Broadband loss-less optical thin-film depolarizing devices

Polarized light is generally considered as an added value and is often used to improve the viewing of scenes and samples through various optimization processes. However, there are a number of situations where the polarization of the light is penalizing, and for which it is important that the polarized light is transformed into an unpolarized light. For illustration, many space applications require on-board detectors to analyze optical flows from the Earth or the environment. These flows are collected after being scattered and reflected by the different elements encountered, which can partially polarize the light studied. This depolarization is not predictable because it strongly depends on the environments crossed (clouds of water or dust, atmosphere ...) whereas it strongly influences the calibration of the instruments. Although the polarization state of light can be easily transformed into another arbitrary state, or from unpolarized light to polarized light, the reverse situation of depolarizing light is less common and often accompanies optical losses and a reduction in spatial or temporal coherence. As a result, different types of devices and systems have been designed and constructed to achieve this depolarization function. Nevertheless, the systems currently used lead to substantial losses of energy or to a division of the beam considered. In addition, these systems generally operate in transmission and are not integrable. Finally, they do not offer a spectral control of the degree of polarization. We proposed an original alternative technique based on the principle of spatial depolarization (with regard to temporal depolarization). It requires an optical multilayer type component having a transverse gradient of optical properties. In this paper, we will show under which conditions this gradient satisfies a spatial depolarization condition, without creating energy loss. This synthesis step simultaneously takes into account the spatial and spectral variations of the optical properties of the filter. These conditions are then refined to minimize the diffraction effects of the depolarized beam. A compromise is sought between the value of the spatial gradient of the filter, and the spectral variation speed of its polarimetric phase shift. The results obtained are remarkable and the technique is extended to provide specular depolarizers operating in broadband or narrowband . The components are finally manufactured by vacuum deposition technologies. A specific method is used to reach the desired spatial gradient. The metrology that we have implemented reveals an excellent agreement with the theoretical predictions, both for depolarization and diffraction over a wide spectral range.