Non-linear compensation high-precision dual-wavelength sweep frequency interferometry ranging method

Abstract

The invention discloses a non-linear compensation high-precision dual-wavelength frequency sweeping interference distance measurement method. The method comprises the following steps: respectively acquiring measurement interference signals and auxiliary interference signals of two sweep frequency laser light sources through a double sweep frequency interference absolute distance measurement system; establishing a differential equation of the optical frequency and the phase based on a Taylor series model, and obtaining change information of the optical frequency by solving the differential equation; and constructing a high-order orthogonal basis according to the change information of the optical frequency, performing orthogonal decomposition on a measurement interference signal by using the high-order orthogonal basis to obtain distance spectrum information, and solving according to the distance spectrum information to obtain a to-be-measured distance value. According to the technical scheme provided by the invention, a relatively good residual nonlinear compensation effect can still be obtained in a complex test environment, and the interference measurement precision is improved; in addition, the method is not limited by fast large-range frequency modulation, and a good effect can still be obtained for the Doppler effect and nonlinear compensation in the middle and long distance range.

Description

Technical Field

[0001] The embodiments of the present invention relate to the technical field of laser interferometry, and in particular to a high-precision dual-wavelength swept-frequency interferometry ranging method with nonlinear compensation. Background Art

[0002] With the rapid development of high-end manufacturing and large scientific facilities, the demand for large-scale three-dimensional precision measurement in various fields such as aerospace, automobiles, wind and hydropower generation, and the construction of large scientific research facilities has increased significantly. Frequency modulated continuous wave (FMCW) laser ranging technology has the advantages of eliminating distance ambiguity, strong anti-interference ability, high ranging accuracy, and applicability to non-cooperative targets. It has been widely used in the development of coherent detection technology. The principle of the FMCW laser radar system relies on the linear modulation of the laser frequency. The beat signal is generated by interfering with the reference signal with a delayed echo signal, and the beat signal is used to determine the absolute distance of the target. However, since the FMCW ranging method is sensitive to the beat signal frequency, the Doppler shift introduced by micron-level vibration will cause millimeter range errors. In the prior art, the forward and reverse dual-wavelength sweep structure (referred to as FMCW dual-sweep structure) is usually used to eliminate and compensate the Doppler effect in the FMCW ranging system. It achieves real-time and accurate distance measurement by synchronously obtaining two beat waveforms with opposite sweep modulation. Because of its simple structure and real-time elimination of the Doppler effect, it is widely used for vibration compensation in the FMCW ranging system.

[0003] However, the frequency modulation of the laser in the FMCW dual-scan structure is often affected by factors such as temperature and carrier concentration, which leads to the deterioration of its frequency scanning linearity, and ultimately causes the spectrum of the measured interference signal to be broadened, thus severely limiting the accuracy of the ranging results. Usually, nonlinear compensation is mainly carried out through the following methods: ① Optoelectronic phase-locked loop: This method performs phase detection on the acquired interference signal and the standard source to obtain the error signal, and then feeds the error signal back to the laser for real-time compensation of the frequency modulation nonlinearity; however, the optoelectronic phase-locked loop requires a complex control module and complex parameter adjustment, and changes in the external environment will also cause it to lose lock. In addition, it is challenging for fast and large-scale frequency modulation; ② Equal optical frequency interval resampling method: This method usually requires hundreds of meters of auxiliary interferometers to resample the measured interference signal, which places high demands on the sampling rate of the system and the calibration accuracy of the auxiliary interferometer. requirements; in addition, it is worth noting that due to the large difference in arm length between the auxiliary interferometer and the measuring interferometer, the final demodulation result will be affected by residual nonlinearity; ③ Phase comparison method: By comparing the phase of the measuring interferometer and the auxiliary interferometer with a known arm length difference, the ranging result can be obtained, but the phase calculation process of this method is greatly affected by the signal-to-noise ratio, so this method has great limitations for the ranging of non-cooperative targets; ④ Non-uniform Fourier change method, this method is based on the principle of maximum likelihood function, and the orthogonal basis constructed by the auxiliary interferometer is used to perform orthogonal decomposition on the measuring interferometer to finally solve the distance value.

