Page 290 - Tunable Lasers Handbook

7 [::[

250 Norman P. Barnes

6
C
.e 100
c
9
0
u)
Q
a
0.3 0.4 0.5 0.6 0.7
Wavelength (micrometers)
FIGURE 1 1 Absorption spectra ofCr:A1,o3.

The upper laser level lifetime of this material is relatively long. being about 3.0
ms at room temperature. Lifetimes of standard concentrations of Cr are about 4.3
ms at 78 K [13]. Having a long upper laser level lifetime allows long pump pulses
to be employed. thereby facilitating the intense pumping required for this material.
Polarized emission spectra of Cr:A1,0, display two line features. usually
referred to as the R, and R, lines. The existence of two lines arises from the fact
that the ?E level is split into two narrowly separated levels. Separation of these
levels is only 29 cm-1. Lasing naturally occurs on the R, line, which has its ori-
gin on the lower of these two levels. Lasing occurs on this line for two reasons.
First, the lower level has a somewhat larger fraction of the population of the
inverted population density by virtue of its lower energy. Second, because the
stimulated emission cross section of this line is higher than of the R, line, the
gain is higher. The emission cross section is higher for radiation polarized per-
pendicular to the optic axis than for radiation polarized parallel to the optic axis.
As such, the laser output from a Cr:A1,03 laser is polarized.
Even though the strongest radiation from Cr:A1,0, is associated with the R,
and R, lines, sidebands had been noted early in the development of this material.
The fraction of the radiation appearing in the R bands is approximately constant
up to a temperature of about 275 K. As expected when considering the vibronic
transitions, the majority of the sideband radiation existed on the long-wavelength
side of the R lines. It is interesting to note that lasing was observed in Cr:A1,03
at 0.767 pm [16]. However. at the time it was not ascribed to lasing on a vibrokic
transition and its appearance was treated mostly as a curiosity.
Although the threshold of a Cr:A1,03 laser can be quite high. performance
of this laser above threshold can be relatively efficient. Operation of a laser can
be characterized by two parameters. the threshold and the slope efficiency. Con-
sider a plot of laser output energy as a function of electrical energy used for
pumping the laser. Electrical energy is usually associated with the energy stored