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Holograms are thermally fixed in photorefractive lithium niobate crystals, i.e., they are recorded at temperatures between 130 and 180 °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 7.5×10−4 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.

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Holographic gratings were thermally fixed in iron-doped photorefractive LiNbO(3) crystals and developed with frequency-doubled Q -switched pulses of a Nd:YAG laser (light wavelength, 532 nm; pulse duration, 3 ns). The saturation values of the diffraction efficiency increased with increasing pulse light intensity. Compared with development with cw light, high-intensity laser pulse development resulted in an enlargement of the diffraction efficiency of a factor of ~2 . A contribution of the intrinsic defect Nb on Li site to the charge transport is most probably the origin of this effect. The results are useful, e.g., for improvements of volume holographic memories and wavelength filters.

2.5 -1.5 -0.5 0.5 1.5 2.5 External fleld [kV/cm] CMB5 Fig. 1 Reconstructed intensity of a planar reflection hologram, recorded with an external field of 2 kV/cm, as the external field is scanned from -2.5 kV/cm to +2.5 kV/cm. 3 -2 -1 -7 * N 0 a -1 v 0 -2 0.7 1.7 2.7 3.7 External Field [kVlcm]LiNb0,:Cu Fig. 2 Reconstructed intensity of 3 multiplexed holograms recorded with an external field of a: 1.7 kV/cm; b 2.7 kV/cm; c: 3.7 kV/cm, as the external reading field is scanned.ternal reading field E, . Because this is a quadratic effect optimal reconstruction occurred at E, = 2 b. Results of EFM of three gratings are presented in Fig. 2. The three gratings were recorded under fields of 0.7, 1.7, and 2.7 kV/cm respectively. Finally, we multiplexed three images of a resolution target at three different angles under fields of 0.7, 1.7, and 2.7 kV/cm respectively. No cross talk was observed during reconstruction of the three images at the respective writing fields. These preliminary results clearly demonstrate that EFM is more effective when performed at the paraelectric phase when the electro-optic effect is quadratic.

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