All-dielectric Filters Research Articles

Deposition error compensation for optical multilayer coatings II Experimental results—sputtering system

A fully automated magnetron-sputtering system for the deposition of complicated optical multilayer structures is described. The process control includes the real-time determination of the deposited layer thicknesses and adjustment of the remaining layer thicknesses to reoptimize the final performance of the multilayer system. With this deposition system it should be possible to produce almost any all-dielectric filter that can be designed, subject only to limitations imposed by time, cost, and mechanical stability of the coatings. To demonstrate the performance of the deposition system, five complex multilayer systems were fabricated. The theoretical and measured spectral transmittance curves of these multilayer systems closely agree with one another over broad spectral regions. The equipment is capable of unattended operation over periods of 24 h or more.

Raman characterization of all-dielectric multilayer SiO_2/TiO_2 optical coatings

Raman spectroscopy can be used to characterize the rutile/anatase phase composition of TiO2 layers in all-dielectric filters and reflectors of arbitrary design. No special specimen preparation is necessary. Interference effects can enhance Raman scattering compared to single-layer coatings of the same total TiO2 thickness.

Thin film narrow band optical filters

Narrow band filters are interesting for a number of reasons. Firstly there is the simple reason that we want to be able to make them better with a higher process yield. Secondly there is the reason that their performance is very sensitive to alterations in material properties and so they could be useful test beds for materials and especially for detecting small instabilities with time. Techniques for design of thin film narrow band optical filters are well established. The filters may be considered as arrangements of tuned cavities: the simplest type, the Fabry-Perot, consists of just one cavity; the double half-wave (DHW) has two cavities; the triple half-wave (THW) consists of three cavities; and so on. The narrowest filters are constructed completely from dielectric materials to keep dissipative losses as low as possible. Almost all the all-dielectric filter designs in common use employ layers that have optical thicknesses which are integral multiples of quarter of the peak wavelength. This is because both the theoretical treatment and the control of deposition are rather simpler for quarter-waves than for non-integral thicknesses. From time to time proposals have been made for new narrow band filter designs which involve nonintegral thicknesses but these do not seem to have been taken up to any extent by manufacturers, probably because the additional complication outweighs any advantage which they might have in performance. The theory and techniques of layer thickness control are apparently rather less well organized than design. These are central to the production of coatings and so manufacturers are naturally reluctant to discuss their methods in detail. There has not been a great deal of interest in this aspect by academics and as a result relatively little has been published. In this review we consider briefly a few of the problems which occur in the production of narrow band all-dielectric thin film optical filters. For the reasons mentioned above we shall consider only designs composed entirely of optical thicknesses which are integral multiples of a quarter-wave. Assuming that we have stable thin film materials with accurately predictable losses then we can estimate the best performance which should be achieved with a particular design. Real manufactured filters will always have poorer performance than this because errors in the layers will disturb the optimum matching. The resulting decrease in peak transmittance will be accompanied by a corresponding increase in reflectance which is always a good test of whether low transmittance is due to losses or to manufacturing inaccuracies. This leads us directly to the idea of tolerances. How accurately

Properties of Weakly Absorbing Multilayer Systems in Terms of the Concept of Potential Transmittance

The general expression for the potential transmittance of a layer, first derived by Berning and Turner [1], is simplified by the use of approximations applicable to weakly absorbing layers, and then used to describe the optical performance of mirrors and narrow-band filters consisting of such layers. In particular, simple expressions are obtained for the transmittance and absorptance of all-dielectric filters in terms of the width of their passband. These provide useful guides to the design of filters with minimum reduction of transmittance due to absorption in the layers, and also allow the absorptance and scattering contributions to be separated in measurements of the total loss.