A method for controlling intensity of laser self-mixing interference optical feedback based on time-frequency spectrum processing

Abstract

The application discloses a kind of based on time-frequency spectrum processing laser self-mixing interference optical feedback intensity control method, comprising the following steps: S1, to self-mixing interference signal using short-time Fourier transform, obtain the numerical matrix of two-dimensional signal time-frequency spectrum;S2, modulus value operation is carried out to numerical matrix, and modulus value is normalized to obtain normalized matrix;S3, the normalized matrix is multiplied to obtain weight matrix multiple times operation, weight matrix is used as the weight between each time-frequency point;S4, weight matrix is multiplied with the numerical matrix of signal time-frequency spectrum, for retaining the main frequency part of signal each time, inhibit harmonic component introduced by optical feedback intensity factor;S5, the matrix obtained after weight matrix and numerical matrix are multiplied is using short-time inverse Fourier transform, obtain time-domain signal, realize algebraic control self-mixing interference signal's optical feedback intensity;The method can reduce the influence of optical feedback intensity on self-mixing interference signal, enhance the measurement stability of self-mixing interferometer.

Description

Technical Field

[0001] This invention relates to the field of optical measurement technology, and specifically to a method for controlling the intensity of laser self-mixing interference light feedback based on time-spectrum processing. Background Technology

[0002] With advancements in wind turbine technology and increasing energy efficiency requirements, long blades and tall towers have become the development direction for wind turbine units. During wind turbine operation, the vibration displacement of the blades reflects their health status, while real-time monitoring of the clearance distance can effectively prevent collisions between the blades and the turbine casing. Therefore, accurate measurement of the vibration displacement and distance of wind turbine blades is of great significance for the healthy operation of wind turbines. Complex geographical environments at wind turbine sites, complex wind conditions, or complex meteorological conditions such as cold waves and typhoons, as well as changes in the surface reflection of the blades, can directly affect the optical feedback intensity factor (C value) of the system parameter in the laser self-mixed interferometry system, making it impossible to accurately measure the vibration displacement of the blades and the absolute distance between the blades and the tower during wind turbine power generation.

[0003] In laser self-mixing interferometry systems, the optical feedback intensity factor (C value) is a very important parameter that affects the laser intensity noise, linewidth broadening, and laser linewidth to varying degrees. It plays a particularly important role in signal displacement reconstruction in self-mixing interferometry systems.

[0004] When using the principle of laser self-mixing interferometry for measurement, the laser focal point changes during the detection process due to the roughness of some target objects and the influence of fluctuations in the measurement environment, causing changes in the optical feedback intensity factor (C value) and making it difficult to calculate accurately. High-precision displacement reconstruction algorithms require the use of non-optical feedback interferometric phase, but this value is affected by the optical feedback intensity factor (C value), which leads to a decrease in measurement accuracy.

[0005] Different feedback levels, i.e., different self-mixing waveforms, require different signal processing methods to obtain the target's displacement information. However, measuring the optical feedback intensity factor (C value) during actual measurement is very time-consuming, and it is difficult to calculate the optical feedback intensity factor (C value) of the self-mixing signal at each moment. Furthermore, maintaining a constant optical feedback intensity during measurement is extremely difficult. Strict control of the optical feedback intensity is required, and there are stringent requirements regarding the surface roughness of the measured target and the measurement environment; this is even more challenging for non-cooperative targets. Summary of the Invention

[0006] The purpose of this invention is to provide a laser self-mixing interferometer optical feedback intensity control method based on time-frequency spectrum processing. This method solves the problem of reduced accuracy caused by the influence of optical feedback intensity factor, reduces the influence of optical feedback intensity on the self-mixing interferometer signal, and enhances the measurement stability of the self-mixing interferometer by processing the signal in the frequency domain.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A laser self-mixing interferometry light feedback intensity control method based on time-spectrum processing includes the following steps:

[0009] S1. Use the short-time Fourier transform on the self-mixed interference signal to obtain the numerical matrix of the two-dimensional signal time spectrum;

[0010] S2. Perform modulus calculation on the numerical matrix and normalize the modulus to obtain the normalized matrix;

[0011] S3. Perform multiple exponentiation operations on the normalized matrix to obtain the weight matrix, and use the weight matrix as the weight between each time frequency point.

[0012] S4. Multiply the weight matrix with the numerical matrix of the signal time spectrum to retain the main frequency part of the signal at each time moment and suppress the harmonic components introduced by the optical feedback intensity factor.

[0013] S5. The matrix obtained by multiplying the weight matrix and the numerical matrix is ​​subjected to short-time inverse Fourier transform to obtain the time-domain signal, thereby realizing the algebraic control of the optical feedback intensity of the self-mixed interference signal.

