously detected analog electrical signal. The analog electrical signal after synchronous detection is input into the analog-to-digital converter for analog-to-digital conversion to obtain the digital difference frequency signal.

4. The method according to claim 2, characterized in that, The step of extracting and processing the digital difference frequency signal to obtain a digital stream signal includes: The digital difference frequency signal is sequentially subjected to Fast Fourier Transform and peak detection processing to obtain the processed digital difference frequency signal; The time-domain envelope, phase information, and frequency-domain features of the processed digital difference frequency signal are extracted to obtain signal feature parameters; The signal characteristic parameters are encoded to obtain the digital stream signal.

5. The method according to claim 2, characterized in that, The signal processor further includes a low-pass filter, a band-pass filter, a notch filter, and a dual-channel differential model. The process of sequentially filtering and nonlinear stripping the digital stream signal to obtain the target denoised signal includes: The digital stream signal is input to the low-pass filter, and the high-frequency random noise in the digital stream signal is filtered out by the low-pass filter to obtain the initial signal; The initial signal is input to the bandpass filter, and the bandpass filter retains the Rayleigh scattering radio frequency band signal in the initial signal to obtain the intermediate signal; The intermediate signal is input to the notch filter, and the periodic interference in the intermediate signal is suppressed by the notch filter to obtain the preliminary signal; The preliminary signal is input into the dual-channel differential model, and the preliminary signal is subjected to nonlinear stripping processing by the dual-channel differential model to obtain the target denoised signal.

6. The method according to claim 2, characterized in that, The step of performing time-domain and frequency-domain correction processing on the denoised target signal to obtain the target Rayleigh scattering signal includes: Perform a Fast Fourier Transform on the target denoised signal to obtain the frequency domain signal of the target denoised signal; The frequency domain signal of the target denoised signal is corrected by using a deconvolution compensation algorithm to obtain the corrected frequency domain signal; The target Rayleigh scattering signal is obtained by performing an inverse fast Fourier transform on the corrected frequency domain signal.

7. The method according to any one of claims 1-6, characterized in that, The step of inputting the preprocessed scattered signal and the coherent local oscillator signal into a mixer for interference processing to obtain a mixed difference frequency signal includes: The coherent local oscillator signal is subjected to optical path slow drift compensation processing to obtain a coherent local oscillator signal with long-term optical path drift compensation. The coherent local oscillator signal with long-term optical path drift compensation is subjected to phase high-frequency jitter compensation processing to obtain a coherent local oscillator signal with phase high-frequency jitter compensation. The coherent local oscillator signal with phase high-frequency jitter compensation and the preprocessed scattered signal are input into a mixer. The mixer performs interference processing on the coherent local oscillator signal with phase high-frequency jitter compensation and the preprocessed scattered signal to obtain the mixed difference frequency signal.

8. The method according to any one of claims 1-6, characterized in that, The method further includes: The femtosecond pulse is input into the signal output model to obtain the simulated Rayleigh scattering signal output by the signal output model; The simulated Rayleigh scattering signal is corrected based on the target Rayleigh scattering signal to obtain correction parameters; The signal output model is corrected according to the correction parameters to obtain the corrected signal output model; the corrected signal output model is used to generate the corrected simulated Rayleigh scattering signal based on the femtosecond pulse.

9. A pure Rayleigh scattering signal acquisition system based on a laser source, characterized in that, The system includes a laser source, a nonlinear crystal, an acousto-optic frequency shifter or an optical phase-locked loop, a mixer, and a signal processor connected in sequence. The laser source is used to output femtosecond pulses; A nonlinear crystal is used to receive the first femtosecond pulse after beam splitting and to perform frequency mixing on the first femtosecond pulse to obtain a preprocessed scattered signal. An acousto-optic frequency shifter is used to receive the second femtosecond pulse after beam splitting and to perform frequency shifting on the second femtosecond pulse to obtain a coherent local oscillator signal; or the optical phase-locked loop is used to receive the second femtosecond pulse after beam splitting and to perform phase-locking on the second femtosecond pulse to obtain a coherent local oscillator signal. A mixer is used to perform interference processing on the preprocessed scattered signal and the coherent local oscillator signal to obtain a mixed difference frequency signal; A signal processor is used to perform multi-level signal processing on the mixed difference frequency signal to obtain the target Rayleigh scattering signal.

10. The system according to claim 9, characterized in that, The signal processor includes a high-bandwidth photodetector, an optical lock-in amplifier, an analog-to-digital converter, a low-pass filter, a band-pass filter, a notch filter, and a dual-channel differential model connected in sequence. The high-bandwidth photodetector is used to perform photoelectric conversion processing on the mixed difference frequency signal to obtain an analog electrical signal; The optical lock-in amplifier is used to perform synchronous detection processing on the analog electrical signal based on the frequency of the femtosecond pulse to obtain the synchronously detected analog electrical signal. The analog-to-digital converter is used to perform analog-to-digital conversion processing on the analog electrical signal after synchronous detection to obtain a digital difference frequency signal; the digital difference frequency signal is then subjected to fast Fourier transform processing and peak detection processing to obtain a processed digital difference frequency signal; the time domain envelope, phase information, and frequency domain features of the processed digital difference frequency signal are extracted to obtain signal feature parameters. The signal feature parameters are encoded to obtain the digital stream signal; The low-pass filter is used to filter out high-frequency random noise in the digital stream signal to obtain the initial signal; The bandpass filter is used to retain the Rayleigh scattering radio frequency band signal in the initial signal to obtain the intermediate signal; The notch filter is used to suppress periodic interference in the intermediate signal to obtain a preliminary signal; The dual-channel differential model is used to perform nonlinear stripping processing on the preliminary signal to obtain the target denoised signal; and to perform a fast Fourier transform on the target denoised signal to obtain the frequency domain signal of the target denoised signal; to perform a correction processing on the frequency domain signal of the target denoised signal using a deconvolution compensation algorithm to obtain a corrected frequency domain signal; and to perform an inverse fast Fourier transform on the corrected frequency domain signal to obtain the target Rayleigh scattering signal.

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