Compensation method and device for Doppler effect in optical time stretching sweep frequency interference measurement

Abstract

The invention discloses an optical time stretching sweep frequency interference measurement Doppler effect compensation method and device, and relates to the technical field of non-cooperative target absolute distance measurement. In order to solve the defect that the absolute distance measurement precision is reduced due to the fact that the Doppler effect cannot be efficiently suppressed in a complex environment in the prior art, the technical scheme provided by the invention comprises the following steps: generating a sweep frequency laser signal and splitting the sweep frequency laser signal to form measurement light, reference light and resampling light; performing time stretching processing on the reference light to obtain a chirp signal; performing interference on measurement light and the chirp signal to obtain a measurement interference signal; performing interference on the reference light and the resampling light to obtain a reference interference signal; performing cross-correlation processing on the measurement interference signal and the reference interference signal, and extracting an optical path difference; and carrying out distance calculation by combining the resampling data, and outputting a target distance after the Doppler effect is compensated. The method is suitable for optical measurement work for performing high-precision absolute distance measurement on a non-cooperative target in a tiny vibration or disturbance 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 Art

[0002] Traditional FMCW ranging methods can only be used in laboratory vibration-isolated environments, such as air-floating vibration isolation platforms. Once vibration is present, even if the amplitude is in the micron range, the Doppler effect can introduce errors as high as hundreds of microns, severely limiting the method's applicability. This makes a single FMCW ranging method unsuitable for most measurement environments. Currently, common FMCW laser ranging solutions for mitigating the Doppler effect introduced by target vibration include sinusoidal / triangular wave frequency modulation, dual-light source inverse frequency modulation, and common-path laser Doppler vibrometer compensation. The following section analyzes the advantages and disadvantages of each method.

[0003] a) Triangle wave frequency modulation Triangular wave frequency modulation is the earliest Doppler effect correction method. The Doppler frequency shift is reduced by utilizing the characteristic that the Doppler frequency shift is equal in magnitude and opposite in direction during forward and reverse frequency modulation of the frequency modulated light source during triangular wave modulation, thereby obtaining accurate target distance information. This technical solution can achieve Doppler effect correction without the need for other hardware, and has important application value in low-cost lidar. However, this method is only applicable to slow and uniform target movement, which makes it impossible to apply it to industrial environments with complex vibrations. Many scholars and teams in China are also conducting extensive research and exploration on the measurement of vibrating targets. Zhang Fumin of Tianjin University and Bo Liu of the University of the Chinese Academy of Sciences also performed Doppler compensation based on triangular wave modulation. In 2020, Tsinghua University proposed a photonics-assisted method using dual-band symmetrical triangular modulation to achieve unambiguous and simultaneous measurement of distance and velocity in multi-target situations.

[0004] b) Double sweep frequency modulation In 2001, German scientists Richard Schneider and others first proposed a solution based on two frequency-modulated lasers to overcome the Doppler effect. The schematic diagram is shown below. In this solution, both lasers are frequency-modulated semiconductor lasers. When the frequency of one laser increases, the frequency of the other decreases. The device uses a phase comparison method with an auxiliary interferometer to correct the frequency-modulated nonlinearity of the two lasers. However, the article only provides simulation analysis, not 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 implemented dynamic distance measurement. They achieved a measurement accuracy of 1.2μm within a 10m 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 100nm and an optimal vibration measurement resolution of approximately 130pm. In 2006, Anthony Slotwinski and others at Nikon proposed a similar dual-semiconductor laser absolute distance measurement structure. In this structure, the two semiconductor lasers use phase-locked loops to perform frequency-modulated nonlinearity correction. The two lasers modulate in opposite directions and at different rates. After combining the two lasers, a common-path configuration is employed. Because the two lasers have different frequency modulation rates, a bandpass filter is required to separate the two interference signals of varying frequencies. The Doppler effect has the same impact on the interference signals corresponding to the two frequency-modulated lasers, canceling each other out and thus providing the absolute distance to the target. In 2014, Armin Reichold of the University of Oxford constructed a dynamic absolute distance measurement system using two external-cavity lasers. A gas absorption chamber was also introduced as a length traceability reference for the ranging system, ultimately achieving a measurement accuracy of 40nm for cooperative targets.

