Spectroscopic Measurements of Optical Elements For Submillimeter Receivers

As submillimeter receivers improve in performance, more attention needs to be given to the design of lenses and windows, because input and coupling losses are responsible for an increasingly larger fraction of the total system noise. In order to optimize the designs, accurate measurements of optical and thermal contraction properties are needed. We discuss techniques of dispersive Fourier transform spectroscopy by which optical constants, thermal contraction coefficients, and surface reflectivities may be measured. Measurements of the optical constants of TPX, Teflon (PTFE), polyethylene, and Teflon-AF, a new material, are presented over the frequency range 0.3 to 2.5 THz. Thermal contraction properties for Teflon are presented. Reflection measurements are presented for a quartz window with a machined matching layer; properties of wire-meshes and wire-grids are also measured.