An approximate mathematical description of tunable metal film reflector filters for the medium and far ir is derived. The absorptive film is characterized by two easily measured quantities, and a mirror reflectivity smaller than one is taken into account. The dependence of reflectivity, phase, and transmissivity on wavelength is studied. Expressions for the filter bandwidth and a lower limit of the bandwidth are obtained. Minimum losses depend primarily on the reflection coefficient of the mirror and not on the film properties. Two interesting configurations can be distinguished: one with high loss and small bandwidth (I) and one with low loss and large bandwidth (II). Measurements of bandwidth and minimum losses at 10 gm using Ge and ZnSe film substrates and gold and aluminum films show very good agreement with theory. Typical experimental results for configuration I yielded a 4.6% bandwidth (given as fraction of the free spectral range) and 2.9% additional reflection losses (caused by the insertion of the film). Smaller losses (≈0.5%) were obtained in configuration II in connection with broader bandwidths. Possible applications include low loss longitudinal mode suppression and rotational line selection in gas lasers.
© 1976 Optical Society of America
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