Periodic-group-delay Dispersion Compensation Reduces Collision-induced Timing Shifts in Dispersion-managed Quasilinear Systems

Collision-induced timing shifts, i.e., changes in the arrival time of a signal due to interaction with signals in neighboring frequency channels, is a major issue in on-off-keying, wavelength-division-multiplexed (WDM) systems. The problem is complex if dispersion management is used: because of the periodic change of the sign of the chromatic dispersion with distance, pulses in different frequency channels undergo a zigzag motion with respect to each other, which results in interactions consisting of a series of “minicollisions.” Furthermore, the low average dispersion value in these systems results in large collision lengths, implying that the interaction unfolds over long distances. Indeed, while it is well known that a moderate amount of dispersion management can be beneficial as regards interchannel interactions, the collision process in extreme cases can result in mutual destruction of the two WDM pulses [1]. Recently, using periodic-group-delay (PGD) modules was proposed as a way to reduce collision-induced timing shifts [2]–[4]. The PGD dispersion-compensating modules [5]–[7] are inserted inside each dispersion map and serve to compensate a fraction of the accumulated dispersion, while the dispersion-compensating fibers still compensate the remaining fraction of dispersion. The advantage of this approach for dispersionmanaged soliton systems was demonstrated both with numerical simulations [2], [4] and with direct transmission experiments [3]. The purpose of the present paper is twofold. First, we derive expressions for the value of the collision-induced frequency and timing shifts in quasilinear return-to-zero systems that use PGDs. Second, with both our analytic results and direct numerical simulations, we show that using PGDs is very effective in reducing the collision-induced timing shifts in quasilinear, return-to-zero systems. As with solitons, the main mechanism for reducing the timing shift is the drastic reduction of the collision length [2]. We show that there is also a significant direct effect from the PGDs themselves, which provide a restoring force to the timing shift.