Page 114 - Tunable Lasers Handbook

4 CO, Isotope Lasers and Their Applications 95

CO? and also the utilization of these narrow Doppler-free resonances for line-
center stabilization of all available regular and hot band CO, lasing transitions.
Since its first demonstration in 1970, this method of line-center stabilization has
attained worldwide use and became known as the Freed-Javan technique.

9. ABSOLUTE FREQUENCIES OF REGULAR BAND LASING
TRANSITIONS IN NINE CO, ISOTOPIC SPECIES

Through the use of optical heterodyne techniques [36,37,56,92-98], beat
frequencies between laser transitions of individually line-center-stabilized C0,-
isotope lasers in pairs can be generated and accurately measured. Measuremenis
of the difference frequencies are then used to calculate the band centers, rota-
tional constants, and transition frequencies by fitting the measured data to the
standard formula for the term values [31,36-38.931 as given here:

The first systematic measurement and really accurate determination of the absolute
frequencies and vibrational-rotational constants of the regular band 12C1601 laser
transitions was accomplished by Petersen et al. of the NBS in 1973 [93.$5]. In
these initial measurements Petersen et al. used 30 adjacent pairs of 12C160, laser
lines in the 10.4-pm regular band and 26 adjacent pairs in the 9.3-pm regular
band. The lasing transitions were generated by tm o grating-controlled 12C16Q7
lasers, which were line-center-stabilized using the standing-wave saturation reso:
nances observed in the 4.3-pm fluorescence band, and the 3240 63-GHz beat fre-
quencies were detected and measured using LHe temperature Josephson junctions.
These measurements, together with the absolute frequencies of the 10.18-pm I-
R(30) and 9.33-pm 11-R( 10) W160, transitions as determined relative to the pn-
mary cesium frequency standard at the NBS in Boulder, Colorado, by Evenson et
al. in 1973 [94], reduced the uncertainties in existing vibrational-rotational con-
stants [92] 20 to 30 times and the additional rotational constant H, was determined
€or the first time with a statistically significant accuracy.
Concurrent with the ongoing work mith 13C1601 lasers at the NBS, we at MIT
Lincoln Laboratory concentrated our effort on measuring the rare CO, isotopic
species using LN,-cooled HgCdTe varactor photodiodes [74.75] and-the two-
channel line-center-stabilized CO,-isotope calibration system illustrated in Fig. 13
and described in Sec. 8. In the initial phase of the MIT Lincoln Laboratory work,
the band centers. rotational constants. absolute frequencies, and vacuum nave