White light OPD scanning polariscope for whole-field optical path retardation and principal angle examinations

Abstract

Fringe-scanning (or TFP/RGB) and phase-stepping are now the most acceptable techniques for examining stressed photoelastic samples; however the former requires a calibration table, which is sensitive to the geometrical and optical parameters of the corresponding polariscope, and concentrates only on phase retardation measurements; the latter suffer from three potential issues: 1) the principal angle is indeterminate when the corresponding phase retardation is close or equal to an integer multiple of π, 2) the obtained phase retardation and principal angle are wrapped, and 3) the phase retardation and principal angle may couple together in certain situations. Free from the disadvantages of the fringe-scanning and phase-stepping polariscopes, a white light OPD (optical path difference) scanning polariscope composed of a white light laser, an OPD modulator, and a dark-field circular polariscope was proposed for whole-field optical path retardation (proportional to the phase retardation) and principal angle determinations. This paper introduces the theory of the polariscope, demonstrates the experimental setup for implementing the polariscope, and presents the results from the uses of the setup. The results manifest a standard deviation of 8 nm when repeatedly measuring an optical path retardation and 1.21ο when repeatedly measuring a principal angle; moreover, the results confirm the polariscope's validity and feasibility, but showing a limitation while adopting the polariscope to evaluate fringe orders < 3.