Large volumes of gas are excited by using internal high reflectance mirrors that are arranged so that the optical path crosses back and forth through the excited gaseous medium. By adjusting the external dielectric mirrors of the laser, the number of paths through the laser cavity can be varied. Output powers were obtained that are substantially higher than the output powers of previous nuclear laser systems

Deyoung, R. J.

Hohl, F.

English

NASA Technical Reports Server

United States Patent [191 11 11 4,300,106 
Hohl et al, BW3t .hmfBaESle copy [45] Nov. lo, 1981 
[54] LARGE VOLUME MULTIPLE-PATH 
NUCLEAR PUMPED LASER 
[75] Inventors: Robert A. Frosch, Administrator of 
the National Aeronautics and Space 
Administration, with respect to an 
invention of Frank Hohl, Newport 
News; Russell J. De Young, 
Hampton, both of Va. 
[73] Assignee: The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
[21] Appl. No.: 41,141 
[22] Filed: May 21, 1979 
[51] Int. Cl.3 ................................................ HOlS 3/05 
[52] U.S. C1. .......................... 33V94.5 C; 33V94.5 P; 
33V94.5 D 
[58] Field of Search ...................... 331/94.5 C, 94.5 P, 
33V94.5 PE, 94.5 G 
1561 References Cited 
U.S. PATENT DOCUMENTS 
3,660,778 5/1972 LeBlanc, Sr 
4,001,705 1/1977 Sinclair et a 
4,110,703 8/1978 Jalufka et a1 
Primary Examiner-William L. Sikes 
Assistant Examiner-Lion Scott, Jr. 
Attorney, Agent, or Firm-Howard J. Osborn; John R. 
Manning; William H. King 
VI ABSTRACT 
A large volume multiple-path nuclear pumper laser 
which has improved characteristics over previous cy- 
lindrical nuclear pumped laser systems. Large volumes 
of gas are excited by using internal high reflectance 
mirrors that are arranged so that the optical path crosses 
back and forth through the excited gaseous medium. By 
adjusting the external dielectric mirrors of the laser, the 
number of paths through the laser cavity can be varied. 
Output powers have been obtained that are substantially 
higher than the output powers of previous nuclear laser 
systems. 
4 Claims, 1 Drawing Figure 
2 
https://ntrs.nasa.gov/search.jsp?R=19820005542 2020-03-21T15:56:53+00:00Z
U.S. Patent Nov. 10, 1981 4,300,106 
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23 4F 
\ 
I I  
VACUUM AND 
LARGE VOLUME MULTIPLE-PATH NUCLEAR 
PUMPED LASER 
ORIGIN OF THE INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA Contract and is sub- 
ject to the provisions of Section 305 of the National 
Aeronautics and Space Act of 1958, Public Law 85-568 
(72 Stat. 435; 42 USC 2457). 
BACKGROUND OF T H E  INVENTION 
The invention relates generally to lasers and more 
specifically concerns improvements in nuclear pumped 
lasers. 
Gaseous lasers pumped directly by charged particles 
from nuclear reactions, called nuclear pumped lasers, 
have been in existence for several years. Typical reac- 
tions used to  pump the laser medium are the 
3He(n,p)3H, "JB(n,a)7Li, and 235U(n,ff)FF nuclear reac- 
tions. In particular, the 3He(n,p)3H reaction has shown 
great promise for higher power nuclear lasers in such 
systems as the 3He-Ar (1.79 pm). Previous nuclear 
pumped lasers consist of a quartz tube surrounded with 
polyethylene moderator and filled with 3He plus some 
minority species gas such as Ar, Xe or Kr. Dielectric 
mirrors at the ends of the tube form the optical laser 
cavity. When this system is exposed to a fast-neutron 
burst, lasing occurs at various wavelengths in Ar, Xe, or 
Kr. For  such cylindrical systems the lasing mode vol- 
ume is relatively small, and thus power output is also 
low. 
The primary object of this invention is to effectively 
increase the lasing mode volume of nuclear pumped 
lasers and to improve the neutron coupling efficiency 
with the external neutron source thereby increasing the 
power output. 
Another object of this invention is to provide means 
for varying the power output of a nuclear pumped laser 
by varying the length of the optical path through the 
excited lasing medium in the nuclear pumped laser. 
Other objects and advantages of this invention will 
become apparent hereinafter and in the drawing. 
SUMMARY OF THE INVENTION 
This invention is a nuclear pumped laser including a 
laser cavity containing the laser medium and two di- 
electric mirrors external to the laser medium. Two 
other high reflectance mirrors are located inside the 