[0014] Preferably, the specific process of step S1 is as follows:

[0015] S11. In a self-mixed interferometric system, the optical feedback interference phase and the non-optical feedback interference phase, i.e., the optical phase change caused by the motion of the object, satisfy the phase equation:

[0016] φ F =φ0-C·sin(φ F +arctanα) (1)

[0017] Signal power equation:

[0018] P(t)=cos(φ F (2)

[0019] Where, φ F φ is the phase of the optical feedback interference signal; φ0 is the phase without optical feedback; C is the optical feedback intensity factor; α is the laser linewidth broadening factor, and α is a constant; P(t) is the self-mixing interference signal;

[0020] S12. Using optical phase-based displacement reconstruction without optical feedback:

[0021] L=λ*φ0 / (4*π)(3)

[0022] Where L is the external target displacement, λ is the laser wavelength, and φ0 is the phase without optical feedback; the effect of the latter term in formula (1) is eliminated by suppressing the optical feedback intensity factor to tend to 0.

[0023] S13. Combining formula (1), the normalized SMI time-domain signal is expressed as: P(t)=cos(φ0-C·sin(φ0-C· ... F Applying +arctanα)) to its Bessel series expansion and then taking the short-time Fourier transform, the Bessel spectral expansion of the SMI signal is obtained as follows:

[0024]

[0025] Where J0(C) is a 0th-order Bessel function of the first kind, J 2n (C) is a Bessel function of the even order of the first kind, J 2n+1 (C) is an odd-order Bessel function of the first kind, C is the optical feedback intensity factor, n is a non-negative integer, δ() represents the unit impulse function, f is the signal frequency, and f0 is the fundamental frequency.

[0026] S14. Using the trigonometric function Bessel spectrum expansion, obtain each frequency component, and transform a class of even and odd-order Bessel functions into a numerical matrix A.

[0027] Preferably, the specific process of step S2 is as follows: the modulus of the numerical matrix A obtained by the even and odd order Bessel functions of a class is calculated to obtain matrix B. After normalization, a normalized matrix M is obtained, where the normalized matrix M serves as the weight between each time frequency point.

[0028] By adopting the above technical solution, the present invention has the following beneficial effects:

[0029] 1. This invention solves the problem of reduced accuracy caused by the influence of optical feedback intensity factor, reduces the influence of optical feedback intensity on self-mixing interference signal, and enhances the measurement stability of self-mixing interferometer by frequency domain signal processing.

[0030] 2. This invention controls the C value of the self-mixed interference signal to an extremely low level, thereby greatly suppressing the displacement reconstruction error introduced by the C value in the phase unwrapping method without performing C value calculation.

[0031] 3. This invention reduces the optical feedback intensity factor from algebraically to near zero, changes the fringe tilt of the self-mixed interference signal, and eliminates the fringe tilt, resulting in a stable self-mixed interference signal with low noise. This solves the problem of the optical feedback intensity factor of the self-mixed interference signal fluctuating over time and being difficult to calculate, which will facilitate subsequent signal processing and improve measurement accuracy. Attached Figure Description

[0032] Figure 1 This is a flowchart of the present invention;

[0033] Figure 2 This is an experimental signal diagram of SMI under weak and moderate feedback levels according to the present invention;

[0034] Figure 3 This is the time-frequency spectrum of the present invention under weak and moderate feedback levels;

[0035] Figure 4 The time-frequency spectrum diagram of the present invention, where the optical feedback intensity factor is approximately zero;

[0036] Figure 5 This is a diagram of the SMI signal with an optical feedback intensity factor of approximately zero, as presented in this invention. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0038] like Figures 1 to 5 As shown, a laser self-mixing interferometry light feedback intensity control method based on time-spectrum processing includes the following steps:

[0039] S1. Use the short-time Fourier transform on the self-mixed interference signal to obtain the numerical matrix of the two-dimensional signal time spectrum;

[0040] The specific process of step S1 is as follows:

[0041] S11. In a self-mixed interferometric system, the optical feedback interference phase and the non-optical feedback interference phase, i.e., the optical phase change caused by the motion of the object, satisfy the phase equation:

[0042] φ F =φ0-C·sin(φ F +arctanα) (1)

[0043] Signal power equation:

[0044] P(t)=cos(φ F (2)

[0045] Where, φ F φ is the phase of the optical feedback interference signal; φ0 is the phase without optical feedback; C is the optical feedback intensity factor; α is the laser linewidth broadening factor, and α is a constant; P(t) is the self-mixing interference signal;

[0046] S12. Using optical phase-based displacement reconstruction without optical feedback:

[0047] L=λ*φ0 / (4*π) (3)

[0048] Where L is the external target displacement, λ is the laser wavelength, and φ0 is the phase without optical feedback; the effect of the latter term in formula (1) is eliminated by suppressing the optical feedback intensity factor to tend to 0.

