Nonlinear Fiber Optics - 4 ed. Agrawal

342 Chapter 9. Stimulated Brillouin Scattering
Figure 9.7: (a) Evolution of pump and Stokes powers (normalized to the input pump power)
along fiber length for b in = 0.001 (solid lines) and 0.01 (dashed lines). (b) Saturated gain as a
function of Stokes output for several values of G A .
shown. The saturated gain is reduced by a factor of two (by 3 dB) when G A b in ≈ 0.5
for G A in the range of 20 to 30 dB. This condition is satisfied when the amplified signal
power becomes about 50% of the input pump power. As typical pump powers are
∼1 mW, the saturation power of Brillouin amplifiers is also ∼1mW.
9.3.2 Amplifier Design and Applications
A semiconductor laser can be used to pump Brillouin amplifiers provided it operates in
a single-longitudinal mode whose spectral width is considerably less than the Brillouingain
bandwidth. Distributed-feedback or external-cavity semiconductor lasers [74] are
most appropriate for pumping Brillouin amplifiers. In a 1986 experiment, two externalcavity
semiconductor lasers, with line widths below 0.1 MHz, were used as pump and
probe lasers [61]. Both lasers operated continuously and were tunable in the spectral
region near 1.5 μm. Figure 9.8 shows the experimental setup schematically. Radiation
from the pump laser was coupled into a 37.5-km-long fiber through a 3-dB coupler. The
probe laser provided a weak input signal (∼10 μW) at the other end of the fiber. Its
wavelength was tuned in the vicinity of the Brillouin shift (ν B = 11.3 GHz) to maximize
the Brillouin gain. The measured amplification factor increased exponentially with the
pump power. This is expected from Eq. (9.3.4). If gain saturation is neglected, the
amplification factor can be written as
G A = I s (0)/I s (L)=exp(g B P 0 L eff /A eff − αL). (9.3.5)
The amplifier gain was 16 dB (G A = 40) at a pump power of only 3.7 mW because of
a long fiber length used in the experiment.
An exponential increase in the signal power with increasing pump powers occurs
only if the amplified signal remains below the saturation level. The saturated gain G s
is reduced by 3 dB when
G A (P in /P 0 ) ≈ 0.5, (9.3.6)