A solar pumped laser is described in which the lasant is a gas that will photodissociate and lase when subjected to sunrays. Sunrays are collected and directed onto the gas lasant to cause it to lase. Applications to laser propulsion and laser power transmission are discussed

Hohl, F.

Lee, J. H.

Weaver, W. R.

English

NASA Technical Reports Server

United States Patent [191 11 11 4,424,592 
Lee et al. WI Jan. 3,1984 
I 
[54] SOLAR PUMPED LASER 1561 References Cited 
[75] Inventors: Ja H. Lee; Frank Hohl; Willard R. 
[73] Assignee: The United States of America as Primary Examiner-James W. Davie 
U.S. PATENT DOCUMENTS 
Weaver, all of Newport News, Va. 4,264,876 4/1981 Jung et al. ............................. 372/79 
Attorney, Agent, or Firm-Howard J. Osborn; John R. 
Manning; William H. King 
1571 ABSTRACT 
A solar pumped laser in which the lasant is a gas that 
will photodissociate and lase when subjected to sunrays. 
Sunrays are collected and directed onto the gas lasant to 
cause it to lase. 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
[2l] Appl. No.: 327,658 
[22] Filed: Dec. 4, 1981 
[51] Int. Cl.3 ......................... HOlS 3/091; HOlS 3/22 
[52] U.S. C1. ......................................... 372/79; 372/55 
[58] Field of Search .............................. 372/79, 55, 70 2 Claims, 8 Drawing Figures 
14 
f 
LASER BEAM 
RECIRCULATION, 
REGENERATION, AND 
HEAT REJECTION 
https://ntrs.nasa.gov/search.jsp?R=19840008474 2020-03-22T08:12:43+00:00Z
ue Jan. 3, 1984 Sheet 1 of 7 9 424,592 
U S .  Patent Jan. 3, 1984 Sheet 2 of 7 
c - z  
v) 
Q 
-I 
a 
Sheet 3 of 7 9 424,592 
- I C -  
aknt Jan. 3, 1984 Sheet 4 of 7 9 5 
I I I 
d N O  
++OM 
O 
Sheet 5 of 7 9 424,592 
U.S. Patent Jan. 3, 1984 Sheet 6 of 7 
aten& Jan. 3, 1984 Sheet 7 of 7 
E e 
8. 
f c 
(3 z 
w 
J w s 
3 
9 424,592 
1 
4,424,592 
I 
SOLAR PUMPED LASER 
ORIGIN O F  THE INVENTION 
The invention described herein was made in the per- 
formance of wcrk 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 O F  THE INVENTION 
The invention relates generally to collecting and 
converting solar energy to high power laser energy and 
more specifically concerns direct solar-pumped gas 
lasers. 
The concept of collecting and converting solar en- 
ergy to high power laser energy has stimulated studies 
of various laser systems and their applications. Impor- 
tant applications considered include laser urouulsion of 
5 
10 
15’ 
. .  
space: br air-borne vehicles and laser power transmis- 
sion to remote spacecraft from a space platform. Also 
laser power transmission is an alternative to the micro- 
wave power transmission from space to Earth. The use 
of lasers in space for power transmission has a signifi- 
cant advantage over microwaves because of small beam 
divergence, resulting from wavelengths that are smaller 
by approximately 104. However, space deployment 
places special requirements on the laser system. Impor- 
tant are: (1) a gas or liquid phase lasant to effect continu- 
ous cooling and recharging for high power (more than 
1 MW) operation; (2) high temperature operation to 
reduce cooling requirements; (3) chemical reversibility 
to renew the lasant in space; and (4) broadband pump- 
ing to efficiently use the solar spectrum. Although 
power transmission by conventional lasers is possible 
with electric power generated in space, a direct solar 
pumped laser offers the further advantage of weight 
reduction by elimination of the electrical power genera- 
tion. 
In the past a solid-state, Nd3+:YAG laser was solar 
pumped. The power output of a solid-statelaser is lim- 
ited by an inability to cool the laser rod; hence, scalabil- 
ity to high powers is precluded. The Nd3+:YAG 
achieved a maximum peak output power of about 5 
watts. Input powers increase in proportion to output 
powers. Eventually the heat transfer characteristics of a 
solid laser rod limit the input power or melting of the 
rod will occur. 
Also, a gas dynamic solar-powered laser has been 
suggested. This type of laser is only sun-powered, not 
