Method for eliminating focusing laser differential interference measurement refraction integral effect

Abstract

The invention discloses a method for eliminating focused laser differential interference to measure refraction integral effect, and belongs to the technical field of optical measurement. In order to solve the problem of measurement distortion caused by mixing of intensity modulation and interference phase modulation caused by refraction in a traditional FLDI, an imaging module and a signal processing module are additionally arranged on the basis of a traditional FLDI optical system, and an interference fringe dynamic video is recorded through imaging equipment; synchronously extracting symmetrical region brightness time sequences (t) and t (t) and a sub-pixel-level stripe transverse displacement time sequence X (t); the refraction interference state is identified through phase analysis, the signal purity is verified in combination with spectral analysis, and finally the density disturbance of the field to be measured is reversely deduced through the Gladstone-Dale relation. According to the invention, the source separation of two signals is realized, the quantitative measurement precision is obviously improved, the application range of the FLDI in a complex flow field is expanded, the hardware transformation cost is low, and the engineering popularization value is high.

Description

Technical Field

[0001] This invention relates to the field of optical measurement, specifically to a method for eliminating the refraction integral effect in focused laser differential interferometry. Background Technology

[0002] Focused laser differential interferometry (FLDI) is a high spatiotemporal resolution, non-contact optical measurement technique that inverts local density variations in a flow field by measuring the optical path difference between two closely spaced probe beams. Traditional FLDI technology is based on the fundamental assumption that the measurement signal originates solely from the interference phase change caused by the optical path difference within the overlapping "sensitive region" of the two beams. However, in practical applications, any refractive index gradient in the beam path (e.g., due to inhomogeneities in temperature or pressure fields) will cause beam deflection (refraction integral effect), resulting in changes in the intensity of the light received by the detector. This intensity modulation caused by refraction and phase modulation caused by interference become intertwined, leading to measurement signal distortion and severely affecting the accuracy of quantitative measurements in complex flow fields using FLDI. Existing FLDI technologies lack effective means to separate these two signals with different physical origins. Summary of the Invention

[0003] The purpose of this invention is to provide a method for eliminating the refraction integral effect in focused laser differential interferometry, thereby solving the problem in the background art where intensity modulation caused by refraction and phase modulation caused by interference are mixed together, resulting in measurement signal distortion.

[0004] To achieve the above objectives, the present invention provides the following technical solution: a method for eliminating the refraction integration effect in focused laser differential interferometry, comprising the following steps:

[0005] S1 Data Acquisition: Data acquisition: The dynamic changes of the interference fringes generated by the focused laser differential interferometry (FLDI) system under the action of the test field are recorded using imaging equipment to obtain the time series of the fringe images;

[0006] S2 signal feature extraction:

[0007] a. Brightness signal extraction: Define fixed regions of interest on both sides of the central fringe in the fringe image, and extract the average brightness values ​​of the two regions respectively to obtain two brightness time series. (t) and t (t);

[0008] b. Displacement signal extraction: For each frame of stripe image, a sub-pixel level signal processing algorithm is used to calculate the lateral position of the center stripe to obtain the lateral displacement time series X(t) of the stripe.

[0009] S3 signal decomposition and identification:

[0010] a. Phase Relationship Analysis: Analyzing the brightness time series (t) and t The phase relationship of (t) is used to determine the existence state of the refraction integral effect in the signal;

[0011] b. Spectrum analysis: Perform spectrum analysis on the displacement time series X(t) to verify the purity of the interference signal.

[0012] Preferably, in step S1, the imaging device is a CMOS camera or a CCD camera, and the frame rate of the imaging device is higher than the characteristic frequency of the field to be measured.

[0013] Preferably, the sub-pixel level signal processing algorithm in step S2b is selected from any one of the following: sub-pixel edge localization algorithm, centroid method, per-row pixel extreme point fitting method, or gradient method.

[0014] Preferably, the criterion for judging the phase relationship analysis in step S3a is: if (t) and t If (t) are out of phase, it indicates that the signal is dominated by fringe displacement caused by interference; if they are in phase, it indicates that there is a significant overall brightness modulation caused by the refraction integral effect.

[0015] Preferably, the verification criterion for the spectrum analysis in step S3b is: the pure interference signal exhibits a significant spectral peak at the target frequency of the field to be measured.

[0016] Preferably, the method further includes a signal inversion step: converting the identified pure displacement time series X(t) into optical path difference ΔOPL(t) based on the system calibration coefficient, and then calculating the density perturbation Δρ(t) of the field to be measured using the Gladstone-Dale relation.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0018] 1. Enhanced anti-interference capability: This invention can effectively separate refraction interference from the mixed original signal, and the extracted displacement signal has a high signal-to-noise ratio at the target frequency, while the refraction interference is suppressed to a negligible level.

