Temporal evolution of the angular response of a holographic diffraction grating in PVA/acrylamide photopolymer

Abstract

Abstract:In this paper we evaluate the temporal evolution, after exposure, of a diffraction grating stored in a PVA/acrylamide photopolymer. We also study the overmodulation of the refractive index inside the hologram, which gives rise to a particular behaviour of the angular response of diffraction efficiency. This evolution takes place in our photopolymer due to the incorporation of dimethylacrylamide (DMAA), which is a liquid at room temperature and so favours diffusion of the species in solution from the zones of greater concentration to those of lower concentration.

Stabilization of volume gratings recorded in polyvinyl alcohol-acrylamide photopolymers with diffraction efficiencies higher than 90%

“…Plates with AA as the only monomer are very unstable at high AA concentrations and noise gratings are formed with thick layers. Another possibility would be to include dimethylacrylamide so as to prevent spontaneous crystallization of the monomer; however, the gratings are not stable for very long [17]. In addition, we know that, if we add BMAA as cross-linker to the same initial monomer concentration, a higher index modulation (around 0.005 compared with 0.003) is obtained [12].…”

“…This type of photopolymer is very sensitive to laboratory conditions owing to the low solubility of BMAA in water; therefore, the drying time must be modified when there are slight variations in temperature or humidity. Once dried, the photopolymer was cut into plates measuring 6.5 cm  6.5 cm to be used in our experimental set-up, and the thickness, absorption constant and refractive index modulation were measured by optical methods [17].…”

“…The experimental device is a typical holographic set-up [17], and it can seen in figure 1. An argon laser at a wavelength of 514 nm was used to store diffraction gratings by means of continuous laser exposure.…”

“…The behaviour of this type of photopolymer during the recording period is well known [13][14][15][16][17] and we can obtain volume holographic gratings with diffraction efficiencies higher than 95%, taking into account the Fresnel losses. However, the stability of these gratings depends strongly on small variations in laboratory conditions (relative humidity and temperature).…”

“…However, the stability of these gratings depends strongly on small variations in laboratory conditions (relative humidity and temperature). The diffraction efficiency is constant only under red light since, if we illuminate the hologram with white light, the residual dye and monomer react [17,18]. For this reason, refractive index modulation is lost and the diffraction efficiency decreases.…”