laser medium such that the optical path through the 
laser medium between the two external dielectric mir- 
rors crosses back and forth between the two high reflec- 
tance mirrors. Consequently, the length of the optical 
path through the laser medium is increased which re- 
sults in an increase in power output. The orientation of 
the two external dielectric mirrors is adjustable so that 
the length of the optical path (number of paths) through 
the laser medium can be changed. 
BRIEF DESCRIPTION OF THE DRAWING 
The sole FIGURE in this application is a schematic 
diagram of the embodiment of the invention selected for 
illustration in the drawing. 
DETAILED DESCRIPTION OF T H E  
INVENTION 
Turning now to the embodiment of the invention 
selected for illustration, thz number 11 in t k  Figure 
5 
10 
15 
20 
25 
9 
4.300,106 
L 
designates the frame for the laser medium. The frame 
can be made from stainless steel. A Brewster angle 
window 12 is mounted in the upper right corner of 
frame 11 over an opening 13 through the frame. A 
Brewster angle window 14 is mounted in the lower left 
corner of frame 11 over an opening through the frame. 
Aluminum cover plates 15 are bolted to the front and 
back of frame 11 and are vacuum sealed with O-rings. 
Only the back cover plate 15 is shown in the drawing; 
the front cover plate is not shown to make it more 
convenient to describe the invention. Polyethylene 
moderators 16 are attached to the outside of the alumi- 
num cover plates 15, both in the front and back. A 
vacuum and gas handling system 17 is connected to the 
laser cavity via a vacuum line 18. The gas handling 
system 17 is used to introduce gas into the laser cavity 
and to maintain the proper pressure of the gas. The 
vacuum system is used to evacuate the gas when de- 
sired. The gas can be 3He-Ar. 
The polyethylene moderators 16 produce near ther- 
mal neutrons inside the laser cavity from the fast neu- 
trons generated by a fast-burst rector 19 used as the 
neutron source for this embodiment of the invention. A 
gold or aluminum plane mirror 20 is mounted on the 
inside of frame 11 below opening 13 and a gold or alu- 
minum plane mirror 21 is mounted on the inside of 
frame 11 above the opening in the frame. Dielectric 
coated mirrors 22 and 23 are located outside frame 11 in 
close Droximitv to Brewster angle windows 12 and 14, 
. 
30 respectively, thus forming an o&cal cavity. Mirror 23 
has a finite transmission which provides the output for 
the laser. 
The optical path 24 of the laser is between mirrors 22 
35 and 23, and folded back and forth by internal mirrors 20 
and 21 through the laser medium. The number of times 
that the optical path crosses the laser medium (nine in 
the embodiment shown) and thus the length of the opti- 
cal path through the laser medium is determined by the 
1o orientations of mirrors 22 and 23. The orientations of 
mirrors 22 and 23 can be changed by any suitable means 
25 and 26, respectively, by tilting or rotating the mirrors 
to the right or  left about the central axes of the mirrors. 
To properly align mirrors 22 and 23, the laser beam 
45 from a He-Ne alignment laser is passed through mirror 
22 and mirror 23. The reflected He-Ne beam is caused 
to reflect back into the He-Ne laser thus describing a 
unique optical path. The positions of mirrors 22 and 23 
can be adjusted thereby varying the number of optical 
The multiple path laser as disclosed in the FIGURE 
is designed to withstand pressures of several atmo- 
spheres of 3He mixed with some minority species gas 
such as Ar  or Xe. Fast neutrons produced by fast-burst 
55 reactor 19 are moderated by the polyethylene modera- 
tor 16 attached to the front and back of the laser. These 
thermalized neutrons enter the "e plus minority spe- 
cies gaseous medium and cause 3He(n,p)3H nuclear 
reactions to occur. These reactions produce charged 
60 particles of high kinetic energy which ionize and excite 
the He-minority species mixture creating a population 
inversion in the minority species gas. The photon flux is 
allowed to amplify in the optical cavity which is formed 
by the dielectric coated external laser mirrors 22 and 23. 
65 The output mirror 23 has a finite transmission which 
allows a portion of the photon flux (laser output) to 
escape from the optical cavity in the form of a laser 