[0005] c) Common optical path laser Doppler vibration measurement-compensation In 2016, Lu Cheng from Harbin Institute of Technology proposed a Doppler effect correction technology based on dual-frequency heterodyne interferometry. This method measures the target vibration and compensates it to the ranging signal by adding a Doppler vibration measurement unit in the common optical path on the basis of the basic FMCW ranging optical path, thereby achieving accurate measurement of non-cooperative vibrating targets. However, due to the limitations of existing digital signal processing methods, when in an industrial environment or when the target surface is tilted or too rough, there is multipath interference in the optical path, resulting in low measurement accuracy. In some cases, distance information cannot even be extracted. In 2022, the research group used this system to propose a compensation method of analytical signal multiplication. The method was verified experimentally to be effective in compensating for non-cooperative target vibrations. For non-cooperative targets at 15m, if the method based on analytical signal multiplication is used for solution, the measurement standard deviation can reach 19.65μm.

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

[0007] The existing technology has the defect of being unable to effectively suppress the Doppler effect in a complex environment where non-cooperative targets have tiny vibrations, resulting in a decrease in the accuracy of absolute distance measurement. Summary of the Invention

[0008] To address the drawbacks of the prior art, which is the inability to effectively suppress the Doppler effect in complex environments with small vibrations of non-cooperative targets, resulting in decreased absolute distance measurement accuracy, the present invention provides the following technical solutions: The Doppler effect compensation method of optical time-stretching swept-frequency interferometry includes: generating a swept laser signal and splitting it into measurement light, reference light, and resampling light; The step of performing time stretching on the reference light to obtain a chirped signal; interfering the measurement light with the chirp signal to obtain a measurement interference signal; The step of interfering the reference light with the resampled light to obtain a reference interference signal; performing cross-correlation processing on the measurement interference signal and the reference interference signal to extract the optical path difference; The steps of performing distance calculation based on the resampled data and outputting the target distance after compensating the Doppler effect are performed.

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

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

[0011] Furthermore, a preferred embodiment is provided in which a reference interference signal is obtained by 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.

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

[0013] Based on the same inventive concept, the present invention also provides an optical time-stretching swept-frequency interferometry Doppler effect compensation device, comprising: Generate a swept laser signal and split it into modules for measurement light, reference light, and resampling light; A module that performs time stretching on the reference light to obtain a chirp signal; A module for interfering the measurement light with the chirp signal to obtain a measurement interference signal; A module for interfering the reference light with the resampled light to obtain a reference interference signal; A module that performs cross-correlation processing on the measurement interference signal and the reference interference signal to extract the optical path difference; A module that combines resampled data to perform distance calculation 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-stretching-based swept frequency interferometry ranging system, which is used to implement the method described above, including: A swept laser, used to output a frequency-modulated laser signal; A light splitting component for dividing the laser signal into measurement light, reference light and resampling light; A time stretching module, provided on a reference optical path, for generating a chirped stretched optical signal; a measuring interferometer, configured to interfere the measuring light with the stretched light signal and output a measuring interference signal; an auxiliary interferometer, configured to interfere the reference light with the stretched light signal and output an auxiliary interference signal; Resampling interferometer to obtain swept frequency nonlinear characteristics; The data processing module is used to process each interference signal, 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. When the computer program is read by a computer, the computer executes the method described above.

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

[0017] Based on the same inventive concept, the present invention also provides a computer program product, which is a computer program. When the computer program is executed, the method described above is implemented.

[0018] Compared with the prior art, the technical solution provided by the present invention is beneficial in that: This solution constructs a bidirectional swept-frequency interferometry optical path based on optical time stretching. A time stretching module is introduced during the measurement process to chirp the interference signal, amplifying the signal on the time axis. This approach leverages the autocorrelation characteristics of broadband chirped signals to achieve natural suppression of the Doppler effect. Compared to traditional bidirectional frequency modulation schemes, which require extremely high symmetry between the upper and lower sweep frequencies, the high-frequency Doppler terms of the time-stretched interference signal are effectively averaged and offset during the autocorrelation process. This allows the system to maintain a stable measurement peak position even when the target undergoes simple harmonic oscillation, significantly improving the system's ability to resist Doppler interference.