[0049] S13. Combining formula (1), the normalized SMI time-domain signal is expressed as: P(t)=cos(φ0-C·sin(φ0-C· ... F Applying +arctanα)) to its Bessel series expansion and then taking the short-time Fourier transform, the Bessel spectral expansion of the SMI signal is obtained as follows:

[0050]

[0051] Where J0(C) is a 0th-order Bessel function of the first kind, J 2n (C) is a Bessel function of the even order of the first kind, J 2n+1 (C) is an odd-order Bessel function of the first kind, C is the optical feedback intensity factor, n is a non-negative integer, δ() represents the unit impulse function, f is the signal frequency, and f0 is the fundamental frequency.

[0052] S14. Using the trigonometric function Bessel spectrum expansion, obtain each frequency component, and transform a class of even and odd-order Bessel functions into a numerical matrix A.

[0053] S2. Perform modulus calculation on the numerical matrix and normalize the modulus to obtain the normalized matrix;

[0054] The specific process of step S2 is as follows: calculate the modulus of the numerical matrix A obtained by the even and odd order Bessel functions of a class of functions to obtain matrix B. After normalization, obtain normalized matrix M, where normalized matrix M serves as the weight between each time frequency point.

[0055] S3. Perform multiple exponentiation operations on the normalized matrix to obtain the weight matrix, and use the weight matrix as the weight between each time frequency point.

[0056] The weight matrix obtained after step S3 will retain the part with the highest energy, while the others will become smaller and smaller.

[0057] S4. Multiply the weight matrix with the numerical matrix of the signal time spectrum to retain the main frequency part of the signal at each time moment and suppress the harmonic components introduced by the optical feedback intensity factor.

[0058] After the processing in step S4, the main frequency part of the original mixed interference signal is retained, and the energy of the remaining harmonic components is suppressed.

[0059] S5. The matrix obtained by multiplying the weight matrix and the numerical matrix is ​​subjected to short-time Fourier inverse transform to obtain the time-domain signal, thereby realizing the algebraic control of the optical feedback intensity of the self-mixed interference signal.

[0060] At this point, the fringe tilt characteristic introduced by the optical feedback intensity factor (C value) in the time domain signal is effectively removed, proving that the C value is effectively suppressed.

[0061] This invention provides a laser self-mixing interference optical feedback intensity control method based on time-spectrum processing. This method achieves the optical feedback intensity of the algebraically controlled signal by controlling the energy of harmonics. The principle is mainly based on:

[0062] After performing a short-time Fourier transform on the signal, we obtain the spectral expression of the signal's Bessel expansion:

[0063] F(f)=cos(φ0)J0(C)δ(f)

[0064]

[0065] Where: J0(C) is a 0th-order Bessel function of the first kind, J 2n (C) is a Bessel function of the even order of the first kind, J 2n+1 (C) is an odd-order Bessel function of the first kind, C is the optical feedback intensity factor, n is a non-negative integer, δ() represents the unit impulse function, f is the signal frequency, and f0 is the fundamental frequency.

[0066] Considering only the DC component and the component with positive frequency f, F(f) is:

[0067]

[0068] The magnitudes of each frequency component of the signal are:

[0069]

[0070] Combining formulas (4) and (5), it can be seen that the frequencies of cos(φ0)δ(f), cos(φ0)δ(f-2nf0), and sin(φ0)δ[f-(2n+1)f0] change due to the influence of the subsequent factor δ.

[0071] The frequency components of the signal change, manifesting as higher harmonics of cos(φ0).

[0072] If we process and retain the dominant frequency component while removing the harmonic components introduced by the C value, we can directly obtain cos(φ). F The change from φ to cos(φ0) is equivalent to the algebraic value of C becoming zero.

[0073] By processing and preserving the dominant frequency, removing the energy of harmonic components, retaining the highest energy portion, and suppressing the remaining higher-order harmonic energies, the feedback level of the algebraic control signal can be achieved by controlling the harmonic energy.

