Page 132 - Introduction to Information Optics

2.7. Processing with Photorefraetive Optics 1 1. 7

space charge field which is shifted by a quarter of a period (or n/2 in phase)
relative to the intensity pattern. Owing to Pocket's effect, the space charge field
induces a change in the refractive index, as given by

(2..00,

where A 1 and A 2 are the amplitude of the two incident waves, An sa, is the
saturation index amplitude, and T is the time constant. We note that Aw sat and
T are material-dependent parameters.
The two beams can, in fact, carry spatial information (e.g., images) with
them. The interaction of the two beams then generates a volume index
hologram in the PR medium. While an optical beam propagates through the
medium, it undergoes Bragg scattering by the volume hologram. If the Bragg
scatterings are perfectly phase matched, a strong diffraction beam will recon-
struct the spatial information. The formation and diffraction of the dynamic
holograms within the PR medium can be explained by nonlinear optical wave
mixing (as briefly discussed in the next section).
PR effect has been found in a large variety of materials. The most commonly
used photorefractive materials in optical processing applications fall into three
categories: electro-optic crystals, semi-insulating compound semiconductors,
and photopolymers.
Lithium niobate (LiNbO 3), barium titanate (BaTiO 3), and strontium bar-
ium niobate (SBN Sr a_, c)Ba, cNb 2O 6) are by far the three most efficient
electro-optic crystals exhibiting PR effects at low intensity levels. Iron-doped
LiNbO 3 has a large index modulation due to the photovoltaic effect. It is also
3
available in relatively large dimensions. For instance, a 3 cm sample used to
record 5000 holograms has been reported. Because of its strong mechanical
qualities, LiNbO 3 has been extended to near infrared (670 nm) wavelengths by
adding Ce dopants.
High diffraction efficiency can be achieved with LiNbO 3 crystals without the
involvement of PR effect. This eliminates the phase distortion of the image
beam and is preferred in some applications. However, BaTiO 3 crystals are not
3
available in large dimensions. A 5 x 5 x 5 mm sample is considered to be big.
Moreover, in order to reach the maximum index modulation, the sample must
be cut at a certain angle of the crystallographic c-axis. This cut is difficult, and
it reduces the size of the sample. A phase transition exists around 13 C;
therefore, the crystal must always be kept above this temperature.
SBN has a large electro-optic coefficient, which can be reached without a
special crystal cut. The phase transition (which can be tuned by doping) is far
from room temperature. It can also be subject to electric or temperature fixing.
The optical quality of SBN obtained up to now is still poorer as compared with
LiNbO, and BaTiO 3 crystals, however.