Method and device for compensating doppler effect in optical time-stretch swept-frequency interferometry

Abstract

The application relates to a method and device for compensating Doppler effect in optical time-stretch swept-frequency interferometry, and relates to the technical field of absolute distance measurement of non-cooperative targets. In order to solve the defects in the prior art that the Doppler effect cannot be effectively suppressed in a complex environment, resulting in the decline of the absolute distance measurement accuracy, the technical scheme provided by the application comprises the following steps: generating a swept-frequency laser signal and branching the swept-frequency laser signal to form a measurement light, a reference light and a resampling light; performing time stretching processing on the reference light to obtain a chirp signal; obtaining a measurement interference signal by interference between the measurement light and the chirp signal; obtaining a reference interference signal by interference between the reference light and the resampling light; performing cross-correlation processing on the measurement interference signal and the reference interference signal to extract an optical path difference; combining the resampling data to perform distance calculation and output the target distance after compensating the Doppler effect. The application is suitable for optical measurement of high-precision absolute distance measurement of non-cooperative targets in a slightly vibrating or disturbed environment.

Description

Technical Field

[0001] It involves the fields of absolute distance measurement of non-cooperative targets, fiber optic sensing, OFDR, swept frequency interferometry, OCT, and lidar three-dimensional measurement technology. Background Technology

[0002] Traditional FMCW ranging methods can only be applied to vibration-isolated environments in laboratories, such as air-bearing vibration isolation platforms. Once vibration is present in the environment, even if the amplitude is on the order of micrometers, the error introduced by the Doppler effect can reach hundreds of micrometers, severely limiting the application of this method. This means that a single FMCW ranging method cannot adapt to most measurement environments. Currently, common methods in FMCW laser ranging schemes to reduce the impact of the Doppler effect introduced by target vibration include sinusoidal / triangular wave frequency modulation, dual-source reverse frequency modulation, and common-path laser Doppler vibration measurement-compensation. The advantages and disadvantages of each method are analyzed below.

[0003] a) Triangle wave frequency modulation Triangular wave frequency modulation (TWFM) is the earliest known method for Doppler effect correction. It utilizes the characteristic that the Doppler frequency shift is equal in magnitude and opposite in direction during forward and reverse frequency modulation of a frequency-modulated light source to reduce the Doppler frequency shift and obtain accurate target distance information. This technique requires no additional hardware to achieve Doppler effect correction and has significant application value in low-cost lidar. However, this method is only suitable for slow and uniform target motion, making it unsuitable for complex vibration environments like industrial environments. Many scholars and teams in China are conducting extensive research on measuring vibrating targets. Zhang Fumin of Tianjin University and Bo Liu of the University of Chinese Academy of Sciences have also used triangular wave modulation for Doppler compensation. In 2020, Tsinghua University proposed a photonics-assisted method that utilizes dual-band symmetrical triangular modulation to achieve unambiguous simultaneous measurement of distance and velocity in multi-target scenarios.

[0004] b) Dual-sweep frequency modulation In 2001, German scientists Richard Schneider et al. first proposed a scheme based on two frequency-modulated lasers to overcome the influence of the Doppler effect, as shown in the schematic diagram. In this scheme, both lasers are frequency-modulated semiconductor lasers; when the frequency of one laser increases, the frequency of the other decreases. The device uses an auxiliary interferometer to correct the frequency modulation nonlinearity of the two lasers by phase comparison. However, the paper only provides simulation analysis and does not provide actual measurement results. In 2022, Liu Guodong's team at Harbin Institute of Technology proposed a Doppler correction algorithm based on the FM kernel function in a dual-wavelength structure and realized dynamic distance measurement. A measurement accuracy of 1.2 μm was achieved within a 10 m ranging range. This method can extract real-time vibration information of the target with a low signal-to-noise ratio, with a measurement error of less than 100 nm and an optimal vibration measurement resolution of approximately 130 pm. In 2006, Anthony Slotwinski et al. from Nikon proposed a similar dual-semiconductor laser absolute distance measurement structure. In this structure, the two semiconductor lasers are corrected for frequency modulation nonlinearity using phase-locked loops. The frequency modulation directions of the two lasers are opposite, and the frequency modulation rates differ to some extent. When two lasers are combined and used in a common-path configuration, a bandpass filter is needed to separate the two interference signals with different frequencies due to their different modulation rates. The Doppler effect has the same effect on the interference signals corresponding to the two frequency-modulated lasers, so they can cancel each other out, thus obtaining the absolute distance information of the target. In 2014, Armin Reichold of Oxford University built a dynamic absolute distance measurement system using two external cavity lasers. A gas absorption chamber was also introduced as the length traceability reference for the ranging system, ultimately achieving a measurement accuracy of 40 nm for cooperative targets.

