TABLE OF CONTENTS For best performance, soliton transmission requires a reliable source of unchirped pulses, whose intensity envelopes are, to good approximation, either of Gaussian or sech 2 shape, and whose repetition rate can be easily adjusted to the desirea bit rate. Many of the early experiments, however, were carried out using mode-locked, electrically driven semiconductor lasers, which tend to produce chirped and asymmetrical pulses with widths that are hard to control. They also require very careful matching of the cavity's c/2 L resonance frequency (where L is the cavity length) to match the microwave drive frequency, since even very small mismatches tend to cause large changes in the pulse characteristics. Harmonically mode-locked erbium fiber ring lasers can provide unchirped, essentially Gaussian pulses, but they require even more complex adjustment, viz., the round-trip times of an internal stabilizing Fabry-Perot etalon and of the fiber ring must be simultaneously adjusted to match, respectively, 1 and an exact integral multiple of the bit period. In sum, both forms of mode-locked laser were much too complex, expensive, and fussy to be viable candidates for use in practical systems, especially where dense WDM may require 100 or more such devices in one and the same transmitter. By contrast, the scheme alluded to in earlier chapters to use a sinusoidally driven LiNbO 3 Mach-Zehnder modulator to carve pulses from the output of a cw laser tends to meet all requirements simply and at relatively low cost.
