A portable instrument denoted a phase-retrieval camera (PRC) is designed to enable the testing of large, lightweight optics in the presence of vibrations and atmospheric turbulence. The development of the PRC was prompted by the fact that conventional optical-testing techniques are not sufficiently accurate or robust for optical testing in high-vibration environments.

The PRC in This Example of a typical application is used to test a concave mirror.

The PRC (see figure) includes a source module and an optics module. The source module contains laser diodes and integrated optical devices that, acting together, constitute a point source of light of three wavelengths. The use of the three wavelengths makes it possible to measure the relative axial displacement (called "piston" in the art) of a segment of an optic under test. The optics module contains a wavefront-sensing (phase-retrieval) camera on a motorized translation stage, a reference (boresight) camera, a short-exposure (millisecond) shutter to "freeze" vibrations and turbulence, selectable calibration optics, attenuators, and other optics. A defocused image of the optic under test is formed on the focal plane of the wavefront-sensing camera.

The PRC also includes a computer programmed with software that provides a graphical user interface, displays data, controls the operation of the instrument, enables data communication and remote control via the Internet, and performs phase-retrieval, phase-unwrapping, and calibration computations. The phase-retrieval algorithm processes multiple short-exposure images from the wavefront-sensing camera and aligns them with each other by use of boresight calibration images acquired simultaneously by the reference camera. The algorithm generates data and imagery that characterize the wavefront generated by (and thus the surface figure of) the optic under test.

This work was done by David Redding, Andrew Lowman, David Cohen, Fang Shi, and Scott Basinger of Caltech for NASA's Jet Propulsion Laboratory.


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Photonics Tech Briefs Magazine

This article first appeared in the March, 2002 issue of Photonics Tech Briefs Magazine.

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