Films made of the halobacterial photochrome bacteriorhodopsin (BR) can be used in a number of holographic real-time applications. Their application as active material in a dual-axis joint-Fourier-transform (DAJFT) real-time correlator was shown recently. The BR films have a strong nonlinear intensity dependence on the light-induced absorption and refractive-index changes. Therefore the holographic diffraction efficiency also shows a nonlinear dependence on the writing intensity. We investigate the effect of this nonlinearity on the result of the correlation process in a bacteriorhodopsin-based DAJFT correlator. Numerical models supporting the experimental observations are presented. It was found that the BR film combines the holographic function for most objects with that of a spatial bandpass filter, whose center frequency is tuned by the writing intensity. This results in smaller peak widths and a suppression of the sidelobes. BR films allow the application of this nonlinear behavior in real time to the all-optical correlation process.
The biological photochrome bacteriorhodopsin (BR) has attractive photophysical properties which allow its use as the photoactive component for dynamic recording media for optical applications. Purple membrane (PM) patches, which contain BR in a two-dimensional crystalline lattice, are isolated from Halobacterium halobium. Polymeric films with embedded PM are well suited reversible media for holographic recording. In addition, artificial derivatives of BR with improved optical properties can be generated by genetic methods and isolated from the mutated halobacterial strains. The high reversibility (> 106 record/erase cycles), the fast timescale of its photoconversions (fs - ms), and the high resolution (> 5000 lines/mm) make these films suitable media for real-time holographic applications. A dual-axis joint-Fourier-transform correlator is described with two liquid crystal television screens as input devices and a BR-film as active holographic material in the Fourier plane. The experimental data presented demonstrate that this system is capable of processing two independent video signals in real-time with a signal-to-noise ratio of 45 dB. The polarization recording properties of BR-films offer an efficient method to separate the correlation signal from scattered light.