Page 84 - Tunable Lasers Handbook

4 CO, Isotope Lasers and Their Applications 45

Relatively low-power (1 to 25 W), easy-to-construct CO, lasers that are also
commercially readily available do, however, play an important role in tunable
laser spectroscopy because of the following characteristics. CO, - isotope lasers
can oscillate in a very large number of vibrational-rotational transitions. These
lasing transitions have inherently high spectral punty and may be line-center
stabilized with a long-term stability comparable to commercial cesium atomic
clocks. Approximately 1500 of the CO, lasing transition frequencies have been
determined thus far, and many more miy be measured if necessary. The accura-
cies of the published frequencies are within a few kilohertz relative to the pri-
mary Cesium frequency standard. Thus CO, isotope lasers can be very conve-
niently used as secondary frequency standards in the 8.9- to 12.3-ym wavelength
region. One can also utilize difference frequencies [16] and harmonics [17] of
C0, lasing transitions to synthesize precisely known reference lines well beyond
the-8.9- to 12.1-pm range, Because of the large number of lasing transitions
measured to date. the average spacing between adjacent lines is only about 3 to 6
GHz, which may well be within the tuning range of moderate-pressure (wave-
guide) CO, lasers, optical frequency shifters, and lead-salt tunable diode lasers.
The intention of this chapter is to give an overview of only those aspects of
CO, laser physics and engineering that most intimately relate to tunable laser
spectroscopy. For all other areas of CO, laser physics, engineering, and applica-
tions, the reader is referred to the vast-array of publications that appeared (and
continue to appear) in textbooks [18.19]. books and book chapters [20-221,
SPIE proceedings [23-281, numerous other scientific publications [29,30], and
conference proceedings, just to name a few.
We should emphasize, however, that many of the most important aspects of
CO, laser physics described in the beginning of this chapter are excerpted from
the seminal papers of C. K. N. Pate1 [1.2,3,5]. On the other hand, vnrtually all of
the experimental results and calculations presented in the latter part of the chap-
ter originate from years of painstaking research performed at MIT's Lincoln
Laboratory, the Time and Frequency Division of the National Institute of Stan-
dards and Technology [NIST previously called the National Bureau of Standards
(NBS)] in Boulder, Colorado, and the National Research Council (NRC) in
Ottawa, Canada. Those individuals whose collaboration I had the privilege to
receive over the years are acknowledged at the end of this chapter.

2. VIBRATIONAL ENERGY-LEVEL STRUCTURE OF THE CO, MOLECULE

The CO, molecule is linear and symmetric in configuration and has three
degrees of vibrational freedom as illustrated in Fig. 1. In the symmetric stretch
mode, denoted by vl, the atoms of the molecule vibrate along the internuclear
axis in a symmetric manner. In the bending mode, denoted by v,, the atoms also
vibrate symmetrically but in planes perpendicular to the internuilear axis. In the