Page 310 - Tunable Lasers Handbook

270 Norman P. Barnes

8. Cr:GSGG, Cr:YSAG, AND Cr:GSAG

Of the plethora of laser materials into which Cr can be incorporated,
Cr:GSGG, Cr:YSAG, and Cr:GSAG are three of the laser materials with the
highest demonstrated potential. An experimental survey of the possible materials
into which Cr could be incorporated was conducted by using laser pumping [46].
Such a procedure has several advantages including the requirement for only
small samples and facile determination of the amount of pump radiation
absorbed by the laser material. Because the amount of pump power absorbed by
the laser material can be determined to good accuracy. the efficiencies of the var-
ious laser materials can be compared on a more nearly equitable basis. Further-
more, by measuring the laser performance as a function of the output mirror
reflectivity, the losses in the laser material can be determined. If these losses are
determined, the effects of laser material quality can also be factored out. Factor-
ing out the losses tends to compensate for the level of development and leaves an
intrinsic slope efficiency. Comparing the intrinsic slope efficiency gives a good
estimate of the potential of various laser materials. Laser material, peak emission
wavelength, and laser-pumped slope efficiency are listed in Table 4. Only the top
10 candidates are listed in the table.
Some of the most promising Cr-doped laser materials appearing in Table 4
are not currently finding wide acceptance for a variety of reasons. Be,Al,(SiO&
or emerald appears to be the most promising material. However, thislaser mate-
rial is a difficult material to grow. Much of the crystal growth has been by the
hydrothermal method. Hydrothermal growth is not widely employed as a
method for large commercial crystal growth and crystal quality has been limited
to date. The next three laser materials, LiCaAlF,. BeA1,0,, and LiSrAlF, have
been covered in previous sections. ScBeAlO, and ScBOi both appear attractive,
however they have not received as much attention as the next group of laser
materials, possibly because of limited availability. Appearing next are four gar-
net laser materials: Gd3Sc,Ga30,, or GSGG, Na3Ga,Li5F, or GFG. Y3Sc,A1,0,,
or YSAG, and Gd3Sc,A130,, or GSAG. Of these. osde crystals tend to be easier
to grow than fluoride crystals. Ease of growth is dependent on material purity as
well as good mechanical and thermal properties. Fluoride materials are often
contaminated, usually with oxides. and tend to have poorer thermal and mechan-
ical properties when compared with oxides. Furthermore, the oxide garnet mate-
rials are available from a variety of vendors. This combination of circumstances
has led to more exploration of GSGG, YSAG, and GSAG than some of the other
laser materials appearing above them in Table 4.
All three of these garnet laser materials have the general chemical formula
A,B,C,O,,. Although in oxide garnets. the three sites have the same valence.
the site-symmetry is completely different. Site symmetry of the A site is dodec-
ahedral. In contrast, site symmetry of the B site is octahedral, and site symme-
try of the C site is tetrahedral. Cr has a proclivity to substitute into octahedrally