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An optimum phase is developed for the synthesis of rugate reflectors by a simple Fourier transform. This phase belongs to a complex function of the desired spectral characteristics and is usually a free parameter. In general, it receives much less attention than the function magnitude, which is not known exactly. The current work shows that phase shaping alone produces surprisingly good results and has other advantages in rugate filter synthesis. In addition, the operating mode of this design procedure is quite unusual and interesting in itself.

Fourier techniques and direct optimization are applied to the synthesis of complex rugate filters. A hybrid approach is presented. The design strategies are illustrated numerically for different spectral shapes.

For the past 50 years, the Thin Films Group of the National Research Council (NRC) in Canada has successfully developed optical interference coatings that have changed the fabric of our lives. The coatings have been used for everything from protecting paper currency against counterfeiting, to increasing the contrast and efficiency of organic LEDs, to forming components of artificial vision systems used in space shuttles. This article reflects on NRC's past successes, current challenges and future directions.

A Fourier transform (FT) approach based on the evaluation of optical-density-bandwidth products in the spectral region of interest was recently proposed for the thickness estimation of reflecting thin-film dielectric filters. For simplicity, the initial discussion was limited to a particular type of immersed coating. The theory is generalized to more realistic filter configurations and confirmed by numerical examples. It is shown that good results are possible although the problem is more complex from a FT point of view.

An empirical procedure based on optical-density-bandwidth products was recently proposed for thickness estimation of dielectric thin film reflectors. A parallel is established with new results derived from the Fourier transform thin film synthesis technique. Two Fourier-transform approaches are proposed and justified by numerical examples.

Gain-flattening filters (GFFs) are key wavelength division multiplexing components in fiber-optics telecommunications. Challenging issues in the design of thin-film GFFs were recently the subject of a contest organized at the 2001 Conference on Optical Interference Coatings. The interest and main difficulty of the proposed problem was to minimize the sensitivity of a GFF to simulated fabrication errors. A high-yield solution and its design philosophy are described. The approach used to control the filter robustness is explained and illustrated by numerical results.

Useful features of an efficient thin-film synthesis technique based on the refinement of inhomogeneous films are adapted to the needle method. As in the former approach, all the layer refractive indices and thicknesses are refined simultaneously, and special attention is given to computation speed and accuracy. In particular, key parameters are calculated from analytical formulas, namely, the gradients used in the optimization routines and the optimum needle positions and refractive indices. Inhomogeneous as well as multilayer solutions are possible. The efficiency of the modified approach is demonstrated by the solution of a complex nonpolarizing, wide-angle antireflection coating problem. The results are compared with those obtained in the past by the conventional needle method and by refinement of inhomogeneous films.

A fast, efficient thin-film synthesis technique is described. Dielectric films are subdivided into thin layers whose refractive indices and thicknesses are varied simultaneously. Because of the large number of optimized parameters, speed and accuracy are given special attention. The efficiency of the method is demonstrated by the successful reproduction of several complex nonpolarizing filters synthesized in the past with the needle method, one of the best approaches currently available.