Page 714 - Introduction to Information Optics

698 12. Networking with Optics

material to create a photon at a lower Stokes frequency and another form of
energy called a phonon. The major difference between Raman scattering and
Brillouin scattering is that an optical phonon is created in Raman scattering
and an acoustic phonon is created in Brillouin scattering.
Although a complete study is very complicated, the initial growth intensity
of the Stokes wave /, in the case of SRS may simply be described by

where l p is the pump intensity and g R is the Raman gain coefficient (% 1 x
13
10~ m/W for silica fibers near 1 /im wavelength). A similar relationship holds
u
for SBS with g R replaced by the Brillouin gain coefficient g B (%6 x 10~ m/W
for silica fibers near 1 /mi). In an optical transmission system the intensity of
the scattered light in both SBS and SRS cases grows exponentially at the
expense of the transmission signal which acts as a pump. This phenomenon
leads to considerable fiber loss. The measure of the power level causing
significant SBS and SRS is commonly called threshold power P th, defined as
the incident pump power at which half of the power is transferred to the Stokes
wave. For SBS, the threshold can be estimated by [24]

2M
- ~". ,12.14)

where A e{{ is the effective mode cross section, often referred to as the effective
fiber core area; and L eff is the effective interaction length [25]

where a is the fiber loss and L is the actual fiber length. For SRS, the threshold
power can be estimated by [24]

l6A ctf
Pth*—r^- (12.16)
0/J^eff
The threshold power of SBS at conventional single mode fiber can be as low
as ~ 1 mW at the most popular 1.55 /mi window. Under the same conditions,
the threshold power of SRS is more than 500 mW, mainly due to the much
smaller Raman gain coefficient.
From the system point of view, it is very important to keep the optical signal
of every channel below the SBS and SRS thresholds. The SBS threshold can