Holograms in photorefractive materials

DETAILED DESCRIPTION OF DRAWINGS Referring now more particularly to the drawing, like numerals indicate like pars and structural features in the various diagrams.
A bismuth silicon oxide (BSO) photorefractive crystal 10 stores multiple volume holograms.
An electric bias field is applied through the crystal 10 along its 110 axis through electric terminals 11 and 12.
It is preferred that the frequency of the bias field be not greater than 100 Hertz.
An argon laser 14 emits monochromatic light at about 5145 Angstroms.
A mask 15 acts as a spatial filter for the laser 14 emitted monochromatic light beam.
Beam splitter 16 splits the laser 14 beam into two component parts of equal light intensities.
The first light beam 17 leaves splitter 16 to be reflected by mirror 18 to the light receiving surface of photorefractive crystal 10.
Beam 17 constitutes a reference path for inducing holographic storage in crystal 10.
The second light beam becomes the information carrying light beam through modulation by light modulator 21, as controlled by information bearing signal supplied to it over line 22.
The output light of modulator 21 travelling over path 23 to crystal is mixed with the reference light of beam 17 within crystal 10 to form a hologram.
When phase conjugation is to be employed, the light intensity of beam 20 is much less than the beam intensity of beam 17, i.
e.
would have a low value of M (0.
0001 or less).
For data recording, the light intensities are equal; the contrast ratio M is equal to unity.
Gallium arsenide solid state laser diode 26 emits a hologram probing beam 27 to the light receiving surface of BSO crystal 10.
The probe beam travels through crystal 10 for sensing the stored hologram.
Transmitted light beam 29 leaves crystal 10 to impinge upon data detector 31.
The light beams 17 and 23 also travel through crystal 10 resulting in light beam 28 impinging upon detector 30.
The output signals of detectors 30 and 31 are applied to utilization means 32 for measuring the diffraction efficiency of BSO crystal 10