Page 497 - High Power Laser Handbook

464 Fi b er L a s er s Pulsed Fiber Lasers 465

optical confinement in the small guiding core of typical long fibers.
The NLE’s “strength,” S NLE, can be expressed as

L P z ()
=
S NLE ∫ peak dz (16.1)
0 A

where L is the fiber length, P peak (z) is the pulse peak power at position z
along the fiber, and A is the in-core guided-mode field area.
Fiber-supported NLEs are quite diverse and include forms of
energy exchange between photons and lattice vibrations of the host
material, as well as parametric processes, which are manifestations of
the irradiance-dependent change in the material refractive index
(optical Kerr effect). In this section, NLEs of special interest for near-
nanosecond-pulse fiber sources will be reviewed, and their impact on
laser and amplifier performance discussed.

Stimulated Brillouin Scattering
Stimulated Brillouin scattering (SBS) consists of the inelastic energy
exchange between incident photons and acoustic phonons in the glass
lattice, which leads to the generation of a phase-conjugated, red-shifted
(Stokes), typically backward-propagating optical beam. Power transfer
from the main beam to the Stokes beam is a major disruption in the
operation of high-power fiber sources, because it reduces the output
optical efficiency and yields a source of power-dependent optical feed-
back that can induce potentially destructive parasitic pulsing behav-
iors. In addition, SBS exhibits very high gain (more than 2 orders of
magnitude larger than stimulated Raman scattering, addressed in the
next subsection), which results in very-low-onset threshold, especially
in the case of powerful and spectrally narrow input beams. Because of
such low threshold and detrimental effects, SBS remains a subject of
intense study, primarily in the context of high-power continuous wave
(CW) fiber lasers, which must exhibit a high spectral quality in order to
enable open-ended power scaling via spectral or coherent beam com-
bining. SBS is also a primary limiting factor for multinanosecond PFLs
and pulsed fiber amplifiers tasked with producing time-bandwidth
products close to the Fourier transform limit—for example, for applica-
tions that involve high-resolution spectroscopic probing and discrimi-
1,2
nation of molecular species.
The importance of SBS, however, is drastically reduced as the pulse
duration is decreased to values less than 2–3 ns, a pulsed regime that is
of primary interest for peak-power maximization. This is due to two
concomitant causes. First, the SBS gain is substantially reduced when
the input beam’s spectral width exceeds the inverse acoustic phonon
lifetime in fused silica, which is usually around 50 MHz. For com-
parison, the Fourier-transform-limited bandwidth of a 3-ns full-width,