directly solar-pumped because it uses the sun only to 
thermally heat the gas lasant and not to directly make 
the gas lase. Extra heavy equipment is required to heat 
and recirculate the gas because in effect the system is a 
subsonic wind tunnel. The gas reaches high tempera- 
tures and velocities which means the gas has to be 
cooled and slowed before it is recirculated. 
“Solar pumping” as used in this specification means 
that the sun directly and immediately makes the gas 
lase. The sunlight photodissociates the lasant to form an 
excited atom in a particular energy state that upon de- 
excitation does so with the emission of coherent radia- 
tion, or lases. The chemistry and physics instantly take 
place in a stationary gas. The sun is not used for heat, 
flowing of the gas is not required for lasing, no super- 
sonic nozzle expansion is required, and no shocking 
down to subsonic flow is required. 
20 
25 
30 
35 
40 
45 
5 0  
55 
6 0  
65 
It is an object of this invention to provide a direct 
solar-pumped gas laser. 
Another object of this invention is to provide direct 
conversion of broadband, incoherent solar radiation 
into coherent laser radiation using a gaseous lasing me- 
dium. 
A further object of this invention is to provide a solar 
pumped laser which has high temperature operation 
thereby reducing cooling requirements. 
Still another object of this invention is to provide a 
solar pumped laser in which a gas lasant is used to effect 
continuous cooling and recharging for high power, 
a still further object of this invention is to provide a 
solar pumped laser which uses a gas lasant that is chemi- 
cally reversible to renew the lasant in space. 
Other objects and advantages of this invention will 
become apparent hereinafter in the specification and 
drawings. 
SUMMARY O F  THE INVENTION 
This invention collects sunlight and directs it onto a 
laser cavity filled with a gas lasant. The gas lasant by 
means of sunlight is made to lase by photodissociation 
of the gas. For example, the gases c3F-11 and (CF3)2 As1 
can be made to lase by the photodissociation of I from 
the gases by sunlight. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a schematic drawing of the embodiment 
selected for illustration in the drawings; 
FIG. 2 is a schematic drawing of the laser cavity 
shown in FIG. 1; 
FIG. 3 is a schematic drawing of an experimental 
device used to prove the feasibility of this invention; 
FIG. 4 is curves showing a comparison of the solar 
simulator output (upper), the laser signal (center), and 
the results of a numerical simulation (lower); 
FIG. 5 is curves showing continuous lasing of c3F-11 
for over 10 ms for single static filling; 
FIG. 6 is curves showing a 30 Hz pulsed c3F-11 laser 
output with the lasant flowing as in a blowdown wind 
tunel; 
FIG. 7 is curves showing laser output as a function of 
c3F-11 pressure; and 
FIG. 8 is absorption curves of perfluoropropylio- 
dides and air mass zero solar spectrum. 
DETAILED DESCRIPTION OF THE 
INVENTION 
Turning now to the embodiment of the invention 
selected for illustration in the drawings the number 11 
in FIG. 1 designates a laser cavity which is shown in 
more detail in FIG. 2. A collector 12 collects sunlight 
13 and directs it onto laser cavity 11 producing a laser 
beam 14. The lasant in laser cavity 11 is a gas in which 
one of the elements of the gas can be photodissociated 
chemically from the gas by means of the sunlight 13 and 
thereby causes the gas lasant to lase. Examples of gas 
lasants that can be used are c3F-11 and (CF3)2AsI. These 
gases lase when the I is photodissociated from the gas 
by sunlight. Even though it is not essential that the gas 
lasant is flowing to produce lasing, a continuous system 
will require some minimum lasant flow through the 
laser cavity by a recirculation, regeneration and heat 
rejection system means 15. The heat rejection means is 
for rejecting waste heat and the regeneration means is 
for rejuvinating the gas lasant. Recirculation, regenera- 
