Page 291 - Tunable Lasers Handbook

6 Transition Metal Solid-state lasers 25 1

on the capacitor in a pulse-forming network. which drives the flashlamp. Electri-
cal energy stored on the capacitor is easily determined by measuring the voltage
to which the capacitor is charged. A plot of the laser output energy as a function
of the electrical energy usually can be well approximated by a linear relarion-
ship. Threshold is defined by the intersection of a linear fit with the abscissa. and
slope efficiency is simply the slope of the line. Threshold occurs for energies on
the order of 2000 to 3000 J. Slope efficiencies can be in excess of 0.01. Conse-
-. laser oscillator
quently, tens of joules can be generated from a single Cr:AI,O,
when operating in the normal mode.
Threshold and slope efficiency are a function of the concentration of Cr in the
A1,0, [17]. Threshold depends on the Cr concentration for two reasons. the
absorption efficiency and the population density of the lower laser level. Absorp
tion efficiency is the fraction of the pump radiation that is transmitted into the laser
material and subsequently absorbed. Obviously, if there is no Cr in the Ai,03,
there will be no absorption. Absorption efficiency increases with increasing Cr
concentration. However, as the laser material becomes opaque, increases in the Cr
concentration further produce diminishingly smaller increases in absorption effi-
ciency. For efficient operation, absorption of the pump radiation should be high.
favoring higher Cr concentrations. Conversely, as the Cr concentration increases.
more energy needs to be absorbed to overcome the population density in the lower
laser level. that 1s to produce an inversion. Thus, threshold depends on these tw0
competing effects. As these two effects compete. the threshold is not critically
dependent on the exact Cr concentration as long as it is near the optimum concen-
[ration. Slope efficiency. on the other hand, tends to favor higher concentrations as
slope efficiency describes what happens above threshold. However. for the concen-
trations commonly used, the absorption efficiency is relatively high, Thus, only
modest increases in the slope efficiency are obtained as the Cr concentration
increases. For a particular application, the Cr concentration can be optimized.
Many of the problems associated with the Cr:A1,03 laser can be obviated by rind-
ing a laser material where Cr can act like a four-l&el laser.
Cr:Al,O, has achieved cw operation at room temperature despite the fact
that it is a-three-level laser [18,19]. Typically, a mercury-arc lamp was used to
optically pump the laser rod. Threshold depends on the size of the laser rod,
being lower for the shorter laser rods [19]. Typically, thresholds are on the order
of 1000 W and slope efficiencies are about 0.001.

5. Cr:5eAl2O4

Cr:BeAl,O, is a laser material that overcame the primary difficulty associ-
ated with Cr:A1,0, lasers. namely, three-level operation. Cr:BeAP,O, is com-
monly referred to as alexandrite, a gemstone that has the same chemical compo-
sition and stiucture as the laser material. Although not a true four-level laser. the
vibronic transition on which this laser usually operates, permits fow-level-like