Two-wavelength digital holographic interferometry for unambiguous range extended measurements in fluid mechanics

Abstract

Non-contact optical methods such as digital holographic interferometry are highly suitable in measurements where the phenomena is fast, performed in transparent or semi-transparent environment and mustn’t be obstructed as when applying local contact techniques. Such specific application can be studying dynamic events during transonic and supersonic blade flutter. Fast, sensitive and rather easy access to the phase information make these techniques very attractive in the study of phase objects/phenomena. However, since light’s phase is bounded to a repetitive cycle of 2π radians, the range of measurement is limited to one cycle of the phase, limiting applications to small gradient phenomena. This paper presents a new interesting way of by-passing this limitation, while still keeping noise values low, by introducing a second laser with a close value wavelength, giving rise to a new interferometric pattern with an extended unambiguous range of measurement. Image acquisition is done simultaneously for both wavelengths and all reconstructions are digitally performed. The principle and preliminary results are included in this paper.