Spatial phase-stepped interferometry using a holographic optical element

We describe two new techniques for spatial phase stepping using a computer-generated holographic optical element (HOE). The techniques are implemented with electronic speckle pattern interferometry, although they could be applied to other optical metrology methods. The first technique uses an HOE that introduces a known phase step between the ±1 diffracted orders, without being translated. The interference phase corresponding to object deformation can be calculated from a single TV frame, and the technique is therefore suitable for the measurement of transient deformations. We apply the technique to measure the phase changes caused by refractive index variations in an evolving thermal plume. The second technique requires the HOE to be used in conjunction with a phase grating. In this technique, the HOE and the phase grating are used sequentially for the two exposures required in speckle interferometry (made before and after object deformation) and is therefore suitable for making dynamic deformation measurements with a double-pulsed interferometer. A root mean square (rms) precision of 2p/20 rad for the phase measurements is obtained.