[0005] c) Common-path laser Doppler vibration measurement and compensation In 2016, Lu Cheng from Harbin Institute of Technology proposed a Doppler effect correction technique based on dual-frequency heterodyne interferometry. This method, by adding a common-path Doppler vibration measurement unit to the basic FMCW ranging optical path, measures and compensates the target vibration to the ranging signal, achieving accurate measurement of non-cooperative vibrating targets. However, limited by existing digital signal processing methods, multipath interference exists in the optical path in industrial environments or when the target surface is tilted or too rough, resulting in low measurement accuracy, and in some cases, even the inability to extract distance information. In 2022, the research group used this system to propose a compensation method based on analytical signal multiplication. Experiments verified the effectiveness of this method for compensating for the vibration of non-cooperative targets. For a non-cooperative target at 15m, if the analytical signal multiplication method is used for calculation, the measurement standard deviation can reach 19.65μm.

[0006] In summary, most current Doppler effect compensation methods adopt a bidirectional (synchronous or asynchronous) frequency sweep optical path structure, and then mix the interference signals to cancel most of the Doppler effect with common-mode properties, thereby suppressing the Doppler effect. However, this method can hardly make the initial optical frequencies of the two beams of light that are swept up and down different, and it is difficult to completely eliminate the Doppler effect.

[0007] Existing technologies have the drawback of being unable to effectively suppress the Doppler effect in complex environments where non-cooperative targets exhibit minute vibrations, leading to a decrease in the accuracy of absolute distance measurements. Summary of the Invention

[0008] To address the shortcomings of existing technologies, such as the inability to effectively suppress the Doppler effect in complex environments with minor vibrations in non-cooperative targets, leading to a decrease in absolute distance measurement accuracy, the present invention provides the following technical solution: Optical time-stretched sweep interferometry Doppler effect compensation methods include: The steps involve generating a swept-frequency laser signal and splitting it into measurement light, reference light, and resampling light. The step of time-stretching the reference light to obtain the chirped signal; The step of interfering the measurement light with the chirped signal to obtain a measurement interference signal; The step of interfering the reference light with the resampled light to obtain the reference interference signal; The steps include cross-correlation processing of the measured interference signal and the reference interference signal to extract the optical path difference; The steps involve calculating the target distance using resampled data and outputting the target distance after compensating for the Doppler effect.

[0009] Furthermore, a preferred embodiment is provided in which the time stretching process includes guiding the reference light into a dispersive fiber or a chirped grating structure, so that the output signal is stretched in time and exhibits linear chirped characteristics.

[0010] Furthermore, a preferred embodiment is provided in which the measurement interference signal is obtained by interfering the target reflected echo with time-stretched light in a measurement interferometer.

[0011] Furthermore, a preferred embodiment is provided in which the reference interference signal is acquired by an auxiliary interferometer and used to detect the nonlinearity of the frequency modulation of the swept laser and the frequency drift between the upper and lower sweeps.

[0012] Furthermore, a preferred embodiment is provided, wherein the cross-correlation processing includes: frequency domain transformation, spectral shift correction, conjugate product operation, and cross-correlation peak extraction of the measured interference signal and the reference interference signal.

[0013] Based on the same inventive concept, the present invention also provides an optical time-stretched sweep frequency interferometry Doppler effect compensation device, comprising: A swept-frequency laser signal is generated and split into modules for measurement light, reference light, and resampling light; A module that performs time-stretching on a reference light to obtain a chirped signal; A module that interferes the measurement light with the chirped signal to obtain a measurement interference signal; The module that obtains the reference interference signal by interfering the reference light with the resampled light; A module that performs cross-correlation processing on the measured interference signal and the reference interference signal to extract the optical path difference; This module calculates the distance using resampled data and outputs the target distance after compensating for the Doppler effect.

[0014] Based on the same inventive concept, the present invention also provides a time-stretched frequency sweeping interferometric ranging system, the system being used to implement the method, comprising: A swept-frequency laser is used to output a frequency-modulated laser signal. A beam splitter is used to split the laser signal into a measurement beam, a reference beam, and a resampling beam. The time stretching module, located on the reference optical path, is used to generate chirped stretching optical signals; A measurement interferometer is used to interfere with the measurement light and the stretching light signal, and output a measurement interference signal. An auxiliary interferometer is used to interfere with the reference light and the stretched light signal, and outputs an auxiliary interference signal. A resampling interferometer is used to obtain swept-frequency nonlinear characteristics; The data processing module is used to process the various interference signals, compensate for the Doppler effect, and output the target distance.

[0015] Based on the same inventive concept, the present invention also provides a computer storage medium for storing a computer program, wherein when the computer program is read by a computer, the computer executes the method described thereon.

[0016] Based on the same inventive concept, the present invention also provides a computer, including a processor and a storage medium, wherein when the processor reads a computer program stored in the storage medium, the computer executes the method described thereon.

