Master thesis and internship[BR]- Master's thesis : Study of Photon Recycling in a Hyperspectral Visible/Infrared Imager[BR]- Integration Internship

Abstract

This study evaluates the possibility of taking advantage of the second-order diffraction efficiency to increase the spectral range of a hyperspectral imager. The target is to cover the VNIR-SWIR range. This so-called second-order recycling involves the multiplication of noisy measurements with the inverse of a spectral response matrix, which maps the wavelengths constituting the observed scene to those measured on the detector. This matrix is ill-conditioned when the wavelength minimizing the first-order diffraction efficiency is part of the incident light. This leads to an amplification of the noise in the measurements during the inversion process. To prevent this, regularization methods are implemented but they do not enable an accurate reconstruction of the incident spectrum at the blaze wavelength and its half. Two solutions are explored. The first consists in reducing the blaze wavelength such that the first-order efficiency minimum is out of the range. It proves successful, at the expense of a reduced spectral range. To overcome this issue, the second solution involves the design of a dual-blazed grating to increase the bandwidth of diffraction efficiency. Those solutions are shown to enable a retrieval of the radiance with a relative error below 5% for most of the spectrum, through direct numerical inversion. The correction of out-of-band transmission is addressed by assessing its effect on the inversion problem. The study reveals that it does not further complicate the inversion, demonstrating that second-order recycling is feasible in realistic applications. By changing some optical parameters, the regularization methods are shown to be useful when the size difference between a pixel and the slit gets larger, thus extending the set of optical configurations for which second-order recycling is feasible. The correction of higher-orders is discussed and found to be feasible only for the single-blazed design, but on a reduced spectral range.