Abstract

In fluid Mechanics, the feasibility of three-wavelength digital holographic interferometry has already been shown in recent years. Results were obtained from an optical system based on a Michelson interferometer. But, when the refractive index gradients are very strong, the light deviation of luminous rays induces a shadow effect which is superimposed to the signal of interference micro fringes. Phase steps appear and cause a significant loss of information in the interferograms processing. The purpose of this paper is to reduce the shadow effect by presenting a new method of digital three-wavelength holographic interferometry with a single crossing of the test section and different of Mach-Zenhder instrument. The presented optical setup uses Wollaston prisms where the interfering beams are highly separated and the two prisms are located at the focusing points of an Z optical setup. The optical arrangement makes it easy to introduce three high spatial carrier frequencies which can be analyzed with and without flow. Fourier spectra obtained on each channel can be filtered easily because the interferograms are recorded with a 3CCD camera. Computed phase difference maps provide refractive index maps with a very good quality and the radial gas density profiles can be extracted assuming the flow to be axisymmetric. A small jet of expanded air is taken as test case.

© 2013 Optical Society of America