All-optical fiber signal processing and regeneration for soliton communications

Ultrafast all-optical signal processing techniques are expected to play a major role in future ultrafast single-carrier soliton systems, because they remove the electronics bottleneck. In this paper, two all-optical devices, the nonlinear optical loop mirror (NOLM) and the Kerr fiber modulator (KFM), are used to achieve major functions related to high bit rate soliton links. At the interface with existing networks, conversions from data at the nonreturn-to-zero (NRZ) format to return-to-zero (RZ) and soliton data, and vice-versa are required. These two conversions are demonstrated through NOLMs, and their limitations investigated. However, the main part of this paper is devoted to in-line soliton regeneration through synchronous modulation. Synchronous modulation requires both clock recovery and in-line optical modulation. In the following, all-optical approaches for these two functions are considered separately, before being associated in a true all-optical regenerator. All-optical clock recovery techniques are first reviewed. An experimental implementation of one of these techniques is described. On the other hand, all-optical modulation can be done either with intensity or phase modulators. We initially proposed the NOLM as all-optical intensity modulator. We analyze it theoretically, both from the component and the system application viewpoints. A modified configuration of the NOLM, having two optical controls, removes some limitations pertaining to the single-control configuration, yielding even higher performance. The other all-optical synchronous modulator considered here is the KFM, which is a pure phase modulator. Its potential is demonstrated in a 20-Gb/s soliton transmission experiment, when driven by an optoelectronic optical clock generation device. Issues specific to the implementation of both types of all-optical fiber-based modulators are discussed. Finally, a true all-optical synchronous regenerator, combining all-optical clock recovery circuit and KFM, is tested in an actual soliton transmission experiment at 20 Gb/s.