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Optical Testing
Published in Rajpal S. Sirohi, Introduction to OPTICAL METROLOGY, 2017
Figure 8.4 shows the experimental arrangement with a single scatter plate. The scatter plate S1 is kept at the center of curvature of the test mirror M. Lens L2 images the interfering beams at the scatter plate on the observation plane where the interferogram is recorded. The scatter-plate interferometer is a common path interferometer and hence is insensitive to vibrations and refractive index variations caused by temperature. Being a common path interferometer, it is very difficult to introduce phase- shifting method. However, there are two possibilities; a tiny mirror may be mounted on a PZT on which the reference beam falls when an annular mirror/surface is to be tested, or the reference and the object beams could be of perpendicular polarization states and the polarization-based phase shifting is introduced. Both these methods have been used in practice. Further, it may be noted that a lateral displacement of the scatter plate introduces tilt fringe whereas the longitudinal displacement introduces defocus.
Rapid phase calibration of a spatial light modulator using novel phase masks and optimization of its efficiency using an iterative algorithm
Published in Journal of Modern Optics, 2020
Amar Deo Chandra, Ayan Banerjee
We study and characterise the phase response of our SLM at 671 nm. Since phase measurements are accurate but are sensitive to vibrations and local turbulence, we generate binary phase masks which are vertically divided into variable gray level and fixed reference gray level in order to alleviate this problem. Our SLM permits 8 bit gray level addressing and we vary grayvalues from 0–255 in steps of 10 gray level (15 for the last step) and record thirty interferograms for each step. Figure 2 shows the montage of interferograms obtained using different binary phase masks displayed on the SLM. This exemplifies that differential phase shifts are produced on displaying different binary gray level masks on the SLM. It is to be noted that the interference contrast or visibility changes with increasing gray level because we cannot avoid a certain extent amplitude modulation when we perform phase modulation in our SLM model. Some earlier studies report existence of coupled amplitude and phase modulation in SLMs [27,28]. We have also shown fringes obtained using a common path interferometer wherein two beams falling on the SLM (generated from the same laser beam by inserting two adjacent pinholes across the beam, as shown in Figure 3) are combined using a Mach-Zehnder interferometer (Figure 3). We would like to point out that in this configuration, the SLM is tilted at an angle which changes its phase response [29] compared to the case when the beam falls at normal incidence. This unwanted phase shift is undesirable for our application using a Michelson interferometer.