Design constraints of high-bit-rate soliton communication systems

Soliton-based optical communication systems are attracting considerable attention because of their potential for significantly increasing the capacity of long-haul lightwave systems. However, even though an ideal fundamental soliton is a robust wavepacket, its propagation through practical fibers can lead to serious limitations on the design of communication systems. In fact, several phenomena can affect soliton transmission: amplified spontaneous emission noise produces Gordon-Haus jitter and reduces the signal-to-noise ratio (SNR) at the receiver; adjacent solitons interact and collide unless sufficiently apart; and the guiding or average soliton undergoes perturbations in power causing nonlocal dispersion/self-phase modulation balancing. The last effect is especially harmful if the amplifier spacing is not much smaller than the soliton period. In this regime, the soliton duration is limited to approximately 10 ps in dispersion shifted fibers. Reduction of soliton widths below this value, while maintaining practical amplifier spacings, leads to increasing emission of dispersive waves that are resonantly amplified periodically, undermining soliton stability. An interesting approach for stable propagation of solitons restores the balance between the group velocity dispersion, (GVD) and self-phase modulation (SPM) in a lossy fiber by changing the dispersion properties of the transmission fiber. Such fibers are called dispersion-decreasing fibers (DDFs) because their GVD must decrease in such a way that it compensates for the reduced SPM experienced by the soliton weakened from fiber loss. Practically undistorted propagation of ultrashort solitons with large amplifier spacings can be achieved by especially designed fibers with tailored-dispersion profile. However, as the soliton width decreases, higher-order nonlinear and dispersive effects become important and ultimately limit the bit rate. In this paper we have examined the main constraints on the design of single-channel, high-capacity soliton communication systems. Regarding the average soliton regime, the Gordon-Haus timing jitter is the principal limitation for 10 Gb/s systems over transoceanic distances.