Thin-film graded optical filters for mini-spectrometers

The thin-film technology allows the fabrication of optical filters which behaviour is not uniform along the surface of the device. Such intentional non uniformity permits the selection of different wavelengths of the spectrum with a single small-dimension optical component. Each wavelength is transmitted at a given position over the component surface and this effect is obtained by depositing a graded coating on a glass substrate. This narrow-band transmission filter is based on the combination of metal and dielectric thin films, in fact this choice allows a wide operating wavelength range with a low number of layers. The optical filter coupled to an array detector gives a spectral sensor that is useful for the fabrication of small-dimension and low-weight spectrometers.