Maintaining sub-pixel alignment for a single-camera two-bucket shearing speckle interferometer

Abstract

In a phase stepped speckle interferometer, phase steps can be realized temporally or spatially. The spatial approach has the advantage of simultaneous acquisition of all phase stepped interferograms which eliminates phase errors due to phase changes that can occur between phase steps when phase is stepped in time. For spatially phase stepped systems based on multiple optical channels, it is important that interferograms are well aligned to the others. Alignment can be achieved and maintained by a system that does not rely strongly on high mechanical stability but is based on measuring the alignment errors and correcting them. The alignment system presented in this work uses speckle correlation to quantify alignment errors with sub-pixel accuracy, and comprises a piezo-electric controlled mirror to achieve precise alignment.The alignment method has been implemented for a two-channel shearing speckle interferometer. It allows for precise initial alignment both for translation and rotation of the two phase stepped speckle patterns on the CCD. Translation adjustments to maintain sub-pixel alignment during operation can be realized very quickly, preceding a test session. The importance of adequate alignment, and the effects of insufficient alignment, are illustrated by experimental results.

An alignment technique based on the speckle correlation properties of Fresnel transforming optical systems

“…4(ii). We note that in this figure the location of the optical axis is at index (3,4), and not at index (3,3) as expected. This is due to the fact that the centre of the optical axis may actually be located outside the active area of the camera for the first iteration of the algorithm.…”

“…Relative optical alignment, in an interferometer for example, can be achieved by iteratively measuring the peak correlation value between two laterally displaced speckle fields [4] . These speckle fields originate from the same object, but are displaced with respect to each other if the paths they travel are not exactly co-incident on the same plane where they recombine prior to capture by a digital camera.…”

Handling unfavourable polarization states in a polarization-based shearing speckle interferometer

“…Mirrors M3 and M4 are set to a position for which the imaging system projects two images of the object side by side onto the CCD camera. Mirror M4 can be fine-adjusted by piezo-elements for sub-pixel alignment of the two images [13]. As a result two speckle interferograms are obtained side by side on a single CCD camera, with a mutual phase step of π/2.…”

Shearography technology and applications: a review

“…Another system that employs spatial phase shifting involves a polarization-based shearing interferometer which divides an incoming image into two and images them side-by-side on a camera with a π relative phase shift [70]. Analysis showed that maintaining sub-pixel image registration was essential to maintain accurate measurements [71]. A more recent paper [70] discusses an improvement of the system when dealing with unfavourable polarization states.…”