Page 303 - Tunable Lasers Handbook

6 Transition Metal Solid-state lasers 3

network. In addition, the radiation from the flashlamp tends to be blue shifted to
n avelengths shorter than ideal for pumping Ti:A1,0,. Nevertheless, flashlamp
pumping has been achieved by utilizing long, low-lo& Ti:A1,0, laser rods. Same-
times a fluorescent converter will be used to increase the pumping efficiency. A
fluorescent converter can absorb ultraviolet radiation and fluoresce in the wave-
length region where the Ti:AI,O, laser material can absorb. Other methods
include using long laser rods so ;hat higher gain can be achieved while the effec-
tive blackbody temperature of the flashlamp can be decreased. Using this arrange-
ment, a slope efficiency threshold of 20 J and of 0.01 have been achieved.
One of the advantages of the Ti:A1,0, laser is the ability to make efficient
amplifiers. Efficient amplifiers can be obtained if the laser induced damage
threshold energy density is several times larger than the saturation energy den-
sity. A large effective stimulated emission cross section provides Ti:A1,03 with a
low saturation energy density while the material properties allow operation ax
high energy densities. As such, Ti:A1,03 can operate as an efficient amplifier.
Amplifier studies have demonstrated the need for lowloss laser material, match-
ing the pump beam and laser beam radii. and for control of parasitic lasing and
ASE [31]. Small-signal gains of 25 were achieved as well as large-signal gains
of 3.0 [35]. In this case high efficiency was not achieved primarily due to the
limited amount of probe energy and the low figure of merit of the material. How-
ever. an analysis of the Ti:,41,0, laser performance indicated that high efficienq
would be achieved if these limitations were removed.
Continuous nave oscillation of Ti:AI,O, can be achieved using laser pumping
[XI. Either Ar ion or frequency-doubled Nd:YAG can be used as the pump source.
Because single-longitudinal-mode operation is often sought, ring resonators are
&en employed. Pump beam radii in the Ti:A1,03 are kept small, on the order of
tens of micrometers, to keep the threshold low. To achieve the small beam radii,
careful attention is given to minimizing astigmatism. By doing this. thresholds can
be well under 1 .O W, and slope efficiencies can be on the order of 0.1 ~

7. Cr:LiCaAIF6 AND Cr:LiSrAIF,

Cr:LiCaA1F6 and Cr:LiSrAIF, fill an important niche between Cr:BeAl,O,
and Ti:A1,0,. Although the former material can be flashlamp pumped. the iain
of this material is low. A primary reason for this is that most of the excited Cr
atoms reside in the ,E manifold rather than the JT2 manifold. It is th: latter mani-
fold from which most of the laser action occurs. On the other hand, the latter
material has a high gain but its short upper laser level lifetime makes flashlamp
pumping difficult. Cr:LiCaA1F6 and Cr:LiSrA1F6 represent a good compromise
between these materials, that is, reasonably high gain but an upper laser level
lifetime long enough for flashlamp pumping. Such a compromise is possible by
selecting a material with a Dq/B ratio of approximately 2.15. In this case, the ,E