INFORMATION TECHNOLOGY

Shear speckle interference: spectral aliasing phase extraction

Shearography is a high-precision optical interference technology proposed on the basis of electron speckle interference (ESPI), which realizes non-destructive testing of objects by detecting the surface stress concentration phenomenon caused by internal defects or deformation of objects under load. Since no additional reference light is required, shear speckle interference has the anti-interference ability that conventional interference technology lacks, making shear speckle interferometry easier to apply to industrial nondestructive testing.

Shear speckle interference obtains the object deformation information by obtaining the phase of the interference fringe, and the measured object is often dynamic in practical applications, and the spatial carrier phase shift method based on a single interferogram to extract the phase information can realize the dynamic phase measurement of shear speckle interference. However, the spatial carrier method often has spectral aliasing, which seriously affects the quality of phase extraction.

Recently, Professor Wang Yonghong’s team from the School of Instrument Science and Optoelectronic Engineering of Hefei University of Technology published a research paper on Light: Advanced Manufacturing with the title of A robust phase extraction method for overcoming spectrum overlapping in shearography

This paper briefly summarizes the causes of spectral aliasing of shear speckle interference space carriers, and realizes high-quality phase extraction during spectral aliasing based on the improved window Fourier ridge algorithm. Simulations and practical experiments are carried out to verify the effectiveness of the proposed method.

Spectral aliasing phenomenon and its cause

In the spatial carrier phase shift shear speckle interference system, the interference light spectrum is divided into three regions, of which the background light is the low-frequency part, and the spectral center coordinates of the object light and its conjugate term are affected by the size of the space carrier and are symmetrical with respect to the spectral center. Since the space carrier phase calculation takes only one object-light spectrum, the ideal scenario is that the three spectra are completely separated (as shown in Figure 1(a)).

In a typical space carrier phase shift shear speckle interference system, the introduction of space carrier generally relies on adjusting the mirror angle, so that the relative angle deflection between the two beams of object light occurs, and the carrier frequency is introduced at the same time as the shear amount. In order to completely separate the object light spectrum from the background light spectrum, a relatively large mirror deflection angle is required, but at the same time, the greater the shear, the shear speckle interferometry sensitivity is too high, the deformation fringe is too dense, and the interferometric measurement area becomes smaller. Although the use of a small aperture diaphragm can reduce the diameter of the interferogram spectral circle and thus do not require a large carrier frequency, it will lead to an increase in speckle particle size and an increase in decorrelation noise in the phase fringe pattern.

Due to the coupling relationship between carrier frequency and shear, spatial carrier speckle interference inevitably occurs in some cases (as shown in Figure 1(b)), such as large object deformation or large measurement area. This spectral aliasing causes the phase extraction quality to deteriorate, resulting in subsequent phase de-wrapping abnormalities, and the accurate deformed phase information cannot be obtained.

Figure 1: Space carrier phase shift speckle interference spectrogram

Improved window Fourier transform ridge phase extraction

In view of the spectral aliasing of shear speckle interference in space, if the phase extraction method can extract high-quality phase, it will be conducive to the practical application of shear speckle interference.

Windowed Fourier ridge (WFR) is a phase extraction method applied in fringe demodulation that has excellent noise immunity and some resistance to spectral aliasing. However, in shear speckle interference, due to the consistency of the spectral size of background light, object light and conjugate light, the spectral aliasing generated is more serious, and the conventional WFR algorithm is difficult to meet the requirements.

For the shear speckle interference system, an improved window Fourier transform ridge phase extraction method is proposed. Since the optical system widely adopts circular aperture diaphragm, the Hough transform is used to analyze the spectrum of speckle interferogram, and the central point coordinates and spectral radius of the object optical spectrum are obtained, and the spectral band range of the window Fourier ridge is roughly determined by the correspondence between the frequency domain coordinates and the image pixel coordinates, which reduces the running time of the algorithm, and can search for the local frequency with a smaller step size to improve the phase extraction quality.

In addition, for the regional characteristics of the optical spectrum when the spectrum is aliased, the use of a large window for Fourier transform will lead to the increase of speckle particles, and the small window will alias the zero-frequency information, so this paper proposes to use a linear transformation ellipse window for window Fourier transform, as shown in Figure 2, when approaching zero frequency, an elliptic window with a long axis parallel to the y-axis direction is used, and the ellipse window with the right shift of the frequency scanning interval is linearly changed to an elliptic window with a long axis flat behavior in the x-axis direction, so as to maximize the use of spectrum information and improve the phase extraction quality.

Figure 2: Schematic diagram of the change of the window Fourier transform window with frequency domain coordinates

Experimental verification

In order to verify the effectiveness of the proposed method for phase extraction, a comparative experiment is carried out. The original speckle interference spectrum information obtained is shown in Figure 1(b), and the spectrogram is extracted by two commonly used phase extraction methods and the method in this paper, and then the phase diagram corresponding to the deformation is obtained after the same filtering and dewrapping operation, as shown in Figure 3.

Figure 3: Comparison of de-wrapping phase results obtained by the three phase extraction methods

From Figure 3, it can be seen that the phase of the solution wrap obtained by the proposed method is very smooth and the quality is good, while the other two methods have obvious fringe breakage caused by the phase anomaly of the solution wrap, which verifies the effectiveness of the proposed method.

prospect

The phase extraction algorithm proposed in this paper can still extract high-quality phases when spectral aliasing occurs in the spatial carrier shear speckle interference system, so that a larger size aperture diaphragm can be used to reduce the speckle reclamation correlation noise, expand the application range of space carrier shear speckle interference, and make it more widely used in industrial nondestructive testing.

Related paper information:https://doi.org/10.37188/lam.2023.007

(Source: Advanced Manufacturing WeChat public account)
 
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