Abstract

There has been a resurgence of interest in the use of Fabry Perots as the narrow band selecting element in high-resolution imaging astronomical spectrometers. The trend has been to solid etalons¹ and away from air-spaced interferometers which often require complex electronics to control parallelism and spacing of the flat plates. These may be manufactured from optically passive material such as silica where tuning is achieved by tilting or electrooptic material such as lithium niobate (LiNbO₃) where the filter can be scanned electrically². Typically the solid etalon has a thickness in the range 0.2-0.5 mm and a diameter in the range 40-70 mm. In this presentation we report on the role that coatings have played in the manufacture of a number of etalons. During the preparation of the substrate, departures from optical thickness uniformity over the aperture have been reduced by depositing layers with the same refractive index as the substrate to selected parts of the surface. Single wavelength and broad band high reflectance coatings have been applied to the surfaces of the etalons. In the case of the electrically tunable etalons, the outside layer of the coatings has consisted of a transparent conducting layer of indium tin oxide (ITO). Ion assisted deposition has been used to prepare hard, dense dielectric oxides layers as well as the ITO. No substrate heating has therefore been required. Measured finesses over the full working aperture have ranged from 15 to 30.

© 1995 Optical Society of America