The application of DOE in uniform illumination for large area digital speckle pattern interferometry

Abstract

Uniform illumination plays an important role in Digital Speckle Pattern Interferometry (DSPI) of measuring a large object. This paper presents a method that by designing a specified DOE with certain algorithm in front of laser diodes, a large and uniform illumination could be achieved at the designed distance. The fluctuation of no more than 10% has been analyzed for its influence on interferogram. Only 6 semiconductor lasers are necessary in the interferometer to lighting its view in theory and the experiment with the simulated illumination has been done. The fringes of deformation at different parts of the object show that this method is suitable for large area detection. 1、INTRODUCTIONWith a laser beam illuminating a rough surface compared with the wavelength speckle will be produced and it has been widely used as a metrology method and investigated for displacement and deformation measurement. Digital Speckle Pattern Interferometry (DSPI), a branch of speckle Metrology and a well-established tool, has been used to detect the deformation of whole-field from static to dynamic [1] [2] . Multiple laser diodes have been used in such non-damaged detection for large area illumination. But it has some defects such as large number of used diodes, coherent superposition of light field, wavelength shifting, etc [3] . To overcome these defects this paper presents a way that by using DOE(diffractive optical element) the flattened Gaussian beam could be achieved and fewer FGBs are necessary to get the uniform illumination of large area at a certain distance. The algorithm for calculating the DOE has been discussed. As in different speckle fields the intensities of speckle pattern are independent of one another, visibility is described by joint probability density function. By changing the different ratios of mean intensities in two adjacent speckle fields, the probability density curve is approximately the same with the one in the uniform lighting field. 2、UNIFORM ILLUMINATION BASED ON DOEThe semi-conductor used in the interferometer has the light intensity in ellipsoid Gaussian shape, which results in a large discrepancy between the horizontal and vertical angles. It has to be changed into the rectangle uniform distribution to fit for the illumination in large area. As a problem of beam shaping, it basically includes three parts: a given input, a designed output and the solution of inverse diffraction between them. With the intensity distribution of input and output lights, the needed phase at the input light field can be calculated. The element with the needed phase is called DOE(Diffractive optical element), which has been proved to be an effective way in beam shaping, for its flexible control