3.5. LASER CHARACTERISTICS 117
where the phase fluctuation∆φ(τ)=φ(t+τ)−φ(t)is taken to be a Gaussian random
process. The phase variance〈∆φ^2 (τ)〉can be calculated by linearizing Eqs. (3.5.26)–
(3.5.28) and solving the resulting set of linear equations. The result is [82]
〈∆φ^2 (τ)〉=Rsp
2 P ̄[
( 1 +βc^2 b)τ+
βc^2 b
2 ΓRcosδ[cos( 3 δ)−e−ΓRτcos(ΩRτ− 3 δ)]]
,
(3.5.36)
where
b=ΩR/(Ω^2 R+Γ^2 R)^1 /^2 and δ=tan−^1 (ΓR/ΩR). (3.5.37)
The spectrum is obtained by using Eqs. (3.5.34)–(3.5.36). It is found to consist of a
dominant central peak located atω 0 and multiple satellite peaks located atω=ω 0 ±
mΩR, wheremis an integer. The amplitude of satellite peaks is typically less than 1% of
that of the central peak. The physical origin of the satellite peaks is related to relaxation
oscillations, which are responsible for the term proportional tobin Eq. (3.5.36). If this
term is neglected, the autocorrelation functionΓEE(τ)decays exponentially withτ.
The integral in Eq. (3.5.34) can then be performed analytically, and the spectrum is
found to be Lorentzian. The spectral linewidth∆νis defined as the full-width at half-
maximum (FWHM) of this Lorentzian line and is given by [82]
∆ν=Rsp( 1 +βc^2 )/( 4 πP ̄), (3.5.38)whereb=1 was assumed asΓRΩRunder typical operating conditions. The linewidth
is enhanced by a factor of 1+βc^2 as a result of the amplitude-phase coupling governed
byβcin Eq. (3.5.28);βcis called the linewidth enhancement factor for this reason.
Equation (3.5.38) shows that∆νshould decrease asP ̄−^1 with an increase in the
laser power. Such an inverse dependence is observed experimentally at low power
levels (<10 mW) for most semiconductor lasers. However, often the linewidth is found
to saturate to a value in the range 1–10 MHz at a power level above 10 mW. Figure 3.24
shows such linewidth-saturation behavior for several 1.55-μm DFB lasers [86]. It also
shows that the linewidth can be reduced considerably by using a MQW design for the
DFB laser. The reduction is due to a smaller value of the parameterβcrealized by such
a design. The linewidth can also be reduced by increasing the cavity lengthL, since
Rspdecreases andPincreases at a given output power asLis increased. Although not
obvious from Eq. (3.5.38),∆νcan be shown to vary asL−^2 when the length dependence
ofRspandPis incorporated. As seen in Fig. 3.24,∆νis reduced by about a factor of
4 when the cavity length is doubled. The 800-μm-long MQW-DFB laser is found to
exhibit a linewidth as small as 270 kHz at a power output of 13.5 mW [86]. It is further
reduced in strained MQW lasers because of relatively low values ofβc, and a value of
about 100 kHz has been measured in lasers withβc≈1 [77]. It should be stressed,
however, that the linewidth of most DFB lasers is typically 5–10 MHz when operating
atapowerlevelof10mW.
Figure 3.24 shows that as the laser power increases, the linewidth not only saturates
but begins to rebroaden. Several mechanisms have been invoked to explain such behav-
ior; a few of them are current noise, 1/fnoise, nonlinear gain, sidemode interaction,
and index nonlinearity [87]–[94]. The linewidth of most DFB lasers is small enough
that it is not a limiting factor for lightwave systems.