[0004] However, the above method is challenging when facing fast and large-range frequency modulation, and as the ranging range increases, the impact of residual nonlinearity will increase exponentially, resulting in the inability to accurately resolve distance information. Summary of the invention

[0005] In order to solve the problems in the prior art, the present invention provides a high-precision dual-wavelength swept frequency interferometry ranging method with nonlinear compensation. The method obtains the optical frequency variation based on the idea of ​​solving differential equations, and then constructs a high-order orthogonal basis based on the solved optical frequency signal to decompose the measured interference signal, thereby obtaining a better residual nonlinear compensation effect. In addition, the method is not limited by fast and large-scale frequency modulation, and can still obtain a good effect for Doppler effect and nonlinear compensation in medium and long distance ranges.

[0006] See also Figure 1 The embodiment of the present invention provides a high-precision dual-wavelength swept frequency interferometry ranging method with nonlinear compensation, comprising:

[0007] S1. Obtain the measurement interference signals I of two frequency-sweeping laser light sources respectively through the dual frequency-sweeping interferometry absolute distance measurement system m1 (t), I m2 (t) and the auxiliary interference signal I a1 (t), I a2(t);

[0008] S2. Based on the Taylor series model, the differential equations of optical frequency and phase are constructed, and the optical frequency change information ΔΩ(k n );

[0009] S3, according to the optical frequency change information ΔΩ(k n ) construct a high-order orthogonal basis, and use the high-order orthogonal basis to perform orthogonal decomposition on the measured interference signal to obtain distance spectrum information, and solve the distance value to be measured based on the distance spectrum information.

[0010] Optionally, the S2 specifically includes:

[0011] S21, obtaining a multiplied signal I of the measurement interference signal that eliminates the Doppler effect mmix (t) and the multiplication signal I of the auxiliary interference signal amix (k n );

[0012] S22, according to the multiplication signal I amix (k n ) and the multiplied signal I amix (k n ) The phase solution formula obtained by unwrapping φ a (k n ) Construct differential equations for optical frequency and phase;

[0013] S23, using improved Euler iteration to solve the differential equation of the optical frequency and phase to obtain the optical frequency change signal ΔΩ(k n ).

[0014] Optionally, the multiplication signal I of the measured interference signal mmix (t) Specifically:

[0015]

[0016] Among them, A m1 and A m2 are the amplitudes of interference signals measured for the two lasers respectively; Δω1(t) and Δω2(t) are the changes in the angular frequencies of the two lasers respectively; τ m is the group delay difference of the measurement interferometer corresponding to the start of the measurement;

[0017] The multiplication signal I of the auxiliary interference signal amix (k n ) Specifically:

[0018]

[0019] Among them, I a1 (kn ), I a2 (k n ) are respectively the formula I a1 (t) and I a2 The variable t in (t) is converted to the corresponding sampling point k n Get; A a1 , A a2 are the amplitudes of the two laser-assisted interference signals respectively; τ a is the group delay of the auxiliary interferometer;

[0020] The phase solution formula of the auxiliary interferometer is:

[0021] φ a (k n )=unwrap[angle(I amix (k n ))]

[0022] Among them, unwrap() represents the unwrapping function, angle() is the phase function, k n is the nth sampling point.

[0023] Optionally, the differential equations of optical frequency and phase in S22 are specifically:

[0024]

[0025] Among them, Ω(k n ) represents the laser frequency corresponding to each sampling point.

[0026] Optionally, the solution formula of the improved Euler iteration in S23 is:

[0027]

[0028] Among them, Ω p (*) is the predicted value of the optical frequency obtained by the Euler method; h is the time difference between adjacent sampling points.

[0029] Optionally, according to the optical frequency change information ΔΩ(k n ) and I amix (k n ) constructs a high-order orthogonal basis, wherein the high-order orthogonal basis is specifically:

[0030]

[0031] Among them, R m is a series of optical path differences, which can be selected near the optical path difference to be measured, where R m，1 is the minimum value of a series of optical path differences, R m,L is the maximum value of a series of optical path differences, and c is the propagation speed of light.

