Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~45~63~1
This invention relates ~o novel llght amplifiers and
resonators of the laser type. More particularly~ this invention
relates to optical amplifiers or lasers using iodine as the lasing
molecule, or a noble gas as ~he lasing substance.
It is known that molecular iodine is potentially use-
ful as a lasing molecule. For example, it has been calculated
by Tellinghuisen (1974) Chem. Phys. Lett. 29~ 359 that population
inversion is possibla in the bands of the iodine molecule in the
region between 3,000 i and 3,300 Angstroms with a gain cross section
of approximately 3 x 10 1 /cm However~ successful iodine lasers
have not been developed.
One serious problem with using the iodine molecule as
a lasing substance is that it has not been possible to put energy
at the appropriate wave lengths into iodine vapour with reasonable
efficiency ~ a sufficient rate to provide a sufficient concen-
tration of excited iodine molecules to give rise to light ampli-
fication. As is well known in the laser art, a "population in-
version", where stimulate~ emission exceeds absorbtion, is
neces~ary to permlt lasingO The efficient obtaining of such a~ ;
population inversion has not been possible wi~h iodine, by
; previously known methods.
It has now been found that iodine can efficiently be
given a population inversion by exciting it by radiation given
off by an excited xenon dimer in a band between 1~650 Angstroms
and 1~790 Angstroms, and centered at 1,720 Angstroms.
The invention will be further described with referencè
to the drawings~ in which;
Figure 1 is a cutaway drawing of one embodiment of the
invention.
Figure 2 is a section through Pigure 1 on the line 2-2~ `
Figure 3 is a cutaway drawing of a second embodiment
of the invention.
Figure 4 is a section through Figure 3 on the line 4-4.
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639
According to the lnvention, Xenon gas at high pressure
is excited by an electron beam sustained discharge to create a
Xenon dimer radiatin~ at 1~650 to 1,790 Angstroms. The amount of
irradiation requi~ed depends on the gain
desired in the iodine. In order to obtain a gain of in the order
of .01 cm 1 in the iodine vapour at a distance of the order of
one centimeter from a window~ an isotropic irradiation of xenon
dimers of the order of at least 3 x 105 watts/cm2 is required.
This figure scales directly with the gain desired. The means that
for a gain of .1 cm 1 at a depth of 1 centimeter the radiation
flux from the excited xenon dimer should be of the order of at
least 3 x 10 watts/cm In order to obtain such an irradiation
density the xenon gas must be present in a number density (number
of molecules per cubic centimeter) of between 10 and 5 x 10
The temperature of the xenon gas is maintained in the range from
about 250K to about 650K. Higher temperatures can also be tol-
erated~ bu~ increased temperatures tend to reduce efficiency. - ~-
The preferred starting temperature is room temperature of about
300Kl. The xenon will heat up as the discharge proceeds but
this is acceptable in most cases without extreme cooling. At
the preferred temperatures, the number densities given above
correspond to a pressure of from 4 to 20 atmospheres. A sustained
discharge of relativistic electrons is directed through the
Xenon gas. The value of the electric field (E) is adjusted so
that the ratio of the electric field to number density (N) of
Xenon molecules (this ratio will hereinafter be called the E/N
value) is between 10 6 and 5 x 10 16. Under these conditions7~ ;
a Xenon dimer which radiates between 1,650 Angstroms and 1,790
Angstroms i5 created. The Xenon dimer can be used alone as a
lasing substance~ A preferred use, however~ is to direct ~the ~ ;
.
radiation into molecular iodine (either alone or mixed with a ~`
buffer gas) where it causes a population inversion in the iodine. ~ -
For repeated operation with several repetitions of dis-
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charge per second, some cooling may be desirable to keep effl-
ciencies high. Generally, it is preferrcd to keep the tempera-
ture as close to 300K as conveniently possible, as stated above.
The n~mber density of electrons created in the discharge
by the electron-beam should be between 10 4 and 1015 per cubic
centimeter. The duration of the discharge pulse should be of
the order of 50 to 200 nanoseconds.
The depth of Iodine to be traversed by the radiation
from the Xenon should be kept as short as possible~ as the acti-
vation drops off rapidly with distance. The maximum acceptable
distance depends upon the gain required, the radiation photon
flux density and the apparatus design as will be understood by
persons skilled in the art. For example, it is preferred that
the total depth of iodine to be traversed be less than 10 centi-
meters for an iodine density of 3 x 10 6, although this distance
can be exceeded for certain designs of apparatus, radiation
photon flux~ and desired results.
