New designs for far-infrared beam splitters

We explore theoretically the design of multilayer beam splitters for the far-infrared that are based on coated pellicles as well as on solid substrates. The specific design criterion considered is as high a value as possible of the efficiency E = 4RT throughout the 10-140-µm spectral region. Here R and T are the reflectance and the transmittance of the beam splitter, respectively. In the numerical study the refractive indices of the substrates and coating materials varied between 1.50 and 4.00. To survey the range of designs, we make a number of simplifying assumptions, the significance of which is later investigated. Various potential manufacturing problems are considered. It is shown that the performance of the beam splitters is not sensitive to the accuracy with which the layer thicknesses can be controlled. However, it does depend strongly on the lowest available refractive index of the coating materials. The performance is particularly sensitive to the extinction coefficients of solid substrate materials. Multilayer designs presented should be useful for use in Fourier-transform spectrometers, as well as in other applications that do not require as high a spectral resolution.