Stabilized recording and thermal fixing of holograms in photorefractive lithium niobate crystals

Holograms are thermally fixed in photorefractive lithium niobate crystals, i.e., they are recorded at temperatures between 130 and 180 degrees C. The setup is actively stabilized during recording against movements or vibrations of the interference pattern which especially occur during long-period writing at enhanced temperatures. Two different techniques are investigated: (1) Interference of the recording beams using one crystal surface as a beamsplitter yields a signal for stabilization. (2) Alternatively, one of the beams is periodically phase modulated and the beam-coupling signal is used for stabilization. Reproducible refractive index changes of thermally fixed holograms up to 7.5 x 10(-4) are obtained with both stabilization techniques. However, the second method is advantageous for multiplexing experiments, because no readjustment of the beam-coupling stabilization system is required if the angles of the recording beams are changed. (C) 1998 American Institute of Physics.