Nonlinear Fiber Optics - 4 ed. Agrawal

9.2. Quasi-CW SBS 339
Figure 9.4: Schematic of the experimental setup used to observe SBS in optical fibers. (After
Ref. [57]; c○1982 IEE.)
Brillouin-gain bandwidth. The experiment employed a 13.6-km-long fiber with losses
of only 0.41 dB/km, resulting in an effective length of 7.66 km. The isolator served
to block the Stokes light from entering the laser. The transmitted and reflected powers
were measured as a function of input power launched into the fiber. Figure 9.5 shows
the experimental data. At low input powers, the reflected signal is due to 4% reflection
at the air–fiber interface. Brillouin threshold is reached at an input power of about
5 mW as manifested through a substantial increase in the reflected power through SBS.
At the same time, the transmitted power decreases because of pump depletion, and
reaches a saturation level of about 2 mW for input powers in excess of 10 mW. The
SBS conversion efficiency is nearly 65% at that level.
In a 1987 experiment, SBS was observed by using a 1.3-μm semiconductor laser,
which utilized the distributed-feedback mechanism to emit light in a single longitudinal
mode with a spectral width ∼10 MHz [58]. The CW pump light was coupled into a 30-
km-long fiber with a loss of 0.46 dB/km, resulting in an effective fiber length of about
9 km. The Brillouin threshold was reached at a pump power of 9 mW. To check if the
Figure 9.5: Transmitted (forward) and reflected (backward) powers as a function of the input
power launched into a 13.6-km-long single-mode fiber. (After Ref. [57]; c○1982 IEE.)