[0019] This solution utilizes an asynchronous or synchronous bidirectional frequency sweeping structure, and designs corresponding optical path systems and resampling interferometers to correct for the nonlinearity of the laser frequency sweeping, while also improving the accuracy of distance extraction. In the asynchronous structure, a triangle wave laser source is combined with a time stretching unit to perform interferometric measurement of the target, and the signal is corrected and processed using an auxiliary interferometer and a resampling interferometer, achieving the effect of extracting a stable interference signal even in the presence of multipath interference. Compared to traditional triangle wave modulation methods that are only applicable to slow targets, this solution is effective for absolute distance measurement in low-frequency vibration scenarios.

[0020] This proposal proposes a modified adaptive frequency-shifted cross-correlation algorithm that accurately estimates the optical path difference through steps such as frequency mixing, spectrum shearing, DFT frequency shifting, and peak extraction. This algorithm incorporates a spectrum shift point estimation mechanism that accurately extracts the peak position of the cross-correlation spectrum even in the presence of laser frequency sweep nonlinearity and system jitter, improving the system's measurement stability in the presence of high-frequency noise or structural perturbations. Compared to 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 solution utilizes a coherent auxiliary interferometer to monitor and correct the reference frequency drift of the upper and lower swept light sources in real time, overcoming the reference drift error caused by inconsistent initial frequencies of the upper and lower light sources in synchronous frequency modulation. This design enables the system to maintain ranging accuracy and consistency even in the presence of frequency source fluctuations or device instability. Compared to symmetrical dual-laser systems requiring strict phase-locking, this solution achieves higher anti-interference capabilities and system robustness with a lower structural complexity.

[0022] The theoretical analysis derived the Doppler immunity of broadband chirp signals in autocorrelation processing, demonstrating that when the target vibration frequency is significantly lower than the sweep repetition rate, the velocity variations caused by the target vibration will not affect the autocorrelation peak position. This theory underpins the data processing foundation of the entire scheme and clearly demonstrates that submicron ranging accuracy can be maintained even in the presence of minor structural vibrations of non-cooperative targets (such as airflow disturbances and equipment resonances), a feature that cannot be reliably achieved with existing FMCW schemes.

[0023] It is suitable for optical measurement work such as high-precision absolute distance measurement of non-cooperative targets in environments with small vibrations or disturbances. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of the optical path for Doppler effect suppression based on time-stretched asynchronous bidirectional frequency sweeping; Figure 2 Schematic diagram of the optical path for Doppler effect suppression based on time-stretched and synchronized bidirectional frequency sweeping; Figure 3 This is a schematic diagram of the time stretch module; Figure 4 Data processing flow for suppressing the Doppler effect based on time-stretched bidirectional frequency sweeping; Figure 5 Flowchart of the modified adaptive frequency shift cross-correlation algorithm. DETAILED DESCRIPTION

[0025] In order to make the advantages and benefits of the technical solution provided by the present invention more clearly reflected, the technical solution provided by the present invention is now further described in detail with reference to the accompanying drawings, specifically: Embodiment 1: This embodiment provides a Doppler effect compensation method for optical time-stretching swept-frequency interferometry, including: generating a swept laser signal and splitting it into measurement light, reference light, and resampling light; The step of performing time stretching on the reference light to obtain a chirped signal; interfering the measurement light with the chirp signal to obtain a measurement interference signal; The step of interfering the reference light with the resampled light to obtain a reference interference signal; performing cross-correlation processing on the measurement interference signal and the reference interference signal to extract the optical path difference; The steps of performing distance calculation based on the resampled data and outputting the target distance after compensating the Doppler effect are performed.

[0026] Time stretching processing involves introducing reference light into a dispersive optical fiber or a chirped grating structure, so that the output signal is stretched in time and exhibits a linear chirp characteristic.

[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 obtained 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] The cross-correlation processing includes: frequency domain transformation, spectrum shift correction, conjugate product operation and cross-correlation peak extraction of the measurement interference signal and the reference interference signal.

[0030] A time-stretching-based swept frequency interferometry ranging system is also provided, which is used to implement the method described above, including: A swept laser, used to output a frequency-modulated laser signal; A light splitting component for dividing the laser signal into measurement light, reference light and resampling light; A time stretching module, provided on a reference optical path, for generating a chirped stretched optical signal; a measuring interferometer, configured to interfere the measuring light with the stretched light signal and output a measuring interference signal; an auxiliary interferometer, configured to interfere the reference light with the stretched light signal and output an auxiliary interference signal; Resampling interferometer to obtain swept frequency nonlinear characteristics; The data processing module is used to process each interference signal, compensate for the Doppler effect and output the target distance.

