5.4. SOURCES OF POWER PENALTY 205
Figure 5.7: Dispersion-induced power penalty for a Gaussian pulse as a function ofBLDσλ.
Source spectrum is also assumed to be Gaussian with an RMS widthσλ.
sensitivity degrades severely when a system is designed to approach it. Most lightwave
systems are designed such thatBLDσλ< 0 .2, so that the dispersion penalty is below
2 dB. It should be stressed that Eq. (5.4.4) provides a rough estimate only as its deriva-
tion is based on several simplifying assumptions, such as a Gaussian pulse shape and
a Gaussian source spectrum. These assumptions are not always satisfied in practice.
Moreover, it is based on the condition 4Bσ=1, so that the pulse remains nearly con-
fined within the bit slot. It is possible to design a system such that the pulse spreads
outside the bit slot but ISI is reduced through pulse shaping at the receiver.
5.4.3 Mode-Partition Noise
As discussed in Section 3.5.4, multimode semiconductor lasers exhibitmode-partition
noise(MPN), a phenomenon occurring because of an anticorrelation among pairs of
longitudinal modes. In particular, various longitudinal modes fluctuate in such a way
that individual modes exhibit large intensity fluctuations even though the total intensity
remains relatively constant. MPN would be harmless in the absence of fiber disper-
sion, as all modes would remain synchronized during transmission and detection. In
practice, different modes become unsynchronized, since they travel at slightly differ-
ent speeds inside the fiber because of group-velocity dispersion. As a result of such
desynchronization, the receiver current exhibits additional fluctuations, and the SNR
at the decision circuit becomes worse than that expected in the absence of MPN. A
power penalty must be paid to improve the SNR to the same value that is necessary to
achieve the required BER (see Section 4.5). The effect of MPN on system performance
has been studied extensively for both multimode semiconductor lasers [74]–[83] and
nearly single-mode lasers [84]–[98].