Page 96 - Tunable Lasers Handbook

4 CO, Isotope Lasers and Their Applications 77

where Avr denotes the separation between the two frequencies at which the
Lorentzian is down to half of its peak value, usually referred to as the FWHM
(full width at half-maximum) linewidth,

The type of broadening described by Eq. (7) is called komogeneoiis broadening
because it describes the response of any of the atoms or molecules. which are
thus indistinguishable from each other.
As mentioned before. homogeneous broadening is often due to the finite
interaction lifetime of the emitting or absorbing atoms or molecules. In the CO,
system the dominant source of homogeneous broadening is gas pressure. At suf-
ficiently high densities, the collisions between the molecules become frequent
enough to dominate the broadening mechanism by shortening the lifetime termi-
nation and phase interruption processes.
The pressure-broadening coefficient in pure CO, is approximately 7.5
MHz/Topr [48]. Thus, the absorption linewidth in a gas reference cell (to be used
for long-term frequency stabilization of CO, lasers) filled with 40 mTorr (typical
pressure) of pure COz is approximately 200 kHz due to self-broadening.
In CO, lasers the usual gas fills also include He, N,. CO, and Xe. The
broadening coefficients due to these four collision partneFs are approximately
5.4, 7.4, 6.5, and 6.7 MHz/Torr [20,49,50], respectively. Because most CO,
lasers contain helium-rich gas mixtures, 6 MHz/Torr (4.6 GHz/atm) is a reason:
ably good assumption for the pressure-broadening coefficient for typical CO,
laser mixtures. In high-pressure CO, lasers, the continuous-tuning range of out-
put frequency is determined by the homogeneous pressure broadening of the
gain profile since the responses of the excited molecules are indistinguishable
from each other.
During the discussion of the rotational energy level substructure in Sec. 3 of
this chapter. we stated that the frequency spacings between adjacent lasing urn-
sitions of the regular band W*60, lines vary from about 30 GHz to somewhat
more than 60 GHz. Hence. assuming about 4.6 GHz/atm pressure broadening of
the gain profile, an overall pressure of about 15 am is required to obtain contin-
uous tunability with a COz laser using a single isotope of CO,. By using several
isotopic species of CO,. continuous tunability can be achieved. of course. at a
lower laser gas fill pressure. Indeed such designs have been proposed and experi-
mented with in the past [Sl-541. However. the construction of a reliably operat-
ing and useful, continuously tunable multiple isotope CO, laser that is still above
atmospheric pressure is far from trivial. and (to the best of my knobledge) is not
commercially available.
There are also physical situations in which the individual atoms or mole-
cules are distinguishable, each having a slightly different transition frequency V.