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Insights into nonlinear material properties of bulk semiconductor optical amplifiers (SOAs) and novel quantumdot (QD) SOA nonlinear materials have led to novel device configuration for all-optical wavelength conversion and signal regeneration. The paper will review latest developments and discuss novel configurations.

All-optical wavelength converters (AOWCs) based on nonlinear processes of semiconductor optical amplifiers (SOAs) have attracted interest to overcome the wavelength blocking issues in future transparent networks. While many schemes work well, pattern effect impairments that are due to the finite lifetime of charge carriers are an issue most of the time. Recently, wavelength conversion and pattern effect mitigation techniques that work by properly shaping the passband of filters following the converter have been introduced. However, due to the necessity of selecting filter slope and position precisely, one would expect that the schemes are extremely sensitive to any drift of the center wavelength. In this work, we demonstrate a 40 Gbit/s SOA-based wavelength converter with more than 15 dB dynamic input power range. In addition, the center wavelength of the converted signal has a tolerance of ~0.2 nm towards the red spectral region and of ~0.1nm towards blue spectral region, respectively. This success is due to combining advantageously pattern effect mitigation techniques connected to the pulse reformatting optical filter, the red-shift and the blue-shift optical filter.

Wavelength converters for phase sensitive modulation formats based on semiconductor optical amplifiers are discussed. Advantages and limits are discussed based on an actual implementation.All-optical regenerators and wavelength converters for phase-sensitive communication formats are of high interest because they potentially increase link-lengths, regenerate degraded signals in point-to-point and next generation meshed transparent networks, or simply overcome wavelength blocking in cross-connects and routers at low costCurrently, Differential Phase-Shift Keying (DPSK) -because of its lower OSNR requirement and robustness towards nonlinear impairments [1] -seems to be the most promising phase-modulation format. In view of the significance of the DPSK format for next generation systems, it would be highly desirable to have small-footprint, low power-consuming all-optical DPSK regenerators and wavelength converters. Solutions exploiting fibernonlinearities [3], [4] or FWM in SOAs have been tested [5]. Unfortunately, all of these solutions only offer a restricted wavelength conversion range and require high input powers. Recently, Sagnac [6] and Mach-Zehnder interferometer (MZI) devices [7] exploiting the more efficient SOA-based cross-phase modulation effect have been demonstrated and their regenerative potential has been shown. Yet, none of the MZI based solutions also showed retiming regeneration, nor was the cascadability or the origin of the regenerative characteristics clarified. This is important, since DPSK is most likely to be used in systems at bit rates of 40 Gb/s and above, where retiming is an issue.In this paper, we show in experiment the feasibility and predict the limits of a retiming and reshaping SOA based MZI all-optical DPSK wavelength converter. We further discuss how these devices can be cascaded an arbitrary number of times. DPSK Wavelength Conversion Scheme and ExperimentThe experimental setup and the all-optical DPSK wavelength converter are depicted in Fig. 1. The setup comprises a push-pull DPSK transmitter, an all-optical wavelength converter and a balanced receiver with a differential encoder.The operation principle of the all-optical wavelength converter is as follows. An incoming DPSK signal is first converted into on-off keying (OOK) and inverted OOK signals in the Delay Interferometer (DI) stage. These on-off signals are then used for controlling the SOAs. They are injected with exact time correlation into the two control inputs of a MZI with SOAs [8]. The relative phase in the SOA arms is then controlled by a high level bit either in the OOK or in the inverse OOK arm, very similar to the push-pull operation of electrically controlled MZI modulators in DPSK transmitters [1]. An optical clock, derived from the input signal, is then introduced at the centre arm of the interferometer. The clock may be set to any wavelength within the SOA gain spectrum. This clock signal then picks up the phase information when travelling through the SOA arms and becomes the new phase encoded signal at th...