Page 289 - Tunable Lasers Handbook

6 Transition Metal Solid-state Lasers 249

TABLE 1 Physical Properties of A1,0,

Parameter \%he Units

Lattice constants
a axis 176.3
c axis 1300.3
Density 3990
Heat capacity 775
Thermal conductivity
a axis 33
c axis 35
Thermal expansion
a axis 1.8
c axis 5.3
Refractive index
a axis 1.7651
c axis 1.7573
Refractive index variation
a axis 13.1
c axis 11.5
Optical transparency 0.154.5
hleltin_g point 2040

of the spectrum, being centered at about 0.405 pm; the other absorption band lies
in the green region of the spectrum, being centered at about 0.551 pm, as shown
in Fig. 11. The spectral bandwidths of these bands are about 0.05 and 0.07 pm,
respectively. Absorption features are associated with transitions between the 'T,
and 4T, levels and the 4A, ground level. Absorption coefficients associated with
these bands are relatively strong and yield absorption coefficients on the order of
200 m-1 for common Cr:Al,O, laser material. Because of the presence of two rel-
atively strong and spectraliy-wide absorption features, Cr can be an efficient
absorber of blackbody radiation in the visible region of the spectrum.
Having absorbed flashlamp radiation in the pump bands, absorbed energy
can be transferred to the upper laser level with a high quantum efficiency. That
is, a quantum of energy absorbed in the pump band has a high probability of
producing a Cr atom in the upper laser level. For Cr:A1,0, operating at 0.694
pm, the upper laser level is the ,E level. Quantum efficiency has been expen-
mentally demonstrated to be a function of temperature. At reduced temperatures,
the quantum efficiency has been measured to be about 1.0; however, it begins to
decrease as the temperature increases. Near room temperature, it has been esti-
mated to be between 0,7 and 1 .0 [ 14,151.