62 CHAPTER 2. OPTICAL FIBERS
(a)(b)Figure 2.18: (a) Raman gain spectrum of fused silica atλp= 1 μm and (b) energy levels partic-
ipating in the SRS process. (After Ref. [75];©c1972 AIP; reprinted with permission.)
signal is in the form of a bit stream. For a single short pulse whose width is much
smaller than the phonon lifetime, no SBS is expected to occur. However, for a high-
speed bit stream, pulses come at such a fast rate that successive pulses build up the
acoustic wave, similar to the case of a CW beam, although the SBS threshold increases.
The exact value of the average threshold power depends on the modulation format (RZ
versus NRZ) and is typically∼5 mW. It can be increased to 10 mW or more by in-
creasing the bandwidth of the optical carrier to>200 MHz through phase modulation.
SBS does not produce interchannel crosstalk in WDM systems because the 10-GHz
frequency shift is much smaller than typical channel spacing.
Stimulated Raman Scattering
Spontaneous Raman scattering occurs in optical fibers when a pump wave is scattered
by the silica molecules. It can be understood using the energy-level diagram shown
in Fig. 2.18(b). Some pump photons give up their energy to create other photons
of reduced energy at a lower frequency; the remaining energy is absorbed by silica
molecules, which end up in an excited vibrational state. An important difference from
Brillouin scattering is that the vibrational energy levels of silica dictate the value of the
Raman shiftΩR=ωp−ωs. As an acoustic wave is not involved, spontaneous Raman
scattering is an isotropic process and occurs in all directions.
Similar to the SBS case, the Raman scattering process becomes stimulated if the
pump power exceeds a threshold value. SRS can occur in both the forward and back-
ward directions in optical fibers. Physically speaking, the beating of the pump and with
the scattered light in these two directions creates a frequency component at the beat fre-
quencyωp−ωs, which acts as a source that derives molecular oscillations. Since the
amplitude of the scattered wave increases in response to these oscillations, a positive
feedback loop sets in. In the case of forward SRS, the feedback process is governed by