Phase shifting interferometery (PSI) can analyze the test wavefront more accurately than the conventional static interferogram anlyzing method. And the lateral shearing interferometer (LSI) is also widely used because of its simplicity and no requirement of the reference wavefront. But its common path nature precludes easy implementation of the PSI in LSI. Therefore, in order to use the PSI method in LSI, more complicated LSI using Mach-Zehnder or Twyman-Green interferometer configuration were used. This thesis describes two simple PSI methods used with the simple wedge plate LSI.
A wedge plate has the thickness-varying characteristic along the wedge direction because of the wedge angle. If a beam is incident on the wedge plate, the optical path of the sheared wavefront depends on the incident position on the wedge plate because of the varying thickness. Because of this feature, PSI method can be easily applied to the wedge plate LSI.
First, to move the wedge plate simply in-plane parallel to the wedge direction becomes a good PSI method. The wedge movement changes the wedge thickness seen by the test beam and changes the optical path of the sheared wavefront. This thickness change gives the phase shift quantity for PSI. This method requires only a single wedge plate and an additional linear translator. So this phase shifting LSI is a very simple and robust interferometer. The required moving distance for the phase shift in this method is the order of millimeter, whereas the typical moving distance in the other method is the order of wavelength. Therefore, this method has an advantage in the moving distance compared with the other methods. The major error sources are the wedge angle error and the moving direction error. But because the wedge angle is predetermined prior to the measuring process, so this error is minimized prior to PSI process and also can be eliminated by $Carré$ algorithm. Incorrect moving direction leads to the lateral displacement of wavefront and the moving distance change. But these values can be made smaller than the wavelength λ with commercial linear translator.
Second, instantaneous PSI method can be also applied to the wedge plate LSI. In order to obtain all the required interferograms for the PSI at the same time, at least three identical test beams is required simultaneously. Multiple test beams separated in regular intervals can be made by two identical transmission gratings. When the test beams are incident on the wedge plate, equal number of interferograms with equally stepped phase shift are obatined simultaneously. So this interferometer becomes a good instantaneous phase shifting LSI. This interferometer uses only a wedge plate and two identical gratings without moving component. Also the alignment error of the two grating gives little phase shift errors. So the major error sources are the wedge angle error and the incident angle error.
