Page 498 - High Power Laser Handbook

466 Fi b er L a s er s Pulsed Fiber Lasers 467

half-maximum (FWHM) Gaussian-shaped pulse is ~150 MHz. Sec-
ond, the length of a sub-3-ns pulse in fiber is less than 1 m, which
means that the spatial overlap (and interaction length) between such
short input pulses and their corresponding counterpropagating, SBS-
generated Stokes pulses is typically only a small fraction of the over-
all fiber length.
This analysis holds generally valid for the frequent, simple cases
in which SBS is not “seeded,” meaning that only the main beam is
present in the fiber and that the Stokes components build from dimin-
utively small parametric photon noise or from ASE occurring at the
Stokes-shifted wavelength. However, due to the relatively small SBS
frequency shift (e.g., ~15 GHz in Yb-doped fibers operated at around
1060 nm), it is possible that the input beam being amplified in (or deliv-
ered through) the fiber medium may fortuitously feature spectral
components of appreciable power at the SBS Stokes frequency. This
may occur, for example, if such a beam is generated by a short-cavity
laser (e.g., certain types of Q-switched or semiconductor lasers) with
longitudinal mode spacing sufficiently close to the SBS shift. In this
case, SBS can set in at lower power and shorter pulses. Another case of
interest is cumulative buildup of SBS in high-repetition-rate coherent
pulse trains, in which the pulse period is significantly shorter than the
acoustic phonon lifetime (~10 ns in fused silica). In such a scenario,
significant SBS can occur even for short pulses, as successive pulses
can constructively interact with the same Stokes wave.
Finally, due to its counterpropagating nature, SBS is strongly
influenced by optical feedback, which may lead to the onset of cha-
otic pulsing behavior superimposed on the normal (CW or pulsed)
operation. This parasitic effect is usually referred to as SBS-induced
modulation instability and is especially detrimental in linear-cavity
3
(standing-wave) lasers.
Stimulated Raman Scattering
Stimulated Raman scattering (SRS) is another important form of
inelastic nonlinear process in fiber, stemming from the interaction
between a laser beam and laser-induced vibrations associated with
intramolecular bonds. This interaction results in a variation of the
electric dipole moment (optical phonons). Such vibrations are more
energetic compared with those involved in SBS; in fused silica, they
result in photon-to-phonon transferred energy of ~55 meV, corre-
sponding to a Stokes red shift in excess of 13 THz. The ensuing large
wavelength difference (e.g., greater than 60 nm at ~1 μm) is unaccept-
able for any applications that are required to maintain some degree of
spectral control.
Because the peak Raman gain (~10 -13 m/W in fused silica fibers)
is approximately 2 orders of magnitude lower compared with SBS,
SRS occurs at much higher power. Unlike SBS, however, the SRS
Stokes beam can copropagate with the main beam, in which case