A SIMPLE QUARTZ BIREFRINGENT QUARTERWAVE PLATE FOR USE AT λ = 3. 39 μM

Abstract

A thin plate of crystalline quartz has been fabricated for use as a quarterwave plate with He -Ne 3.39 -um lasers and has been used to determine the birefringence of the quartz at that wavelength. With a single thin plate, first -order relative retardation is achieved without recourse to an air - spaced pair of thicker plates having opposing retardation. Compared with such thick, air-spaced plates, thin quartz plates are much less costly to make, and they allow use of this excellent optical material much farther out into the infrared region. Because the birefringence at 3.39 um had not yet been measured, an extrapolated value was used to determine an approximate thickness for the plate that was to be fabricated. Using the plate thus fabricated (0.128 mm thick), two methods were followed to independently determine the birefringence: First, with the plate normal to a linearly polarized 3.39 -um laser beam, the state of polarization of the transmitted beam was measured, yielding the relative retardation, and hence the birefringence, after determination of the plate thickness. The second independent determination of birefringence was obtained by measuring the plate tilt necessary to produce exactly circularly polarized light. The average of the 3.39 -um birefringence values obtained from the two methods was .0065 t .0001, corresponding to a quarterwave plate thickness of .1304 mm. The possibility of using thin crystalline quartz for infrared wave plates is attractive. However, one must consider both anisotropic absorption and anisotropic Fresnel reflection, which vary with wavelength. Only if these anisotropic losses can be balanced or made negligible (as by anti - reflection coating) can a perfect waveplate be made.