[0017] Based on the same inventive concept, the present invention also provides a computer program product, which, when executed, implements the method described.

[0018] Compared with the prior art, the advantages of the technical solution provided by the present invention are as follows: This scheme constructs a bidirectional sweep frequency interferometry optical path structure based on optical time stretching. A time stretching module is introduced during the measurement process to chirp-modulate the interference signal, amplifying it along the time axis. This utilizes the autocorrelation characteristics of the broadband chirped signal to naturally suppress the Doppler effect. Compared to traditional bidirectional frequency modulation structures that require extremely high symmetry between the upper and lower sweep frequencies, the time-stretched interference signal effectively averages and cancels out the high-frequency Doppler terms during autocorrelation processing. This ensures that the measured peak position remains stable even when the target undergoes simple harmonic vibrations, significantly improving the system's resistance to Doppler interference.

[0019] This scheme employs an asynchronous or synchronous bidirectional frequency sweep structure, with corresponding optical path systems and resampling interferometers designed to correct the nonlinearity of the laser frequency sweep and improve the accuracy of distance extraction. In the asynchronous structure, a triangular wave laser source combined with a time-stretching unit is used to perform interferometric measurements on the target. The signal is then corrected using an auxiliary interferometer and a resampling interferometer, achieving the ability to extract stable interferometric signals even under multipath interference conditions. Compared to traditional triangular wave modulation methods, which are only applicable to slow-moving targets, this scheme can effectively handle absolute distance measurements in low-frequency vibration scenarios.

[0020] This scheme proposes a modified adaptive frequency-shifting cross-correlation algorithm, which accurately estimates the optical path difference through steps such as mixing, spectrum clipping, DFT frequency shifting, and peak extraction. The algorithm introduces a spectrum shift point estimation mechanism, which can accurately extract the peak position of the cross-correlation spectrum even in the presence of laser sweep nonlinearity and system jitter, improving the measurement stability of the system in the presence of high-frequency noise or structural disturbances. Compared with traditional methods that rely on direct Fourier transform demodulation and are susceptible to Doppler modulation, this algorithm effectively reduces ranging errors in dynamic environments by guiding the cross-correlation to approximate autocorrelation.

[0021] This scheme overcomes the reference frequency drift problem caused by the inconsistency of the initial frequencies of the upper and lower sweeping light sources in synchronous frequency modulation structures by introducing a co-source auxiliary interferometer to achieve real-time monitoring and correction of the reference frequency drift. This design allows the system to maintain ranging accuracy and consistency even when there are fluctuations in the frequency source or device instability. Compared to the symmetrical dual-laser system which requires strict phase-locked loop design, this scheme achieves higher anti-interference capability and system robustness with lower structural complexity.

[0022] The theoretical part derives the Doppler immune characteristics of broadband chirped signals in autocorrelation processing, indicating that when the target vibration frequency is much lower than the sweep frequency repetition frequency, the velocity changes caused by the target vibration will not affect the position of the autocorrelation peak. This theory supports the construction of the data processing foundation for the entire scheme, clearly demonstrating that even in the presence of minor structural vibrations (such as airflow disturbances, equipment resonance, etc.) on non-cooperative targets, sub-micron-level ranging accuracy can still be maintained, a feat that existing traditional FMCW schemes cannot reliably achieve.

[0023] It is suitable for optical measurement work that performs high-precision absolute distance measurement of non-cooperative targets in the presence of minor vibrations or disturbances. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the optical path for suppressing the Doppler effect based on time-stretched asynchronous bidirectional frequency sweep. Figure 2 This is a schematic diagram of the optical path for suppressing the Doppler effect based on time-stretched synchronous bidirectional frequency sweep. Figure 3 This is a schematic diagram of the time stretching module; Figure 4 This is a data processing workflow for suppressing the Doppler effect based on time-stretched bidirectional frequency sweep. Figure 5 The flowchart shows the modified adaptive frequency shift cross-correlation algorithm. Detailed Implementation

[0025] To make the advantages and benefits of the technical solution provided by the present invention clearer, the technical solution provided by the present invention will now be described in further detail with reference to the accompanying drawings, specifically: Implementation Method 1: This implementation method provides a Doppler effect compensation method for optical time-stretched sweep frequency interferometry, including: The steps involve generating a swept-frequency laser signal and splitting it into measurement light, reference light, and resampling light. The step of time-stretching the reference light to obtain the chirped signal; The step of interfering the measurement light with the chirped signal to obtain a measurement interference signal; The step of interfering the reference light with the resampled light to obtain the reference interference signal; The steps include cross-correlation processing of the measured interference signal and the reference interference signal to extract the optical path difference; The steps involve calculating the target distance using resampled data and outputting the target distance after compensating for the Doppler effect.

[0026] Time stretching involves guiding reference light into a dispersive fiber or a chirped grating structure to stretch the output signal in time and give it linear chirped characteristics.

