Page 157 - Tunable Lasers Handbook

4 CO, isotope lasers and Their Applicaticns 137

To assess the frequency differences between the results published by
Bradley er al. [37] and those to be published by Maki et al. [38]. I compiled
Tab'le 11. which shows the frequency differences in kilohertz for the regular
band lasing transitions (differing by A/ = 8 or 10) in the four CO, isotopic
species to be published by Maki er a]. [38]. Similar to the case in-Tables 2
through IO, the horizontal lines in Table 11 demarcate the boundaries in each
vibrational-rotational branch beyond which higher J lines were not measured in
the Bradley et al. database.
Table 11 clearly indicates that within the database given in Bradley er al.
only one transition. the II-R(50) of 12C1807, differs by more than 11 kHz. For
most other transitions within the measured database in [37] the frequency differ-
ences are only a few kilohertz and would be even less had we taken into account
the -2.9-kHz correction to be applied to the I-R(30) WlSO, absolute frequency
reference used in Bradley et al. [37].
At this stage of development it appears that even more refined techniques
will be necessary to attain another order of magnitude improvement in the preci-
sion and accuracy of CO, beat frequency measurements than was obtained with
the relatively simple two-channel heterodyne system depicted in Fig. 13. Such an
improved system was developed at MIT Lincoln Laboratory in order to obtain
reliable measurements of pressure shifts in the CO, laser system [76.111.112]. A
brief outline of the improved heterodyne setup and the results of pressure shift
measurements is given in the next section. However, before leaving the subject of
absolute frequency calibration of CO, laser transitions, I would like to repeat here
the dedication written for the paper b; Bradley et al. [37]:
The authors nould like to dedicate this Lvork to th2 memory of the late Russell
Petersen, who did so much for the measurement of absolute frequencies at optical wave-
lengths. and uhos2 work has been an essential foundation stone for this paper. Russ was
also a true friend, and his premature death leaves a large gap in the lives of psople who
were privileged to ho~v him.
I was gratified to see a very similar dedication to F. R. Petersen in the forthcom-
ing paper by Maki et al. [38].

7 0. PRESSURE SHIFTS IN LINE-CENTER-STABILIZED CO, LASERS

In the very first publication on the standing-wave saturation resonances
observed in the 4.3-pm fluorescence band of CO,, Freed and Javan drew atten-
tion to the phenomenon (see Fig. 1 in [48]) that the center frequency of the
standing-wave saturation resonance shifted by about 0.33 MHz on the low-fre-
quency side of the peak in the broad background curve. (Note that in the actual
Appl. Phys Lett. publication exactly the reverse direction was statcd and indi-
cated by the arrou s. This error was caught shortly after publication and a correc-
tion erratum was included with reprints.) The two-mirror laser (shown in Fig. 9)