Page 302 - Tunable Lasers Handbook

262 Norman P. Barnes

oscillator to the pumped region, the gain of the lasing mode can be high and thus
the threshold can be low [28]. However, use of a small pump beam radius will
limit the amount of energy available from the Ti:A1203 laser. Output from a
Ti:A1203 laser with a tightly focused pump beam will be limited by the laser
induced damage threshold of the laser material. It is simply not feasible to
expose very small areas of the Ti:A1,0, laser material to high energy pump
pulses without incurring laser induced damage. Although the laser induced dam-
age threshold of this material is relatively high, the small pump beam radii will
limit the amount of pump energy that can be used and thus the amount of laser
output energy. Consequently, the pumped beam radius is adjusted to accommo-
date the desired laser output energy without incurring laser induced damage.
Using a frequency-doubled Nd:YAG pump laser has the additional benefit
of producing a short Ti:A1,0, laser output pulse, much like a &-switched pulse.
To achieve efficient frequency doubling, the Nd:YAG laser is usually Q-
switched. As such, the pump pulse is short compared with the pulse evolution
time interval of the Ti:A1,0, laser. A short pump pulse produces gain-switched
operation. Gain switching is different than Q-switching: however, the effect is
the same. With gain switching, the gain varies quickly while with Q-switching
the loss varies quickly. In either case, a short laser output pulse is produced. For
all practical purposes, the dynamics of the pulse evolution for gain-switched or
Q-switched operation can be described using the same formalism. The desirabil-
ity of a short gain-switched pulse often excludes pumping of a Ti:A1,03 with a
flashlamp pumped dye laser even though they could be tuned to the absorption
peak of Ti:A1,0,. The pulse lengths of these devices are relatively long in com-
parison to either the upper laser level lifetime or the pulse evolution time inter-
val, making either Q-switching or gain switching less efficient.
The slope efficiency of a frequency-doubled Nd:YAG laser-pumped Ti:A120,
laser is limited primarily by the ratio of the photon energies. In the ideal case, one
pump photon, with a wavelength of 0.532 pm, produces one photon with a wave-
length of about 0.795 pm. Slope efficiencies are limited by the ratio of photon
energies to about 0.67. In actuality. not all of the pump beam will be absorbed,
not all of the population inversion will be extracted, and not all of the extracted
energy appears as laser output energy. In many situations, the slope efficiency is
only about 0.4, somewhat more than half of the maximum slope efficiency.
Flashlamp-pumped Ti:Al,O, lasers can be achieved in spite of the short
upper laser level lifetime [32,?33]. For most solid-state lasers, efficient pumping
can occur over time intervals on the order of 100 ps or more. Efficient energy
storage over a time interval this long facilitates the achievement of threshold by
allowing high population inversions to be attained. However, pumping longer
than a few times the upper laser level lifetime produces a negligible increase in
the population inversion. Thus the pump intensity must be high enough to pro-
duce threshold in Ti:A1,0, in about 10 ps. Flashlamp pulses this short can be pro-
duced, but careful attention must be given to the inductance in the pulse-forming