Page 467 - High Power Laser Handbook

434 Fi b er L a s er s Intr oduction to Optical Fiber Lasers 435

1.0E+02

SPM, L = 1 m, B < π
eff

Pulse width (ns) 1.0E+01 Raman, L = 1 m

eff

1.0E+00
NSF

Damage
1.0E−01
0 1000 2000 3000 4000 5000
2
Effective mode area (µm )
Figure 15.18 Lower bounds of pulse duration for meeting F and various
sat
3+
limits in Yb -doped fibers. NSF: nonlinear self-focusing.

limits for SRS, SPM, and damage threshold all have the same depen-
dence on A as the saturation pulse energy does, A cancels out in
eff
eff
the calculation for minimum pulse duration. The minimum allow-
able pulse duration for reaching F thus becomes independent of A
sat
eff
for SRS, SPS, and damage threshold (see Fig. 15.18). The optical dam-
age limit, which is less than 480 GW/cm , requires a pulse duration
2 24
∆τ > 40 ps; the Raman scattering limit, assuming L = 1 m, requires
eff
∆τ > 6 ns; and SPM, assuming L = 1 m and B < π, requires ∆τ > 50 ns.
eff
The only exception is the nonlinear self-focusing (NSF) limit, which is
independent of A , and the minimum allowable pulse duration is
eff
consequently proportional to A .
eff
15.2.4 Fibers for High-Power Fiber Lasers
Conventional Large Mode Area Fibers
A conventional step-index fiber operates in the single-mode regime
when normalized frequency V < 2.405. Single-mode fibers with core
diameters in the 2 to 8 µm range are regularly made for telecommu-
nication applications. For high-power fiber lasers, the large effective
mode area of a large fiber core is desirable for reducing nonlinear and
optical damage related effects. Single-mode operation is also required
to maintain good mode quality. The easiest way to extend the core
diameter, while remaining in the single-mode regime, is to mini-
mize a fiber’s NA. The consequences of this approach are weak
guidance and high bend loss. As will become clear later, this is a