Control of Optical Solitons

Abstract

Under the ideal conditions, isolated optical solitons can propagate over infinite distances without distortion. In actual transmission systems, however, a variety of effects perturb the soliton propagation so that the transmission distance and speed are limited. Such perturbations include interaction between neighboring solitons [37, 38, 54], local imbalance of fiber dispersion and nonlinearity due to fiber loss and lumped amplification [45–47,55], noise-induced timing jitter (the Gordon–Haus effect [48]), higher-order effects such as third-order dispersion and stimulated Raman effect [16, 56, 57], and randomly varying birefringence [58]. In WDM systems, four-wave mixing and cross-phase modulation between pulses in different wavelength channels give further restriction to the system performance [52, 59, 60]. Although the pulse shape tends to be preserved even under these perturbations owing to the particle nature of the soliton, the pulse parameters such as amplitude, frequency, and temporal position are fluctuated, and dispersive radiations are shed from the soliton, which gives rise to bit errors. Transmission controls by means of narrowband filters and/or synchronous amplitude and phase modulators are quite effective in reducing the fluctuations and extending the error-free transmission distance [50,51,61]. These controls utilize the nonlinear nature of the pulse to their advantage. In some of the soliton transmission controls, not only soliton signals are controlled and stabilized but also low-power noise is continuously removed from the signal, which is a unique property not available in linear transmission systems [62,63].