9.2. SOLITON-BASED COMMUNICATIONS 417
Figure 9.8: Schematic of (a) the device and (b) the package for a hybrid soliton pulse source.
(After Ref. [41];©c1995 IEEE; reprinted with permission.)
laser has produced 3.5-ps pulses at a repetition rate of 40 GHz [43]. An electroab-
sorption modulator, integrated with the semiconductor laser, offers another alterna-
tive. Such transmitters are commonly used for nonsoliton lightwave systems (see Sec-
tion 3.6). They can also be used to produce a pulse train by using the nonlinear nature of
the absorption response of the modulator. Chirp-free pulses of 10- to 20-ps duration at
a repetition rate of 20 GHz were produced in 1993 with this technique [44]. By 1996,
the repetition rate of modulator-integrated lasers could be increased to 50 GHz [45].
Thequantum-confinement Stark effectin a multiquantum-well modulator can also be
used to produce a pulse train suitable for soliton transmission [46].
Mode-locked fiber lasers provide an alternative to semiconductor sources although
such lasers still need a semiconductor laser for pumping [47]. An EDFA is placed
within the Fabry–Perot (FP) or ring cavity to make fiber lasers. Both active and passive
mode-locking techniques have been used for producing short optical pulses. Active
mode locking requires modulation at a high-order harmonic of the longitudinal-mode
spacing because of relatively long cavity lengths (>1 m) that are typically used for
fiber lasers. Such harmonically mode-locked fiber lasers use an intracavity LiNbO 3
modulator and have been employed in soliton transmission experiments [48]. A semi-
conductor optical amplifier can also be used for active mode locking, producing pulses
shorter than 10 ps at a repetition rate as high as 20 GHz [49]. Passively mode-locked
fiber lasers either use a multiquantum-well device that acts as a fast saturable absorber
or employ fiber nonlinearity to generate phase shifts that produce an effective saturable
absorber.
In a different approach, nonlinear pulse shaping in a dispersion-decreasing fiber is