Henri Rajbenbach

Two‐beam coupling in photorefractive Bi12SiO20 crystals with moving grating: Theory and experiments

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)(μe/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.

High photorefractive gain in two-beam coupling with moving fringes in GaAs:Cr crystals

We report two-beam coupling experiments at 1.06-microm wavelength using photorefractive semi-insulating GaAs:Cr in the moving-grating recording mode. The steady-state gain coefficient r is measured as a function of fringe spacing Lambda, fringe velocity v, applied voltage V0, and incident beam ratio beta. A maximum Gamma as high as 6-7 cm(-1) is observed for Lambda approximately 18 microm, v approximately 1 mm/sec, beta > 10(3), and V(0) approximately 5 kV (interelectrode distance 5.8 mm). The agreement with the theory of two-beam coupling with moving fringes is discussed. In particular, a set of numerical values is obtained for some crystal parameters. Image amplification is demonstrated.

Noise suppression in photorefractive image amplifiers

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.

Compact photorefractive correlator for robotic applications.

We design, implement, and test a multichannel photorefractive optical joint transform correlator that is capable of performing sorting tasks for robotic applications. The use of mini-YAG lasers and liquid-crystal spatial light modulators, in conjunction with updatable holographic BSO crystals, results in a compact correlator (600 mm x 300 mm x 300 mm) with real-time capabilities (100-ms recognition speed). Flexibility is a built-in feature, and correlation is demonstrated for various applications. Electronic and optical preprocessing and postprocessing for improving demonstrator performances are also discussed.

Performance of a photorefractive joint transform correlator for fingerprint identification

A photorefractive joint transform correlator is connected to a PC-based image processing board and tested for fingerprint recognition. Distortions due to rotation, scale, and partial hiding are considered: ± 4 deg in rotation, ± 7% in scale, and up to 80% hiding can be handled by the correlator. The performance of the correlator operating with a realtime fingerprint acquisition camera is then measured. The captured fingerprint is generally distorted with respect to its original counterpart stored in the PC memory. These distortions are mostly due to the positioning and pressure of the finger on the glass prism of the camera. We evaluate the recognition performance qualitatively and statistically with these distortions on a limited data bank and demonstrate 80% successful recognition rate.

Low-noise amplification of ultraweak optical wave fronts in photorefractive Bi 12 SiO 20

We propose and demonstrate a technique for suppressing the optical noise of the photorefractive amplifiers that exhibit a sharp gain resonance around an optimum pump-to-signal angle. The amplification of extremely weak images is achieved when the entrance face of the crystal is tilted with respect to the pump beam direction. In Bi(12)SiO(20) subpicowatt optical signals are detected with this technique.

Optical implementation of an iterative algorithm for matrix inversion

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.

ENHANCEMENT OF THE SIGNAL-TO-BACKGROUND RATIO IN PHOTOREFRACTIVE TWO-WAVE MIXING BY MUTUALLY INCOHERENT TWO-BEAM COUPLING

We present and experimentally demonstrate a new method for enhancing the signal-to-background ratio of two-wave mixing in photorefractive crystals. The method uses a mutually incoherent third beam to suppress the fanning in a dark ring-shaped region in which the amplified signal is located. A 20-fold improvement of the signal-to-background ratio is measured in BaTiO(3) at lambda = 514 nm. The extension of this principle to wide-field-of-view heterodyne detection is discussed.

Array modes of multiple‐stripe diode lasers: A broad‐area mode coupling approach

Transverse‐mode properties of free‐running diode laser arrays are investigated through high‐resolution spectrally resolved imaging. Observations are shown not to agree with the supermode theory. The coupling of broad‐area modes by gain and temperature perturbations accurately describes the observed modal behavior. Theoretical results presented are in close agreement with the experiments.

Near-infrared four-wave mixing with gain and self-starting oscillators with photorefractive GaAs

We report high-reflectivity four-wave mixing in photorefractive GaAs:Cr crystals at 1.06 microm using a nearly degenerate interaction with an externally applied dc electric field (moving-grating recording mode). The highest reflectivities (R approximately 5) are observed for the positive-readout configuration with cross-polarized pump beams. The variation in reflectivity with the pump-beam ratio is presented for two pump-beam polarization states and compared with theory. Self-starting optical resonators are also described.