Optical Shop Testing

Chapter 7 - Multiple-Pass Interferometers

This chapter discusses some variants of the conventional interferometers used for
optical testing in which one (or more) of the wavefronts is sent back and makes two or
more traverses of either the whole system or a part of it. Such double- or multiple-pass
interferometers offer definite advantages for some testing applications.


7.1.1.   Separation of Aberrations

The interference pattern obtained with a lens in a Twyman–Green interferometer
gives a contour map of the wavefront leaving the lens aperture; however, when many
aberrations are present, estimation of the individual aberrations becomes difficult.
This problem can be simplified if the Twyman-Green interferometer is used in a
double-pass configuration (Hariharan and Sen, 1961d) so that the symmetrical and
the antisymmetrical parts of the wave aberration (see Chapter 13) are displayed in
separate interferograms.

As shown in Figure 7.1, the beams emerging from the interferometer through the
lens L2 are reflected back through it by the plane mirror M3 placed at its focus, and
the double-pass beams emerging from L1 are brought to a focus at the eye stop by the
auxiliary beam divider S2. If the source is shifted very slightly sideways, the two
images formed at the eye stop move off the axis in opposite directions, and it is
possible to view either the fringes produced by the double-pass beams or the normal
interference pattern.

The four double-pass rays derived from a ray incident on the beam divider S1 at O
can be identified as the AA' ray (SOAOM3O'A'O'S), the AB' ray (SOAOM3O'B'O'S),
the BA' ray (SOBOM3O'A'O'S), and the BB' ray (SOBOM3O'B'O'S), corresponding
to the paths they follow on the outward and return journeys.


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