[0074] Case Analysis

[0075] A vibration measurement example based on laser self-mixing interferometry with time-spectrum processing was conducted here. The results collected in the experiment were as follows: Figure 2 The weak feedback vibration signal shown in (a) and Figure 2 The self-mixing signal at a moderate feedback level is shown in (b). It can be seen from... Figure 2 The results show that the experimentally acquired SMI signal is fluctuating. Furthermore, the self-mixed signal under moderate feedback is a sawtooth-like waveform with increased fringe tilt. Next, by selecting an appropriate window function length and performing a short-time Fourier transform (STFT), the window function slides across the entire signal range with a certain step size. The transformed results of the self-mixed signals under weak and moderate feedback are as follows: Figure 3 (a) and (b) are given respectively. Figure 3 The time-spectrum diagrams are presented for weak and moderate feedback levels. These diagrams include not only the energy of the dominant frequency but also the energy of harmonic components, yielding the corresponding numerical matrix A. By repeatedly exponentiating and normalizing the magnitude matrix of numerical matrix A, a weighting matrix representing the adjusted time-spectrum energy is obtained. Multiplying this weighting matrix by the numerical matrix A removes the harmonic components. The results are then obtained from... Figure 4 (a) and Figure 4 The time spectrum at this point only retains the frequency component of the dominant frequency. An inverse transform of this spectrum into the time domain yields the following result: Figure 5 The results are given in (a) and (b). At this point, the process of... Figure 2 The weak feedback strength and moderate feedback strength factors in (a) and (b) decrease algebraically to near zero.

[0076] pass Figure 2 and Figure 5 The comparison of self-mixed interference signals shows that reducing the optical feedback intensity factor from algebraically to near zero changes the fringe tilt of the self-mixed interference signal, and the fringe tilt disappears. Moreover, the resulting self-mixed interference signal is stable and has less noise. This solves the problem of the optical feedback intensity factor of the self-mixed interference signal fluctuating over time and being difficult to calculate, which will facilitate subsequent signal processing and improve measurement accuracy.

[0077] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A laser self-mixing interferometry light feedback intensity control method based on time-spectrum processing, characterized in that, Includes the following steps: S1. Use the short-time Fourier transform on the self-mixed interference signal to obtain the numerical matrix of the two-dimensional signal time spectrum; S2. Perform modulus calculation on the numerical matrix and normalize the modulus to obtain the normalized matrix; S3. Perform multiple exponentiation operations on the normalized matrix to obtain the weight matrix, and use the weight matrix as the weight between each time frequency point. S4. Multiply the weight matrix with the numerical matrix of the signal time spectrum to retain the main frequency part of the signal at each time moment and suppress the harmonic components introduced by the optical feedback intensity factor. S5. The matrix obtained by multiplying the weight matrix and the numerical matrix is ​​subjected to short-time inverse Fourier transform to obtain the time-domain signal, thereby realizing the algebraic control of the optical feedback intensity of the self-mixed interference signal.

2. The laser self-mixing interferometry light feedback intensity control method based on time-spectrum processing as described in claim 1, characterized in that, The specific process of step S1 is as follows: S11. In a self-mixed interferometric system, the optical feedback interference phase and the non-optical feedback interference phase, i.e., the optical phase change caused by the motion of the object, satisfy the phase equation: f F =φ0-C·sin(φ F +arctanα) (1) Signal power equation: P(t)=cos(φ F ) (2) Where, φ F φ is the phase of the optical feedback interference signal; φ0 is the phase without optical feedback; C is the optical feedback intensity factor; α is the laser linewidth broadening factor, and α is a constant; P(t) is the self-mixing interference signal; S12. Using optical phase-based displacement reconstruction without optical feedback: L=λ*φ0 / (4*π) (3) Where L is the external target displacement, λ is the laser wavelength, and φ0 is the phase without optical feedback; the effect of the latter term in formula (1) is eliminated by suppressing the optical feedback intensity factor to tend to 0. S13. Combining formula (1), the normalized SMI time-domain signal is expressed as: P(t)=cos(φ0-C·sin(φ0-C· ... F Applying +arctanα)) to its Bessel series expansion and then taking the short-time Fourier transform, the Bessel spectral expansion of the SMI signal is obtained as follows: Where J0(C) is a 0th-order Bessel function of the first kind, J 2n (C) is a Bessel function of the even order of the first kind, J 2n+1 (C) is an odd-order Bessel function of the first kind, C is the optical feedback intensity factor, n is a non-negative integer, δ() represents the unit impulse function, f is the signal frequency, and f0 is the fundamental frequency. S14. Using the trigonometric function Bessel spectrum expansion, obtain each frequency component, and transform a class of even and odd-order Bessel functions into a numerical matrix A.

3. The laser self-mixing interference light feedback intensity control method based on time-spectrum processing as described in claim 2, characterized in that, The specific process of step S2 is as follows: the modulus of the numerical matrix A obtained by the even and odd order Bessel functions of a class is calculated to obtain matrix B. After normalization, the normalized matrix M is obtained, where the normalized matrix M serves as the weight between each time frequency point.

Images