Afocal coupled etalons DEM: a high-resolution double-etalon modulator spectrometer

Abstract

The properties of a spectroscopic device consisting of two etalons coupled by an afocal system, and which behaves as a high-luminosity single-etalon single-aperture Fabry-Perot spectrometer, have been investigated. Although the high-luminosity behavior is one of the many states in which a coupled dual-etalon system can exist, mainly the high-luminosity condition has been studied in detail here. This condition is reached when the orders of the etalons comprising the device are related by n(01) = n(02)eta(-2), where the coupling constant is defined as eta = micro(1)micro(-1)(2)f(1)f(-1)(2) micro(i) and f(i) are the indices of refraction and coupling lens focal length associated with the ith etalon. In high-luminosity conditions, one of the etalons serves as a multiple annuli mask for the other thus making it possible to use many of these annuli to achieve the desired high throughput. The limitation on the number of orders usable is given by the (small) nonlinearity in the matching of these orders, because of the different etalon gaps. The results of the study show that luminosity gains near 100 (when compared with a single-etalon single-aperture Fabry-Perot spectrometer) are possible at high resolving powers. This occurs when the coupling constant eta is not too far from unity. It is also shown that this device, or double-etalon modulator (DEM), is a compensated spectrometer since the gain increases with increasing resolving power. Other practical limitations, such as the beam walk-off from the edges of finite size etalons, are discussed.