beam. 
50 paths traversing the laser medium. 
4,300,106 
3 4 
One set of dimensions of a laser constructed in accor- 
dance with this invention is: 5 cm thick polyethylene 
moderator 16, 3 cm thick frame 11 with the internal 
dimensions of the frame being 40 cm between mirrors, 
30 cm high and 3 cm wide. 
The invention as disclosed is in the form of a rectan- 
gular structure. Alternate forms of the invention would 
include a cylindrical geometry where the laser cavity is 
made to reflect the laser emission around a cylindrical 
fast-burst reactor. The aluminum cover plates could 
also be coated with either 235U or ‘OB or some other 
solid fissionable material, thus providing a wall source 
of excitation. The use of 235UFg will allow operation as 
a self-critical volume pumped gaseous reactor-laser. 15 
The external laser cavity mirrors could also be mounted 
internally, thereby reducing optical loss from the Brew- 
ster angle windows. 
By replacing the aluminum cover plates with a trans- 
parent window material, the invention could be used to 20 
pump a laser medium such as iodine by a photo flux. A 
photon flux, generated by a Xe flashlamp or  carbon arc 
would pass through the window, photolyzing the gase- 
ous medium. Lasing would occur transverse to the 
photon flux, just as in the nuclear excitation system. 25 
With this system, a highly absorbing laser gas can be 
pumped homogeneously. 
The advantages of this invention are numerous: It has 
a higher laser output power with lower 
input of thermal neutron flux when compared to previ- 
ous cylindrical laser systems; it has a more efficient use 
of the reactor neutron flux by increasing the neutron 
capture area; it has a laser cavity path length that is 
where the optical path length is fixed; the reactor neu- 
tron fluxes needed for lasing are approaching steady- 
State  actor fluxes; l3’U wall coatings as well as 
235UF6 and 3He volumetric sources of excitation can be 
used; its physical system can be scalable to larger vel- 40 
umeS and higher laser output powers; by using transpar- 
ent cover plates, a large volume laser system could be 
homogeneously pumped by photons; and by careful 
?-’5UFg and 3H volumetric sources of excitation a self- 
critical laser reaction can be achieved. 
What is claimed is: 
1. A laser comprising: 
an enclosure with first and second windows and con- 
taining a lasing medium; 
a first mirror with zero transmission located outside 
said enclosure adjacent said first window; 
a second mirror with finite transmission located out- 
side said enclosure adjacaent said second window 
for providing the output for said laser; 
5 
10 
a third flat mirror; 
a fourth flat mirror; 
pumping pumping radiation in to  the lasing 
medium to lasing along an optical path be- 
tween the first and second mirrors; 
said first, second, third and fourth mirrors and said 
first and second windows being located and ori- 
ented such that the optical path is from the first 
mirror through the first window, through the las- 
ing medium to the third mirror, through the lasing 
medium to the fourth mirror, back and forth sev- 
era1 times between the third and fourth mirrors, 
through the second window to the second mirror, 
and then along the Same Path back to the first mir- 
ror whereby the optical path of the laser has a 
distance several times the distance across the laser 
medium; and 
means for changing the orientations of said first and 
second mirrors to alter the number of times the 
optical path goes back and forth between the third 
and fourth mirrors enroute between the first and 
second mirrors thereby changing the length of the 
optical path. 
30 
as Opposed to the cy1indrica1 laser systems 35 2. A laser according to  claim 1 wherein said third and 
fourth mirrors are located inside said enclosure. 
3. A laser according to claim 2 wherein said third and 
fourth mirrors are parallel to each other and are fixed 
relative to the enclosure. 
4. A laser according to claim 1 wherein said pumping 
means is a fast-burst reactor means for generating fast 
neutrons to pump the laser whereby the laser is a nu- 
clear pumped laser. 
design of moderators and use of 235H wall coatings or * * * * *  
45 
50 
60 
65 


Large volume multiple-path nuclear pumped laser

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