Optical Shop Testing

Chapter 2.6.2 - Improving the Fringe Stability

2.6.2.   Improving the Fringe Stability

The unequal-path interferometer frequently has the problem that the fringes are
very unstable due to vibrations of the surface under test. Most mirror vibrations
have mainly tip and tilt components more than a piston component. Bending is not
usually important mainly if the optical elements are thick enough. Thus, one obvious
approach is to configure the optical system so that the antisymmetric components of
the wavefront from the vibrating system under test are canceled out. The disadvantage
of this is that the antisymmetric aberrations, such as coma, are not detected.

One possible implementation of this concept is the arrangement in Figure 2.33.
The diverging beam of light going out from the interferometer illuminates the
concave surface under test and then the reflected convergent beam gets reflected in
a small flat mirror near the center of curvature. Then, the light returns to the mirror,
but the returning wavefront has been rotated 180o with respect to the incident

FIGURE 2.32. Kocher’s unequal path interferometer.


FIGURE 2.33. Stabilization of interference fringes by eliminating antisymetric errors in the wavefront by means of a reflection in a small flat mirror.

wavefront. This rotation of the wavefront eliminates all antisymmetric components
of the wavefront error, remaining only the symmetric components.

The mirror vibrations may also be eliminated from the interferogram by introducing
exactly the same vibrations in the reference wavefront. This is done with an
arrangement described by Hardesty (1979), where the reference arm of the interferometer
is made as long as the test arm and the reference mirror is placed as close as
possible to the tested element, so that they vibrate together.

A third method to dampen the vibrations is by sensing this vibrations with a light
detector and with this signal move the reference mirror in the opposite sense (Cole
et al., 1997). This is a closed loop servo system that uses a phase smaple frequency of
several kilo Hertz.

A fourth method is to capture the interferogram image as fast as possible, so that
vibrations do not affect the image. This can be done, but the limitation might come if
several images with different phase are required, as in phase shifting interferometry.
Some interferometers had been designed that allow the simultaneous measurement
of four interferograms with different phase.

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