Page 131 - Introduction to Information Optics
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16 2. Signal Processing with Optics
Beam 1 Beam 2
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Intensity
vAAAAA; .
Charge Density
Space Charge
Field
Refractive Index An(x)
Variation
Fig. 2.35. Refractive index grating produced by interference fringes in a PR medium.
maximum intensity, photoionized charges are generated by the absorption of
photons. These charge carriers diffuse away from the bright region and leave
behind positively charged donor impurities. If these charge carriers are trapped
in the dark region, they will remain there because there is almost no light to
reexcite them. This leads to the charge separation as shown in Fig. 2.35. Due
to the periodic intensity distribution within the PR medium, a space charge is
created with the dark regions negatively charged and the bright regions
positively charged. The space charge continues building up until the diffusion
current is counterbalanced by the drift current. The space charges produce a
