Page 72 - High Power Laser Handbook

44 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 45

3. The supersonic gas flow’s low density makes it relatively
easy to minimize refractive index gradients and achieve
acceptable beam quality characteristics.
The following sections provide a brief background to chemical lasers,
followed by detailed discussions of the two most common chemical
lasers—HF or DF and COIL. The chapter concludes with a short discus-
sion of other chemical lasers that have been demonstrated to date.

3.2 General Background
1
In 1961, Penner and Polanyi first suggested that chemically produced
inversion could be used to create infrared lasing based on studies of
low-pressure H + Cl flames (low-pressure atomic hydrogen [H] and
2
molecular chlorine [Cl2] flames). The first demonstration of such a
chemical laser, which was conducted by Kasper and Pimental at the
University of California at Berkeley in 1965, consisted of a photolyti-
2
cally initiated hydrogen chlorine explosion.
Either exothermal chemical reactions directly produce the excited las-
ing species or reaction-produced excited species transfer energy to
another laser species. In practice, however, very few such examples have
proved to be scalable to high power. The set of such lasers could be sig-
nificantly increased if one were willing to expand the definition of chem-
ical lasers to include systems that rely on electrically or photolytically
produced species to initiate the reaction chemistry or that actually supply
a major reactant. An additional expansion would be possible if one were
to include gas dynamic lasers (GDLs). GDLs use chemical combustion to
produce hot gas mixtures in thermal equilibrium; they then expand those
mixtures to supersonic conditions to exploit differences in molecular

Type Example
Three-atom exchange F + H → HF* + H
2
O + CS → CO* + S
Abstraction F + CH → HF* + CH
4 3
Photodissociation CF I + hν → CF + I( P 1/2 )
2
3
3
Elimination
Radical combination CH + CF → HF* + CH CF
3 3 2 2
Insertion O(D) + CH F → HF* + OCH n–1 3–n
F
n 4–n
Addition NF + H CCH → HF* + CH C--N
2 2 3
Photoelimination H C – CHCl + hν → HCl* + HCCH
2
Table 3.1 Chemical Laser Classifications by Pimental
4