Rectangular-bore tube improves population inversion obtained from upper and terminal laser states, resulting in a significant increase in unsaturated gain factor. Radial field produces efficient pumping of upper laser state. Narrow tube dimensions cause increased diffusion flow of neon is metastable states to tube walls

Mollo, R. A.

English

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June 1969	 Brief 69-10193 
NASA TECH BRIEF 
NASA Teèh Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Rectangular-Bore, High-Gain Laser Plasma Tube 
The small signal unsaturated gain (gain factor) of 
the 3s2-2p4 neon transition (6328 angstroms) in a 
conventional circular-bore laser discharge tube is 
limited by the degree of population inversion that 
can be obtained between the upper laser state (3S2) 
and the terminal laser state (2p 4 ). An experimental 
tube of rectangular bore, which has been constructed, 
provides a method for improving the population in-
version, resulting in a significant increase in the un-
saturated gain factor. 
It had been reported previously that for circular-
bore tubes the gain factor for the 3s2-2p4 neon transi-
tion increases in direct proportion to a reduction in 
the bore diameter. The large reduction in the volume 
of the active gas which results from a straight reduc-
tion in bore diameter makes this solution unattractive 
for laser amplifier applications. Also, for laser oscil-
lator applications, a reduced bore diameter results 
in a proportionate increase in the diffraction losses, 
if the length of the discharge tube is not altered. Thus 
this solution for circular-bore plasma tubes is of limited 
practical value. 
The electron distribution and subsequently the 
density of 3s2 neon states is influenced by the plasma 
tube geometry. As a result, a rectangular-bore plasma 
tube with a large height-to-width ratio exhibits a 
radial and an axial electric field distribution which 
is quite different from that of an equivalent cross-
sectional area circular-bore tube. By appropriately 
shaping the radial field, more efficient pumping of 
the upper laser state (3s2) can be obtained. Also, the 
narrow dimension of a rectangular-bore tube should 
result in an increased diffusion flow of neon Is meta-
stable states to the tube walls because of an increase 
in the concentration gradient. For this transition, a
buildup of metastable Is neon states produces a 
bottleneck in the decay chain for the lower laser 
state 2 4. The partial removal of this bottleneck 
through radial diffusion flow processes results in a 
larger population inversion and, thus, an increased 
gain factor. 
As a part of a study of the discharge current (pump-
ing power) induced saturation of the 6328 angstrom 
transition, a 100-cm-long rectangular-bore glass tube 
with a 2.5 by 10.0 mm cross-sectional area was con-
structed. The tube was terminated with the usual 
quartz Brewster angle windows. An experimental 
investigation of the gain factor associated with this 
tube confirmed the predicted increase in the gain 
factor. Under optimum operating conditions (5—I 
He—Ne, 1.07 torr at an excitation current of about 
50 mA), a measured gain factor of 1.20 dB/rn (about 
32%/rn) was recorded. This gain factor measurement 
represents the highest value obtained to date for this 
useful transition. 
The favorable gain factor attained with this am-
plifier tube was quite evident when it was converted 
into a laser oscillator. The incremental gain was suf-
ficient to allow oscillation to occur within a relatively 
high-loss optical cavity formed by a 2- and 3-meter 
radius-of-curvature mirror. It lased quite easily and 
remained on even though coherent radiation was 
noticeably being scattered from the side walls of the 
discharge tube at the junction of the rectangular-bore 
tubing with the short, terminal, circular-bore tubing. 
The rather complex mode pattern of this laser is 
approximately rectangular (with rounded corners). 
Under the optimum gas conditions, this laser ex-
hibited an output power of about 70 milliwatts. This 
figure is by no means the best output power that could 
(continued ovecleufl 
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I 
be obtained. Because of the limited number of optical 
elements available at the time of this study, no serious 
attempt could be made to obtain the maximum output 
power. The rectangular mode pattern of this laser 
could be quite useful as a coherent, high-power illumi-
nator for optical experiments involving monochroma-
tors and/or other optical instruments that utilize an 
entrance slit. The high gain factor shown by this 
plasma tube, coupled with a cross-sectional geometry 
that is quite favorable to signal-beam folding, re-
moves one of the most serious previous objections 
to the construction of practical traveling-wave laser 
amplifiers for this transition.
Note: 
Documentation is available from: 
Clearinghouse for Federal Scientific 
and Technical Information 
Springfield, Virginia 22151 
Price $3.00 
Reference: TSP69 -10193 
Patent status: 
No patent action is contemplated by NASA. 
Source: R. A. Mollo 
of The Pennsylvania State University 
under contract to 
Headquarters, NASA 
(HQN- 10234)
. 
Brief 69-10193	 Category 02


Rectangular-bore, high-gain laser plasma tube

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