Design of broadband dielectric coatings for near-infrared Fabry-Perot interferometer

Abstract

Fabry-Perot interferometer has an important effect on near-infrared high spectral resolution spectrograph. In 1896, Ch.Fabry and Alfred Perot designed and used the Fabry-Perot interferometer for the first time. Since then the instruments using Fabry-Perot interference phenomena have been applied broadly to multi-field, such as astronomy, laser, and fiberoptic transmission. Fabry-Perot interferometer has many advantages such as narrow passband, high spectral resolution,high throughput, easy wave-length adjustment, simple structure and large aperture. Comparing with traditional visible light, the solar observation in near-infrared has many advantages: for example, weaker magnetic field strength can be more precisely measured with near-infrared spectrum .So developing the key technology of near-infrared high spectral resolution spectrograph Fabry-Perot interferometer has become urgent. For developing near-infrared Fabry-Perot interferometer, there are four difficulties: producing high quality optical plane: peak-to valley surface flatness better than λ/100; coating Fabry-Perot interferometer plates with broadband multilayer dielectric films(including spectrum performance, thickness uniformity and stress effects); controlling the distance of interference cavity; keeping constant temperature. In this paper, the process of designing broadband dielectric reflective and antireflective coatings applied in near-infrared Fabry-Perot is described and some problems of designing Fabry-Perot interferometer are discussed: the design of broadband dielectric mirror is described with reflectivity of 93.9±1.0% over spectralranges from 1.0μm to 1.7μm; by reflective phase shifts in the design of mirror coating, computing the required film thickness uniformity atλ/100 of peakto-valley surface flatness;degradation of surface figure is perhaps more thanλ/100 even if the soft coating materials-zinc sulfide and cryolite are used, and in order to reduce the degradation of surface figure brought by the stress of dielectric mirror coating, antireflective coating adopts same materials of dielectric mirror coating, ZnS and Na3AlF6, and similar film total thickness.