4424,592 
3 
tion aqd heat rejection means are well known and there- 
fore not disclosed in detail in this specification. 
Laser cavity 11 as shown in FIG. 2 includes a first 
mirror 16 that has approximately 100% reflectivity and 
a second mirror 17, the laser output mirror, which has 
approximately 2% transmission, that is the output of the 
laser. A first large transparent tube 18 with Brewster 
angle windows at each of its ends contains the gas las- 
ant. The gas lasant is pumped into tube 18 through a 
second tube 19 and out of tube 18 through a third tube 
20. Openings 21 and 22 throughout the lengths of tubes 
18, 19 and 20 and between tube 18 and tubes 19 and 20 
allow the gas lasant to flow into and out of tube 18 
throughout the length of tube 18. Consequently, the gas 
flows across tube 18 and for a very short distance in 
tube 18. Alternately, the slits or openings 21 and 22 
could be holes spaced along the tubes. 
In operation sunlight 13 is collected by collector 12 
and directed onto tube 18. As a result the gas in tube 18 
lases thereby producing the laser beam 14. Means 15 
pumps the gas through tube 18, regenerates the gas 
(when C3F7I is used the I is photodissociated from the 
gas forming 12 which in sufficient amounts diminishes 
lasing) and cools the gas (most heat from the sun passes 
through the cavity for return to space, some absorbed 
by the gas and cavity walls has to be dissipated). 
In an experiment, FIG. 3, to prove the feasibility of 
the present invention, solar energy is simulated by the 
discharge from a continuous 40-kW xenon arc 23. The 
discharge from the xenon arc is collected by a reflector 
24 to a point at a variable-speed optical chopper 25. The 
purpose of chopper 25 is to obtain fast rise time pulsed 
pumping to minimize the quenching effect of molecular 
iodine. a breakdown component of the lasant. The 
pulsed light beam from chopper 25 is collected by a 
highly polished aluminum cone 26 to form a focal line 
along the cone centerline. The laser tube is an ultravio- 
let grade quartz tube 27 which has a 7 mm inside diame- 
ter, a 1 mm wall thickness, and is 30 cm long. Tube 27 
is located on the focal line of cone 26. Cone 26 has a 12.5 
degree half-angle, an entrance aperture of 9.6 cm, and is 
18 cm long. The laser cavity consists of tube 27 includ- 
ing Brewster angle windows 28 and 29, a dielectric rear 
mirror 30 with maximum reflectance (99.97 percent at 
1.31 pm) and a front mirror 31 with a reflectance of 98 
percent. A germanium detector 32 operated at room 
temperature senses the laser output. 
The 4-kW outDut of xenon arc 23 is transformed in 
5 
10 
15 
20 
25 
3 0  
35 
40 
45 
the experiment h t o  a line focus with an intensity of 
10,OOO solar constants (half width) for 5 cm of length. 50 
The lasant n-C3F7I, a perfluoropropyliodide, absorbs 
the radiation between 250 and 290 nm. Photolysis of the 
lasant produces the upper level I(52P1/2) of the laser 
transition of C3F7I +h~-+C3F7+1(5~P1/2), and the 
lower level is the ground state of the iodine atom 55 
I(s2P3/2). Laser output is at 1.315 pm for as long as 10 
ms for a single static fill at 10 Torr, and peak power 
outputs up to five watts were obtained at pressures near 
20 Torr. 
gether with the output signal from the FIG. 3 simulator 
(upper trace) and a result of the numerical simulation 
(lower trace). The lasing lasts for 1 ms with the peak 
power of approximately three watts. The output power 
was determined with the detector’s absolute calibration 65 
data and the spot size from the beam profile measure- 
ment. The threshold of lasing is reached when the solar 
simulator output increases to about 50% of maximum 
A typical laser signal output is shown in FIG. 4 to- 60 
4 
intensity. The initial spikes, caused by gain dumping 
near threshold, and subsequent continuous output (ap- 
proximately two watts) of the experimental results 
agree well with those of the numerical simulation. The 
highest laser peak power output recorded on other tests 
exceeded four watts, which is comparable to the maxi- 
