Abstract

There are two geometries that are generally used for real-time interferometry, the four wave mixing (FWM)^1,2 and two beam coupling (TBC)^3,4 geometries. There is, however, a geometry which incorporates features of both FWM and TBC proposed by Petrov et.al.⁵ and it is this geometry which is easiest to implement and gives good results. In this geometry the anisotropic self diffraction properties of sillenite crystals such as Bi₁₂(Ge, Si, Ti)O₂₀ are used to produce a polarization rotation between the incident and diffracted beams. Thus if one of the above crystals is placed in the standard FWM orientation, with the [110] crystalographic direction in the plane of the incident beams, such that the axes of charge induced birefringence are at ±45° to the [001] direction (Fig.1), then the transmitted object (signal) beam can be cancelled by using two orthogonal polarizers placed in front of and behind the crystal. This enables the diffracted image to be isolated from the signal so reducing the effective noise in the output hologram. Rather good time-average and double exposure interferograms can be produced^6,7, (Fig.2), with, however, a restriction on the size of the object that can be investigated, when the object is diffusely reflecting. In the case of the time-average interferogram shown in (Fig.2) the object was only 40 mm in diameter. The restriction on object size is partly due to geometrical considerations but is also strongly dependent on the noise and sensitivity characteristics of the crystal used. For this reason we present here an analysis of two samples of Bi₁₂SiO₂₀ (BSO) crystals grown in different laboratories, based on a simple theory for the determination of maximum viewable object size, from material considerations.

© 1991 Optical Society of America