Thin-film filters for a high resolution miniaturised spectrometer

摘要

A lightweight and compact spectrometer with spectral resolution in the order of 0.1 nm, in a spectral range from UV to NIR, can be implemented using a spectral pre-selection technique at the spectrometer entrance, through transmission variable filters. Such filters, based on thin-film optical coatings, have either a transmission peak or a transmission edge that moves along one direction of the filter surface. Depending on the spectrometer design, different configurations of the pre-selection device are possible. The operating spectral range (240-800 nm) is divided in a number of sub-ranges and in each of them the filter transmission peak, or edge, is displaced from the minimum to the maximum wavelength over a distance of few mm. Two cases are considered: a configuration with both a narrow-band transmission filter and a band-pass filter having a linear spatial variation and a configuration with an edge filter having a non-linear spatial variation. To obtain the required spatial profile of filter performance, a graded coating is deposited on a fused silica substrate, by r.f. sputtering with a moving mask. Details on filter requirements and fabrication technology are reported.