5.4. SOURCES OF POWER PENALTY 207
Figure 5.8: MPN-induced Power penalty versusBLDσλfor a multimode semiconductor laser
of RMS spectral widthσλ. Different curves correspond to different values of the mode-partition
coefficientk.
One may think that MPN can be avoided completely by using DFB lasers designed
to oscillate in a single longitudinal mode. Unfortunately, this is not necessarily the
case [88]–[91]. The reason is that the main mode of any DFB laser is accompanied
by several side modes of much smaller amplitudes. The single-mode nature of DFB
lasers is quantified through themode-suppression ratio(MSR), defined as the ratio of
the main-mode powerPmto the powerPsof the most dominant side mode. Clearly,
the effect of MPN on system performance would depend on the MSR. Attempts have
therefore been made to estimate the dependence of the MPN-induced power penalty on
the MSR [84]–[98].
A major difference between the multimode and nearly single-mode semiconduc-
tor lasers is related to the statistics associated with mode-partition fluctuations. In a
multimode laser, both main and side modes are above threshold and their fluctuations
are well described by a Gaussian probability density function. By contrast, side modes
in a DFB semiconductor laser are typically below threshold, and the optical power
associated with them follows an exponential distribution given by [84]
p(Ps)=P ̄s−^1 exp[−(Ps/P ̄s)], (5.4.8)whereP ̄sis the average value of the random variablePs.
The effect of side-mode fluctuations on system performance can be appreciated
by considering an ideal receiver. Let us assume that the relative delay∆T=DL∆λ
between the main and side modes is large enough that the side mode appears outside
the bit slot (i.e.,∆T> 1 /BorBLD∆λL>1, where∆λLis the mode spacing). The
decision circuit of the receiver would make an error for 0 bits if the side-mode powerPs
were to exceed the decision threshold set atP ̄m/2, whereP ̄mis the average main-mode