Using two beam coupling geometry, high order copropagating and contrapropagating isotropic and copropagating anisotropic self-diffraction are demonstrated using photorefractive cerium doped barium titanate. At small incident angles, typically less than 0.015 radians, both codirectional isotropic self-diffraction (CODIS) and contradirectional isotropic self-diffraction (CONDIS) orders are generated simultaneously. At larger incident angles, typically approximately more than 0.2094 radians, only codirectional anisotropic-self diffraction (CODAS) orders are generated. Ongoing work on image auto/cross correlation results are also shown.
Using barium titanate as the photorefractive material, we show self-pumped phase conjugation, beam coupling, higher non-Bragg order generation yielding forward phase conjugate, and image correlation using higher order diffraction. It is shown that 2<b>K</b> gratings are primarily responsible for higher order diffraction.
In this paper, we present experimental results of dynamic aberration correction based on gradient descend algorithms. The experimental setup included a 37-actuators piezoelectric deformable mirror to distort dynamically an input laser beam and a 37-element micromachined deformable membrane mirror to correct the resulting wavefront distortions. We generated time-varying aberrations using the first mirror and used the light power focused onto a pinhole as our optimization matrix. We programmed a computer to maximize this metric and control the shape of the micromachined deformable membrane mirror for wavefront correction. We implemented in this computer a simple gradient descend algorithm and a stochastic perturbation gradient descent algorithm. We present experimental data on the convergence and stability of this adaptive system for various conditions of dynamic turbulence.
Laser beam propagation through atmospheric turbulence may result in strong intensity fluctuations at the receiver plane of a free-space communication link. To minimize intensity fluctuations, we consider an approach based on the use of a dynamic random phase diffuser placed in the transmitter plane. To create a phase diffuser having controlled statistical properties, a nonlinear optical system with 2D feedback (artificial optical turbulence generator) is used. This system can provide real-time laser beam spatial coherence control for a laser transmitter in order to optimize laser communication link performance.
An adaptive system for active imaging and radiation focusing on extended rough-surfaced objects in the presence of phase distortions was experimentally demonstrated. This technique is based on the use of two laser beams having different wavelengths, here a He-Ne and Argon laser. The Argon laser beam was used for adaptive radiation focusing on the rough target surface. The second laser beam served for target active imaging. The imaging system included the same wavefront correctors used for the radiation focusing system. For adaptive control of the wavefront correctors, a gradient technique based on a radiation focusing quality criterion optimization was applied. Statistical properties of the speckle field scattered by the target surface were used as the radiation focusing quality metric. To measure this criterion a single photodiode with a simple signal analysis method was employed. Adaptive correction was implemented with a nine electrode bimorph mirror and a liquid crystal spatial phase modulator having 127 individually addressed hexagonal liquid crystal cells.