Optical Shop Testing

Chapter 2.7.2 - Oblique Incidence Interferometers

2.7.2.   Oblique Incidence Interferometers

Another kind of two beam interferometers have triangular paths, so that one of the
beams is obliquely reflected on the flat surface under test. It may easily be proved that
under those conditions a small error with height h on the surface under test introduces
an error equal to 2 h cos θ, where θ is the incidence angle. Thus, the interferometer is
desensitized by a factor cos θ. Another consequence of the oblique incidence is that
the reflectivity of the surface under test is greatly increased. Thus, an interferometer
with oblique incidence is ideally suited for testing ground or mate flat surfaces,

FIGURE 2.35. Mach-Zehnder interferometer used to test the wavefront quality of a laser diode.


FIGURE 2.36. Triangular path interferometer.

whose flatness and reflectivity are not good enough to be tested by conventional
interferometry.

Oblique incidence interferometers have been described by Linnik (1942),
Saunders and Gross (1959), by Birch (1973,1979), by Hariharan (1975), and by
MacBean (1984). Some of these interferometers use diffraction gratings as beam
splitters, as the one designed by Hariharan, as shown in Figure 2.37. Small ground
and almost flat aspherical surfaces may be tested with oblique incidence interferometers
as shown by Jones (1979).

FIGURE 2.37. Grazing incidence Interferometer using a diffraction grating beam splitter.

 

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