Page 92 - Tunable Lasers Handbook

4 CO, Isotope Lasers and Their Applications 7

deexcitation processes in CO, lasers [1,2,4.5]. By 1965, however. Patel et al. had
demonstrated that the addition of He to the C0,-N, gas fill can be very effective
in the deexcitation of the (0100) level [39] anh icbecame clear that “the CO,
laser was.” in Patel’s words, “a quintessential collision laser in which all the
excitation and de-excitation mechanisms depended crucially on collisions.”
The addition of He into the CO, laser gas mixture is also very effective in
the cooling of the discharge gas because the thermal conductivity of He is signif-
icantly higher than that of CO, or N,. The resulting increase of heat transfer to
the cooled wall of the dischar& tubehnables laser operation at higher excitation
current. which leads to greater power output. Helium typically constitutes at
Least 50% of a CO, laser’s gas fill.
During the initial three years of CO, laser development, all lasers utilizing
He-N,-CO, gas mixtures had to be open, continuous-flow systems, in order to
replenish the CO, that rapidly dissociated in the gas discharge.
Sealed-off CO. laser operation was first reported in 1967 by Witternan [40!
in Holland and by Carbone [41] at MIT Lincoln Laboratory. Witternan used plat-
inum electrodes and the admixture of a small amount (-0.15 Torr) of H,O vapor
or H, as catalysts, whereas Carbone utilized oxidized Ni cathodes. Both of them
reported sealed-off operating lifetimes exceeding 1000 hours.
Shortly thereafter. significant reductions in the CO, dissociation rate and
simultaneous enhancement of the efficiency and/or pow& output of sealed-off
CO, lasers by Xe gas additive was reported by Paanannen [42] at Raytheon
Research Laboratory and by Clark and Wada [43] at Hughes Research Labora-
tory. Wieder and McCurdy [44] were the first to obtain laser operation with a
rare CO, isotope in 1966.
The-addition of Xe has a significant influence on the gas discharge. Xenon
changes the electron energy distribution by increasing the number of electrons
with lower energy and decreasing the number of those with higher energy. This
change in electron energy distribution has a favorable effect on the vibrational
excitation of CO, and N, and also reduces the dissociation rate of CO,. Also. the
ionization potential of Xe is several electron-volts less than that of the other gas
constituents. The low ionization potential facilitates the production of electrons
for maintaining the discharge with a lower longitudinal electric field while main-
taining the same excitation current through the discharge. This, in turn, leads to
either an increase in laser efficiency or an increase in output power or a combi-
nation of both.
During the last 10 to 15 years the steadily increasing interest in the use of
very long-life sealed-off CO, lasers for remote and satellite-borne applications
and in large systems [45] using rare (and expensive) CO, isotopes of limited
availability, greatly spurred the research and development of both homogeneous
and heterogeneous catalysts to be used in long-life CO, lasers. Valuable addi-
tional information may be found in the proceedings of-several topical confer-
ences that were held an this subject during the last decade [4547].