[0031] Implementation Method 2: This implementation method further limits the technical solution provided in Implementation Method 1, including: A Doppler effect compensation method and device based on optical time stretching and frequency-sweeping interferometry is suitable for achieving high-precision absolute distance measurement of non-cooperative targets in complex vibration environments. This solution, based on optical time stretching, a bidirectional frequency-sweeping structure, an auxiliary interferometry compensation mechanism, and a modified cross-correlation algorithm, systematically suppresses the influence of the Doppler effect on measurement results, significantly improving ranging accuracy and stability.

[0032] The specific implementation process includes the following steps: Step 1: Build a time-stretched bidirectional frequency-sweeping interferometer optical system Construct an optical path system including a measurement interferometer, an auxiliary interferometer and a resampling interferometer, and configure a swept laser source, a time stretching module and a detector.

[0033] Detailed description: Configure a swept laser as the light source, and choose a triangle wave modulated single source or a dual source system consisting of two synchronous swept lasers; The light source output is divided into multiple light paths in sequence through the optical splitter; The first part of the light enters the measurement interferometer for detecting the target. The specific path includes an optical circulator, a lens, a target reflection path, a return optical circulator, a beam combiner, and a double-balanced detector. After the second part of the light is processed by the time stretch module, one part enters the measurement interferometer to interfere with the target reflected light, and the other part enters the auxiliary interferometer; The third part enters the reference arm through the delay line and interferes with the time-stretched beam in the auxiliary interferometer; The fourth part is used to resample the interferometer to obtain the nonlinear changes during the frequency sweep process for subsequent nonlinear correction; All interference signals are collected 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 Collect and distinguish multiple interference signals obtained during the up and down scanning process, providing a basis for subsequent independent processing.

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

[0036] Step 3: Perform mixing processing and spectrum calculation The measurement signal is mixed with the reference signal and filtered, the interference envelope is extracted and the spectral characteristics are calculated.

[0037] Detailed description: Perform dot multiplication on the measured interference signal and the reference interference signal to obtain a mixed signal; Apply digital low-pass filtering to the mixed signal to retain the interference envelope signal in the useful frequency range; Perform fast Fourier transform (FFT) on the measured interference signal and the reference interference signal to extract frequency domain features; The spectrum data output from this step is prepared for cross-correlation frequency shift point estimation and spectrum shifting.

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

[0039] Detailed description: Find the peak position of the narrowband pulse in the reference interference signal spectrum, which corresponds to the time delay between the interference signals; Convert the delay time into the corresponding frequency shift points in the frequency domain; Perform a shearing operation on the spectrum, removing the high-frequency part at the beginning of the spectrum and padding the end with zeros to keep the spectrum length unchanged; If it is a bidirectional sweep frequency structure, the upper and lower sweep frequency spectra are corrected separately to ensure their mutual correlation; The output is the corrected spectrum data, which is used to calculate the cross-correlation spectrum.

[0040] Step 5: Calculate the cross-correlation spectrum and extract the peak position The cross-correlation spectrum is calculated using the corrected spectrum, from which the corresponding delay peak is extracted.

[0041] Detailed description: Take the complex conjugate of the corrected reference spectrum and multiply it by the measured spectrum; Clip the product result and keep only the non-zero part of the spectrum; Perform short-time Fourier transform (DFT) on the clipped spectrum to obtain the cross-correlation spectrum; Perform peak search in the cross-correlation spectrum and record the peak position; The peak position corresponds to the optical path difference between the measuring arm and the reference arm in the measuring optical path.

[0042] Step 6: Calculate the absolute distance to the target by combining the rough and fine measurement results The absolute distance of the target is finally calculated by combining the cross-correlation calculation results and the coarse measurement information measured by the resampling interferometer.

[0043] Detailed description: The coarse measurement results provided by the resampling interferometer give an approximate ranging interval; The cross-correlation peak position provides the precise relative delay; Combine the rough measurement and fine measurement results and convert them into absolute distance output; The output here is the target distance with sub-micron accuracy.