[0027] The measurement interference signal is obtained by the interference of the target reflected echo and the time-stretched light in the measurement interferometer.

[0028] The reference interference signal is acquired through an auxiliary interferometer and is used to detect the nonlinearity of the frequency modulation of the swept laser and the frequency drift between the upper and lower sweeps.

[0029] Cross-correlation processing includes: frequency domain transformation, spectral shift correction, conjugate product operation, and cross-correlation peak extraction of the measured interference signal and the reference interference signal.

[0030] A time-stretched frequency-sweeping interferometric ranging system is also provided, which is used to implement the method, including: A swept-frequency laser is used to output a frequency-modulated laser signal. A beam splitter is used to split the laser signal into a measurement beam, a reference beam, and a resampling beam. The time stretching module, located on the reference optical path, is used to generate chirped stretching optical signals; A measurement interferometer is used to interfere with the measurement light and the stretching light signal, and output a measurement interference signal. An auxiliary interferometer is used to interfere with the reference light and the stretched light signal, and outputs an auxiliary interference signal. A resampling interferometer is used to obtain swept-frequency nonlinear characteristics; The data processing module is used to process the various interference signals, compensate for the Doppler effect, and output the target distance.

[0031] Implementation Method Two: This implementation method further defines the technical solution provided in Implementation Method One, including: A Doppler effect compensation method and apparatus based on optical time-stretching sweep frequency interferometry is proposed, suitable for high-precision absolute distance measurement of non-cooperative targets in complex vibration environments. This scheme, based on optical time stretching, a two-way sweep frequency structure, an auxiliary interferometric compensation mechanism, and a modified cross-correlation algorithm, systematically suppresses the influence of the Doppler effect on the measurement results, significantly improving ranging accuracy and stability.

[0032] The specific implementation process includes the following steps: Step 1: Constructing a time-stretched bidirectional sweeping interferometric optical path system An optical path system was constructed, including a measurement interferometer, an auxiliary interferometer, and a resampling interferometer, and a swept laser source, a time stretching module, and a detector were configured.

[0033] Detailed description: A swept-frequency laser can be configured as the light source, and a triangular wave modulated single-source system or a dual-source system composed of two synchronously swept-frequency lasers can be selected. The light source output is split into multiple optical paths sequentially by a beam splitter; The first part of the light enters the measurement interferometer for target detection. The specific path includes an optical circulator, a lens, a target reflection path, a return optical circulator, a beam combiner, and a dual-balanced detector. After being processed by the time-stretching module, part of the second portion of the light enters the measuring interferometer to interfere with the target reflected light, and the other portion enters the auxiliary interferometer. The third part enters the reference arm via the delay line and interferes with the time-stretched beam in the auxiliary interferometer. The fourth part is used for the resampling interferometer to obtain the nonlinear changes during the frequency sweep process, which are then used for subsequent nonlinear correction. All interference signals are acquired in parallel by high-speed sampling equipment and used as input for subsequent data processing.

[0034] Step 2: Acquire and separate the bidirectional swept frequency interference signal Multiple interference signals obtained during the up and down scanning processes are collected and distinguished, providing a basis for subsequent independent processing.

[0035] Detailed description: For the asynchronous bidirectional frequency sweep structure, record one complete up-sweep and down-sweep process, and acquire the corresponding measurement interference signal and reference interference signal; For synchronous bidirectional structures, the signals output by the two lasers are collected separately and distinguished using a wavelength division multiplexer; The acquired interference signal is divided according to the frequency sweep direction to obtain the up-sweep measurement signal, the down-sweep measurement signal, the up-sweep reference signal, and the down-sweep reference signal; The output of this step is a digital data sequence of four interference signals, which is then input to the subsequent mixing processing module.

[0036] Step 3: Perform mixing and spectrum calculation The measured signal and the reference signal are mixed and filtered to extract the interference envelope and calculate the spectral characteristics.

[0037] Detailed description: The measured interference signal and the reference interference signal are multiplied by a dot to obtain the mixed signal; A digital low-pass filter is applied to the mixed signal to preserve the interference envelope signal within the useful frequency range; Fast Fourier Transform (FFT) is performed on both the measured interference signal and the reference interference signal to extract frequency domain features; The spectral data output in this step prepares for cross-correlation frequency shift estimation and spectrum shifting.

[0038] Step 4: Estimate the time delay difference and perform frequency shift correction. By analyzing the spectrum of the reference signal, the time delay difference is estimated and the spectrum is frequency-shifted to correct the effect of the Doppler effect.

