Page 465 - High Power Laser Handbook

432 Fi b er L a s er s Intr oduction to Optical Fiber Lasers 433

1.E+01

Upper bound
1.E+00
P cr /A eff (MW/µm 2 ) 1.E−01

1.E−02

1.E−03
Lower bound
1.E−04
1.E−15 1.E−12 1.E−09 1.E−06
Pulse duration (s)
Figure 15.17 Damage threshold of silica optical fibers. Lower bound is from
Eq. (15.27); upper bound is from Smith, Do, and Söderlund. 24

threshold is much lower than that of the bulk. Recently, new observa-
tions indicated that the optical damage threshold is independent of
pulse width for pulse durations above 50 ps and is close to 480 GW/cm ,
2
2 24
or 4.8 kW/µm , limited by electron avalanche. For shorter pulses
(less than 50 ps), the avalanche process evolves more slowly than the
pulse envelope, leading to a slightly higher threshold. Both limits are
plotted in Fig. 15.17.
Energy Extraction
For amplifiers operating at repetition rates much greater than the
inverse of upper-state lifetime—that is, for pulse repetition rates
greater than 10 kHz—and for output pulse fluence much less than
F , where F is the saturation fluence given by
sat
sat
hν
F = (15.28)
sat σ s + σ s
e a
we can generally ignore intrapulse saturation characteristics, and
amplifier performance can be modeled using average values of signal
and pump power. High repetition rates are typically used in micro-
machining applications. For counterpumped amplifiers, energy
extraction efficiencies approaching 100 percent can be obtained even
for high-gain amplifiers, which is one of the main advantages of fiber,
compared with solid-state, amplifier technology. The main reason for