Optical Shop Testing

Chapter 3 - Common-Path Interferometers

3.1.  INTRODUCTION

In the general type of interferometer, such as the Twyman–Green or Mach–
Zehnder, the reference and test beams follow widely separated paths and are,
therefore, differently affected by mechanical shocks and temperature fluctuations.
Thus, if suitable precautions are not taken, the fringe pattern in the observation
plane is unstable and measurements are not possible. The problems are particularly
acute when optical systems of large aperture are being tested.Most of the difficulty
can be avoided by using the so-called common-path interferometers, in which the
reference and test beams traverse the same general path. These interferometers
have the additional advantage that they do not require perfect optical components
(the master) of dimensions equal to those of the system under test for producing the
reference beam. Furthermore, the path difference between the two beams in the
center of the field of view is, in general, zero, making the use of white light
possible.

In certain common-path interferometers, the reference beam is made to traverse a
small area of the optical system under test and is, therefore, unaffected by system
aberrations. When this beam interferes with the test beam, which has traversed the
full aperture of the optical system, explicit information about the system defects is
obtained. However, in most common-path interferometers both the reference and
test beams are affected by the aberrations, and interference is produced by shearing
one beam with respect to the other. The information obtained in this case is implicit
and some computations are needed to determine the shape of the aberrated
wavefront.

The beam splitting is brought about by amplitude division with the help of a
partially scattering surface, a doubly refracting crystal, or a semi-reflecting surface.
We consider a few examples of these instruments in this chapter.

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