[0019] 2. High application value: This method provides a universal solution to the refraction interference problem that is common in FLDI and other interferometric measurement techniques in complex flow fields. It does not require expensive hardware upgrades and can significantly improve data quality through algorithm processing. It has high engineering application value and great prospects for promotion. Attached Figure Description

[0020] Figure 1 A schematic comparison of a traditional FLDI system and the anti-reflection FLDI system of this invention;

[0021] Figure 2 A comparison diagram of typical image features of interference effect (fringe displacement) and refraction integral effect (brightness modulation);

[0022] Figure 3 This is a comparison chart of the measurement results along the acoustic standing wave axis and the theoretical values ​​before and after using the method of the present invention. Detailed Implementation

[0023] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] Taking the measurement of ultrasonic standing wave field in water as an example, the specific implementation method is as follows:

[0025] 1. Set up the FLDI optical path: Set up a standard FLDI optical system, including a He-Ne laser, beam expander, Wollaston prism, focusing lens, etc.

[0026] 2. Introduce an imaging module: Replace the photodetector in the traditional FLDI system with a diffuse reflection screen. Use a high-speed CMOS or CCD camera aligned with this screen to record the dynamic changes in the interference fringes. Ensure the camera frame rate is much higher than the frequency of the sound wave being measured (e.g., above 100kHz).

[0027] 3. Data Acquisition: Excite a 20kHz ultrasonic standing wave. Control a high-speed camera to record a video of the stripe changes over a period of time.

[0028] 4. Signal Processing:

[0029] 1) Import the video frame by frame into your computer.

[0030] 2) Write or use an image processing program to perform the following operations on each frame of the image:

[0031] a. Define two symmetrical regions of interest, one on the left and one on the right.

[0032] b. Calculate the average grayscale value of the two regions and store it. (t) and t (t).

[0033] c. Fit the sub-pixel position X(t) of the center stripe using any one of the following methods: sub-pixel edge localization algorithm, centroid method, per-row pixel extreme point fitting method, or gradient method.

[0034] 5. Signal Analysis and Inversion:

[0035] 1) Drawing (t) and t By observing the phase relationship of the time series plot of (t), the existence of the refraction integral effect can be confirmed.

[0036] 2) Perform a fast Fourier transform on X(t), and a clear spectral peak should be observed at 20kHz.

[0037] 3) Based on the system calibration coefficients, X(t) is converted into optical path difference ΔOPL(t), and then the density perturbation Δρ(t) is calculated using the Gladstone-Dale relationship.

[0038] Verification: such as Figure 3 As shown, by moving the measurement point axially, the entire sound pressure distribution is plotted and compared with the theoretical simulation results. Judging from the curve trend, the two curves should be highly coincident with minimal deviation. This indicates that the measurement data obtained by the method of this patent is almost consistent with the "ideal interference-free theoretical result", and the measurement results are accurate and reliable.

[0039] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for eliminating the refraction integration effect in focused laser differential interferometry, characterized in that, Includes the following steps: S1 Data Acquisition: An imaging device is used to record the dynamic changes of the interference fringes generated by the focused laser differential interferometry system under the action of the test field, and a time series of fringe images is obtained; S2 signal feature extraction: a. Brightness signal extraction: Define fixed regions of interest on both sides of the central fringe in the fringe image, and extract the average brightness values ​​of the two regions respectively to obtain two brightness time series. (t) and t (t); b. Displacement signal extraction: For each frame of stripe image, a sub-pixel level signal processing algorithm is used to calculate the lateral position of the center stripe to obtain the lateral displacement time series X(t) of the stripe. S3 signal decomposition and identification: a. Phase Relationship Analysis: Analyzing the brightness time series (t) and t The phase relationship of (t) is used to determine the existence state of the refraction integral effect in the signal; b. Spectrum analysis: Perform spectrum analysis on the displacement time series X(t) to verify the purity of the interference signal.

2. The method for eliminating the refraction integral effect in focused laser differential interferometry according to claim 1, characterized in that, In step S1, the imaging device is a CMOS camera or a CCD camera, and the frame rate of the imaging device is higher than the characteristic frequency of the field to be measured.

3. The method for eliminating the refraction integration effect in focused laser differential interferometry according to claim 1, characterized in that, In step S2b, the subpixel-level signal processing algorithm is selected from any one of the following: subpixel edge localization algorithm, centroid method, extreme point fitting method for each row of pixels, or gradient method.

4. The method for eliminating the refraction integration effect in focused laser differential interferometry according to claim 1, characterized in that, The criterion for judging the phase relationship analysis in step S3a is: if (t) and t If (t) are out of phase, it indicates that the signal is dominated by fringe displacement caused by interference; if they are in phase, it indicates that there is a significant overall brightness modulation caused by the refraction integral effect.

5. The method for eliminating the refraction integration effect in focused laser differential interferometry according to claim 1, characterized in that, The verification criterion for the spectrum analysis described in step S3b is that the pure interference signal exhibits a significant spectral peak at the target frequency of the field to be measured.

6. The method for eliminating the refraction integration effect in focused laser differential interferometry according to claim 1, characterized in that, It also includes a signal inversion step: converting the identified pure displacement time series X(t) into optical path difference ΔOPL(t) based on the system calibration coefficients, and then calculating the density perturbation Δρ(t) of the field to be measured through the Gladstone-Dale relation.

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