Multiple-beam interference fringes between two closely spaced highly reflecting surfaces have long been used for sensitive examination of surface contours. The spacing between the two surfaces has been limited to a few millimeters at most to minimize the walk-off effect of the beam between the two slightly angled surfaces and to obtain fringes having a narrow width compared to their spacing, usually referred to as a high finesse.
A system consisting of two spherical surfaces with a lens between them gives sharp multiple-beam fringes at a spacing of 50 cm. Fringes with a contour interval of λ/6 result from the use of the separate wavelengths of a helium–neon laser as a source.
This long-path system can be used to examine the index homogeneity in a sizeable thickness of transparent solids, plasmas, or gases. The system should be useful for examining thermal, pressure, or composition gradients in wind tunnels, shock tubes, or plasma studies with about ten times normal sensitivity.
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