Abstract
This article presents a digital three-wavelength holographic interferometer based on the use of two Wollaston prisms. This provides an in-line setup with quasi common-path and as a consequence there is no additional independent reference wave to be added. Thus, immunity to external perturbations such as vibrations or thermal perturbations is achieved. Furthermore, the set-up exhibits a single shot and real time capability which is very useful to study dynamic events. By using the two Wollaston prisms in an astigmatic configuration, spatial carrier frequencies can be adjusted both in amplitude and orientation. The digital hologram processing is based on Fourier processing and filtering around the carrier spatial frequency so that phase shifting is not required. The use of three wavelengths leads to visualizing directly the zero order fringe and regions for which there is no air density variation in a dynamic flow. Experimental proof of concept is demonstrated with a supersonic jet when the injection pressure varies.
Highlights
This article presents a digital three-wavelength holographic interferometer based on the use of two Wollaston prisms
The introduction of the spatial carrier frequency by the Wollaston prism leads to the recording of one color hologram at each instant, since there is no need for phase shifting
This is due to the rotation of the Wollaston prism which has a limited square aperture (15 mm2)
Summary
A spatial carrier frequency to the hologram) can be performed according many different ways: tilting one mirror in a Michelson [1,10,16] or a Mach-Zehnder interferometer [19,24], using a Lloyd mirror [4], or using a diffraction grating associated to a spatial filtering [5,7,9]. In a Michelson and a Mach-Zehnder set-up, the optical paths are quite different. When using a diffraction gating, the optical paths are quasi-common, but the filtering induces losses which reduce the light efficiency of the set-up. Note that the use of an independent reference wave induces sensitivity to external perturbations such as vibrations, temperature changes, etc., and leads to an increase in the set-up complexity. The proposed set-up is based on a “Z” architecture and the use of two Wollaston prisms This provides an in-line setup with quasi commonpath and as a consequence there is no additional independent reference wave to be added and no losses in the light transmission.
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