5.4. SOURCES OF POWER PENALTY 209
Figure 5.9: Effect of MPN on bit-error rate of DFB lasers for several values of MSR. Intersection
of the dashed line with the solid curves provides MPN-induced power penalty. (After Ref. [85];
©c1985 IEEE; reprinted with permission.)
5.4.4 Frequency Chirping
Frequency chirping is an important phenomenon that is known to limit the performance
of 1.55-μm lightwave systems even when a DFB laser with a large MSR is used to gen-
erate the digital bit stream [99]–[112]. As discussed in Section 3.5.3, intensity modula-
tion in semiconductor lasers is invariably accompanied by phase modulation because of
the carrier-induced change in the refractive index governed by the linewidth enhance-
ment factor. Optical pulses with a time-dependent phase shift are called chirped. As a
result of the frequency chirp imposed on an optical pulse, its spectrum is considerably
broadened. Such spectral broadening affects the pulse shape at the fiber output because
of fiber dispersion and degrades system performance.
An exact calculation of the chirp-induced power penaltyδcis difficult because fre-
quency chirp depends on both the shape and the width of the optical pulse [101]–[104].
For nearly rectangular pulses, experimental measurements of time-resolved pulse spec-
tra show that frequency chirp occurs mainly near the leading and trailing edges such
that the leading edge shifts toward the blue while the trailing edge shifts toward the
red. Because of the spectral shift, the power contained in the chirped portion of the
pulse moves out of the bit slot when the pulse propagates inside the optical fiber. Such