Page 33 - High Power Laser Handbook

6 G a s , C h e m i c a l , a n d F r e e - E l e c t r o n L a s e r s Carbon Dioxide Lasers 7

CO , can be explained by the larger difference in energy levels and
2
the fact that CO has a dipole moment and thus has spontaneous
decay.
The energy transfer via N and CO to CO is much more efficient
2
2
than the direct excitation of the CO molecule; this is due to the much
2
larger cross sections for vibrational excitation of N and CO by elec-
2
tron impact. According to Hake and Phelps (1967), vibrational excita-
tion of CO molecules by electron impact is only efficient for a narrow
2
6
range of electron energies. The vibrational excitation of CO and N
2
by electron impact, however, is quite efficient for a wide range of elec-
tron energies. For optimum excitation of CO and N , the electron
2
energies should range from 1 to 3 electron volts (eV). The range of
electron energies can be adjusted by changing the pressure and com-
2
position of the laser gas mix. Figure 1.3 shows the calculated small
signal gain for various transitions in CO .
2
The gas discharge of CO lasers is typically a Townsend discharge,
2
which is a gas ionization process in which an initially very small
amount of free electrons, accelerated by a sufficiently strong electric
field, gives rise to electrical conduction through a gas by avalanche
multiplication. When the number of free charges drops or the electric
field weakens, the phenomenon ceases.
Rf discharges can be subdivided into inductive and capacitive dis-
charges. For most lasers, only the capacitive discharges are relevant.
The two important forms of the capacitively coupled RF discharge are
named α- and γ-discharge, according to the Townsend coefficients
α and γ, which describe where the electrons are generated. The main
4
difference between the two is the impedance of the sheaths, the power

20
0.5
10R 20 10P
9R 9P
20
0.4
20
10 10
g 0 (m −1 ) 0.3 10 10 J = 40 40

0.2 40 40
50
0.1
60
0.0
9.2 9.4 9.6 9.8 10.0 10.2 10.4 10.6
λ (µm)
Figure 1.3 Calculated small-signal gain for the regular bands of the CO 2
–3
laser at T = 520 K; RF density = 5 Wcm ; He = 73%; and N 2 : CO 2 = 2.75. 19