Nonlinear Fiber Optics - 4 ed. Agrawal

340 Chapter 9. Stimulated Brillouin Scattering
Figure 9.6: Output spectra in the forward (upper trace) and backward (lower trace) directions
showing multiple orders of Stokes and anti-Stokes waves generated without isolation between
laser and fiber. (After Ref. [59]; c○1980 Elsevier.)
bandwidth of the pump laser affected the SBS threshold, the measurements were also
performed using a Nd:YAG laser having a bandwidth of only 20 kHz. The results were
virtually identical with those obtained with the semiconductor laser, indicating that the
Brillouin-gain bandwidth Δν B was considerably larger than 15 MHz. As mentioned in
Section 9.1.2, Δν B is considerably enhanced for optical fibers from its value in bulk
silica (≈22 MHz at 1.3 μm) because of fiber inhomogeneities. The threshold power
is smaller (∼1 mW) near 1.55 μm because of relatively low losses of optical fibers
(0.2 dB/km) at that wavelength.
In most experiments on SBS, it is essential to use an optical isolator between the
laser and the fiber to prevent the Stokes signal from reentering into the fiber after reflection
at the laser-cavity mirror. In the absence of an isolator, a significant portion of the
Stokes power can be fed back into the fiber. In one experiment, about 30% of the Stokes
power was reflected back and appeared in the forward direction [59]. It was observed
that several orders of Stokes and anti-Stokes lines were generated in the spectrum as
a result of the feedback. Figure 9.6 shows the output spectra in the forward and backward
directions for SBS occurring in a 53-m-long fiber. The 34-GHz spacing between
adjacent spectral lines corresponds exactly to the Brillouin shift at λ p = 514.5 nm. The
anti-Stokes components are generated as a result of four-wave mixing between the copropagating
pump and Stokes waves (see Section 10.1). The higher-order Stokes lines
are generated when the power in the lower-order Stokes wave becomes large enough
to satisfy the threshold condition given in Eq. (9.2.4). Even when optical isolators are
used to prevent external feedback, Rayleigh backscattering, occurring within the fiber,
can provide feedback to the SBS process. In a 1998 experiment, this internal feedback
process in a high-loss fiber (length 300 m) was strong enough to produce lasing when
the fiber was pumped using 1.06-μm CW radiation [60].
9.3 Brillouin Fiber Amplifiers
Similar to the case of SRS, the SBS-produced gain in an optical fiber can be used to amplify
a weak signal whose frequency is shifted from the pump frequency by an amount