Page 309 - Tunable Lasers Handbook

6 Transition Metal Solid-state Lasers 269

Normal mode thresholds for flashlamp-pumped Cr:LiSrA1F6 are consider-
ably lower than they are for Cr:LiCaA1F6, reflecting the higher gain of the for-
mer laser material. Experimental results are available for 6,35-mm-diameter
laser rods. The length of the Cr:LiCaAlF, was 80 mm: the length of the Cr:Li-
SrAlF, was 100 mm. Although the experimental arrangements are somewhat dif-
ferent, including the exact pump cavity. there are enough similarities for a com-
parison. Threshold for Cr:LiCaAlF, varied from about 55 to 82 J as the output
mirror reflectivity decreased from 0.944 to 0.62 [43]. On the oiher hand, the
threshold for Cr:LiSrA1F6 varied from 15 to 32 J as the output mirror reflectivity
decreased from 0.985 to 0.43 [44]. From these measurements, the double-pass
loss was deduced as 0.49 for Cr:LiCaAlF, and 0.39 for the Cr:LiSrA1F6. Consid-
erably lower thresholds can be expected if this relatively high loss can be
reduced. Because both of these laser materials can be considered to be new
materials at this time, a relatively high loss is not surprising.
Even at this stage of development, normal mode slope efficiencies can be
relatively high. Slope efficiency of a normal mode Cr:LiCaAlF, increased
monotonically with a decrease in the output mirror reflectivity. As the output
mirror reflectivity decreased from 0.944 to 0.62, the slope efficiency increased
from 0.0035 to 0.0155 [43]. Such an increase is consistent with a nonsaturable
passive loss. Slope efficiency of a normal mode Cr:LiSrA1F6 also displayed a
monotonic increase in the slope efficiency as the output mirror reflectivity
decreased. For this laser material, the slope efficiency increased from 0.0043 to
0.050 as the output mirror reflectivity decreased from 0.985 to 0.43 [44]. Slope
efficiency is expected to increase as the quality of the laser materid increases
and more nearly optimum configurations are developed.
Tuning of these laser materials has been demonstrated over wide spectral
regions. Cr:LiCaA1F6 has operated between about 0.72 and 0.85 pm [45]. Short-
wavelength operation appears to be limited primarily by ground state absorption.
Long-wavdength operation is limited by a combination of effects including the
decreasing effective stimulated emission cross section and the increasing excited
state absorption. Cr:LiSrA1F6 has lased over the approximate range between
0.78 and 1.01 pm [44]. Limits on the short-wavelength operation are probably
caused by ground state absorption, a decrease in effective stimulated emission
cross section. and possibly excited state absorption. Long-wavelength operation
is also limited by a decreasing effective stimulated emission cross section.
Continuous wave operation of these laser materials has been demonstrated
using a 0.647-pm Kr ion laser pump. Slope efficiencies €or Cr:LiCaA1F6 and
Cr:LiSrAAlF6 were measured as 0.54 and 0.36, respectively [43]~ Slope efficiency
is limited by the ratio of the pump wavelength to the lasing wavelength. Based
on this. slope efficiencies should be 0.83 and 0.78, respectively. Part of the dif-
ference between the observed slope efficiency and the maximum slope efficiency
can be attributed to the losses in the resonator. However, because the observed
slope efficiency of Cr:LiSr,41F6 is less than half of the limit, an excited state
absorption mechanism has been used to explain the difference.