10.5. SENSITIVITY DEGRADATION 505
Figure 10.14: Dispersion-induced power penalty as a function of the dimensionless parameter
|β 2 |B^2 Lfor several modulation formats. The dashed line shows power penalty for a direct-
detection system. (After Ref. [95];©c1988 IEEE; reprinted with permission.)
coherent systems although its impact is less severe than for IM/DD systems [93]–[97].
The reason is easily understood by noting that coherent systems, by necessity, use a
semiconductor laser operating in a single longitudinal mode with a narrow linewidth.
Frequency chirping is avoided by using external modulators. Moreover, it is possible
to compensate for fiber dispersion (see Section 7.2) through electronic equalization
techniques in the IF domain [98].
The effect of fiber dispersion on the transmitted signal can be calculated by using
the analysis of Section 2.4. In particular, Eq. (2.4.15) can be used to calculate the
optical field at the fiber output for any modulation technique. The power penalty has
been calculated for various modulation formats [95] through numerical simulations
of the eye degradation occurring when a pseudo-random bit sequence is propagated
through a single-mode fiber and demodulated by using a synchronous or asynchronous
receiver. Figure 10.14 shows the power penalty as a function of the dimensionless
parameter combination|β 2 |B^2 Lfor several kinds of modulation formats. The dashed
line shows, for comparison, the case of an IM/DD system. In all cases, the low-pass
filter (before the decision circuit) is taken to be a second-orderButterworth filter[99],
with the 3-dB bandwidth equal to 65% of the bit rate.
As seen in Fig. 10.14, fiber dispersion affects the performance of a coherent light-
wave system qualitatively in the same way for all modulation formats, although quan-
titative differences do occur. The power penalty increases most rapidly for CPFSK
and MSK formats, for which tone spacing is smaller than the bit rate. In all cases
system performance depends on the productB^2 Lrather thanBL. One can estimate