[0044] Step 7: Eliminate the error caused by the inconsistency of the initial frequencies of the upper and lower sweeps The initial frequencies of the upper and lower sweeps are 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 homologous interference reference; The reference drift error is quantified by comparing the relative offsets of the initial frequencies of the upper and lower sweeps in the auxiliary interferometer; Feed the error value back to the distance calculation module to correct the final distance output; Achieve stable compensation for inconsistent references in the upper and lower sweep structures, and improve the long-term measurement consistency of the system.

[0046] In terms of optical path: Figure 1 :Doppler effect suppression optical path structure based on time-stretched asynchronous bidirectional frequency sweeping TLS (Tunable Laser Source) As a swept frequency light source, it outputs a frequency-variable laser signal modulated by a triangle wave.

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

[0048] The first path goes directly to the measuring interferometer for target detection.

[0049] The second path is fed into a 1×2 time stretch module for spectral chirping processing.

[0050] The third path is separated and used as a reference signal for 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] Measuring interferometer (upper right) Includes a reflective path (for illuminating and receiving light reflected from a target) Interference with the time-stretched reference light The output signal is recorded by the detector for subsequent ranging analysis Auxiliary interferometer (middle right of the picture) Receives light from the time stretch module and other split paths Compensate for the initial frequency difference between upper and lower sweeps Used to stabilize the system benchmark and reduce benchmark drift error Resampling interferometer (bottom right) Mainly used to monitor the nonlinear changes of laser frequency sweep Provide time axis correction data Output for subsequent data resampling and spectrum correction Figure 2 :Doppler effect suppression optical path structure based on time-stretched synchronous bidirectional frequency sweeping TLS1 vs. TLS2 Two tunable lasers perform up-sweep and down-sweep operations respectively, and output sweep signals with opposite directions of optical frequency change.

[0053] photosynthesizer The output light of TLS1 and TLS2 is combined spatially or by fiber to form a coaxial optical path and enter the system backbone uniformly.

[0054] 1×2 time stretch module Similar to Figure 1 The structure performs time stretching on the combined signal to improve the ability to analyze frequency modulation.

[0055] Measuring interferometer (upper right) and Figure 1 Same, used to collect target echo and interfere with time-stretched light Output for measuring target distance Auxiliary interferometer (middle right of the picture) The two lasers interfere with the reference arm respectively Used to monitor the initial frequency fluctuation between two light sources in real time Resampling interferometer (bottom right) Monitor the frequency sweep nonlinearity of each laser Provide resampling benchmark information to improve ranging accuracy Compared with the graph structure 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 branching structure This diagram shows a basic time stretching implementation with the following structural features: Input optical port The swept laser signal enters the module from the TLS and is introduced into the structure through optical fiber.

[0057] 1×2 Splitter Split the input signal into two paths: All the way into the time stretch path; One path is used as a bypass light (or used for interference reference with the stretched path output).

[0058] Time Stretch Path Contains dispersion devices, such as dispersion-compensating fiber (DCF) or chirped grating pairs, to temporally stretch the frequency-modulated signal. This path converts the frequency-modulated signal into a chirped signal that is linearly stretched in time.

[0059] Output Port The stretched signal is output from the module for subsequent use in the measurement interferometer or auxiliary interferometer interference.

[0060] The time axis of the frequency sweep signal is expanded through the splitter + dispersion element, and the output is an optical signal with more obvious chirp characteristics and is easier to analyze and process.

[0061] Figure 3 (b): Time stretching module based on echo structure (reverberation structure) This figure shows a time stretching method for a double-pass dispersion structure, which has the following structure: Input optical port After the laser signal is input, it is introduced into the module through optical fiber.

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

[0063] Dispersion path + mirror (or reflector) combination After the optical signal enters the dispersion path, it is reflected by a highly reflective mirror (or reflective grating) at the end of the path; Two-way propagation is achieved, that is, the signal undergoes two dispersion effects in the dispersion path; The time stretching factor is enhanced, which improves the resolution between modulation frequency and time.

[0064] Output Path The reflected and stretched optical signal returns along the original path, is separated by an optical circulator or coupler, and output to the interferometer.

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

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

[0067] When the target speed In the sweep cycle The internal variation is large, and the interference signal expression is as follows: (1) Indicates the sweep speed, Indicates the initial position of the target at the beginning of the measurement. The target moves at a speed of Do amplitude ,frequency The simple harmonic motion of is expressed as: (2) Substituting it into formula (1) we can get the original signal phase: (3) The autocorrelation function of the signal after time amplification is defined as: (4) make , the phase difference can be expressed as: (5) They represent the Doppler integral phase difference, linear phase difference and quadratic phase difference respectively.

