Page 136 - Introduction to Information Optics

2.7. Processing with Photorefractive Optics I")'
by the incident angle of the reference beam. To change the angle of the
reference beam, a mirror mounted on a rotating step motor can be utilized.
For rapid access to all of the stored images, acousto-optic cells can be used to
deflect the reference beam. Note that two acousto-optic cells must be used to
compensate for the Doppler shift in frequency.
For wavelength multiplexing, both the object and reference beams are fixed
and only their wavelengths are changed. The first demonstration of wavelength
multiplexing with PR materials was made to record three primary-color
holograms from a color object. The simultaneous replay of the three holograms
reconstructs the colored image. The application of wavelength multiplexing has
stimulated the development of solid-state tunable laser diodes and specially
doped PR crystals that are sensitive to laser diode wavelength range.
In phase-code multiplexing, the reference beam consists of multiple plane
wavefronts. The relative phases among all these wavefronts are adjustable and
represent the addresses of the stored images. Each image can be retrieved by
illuminating the holograms with the exact same phase code used for recording
the image. The merits of phase-code multiplexing include fast access, high light
efficiency, and the elimination of beam steering.

2.7.3. BRAGG DIFFRACTION LIMITATION

Bragg diffraction limitation in a thick PR crystal can be explained with the
k vector diagram, as depicted in Fig. 2.38. The recorded spatial grating vector
kis

k = k 0 - k t , (2.104)

where k 0 and k t are the writing wave vectors. If the recorded hologram is read
out by a wave vector k 2 (where the scattered wave vector is denoted by k 3),
then the optical path difference (OPD) of the scattered light from two points
within the crystal can be written as

OPD = k • r - (k 3 - k 2 ) • r = Ak • r, (2.105)

Fig. 2.38. Bragg diffraction vectors.