[0039] Detailed description: Find the peak position of the narrowband pulse in the spectrum of the reference interference signal, which corresponds to the time delay between the interference signals; Convert the delay time into the corresponding number of frequency shift points in the frequency domain; The spectrum is trimmed by removing the high-frequency portion at the beginning and zero-padding at the end, while keeping the spectrum length unchanged. If it is a bidirectional sweep frequency structure, the upper and lower sweep frequencies are corrected separately to ensure their cross-correlation. The output is the corrected spectral data, which is used to calculate the cross-correlation spectrum.

[0040] Step 5: Calculate the cross-correlation spectrum and extract the peak positions. The cross-correlation spectrum is calculated using the modified spectrum, and the corresponding time delay peak is extracted from it.

[0041] Detailed description: Take the complex conjugate of the corrected reference spectrum and multiply it by the measured spectrum; The product result is clipped, retaining only the non-zero part of the spectrum; Perform a short-time Fourier transform (DFT) on the cut spectrum to obtain the cross-correlation spectrum; Peak-finding processing is performed on the cross-correlation spectrum, and the peak positions are recorded; The peak position corresponds to the optical path difference between the measuring arm and the reference arm in the measurement optical path.

[0042] Step Six: Calculate the absolute distance to the target by combining the coarse and fine measurement results. By combining the cross-correlation calculation results and the coarse measurement information obtained by the resampling interferometer, the absolute distance to the target is finally calculated.

[0043] Detailed description: The coarse measurement results provided by the resampling interferometer give an approximate ranging range. The position of the cross-correlation peak provides a precise relative delay; The coarse and fine measurement results are combined and converted into an absolute distance output; The output here is the target distance, with submicron level accuracy.

[0044] Step 7: Eliminate errors caused by inconsistencies in the initial frequencies of the up and down sweep frequencies. The initial frequency of the up and down sweep is monitored by an auxiliary interferometer to compensate for the ranging error caused by the reference drift.

[0045] Detailed description: The auxiliary interferometer uses the same light source as the measuring interferometer and introduces a common interference reference. The reference drift error is quantified by comparing the relative offsets of the initial frequencies of the up and down sweeps in the auxiliary interferometer. The error value is fed back to the distance calculation module to correct the final distance output; Achieve stable compensation for inconsistencies in the reference in the upper and lower frequency sweep structures, thereby improving the long-term measurement consistency of the system.

[0046] Regarding optical paths: Figure 1 Optical path structure based on time-stretched asynchronous bidirectional frequency sweep to suppress the Doppler effect. TLS (Tunable Laser Source) As a frequency-sweeping light source, it outputs a variable-frequency laser signal modulated by a triangular wave.

[0047] Multiple optical couplers The laser signal is split into multiple paths by multiple optical couplers (three ellipses in the figure).

[0048] The first path directly enters the measurement interferometer for target detection.

[0049] The second path sends the data to a 1×2 time stretching module for spectral chirping processing.

[0050] The third path, once separated, serves as a reference signal for use by the auxiliary interferometer and the resampling interferometer.

[0051] 1×2 Time-stretching Module It is used to convert part of the laser signal into a time-stretched chirped signal, thereby improving the ability to resolve the phase of the interference signal.

[0052] Measurement interferometer (top right of the image) Includes the reflection path (used to illuminate and receive the light reflected from the target). Interference with time-stretched reference light The output signal is recorded by the detector and used for subsequent ranging analysis. Auxiliary interferometer (middle right of the image) Receives light from the time stretching module and other beam splitting paths Compensation for the initial frequency difference between up and down sweeps Used to stabilize the system reference and reduce reference drift error. Resampling interferometer (bottom right of the image) Primarily used for monitoring nonlinear changes in laser sweep frequency. Provide time axis correction data The output is used for subsequent data resampling and spectral correction. Figure 2 Optical path structure based on time-stretched synchronous bidirectional frequency sweep to suppress the Doppler effect. TLS1 and TLS2 Two tunable lasers perform up-scan and down-scan operations respectively, outputting frequency sweep signals with opposite directions of optical frequency change.

[0053] Optical beam combiner The output light from TLS1 and TLS2 is spatially or optically combined to form a coaxial optical path, which is then fed into the system backbone.

[0054] 1×2 Time Stretching Module Similar to Figure 1 The structure performs time stretching on the combined signal to improve the resolution of frequency modulation.

[0055] Measurement interferometer (top right of the image) and Figure 1 Similarly, it is used to collect target echoes and interfere with time-stretched light. Output is used to measure target distance Auxiliary interferometer (middle right of the image) The two lasers interfere with the reference arm respectively. Used for real-time monitoring of initial frequency fluctuations between two light sources. Resampling interferometer (bottom right of the image) Monitoring the sweep frequency nonlinearity of each laser Provides resampling reference information to improve ranging accuracy. The structure of this diagram is relatively... Figure 1 It is more suitable for synchronous control environments, has higher requirements for light source consistency, but also has stronger compensation capabilities.

