Page 92 - High Power Laser Handbook

62 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 63

(1) mixing of large-scale structures, such as jets; and (2) local, diffu-
sional mixing between large-scale structures, whose mixing may be
somewhat augmented by local turbulence and simple diffusion.
Pressure Recovery
CW laser devices typically operate at relatively low pressures,
which means that in mobile systems, one must either use a chemical
or microporous absorbent pump, such as a zeolite, or use an exter-
nal pump, such as a single- or multiple-stage ejector system, to
maintain the required low operating pressures. As shown in Fig. 3.16,
an ejector consists of several sections, including a gas generator;
supersonic mixing nozzles, which inject the gas into the subsonic
laser flow; a constant-area supersonic diffuser region, in which the
mixture is converted from supersonic to subsonic flow via two- or
three-dimensional shock interactions; and a subsonic diffuser
expansion region, which further slows the flow to yield an addi-
tional pressure increase. To gain added pressure recovery, the laser
itself also contains supersonic and subsonic diffusers in series that
work on the same principle. These diffusers, however, rely on the
laser cavity itself to supply the mixed supersonic flow.

3.3.6 Variations on Continuous Wave HF and DF Devices

HF and DF Overtone Lasers
In addition to lasing on fundamental HF transitions (∆v = 1), it is
also possible to lase on overtone (∆v = 2) transitions. However, the
gain is reduced due to substantial reductions in the associated Ein-
stein coefficients. Furthermore, it is necessary to use resonator con-
cepts that accommodate the substantially reduced gain, while
simultaneously suppressing the higher-gain fundamental laser
transitions. Moderate-sized CW devices have been constructed
using approaches somewhat similar to those used for more conven-
tional low-pressure (∆v = 1) devices. Note that the above discussion
has primarily used HF as an example; however, DF device
approaches are very similar to HF ones.

Supersonic nozzles Supersonic diffuser
Laser diffusers
Laser
flow
(subsonic)
Mixing region Subsonic diffuser

Gas generator
Figure 3.16 Schematic drawing of a typical laser pressure recovery system.