The Island CGH, a new coding scheme: concept and demonstration.

Computer generated holograms (CGHs) are powerful optical elements used in many fields, such as wavefront shaping, quality testing of complex optics, and anti-counterfeiting devices. The Lee algorithm is the most used to generate binary amplitude Fourier holograms. Grayscale CGHs are known to give a higher reconstruction quality than binary holograms, but they usually require a cumbersome production process. A very simple and straightforward method of manufacturing rewritable grayscale CGHs is proposed here by taking advantage of two key components: a digital micro-mirror device (DMDs) and a photochromic plate. An innovative algorithm, named Island algorithm, able to generate grayscale amplitude Fourier CGHs, is reported and compared with the standard Lee approach, based on 9 levels. A crucial advantage lies on the fact that the increase or decrease of the quantification does not affect the spatial resolution. In other words, the new coding leads to a higher spatial resolution (for a given CGH size) and a reconstructed image with an order of magnitude higher contrast with respect to the classical Lee-coded hologram. In order to show the huge potential of our approach, a 201 level Island hologram is designed, produced and reconstructed, pushing the contrast to values higher than104. These results reveal the potential of our process as well as our algorithm for generating programmable grayscale CGHs.

Programmable CGH on photochromic plates coded with DMD generated masks.

Computer Generated Holograms (CGHs) are used for wavefront shaping and complex optics testing. Present technology allows for recording binary CGHs. We propose a Digital Micro-mirror Device (DMD) as a reconfigurable mask, to record rewritable binary and grayscale CGHs on a photochromic plate. Opaque at rest, this plate becomes transparent when it is illuminated with visible light of suitable wavelength. We have successfully recorded the very first amplitude grayscale CGH, with a contrast greater than 50, which was reconstructed with a high fidelity in shape, intensity, size and location. These results reveal the high potential of this method for generating programmable/rewritable grayscale CGHs, which combine DMDs and photochromic substrates.

Optical characterization of fully programmable MEMS diffraction gratings.

We have fabricated and characterized fully programmable diffraction gratings consisting of 64 silicon micro-mirrors. The mirrors are 700µm long and 50µm wide with a fill factor of 90%. They are actuated electrostatically and move down by 1.25µm while showing negligible cross-talk and bowing as small as 0.14µm over 700µm. Extinction ratio up to 100 has been achieved by adjusting only 3 adjacent micro-mirrors. The gratings could operate either as light modulators up to 5µm or spectra generators up to 2.5µm.

Surface characterization of micro-optical components by Foucault's knife-edge method: the case of a micromirror array.

As micro-optical components are introduced into optical systems, accurate surface characterization becomes important. We describe a method for quantitative evaluation of surface deformations based on Foucault's knife-edge test. By measurement of local slopes, the surface shape of each mirror in a micromirror array has been reconstructed with a subnanometer accuracy. In addition to low-order deformation (tilt, curvature, astigmatism), each mirror is seen to be palm-tree shaped. This may be explained by strain relaxation in the fabrication process. Measurement on a conventional concave mirror confirms our method.

Characterization by guided wave of instabilities of optical coatings submitted to high-power flux: thermal and third-order nonlinear properties of dielectric thin films.

We study the reversible refractive index variations of optical thin films submitted to a high-power light flux. As a preliminary step, we study the thermorefractive coefficient ?n/?T and the laser damage threshold of our materials. From the hypothesis of a localized optical Kerr-type effect, we use the m-line technique to estimate the nonlinear refractive-index coefficients n(2) of TiO(2), Ta(2)O(5), and ZnS films with continuous illumination. Very large values of n(2) are found for the films obtained by conventional evaporation condensation. On the other hand, the study of the change in the decoupling direction of a low-power light guided in a layer disturbed by a pulsed YAG laser gives a first analysis of the phenomena versus time.