8.5. SUBCARRIER MULTIPLEXING 387
Figure 8.31: Predicted and measured crosstalk acquired over 25 km of fiber at 11-mW average
power. The CW laser acts as a probe and its wavelength is (a) lower or (b) higher by 8.5 nm than
the signal wavelength. (After Ref. [245];©c1999 IEEE; reprinted with permission.)
2000 experiment in which a broadcast-and-select network was capable of delivering
10,000 channels, each operating at 20 Gb/s [242]. The network used 32 wavelengths
(on the ITU grid) each of which could carry 310 microwave subcarriers by modulating
at a composite bit rate of 20 Gb/s.
The limiting factor for multiwavelength SCM networks is interchannel crosstalk re-
sulting from both the linear and nonlinear processes [244]–[246]. The nonlinear effects
that produce interchannel crosstalk are SRS and XPM, both of which have been ana-
lyzed. Figure 8.31 shows the crosstalk measured in a two-channel experiment together
with the theoretical prediction of the SRS- and XPM-induced crosstalk levels [245].
One channel is modulated and carries the actual signal while the other operates contin-
uously (CW) but its power is low enough that it acts as a probe. The wavelength dif-
ferenceλmod−λCWis±8.5 nm in the two cases shown in Fig. 8.31. The probe power
varies with time because of SRS and XPM, and the crosstalk is defined as the ratio
of radio-frequency (RF) powers in the two channels. The XPM-induced crosstalk in-
creases and the Raman-induced crosstalk decreases with the modulation frequency but
each has the same magnitude in the two cases shown in Fig. 8.31. The two crosstalks
add up in phase only whenλmod<λCW, resulting in a larger value of the total crosstalk
in that case. The asymmetry seen in Fig. 8.31 is due to SRS and depends on whether
the CW probe channel is being depleted or is being amplified by the other channel.
The linear crosstalk results from the phenomenon of optical beat interference. It oc-
curs when two or more users transmit simultaneously on the same optical channel using
different subcarrier frequencies. As the optical carrier frequencies are then slightly dif-
ferent, their beating produces a beat note in the photocurrent. If the beat-note frequency
overlaps an active subcarrier channel, an interference signal would limit the detection
process in a way similar to IMD. Statistical models have been used to estimate the
probability of channel outage because of optical beat interference [244].
Multiwavelength SCM systems are quite useful for LAN and MAN applications
[239]. They can provide multiple services (telephone, analog and digital TV channels,
computer data, etc.) with only one optical transmitter and one optical receiver per user
because different services can use different microwave subcarriers. This approach low-
ers the cost of terminal equipment in access networks. Different services can be offered