Page 112 - Tunable Lasers Handbook

4 CO, Isotope Lasers and Their Applications 93

significant change in the beat frequency after interchanging the two stabilizing
cells, which had very different internal geometries and volumes, and (within the
frequency resolution of our system) no measurable effects due to imperfect
and/or slightly truncated TEMoo, beam profiles.
We have used external stabilizing cells with 2-cm clear apertures at the beam
entrance window. Inside the cell, the laser beam was turned back on itself (in
order to provide a standing wave) by means of a flat, totally reflecting mirror.
Slight misalignment of the return beam was used as a dispersion-independent
means of avoiding optical feedback. External stabilizing cells were used, instead
of an internal absorption cell within the laser cavity, in order to facilitate the opti-
mization of SNIP, in the 4.3-pm detection optics, independent of laser design con-
straints. External cells w-ere also easily portable and usable with any available
laser. The FWHM of the saturation resonance dip ranged from 700 kHz to 1 or 2
MHz as the pressure was varied from 10 to about 200 to 300 mTorr within the
relatively small (2-crn clear aperture) stabilizing cells employed in our experi-
ments. By using a 6.3-cm-diameter cell, 164-kHz RVHM saturation resonance
dips were reported by Kelly [86]. Because the FWHM of the CO, saturation res-
onance due to pressure is about 7.5 kHz/mTorr. much of the lin&idth broaden-
ing is due to other causes such as power and transit-time broadening, second-
order Doppler shift. and recoil effects. More detailed discussions of these causes
can be found in [76,112], and in the literature on primary frequency standards but
any further consideration of these effects is well beyond the scope of this chapter.
The saturated 4.3-pm fluorescence frequency stabilization method has been
recently extended to sequence band CO, lasers by Chou et al. [87,88]. The sequence
band transitions in CO, are designated as (000~)-[100(u- 1). 020 (u- l)lI.*. where
li > 1 (u = 1 defines the-regular bands discussed in this and previous sections of this
chapter). Sequence band lasers were intensively studied by Reid and Siemsen at the
NWC in Ottawa beginning in 1976 [89,90]. Figure 17 shows the sinnplified vibra-
tional energy-level diagram of the CO, and N, molecules, with solid-line arrows
showing the various cw lasing bands observed so far. The dotted-line arro\vs show
the 43-pm fluorescence bands that were utilized for line-center stabilization of the
great multitude of individual lasing transitions.
Figure 17 clearly shows that for the (0002)-[1001, 0201],,, first sequence
band transitions the laver laser levels are approximately 2300 crn-1 above those
of the regular band transitions and therefore the population densities of the first
sequence band laser levels are about four orders of magnitude less than in the
corresponding regular band laser levels. Chou er al. overcame this problem by
using a heated longitudinal C07 absorption cell (L-cell) in which the 4.3-ym
fluorescence was monitored through a 3.3yrn bandpass filter in the direction of
the laser beam [87,88]. Due to the increased CO, temperature, photon trapping
[82,83,87] was reduced. and by increasing the fluorescence collecting length
they increased the intensity of sequence band fluorescence so that z. good enough
SNR was obtained at relatively low cell temperatures.