A confocal Fabry-Perot processor, with coherent image amplification provided by a photorefractive BaTiO(3) crystal in the feedback path, is analyzed and implemented to perform the iterative algorithm based on the relation B(-1) = (I - A)(-1) = (infinity)Sigma(k=0) A(k), where B is the matrix to be inverted and I is the identity matrix. Both A and B are large size matrices. When the feedback loop contains a coherent matrix-vector multiplier (AX) and the input vector is sequentially scanned from one element to another, the columns of B(-1) can be sequentially generated at the output. The photorefractive BaTiO(3) amplifier provides loss compensation and coherence restoration of the feedback signal, thereby increasing the effective number of iterations in the algorithm. Thus it becomes possible to use this technique to implement slowly (as well as rapidly) converging algorithms. Experimental verification of the matrix inversion algorithm is presented, along with an analysis of possible real-time operations.
We propose and experimentally demonstrate a simple technique capable of significantly enhancing the signal-to-noise ratio of photorefractive amplifiers. The optical noise due to amplified scattered light and multiple interface reflections is removed by performing two-wave mixing in off-axis-rotating BaTiO(3) and Bi(12)SiO(20) crystals. A 20-fold improvement of the signal-to-noise ratio is achieved, and virtually noise-free image amplifiers are demonstrated.
The results of an experimental optical technique for imaging the electrical domain repartition in semi-insulating GaAs:Cr are reported. The technique is based on the use of the crystal as the active component of a transverse electro-optic two-dimensional light modulator. Under dc applied voltage, the electrical domains are traveling from the cathode to the anode at a velocity that increases with the applied voltage and with the incident illumination (v≂10–100 mm/s). Results for ac applied voltages are also presented. In particular, the observation of stationary and periodically distributed high-field domains in GaAs:Cr is reported for sawtooth applied voltages (1 kV, 50–250 Hz). These high-field domains induce a phase structure whose period is shown to be electrically controllable. This is the first reported demonstration of the possibility of a variable grating mode operation in semiconductors.
Large values of the exponential gain coefficient Γ are obtained (Γ≂8–12 cm−1) when recording with a moving grating in photorefractive BSO crystals (nearly degenerate two-wave mixing; drift recording mode). The resolution of the Kukhtarev’s equations with a moving grating shows a resonance effect which at the optimum velocity makes the modulation of the photoinduced space charge field Esc higher. An optimum of the grating spacing also exists: Λopt:2π(E0/NA)(με/eγR)1/2. In such conditions, the space charge field is phase shifted by π/2 with respect to the incident fringe pattern; this allows an efficient beam coupling between the two recording beams. The dependence of the gain Γ versus the incident beam ratio β of the two interfering waves is interpreted by including the second-order term in the Fourier development of Esc. The conditions allowing one to obtain a reasonable agreement between the theory and experiments are presented and discussed, as well as the adopted values of the crystals’ parameters.
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