The iodine molecules should be present in a number
density of from 1016 to 5 x 1017. The temperature of the iodine
should be maintained by external heating or cooling at a level
high enough so that the desired number density of iodine molecules
is present in vapour form. Such temperature will be obvious,
having regard to the membqr- density desired. However~ tempera-
tures~higher than necessary should be avoided, as efiiciency
drops ~1thhincreased temperature. In the range of pressures
states above the inversion will occur between what is known as
the D to X transition in iodine. In the beginning of the dis-
charge optical gain is obtained in the region 3,000 to 3,300
Angstroms, This gain will diminish during the discharge.
However gain is also available in a band between appr~ximately
3~700 and h~100 Angstr~ms which is thought to correspond from the
D to the og~(3~t) repulsive state according to Tellinghuisen~ Some
other transitions may also be present in this wave length range.
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Particular examples of thc operation of the invention
will now be described~ with reference to the drawings.
Example 1
~ igures 1 and~2 show a suitable apparatus for the cre-
ation of Xenon dimers and their use in exciting iodine vapour.
The apparatus comprises a transparent cylindrical tube 5~ made
from ultra~iolet transmitting silica, and has two transparent
end windows 14. These end windows are arranged at the Brewster
angle with respect to the axis of the tube, to minimize reflection,
as is well known in the laser art. Surrounding the tube 5 is a
concentric tube 3~ which is formed of opaque material permeable
to a beam of relativistic electrons~ Insulating end portions 18
are provided for the concentric tube 3. The portions of tube 5
which extend beyond concentric tube 3 are rendered opaque, as
shown at 11. An electrode 9 is providecl adjacent the interior
wall of tube 3 and another electrode 10 is provided ad~aceht the
exterior wall of tube 5. These electrodes are connected by elec-
trical conductors 16 to aisuitable source of electrical potential~
such as a capacitor 17.
The volume between tubes 3 and 5 (indicated as 2) is
filled with Xenon gas~ in a number density of 10 . The volume
within the tube 5 li~dicated as 6) is filled wi~h a mixtùre of
iodine gas with a total number density of 1017/cm . The tempera-
ture of the apparatus is maintained at a temperature sufficient
to obtaln the given number density by means of heating/cooling
coils (not sho~m).
A voltage is applied across the electrodes 9 and 10,
such that an ~/N of 2 x 10 16 is maintained and a beam of elec~
trons at 5 x 105 electron-volts bombards the concentric tubes
from outside wall 3~ as indicated diagrammatically at 1~ at a
; current density ~ufficient to obtain an electxon density around
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5 x 10 . The electrodes 9 and 10 are separated by a distance
of one centimeter. The voltage of the beam of electrons can
easily be ad~usted or other electrode separations and xenon
pressures by someone versed in the art.
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11)4~39
A xenon dimer is formed which radiates in a band centred
abo~t 1,720 Angstromsr The radiation passes through holes in the
mesh electrode 10 and through the transparent wall 5 into the
iodine mixture, as indicated schematically be lines 12.
The radiation entering volume 6 excites the iodine
molecules, creating a population inversion. With the temperatures
and number densitles present in this example, the D level is the
; predominant excited state. The iodine can be used as an optical
amplifier for light of wave length around 3,000-3,300 Angstroms
and 3~700-~000 Angstroms by passing a weak beam of such light,
in the direction shown as 13~ down the a~is of the tube. An
amplified beam of light exits at 18. Alternately~ if it
is desired to use ~he appratus as a reson~tor~ mirror 15 and
partial mirror 19 are placed outside wlndows 14 as is known in
the art. The resonator frequency can be tuned by known methods,
such as a diffraction grating or prism. ~ ~ ~
Example 2 ~ -
A further embodiment of the invention is illustrated
`i in Figures 3 and 4. Figure 3 shows a chamber generally indicated~ 20 as 27, having an insulated sids wall and end walls 24. Top and ;
. ~ ~
bottom walls 30 are electron permeable, and are provided with
electrodes 35 and 36. The remaining side wall 22 is formed of
a substance transparent to ultraviolet radiation, such as silica
of high transmissiveness for light of about 1,720 Angstroms.
