332 CHAPTER 8. MULTICHANNEL SYSTEMS
TxTxTxFigure 8.2: Multichannel point-to-point fiber link. Separate transmitter-receiver pairs are used
to send and receive the signal at different wavelengths
channels at bit ratesB 1 ,B 2 ,..., andBNare transmitted simultaneously over a fiber of
lengthL, the total bit rate–distance product,BL, becomes
BL=(B 1 +B 2 +···+BN)L. (8.1.1)For equal bit rates, the system capacity is enhanced by a factor ofN. An early experi-
ment in 1985 demonstrated theBLproduct of 1.37 (Tb/s)-km by transmitting 10 chan-
nels at 2 Gb/s over 68.3 km of standard fiber with a channel spacing of 1.35 nm [3].
The ultimate capacity of WDM fiber links depends on how closely channels can
be packed in the wavelength domain. The minimum channel spacing is limited by
interchannel crosstalk, an issue covered in Section 8.3. Typically, channel spacing∆νch
should exceed 2Bat the bit rateB. This requirement wastes considerable bandwidth.
It is common to introduce a measure of thespectral efficiencyof a WDM system as
ηs=B/∆νch. Attempts are made to makeηsas large as possible.
The channel frequencies (or wavelengths) of WDM systems have been standard-
ized by the International Telecommunication Union (ITU) on a 100-GHz grid in the
frequency range 186–196 THz (covering the C and L bands in the wavelength range
1530–1612 nm). For this reason, channel spacing for most commercial WDM systems
is 100 GHz (0.8 nm at 1552 nm). This value leads to only 10% spectral efficiency at the
bit rate of 10 Gb/s. More recently, ITU has specified WDM channels with a frequency
spacing of 50 GHz. The use of this channel spacing in combination with the bit rate of
40 Gb/s has the potential of increasing the spectral efficiency to 80%. WDM systems
were moving in that direction in 2001.
What is the ultimate capacity of WDM systems? The low-loss region of the state-
of-the-art “dry” fibers (e.g, fibers with reduced OH-absorption near 1.4μm) extends
over 300 nm in the wavelength region covering 1.3–1.6μm (see Fig. 8.1). The min-
imum channel spacing can be as small as 50 GHz or 0.4 nm for 40-Gb/s channels.
Since 750 channels can be accommodated over the 300-nm bandwidth, the resulting
effective bit rate can be as large as 30 Tb/s. If we assume that the WDM signal can be
transmitted over 1000 km by using optical amplifiers with dispersion management, the
effectiveBLproduct may exceed 30,000 (Tb/s)-km with the use of WDM technology.