Page 481 - High Power Laser Handbook

448 Fi b er L a s er s Intr oduction to Optical Fiber Lasers 449

1 κ = 1000 mm −1

500 mm −1
0.8
200 mm −1
Reflection 0.6 100 mm −1 −1
50 mm
0.4

0.2

0
−100 −75 −50 −25 0 25 50 75 100
Frequency detuning (GHz)
(b)
Figure 15.28 (Continued )

where f and f are signal and pump frequencies, respectively, and
s
p
where P and P are absorbed pump power and pump threshold,
th
abs
respectively. The slope efficiency η s
Tf
0
η = α f s (15.42)
s
t p
increases with output mirror transmission T . In cases of typical opti-
0
cal fiber lasers with FBGs as reflectors, in which excessive intracavity
loss is low—that is, α ≈ T —the slope efficiency becomes η = f /f ,
p
s
s
0
t
which is the quantum defect, and it is independent of output mirror
transmission. In Yb -doped optical fiber lasers, a slope efficiency of
3+
~80 percent is regularly achieved, making them extremely attractive
for power scaling to high average powers.
One drawback of cladding pumping configurations is the much
reduced pump overlap with the rare-earth-doped core. Conse-
quently, a much longer fiber must be used for efficient pump absorp-
tion as compared with that for a direct core-pumped configuration.
Most nonlinear effects scale with fiber length, which imposes lower
nonlinear thresholds. This is especially detrimental for power scal-
ing lasers with high peak powers limited by SPM or SRS and lasers
with narrow spectral width limited by SBS. Because ytterbium can
be doped in relatively higher concentrations in silica glass compared
with other rare earth ions, it is a favorite candidate for cladding
pumped optical fiber lasers.