All-Optical Regeneration Using InP-Based Photonic Integrated Circuits

During the last years we have pioneered several new concepts for 2R optical regeneration. Two of these concepts, an active 2x2 multi mode interference (MMI) coupler and a Mach Zehnder interferometer with a regular semiconductor optical amplifier in one arm and an active 1x1 MMI in the other arm, have been fabricated as photonic integrated circuits. A third regenerator, a MachZehnder interferometer with Gain-Clamped Semiconductor Optical Amplifiers (GCSOA) in both arms, has been implemented using a fiber interferometer and commercial GCSOA’s. An extinction ratio improvement of around 15 dB as well as noise reduction has been demonstrated with all 3 concepts. In addition, one of the concepts has been demonstrated at 10 Gb/s. Introduction Signals propagating over optical fiber networks are significantly distorted due to several causes such as fiber dispersion, noise accumulation in optical amplifiers and crosstalk addition in switches. This is especially the case in advanced WDM-based networks, where many channels at different wavelengths and each carrying signals of up to 40 Gb/s are sent through long stretches of fiber and are traversing several optical amplifiers and optical switches. The resulting deterioration of the signals may require that signals are regenerated at intermediate nodes. This regeneration can be either 2R regeneration, consisting of signal reshaping, or 3R regeneration, consisting of signal reshaping and retiming. Although 3R regeneration will be required in most cases, 2R regeneration may be useful in parts of an optical network. Moreover, a 2R regenerator is itself an indispensable part of a 3R regenerator. Although regeneration is perfectly possible using opto-electronic conversion, all-optical regeneration has many advantages, e.g. in terms of cost and power consumption, particularly as bitrates increase or if multi channel regeneration is required [1]. Many different solutions have already been proposed (cf. [2-4]), but the optical 2R regenerators proposed so far have the disadvantage that they are either requiring an external pump signal, giving a poor regeneration or requiring a complex interferometric structure. Some alternative 2R regenerator structures have been devised at the Department of Information Technology of Ghent University. These concepts, as well as experimental results obtained with them, will be discussed below. Interferometer with Gain-Clamped Semiconductor Optical Amplifiers (GCSOAs) A first concept for a 2R optical regenerator is a Mach-Zehnder interferometer (MZI) with equal GCSOAs in each arm, as depicted on the l.h.s. in Figure 1. The operation of this regenerator is based on the specific property of a GCSOA that its amplification in the linear regime is independent of the injected current, whereas the saturation power increases linearly with the injected current [5]. The two GCSOAs in Figure 1 are identical but are fed different bias currents such that they provide the same amplification in the linear regime but exhibit a different saturation inand output power. For input powers below the saturation power of both GCSOAs, the equal gain in both arms and the phase difference of π between both arms result in a complete destructive interference and a very low output power. Beyond the saturation power of both GCSOAs, both arms provide a constant (input power independent) output power (that is however different for both arms). The phase difference between both arms is also constant, but now different from π, such that a constant output power is also obtained at the output of the interferometer.