[0032] Optionally, the distance spectrum information in S3 is:

[0033]

[0034] Where N represents the total number of sampling points of the signal, n air represents the refractive index of air, R d Indicates the distance to be measured. When R m =2n air R d When X(R m ) takes the maximum value, by m ) to find the peak to obtain the distance R to be measured d .

[0035] Optionally, the S1 specifically includes:

[0036] The output lasers of the two lasers first pass through the isolator and are combined through the 50:50 coupler ①, and then are split through the 99:1 coupler ②; one of the beams enters the measuring interferometer and is split through the 99:1 coupler ③, where 99% of the light is focused on the target surface through the focusing system, and 1% of the light is used as the reference light of the measuring interferometer. The light reflected by the target and the reference light are finally combined and interfered through the 50:50 coupler ④; then they are split at different wavelengths through the first wavelength division multiplexer WDM1 and the second wavelength division multiplexer WDM2, and finally the first balanced photodetector BPD1 and the second balanced photodetector BPD2 and the acquisition card are used for photoelectric conversion and data signal acquisition to obtain the measurement interference signal I m1 (t), I m2 (t); Similarly, after being split by the 99:1 coupler ②, another beam of light enters the auxiliary interferometer, and after being split by the 50:50 optical coupler ⑤, it is used as the measurement path and reference path of the auxiliary interferometer respectively, and then passes through the 50:50 coupler ⑥ again for beam combining interference; then it is split at different wavelengths by the third wavelength division multiplexer WDM3 and the fourth wavelength division multiplexer WDM4, and finally the third balanced photodetector BPD3 and the fourth balanced photodetector BPD4 and the acquisition card are used for photoelectric conversion and data signal acquisition to obtain the auxiliary interference signal I a1 (t), I a2 (t).

[0037] The present invention first obtains the measurement interference signal and auxiliary interference signal of two frequency-sweeping laser light sources respectively through a dual frequency-sweeping interferometric absolute distance measurement system, then establishes the differential equation of optical frequency and phase based on the Taylor series model, obtains the change information of optical frequency by solving the differential equation, then constructs a high-order orthogonal basis based on the solved optical frequency signal, decomposes the measurement interference signal, and obtains a better residual nonlinear compensation effect, thereby improving the interference measurement accuracy. In addition, the method is not limited by fast and large-scale frequency modulation, and can still obtain better effects for Doppler effect and nonlinear compensation in medium and long distance ranges. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 A flow chart of a high-precision dual-wavelength swept-frequency interferometry ranging method with nonlinear compensation provided by an embodiment of the present invention;

[0039] Figure 2 A structural diagram of a dual-wavelength swept frequency interferometry absolute distance measurement system provided by an embodiment of the present invention;

[0040] Figure 3 A flow chart of the Euler-NUDFT algorithm based on the FMCW dual-sweep structure provided in an embodiment of the present invention;

[0041] Figure 4 A result diagram of the uncorrected Doppler, first-order NUDFT method and Euler-NUDFT method for solving the range spectrum provided by the embodiment of the present invention;

[0042] Figure 5 This is a diagram of multiple measurement results of the uncorrected Doppler, first-order NUDFT method and Euler-NUDFT method provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0043] The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It is to be understood that the specific embodiments described herein are only used to explain the present invention, rather than to limit the present invention. It should also be noted that, for ease of description, only parts related to the present invention, rather than all structures, are shown in the accompanying drawings.

[0044] Example

[0045] Figure 1 A flowchart of a high-precision dual-wavelength swept frequency interferometry ranging with nonlinear compensation provided by an embodiment of the present invention specifically includes the following steps:

[0046] S1. Obtain the measurement interference signals I of two frequency-sweeping laser light sources respectively through the dual frequency-sweeping interferometry absolute distance measurement system m1 (t), I m2 (t) and the auxiliary interference signal I a1 (t), Ia2 (t);

[0047] S2. Based on the Taylor series model, the differential equations of optical frequency and phase are constructed, and the optical frequency change information ΔΩ(k n );

[0048] S3, according to the optical frequency change information ΔΩ(k n ) construct a high-order orthogonal basis, and use the high-order orthogonal basis to perform orthogonal decomposition on the measured interference signal to obtain distance spectrum information, and solve the distance value to be measured based on the distance spectrum information.

