Page 55 - Tunable Lasers Handbook

36 R. C. Sze and D. G. Harris

TABLE 3 Oscillator/Amplifier and Master Oscillator/Forced Oscillator
Escimer Lasers

Oscillator Output
Laser medium configuration Secondary stage Linewidth energy (mJj Reference

KrF GI Amplifier 1 GHz 50
XeCl Double etalon Amplifier 599 MHz 310
XeCl GI AmplifieP 4.5 GHz
XeCl IVPL Amplifier 15 GHz 300
XeF Dye laser Amplifier 6 GHz 450-750
(C+N
KrF 3 etalons Forced oscillator 3 GHz UK)
AIF Prism expander grating Forced oscillator 9 GHz 100
KrF 6 GHz 200
XeCl 9 GHz 120
oRegenerative.

can be used to explain their spectroscopic features with regard to frequency nar-
rowing and tunability of the lasing spectrum.
Excimers are a class of molecules in which an electronically excited molec-
ular state is formed by one atom in an electronically excited state associating
with a second atom in its ground state. The molecular ground state is unbound or
only weakly bound (by van der Waals forces). Consequently. a population inver-
sion is automatically established when the excited state is formed. A photon is
emitted and the resulting ground state molecule dissociates. along the lower
potential curve, in a time comparable to one vibrational period (-10-12 sec) (Fig.
1). The practical advantage of such a system is that one photon can be extracted
from each excited molecule produced. rather than the situation in conventional
laser media in which only enough photons can be extracted to equalize the popu-
lations in the upper and lower levels. The emission from the bound repulsive
transition is typically a broad coritinuum resulting from the lack of vibrational
structure and the steepness of the unbound ground state. Emissions from
excimers with a weakly bound ground state. most notably XeCl and XeF, show a
more conventional vibrational and rotational structure.
Using laser rate equations and semiclassical theory, one can go quite far with
elementary derivations toward describing the behavior of excimers. Indeed calcula-
tions of the gain coefficient, saturation intensity, stimulated emission cross sections
and even modeling of the ground state can be quite easily accomplished [27, 27aI.
Care must be taken not to rely completely on these models, because these parame-
ters can vary quite differently depending on the experimental conditions. For
instance, the saturation parameter may vary bj7 a factor of 2 or more depending on