**4.7.1 Polarization multiplexing holograms**

Polarization multiplexing is based on the photo-induced anisotropic property of the material, at the same location of sample different kinds of polarization holograms are recorded. Todorov et al were first to show that two holographic recording could be stored independently inside the same film when using different combinations for the polarization states of the reference and the object beam during recording. Su et al presented a technique for polarization multiplexing in LiNbO3. Koek et al have presented a technique for simultaneous readout polarization multiplexing in bacteriorhodopsin.

Here different kinds of polarization multiplexing holographic storage were realized in fulgide film, including linearly polarization multiplexing and circularly polarization multiplexing, optical setup as shown in Fig.28. In circularly polarization multiplexing experiments, at the same location of the sample parallel circularly polarization hologram and orthogonal circularly polarization hologram were recorded. By adjusting the Q3 and P, the diffraction images of parallel circularly polarization hologram and of orthogonal circularly polarization hologram can be obtained individually and together, like shown in Fig.35. So it can be seen that the storage density can be doubled by using polarization multiplexing.

Fig. 35. The reconstruction images in polarization multiplexing holographic storage in fulgide film: reconstruction image (a) of parallel circular polarization hologram; (b) of orthogonal circular polarization hologram; (c) of both holograms