[0056] Figure 3 (a): Time stretching module based on 1×2 split structure This diagram illustrates a basic time stretching implementation, which has the following structural characteristics: Input optical port The swept-frequency laser signal enters the module from the TLS and is then guided into the structure through optical fiber.

[0057] 1×2 splitter The input signal is split into two paths: Entering the time stretching path; One path serves as a bypass light (or as an interference reference with the output of the stretching path).

[0058] Time stretch path It includes dispersion-compensating fiber (DCF) or chirped grating pairs to stretch the frequency-modulated signal over time. This path converts the frequency-modulated signal into a time-linearly expanded chirped signal.

[0059] Output port The stretched signal is output from the module and used for subsequent interference by the measuring interferometer or auxiliary interferometer.

[0060] By using a splitter and dispersive element to expand the time axis of the swept frequency signal, the output is an optical signal with more obvious chirp characteristics that is easier to analyze and process.

[0061] Figure 3 (b): Time stretching module based on echo structure (reflection structure) This figure illustrates a time-stretching method for a two-way dispersion structure, as shown below: Input optical port After the laser signal is input, it is transmitted into the module through an optical fiber.

[0062] Optical splitter or optical circulator (such as OC or CIR) Control the direction of incoming and outgoing signals to ensure that light can propagate in a directional manner within the structure.

[0063] Dispersion path + mirror (or reflector) combination After the light signal enters the dispersive path, it is reflected at the end of the path by a highly reflective mirror (or a reflective grating); It achieves two-way propagation, meaning the signal undergoes two dispersion processes in the dispersive path; The time stretch factor has been increased, improving the resolution between the modulation frequency and time.

[0064] Output path The reflected and stretched light signal returns along its original path, is split off through an optical circulator or coupler, and is output to the interferometer.

[0065] By increasing the dispersion path through the reflection structure, the time stretching effect of the chirped signal is made more significant, making it suitable for systems with higher precision requirements.

[0066] Implementation Method 3: Combination Figure 1-5 This embodiment describes the technical solution provided above in further detail through specific examples. Specifically: Based on the Doppler effect compensation method of bidirectional sweep frequency optical time stretching, a chirped interference signal is generated by time stretching technology. The suppression characteristics of the autocorrelation spectrum of broadband chirped signal on Doppler frequency shift are proposed and proved. Based on this characteristic, multiple optical path structures for compensating for the Doppler effect and realizing high-precision absolute distance measurement are designed. Corresponding signal processing methods are designed according to the optical path structures.

[0067] When the target speed During the frequency sweep cycle The internal variation is relatively large, and the expression of the interference signal is as follows: (1) Indicates the frequency sweep speed. This indicates the initial position of the target at the start of the measurement, and the target's velocity... Make amplitude ,frequency The simple harmonic motion of is expressed as: (2) Substituting it into formula (1) yields the phase of the original signal: (3) Define the autocorrelation function of the time-amplified signal as follows: (4) make The phase difference can be expressed as: (5) These represent the phase difference of the Doppler integral terms, the linear phase difference, and the quadratic phase difference, respectively.

[0068] (6) in, , because = The above formula can be approximated as: (7) Linear phase difference: (8) Second phase difference: (9) Substituting the phase difference above into (4), we get: (10) Expanding the sine term in the above equation using Bessel functions, let: (11) Substituting it into (10), we get: (12) Because the target vibration frequency and the sweep bandwidth satisfy the following relationship... The integration interval is much larger than the period of the sine term. Therefore, the high-frequency oscillation term in the integral They will cancel each other out due to rapid oscillations, leaving only [the remaining portion]. The DC term dominates. At this point, (12) can be simplified to: (13) The first term in the above equation is the complex phase, whose effect on the magnitude is negligible; the second term is the zeroth-order Bessel function, which is related to the amplitude and frequency of the target's simple harmonic motion and has an effect on... The amplitude changes at a certain point, but it does not affect the symmetry about the y-axis; the third term is a sinc function, whose pulse width is inversely proportional to the sweep bandwidth. Based on the above derivations, we can conclude that when the target's simple harmonic motion frequency... The peak position of the autocorrelation of the broadband signal is not affected by the target Doppler effect.

[0069] Furthermore, assuming that the target's minute vibrations originate only from low-frequency vibrations such as building structure vibrations, airflow disturbances, equipment resonance, and micro-seismic vibrations, with a vibration frequency less than 100Hz, while the high-speed sweep frequency light source has a repetition frequency greater than 10kHz, which is much greater than the target's vibration frequency, then the target's velocity within one cycle... It can be approximated as a constant. Under this condition, the modulation of the phase by the target Doppler effect can be simplified by equation (6) as follows: (14) The linear phase difference and quadratic phase difference are the same as above. Substituting the above equation into (4), we get: (15) Similarly, the Doppler effect caused by target vibration does not affect the position of the autocorrelation peak.

