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CN-121994446-A - Method for eliminating refraction integral effect of focused laser differential interferometry

CN121994446ACN 121994446 ACN121994446 ACN 121994446ACN-121994446-A

Abstract

The invention discloses a method for eliminating refraction integral effect of differential interferometry of focused laser, and belongs to the technical field of optical measurement. Aiming at the measurement distortion problem caused by the mixing of intensity modulation and interference phase modulation caused by refraction in the traditional FLDI, the invention adds an imaging module and a signal processing module on the basis of the traditional FLDI optical system, records interference fringe dynamic video through imaging equipment, and synchronously extracts the time sequence of brightness in a symmetrical area (T) and t And (t) and a sub-pixel level stripe transverse displacement time sequence X (t), identifying a refraction interference state through phase analysis, verifying signal purity through spectrum analysis, and reversely deducing field density disturbance to be detected through Gladstone-Dale relation. The invention realizes the source separation of two signals, obviously improves the quantitative measurement precision, expands the application range of FLDI in a complex flow field, and has low hardware transformation cost and high engineering popularization value.

Inventors

  • ZHU YIDING

Assignees

  • 北京大学

Dates

Publication Date
20260508
Application Date
20260224

Claims (6)

  1. 1. A method for eliminating the refraction integral effect of focused laser differential interferometry, comprising the steps of: S1, data acquisition, namely recording a dynamic change video of interference fringes generated by a focusing laser differential interference system under the action of a field to be detected by adopting imaging equipment to obtain a fringe image time sequence; S2, extracting signal characteristics: a. the brightness signal extraction, namely defining the fixed interested areas on two sides of the central stripe in the stripe image, and extracting the average brightness value of the two areas respectively to obtain two brightness time sequences (T) and t (t); B. extracting displacement signals, namely calculating the transverse position of a central stripe by adopting a sub-pixel level signal processing algorithm for each frame of stripe image to obtain a stripe transverse displacement time sequence X (t); S3, signal decomposition and identification: a. analysis of phase relation, analysis of luminance time series (T) and t (T) determining the presence of refraction integration effects in the signal; b. and (3) performing spectrum analysis on the displacement time sequence X (t) to verify the purity of the interference signal.
  2. 2. The method of eliminating refraction integration effect of differential interferometry of focused laser light according to claim 1, wherein 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. 3. The method for eliminating refraction integration effect of focused laser differential interferometry according to claim 1, wherein the sub-pixel level signal processing algorithm in step S2b is selected from any one of a sub-pixel edge positioning algorithm, a gravity center method, a pixel extreme point fitting method per line, or a gradient method.
  4. 4. The method for eliminating refraction integral effect of focused laser differential interferometry according to claim 1, wherein the judgment criteria of the phase relation analysis in step S3a is that if (T) and t And (t) the opposite phase indicates that the signal is mainly fringe displacement caused by interference, and if the two signals are in phase, the signal indicates that the integral brightness modulation caused by the obvious refraction integral effect exists.
  5. 5. The method for eliminating refraction integration effects of focused laser differential interferometry according to claim 1, wherein the spectral analysis in step S3b is verified by a clean interference signal exhibiting significant spectral peaks at the target frequency of the field under test.
  6. 6. The method for eliminating refraction integral effect of focusing laser differential interferometry according to claim 1, further comprising a signal inversion step of converting the identified pure displacement time sequence X (t) into an optical path difference ΔOPL (t) according to a system calibration coefficient, and calculating to obtain density disturbance Δρ (t) of the field to be measured through Gladstone-Dale relation.

Description

Method for eliminating refraction integral effect of focused laser differential interferometry Technical Field The invention relates to the field of optical measurement, in particular to a method for eliminating refraction integral effect of focused laser differential interferometry. Background The Focused Laser Differential Interferometry (FLDI) is a high spatial-temporal resolution, non-contact optical measurement technique that inverts the local density variation of the flow field by measuring the optical path difference between two closely spaced probe lights. The conventional FLDI technique is based on the basic assumption that the measurement signal originates only from the interference phase variations caused by optical path differences in the "sensitive area" where the two beams overlap. However, in practical applications, any refractive index gradient (e.g., due to temperature field, pressure field non-uniformity) present in the beam path may cause beam deflection (refraction integration effect), resulting in a change in the intensity of light received by the detector. This refraction-induced intensity modulation mixes with interference-induced phase modulation, resulting in distortion of the measurement signal, severely affecting the accuracy of FLDI quantitative measurements in complex flow fields. The prior FLDI technology lacks an effective means to separate these two physically distinct signals. Disclosure of Invention The invention aims to provide a method for eliminating the refraction integral effect of focused laser differential interferometry, which is used for solving the problem that in the background art, intensity modulation caused by refraction and phase modulation caused by interference are mixed together, so that a measurement signal is distorted. In order to achieve the purpose, the invention provides a method for eliminating the refraction integral effect of the differential interferometry of the focused laser, which comprises the following steps: S1, data acquisition, namely recording a dynamic change video of interference fringes generated by a Focused Laser Differential Interference (FLDI) system under the action of a field to be detected by adopting imaging equipment to obtain a fringe image time sequence; S2, extracting signal characteristics: a. the brightness signal extraction, namely defining the fixed interested areas on two sides of the central stripe in the stripe image, and extracting the average brightness value of the two areas respectively to obtain two brightness time sequences (T) andt(t); B. extracting displacement signals, namely calculating the transverse position of a central stripe by adopting a sub-pixel level signal processing algorithm for each frame of stripe image to obtain a stripe transverse displacement time sequence X (t); S3, signal decomposition and identification: a. analysis of phase relation, analysis of luminance time series (T) andt (T) determining the presence of refraction integration effects in the signal; b. and (3) performing spectrum analysis on the displacement time sequence X (t) to verify the purity of the interference signal. 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. Preferably, the subpixel level signal processing algorithm in step S2b is selected from any one of a subpixel edge positioning algorithm, a gravity center method, a pixel extreme point fitting method of each row, and a gradient method. Preferably, the judgment criterion of the phase relation analysis in the step S3a is that if(T) andt And (t) the opposite phase indicates that the signal is mainly fringe displacement caused by interference, and if the two signals are in phase, the signal indicates that the integral brightness modulation caused by the obvious refraction integral effect exists. Preferably, the verification criteria of the spectrum analysis in step S3b is that the clean interference signal exhibits a significant spectral peak at the target frequency of the field to be measured. Preferably, the method further comprises a signal inversion step of converting the identified pure displacement time sequence X (t) into an optical path difference delta OPL (t) according to a system calibration coefficient, and calculating to obtain the density disturbance delta rho (t) of the field to be measured through a Gladstone-Dale relation. Compared with the prior art, the invention has the following beneficial effects: 1. the invention can effectively separate the refraction interference from the mixed original signal, the signal-to-noise ratio of the extracted displacement signal at the target frequency is high, and the refraction interference is restrained to a negligible level. 2. The method provides a universal solution for solving the problem of refraction interference commonly existing in a complex flow field by FLDI or e