Page 304 - High Power Laser Handbook

272 So l i d - S t at e La s e r s Heat-Capacity Lasers 273

cooldown time to get the hot slabs from their elevated temperature
down to room temperature can vary from tens of seconds to several
minutes, depending on the type of cooldown system used. The actual
application of the laser and how it will be required to operate (e.g.,
duty cycle, etc.) will dictate the most suitable type of cooling system.
The HCL’s flexibility with regard to how it is cooled is another key
attribute for using this type of laser architecture, particularly for use
in real-world applications.

11.3 Laser Performance Modeling

11.3.1 Pump Absorption, Gain, and Extraction
The geometry of the HCL as implemented in our model is shown in
Fig. 11.7. The one-dimensional laser medium is composed of m slabs,
each with a thickness of l, between two mirrors of reflectivity R and
1
R . The cavity defined by the mirrors has length L , and the total
cav
2
length of the active medium is L slab = ml. Each slab surface is pumped
at intensity i (,l t) W/cm -nm. For simplicity, we’ll assume the laser
2
p
output is on a single line. Within the medium are circulating intensi-
±
ties I W/cm , and the medium has bulk loss a cm .
2
–1
L
The absorption cross section of Nd:YAG in the vicinity of the
808-nm pump band is shown in Fig. 11.8, along with the time-integrated
pump spectrum. As may be seen, the spectrum has numerous peaks
and valleys. In addition, the pump laser diodes have a time-dependent
center wavelength and spectral width. It is therefore important to use
the more general expression for the pump rate into the upper laser
level—that is,
l
Rz t)∝ p (, a ( l i )( , t)[exp( − ∫ ls N sl z ))
(
0
p
a
(11.1)
−
+ exp( N sl()(l − z dl
))]
N
a
0
I + L
I −
� L
+ −
i (λ, t) i (λ, t)
p
p
R 1 R 2
Output coupler
m slabs
Figure 11.7 Geometry of the HCL used in the model.