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Abstract-We investigate nonlinear carrier dynamics in a multiquantum-well semiconductor optical amplifier (SOA) in the context of ultrafast all-optical logic. A rate-equation model is presented that accounts for two-photon absorption, free-carrier absorption, self-and cross phase modulation, carrier heating, spectral, spatial hole burning, and self-and cross polarization modulation. The nonlinear refractive index dynamics is investigated theoretically and experimentally. We find nonlinear phase changes larger than radians, which recovers on a timescale in the order of 1 ps. We also investigate a nonlinear AND gate that consists of an SOA that is placed in an asymmetric Mach-Zehnder interferometer. We show that the gate can be operated using 800-fJ optical pulses with duration of 200 fs while having a contrast ratio larger than 11 dB.Index Terms-Optical logic, optical signal processing ultrafast carrier dynamics, semiconductor optical amplifier.

Abstract-In this paper, we review the recent progress in transmission experiments by employing optical phase conjugation (OPC) for the compensation of chromatic dispersion and nonlinear impairments. OPC is realized with difference frequency generation (DFG) in a periodically poled lithium-niobate (PPLN) waveguide, for transparent wavelength-division multiplexed (WDM) operation with high conversion efficiency. We discuss extensively the principle behind optical phase conjugation and the realization of a polarization independent OPC subsystem. Using OPC for chromatic dispersion compensation WDM 40-Gb/s long-haul transmission is described. As well, transmission employing both mixed data rates and mixed modulation formats is discussed. No significant nonlinear impairments are observed from the nonperiodic dispersion map used in these experiments. The compensation of intrachannel nonlinear impairments by OPC is described for WDM carrier-suppressed return-to-zero (CSRZ) transmission. In this experiment, a 50% increase in transmission reach is obtained by adding an OPC unit to a transmission line using dispersion compensating fiber (DCF) for dispersion compensation. Furthermore, the compensation of impairments due to nonlinear phase noise is reviewed. An in-depth analysis is conducted on what performance improvement is to be expected for various OPC configurations and a proof-of-principle experiment is described showing over 4-dB improvement in Q-factor due to compensation of nonlinear impairments resulting from nonlinear phase noise. Finally, an ultralong-haul WDM transmission of 22 × 20-Gb/s return-tozero differential quadrature phase-shift keying (RZ-DQPSK) is discussed showing that OPC can compensate for chromatic dispersion, as well as self-phase modulation (SPM) induced nonlinear impairments, such as nonlinear phase noise. Compared to a "conventional" transmission link using DCF for dispersion compensation, a 44% increase in transmission reach is obtained when OPC is employed. In this experiment, we show the feasibility of using only one polarization-independent PPLN subsystem to compensate for an accumulated chromatic dispersion of over 160 000 ps/nm. Index Terms-Dispersion compensation, differential phaseshift keying (DPSK), differential quadrature phase-shift keying (DQPSK), duobinary, fiber-optics communications, nonlinear phase noise, phase conjugation, phase-shift keying, periodically poled lithium niobate (PPLN), spectral inversion.

We demonstrate error-free 320 Gb/s SOA-based optical wavelength conversion. By utilizing optical filtering, an effective recovery time of less than 1.8 ps is achieved in an SOA, which ensures 320 Gb/s operation. OCIS codes: (190.5970) Semiconductor nonlinear optics, (250.5980) Semiconductor optical amplifier 1. Introduction All-optical wavelength converters (AOWCs) are considered as important building blocks in the future high-capacity wavelength-division-multiplexed networks. AOWCs that utilize nonlinearities of semiconductor optical amplifiers (SOAs) have attracted considerable research interest due to the integration ability and power efficiency [1]. A number of SOA-based AOWCs have been demonstrated [2][3][4][5]. However, the slow SOA recovery time (typically several tens to hundred ps) can cause unwanted pattern effects in the converted signal, which limits the maximum operation speed.In this paper, we present for the first time an error-free and pattern-independent 320 Gb/s wavelength conversion using a single SOA. To our best knowledge, this is the highest operation speed for SOA-based wavelength conversion. The wavelength converter is constructed by using commercially available fiber pigtailed components. The SOA in the experiment is a commercial product (Kamelian nonlinear SOA), having an initial fully gain recovery time of 56 ps. We demonstrate that the effective recovery time of the SOA can dramatically shorten to less than 1.8 ps by using optical filtering. A delayed-interferometer is utilized to change the inverted signal into noninverted signal. The wavelength converter has a simple configuration, operates at low optical power, and this concept allows photonic integration. The work was funded by STW EET6491 and IST-LASAGNE (FP6-507509).

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