Francisco Javier Martínez Guardiola

Linearity in the response of photopolymers as optical recording media

Photopolymer are appealing materials for diffractive elements recording. Two of their properties when they are illuminated are useful for this goal: the relief surface changes and the refractive index modifications. To this goal the linearity in the material response is crucial to design the optimum irradiance for each element. In this paper we measured directly some parameters to know how linear is the material response, in terms of the refractive index modulation versus exposure, then we can predict the refractive index distributions during recording. We have analyzed at different recording intensities the evolution of monomer diffusion during recording for photopolymers based on PVA/Acrylamide. This model has been successfully applied to PVA/Acrylamide photopolymers to predict the transmitted diffracted orders and the agreement with experimental values has been increased.

Analysis of the fabrication of diffractive optical elements in photopolymers

Recently the possibility to record phase diffractive optical elements (DOEs) onto photopolymers has been explored. Two of their properties when they are illuminated are useful to this goal: the relief surface changes and the refractive index modifications. The recording intensity distribution with a sinusoidal profile is the easiest profile to record in a holographic recording material, i.e. it can be obtained by the simple interference of two plane wave beams or alternatively using a spatial light modulator. This second method is more flexible and opens the possibility to record a wide range of diffractive elements such as binary, blazed gratings, diffractive lenses, etc. Sharp profiles may as well be recorded. In general they present a clear smoothing of the edges due to various reasons: the cut-off frequency (a low pass filtering) of the optical system, quality of the spatial light modulation, inhibition period, finite size of polymer chains, monomer diffusion, and non-linearities in the recording process. In this work we have analyzed the importance of some of these aspects of the photopolymer and the experimental set-up in order to record high quality DOEs. The photopolymer analyzed is based on polyvinylalcohol/acrylamide. To achieve this goal we have used a diffusion model to simulate the DOE’s recording with different recording intensities distributions.

Polarimetric and diffractive evaluation of 3.74 micron pixel-size LCoS in the telecommunications C-band

Liquid-crystal on Silicon (LCoS) microdisplays are one of the competing technologies to implement wavelength selective switches (WSS) for optical telecommunications. Last generation LCoS, with more than 4 megapixels, have decreased pixel size to values smaller than 4 microns, what increases interpixel cross-talk effects such as fringing-field. We proceed with an experimental evaluation of a 3.74 micron pixel size parallel-aligned LCoS (PA-LCoS) device. At 1550 nm, for the first time we use time-average Stokes polarimetry to measure the retardance and its flicker magnitude as a function of voltage. We also verify the effect of the antireflection coating when we try to characterize the PA-LCoS out of the designed interval for the AR coating. Some preliminary results for the performance for binary gratings are also given, where the decrease of modulation range with the increase in spatial frequency is shown, together with some residual polarization effects.

Anamorphic characterization of a PA-LCoS based holographic data storage system

We have included a Parallel Aligned Liquid Crystal on Silicon (PA-LCoS) microdisplay in a Holographic Data Storage System (HDSS). This novel display, widely accepted as Spatial Light Modulator (SLM), presents some advantages and disadvantages. One of these disadvantages is the anamorphic and frequency dependent effect. In this work we want to test this effect and see its effects in the complete optical process involved in the HDSS. We will use stripe-based patterns with different orientation (vertical and horizontal). To check the limits, we will increase the data density by decreasing the minimum stripe width. For evaluating the degradation suffered by the data page, we use the Bit Error Rate (BER) as figure of merit. We make a BER calculation from the statistical analysis of the histogram. In addition to the anamorphic effects we evaluate the degradation effects introduced by the non-uniformity in the illumination. To this goal we divide the image in several regions that are processed in the same way that the entire image. The error analysis of the entire optical system is useful for its calibration and fine adjustment. Once we have characterized the experimental setup we introduce the holographic material. Thus, by making the same analysis, we can evaluate the errors introduced by the material. As holographic material we use Polyvinyl Alcohol Acrylamide (PVA/AA), that has been characterized and developed in previous works by our group.