Optical spectral filtering with segmented diffractive optical elements

This thesis presents a study of optical spectral filtering with segmented Diffractive Optical Elements (DOEs). Instead of moving a DOE as a whole, as is done with a scanning monochromator, it is possible to segment a DOE in several parts that can be displaced individually. This principle gives much more freedom in the way the filter transmission can be controlled. However, typical dimensions of DOEs and the accuracy in the displacements require a level of miniaturisation that was not achievable at a reasonable cost. The development of micro-technologies has changed this context, making possible the fabrication of both the DOE and the actuation mechanism on a single micro-mechanical chip. The present work describes in detail several implementations of micro-mechanical segmented DOE filters, which we believe to be especially well-suited to the synthesis of spectral filters with arbitrary transmissions. The spectral transmission of those DOE filters is derived, using the scalar diffraction theory. We also study several algorithms that give the positions of the segments, so that the synthetic filter transmission matches a target. The performances of these algorithms are assessed both theoretically and with computer simulations. Based on those results, we compare DOE filters to other types of optical filters with controllable or adjustable spectral transmission, and show that segmented DOE filters have inherent advantages and limitations. We came to the conclusion that segmented DOE filters are especially well-suited to some tasks in spectroscopy, and to this effect we fabricated a prototype of a micro-mechanical diffractive Fresnel lens. This work encompasses the fabrication of the prototype, as well as an experimental characterisation that has a value of proof of principle.