[0070] In the design of the scheme: An optical path for suppressing the Doppler effect based on time-stretched asynchronous bidirectional frequency sweep, such as... Figure 1 As shown, the system comprises three parts: a measurement interferometer, an auxiliary interferometer, and a resampling interferometer. It employs a triangular wave modulated swept-frequency laser. After passing through OC1, OC2, and OC3, the light source is divided into four parts. The first part enters the measurement arm of the measurement interferometer to detect the target, following the route: TLS-OC1-CIR1-Lens-Target-Lens-CIR1-OC4-BPD1. The second part enters a 1×2 time-stretching module to chirp-modulate the light source. A portion of the emitted light enters the measurement interferometer, interfering with the probe light to obtain the measurement signal. Another portion enters the auxiliary interferometer; the third portion, after passing through DL1, enters OC5 and, together with the stretched light mentioned above, enters BPD2 and forms interference, denoted as the reference signal. The third part is a resampling interferometer, which corrects the sweep frequency nonlinearity of the laser.

[0071] Figure 2Another Doppler effect suppression optical path based on time-stretched synchronous bidirectional frequency sweep is also composed of three parts: a measurement interferometer, an auxiliary interferometer, and a resampling interferometer. It uses two tunable lasers to perform up and down scanning synchronously. The output light from the two lasers is combined through OC1 and finally filtered out by fiber wavelength division multiplexers WDM1, WDM2, WDM3, and WDM4 respectively. The two lasers have different operating wavelengths to ensure that the two output channels that can be WDM do not produce aliasing.

[0072] The data processing flow of this method is as follows: Figure 4 As shown, firstly, when adopting an asynchronous bidirectional frequency sweep structure, it is necessary to process the acquired interference signals. and Perform up and down frequency scanning to obtain and , and If a synchronous bidirectional frequency sweep structure is used, it will directly yield... and , and Four interference signals.

[0073] Then, the two pairs of signals are subjected to modified frequency-shift cross-correlation processing, and the algorithm flow is as follows: Figure 5 As shown, the adaptive frequency shift cross-correlation algorithm is described in detail: right and (The signal from the measuring interferometer or auxiliary interferometer) is mixed, and the mixed signal is then passed through a digital low-pass filter to obtain... .

[0074] At the same time , and Perform a Fourier transform (FFT) to obtain their respective spectra.

[0075] Frequency domain frequency shift point estimation. The spectrum contains a narrow pulse with a high amplitude, and the horizontal axis corresponding to its maximum value is... and The time delay difference between two signals can be estimated by finding the peak of a narrow pulse. Delay difference estimate The corresponding number of points is FSR represents the free spectral range of the resampling interferometer. , This represents the time delay difference between the measurement arm and the reference arm of the resampling interferometer.

[0076] The corrected spectrum shift. Spectrum shifting involves truncating the spectrum on the left and zero-padding at the end to keep the total number of points N constant. However, due to the bidirectional frequency sweep, adjustments are needed... Adjustments were made to the actual number of points moved. The frequency-shifted signal is denoted as .

[0077] Inverse Fourier Transform. The FFT result is conjugate, and in The FFT results are multiplied, and then the product sequence is cut to retain only the spectral sequence of the non-zero region. The cut sequence is then subjected to DFT calculation with L-point input and M-point output to obtain the cross-correlation spectrum.

[0078] Find the peaks in the cross-correlation spectrum and denote the peak positions as... , can be obtained corresponding distance , This indicates the optical path difference within the optical fibers of the two fibers.

[0079] Combining the coarse measurement results from step 3 and the fine measurement results from step 6, the final distance measurement result can be obtained: .

[0080] The frequency-shifted signal can be expressed by the formula: (16) Its and The time delay difference between them is theoretically less than Therefore, for the signal and Cross-correlation can be approximated as autocorrelation. As can be seen from the derivation of the principle, this approximate autocorrelation can be used to suppress the Doppler effect.

[0081] Finally, it is necessary to suppress the measurement reference drift during the frequency sweep process. This part is as follows: there is a relative fluctuation in the initial optical frequency of the upper and lower frequency sweep light sources. The measurement error caused by this fluctuation is: (17) To suppress measurement errors caused by reference drift, an auxiliary interferometer of the same origin can be introduced.

