All-optical logic XNOR gate is realized by a series combination of XOR and INVERT gates. This Boolean function is realized by using Mach-Zehnder interferometers (MZIs) and exploiting the nonlinear effect of two-photon absorption (TPA) in semiconductor optical amplifiers (SOAs). The employed model takes into account the impact of amplified spontaneous emission (ASE), input pulse energy, pulsewidth, SOAs carrier lifetime, and linewidth enhancement factor (α-factor) on the gate’s output quality factor (Q-factor). The outcome of this study shows that the all-optical XNOR gate is indeed feasible with the proposed scheme at 250？Gb/s with both logical correctness and acceptable quality. 1. Introduction The development of all-optical logic technology is important for a wide range of applications in all optical networks, including high speed all-optical packet routing and optical encryption [1]. An important step in the development of all-optical logic technology, which includes key functionalities in fundamental and system-oriented level such as buffering, demultiplexing, clock recovery, packet processing, wavelength conversion, data regeneration, and optical encryption/decryption, is the demonstration of optical logic elements that can operate at ultrahigh speeds. All-optical logic gates based on several different schemes have been demonstrated and reported, including that based on dual semiconductor optical amplifier (SOA) Mach-Zehnder interferometer (MZI) [2, 3], semiconductor laser amplifier loop mirror (SLALM) [4], ultrafast nonlinear interferometer (UNI) [5], four-wave mixing (FWM) process in SOA [6], or cross-gain modulation (XGM) or cross-phase modulation (XPM) in nonlinear devices [7]. All-optical logic XNOR gate using SOA is described and demonstrated earlier [8–13]. However the main limitation imposed on most of these schemes is that they cannot keep conveniently pace with the upgrades of single channel data rates in the effort to satisfy the unceasing bandwidth demand [14]. Thus in this work, we propose to address this critical issue by exploiting two-photon absorption (TPA) in SOAs, which are placed in the two arms of MZI operated in probe-dual pump mode. According to relevant pump-probe experiments, when a data modulated pump beam of appropriate intensity is launched into a SOA, the phase induced through TPA on a weak probe signal can change as fast as 1？ps, which subsequently can enable ultrafast interferometric switching of the same order [15]. Pump-probe experiments have shown that phase change takes place in duration ~1？ps or less when the
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