Adaptive Holographic Fiber-Optic Interferometer

Abstract

ABSTRACT Interaction of phase-modulated light beams in photorefractive local inertial responce media was analysed. Interaction of this type allows to registrate phase-modulated signals adaptively under low frequency phase disturbtion. The experiments on multimode fiber-optic interferometer with demodulation element based on photorefractive bacteriorhodopsin-doped polimer film are described. As the writing of dynamic phase hologram is an inertial process the signal fluctuations with the frequencies up to 100 Hz can be canceled. The hologram efficiencies are enough to registrate high frequency phase shifts ~ lO*1* radn. 1.INTRODUCTION The usual method to registrate phase-modulated signals by a fiber-optic sensor is the holographic real-time interferometry. It allows to get rather contrast patterns due to interference of specle-inhomogenious light waves easily?*2 The main restriction of this method is .connected with the strong influence of low frequency noise which leads to the random modulation of the output signal. In a mirror-type interferometer (case of a quasi-plane waves) the fluctuations can be decreased by the stabilization of the initial phase difference. The usual way is an electronical autotuning of the mirror?^ In a multimode optical fiber interferometer based on a static phase hologram the tuning is practically impossible. The reason of this is the disturbing of light beam - hologram tuning due to strong effect of any external influence to the specie structure of the interacting beams. The way to eleminate the effect of low frequency noise is to adapt the interferometer by the dynamic hologram with the responce time less then the average period of the fluctuations.The dynamic holograms were applied to the interferometric structures with quasiplane5 and diffusively scattered light waves.' The prevalent of dynamic holography media are photorefractive crystals ,8*9 Such crystals are studied theoretically and experimentally rather good. The typical feature of photorefractive crystals is the non-local photoresponce with large time constant. Due to this feature applications of the dynamic holography are limited by the difficulties of low-frequency noise adaptation and low efficiency of phase-to-amplitude signal transformation.The interesting local-responce fast photorefractive matherials are ones containing the photosensitive molecules of bacteriorhodopsin.'0 Regardless of the relatively low diffraction efficiency ofphase holograms those media allow to construct rather sensitive interferometers by the simplest way. The present paper studies the theoretical model of phase-modulated beam interaction in a media with the local inertial responce. The results of the experiments on multimode optical fiber interferometer with the dynamic phase hologram in bacteriorhodopsin-doped polimer- film are presented.