On the other side of wall 22 is a chamber indicated as
28. One wall of this chamber is formed by wall 22, !-The top and
;i bottom walls and opposite side wall (all indicated generally as
37) are opaque. The distance across chamber 28 from wall 22 to
opposing side wall 37 is 1 eentimeter.
The ends of chamber 28 are formed by windows 31, which
are optically transparent and oriented at the Brewster angle to
reduce reflection as known in the art.
In operation, Xenon gas in a number density of 102 is
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963~
placed in chamber 27. The chamber 28 is fiLlcd with a mixture
of iodine gas. The total nurnber density of molecules present -ln
chamber 28 is 10 Icm .
A voltage such that E/N is maintained at 2 x 10 is
placed across electrodes 35 and 36 by means of power source 26
and wiring 25, and a sustained electron discharge at 5 x 10
electron-volts is passed through chamber 27 in the direction in-
dicated by arrow 20. The temperature of the iodine chamber is
maintained by external heating/cooling coils at a temperature
sufficient to maintain the number density oE iodine. Xenon
dimers are formed~ and these radiate in a band centered at 1,720
Angstroms through transparent wall 22~ as shown schematically at
38. The radiation passing into chamber 28 excites the iodine
molecules. Under the pressure conditions arising from the number
density specified above, the iodine radiate preferably at 3,000~
3,300 and 3,700-4,000 Bngstroms, as the predominant excited state
is the D st~te. In the embodiment shown~ iLthe apparatus is designed
as an oscillator, and is provided with external mirror 32 and
external part transparent mirror 39. The radiation emitted by the
excited iodine molecules oscillates as shown by arrow 33 until it
achieves sufficient intensity to escape through mirror 39 as a
coherent beam. The oscillatorOcan be turned as known in the art~
as for example by diffraction gratings or prisms. Alternately, the
same apparatus can be used as an amplifier, by removing mirrors
;~ 32 and 39, and passing a weak beam of light of wave length around
3~200 Angstroms into volume 28 through one of the windows 31~ in
the direction of either head of arrow 33.
The invention also comprises a method of stimulating
production of the xenon dimer by means of light emission ~ ~ -
from krypton in an adjacent chamber, through formation of an
emitting krypton dimer. The same method can be used to stimulate
production of the krypton dimer by forming a radiating Argon
dimer in an adjacent chamber.
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rhe preparatlon of Xenon dlmers by this method (as
opposed to irradlation by hlgh-energy electrons as described
earller) ls preferred as the temperature rise in the Xenon is
considerably lower, and the excitation is considerably more
uniform than i5 possible with electron beam pumping. It has not
been previously recognized that broad band radiation coincident
with substantial]y broadened atomic resonance line will deposit
a substantial fraction oE its energy (i.e., greater than 10%)
at a substantial distance (of the order of 1 to 10 cm) even at
a pressure of the order of 1 atmosphere~ This is due to the large
percentage of emission occuring in the far wings of the absorbtion
profile. Such wings have an absorption coefficient which varies
approximately fro~ 3 x 10 39 to 10 40 in the noble gases at a
distance from the line centre of 2~000 to 4~000 cm on the long
wave length side.
According to this aspect of the invention, a buffer
gas selected from the group consisting of krypton, argon, neon or
.
helium must be added to the Xenon~ in order to permit a decreased
number density o~ xenon atoms to be present~ while providing a
large enough total number density for three body formation of
` Xenon dimers. The amount of buffer gas present should meet
the following criteria:
Nxe X (Nxe + B) ~ 4 x 10 9 cm~6
where NXe is the~number density of the Xenon present, and NB is
the number density of the buffer gas.
It is preferred, for practical applications) that an ~ `
isotropic flux density of radiation from the krypton greater
than 106 watts/cm2 sec. be maintained. The Krypton gas must be
present in a number density (number of molecules per cubic centi-
meter) of between 102 and 5 x 102. Temperature of the krypton
is not highly critical, although efficiency drops as temperature
increases. Generally, the initial temperature of the krypton
gas is maintained preferably at 300K as this is approximately
room temperature and is hence easy to obtain in the system.
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639
At the preferred temperature, the number densities given above
correspond to a pressure of from 4 to 20 atmospheres. A beam
of relativistic electrons is directed through the Krypton gas,
to create and maintain an electron density between 1014 and 1015
per c~c. The value of the electric field is adjusted so that the
ratio of the electric field E to the number density N of Krypton
molecules (hereinafter called the E/NXr value) i5 from 2 x 10
to 5 x 10 16. Under these conditions, a Krypton dimer, which
radiates wavelengths from between 1395 Angstroms and 1,515 Ang-
stroms is-created. This radiation-can be passed into molecular !
