Abstract

A major limitation to the transmission capacity of soliton-based periodically amplified fiber links [1] is provided by the fluctuations of the pulse arrival time due to the accumulation of small frequency deviations from the amplifier noise [2]. The output time jitter (and the resulting bit-error-rate) may be reduced by means of bandwidth limited amplification [3] and sliding frequency filters [4-5]. In wavelength division multiplexed (WDM) soliton systems, the periodic power variation of the solitons between amplifiers leads to asymmetric soliton collisions [6]. Whenever a soliton in, say, the first channel experiences collisions with solitons of an adjacent channel, two distinct sources of randomness arise. First, the information-bearing sequence of solitons in the second channel is random. Since each collision leads to a small time displacement of the soliton center, multiple collisions lead to a random net time shift at the system output, which however is nearly the same for adjacent solitons in the first channel, and no significant bit error rate results. The second source of randomness is much more harmful and is due to the collision asymmetry whenever the collision distance is close to the amplifier spacing (resonance condition). In fact, the center of the collision is located at random in between amplifiers and originates a small residual positive or negative soliton frequency shift. The accumulation of these random shifts along the link produces a jitter of the soliton output time position as in the case of ref. [2]. We analyse here the effects of strong filtering on the resonant dynamics of multiple soliton collisions between solitons of different wavelengths. We show by perturbation theory and full numerical simulations that the ultimate capacity of WDM transoceanic soliton systems may be much increased by means of in-line filters, and in particular with sliding filters, irrespectively of initial time overlaps between the channels.

© 1995 Optical Society of America