Page 264 - Tunable Lasers Handbook

224 Norman P. Barnes

transition metal solid-state lasers. Basic material properties and laser perfor-
mance details are given for each laser material. Oscillator performance, as well
as amplifier performance where appropriate, is also presented.
Laser performance will be characterized by a threshold and slope efficiency.
If the laser output energy is plotted versus the pump energy, very often a linear
relationship can be obtained. That is, the laser output energy ELo is approxi-
mately related to the pump energy, Ep, by a relationship of the form

where os is the slope efficiency and EPrh is the threshold energy. Pumping of
the transition metal solid-state lasers can be accomplished using either a flash-
lamp or another laser. In either case. the linear relationship is approximately
true. For some laser materials, both types of optical pumping have been suc-
cessfully employed. Furthermore. this approximate relation is true for either
normal mode or Q-switched operation of the laser. Thus, rather than presenting
typical laser output energy versus pump energy data. the threshold and slope
efficiency are given.
When selecting a laser for a particular application, several factors need to be
considered including tuning range, threshold, slope efficiency, energy per pulse,
and average power. Tuning range is often the first selection criterion, that is, the
laser must be able to produce the desired wavelength. Toward this end, the emis-
sion spectra for the various laser materials are given.
A low threshold is necessary to efficient laser operation, particularly if cw
operation is desired. A primary consideration is whether the laser operates as a
three- or four-level laser, the latter being vastly preferred. Threshold depends on
the absorption efficiency of the laser material and the product of the effective
stimulated emission cross section and the lifetime. Absorption efficiency
depends on the characteristics of the optical pump and the absorption properties
of the laser material. If a narrow spectral bandwidth optical pump is used, such
as a laser diode or another laser, relatively narrow absorption features can be
used to advantage. However, if a broad spectral bandwidth pump is used, such as
a flashlamp, broad absorption features become advantageous. To assess the
absorption efficiency, absorption spectra have been included. Threshold is
roughly inversely proportional to the product of the effective stimulated emis-
sion cross section and the upper laser level lifetime. Both parameters are given
in this chapter for the various laser materials.
Slope efficiency depends on the absorption efficiency, as does the threshold,
and on the overlap of the laser mode volume with the pumped volume as well as
the losses. Overlap of the laser mode volume and the pumped volume is particu-
larly important if good beam quality is also important. Good overlap is depen-
dent on the particular laser design but as a general rule is easier to achieve when