Page 311 - Tunable Lasers Handbook

6 Transition Metal Solid-state lasers

TABLE 4 Measured Slope Efficiencies of Cr Lasers
~~~
Laser material Peak wavelength Slope efficiency

Be+,( SiO;), 0.768 5.64
LiCa.L\lF, 0.780 0.51
Be.i\l,O, 0.752 0.3 1
LiSrXlF, 0.825 0.36
ScBeXlO, 0.797 0.30
ScBO, 0.843 0.29
Gd,Sc,Ga,O 0.785 0.28
Na,Ga,Li3FI , 0.791 0.23

~-,sc,w,2 0.767 0.27
Gd,Sc2A1,0,, 0.784 0.19

coordinated sites. Thus, Cr tends to occupy the B site. which enjoys the octahe-
dral symmetry. The Dq/B parameters have been estimated for GSGG and
GSAG to be 2.45 and 2.55, respectively. Consequently, the ‘T? manifold
appears above the IE manifold for these laser materials. However.-both laser
materials are expected to have a larger fraction of the excited Cr atoms in the
AT7 manifold than Cr:BeAl,O,. Because laser action usually occurs from this
manifold. having a larger fraction of the excited state population increases the
gain of the laser material. All three of these materials utilize Sc in the octahe-
dral site rather than A1 to provide a better lattice match for the Cr. Cr usually
substitutes for Sc. however, it can also substitute for Ga in GSGG. By expand-
ing the octahedral site with Sc atoms? crystals with reasonable concentrations of
Cr can be grown with high optical quality. Availability of high optical quality
laser material is a contributing factor to the interest in these laser materials.
Although a variety of Cr concentrations is available in these laser materials,
absorption bands are strong enough so that concentrations are usually kept in
the range of 0.01 atomic or less.
Garnets are desirable materials from which to make lasers, however, the
thermal and mechanical properties are not usually as good as YAG. Transparency
of these garnets in the ultraviolet is less than that of YAG: typically substantial
absorprion occurs at wavelengths shorter than 0.3 ym. Lack of good ultraviolet
transmission is not surprising in view of the strong absorption of Gd in this
region. Low ultraviolet transmission does not seriously degrade the absorption of
Cr. However. absorption of short-wavelength flashlamp radiation by the laser
material may detract seriously from the laser performance as described later.
Substitution of Gd. Sc. and Ga for Y and AI in YAG degrades the thermal proper-
ties of these materials. Specifically, the thermal conductivity is considerably