Page 319 - Tunable Lasers Handbook

6 Transition Metal Solid-State Lasers 279
Quantum efficiency data for these laser materials are complicated by the
varying definitions found in the literature. However, quantum efficiency, defined
as the fraction of the active atoms in the upper laser manifold that decay by
direct emission of a photon, is expected to be low. For example, for a similar
laser material, Co:KMgF3. the quantum efficiency even at cryogenic tempera-
tures has been estimated as only 0.7 [59]. Low values of the quantum efficiency
tend to indicate the presence of strong nonradiative decay.
Upper laser level lifetime is strongly temperature dependent. For Ni:MgF,.
the upper laser level lifetime drops precipitously as the temperature increases.
Lifetimes are 12.8. 11.5, and 3.7 ms at 20, 77, and 295 KI respectively [60]. At
very low temperatures, the upper laser level lifetime of Co:MgF7 is relatively
long, about 1.8 ms. However, above about 80 K, the lifetime begins to fall as
shown in Fig. 29 [61]. At room temperature. the upper laser level lifetime is only
about 36 ps.
Polarized emission spectra of Ni:MgF, - (Fig. 30j extend between 1.5 and 1.9
pm. At cryogenic temperatures, a strong emission feature occurs for both polar-
izations at about 1.54 pm [62]. This emission feature is associated with a pure
electronic transition. On the long-wavelength side of the electronic transition are
vibronic transitions. At low temperatures the TC polarized spectra is stronger than
the 0 polarized spectra. Relatively broad peaks can be observed in the nominal
vibronic transitions. As the temperature increases, these peaks tend to disappear
and the emission spectra become smoother.
Polarized emission spectra of CoMgF, (Fig. 31) display broad peaks
around 1.9 pm and relatively sharp line spectia only at cryogenic temperatures.
At cryogenic temperatures, around 80 K, six peaks in the fluorescence can be
observed. These peaks are associated with the six components of the ground

100,000 =-
rn Co: MgFr
A
n" 10,000~
f
0
u
3 ,,ooo,
2
0
._
5. 100:
E"
._
c '0:
0
!e
-I
1 --
I I I I
100 200 300 400 0
Temperature (K)
FIGURE 29 Upper laser level lifetime of Co:hlgF,. Co:KhlgF,, Ni:hfgF,. and V:M~F~ versus
temperature. (Courtesy of P. E Moulton, Schwanz Elecro-Optics.)