The characteristics and the applications of the photorefractive $Bi_12(Si;Ge)O_20$ Single Crystal is investigated both in a transverse electrooptic modulator type specimen and in a longitudinal electrooptic modulator type specimen.
An incoherent image is converted to a coherent image in the incoherent-to-coherent image converter system. The image conversion is achieved in the transmission type Pockels Readout Optical Modulator(PROM) system which is the transverse electrooptic modulator type. The system employs $Bi_12SIO_20$ crystal of (001) cut and the thickness of the crystal is 800㎛, and the applied electric field intensity is 4 KV/cm. The real time incoherent-tocoherent image conversion is successfully accomplished proving experimentally the theoretical prediction.
The real time holographic image recording in the longitudinal electrooptic modulator is made in photorefractive $Bi_12GeO_20$(BGO) crystal using $Ar^+$ laser (5145 $À$) beam, and both the diffraction efficiency and image recording and erasure times are measured by using He-Ne laser(6328Å). The dimension of BGO crystal is 6.05mm (110) × 1.60mm (001) × 5.65mm (-110) and applied electric field intensity $E_o$ is 0-9.90KV/cm. The diffraction efficiency increase from $2.08×10^{-3}$($E_o = 0$) to $3.85×10^{-2}$ ($E_o = 9.90KV/cm$) as applied electric field intensity is increased, and the image recording time and the image erasure time are dependent on $E_o$ and they are 60msec-90msec for the same applied voltage range. The good quality of the reconstructed image is obtained.
Also, phase conjugate wave is generated from a degenerate four wave mixing in photorefractive $Bi_12SiO_20$ (BSO) crystal without applied voltage. Experiments are performed using the linearly polarized $Ar^+$ laser beam (λ = 5145A) and BSO crystal. The size of this crystal is 10mm (110) × 10mm (001) × 3mm (-110). The maximum intensity of the signal beam is obtained at the pump ratio of r = 1 and the intensity ratio of R = 0.4 between forward pump beam and probe beam. And we have observed that the distorted probe beam is corrected by the wavefront restoration property of the phase conjugate wave.
In a degenerate four wave mixing system, the phase conjugate signal from a transmission grating recorded in a $Bi_12SiO_20$ crystal is measured at various temperatures and the results are compared with theory. The diffraction efficiency η has a maximum value at a temperature above room temperature. The dynamic range in η vs $I_t$ increases with increasing temperature, where $I_t$ is the total intensity of the laser beam incident on the crystal. The experimental results are interpreted in terms of the temperature dependence of the effective modulation depth of the transmission grating and the diffusion field in the crystal. Further works on those temperature dependences and that property in the Pockels' coefficients should give more accurate understanding of temperature dependent behavior of the diffraction efficiency of $Bi_12SiO_20$ crystal.
