All-optical processing of optical-network signals using distributed feedback amplifiers

Thesis (Ph. D.)--University of Rochester. Institute of Optics, 2001.We study the nonlinear response and signal-processing capabilities of distributed feedback semiconductor optical amplifiers, and seek to advance their application to optical communication networks. Bistability occurring for optical signals tuned near a Bragg resonance is useful for switching and memory applications, but traditionally exhibits a limited wavelength range. We relax this constraint by varying the grating pitch along the length of the distributed feedback amplifier. A transfer-matrix method is developed for simulating this improvement, and for studying changes in the shape of the hysteresis curve throughout this wavelength range. We predict a new hysteresis-curve shape on reflection, and show how the grating-pitch variation can suppress or enhance this shape. Optical memory based on bistability is useful for sequential signal-processing applications, but previous control techniques operate with wavelengths only in the vicinity of the bistable-signal wavelength. We propose, model, and demonstrate control techniques via auxiliary optical signals that exhibit a very wide wavelength range. Set and reset signals vary the refractive index in opposite ways and shift the upward- and downward-switching thresholds, respectively, of the hysteresis curve through the holding-beam input power, which is kept constant. We develop a numerical model and an experimental system to investigate the performance of the all-optical flip-flop pertaining to speed, power, polarization, and response to back-to-back 'set' pulses. We propose and numerically simulate a sequential processing application to fiber-optic networks - data format conversion from high-speed, return-to-zero signals to low-speed, non-return-to-zero signals. We demonstrate data-wavelength conversion to a signal wavelength of 1547 nm (in the vicinity of the Bragg wavelength) from initial data signals at 1306 nm, 1466 nm, and 1560 nm. This research demonstrates that cross-phase-modulation-based conversion using signals that generate charge carriers (e.g., those at 1306 and 1466 nm) can be implemented in gain-biased amplifiers, a principle that is applicable to other semiconductor-optical-amplifier-based data-wavelength converters. We also demonstrate how to select the converted-data polarity and to achieve a digital-like transfer function