Page 167 - Tunable Lasers Handbook

146 Charles Freed

and show excellent agreement with the corresponding values of Silver et al.
More importantly. however, Table 13 gives a quick previe\t of the significant dif-
ferences between corresponding I and I1 band transitions of a given isotope and
also among corresponding transitions of the various CO, isotopic species. The
procedure followed by Freed et al. in the Lincoln Laboratory experiments in
1981 was based on the method developed by Christensen et al. in 1969 [126].
In a typical gain measurement sequence, the laser oscillator was first fre-
quency locked to the line center of the transition to be measured, and the ampli-
fier gain was then determined for several input power levels.
The TEMOo, mode output beam of the COz oscillator \vas recollimated into
the amplifier in a confocal configuration, with the position of the beamwaist at
the center of the amplifier. The water-cooled. sealed-off amplifier had an inside
diameter of 1.3 cm and an active length of 203 cm. The computed average
probe-beam diameter within the amplifier was 21: = 0.35 cm at the e-1 point of
intensity. Under these conditions typically 8.5% of the probe beam vas trans-
mitted through the unexcited amplifier. About half of the insertion loss could
be attributed to attenuation of the gas mix. The remaining attenuation was
caused by window loss. aperturing, and scatter in the amplifier bore due to
slight misalignments.
The gas mixtures used were identical for all CO, isotopes and consisted of
59.2% He, 20% CO,, 14.5% N,, 5.5% Xe, and -1.3% H, at a total pressure of
11.75 Torr. The sealed-off volume of the amplifier was 830 cm3, of which 310
cm3 (37% of the entire volume) was occupied by the excited discharge. After a
fresh fill of the amplifier, the discharge was turned on for at least several hours to
allow the CO, dissociation-recombination process and gas mixing to come to
equilibrium before commencing with the measurements.
The gain was determined by taking the ratio of the output power measured
with the amplifier discharge on, to the output power with the discharge off. True
amplifier gain is, of course, defined as the ratio of power output to pouer input
and in this sense the values of gain we determined are overestimated. but by no
more than a few percent. This overestimate of the measured gain is probably
more than counterbalanced by the fact that the experimental parameters were not
optimized for each individual transition of the various isotopic gas mixtures.
The gain was measured for five transitions (J = 12, 16, 20. 24. 28) in each of
the four rotational branches of the (0001)-[ 1000, 0200],,,, vibrational bands.
Thus, 20 individual vibrational-rotational transitions were measured for each
CO, - isotopic gas mixture.
The data gathering for a given isotopic mixture was carried to completion
with a single gas fill of the amplifier. The amplifier power output readings were
taken within about 2 min after turning on the amplifier discharge. The measured
gain had excellent day-to-day repeatability.
The 10 f 1 mA excitation current in our experiments was optimized for
maximum small-signal gain and was substantially lower than one would find in