Page 76 - High Power Laser Handbook

Chemical Lasers 47

factor of 10. The reactions in the combustor and in the lasing cavity
correlate with the reactions in Fig. 3.1.
An optical resonator then produces laser output based on the
resultant gain. Figure 3.2 shows a typical high-power resonator sys-
tem. The laser beam sizes are indicated in cross section at various
locations. A key aspect of high-power chemical lasers is the use of an
aerowindow to isolate the laser cavity (~10 torr) from the atmosphere.
This isolation is typically achieved by creating a focus where the
beam exits the cavity, as shown in Fig. 3.2.
The following discussion begins with a review of energy-level
structure and small signal gain equations of HF and DF lasers; it then
turns to population inversion generation and associated general fluid
mechanic considerations. Finally, performance features associated
with HF and DF laser operation are discussed.

3.3.1 Energy Levels
The lasing species in hydrogen fluoride and deuterium fluoride sys-
tems are the diatomic molecules HF and DF, respectively. The mole-
cules are in their electronic ground state, and the energy levels of
interest are the vibrational and rotational levels of the molecules.
General diatomic molecule behavior is illustrated in Fig. 3.3. The pos-
sible motions of diatomic molecules can be divided into three pri-
mary components:
1. Translational motion of the center of mass: This motion, which
can be treated as classical, is generally well characterized by
a local static temperature. It can be ignored when considering
laser transition energies, except for small secondary effects,
such as Doppler broadening.
2. Rotational motion about the various axes: Quantization is
required and is based on a rotational energy term of the type
2
(1/2)Iω , where I is the moment of inertia and ω is the angular
frequency. Because the moment of inertia is small for the axis
passing through both atoms, it can be ignored. Rotation about
the other two axes produces significant contributions.
Neglecting higher-order terms, the associated quantum lev-
els are B × J × ( J + 1), where B is the molecule’s rotational
J
J
constant and J is the rotational quantum number. In the
absence of lasing, the molecules are nearly in thermal equilib-
rium at the translational gas temperature statistical mechan-
ics which implies that the fraction of the population in the Jth
level F is given by Eqs. (3.3a) and (3.3b):
J
× J × ( J +1)/(kT)]
(2J + 1) e [–B J /Z (3.3a)
× i × (i +1)/(kT)]
where Z = Σ(2i + 1) e [–B J (3.3b)
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