Abstract

We were presented with the problem of producing a series of narrow bandbass filters in the 8 to 12 micrometer spectral band of high durability with 1.0% tolerances on the edges and bandwidths. After some design study, it was decided to use germanium (Ge) and thorium fluoride (ThF2) as the coating materials and germanium for the substrates. The available coating chamber was equiped with an optical monitor using a photomultiplier whose response was adequate to 1064 nanometers (nm) but not at much longer wavelengths. Germanium films absorb significantly at 1064nm but it is practical to deposit 15 or more quarter waves (QWOT,s) before the optical monitor signal becomes too attenuated to make reasonable layer terminations. Figure 1 shows a typical example of the monitor signal in reflection for a series of ThF4 and Ge layers. This structure could not be monitored directly in transmittance because the monitoring signal would be too severely attenuated after the first or second Ge layer. However, the whole layer system can be monitored directly in reflection if certain design conditions are satisfied. There is also some flexibility benefit to reflection monitoring in this case where each thick Ge layer masks the reflection characteristics of the underlying layers. Each low index layer has somewhat of a fresh start on the monitor. This approach cannot take advantage of error compensation features as described by Macleod and Pelletier(1) and Zhao(2), but the fact that the layers are being monitored at 1/10th the passband wavelength gives enough sensitivity for good spectral control.

© 1988 Optical Society of America