[0049] Specifically, the embodiment of the present invention adopts a dual semiconductor laser frequency scanning interferometer absolute distance measurement system structure, and the system structure diagram is as follows: Figure 2 As shown. Exemplarily, the two distributed feedback lasers DFB1 and DFB2 in the figure are both frequency-tunable semiconductor lasers, with operating wavelengths of 1550nm and 1560nm respectively, and a sweep range of about 1nm. The output lasers of the two lasers first pass through the isolator and are combined through a 50:50 coupler ①, and then are split through a 99:1 coupler ②; one of the beams enters the measurement interferometer and is split through a 99:1 coupler ③, where 99% of the light is focused on the target surface through a focusing system, and 1% of the light is used as the reference light of the measurement interferometer. The light reflected by the target and the reference light are finally combined and interfered through a 50:50 coupler ④; then they are split at different wavelengths through the first wavelength division multiplexer WDM1 and the second wavelength division multiplexer WDM2, and finally undergo photoelectric conversion and data signal acquisition through the first balanced photodetector BPD1 and the second balanced photodetector BPD2 and the acquisition card to obtain the measurement interference signal I m1 (t), I m2 (t); Similarly, after being split by the 99:1 coupler ②, another beam of light enters the auxiliary interferometer, and after being split by the 50:50 optical coupler ⑤, it is used as the measurement path and reference path of the auxiliary interferometer respectively, and then passes through the 50:50 coupler ⑥ again for beam combining interference; then it is split at different wavelengths by the third wavelength division multiplexer WDM3 and the fourth wavelength division multiplexer WDM4, and finally the third balanced photodetector BPD3 and the fourth balanced photodetector BPD4 and the acquisition card DAQ are used for photoelectric conversion and data signal acquisition to obtain the auxiliary interference signal I a1 (t), I a2 (t).

[0050] Among them, DFB1 and DFB2 are used as swept frequency laser sources, and their output optical signals can be expressed as:

[0051]

[0052] Wherein, E1 and E2 represent the amplitudes of the optical signals output by DFB1 and DFB2 respectively. and are the real-time phases of the output optical signals of the DFB1 and DFB2 frequency-sweeping laser sources, respectively. The optical signal frequencies corresponding to DFB1 and DFB2 can be expressed as:

[0053]

[0054] ω1(t) and ω2(t) are the real-time optical frequencies of the optical signals output by the DFB1 and DFB2 frequency sweeping laser sources, respectively. Since the frequency sweeping directions of DFB1 and DFB2 are opposite, the signs of ω1(t) and ω2(t) are opposite.

[0055] Considering that the target will vibrate in a complex industrial measurement environment, the signals of the auxiliary interferometer and the measurement interferometer of DFB1 are expressed as follows:

[0056]

[0057] Among them I a1 and I m1 are the auxiliary interference signal and the measurement interference signal corresponding to the DFB1 laser, with amplitudes of A and a1 and A m1 , τ a is the group delay of the auxiliary interferometer, τ m is the group delay difference of the measuring interferometer corresponding to the start of the measurement, Δτ m (t) is the time variation of the group delay due to target vibration.

[0058] Similarly, for DFB2, the signals of the auxiliary interferometer and the measuring interferometer are expressed as:

[0059]

[0060] Among them I a2 and I m2 are the auxiliary interference signal and the measurement interference signal corresponding to the DFB2 laser, with amplitudes of A and a2 and A m2 .

