Optical EngineeringNovel all-optical flip-flop using semiconductor optical amplifiers in innovative frequency-shifting inverse-threshold pairs
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All-optical digital sequential logic reduces the expense and bit rate constraints associated with optical-electronic-optical (OEO) conversion in telecommunications. Two semiconductor optical amplifiers (SOAs) are directly coupled by connecting the output of each to the input of the other. This new configuration provides positive feedback to create a logical NOT or inverse for complete logic. An inverted optical threshold improves the quality of bit modulation. At the same time, frequency shifting allows improvement in the quality of the optical carrier. The SOA pair is analyzed in closed form to show that the positive feedback provides an output-input slope steeper than –1, allowing connection of two pairs to form a flip-flop. Two SOA pairs are cross-connected by connecting the output of each pair to the input of the other, to construct a novel R-S flip-flop. Closed form analysis shows, for the first time, that there is an unstable state and two stable states forming bistability for this configuration. Simulation demonstrates setting and resetting the reset-set (R-S) flip-flop at more than 5 Gbps. Agreement between closed form analysis and numerical simulation provides verification of the validity of the results obtained.