All-Optical Signal Processing for over-100-Gb/s Optical-TDM Networks

1. Current OTDM Component Technology The large and ultrafast optical nonlinearity of millimeter-size semiconductor optical amplifiers (SOAs) has led to the study of several types of gate functions at ultrahigh bit-rates beyond 40 Gb/s. Among these functions, optical demultiplexing has been the most successfully demonstrated with 336 Gb/s RZ signals [1]. The slow-recovery-induced phenomena that accompany the ultrafast nonlinear response inside each SOA do not slow down the gating speed, because their effects on the optical signals are dramatically removed by an optical interferometer incorporated in each ultrafast gate structure (e.g., SMZ, TOAD, PD-SMZ, UNI, or DISC). Despite the use of optical interference, their stability will not be a problem in the building of network systems. Integrated gates have shown excellent stability. Even when ultrafast gating with large extinction ratios is required in long terms, the optical interference will precisely be stabilized with low-cost slow optoelectronics inside each gate module (Fig. 1) [2]. One gate function that has attracted considerable attention from both OTDM and 40-Gb/s researchers is optical 3R gating. Its noise-tolerant, jitter-tolerant regenerative properties have been observed experimentally with 40to 80-Gb/s signals by several research teams (Fig. 2) [3-6]. 3R regenerative gating with 160-Gb/s signals is currently a particularly attractive research goal. All-optical wavelength conversion for cross-connecting OTDM signals has also been demonstrated with one of the above gate structures. To date, error-free wavelength conversion of 168-Gb/s signals has been reported [7]. Importantly, all-optical gates in the wavelength-conversion mode, as well as those in the 3R-gating mode, are driven with ultrafast-line-rate RZ signals, in good contrast to those gates in the demultiplexing mode. The experimental demonstration of 168-Gb/s conversion, consequently, is considered a milestone along the way to 160-Gb/s 3R gating.