Page 197 - High Power Laser Handbook

166 So l i d - S t at e La s e r s Intr oduction to h igh-Power Solid-State Lasers 167

lasers, σ must be sufficiently high to provide reasonable levels of gain
and to prevent the saturation fluence (F = hν/σ) from exceeding the
sat
material’s damage threshold.

7.2.2 Host Materials
The choice of host material is of particular importance for high-power
SSLs. The highest grade of optical material purity is critical, both for
purposes of laser damage resistance and to minimize transmission
losses of the high-power extracting laser beam—in particular, absorp-
tion losses, which deposit excess heat in the material. The material must
be able to be cut and polished to laser-grade specifications (typically
better than 1/10 wave surface figure and 10/5 scratch-dig) with reason-
able effort and yield. The mechanical properties of the host material are
also of key importance for high power. High thermal conductivity will
minimize the temperature increase associated with a given volumetric
heat load that arises from lasing. The fracture toughness—that is, the
peak surface tensile stress that the material can withstand—will deter-
mine the ultimate power density allowed for a particular geometry.
Finally, the host material’s thermo-optic properties (i.e., the change in
index with temperature dn/dT and the coefficient of thermal expansion,
CTE or α) drive the magnitude of laser wavefront distortion and depo-
larization for any given temperature increase. All these properties work
together to determine the performance of a particular architecture.
Many of these considerations apply not only to the laser gain
materials but also to any optical materials or coatings upon which the
high-power laser beam is incident. However, laser gain materials are
typically far more difficult to engineer or select than passive optical
materials. First and foremost, this is because host selection is limited
to those host materials that provide adequate lattice matches such
that active ions can be doped in high concentrations. Moreover, the
host material must be able to withstand the laser waste heat loads,
which are typically order(s) of magnitude greater than the heat loads
resulting from trace absorption of the high-power laser beam that
may occur in passive optical elements.
The most successful and ubiquitous host material used in HAP
SSLs is yttrium aluminum garnet (Y Al O , or YAG), which pos-
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12
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sesses a fortuitous mix of high thermal conductivity, mechanical
5
strength, and excellent optical quality. Most of the active lasing rare
earth (RE) elements can be readily substituted for Y in the YAG crys-
tal lattice, enabling high dopant concentrations. YAG is also readily
manufacturable and, despite its hardness, can be cut and polished to
exacting laser-grade tolerances.
One of the primary limitations of high-power SSL host materials
has been imposed by their crystalline nature, which limits the size to
which they can be grown. For example, a grown boule of crystalline
YAG is limited to a diameter of ~10 cm by accumulated growth stresses