Focal Plane Array Spectrometer FPAS: preliminary development results and recommendations

Abstract

The focal plane array spectrometer (FPAS) is a miniaturization concept for imaging spectrometers compared to classical dispersive and FTS instruments. FPAS is an imaging implementation of interferogram integrated FTS, targeting space-borne and commercial applications. It is based on a bi-dimensional array of waveguide spectrometers which can be assembled in small size, and form a compact package of single spectrometers. When this system is positioned in the focal plane of an objective (in a similar way as a CMOS detector array or a CCD in a camera), it will allow imaging spectrometry of the observed surface (objects). The instrument is therefore reduced to the imaging optics and the FPAS in the focal plane, which takes over the role of spectrometer and detector array. This is a breakthrough concept enabling imaging spectroscopy in a reduced volume with low power consumption. This article describes the preliminary development results of a FPAS based on arrays of single-mode waveguides. The developed linear array forms an acquisition line to image with lateral scanning. The light is injected with micro-lenses into several optical polymer waveguides. In order to circumvent the sub-sampling limitation due to nano-sampler spacing and to expand the spectral bandwidth, the basic principle of a static Lippmann spectrometer is combined with a dynamic Fourier-transform spectrometer, by adopting a piezo actuated movable mirror located at the waveguide end-facet. This waveguide spectrometer is designed for a nominal bandwidth of 25 nm at central wavelength of 762.5 nm and a spectral resolution of about 0.06 nm. The throughput and SNR in this preliminary linear array prototype are analysed and design limitations are discussed. The authors then introduce a new method for the realization of FPAS aiming at enhancing throughput and bandwidth of the integrated device. This new concept uses a bundle of single-mode waveguides, instead of only one, at the focal distance of the optical element. The novel FPAS concept is elaborated and its characteristics are described.