Abstract

Already in 1974 the use of multiphoton processes was proposed for hologram re cording in photorefractive crystals. Then nondestructive readout of information is possible via one-photon effects, eliminating the need for fixing processes while the versatility of optical erasure is maintained. If impurity ions exhibiting real intermediate states with relatively long life times can be used a considerable reduction of the peak intensity required for two-photon recording is possible, though the intensity has to be still much larger than in the case of usual one-step recording. In 1984 nondestructive readout of holograms recorded by two-step excitations in LiTaO₃:Fe was demon strated. The holograms were written by simultaneous illumination with 30 ps pulses of a mode-locked Nd:Yag laser at 1064 nm forming the interference pattern and with spatially homogeneous frequency-doubled pulses of the same laser at 532 nm. From the experimental accuracy it was concluded that more than 10000 read-out processes are possible. Similar results were obtained for LiNbO₃:Cr using 40 ns pulses of a Q-switched ruby laser at 694 nm.

© 2000 IEEE