Page 82 - High Power Laser Handbook

52 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 Chemical Lasers 53

28
24
20
Amplitude (a.u.) 16 v = 1 --> 0

v = 2 --> 1
12

4 8
0
2.5 2.6 2.7 2.8 2.9 3.0 3.1
Wavelength (µm)
Figure 3.5 Typical lasing spectrum of HF continuous wave (CW) lasers.

20
18
16
Amplitude (a.u.) 12 8 v = 1 --> 0
14
10
v = 2 --> 1
v = 3 --> 2
4 6
2
0
3.5 3.6 3.7 3.8 3.9 4.0 4.1
Wavelength (µm)
Figure 3.6 Typical lasing spectrum of DF CW lasers.

g = upper-level degeneracy factor
U
g = lower-level degeneracy factor
L
λ = laser wavelength
t = upper-state spontaneous lifetime
spont
g(ν) = normalized line shape
Limiting the discussion specifically to HF, we first consider the term
in brackets and the transition (v + 1, J – 1) → (v, J). N can be expressed
U
as the product of the total HF number density N and the fraction of
these molecules in the v + 1 and J – 1 states.
One assumption that is frequently satisfied in the absence of las-
ing, and that is approximately satisfied during lasing, is that the rota-
tional levels are in thermal equilibrium with an absolute temperature T