8.5. SUBCARRIER MULTIPLEXING 381
sary to compensate for the dispersion slope (third-order dispersionβ 3 ) as 1-ps pulses
were severely distorted and exhibited oscillatory tails extending to beyond 5 ps (typ-
ical characteristic of the third-order dispersion) in the absence of such compensation.
Even then, the transmission distance was limited to 40 km. In a 2000 experiment, a
1.28-Tb/s ODTM signal could be transmitted over 70 km but it required compensation
of second-, third-, and fourth-order dispersion simultaneously [227]. In a 2001 field
trial, the bit rate of the OTDM system was limited to only 160 Gb/s but the signal was
transmitted over 116 km using a standard two-fiber dispersion map [228].
A simple method for realizing high bit rates exceeding 1 Tb/s consists of combin-
ing the OTDM and WDM techniques. For example, a WDM signal consisting ofM
separate optical carriers such that each carrier carriesNOTDM channels at the bit rate
Bhas the total capacityBtot=MNB. The dispersion limitations of such a system are set
by the OTDM-signal bit rate ofNB. In a 1999 experiment, this approach was used to
realize a total capacity of 3 Tb/s by usingM=19,N=16, andB=10 Gb/s [219]. The
channels were spaced 450 GHz apart (about 3.6 nm) to avoid overlap between neigh-
boring WDM channels at the 160-Gb/s bit rate. The 70-nm WDM signal occupied
both the C and L bands. The total capacity of such OTDM/WDM systems can exceed
10 Tb/s if the S band is also used although many factors such as various nonlinear ef-
fects in fibers and the practicality of dispersion compensation over a wide bandwidth
are likely to limit the system performance.
OTDM has also been used for designing transparent optical networks capable of
connecting multiple nodes for random bidirectional access [215]. Its use is especially
practical for packet-based networks employing the ATM and TCP/IP protocols. Similar
to the case of WDM networks, both single-hop and multihop architectures have been
considered. Single-hop OTDM networks use passive star couplers to distribute the
signal from one node to all other nodes. In contrast, multihop OTDM networks require
signal processing at each node to route the traffic. A packet-switching technique is
commonly used for such networks. Considerable effort was under way in 2001 for
developing packet-switched OTDM networks [229]. Their implementation requires
several new all-optical components for storage, compression, and decompression of
individual packets [230].
8.5 Subcarrier Multiplexing.........................
In some LAN and MAN applications the bit rate of each channel should be relatively
low but the number of channels can become quite large. An example is provided by
common-antenna (cable) television (CATV) networks that have used historically elec-
trical communication techniques. The basic concept behindsubcarrier multiplexing
(SCM) is borrowed from microwave technology, which employs multiple microwave
carriers for transmission of multiple channels (electrical FDM) over coaxial cables or
free space. The total bandwidth is limited to well below 1 GHz when coaxial cables are
used to transmit a multichannel microwave signal. However, if the microwave signal
is transmitted optically by using optical fibers, the signal bandwidth can easily exceed
10 GHz for a single optical carrier. Such a scheme is referred to as SCM, since mul-
tiplexing is done by using microwave subcarriers rather than the optical carrier. It has