This text presents several new thin-film design methods that can produce multiple stopbands as well as passbands. It is written for thin-film designers and students with advanced knowledge of multilayer, optical thin-film coatings. The text focuses on coatings that have high reflectance performance requirements in more than one spectral wavelength band or region.
The design of ideal infrared thin-film polarization preserving reflectors requires the equalization of the p and s polarization reflectances and zero differential phase shift between them. Depending on system design requirements for absolute reflectivity, either non-absorbing substrates such as zinc selcnide, or metallic films such as silver or gold are commonly utilized. In addition, a few dielectric layers are deposited onto the substrate for reflection enhancement and phase correction. This paper will investigate the design of enhancement layers with refractive indices n<sub>1</sub> and n<sub>2</sub>, onto various substrates for a single wavelength and in most cases, 45 degrees angle of incidence. Indices of actual film materials such as thorium fluoride, germanium and zinc sulfide will be utilized to demonstrate actual design performance. Also, an equation is presented that is used to predict the differential phase shift sensitivity to wavelength centering of a quarter wave stack. In this case, the film indices determine the incidence angle sensitivity. Next, the incidence angle sensitivity of some designs is investigated. Some designs act as polarization preserving reflectors from normal incidence to nearly 80 degrees angle of incidence. A brief summary of conventional enhanced metal coatings is presented, along with design methods. Some applications of polarization preserving reflectors are described, especially for C0<sub>2</sub> lasers.
Output coupler (OC) and high reflector (HR) thin-film coatings and substrates that are employed in 632.8 nm helium-neon (HeNe) lasers are investigated for optical scatter. The measurement of scatter in this paper is in terms of bidirectional reflectance distribution function and calculated total integrated scatter, or BRDF and CTIS, respectively. Laser output power will be briefly reviewed as a function of total scatter loss from the OC and HR. Increasing amounts of loss (scatter), in individual sets of OC's and HR's, reduces the output power of each laser tube from a theoretical optimum output, for the same amount of loss. Sample optics were measured for BRDF and compared with visual microscope inspection. Statistical analysis of many thousands of coated optics provides insight to controlling causes of scatter in the manufacturing process. Here, the average and standard deviation of BRDF scatter data for many optics provide important data for process control. Various scatter data is presented form OC and HR production runs.