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Interferometer Detects Microscopic Flaws on Optical Surfaces

Photonics Spectra
Jun 1998
A team of scientists at Lawrence Livermore National Laboratory in Livermore, Calif., has developed an absolute interferometer that can detect imperfections less than 1-nm deep in highly polished surfaces, such as lenses or mirrors. The device could advance UV lithography used in semiconductor manufacturing.
The interferometer operates by aiming a frequency-doubled Nd:YAG laser emitting at 532 nm into two single-mode optical fibers. The light diffracts on exiting the fibers and forms a spherical wavefront. The wavefront passes through the optical surface under scrutiny, causing aberrations in the light, and then hits a metallic film, interfering with a reference wavefront. This creates an interference pattern that a charged-coupled device camera records. Software processes the data to determine the magnitude and location of the flaws.
Unlike conventional interferometers, the device performs measurements in a single-pass transmission.

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