110 CHAPTER 3. OPTICAL TRANSMITTERS
3.5.2 Small-Signal Modulation
The modulation response of semiconductor lasers is studied by solving the rate equa-
tions (3.5.1) and (3.5.2) with a time-dependent current of the form
I(t)=Ib+Imfp(t), (3.5.14)whereIbis the bias current,Imis the current, andfp(t)represents the shape of the
current pulse. Two changes are necessary for a realistic description. First, Eq. (3.5.3)
for the gainGmust be modified to become [2]
G=GN(N−N 0 )( 1 −εNLP), (3.5.15)whereεNLis a nonlinear-gain parameter that leads to a slight reduction inGasPin-
creases. The physical mechanism behind this reduction can be attributed to several
phenomena, such as spatial hole burning, spectral hole burning, carrier heating, and
two-photon absorption [62]–[65]. Typical values ofεNLare∼ 10 −^7. Equation (3.5.15)
is valid forεNLP1. The factor 1−εNLPshould be replaced by( 1 +P/Ps)−b, where
Psis a material parameter, when the laser power exceeds far above 10 mW. The expo-
nentbequals^12 for spectral hole burning [63] but can vary over the range 0.2–1 because
of the contribution of carrier heating [65].
The second change is related to an important property of semiconductor lasers. It
turns out that whenever the optical gain changes as a result of changes in the carrier
populationN, the refractive index also changes. From a physical standpoint, ampli-
tude modulation in semiconductor lasers is always accompanied by phase modulation
because of carrier-induced changes in the mode index ̄n. Phase modulation can be
included through the equation [2]
dφ
dt=
1
2
βc[
GN(N−N 0 )−
1
τp]
, (3.5.16)
whereβcis the amplitude-phase coupling parameter, commonly called thelinewidth
enhancement factor, as it leads to an enhancement of the spectral width associated
with a single longitudinal mode (see Section 3.5.5). Typical values ofβcfor InGaAsP
lasers are in the range 4–8, depending on the operating wavelength [66]. Lower values
ofβcoccur in MQW lasers, especially for strained quantum wells [5].
In general, the nonlinear nature of the rate equations makes it necessary to solve
them numerically. A useful analytic solution can be obtained for the case of small-
signal modulation in which the laser is biased above threshold (Ib>Ith) and modulated
such thatImIb−Ith. The rate equations can be linearized in that case and solved
analytically, using the Fourier-transform technique, for an arbitrary form offp(t).The
small-signal modulation bandwidth can be obtained by considering the response of
semiconductor lasers to sinusoidal modulation at the frequencyωmso thatfp(t)=
sin(ωmt). The laser output is also modulated sinusoidally. The general solution of Eqs.
(3.5.1) and (3.5.2) is given by
P(t)=Pb+|pm|sin(ωmt+θm), (3.5.17)
N(t)=Nb+|nm|sin(ωmt+ψm), (3.5.18)