CA1049639A - Lasing device and method - Google Patents

Abstract

ABSTRACT

Light amplification for use in an optical amplifier or optical resonator can be obtained by exciting iodine vapour with radiation given off by an excited xenon dimer. The xenon dimer itself can be used for light amplification as well. The xenon dimer can be excited by irradiating xenon gas with an electron beam sustained discharge, or with radiation given off by excited krypton dimers. Krypton dimers can be excited by irradiating krypton gas with an electron beam sustained discharge or with radiation given off by excited argon dimers, which are excited by an electron beam sustained discharge.

Description

~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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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 ~ ;
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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 4_ :~ . . - . .
.

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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.

-4a-., , . :, ,., . ',, 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 ;
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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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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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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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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.

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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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Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of obtaining optical amplification in the region 3,000-3,300 and 3,700-4,000 Angstroms comprising:
a) providing in a discharge chamber a mixture comprising from 1016 to 5 x 1017 molecules of gaseous iodine per cubic centimeter, b) bombarding said molecules with energizing radiation of wave length in the range of 1,650 Angstroms to 1,790 Angstroms, said energizing radiation being emitted by excited xenon dimers in a separate chamber adjacent said discharge chamber and separated therefrom by a window sub-stantially transparent to said energizing radiation.

2. A method as claimed in claim 1, in which the xenon dimers are formed by a process which comprises providing xenon gas in a number density of from 1020 to 5 x 1020 molecules per cubic centimeter, and exposing said xenon molecules to an electron beam providing an electron density of from 10 4 to 1015 electrons per cubic centimeter at an E/N value of from 10-16 to 5 x 10-16 volts-cm2.

3. A method as claimed in claim 1 or 2, in which the method is carried out at a temperature of approximately 300°K.