8.4. TIME-DIVISION MULTIPLEXING 375
the same factor unless the gain spectrum is flat over the entire bandwidth of the WDM
signal. Although gain-flattening techniques are commonly employed, channel powers
can still deviate by 10 dB or more when the WDM signal passes through many optical
amplifiers before being detected. It may then become necessary to control the power
of individual channels (through selective attenuation) at each node within a WDM
network to make the channel powers nearly uniform. The issue of power management
in WDM networks is quite complex and requires attention to many details [210]–[212].
The buildup of amplifier noise can also become a limiting factor when the WDM signal
passes through a large number of amplifiers.
Another major issue in the design of WDM systems concerns dispersion manage-
ment. As discussed in Chapter 7, dispersion-management techniques are commonly
used for WDM networks. However, in a reconfigurable network the exact path of a
WDM channel can change in a dynamic fashion. Such networks will require compen-
sation of residual dispersion at individual nodes. Network management is an active
area of research and requires attention to many details [213].
8.4 Time-Division Multiplexing
As discussed in Section 1.2, TDM is commonly performed in the electrical domain
to obtain digital hierarchies for telecommunication systems. In this sense, even single-
channel lightwave systems carry multiple TDM channels. The electrical TDM becomes
difficult to implement at bit rates above 10 Gb/s because of the limitations imposed by
high-speed electronics. A solution is offered by theopticalTDM (OTDM), a scheme
that can increase the bit rate of a single optical carrier to values above 1 Tb/s. The
OTDM technique was studied extensively during the 1990s [214]–[219]. Its commer-
cial deployment requires new types of optical transmitters and receivers based on all-
optical multiplexing and demultiplexing techniques. In this section we first discuss
these new techniques and then focus on the design and performance issues related to
OTDM lightwave systems.
8.4.1 Channel Multiplexing
In OTDM lightwave systems, several optical signals at a bit rateBshare the same
carrier frequency and are multiplexed optically to form a composite bit stream at the
bit rateNB, whereNis the number of channels. Several multiplexing techniques can
be used for this purpose [219]. Figure 8.26 shows the design of an OTDM transmitter
based on the delay-line technique. It requires a laser capable of generating a periodic
pulse train at the repetition rate equal to the single-channel bit rateB. Moreover, the
laser should produce pulses of widthTpsuch thatTp<TB=(NB)−^1 to ensure that
each pulse will fit within its allocated time slotTB. The laser output is split equally into
Nbranches, after amplification if necessary. A modulator in each branch blocks the
pulses representing 0 bits and createsNindependent bit streams at the bit rateB.
Multiplexing ofNbit streams is achieved by a delay technique that can be imple-
mented optically in a simple manner. In this scheme, the bit stream in thenth branch is
delayed by an amount(n− 1 )/(NB), wheren= 1 ,...,N. The output of all branches is