Page 201 - High Power Laser Handbook

170 So l i d - S t at e La s e r s Intr oduction to h igh-Power Solid-State Lasers 171

seem attractive for pulse energy storage, the low cross section leads
2
to saturation fluence F = hν/σ = 8.8 J/cm , making efficient extrac-
sat
tion without damage difficult outside clean room environments.
Cryogenic cooling of Yb:YAG is also commonly employed to reach
13
higher powers than are easily attainable at room temperature. Cool-
ing the material to near-liquid nitrogen temperature (77 K) provides a
number of benefits. First, the thermo-optic and thermomechanical
properties of the YAG host material improve dramatically, with a factor
of ∼4 increase in thermal conductivity and a lowering of both dn/dT
and CTE by factors of ∼7 and 3, respectively. Second, the terminal laser
level population is frozen to near zero, and the material becomes a true
four-level laser with low lasing threshold. Finally, thermal broadening
of the Yb gain spectrum that occurs at room temperature is eliminated,
sharply narrowing the line width and increasing the peak gain by a
factor of ∼7. These benefits must be weighed against the cumbersome
engineering required for implementation of cryo-cooling, which either
requires a reservoir of liquid nitrogen with associated limited runtime,
or integration of closed-loop chillers that are noisy and bulky and that
severely affect overall system efficiency.

7.2.4 High Pulse-Energy and Peak-Power SSL Materials
Whereas Nd:YAG and Yb:YAG are the bases for most HAP SSLs emit-
ting near 1 µm, other materials can provide improved performance
for pulsed operation. For lasers intended to scale to high pulse ener-
gies or peak powers, the average power (i.e., the pulse repetition rate)
is often of secondary importance. This opens the door to the use of
host materials that are less thermally advantageous than YAG. The
key material considerations for pulsed lasers are the energy storage
and extraction capability; the ability to obtain large clear apertures
free from any defects that might provide seeds for damage; and an
emission bandwidth that can support short pulses.

Nd:glass
Nd-doped glasses have long been the material of choice for ultrahigh-
energy pulsed lasers, such as the National Ignition Facility (NIF) laser
(Chap. 14). Laser glass can be fabricated in meter-class apertures,
which are beyond even the capability of ceramics. This enables large
apertures to spread the laser energy to avoid damage, while also
providing a large gain volume of Nd in which to store energy.
Nd:glass’s broad absorption spectrum allows for economical flash-
lamp pumping, and its inhomogeneously broadened emission spec-
trum of several nanometers can support subpicosecond pulses.
14
Inhomogeneous broadening from the glass host also reduces the
peak emission cross section by nearly an order of magnitude, as
compared with Nd:YAG (c.f., Fig. 7.1), hence allowing more stored
energy without ASE depletion. However, the glass host material has