Page 222 - High Power Laser Handbook

190 So l i d - S t at e La s e r s Zigzag Slab Lasers 191

t/2 Parameter Factor Change
Thickness, t 1 1/2
t
Height, h 1 2
Heat density, Q 1 1
h 2h ∆T 1 1/4
Index change, ∆n 1 1/4
Focal length 1 1
Stress, σ 1 1/4
Gain, g 0 1 1
Figure 8.5 The table highlights the advantages of aspect-ratio scaling of a slab.

offers two degrees of freedom when it comes to slab sizing. By mak-
ing a slab taller and thinner, its center-to-edge temperature difference
∆T and the stresses can be reduced, enabling the slab to scale to higher
powers. Figure 8.5 shows a comparison of two slabs with the same
area and overall output power. The first one has a cross-sectional
aspect ratio of 2:1, while the second one is a factor of 2 thinner and
taller with an aspect ratio of 8:1. According to equations in Figs. 8.3
and 8.4, both ∆T and the stress decrease by a factor of 4, enabling a
factor of 4 scaling in power.
Aspect-ratio scaling also has limitations. For traditional side-
pumped slabs, as the slab becomes taller and thinner, pump absorp-
tion efficiency begins to suffer. In addition, losses due to diffraction
within the slab become a factor that limits the length of the slab, and
maintaining fabrication tolerances on TIR surface figures becomes
increasingly more difficult. Finally, for crystalline slabs, there may be
growth limits on the height of the slab. A brief discussion for each of
these limitations follows.

Pump Absorption
Pump absorption efficiency for traditional side-pumped thin
slabs is an issue primarily for Nd:YAG, because the Nd-doping
concentrations are limited to ~1 percent. Higher concentrations of
Nd in YAG result in low-optical-quality crystals and a rapid deg-
radation in the fluorescence lifetime. For diode arrays centered
on the 807-nm absorption band of Nd:YAG with a bandwidth of
~4 nm, the required slab thickness for greater than 80 percent absorp-
tion efficiency is ~6 mm. This efficiency follows Beer’s law and
degrades for thinner slabs. To overcome this problem, alternate
edge-pumping and end-pumping techniques have been devel-
oped (discussed later in this section) that provide much longer
absorption distances.