Page 90 - Tunable Lasers Handbook

4 CO, Isotope Lasers and Their Applications 7

4. PROCESSES GOVERNING THE EXCITATION OF REGULAR BAND
LASER TRANSITIONS IN CO,

The most basic configuration of a CO, laser consists of an amplifying gain
section within an optical cavity. In its simpfest form the optical cavity consists of
two mirrors: however, a diffraction grating is often substituted for one of the
mirrors in order to provide frequency dispersion and easily controllable line
selectivity among the many lasing transitions that could otherwise oscillate. The
gain medium is a gas discharge, which uses a (typical) mixture of CO,, N,. He.
Xe, and a small amount of H, or H,O vapor in the sealed-off CO, isotope lasers
that are most useful in laser spectroscopy. The dominant processes that govern
the regular band CO, laser transitions are depicted in Fig. 1. Gain will occur due
to either complete 0; partial inversion of the vibrational population densities of
the (0001) upper laser level over the (1000) or (0200) lower laser levels [1.2,4,51.
Up to 50% of all N, molecules may become vibrationally excited in the gas
discharge and transfer most of their vibrational energy to the CO, molecules by
exciting the vibrational levels of the vj mode, primarily the (0001) level of
12C160,, which is only about AE = 18 cm-1 higher than the (U = 1) level of IAN,.
This 18-cm-1 energy difference is much smaller than the average kinetic eneri,
0 .

RECOMBINATION
ANDCASCADES
-
u=l

2000 VIBRATIONAL
ENERGY
TRANSFER
DURING
COLLISION
1000 - 9.4 pm
z
h
- F
r 0
5
2
$ 1000 :
9 W
z
W
F\ ELECTRON
IMPACT
+DECAY EXCITATION
-
u=o
C02 GROUND STATE (OOoO) Np GROUND
STATE
FIGURE 4 The vibratimal energy levels most relevant to the regular band CO, laser transitions
(After C. K. N. Patel.)