[0061] For formulas (5), (6), (7), and (8), Performing Taylor expansion, formula (5) can be expressed as:

[0062]

[0063] Similarly, formulas (6), (7), and (8) can be expressed as:

[0064]

[0065]

[0066] It can be seen from formulas (9) to (12) that if the laser frequency is modulated linearly in time, the interference signal is a standard cosine signal that varies with time, and the time domain signal can be directly Fourier transformed to determine the group delay. However, in practical applications, it is a challenge to achieve complete linear modulation of the laser source frequency, so the nonlinear effect needs to be corrected. Through Taylor expansion, it is known that the high-order terms introduced by the frequency sweep nonlinearity have little effect, so the demodulation model of the traditional method usually only considers the first order of Taylor expansion, from which it can be approximately concluded that the phase of the interference signal is linearly related to the optical frequency. Therefore, first calibrate the delay difference τ of the auxiliary interferometer a , and then demodulate to obtain the linear multiple relationship of the phase or frequency information of the auxiliary interferometer and the measuring interferometer, and further solve to obtain the measured distance.

[0067] However, in the actual measurement process, the arm length difference of the auxiliary interferometer is constant. As the delay difference τ of the measuring interferometer arm length increases, m As the measured distance increases, ignoring the higher-order terms will lead to larger errors, that is, the influence of residual nonlinearity will gradually increase, further deteriorating the distance measurement results. Therefore, it is necessary to retain the second order of Taylor expansion, but due to the third order τ m The cubic magnitude of is extremely small and can be completely ignored for medium and long range measurements.

[0068] In view of the above problems, the present invention proposes a high-precision distance measurement method with nonlinear compensation. The method first constructs a differential equation based on the Taylor series model, and further obtains the change information of the optical frequency by solving it through the Euler method. Then, a high-order Fourier transform (abbreviated as: Euler-NUDFT) is performed on the optical frequency domain of the measuring interferometer to achieve higher distance measurement accuracy. This method can effectively reduce nonlinear errors, thereby providing more accurate measurement results.

[0069] For the measured interferometer signal I corresponding to DFB1 in formula (10), m1 (t), where the first term contains information about the target distance; the second term is related to the Doppler effect, where the angular frequency ω of the laser 01 is about 2π×200THz, which is much larger than the change in angular frequency Δω1(t). Therefore, the second term is the group delay Δτ mThe change of (t) is very small, but it cannot be ignored, indicating that the Doppler effect deteriorates the measurement accuracy; the third term is high-order nonlinear information, which is used to correct residual nonlinearity and cannot be ignored; the fourth and fifth terms are extremely small in magnitude and can be ignored; the sixth term is a constant term and can be ignored. Therefore, formula (10) can be expressed as:

[0070]

[0071] Similarly, formula (12) can be expressed as:

[0072]

[0073] Since the signs of Δω1(t) and Δω2(t) are opposite, multiplying the measured interference signals corresponding to the two DFB lasers can eliminate the Doppler effect and obtain the distance information of the final target. The formula is expressed as:

[0074]

[0075] Since Δω1 and Δω2 are unknown and have nonlinear characteristics, the signal of the auxiliary interferometer is needed to correct the frequency modulation nonlinearity. Next, the double-scan data is processed based on the proposed Euler-NUDFT algorithm to obtain more accurate ranging results.

[0076] First, the two auxiliary signals are multiplied, and then the multiplied signal is unwrapped to obtain its phase, let: ΔΩ(t) = Δω1(t)-Δω2(t), and t is converted to the corresponding sampling point k n (k1<k n <k N ), as shown in formulas (16) and (17):

[0077]

[0078] φ a (k n )=unwrap[angle(I amix (k n ))] (17)

[0079] Among them, unwrap() represents the unwrapping function, and angle() is the phase function;

[0080] Then, the differential equations of optical frequency and phase are constructed based on equations (16) and (17):

[0081]

[0082] The improved Euler iteration is used to solve the differential equation and obtain the optical frequency signal ΔΩ(k n), the specific solution formula is:

[0083]

[0084] The optical frequency variation information ΔΩ(k n ) and combined with formula (16) to form a high-order orthogonal basis, namely:

[0085]

[0086] Ω(k n ) relative to k n It changes nonlinearly, which is caused by the nonlinearity of laser frequency modulation. air represents the refractive index of air, R d represents the distance to be measured, τ m Represents the group delay difference of the measurement interferometer.

