STABILIZED RECORDING AND THERMAL FIXING OF HOLOGRAMS IN PHOTOREFRACTIVE LITHIUM-NIOBATE CRYSTALS

Holograms are thermally fixed in photorefractive lithium niobate cryst als, i.e., they are recorded at temperatures between 130 and 180 degre es C. The setup is actively stabilized during recording against moveme nts or vibrations of the interference pattern which especially occur d uring long-period writing at enhanced temperatures. Two different tech niques are investigated: (1) Interference of the recording beams using one crystal surface as a beamsplitter yields a signal for stabilizati on. (2) Alternatively, one of the beams is periodically phase modulate d 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 seco nd method is advantageous for multiplexing experiments, because no rea djustment of the beam-coupling stabilization system is required if the angles of the recording beams are changed. (C) 1998 American Institut e of Physics.