Interferometric Analysis of Cellophane Sheet for Broad Band Application in Phase Retardation
Kinyua, Dickson Mwenda
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The analysis of cellophane sheet as a phase retarder is reported. Several cellophane sheets were tested for birefringence and compared with a purpose built half wave plate (HWP). A Mach-Zehnder interferometer was used to generate interference patterns when the cellophane sheet was half-way mounted on one arm of the interferometer. Analysis of the interferograms revealed a clear phase shift which was calculated to be 0.98π. The fast axis was determined to be along the length of the cellophane sheet. By varying the angle of the cellophane fast axis with respect to an incident linearly polarized beam, the fringe separation was obtained and used to calculate the ordinary and extraordinary refractive indices. The ordinary and extraordinary refractive indices were calculated as 1.4721 and 1.4680 respectively. Using an air-wedge set-up, the thickness of the birefringent cellophane sheet was found to be 24 μm on average. The measurement of thickness of the cellophane sheet by air wedge provided a simple and sensitive optical interferometric method. The surface quality of the cellophane sheet was determined to be 0.087λ peak to valley and the resulting point spread function (PSF) map and surface obtained. Obtaining the values of the refractive index and thickness gave a complete optical analysis of the cellophane sheet which revealed that the correct choice of cellophane sheet can perform as a purpose built half wave plate. The measurement of the transmitted power values with cellophane sheet and purpose built HWP revealed similar behavior for both materials in rotating the direction of polarization of laser beam. Although the cellophane sheet shows similar HWP behavior, it has the advantage of low cost and is easily available in comparison to commercial HWP.