[0082] The above description of several specific embodiments further details the technical solution provided by the present invention in order to highlight the advantages and benefits of the technical solution provided by the present invention. However, the above-described specific embodiments are not intended to limit the present invention. Any reasonable modifications and improvements to the present invention, combinations of embodiments, and equivalent substitutions based on the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for compensating for the Doppler effect in optical time-stretched sweep interferometry, characterized in that, include: The steps involve generating a swept-frequency laser signal and splitting it into measurement light, reference light, and resampling light. The step of time-stretching the reference light to obtain the chirped signal; The step of interfering the measurement light with the chirped signal to obtain a measurement interference signal; The step of interfering the reference light with the resampled light to obtain the reference interference signal; The steps include adaptive frequency-shifting cross-correlation processing of the measured interference signal and the reference interference signal to compensate for the Doppler effect and simultaneously extract the optical path difference; The steps involve calculating the distance using resampling interferometer parameters and outputting the target distance after compensating for the Doppler effect. The steps for cross-correlation processing of the measured interferometric signal and the reference interferometric signal to extract the optical path difference include: mixing the measured interferometric signal and the reference interferometric signal; passing the mixed signal through a digital low-pass filter to obtain a mixed signal; simultaneously performing Fourier transforms on the mixed signal, the measured interferometric signal, and the reference interferometric signal to obtain their respective spectra; finding the peak position of high-amplitude narrow pulses in the spectrum of the mixed signal to obtain an estimated value of the time delay difference between the measured interferometric signal and the reference interferometric signal; and obtaining the corresponding number of frequency-domain shift points based on the estimated time delay difference. The frequency spectrum is shifted according to the specified frequency shift points, and the shifted signal is denoted as the frequency-shifted signal. The Fourier transform result of the reference interference signal is conjugate and multiplied with the Fourier transform result of the frequency-shifted signal. The product sequence is then truncated, retaining only the spectral sequence of the non-zero region. The truncated spectral sequence is subjected to DFT calculation to obtain the cross-correlation spectrum, thereby compensating for the Doppler effect. The peaks of the cross-correlation spectrum are then located to obtain the peak positions. The corresponding precise distance is obtained based on the peak position. ; The steps for calculating the target distance by combining the parameters of the resampling interferometer and outputting the Doppler-compensated target distance include: calculating the free spectral range (FSR) of the resampling interferometer and the time delay difference. , The frequency shift points mentioned above The corresponding coarse measurement results and the fine measurement distance ,according to Obtain the final target distance ,in, c represents the speed of light, and N represents the total number of points in the spectrum. Indicates the refractive index of air. This represents the optical path difference within the optical fibers of the two arms, specifically the internal optical path difference between the reference arm fiber path and the measurement arm fiber path.

2. The Doppler effect compensation method for optical time-stretched sweep interferometry according to claim 1, characterized in that, Time stretching involves guiding reference light into a dispersive fiber or a chirped grating structure to stretch the output signal in time and give it linear chirped characteristics.

3. The Doppler effect compensation method for optical time-stretched sweep interferometry according to claim 1, characterized in that, The measurement interference signal is obtained by interfering the target reflected echo with the time-stretched light in the measurement interferometer.

4. The Doppler effect compensation method for optical time-stretched sweep interferometry according to claim 1, characterized in that, The reference interference signal is acquired through an auxiliary interferometer and is used to detect the nonlinearity of the frequency modulation of the swept laser and the frequency drift between the upper and lower sweeps.

5. The Doppler effect compensation method for optical time-stretched sweep interferometry according to claim 1, characterized in that, Cross-correlation processing includes: frequency domain transformation, spectral shift correction, conjugate product operation, and cross-correlation peak extraction of the measured interference signal and the reference interference signal.

6. An optical time-stretched sweep frequency interferometry Doppler effect compensation device, characterized in that, To implement the method of claim 1, the method comprises: A swept-frequency laser signal is generated and split into modules for measurement light, reference light, and resampling light; A module that performs time-stretching on a reference light to obtain a chirped signal; A module that interferes the measurement light with the chirped signal to obtain a measurement interference signal; The module that obtains the reference interference signal by interfering the reference light with the resampled light; A module that performs cross-correlation processing on the measured interference signal and the reference interference signal to extract the optical path difference; This module calculates the distance using resampled data and outputs the target distance after compensating for the Doppler effect.

7. A time-stretched frequency sweeping interferometric ranging system, characterized in that, The system is used to implement the method of claim 1, comprising: A swept-frequency laser is used to output a frequency-modulated laser signal. A beam splitter is used to split the laser signal into a measurement beam, a reference beam, and a resampling beam. The time stretching module, located on the reference optical path, is used to generate chirped stretching optical signals; A measurement interferometer is used to interfere with the measurement light and the stretching light signal, and output a measurement interference signal. An auxiliary interferometer is used to interfere with the reference light and the stretched light signal, and outputs an auxiliary interference signal. A resampling interferometer is used to obtain swept-frequency nonlinear characteristics; The data processing module is used to process the various interference signals, compensate for the Doppler effect, and output the target distance.

8. A computer storage medium for storing computer programs, characterized in that, When the computer program is read by the computer, the computer executes the method of claim 1.

9. A computer, comprising a processor and a storage medium, characterized in that, When the processor reads the computer program stored in the storage medium, the computer executes the method of claim 1.

10. A computer program product, as a computer program, is characterized by: When the computer program is executed, it implements the method of claim 1.

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