8.3. SYSTEM PERFORMANCE ISSUES 369
Figure 8.24: Output signal power (solid circles) and reflected SBS power (empty circles) as a
function of power injected. (After Ref. [180];©c1992 IEEE; reprinted with permission.)
8.3.4 Stimulated Brillouin Scattering
Stimulated Brillouin scattering (SBS) can also transfer energy from a high-frequency
channel to a low-frequency one when the channel spacing equals the Brillouin shift.
However, in contrast with the case of SRS, such an energy transfer is easily avoided
with the proper design of multichannel communication systems. The reason is that the
Brillouin-gain bandwidth is extremely narrow (∼20 MHz) compared with the Raman-
gain bandwidth (∼5 THz). Thus, the channel spacing must match almost exactly the
Brillouin shift(about 10 GHz in the 1.55-μm region) for SBS to occur; such an exact
match is easily avoided. Furthermore, as discussed in Section 2.6, the two channels
must be counterpropagating for Brillouin amplification to occur.
Although SBS does not induce interchannel crosstalk when all channels propagate
in the forward direction, it nonetheless limits the channel powers. The reason is that a
part of the channel power can be transferred to a backward-propagating Stokes wave
generated from noise when the threshold conditiongBPthLeff/Aeff≈21 is satisfied (see
Section 2.6). This condition is independent of the number and the presence of other
channels. However, the threshold for each channel can be reached at low power levels.
Figure 8.24 shows how the output power and power reflected backward through SBS
vary in a 13-km-long dispersion-shifted fiber as the injected CW power is increased
from 0.5 to 50 mW [180]. No more than 3 mW could be transmitted through the fiber
in this experiment after theBrillouin threshold. For a fiber withAeff= 50 μm^2 and
α= 0 .2 dB/km, the threshold power is below 2 mW when the fiber length is long
enough (>20 km) thatLeffcan be replaced by 1/α.
The preceding estimate applies to CW signals as it neglects the effects of signal
modulation resulting in a random sequence of 0 and 1 bits. In general, the Brillouin
threshold depends on the modulation format as well as on the ratio of the bit rate to
the Brillouin-gain bandwidth [181]. It increases to above 5 mW for lightwave systems
operating near 10 Gb/s. Some applications require launch powers in excess of 10 mW.