PROCEEDINGS ARTICLE | November 1, 1997
Proc. SPIE. 3134, Optical Manufacturing and Testing II
KEYWORDS: Mirrors, Polarization, Birefringence, Interferometers, Glasses, Wavefronts, Collimators, Aspheric lenses, Phase shifts, Fizeau interferometers
The goal of this 24 inch phase shifting Fizeau interferometer design is to measure the wavefront of an optical window at Brewster's angle. There are two important requirements: a small wavefront slope error and a high optical resolution. To test the sample in transmission, each pencil of light returned from the RF must go back through the window from which it was previously transmitted. Therefore, the slope of the wavefront transmitted through the TF has to be less than a few arc seconds, especially for a long cavity length. For example, for a 2 meter round trip, a 5 arc second slope causes the beam to deviate 0.05 mm. For a 431-mm sample imaged onto a 1000 pixel array, a 0.05 mm displacement corresponds to a 0.116 pixel, which is negligible. However, when a 100 mm sub-aperture is imaged, a 0.05 mm displacement is significant. A shorter round-trip distance can effectively reduce the displacement. The deviations due to a 5-arc second wavefront slope is 0.12 pixel for 2-meter round trip in the full aperture and 0.10 pixel for 0.4-meter round trip in the sub-aperture imaging. Because the phase of the optical window is to be measured and not the amplitude or intensity, the MTF is ont suitable for evaluating the interferometer's resolution. A phase object was measured to determine the system transfer function. The fidelity of the measurement is required to be within 60 percent amplitude for a specified spatial frequency range. For example,for a sinusoidal phase object with a phase undulation of 0.01 wave p-v, the measured result should not be less than 0.006 wave. From theory, a phase object with a smaller phase undulation can be imaged with good fidelity. Because the wavefront slope and optical resolution requirements are very tight, to ensure the interferometer meets these requirements, theoretical errors were thoroughly analyzed and the design implementation was carefully studied.