[0068] (6) in, , due to = , the above formula can be approximated as: (7) Linear phase difference: (8) Secondary phase difference: (9) Substituting the above phase difference into (4) we can get: (10) Use Bessel function to expand the sine term in the above formula, let: (11) Substituting it into (10), we get: (12) Since the target vibration frequency meets the sweep bandwidth , the integration interval is much larger than the period of the sine term , so the high-frequency oscillation term in the integral will cancel each other out due to rapid oscillations, leaving only The DC term dominates. At this time, (12) can be simplified as: (13) The first term in the above formula is the complex phase, and its influence on the modulus can be ignored; the second term is the zero-order Bessel function, which is related to the amplitude and frequency of the target simple harmonic motion and affects The amplitude changes at , but does not affect the symmetry characteristics about the y-axis; the third term is the sinc function, whose pulse width is inversely proportional to the sweep bandwidth. Combining the above derivations, we can draw the following conclusions: when the target simple harmonic motion frequency , the peak position of the broadband signal autocorrelation is not affected by the target Doppler effect.

[0069] Furthermore, assuming that the target's tiny vibration is only caused by low-frequency vibrations such as building structure vibration, airflow disturbance, equipment resonance, and micro-earthquake vibration, and the vibration frequency is less than 100Hz, and the repetition frequency of the high-speed sweep light source is greater than 10KHz, which is much greater than the target's vibration frequency, then the target speed in one cycle is can be approximated as a constant. Under this condition, the modulation of the phase by the target Doppler effect can be simplified from Equation (6) as follows: (14) The linear phase difference and quadratic phase difference are consistent with the above. Substituting the above formula into (4), we can get: (15) Likewise, the Doppler effect caused by target vibration does not affect the autocorrelation peak position.

[0070] In the design of the program: A Doppler effect suppression optical path based on time-stretched asynchronous bidirectional frequency sweeping, such as Figure 1 As shown in the figure, the system consists of three parts: measurement interferometer, auxiliary interferometer and resampling interferometer. It uses a triangular wave modulated swept laser. The light source is divided into four parts after passing through OC1, OC2 and OC3. The first part enters the measuring arm of the measurement interferometer to detect the target. The specific route is: TLS-OC1-CIR1-Lens-Target-Lens-CIR1-OC4-BPD1; secondly, the second part enters the 1×2 time stretch module to chirp modulate the light source. Part of its output light enters the measurement interferometer, which interferes with the detection light to obtain the measurement signal. , another part enters the auxiliary interferometer; the third part enters OC5 after passing through DL1, and enters BPD2 together with the stretched light above to form interference, which is recorded as the reference signal ; The third part is the resampling interferometer, which corrects the laser's frequency scanning nonlinearity.

[0071] Figure 2This is another Doppler effect suppression optical path based on time-stretched synchronized bidirectional frequency sweeping. The system also consists of three parts: a measurement interferometer, an auxiliary interferometer, and a resampling interferometer. Two tunable lasers are used to synchronously perform up and down scanning. The output light of the two lasers is combined through OC1 and finally filtered out respectively through fiber wavelength division multiplexers WDM1, WDM2, WDM3 and WDM4. The operating wavelengths of the two lasers are different to ensure that there is no aliasing in the two output channels of the WDM.

[0072] The data processing flow of this method is as follows Figure 4 As shown, first, when adopting the asynchronous bidirectional sweep frequency structure, the collected interference signal needs to be and Perform up and down sweep frequency division to obtain and , and If a synchronous two-way sweep frequency structure is used, the result will be 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 in the figure, the adaptive frequency shift cross-correlation algorithm is introduced in detail: right and (from the measuring interferometer or auxiliary interferometer) for mixing, and the mixed signal passes through a digital low-pass filter to obtain .

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

[0075] Estimation of the number of frequency shift points in the frequency domain. The spectrum contains a narrow pulse with a high amplitude, and the horizontal coordinate corresponding to its maximum value is and The delay difference between two signals can be estimated by searching for peaks in narrow pulses. , delay difference estimation value The corresponding points are , FSR represents the free spectral range of the resampling interferometer, , is the time delay difference between the measurement arm and the reference arm of the resampling interferometer.

