282 CHAPTER 7. DISPERSION MANAGEMENT
Figure 7.1: Schematic of the prechirp technique used for dispersion compensation: (a) FM
output of the DFB laser; (b) pulse shape produced by external modulator; and (c) prechirped
pulse used for signal transmission. (After Ref. [9];©c1994 IEEE; reprinted with permission.)
C= 1 /
√
- These features clearly illustrate that the prechirp technique requires careful
optimization. Even though the pulse shape is rarely Gaussian in practice, the prechirp
technique can increase the transmission distance by a factor of about 2 when used
with care. As early as 1986, a super-Gaussian model [2] suitable for nonreturn-to-zero
(NRZ) transmission predicted such an improvement, a feature also evident in Fig. 2.14,
which shows the results of numerical simulations for chirped super-Gaussian pulses.
The prechirp technique was considered during the 1980s in the context of directly
modulated semiconductor lasers [2]–[5]. Such lasers chirp the pulse automatically
through the carrier-induced index changes governed by the linewidth enhancement fac-
torβc(see Section 3.5.3). Unfortunately, the chirp parameterCis negative (C=−βc)
for directly modulated semiconductor lasers. Sinceβ 2 in the 1.55-μm wavelength re-
gion is also negative for standard fibers, the conditionβ 2 C<0 is not satisfied. In
fact, as seen in Fig. 2.12, the chirp induced during direct modulation increases GVD-
induced pulse broadening, thereby reducing the transmission distance drastically. Sev-
eral schemes during the 1980s considered the possibility of shaping the current pulse
appropriately in such a way that the transmission distance improved over that realized
without current-pulse shaping [3]–[5].
In the case of external modulation, optical pulses are nearly chirp-free. The prechirp
technique in this case imposes a frequency chirp with a positive value of the chirp pa-
rameterCso that the conditionβ 2 C<0 is satisfied. Several schemes have been pro-
posed for this purpose [6]–[12]. In a simple approach shown schematically in Fig. 7.1,
the frequency of the DFB laser is first frequency modulated (FM) before the laser out-
put is passed to an external modulator for amplitude modulation (AM). The resulting
optical signal exhibits simultaneous AM and FM [9]. From a practical standpoint, FM