mum peak power output (five watts) of the well devel- 
oped solid-state Nd3+:YAG solar laser. For the highest 
laser output of the present research, the ratio of power 
out to power in is 0.1 %, which is already one-half of the 
maximum intrinsic, or thermodynamic efficiency and 
the extraction efficiency, or the ratio of energy depos- 
ited to energy extracted from the gas is 20 percent. 
Continuous lasing was obtained for over the entire 
pump period, 10 ms, at a reduced fill pressure of about 
10 Torr (FIG. 5). When the lasant was flowed through 
the laser tube as in a blowdown wind tunnel, a 3 0 - H ~  
pulsed output was observed in excess of 150 ms (See 
FIG. 6). The decrease of the laser output with time in 
FIG. 6 is mainly due to insufficient flow speed. 
FIG. 7 shows the variation of the laser output as a 
function of lasant pressure. The pressure range from 2 
to 2.7 kPa (15-20 Torr) produced the maximum laser 
output, a result which also agrees well with the numeri- 
cal simulation result of 2 kPa. 
The experimental results presented here are signifi- 
cant because the lasant is in gaseous form which can be 
circulated for external cooling and reprocessing, the 
iodine laser is a high power laser which has already 
been developed to terawatt levels as a laser-fusion 
driver indicating virtually unlimited scalability, and the 
lasing wavelength of 1.315 pm allows the use of well 
developed optical materials for high power infrared 
lasers. Furthermore, the iodine laser can be operated in 
a high temperature (up to 700 K.) environment which 
significantly reduces cooling requirements in space. 
Two factors limiting the laser output of the FIG. 3 
experiment are power input and mismatch between the 
lasant absorption band and the peak of the solar spec- 
trum. The first factor is not a limitation in space where 
a large solar collector could be placed. Indeed, the 
beam power used in the experiment could be collected 
with only a 2-m diameter collector. The second factor 
can be improved with an alternate iodide such as 
(CF3)zAsI which absorbs seven times the amount of the 
solar spectrum as the C3F7I (see FIG. 8). In addition, 
the reduced output in successive pulses (FIG. 6) of the 
present laser can be overcome by a closed cycle flow 
system that removes the accumulated I2 and replenishes 
the iodide consumed, the two causes of quenching. 
The advantages of this invention are: it provides di- 
rect conversion of broadband, incoherent solar radia- 
tion into coherent laser radiation using a gaseous lasing 
medium, it provides high temperature operation 
thereby reducing cooling requirements, it uses a gas 
lasant that is chemically reversible to renew the lasant 
in space and since a gas lasant is used the lasant can be 
cooled and recharged for high power operation. 
Other gas phase lasants such as iodine bromide or 
nitrosyl chloride can be used as the gas lasant since 
these lasants are also excited by photodissociation re- 
sulting from absorption of wavelengths within the solar 
spectrum. 
\ 
What is claimed is: 
1. A method of direct solar pumping a laser compris- 
providing in a laser cavity a gas lasant (CF$2AsI 
which will lase when subjected to sunlight by pho- 
ing the steps of: 
5 
4,424,592 
6 
todissociation of the element I from the gas lasant; 
and 
collecting sunlight and directing the collected sun- 
light onto the laser cavity to cause lasing of the gas 
lasant by photodissociation. 
a gas lasant (CF3)zAsI which can be photodis- 
sociated by sunlight to produce lasing; 
for recirculating the gas lasant through the 
laser cavity; and 
means for collecting sunlight and directing it onto the 
walls of the chamber to cause the gas lasant to lase. 
2. A direct solar pumped laser system comprising: 
a laser cavity including two mirrors and a chamber 
having walls transparent to sunlight and filled with * * * * *  
10 
15 
20 
25 
30 
-, 
35 
45 
50 
55 
60 
65 


Solar pumped laser

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server