Bruno Viaris de Lesegno

In-situ SLM correction by birefringence mapping

Liquid crystal spatial light modulators (SLM) often suffer from defects that need to be compensated for demanding phase modulation applications. Usually the calibration map used to correct the SLM are determined at the factory or on a dedicated optical bench, but either way the measurement is done away from the experimental setup. So this correction map can’t reflect any temporal modification of defects, or correct for defects induced by the experimental setup itself or the environment. With a liquid crystal SLM, the read-out beam phase is modulated by tuning locally the birefringence. We present here a method where we record the birefringence map with a second superimposed light beam which uses the SLM in intensity modulation. In a first experiment we use the birefringence map to deduce the complete phase response of the SLM and optimize its parameters. In a second experiment we demonstrate the correction of externally induced defects: after a comparison between the measured and desired birefringence maps, SLM defects are compensated via a feed-back on the addressed hologram. As SLM monitoring is done in-place we can control time-dependant defects like those induced by a powerful read-out beam or a thermal drift. This method allows us to measure the defects of the SLM with spatial and phase resolutions comparable to interferometric methods. As it relies on polarization modulation, vibrations and misalignments are not critical, therefore supplying robustness. Furthermore, this method provides in-situ measurement, so that it’s easy to compensate day to day defects variation or aging. Finally the demonstrated method is a way open to closed-loop phase correction.

Cold atoms in holographically shaped optical traps

We are exploring the possibilities offered by Spatial Light Modulators to create dipole potentials with arbitrary shape in order to apply to cold atoms. As a first experiment we have realized a guide for atoms by shaping the laser beam into a Laguerre-Gaussian mode, creating a sort of tube, empty of light, where the atoms rest confined during their flight in the gravity field. We are, now, creating new potentials by modifying the Laguerre-Gaussian hologram in order to create new original traps for atoms.

Improvements on the refresh rate and dynamical properties of a SLM by sequential readout using an acousto-optic modulator

We demonstrate the use of an acousto-optic modulator to enhance the refresh rate and dynamic properties of a liquid-crystal spatial light modulator (SLM). The useful area of the SLM surface is split in several zones which are addressed separately, and read in a sequence by a steered laser beam. This configuration allows to increase the refresh rate by five orders of magnitude. Furthermore, improvements on the nature of the transition between different holograms are experimentally shown. The advantages of this technique are discussed in the particular context of cold atom manipulation with holographic optical tweezers.