Optical Shop Testing

Chapter 2.7.1 - Open Path Interferometers

2.7.   OPEN PATH INTERFEROMETERS

An optical element under test in a Twyman–Green interferometer is traversed twice.
However, sometimes it is necessary that the sample is traversed by the light beam
only once. This can be done if the interferometer configuration is modified by
unfolding the light paths. Several possible configurations including the well-known
Mach-Zehnder will be described.

2.7.1.   Mach-Zehnder Interferometers


As pointed out before, the Mach-Zehnder configuration shown in Figure 2.34 has
some advantages with respect to the Twyman–Green configuration. For example, if
the sample under test has a large aberration, it is better to pass the beam of light only
once through it. Another advantage some times is that this interferometer is automatically
compensated because it has two beam splitters.

FIGURE 2.34. Basic Mach-Zehnder interferometer configuration.

A commercial version of this interferometer configuration, manufactured by
Wyko Corp., uses a pinhole in one of the branches to generate a perfect wavefront,
in order to test the wavefront quality of the light source, as shown in Figure 2.35.
(Leung and Lange, 1983; Creath, 1987). A Mach–Zehnder has also been used to test
off-axis paraboloids (Gerth et al., 1978). Cuadrado et al. (1987) have described a
method to align a Mach–Zehnder interferometer using equilateral hyperbolic zone
plates and Flack (1978) has analyzed the errors that result from a test section
misalignment.

Sometimes, to save one mirror, a triangular configuration as illustrated in
Figure 2.36 is used.

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