Xenon, present in a number density of 1019 to 4 x 1019 at a
temperature of 300K in order to obtain the Xenon dimer discussed
above.
In the case where the radiation from the-Krypton-dimer
is to be used to excite-Xenon to form radiating Xenon dimers, care
s-hould be taken -that~the path to be travelled by the-radiation
is as short as possible, and-that the depth of the Xenon to be
activated is small. The gain (or amount of population inversion)
obtainable for a given photon flux of radiation drops rapidly as
the distance-increased. The maximum distance which is acceptable ~`
- 20 wiIl depend upon the particular-geometry of the apparatus being
used,-the degree of population inversion sought and the xenon ;
density. However, for good results, it is preferred that the
total depth of window and xenon to be traversed shouId not exceed
5 centimeters. -~he window-~between-the krypton and-xenon should
; be made of material transparent to radiation at 1,470 Angstroms
such as lithium-fluoride.
Example 3
The arrangement of Figures 3-and 4, previously described,
is convenient for preparing Xenon dimers by excitation with radi-
ation from Krypton dimers. The Krypton is placed in chamber 27in a number density of 2 x 10 . An E/N value of 3 x 10 volts
; is created across electrodes 25 and 36. The initial temperature
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1~963~ ~
is controlled at approximately 300K. Xenon in a number density
of 2 x 1019 is placed in chamber 28. A beam of electrons at an
energy of the order of 5 x 105 electron-volts is passed into
chamber 27 along the path shown by arrow 20. A light centered
about 1,730 Angstroms oscillates between mirrors 32 and 39,
eventually passing through mirror 39 as a coherent beam. The
optical gain in some cases within the range of parameters here-
inbefore descri.bed can be as low as .01 cm 1. The mirror
reflectivity in the ultraviolet can sometimes be lower than 90%.
10 -These factors should be taken into consideration in-the design
of the apparatus, as which is well known in the laser art. -
Also according to the invention, krypton dimers can be
prepared by excitation with radiation from Argon dimers. The
parameters are the same as described previously with respect to :
- the excitation of Xenon dimers by krypton dimers, except that
~ ~ argon rep-laces-krypton and ~rypton replaces Xenon. .I.n *his case, .:
the:~buffer gas is.. ~.s~-lected from helium,.. neon or argon. .. Lithium ~
fluoride is suitable as a material for window 22, as it is trans- ~ -
parent to the radiation produced by the Argon dimer, which is in ~
a wave length band-between 1,205 and 1,315 Angstroms. ~; .
--Example 4
A beam of radiation of wave length centered about 1,470
Angstroms is produced when argon is placéd in chamber 27 and krypton
is placed in chamber 28, in the same number densities as in Example
37 and the--other--par~eters.are:kept.~he-same as:in~-Example~3 with
the eXception that the buffer gas in chamber 28 is helium. The
radiation produced in chamber 27 is in a band between 1,205-1,315
Angstroms approximately. Also an emission centered about 1,470
Angstroms is produced between mirrors 32 and 39.
30 If desired, the apparatus of Figures 1 and 2 can be
: used for the operations of each of the preceeding examples with
similar results.
1(34~3~
Similar excitation of argon by neon can be expected to
occur. However, the inventor is unaware of a suitable material
for the making of a window 22 which is transparent to the wave
length of the radiation given off by the neon.
In all apparatus described herein, care should be taken
to prevent surface tracking across the insulating side walls of
the chamber. As known in the art, this can be prevented by a
variety of well-known means, such as having the insulated walls
set back from the path of the discharge. Also, care must be
taken to prevent a premature static breakdown within the dis-
charge chamber. As known in the art, this-can be prevented by
commencing the electron beam before a voltage is applied across
the electrodes.
It is understood that the foregoing describes particular
embodiments of the invention, and that many modifications thereof
will be obvious to one skilled in the art. -For example,many
other geometries of discharge vessel can be used if desired, and
the discharge according to the invention can be used for many
different purposes, as will be obvious to skilled persons. It
20 is therefore understood that the specific examples of the dis- 1
closure are not intended to limit the scope of invention, which
is as defined in the appended claims.
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