[0087] Where R m is a series of optical path differences, which can be selected near the optical path difference to be measured. Therefore, the above-mentioned orthogonal basis can be used to decompose the interferometer signal in the optical frequency domain to obtain the final high-precision distance spectrum information, which is:

[0088]

[0089] Where N represents the total number of signal points, and finally X(R m ) is a standard sinc function. m =2n air R d When X(R m ) takes the maximum value. Therefore, for the distance spectrum X(R m ) Find the peak and you can get the distance R to be measured d , eliminating the influence of frequency modulation nonlinearity and residual nonlinearity.

[0090] Different from the traditional nonlinear compensation algorithm, the Euler-NUDFT method proposed in this invention establishes a high-order model of the beat frequency signal and the optical frequency based on the FMCW double-scan structure, and uses the obtained optical frequency signal to construct a high-order orthogonal basis to further orthogonally decompose the measured interference signal, which greatly reduces the influence of residual nonlinearity and finally obtains an ultra-high-precision ranging result. For further reference Figure 3 , Figure 3 This is a flow chart of the Euler-NUDFT method based on the FMCW double-sweep structure.

[0091] Experimental verification

[0092] The Euler-NUDFT based on solving differential equations proposed in the present invention is compared with the first-order NUDFT used in the prior art.

[0093] Specifically, the arm length difference of the auxiliary interferometer is set to 1m, the arm length difference of the measuring interferometer is set to 50m (corresponding to a measuring distance of 25m), and the vibration frequency and vibration amplitude of the target are 5KHz and 1μm respectively. Figure 4 It can be seen that the distance spectrum leakage of the uncorrected Doppler effect is extremely serious, and the target distance information can no longer be accurately extracted; using the first-order NUDFT method, due to the influence of residual nonlinearity, the distance value obtained by the solution is 25.000433m, and the deviation from the true value of 25m is 433μm; the final distance value obtained by Euler-NUDFT is 24.999980m, and the deviation from the true value is only 20μm. This result shows that the use of Euler-NUDFT can have a better compensation effect on nonlinearity to obtain a more accurate distance value. In order to verify the repeatability of the present invention, 50 multiple measurements were performed, see Figure 5 , the average and standard deviation of multiple measurements of the first-order NUDFT are 25.000430m and 3.44μm respectively; the average and standard deviation of multiple measurements of Euler-NUDFT are 24.999980m and 2.25μm respectively. In summary, based on the FMCW dual-scan ranging structure, the Euler-NUDFT method can obtain more accurate and higher precision measurement results than the first-order NUDFT.

[0094] Note that the above are only preferred embodiments of the present invention and the technical principles used. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described herein, and that various obvious changes, readjustments and substitutions can be made by those skilled in the art without departing from the scope of protection of the present invention. Therefore, although the present invention has been described in more detail through the above embodiments, the present invention is not limited to the above embodiments, and may include more other equivalent embodiments without departing from the concept of the present invention, and the scope of the present invention is determined by the scope of the appended claims.

Claims

1. A high-precision dual-wavelength swept frequency interferometry ranging method with nonlinear compensation, characterized in that: include: S1. Obtain the measurement interference signals I of two frequency-sweeping laser light sources respectively through the dual frequency-sweeping interferometry absolute distance measurement system m1 (t), I m2 (t) and the auxiliary interference signal I a1 (t), I a2 (t); S2. Based on the Taylor series model, the differential equations of optical frequency and phase are constructed, and the optical frequency change information ΔΩ(k n ); S3, according to the optical frequency change information ΔΩ(k n ) construct a high-order orthogonal basis, and use the high-order orthogonal basis to perform orthogonal decomposition on the measured interference signal to obtain distance spectrum information, and solve the distance value to be measured based on the distance spectrum information.

