IEEE JOURNAL ON SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 8, NO. 3, MAY/JUNE 2002 591 Multichannel Soliton Transmission and Pulse Shepherding in Bit-Parallel-Wavelength Optical Fiber Links Elena A. Ostrovskaya, Yuri S. Kivshar, Dumitru Mihalache, and Lucian-Cornel Crasovan Abstract—We study basic principles of the bit-parallel-wave- length (BPW) pulse transmission in multichannel single-mode op- tical fiber links for high-performance computer networks. We de- velop a theory of the pulse shepherding effect that allows simulta- neous propagation of pulses in parallel bit slots by binding them into a multicomponent BPW soliton. We describe families of the BPW solitons and bifurcation cascades in a system of incoher- ently coupled nonlinear Schrödinger equations that model the mul- tichannel multiwavelength transmission in a single-mode optical fiber. We demonstrate high robustness of the composite BPW soli- tons, due to their underlying linear stability, to a moderate pulse walkoff. Index Terms—Optical solitons, wavelength-division multi- plexing, bit-parallel pulse transmission, soliton bifurcation diagram, pulse walkoff. I. INTRODUCTION A GROWING demand for high-speed computer commu- nications requires effective and inexpensive parallel computer interconnects that eliminate bottlenecks caused by parallel-to-serial conversion. Bit-parallel-wavelength (BPW) single-fiber optical links were proposed as possible high-speed computer interconnects for local networks [1]. Functionally, the BPW link is just a single-medium parallel fiber ribbon cable (see Fig. 1, top). In a conventional ribbon cable, parallel bits coming from a computer bus are transmitted by pulses traveling in separate fibers, thus implementing the so-called space-division-multiplexing (SDM) scheme. In a BPW scheme, all bits are wavelength multiplexed and transmitted by time-aligned pulses, ideally solitons through a single (and single-mode) optical fiber (see Fig. 1, bottom). For any bit-parallel optical fiber transmission, the crucial problem is maintaining the alignment of pulses corresponding to parallel bits of the same word. Unlike the fiber ribbon cable, a single-fiber BPW link presents a unique possibility of dynamical control of the pulse alignment by employing the so-called pulse shepherding effect [2], [3]. This effect Manuscript received January 2, 2002; revised April 11, 2002. The work of E. A. Ostrovskaya and Yu. S. Kivshar was supported in part by the Performance and Planning Fund of the Institute of Advanced Studies, the Australian National University, and the Australian Photonics Cooperative Research Centre. E. A. Ostrovskaya and Yu. S. Kivshar are with the Nonlinear Physics Group, Research School of Physical Sciences and Engineering, the Australian National University, Canberra, Australia (e-mail: ost124@rsphysse.anu.edu.au). D. Mihalache and L.-C. Crasovan are with the Department of Theoretical Physics, National Institute of Physics and Nuclear Engineering, Bucharest, Ro- mania. Publisher Item Identifier S 1077-260X(02)05473-4. Fig. 1. Schematics of the parallel bits transmission through a conventional fiber ribbon link (top) and a bit-parallel single fiber link (bottom). arises from the interaction of copropagating pulses through the nonlinear cross-phase-modulation (XPM) effect. In the wavelength-division multiplexed lines, such an interaction is avoided by all possible means as it strongly degrades the bit rate. However, in a bit-parallel transmission scheme it enables for a strong “shepherding” pulse to trap and constrain a sequence of copropagating weaker pulses to their required time slot. Experimentally, the reduction of the misalignment due to the group-velocity mismatch of two pulses in the presence of the shepherding pulse has been observed in a dispersion-shifted Corning fiber [4], [5]. In this paper, we develop a rigorous theory of the shepherding effect. We show that due to the XPM interaction of pulses trans- mitted through the same fiber, they can be treated as funda- mental modes of different colors trapped and guided by an ef- fective waveguide induced by the the strong shepherd pulse. As a result, the shepherding pulse and all other (weaker) pulses form a new entity—a multicomponent soliton pulse. The BPW soliton propagates in the fiber preserving the time alignment of its constituents, and thus enables multichannel bit-parallel trans- mission. For the first time to our knowledge, we analyze mul- ticomponent BPW solitons and describe a mechanism for the growth of the BPW soliton complex via the bifurcation cascades in a model of incoherently coupled nonlinear Schrödinger (NLS) equations. We also discuss the effect of the group-ve- locity mismatch on the time alignment of the constituents of the multicomponent pulse and present numerical analysis of the linear stability of the BPW pulses in the presence of the walkoff effect. 1077-260X/02$17.00 © 2002 IEEE