Page 315 - Tunable Lasers Handbook

0 Transition Metal Solid-state Losers 275

0.003. Threshold and slope efficiency for GSAG are 35 J and 0.0012, respec-
tively [50]. Effects of filtering the flashlamp were not performed with this
material although increases in performance are expected based on the measured
flashlamp-induced loss. Gain in flashlamp-pumped YSAG has been observed but
the gain was insufficient to produce lasing [53].
Laser-pumped lasing has been achieved in GSGG, YSAG, and GSAG,
Commonly a Kr ion laser is used as the pump source [46,54,55]. Its wavelength,
0.647 pm, corresponds well to the long-wavelength absorption band of these
laser materials. Low thresholds are achieved using laser pumping by focusing
the pump laser to a small beam radius, often to a pump beam radius as small as
25 pm. As such, the threshold is a critical function of the degree of focusing, A
more fundamental parameter of these laser materials is the slope efficiency.
Slope efficiencies of GSGG, YSAG, and GSAG are 0.23, 0.22. and 0.19 as listed
in Table 1, Slope efficiency will be limited by the ratio of the pump wavelength
to the lasing wavelength. about 0.33 for these materials. Because these laser
materials have a slope efficiency so much lower than the limiting value, a serious
loss mechanism is indicated. Excited state absorption has been identified as a
possible source of this loss mechanism.

9. Co:MgF2, Ni:MgF2, AND V:MgF,

Co:MgF,, Ni:MgF,, and VMgF2 are among the earliest solid-state lasers
discovered: hornsever, a- low effective stimulated emission cross section con-
tributed to their slow development. Initial laser experiments performed 1% ith
these laser materials utilized flashlamp pumping and were conducted at cryo-
genic temperatures, 40 K [56]. LOM -temperature operation increased the upper
laser level lnfetime and the gain of these laser materials, which promoted laser
operation. While low-temperature operation is feasible, room-temperature opera-
tion is vastly preferred. The advent of laser pumping refocused attention on
these laser materials. By using laser pumping, the pump radiation could be
focused into small volumes. thus compensating in part for the low effective stirr-
dated emission cross section. In addition, the use of laser pumping allowed for
rapid increases in the population inversion. A rapid increase in the population
inversion mitigates the effect of the decrease in the upper laser level lifetime
with increasing temperature.
Co:MgF,, Ni:MgF,, and VMgF, have manifolds that are labeled using a
nomenclature- associated with octahedral symmetry. A strong interaction of the
active atoms with the crystal field is in effect. One result of this strong interaction
is a difference in the lattice configuration of the laser material for the ground and
excited states in some instances. Even though the crystal field does not have strict
octahedral symmetry, the states are still labeled using the octahedral nomencla-
ture. Doubly ionized V has the same energy-level diagram of tnply ionized Cr. As