What Is Holographic Interferometry?

Since holography was invented by Gabor in 1948, it has continuously developed by virtue of its outstanding characteristics of recording and reproducing the amplitude and phase of original light waves. In the 1980s and 1990s, with the rapid development of computer technology and the continuous advancement of digital device manufacturing processes, digital holography and digital holographic interferometry have also made rapid progress and have become an important branch of information optics. Not only has made substantial progress in recording media and recording methods, but also the application fields have evolved from three-dimensional reproduction and information storage to displacement and deformation measurement and topography measurement, and there are various methods. ^[1]

Since holography was invented by Gabor in 1948, it has continuously developed by virtue of its outstanding characteristics of recording and reproducing the amplitude and phase of original light waves. In the 1980s and 1990s, with the rapid development of computer technology and the continuous advancement of digital device manufacturing processes, digital holography and digital holographic interferometry have also made rapid progress and have become an important branch of information optics. Not only has made substantial progress in recording media and recording methods, but also the application fields have evolved from three-dimensional reproduction and information storage to displacement and deformation measurement and topography measurement, and there are various methods. ^[1]

General holographic interferometry uses a single laser wavelength, and its measurement error can reach the wavelength level. However, when interferometric measurement is performed on a wavefront with a large wavefront deformation, the interference fringe is too dense, making the obtained interference pattern unreadable. To this end, a dual-wavelength holographic interferometry is proposed.

Two-wavelength Holographic Interferometry (TWHI)

Poisson's ratio is a constant that reflects the elastic properties of a material, and characterizes the degree of shrinkage in the transverse direction when the sample is stretched. It is usually used for numerical pressure analysis of engineering parts. Commonly used electronic and mechanical methods, such as measuring the lateral and longitudinal deformation of the sample with an extensometer to obtain Poisson's ratio. This method is inevitable to drift when measuring the long-term performance of the material, and the extensometer's own weight and clamping force can cause additional deformation of the soft sample, so it is only suitable for hard samples. The Poisson's ratio can also be measured by attaching a resistance strain gauge to the sample, but the deformation range measured by this method is limited, and the stiffness of the sample with the attached piece will cause certain errors. In addition, traditional optical measurement methods, such as holography, speckle method, and image moiré method, calculate the Poisson's ratio from the obtained interference pattern, but these methods require physical chemistry such as exposure and development necessary for the recording medium. The processing process and the reproduction process are complicated and the cycle is long. Some also need to grind the curved surface of the sample to be tested into a mirror surface, which is almost impossible for non-metal materials.

Digital holographic interferometry is widely used. At the same time, various mathematical processing methods can be conveniently added to perform various mathematical operations between two or more holograms and complex amplitudes in the true sense and eliminate the effects of zero-order diffraction images. ^[4]