High Diffraction Efficiency Gratings In Dichromated Gelatin For Near Infrared Wavelengths

Abstract

Various applications of holographic optical elements (HOEs) have been widely studied in the litterature. They include array illuminators for input, output and offset beams on nonlinear optical arrays, connectors in optical cellular processors, head up displays, etc... Thick holograms show the potential of very high diffraction efficiencies and therefore attract particular attention. Photopolymers and dichromated gelatin (DCG) are among the best materials for a large index modulation extending over a thickness of several microns. Three practical and theoretical problems are associated with their use for thick HOEs: . chromatic aberration results from the wavelength change between recording -usually in the blue part of the spectrum- and reconstruction which often involves near infrared lines; . field aberrations in the case of reconstruction by more than one source point; . optimization of diffraction efficiency. In the present communication, we concentrate on diffraction efficiency; we elude the problem of aberrations by considering only plane waves holograms, i.e. holographic gratings. Firstly,we review previous approaches to diffraction modelling and reformulate them using an alternate rigourous analysis. Secondly, we discuss some issues of practical interest with reference to the various models and to our experimental results, such as Bragg peaks profile, optimal index modulation depth as a function of wavelength. Emphasis is put on the obtention of high efficiency in the near infrared. Thirdly, we apply our analysis to study the influence of polarisation and of index profile nonlinearities.