374 CHAPTER 8. MULTICHANNEL SYSTEMS
zero-dispersion wavelength of the fiber was close to 1548 nm, resulting in near phase
matching of many FWM components. Nonetheless, the system performed quite well
with less than 1.5-dB power penalty for all channels.
The use of a nonuniform channel spacing is not always practical because many
WDM components, such as optical filters and waveguide-grating routers, require equal
channel spacings. A practical solution is offered by the periodic dispersion-management
technique discussed in Section 7.8. In this case, fibers with normal and anomalous
GVD are combined to form a dispersion map such that GVD is high locally all along
the fiber even though its average value is quite low. As a result, the FWM efficiencyηF
is negligible throughout the fiber, resulting in little FWM-induced crosstalk. The use of
dispersion management is common for suppressing FWM in WDM systems because of
its practical simplicity. In fact, new kinds of fibers, called nonzero-dispersion-shifted
fibers (NZDSFs), were designed and marketed after the advent of WDM systems. Typ-
ically, GVD is in the range of 4–8 ps/(km-nm) in such fibers to ensure that the FWM-
induced crosstalk is minimized.
8.3.7 Other Design Issues
The design of WDM communication systems requires careful consideration of many
transmitter and receiver characteristics. An important issue concerns the stability of the
carrier frequency (or wavelength) associated with each channel. The frequency of light
emitted from DFB or DBR semiconductor lasers can change considerably because of
changes in the operating temperature (∼10 GHz/◦C). Similar changes can also occur
with the aging of lasers [203]. Such frequency changes are generally not of concern for
single-channel systems. In the case of WDM lightwave systems it is important that the
carrier frequencies of all channels remain stable, at least relatively, so that the channel
spacing does not fluctuate with time.
A number of techniques have been used for frequency stabilization [204]–[209].
A common technique useselectrical feedbackprovided by a frequency discriminator
using an atomic or molecular resonance to lock the laser frequency to the resonance
frequency. For example, one can use ammonia, krypton, or acetylene for semicon-
ductor lasers operating in the 1.55-μm region, as all three have resonances near that
wavelength. Frequency stability to within 1 MHz can be achieved by this technique.
Another technique makes use of theoptogalvanic effectto lock the laser frequency to
an atomic or molecular resonance. A phase-locked loop can also be used for frequency
stabilization. In another scheme, a Michelson interferometer, calibrated by using a
frequency-stabilized master DFB laser, provides a set of equally spaced reference fre-
quencies [205]. A FP filter, an AWG, or any other filter with a comb-like periodic
transmission spectrum can also be used for this purpose because it provides a reference
set of equally spaced frequencies [206]. A fiber grating is useful for frequency stabi-
lization but a separate grating is needed for each channel as its reflection spectrum is
not periodic [207]. A frequency-dithered technique in combination with an AWG and
an amplitude modulator can stabilize the channel frequency to within 0.3 GHz [209].
An important issue in the design of WDM networks is related to the loss of signal
power that occurs because of insertion, distribution, and transmission losses. Optical
amplifiers are used to compensate for such losses but not all channels are amplified by