3.5.1 Introduction

In mid-1991, two groups independently suggested the idea that the Gordon-Haus jitter and other noise effects could be significantly suppressed in soliton transmission systems simply through a narrowing of the amplifier gain-bandwidth [ ^[65], ^[66]]. In practice this means the use of narrow-band filters, typically one per amplifier. Figure 3.28 shows appropriate filter response curves in comparison with the spectrum of a 20-ps-wide soliton. The fundamental idea is that any soliton whose central frequency has strayed from the filter peak will be returned to the peak, in a characteristic damping length ?, by virtue of the differential loss the filters induce across its spectrum. The resultant damping of the frequency jitter leads in turn to a corresponding damping of the jitter in pulse arrival times. For example, in Eq. (3.35) for the variance of the Gordon-Haus jitter, when guiding filters are used, the quantities z ² _n in the sum are all replaced with the common factor ? ². Thus, the factor Z ³/3 in the final expression is replaced by the (potentially much smaller) factor Z ? ².

Figure 3.28: Intensity response curves of practical etalon guiding filter, Gaussian with same peak curvature, and spectrum of 20-ps soliton. The etalon mirrors have R = 9% and their 2.0-mm spacing creates the 75-GHz free spectral range.

The filters also cause a reduction in amplitude jitter. Consider, for example, a pulse with greater than normal power;