Claus Benkert

Self-organizing photorefractive frequency demultiplexer

We demonstrate a self-organizing photorefractive circuit that demultiplexes a beam that has two signals imposed on separate optical carrier frequencies into two beams, each containing one of the signals on its carrier. Unlike conventional demultiplexing techniques, this method requires little a priori knowledge about the carrier frequencies. The signal channels must be spatially uncorrelated, and their frequency separation must be more than the photorefractive response bandwidth (hertz to kilohertz). The optical circuit uses no local oscillator, and the photorefractive response bandwidth places no upper bound on the channel bandwidth. Experimental results for demultiplexing a beam that has two signals, with a BaTiO(3) circuit, show contrast ratios of better than 40:1 at the outputs.

Photorefractive flip-flop

We demonstrate an optical flip-flop that consists of two unidirectional ring resonators that are coupled competitively through two-beam interactions in a photorefractive BaTiO(3) crystal. This crystal provides an active loss for each ring that depends on the oscillation intensity of the other. The coupled rings exhibit flip-flop bistability when the net gain, given by the product of the passive cavity losses and the gain, is greater than unity but is less than the small-signal active loss. The state of the system can be switched with an injected signal.

The transfer function and impulse response of photorefractive two-beam coupling

The transfer function and impulse response of photorefractive two-beam coupling are derived in the undepleted pump approximation. For sufficiently strong coupling Gamma L, the impulse response features a broad delayed output pulse. In the limits of negligible and strong absorption alpha L, this coupling threshold reads Gamma L/sub thr/=4 and Gamma /sub thr/=2 alpha , respectively. The time delay and pulse height are functions of the coupling Gamma L, the photorefractive time constant tau , and the effective absorption alpha L. Experiments on a BaTiO/sub 3/ crystal measuring the absoluted square of its transfer function and the impulse response are used to determine the coupling and time constant. >

Competitive and cooperative multimode dynamics in photorefractive ring circuits

Publisher Summary This chapter describes competitive and cooperative multimode dynamics in photorefractive ring circuits. Competitive and cooperative interactions play a fundamental role in many neural network models. It has been shown that groups of neurons in the brain become maximally sensitive for different sensory inputs. Such a specialization of neurons is not only genetically predetermined but can also develop dynamically over time. This self-organizing process requires a competitive interaction among the neurons to determine which group becomes the activity center for a given external stimulus. Competitive interactions have been shown to solve a universal problem concerning the evaluation of information by a neural network in the presence of noise. This problem is known as the noise-saturation dilemma and results from the fact that small signals can get lost in noise whereas large signals can saturate the systems response. However, through a competitive coupling among the processing units, the system can automatically retune itself to avoid both noise and saturation.