2. The method according to claim 1, characterized in that The S2 specifically includes: S21, obtaining a multiplied signal I of the measurement interference signal that eliminates the Doppler effect mmix (t) and the multiplication signal I of the auxiliary interference signal amix (k n ); S22, according to the multiplication signal I amix (k n ) and the multiplied signal I amix (k n ) The phase solution formula obtained by unwrapping φ a (k n ) Construct differential equations for optical frequency and phase; S23, using the improved Euler method to iteratively solve the differential equation of the optical frequency and phase to obtain the optical frequency change signal ΔΩ(k n ).

3. The method according to claim 2, characterized in that The multiplied signal I of the measured interference signal mmix (t) Specifically: Among them, A m1 and A m2 are the amplitudes of interference signals measured for the two lasers respectively; Δω1(t) and Δω2(t) are the changes in the angular frequencies of the two lasers respectively; τ m is the group delay difference of the measurement interferometer corresponding to the start of the measurement; The multiplication signal I of the auxiliary interference signal amix (k n ) Specifically: Among them, I a1 (k n ), I a2 (k n ) are respectively the formula I a1 (t) and I a2 The variable t in (t) is converted to the corresponding sampling point k n Get; A a1 , A a2 are the amplitudes of the two laser-assisted interference signals respectively; τ a is the group delay of the auxiliary interferometer; The phase solution formula of the auxiliary interferometer is: φ a (k n )=unwrap[angle)I amix (k n )] Among them, unwrap() represents the unwrapping function, angle() is the phase function, k n is the nth sampling point.

4. The method according to claim 3, characterized in that The differential equations of optical frequency and phase in S22 are specifically: Among them, Ω(k n ) represents the laser frequency corresponding to each sampling point.

5. The method according to claim 4, characterized in that The solution formula of the improved Euler iteration in S23 is: Among them, Ω p (*) is the predicted value of the optical frequency obtained by the Euler method; h is the time difference between adjacent sampling points.

6. The method according to claim 5, characterized in that According to the optical frequency variation information ΔΩ(k n ) and I amix (k n ) constructs a high-order orthogonal basis, wherein the high-order orthogonal basis is specifically: Among them, R m is a series of optical path differences, which can be selected near the optical path difference to be measured, where R m，1 is the minimum value of a series of optical path differences, R m,L is the maximum value of a series of optical path differences, c is the propagation speed of light, and m and L are variables.

7. The method according to claim 6, characterized in that The distance spectrum information in S3 is: Where N represents the total number of sampling points of the signal, n air represents the refractive index of air, R d Indicates the distance to be measured. When R m =2n air R d When X(R m ) takes the maximum value, by the distance spectrum X)R m _Search for the peak to get the distance R to be measured d .

8. The method according to claim 1, characterized in that The S1 specifically includes: The output lasers of the two lasers first pass through the isolator and are combined through the 50:50 coupler ①, and then are split through the 99:1 coupler ②; one of the beams enters the measuring interferometer and is split through the 99:1 coupler ③, where 99% of the light is focused on the target surface through the focusing system, and 1% of the light is used as the reference light of the measuring interferometer. The light reflected by the target and the reference light are finally combined and interfered through the 50:50 coupler ④; then they are split at different wavelengths through the first wavelength division multiplexer WDM1 and the second wavelength division multiplexer WDM2, and finally the first balanced photodetector BPD1 and the second balanced photodetector BPD2 and the acquisition card are used for photoelectric conversion and data signal acquisition to obtain the measurement interference signal I m1 (t), I m2 (t); Similarly, after being split by the 99:1 coupler ②, another beam of light enters the auxiliary interferometer, and after being split by the 50:50 optical coupler ⑤, it is used as the measurement path and reference path of the auxiliary interferometer respectively, and then passes through the 50:50 coupler ⑥ again for beam combining interference; then it is split at different wavelengths by the third wavelength division multiplexer WDM3 and the fourth wavelength division multiplexer WDM4, and finally the third balanced photodetector BPD3 and the fourth balanced photodetector BPD4 and the acquisition card are used for photoelectric conversion and data signal acquisition to obtain the auxiliary interference signal I a1 (t), I a2 (t).

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