[0076] Modified spectrum shifting. Spectrum shifting is to cut off the spectrum on the left and fill the spectrum at the end to keep the total number of points N unchanged. However, due to the use of bidirectional frequency sweeping, it is necessary to Correction, actual movement points , the frequency-shifted signal is recorded as .

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

[0078] Find the peak of the cross-correlation spectrum and record the peak position as , we can get Corresponding distance , Indicates the optical path difference between the two optical fibers.

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

[0080] The frequency-shifted signal can be expressed as follows: (16) Its The delay difference between them is theoretically less than Therefore, the signal and The cross-correlation can be approximated as autocorrelation. From the derivation of the principle part, it can be known that the Doppler effect can be suppressed by using this approximate autocorrelation.

[0081] Finally, it is necessary to suppress the drift of the measurement reference during the frequency sweep process. The content of this part is as follows: There is a relative fluctuation in the initial optical frequency of the upper frequency sweep light source and the lower frequency sweep light source. , the measurement error caused by this fluctuation is: (17) In order to suppress the measurement error caused by reference drift, a homologous auxiliary interferometer can be introduced.

[0082] The above further describes the technical solution provided by the present invention in detail through several specific embodiments in order to highlight the advantages and benefits of the technical solution provided by the present invention. However, the several specific embodiments described above are not intended to limit the present invention. Any reasonable modification and improvement of the present invention, combination of embodiments and equivalent replacement based on the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for compensating the Doppler effect by optical time-stretching and frequency-sweeping interferometry, characterized in that: include: generating a swept laser signal and splitting it into measurement light, reference light, and resampling light; The step of performing time stretching on the reference light to obtain a chirped signal; interfering the measurement light with the chirp signal to obtain a measurement interference signal; The step of interfering the reference light with the resampled light to obtain a reference interference signal; performing cross-correlation processing on the measurement interference signal and the reference interference signal to extract the optical path difference; The steps of performing distance calculation based on the resampled data and outputting the target distance after compensating the Doppler effect are performed.

2. The Doppler effect compensation method for optical time-stretching swept-frequency interferometry according to claim 1, characterized in that: Time stretching processing involves introducing reference light into a dispersive optical fiber or a chirped grating structure, so that the output signal is stretched in time and exhibits a linear chirp characteristic.

3. The Doppler effect compensation method for optical time-stretching swept-frequency interferometry according to claim 1, characterized in that: The measurement interference signal is obtained by the interference of the target reflected echo and the time-stretched light in the measurement interferometer.

4. The method for compensating the Doppler effect of optical time-stretching swept-frequency interferometry according to claim 1, wherein: The reference interference signal is obtained 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 method for compensating the Doppler effect of optical time-stretching swept-frequency interferometry according to claim 1, wherein: The cross-correlation processing includes: frequency domain transformation, spectrum shift correction, conjugate product operation and cross-correlation peak extraction of the measurement interference signal and the reference interference signal.

6. Optical time-stretching swept-frequency interferometry Doppler effect compensation device, characterized in that: include: Generate a swept laser signal and split it into modules for measurement light, reference light, and resampling light; A module that performs time stretching on the reference light to obtain a chirp signal; A module for interfering the measurement light with the chirp signal to obtain a measurement interference signal; A module for interfering the reference light with the resampled light to obtain a reference interference signal; A module that performs cross-correlation processing on the measurement interference signal and the reference interference signal to extract the optical path difference; A module that combines resampled data to perform distance calculation and outputs the target distance after compensating for the Doppler effect.

7. A time-stretching based swept frequency interferometry ranging system, characterized in that: The system is used to implement the method according to claim 1, comprising: A swept laser, used to output a frequency-modulated laser signal; A light splitting component for dividing the laser signal into measurement light, reference light and resampling light; A time stretching module, provided on a reference optical path, for generating a chirped stretched optical signal; a measuring interferometer, configured to interfere the measuring light with the stretched light signal and output a measuring interference signal; an auxiliary interferometer, configured to interfere the reference light with the stretched light signal and output an auxiliary interference signal; Resampling interferometer to obtain swept frequency nonlinear characteristics; The data processing module is used to process each interference signal, compensate for the Doppler effect and output the target distance.

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

10. A computer program product, being a computer program, characterized in that When the computer program is executed, the method according to claim 1 is implemented.

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