CA2060180A1 - Laser - Google Patents

Abstract

It has been discovered that short-lived dimer ions are created in a noble gas under high energy conditions, such as during an electrical discharge. These noble gas dimer ions have excited energy states which make them suitable for use as the gain medium in a laser. A laser created using noble gas dimer ions as the gain medium exhibits a high degree of efficiency, is tunable over a broad range of wavelengths, and may be capable of being operated continuously.

Description

~091/038~0 PCT/AU~/~03 ~, -.
A LASER 2 ~

This invention relates to a laser. It relates particularly to a laser in which the gain medium ~onsists principally of a noble gas.
A laser comprises a means for the controlled excitation and de-e~citation of particles in a gain medium in such a way that the de-excitation is accompanied by the emission of highly coherent electro-magnetic radiation (light). The operation of a laser generally depends upon the ability of the individual particles of the gain medium to adopt at least three separate energy states. The first energy state is an excited state which is sufficiently stable to allow a substantial number of particles to achieve that energy level simultaneously. The second energy state is a lower energy state than the first and is generally unstable to enable a low concentration of that species. In the process of stimulated emission, photons of an energy equal to the energy difference between the first and second energy level, can induce a transition from the first state to the second state that produces a second photon of identical wavelength. As this stimulated emission process is proportional to the number of photons, the medium amplifies the light. The rate of stimulated ~5 emission is therefore dependent on the amount of particles in the first energy level and the number of photons of the appropriate energy into the medium.
However, because the photons are capable of being absorbed by particles in the second energy state, reinstating the particles to the first energy state and :- - - ~ . . .
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., ... i , WO91/03850 PCT/AU90/0~03 ~ Q;~ - 2 -demi~lshing and preventing amplification of the light, it is desirable that the second energy state be unstable so that the particles quickly decay to a third energy state which is transparent to the photons.
In order to achieve lasing, it is important to provide conditions which favour stimulated emission of radiation as previously described, rather than spontaneous emission. This is commonly achieved by setting up standing waves in the gain medium at the wavelength of the photons emitted by the relevant energy state transitions to increase the number of photons of the relevant wavelength. Such standing waves are normally created by positioning reflective elements at opposite ends of the gain medium. The elements may be planar and parallel, they may be confocal and spherical, or they may be any other suitable configuration. The elements are generally constructed or coated to be selectively reflective of photons of the relevant wavelength. This permits the elements to confine radiation of the appropriate wavelength while allowing transmission of radiation of other wavelengths.
Transmission of laser radiation from such a system is usually effected either by allowing transmission of about 2~ of radiation of the relevant wavelength through one of the reflective elements or by inserting a beam-splitting element between the two existing elements to split out approximately 2% of the radiation.
The energy level transitions which are useful particularly for gaseous atomic lasers in producing laser radiation usually result in the production of photons WO9l/03850 _ 3 _ 2 ~ S ~ /AU~to~o3 having a wavelength range of a few ~ngstroms. It is possible to tune the wavelength of the standing wave within this wavelength range to ensure that the laser radiation produced has a specific wavelength. Numerous different techniques are available for tuning of the wavelength. These include the use of prisms such as those described in Australian patent 284799, or the use of filters and diffxaction gratings, such as lithrow mount diffraction gratings. Another suitable means for tuning comprises the use of another laser, which is already providing laser radiation of the desired wavelength, as incident radiation. Providing that the incident radiation is within ths available wavelength range, the standing wave will then tend to adopt an identical wavelength.
However, the ability to tune a laser is limited by the range of wavelengths which the relevant energy level transition is capable of producing, and this range is generally very small, particularly those involving atomic energy levels.
Lasers whose gain medium is a combination of gases have been known for some time. Australian patent number 254702 describes in some detail examples of such lasers, including in particular lasers the gain medium of which is a combination of helium and neon.
The helium-neon laser takes advantage of the fact that helium has a metastable excited energy level ~23Sl) corresponding with an excited energy level of neon (2s5). Neon also has a lower energy level (2pg), and the transition between the two energy levels of neon is the relevant one for the purposes of lasing.

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. i l wosl/03850 ~ ~r3~ ~Q ~ PCT/AU90/00403 ~!. , ~ , ' When the mixture of unexcited helium and neon is placed in a radiation field so that partial ionisation of helium occurs, free electrons of relatively high energy are produced. These free electrons, hy collision processes, excite to higher states un-ionised atoms predominantly of helium but incidentally also of neon. In particular, atoms of helium tend to be excited predominantly to the 23Sl level either directly or by relaxation to that state from a higher state to which they were excited. When the excited helium atoms collide with ground state neon atoms, some neon atoms are excited to the 2s5 state because transfer excitation occurs between colliding atoms whose energy levels are closely matched, so that there is established a population inversion or negative temperature between the 2s5 state and the 2pg state of the neon gas, and this permits the stimulation of emission of radiation.
However, the helium-neon lasing process is inefficient for a number of reasons. The major inefficiencies are attributable to the energetic electrons created by the ionisation of helium, inhibiting the e~ficient formation of the desired population inversion by promoting reactions which increase the number of neon atoms at levels other than the 2s5 state or by accelerating reactions which tend to restore the system to e~uilibrium.
Australian patent number 254702 sought to overcome these inefficiencies by pulsing the radiation used to cause the ionisation o~ helium so that the creation of high energy free electrons is periodic and the lifetime of - . . - . : . : . : , : . , :

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helium atoms in the 23Sl level is longer than the lifetime of the free electrons. By appropriate choice of pulsing parameters it is possible to induce stimulated emission which approximates continuous lasing, but true continuous lasing is never achieved and the helium-neon laser remains inefficient.
Another type of commonly used gas laser is the excimer laser. This type of laser uses as its gain medium a noble gas in conjunction with a halide gas, and produces photons which are in the ultraviolet range. Examples of excimer lasers are lasers which comprise neon and chlorine or xenon and fl~orine. Such lasers produce only a narrow range of wavelengths, typically having a range of about 1~. Excimer lasers suffer the disadvantage that the halide gases have a corrosive effect on the vessel in which they are contained, so that frequent restoration or replacement of the vessel is necessitated. In practice, this occurs after about ten days of continuous operation.
The gases are also dangerous, requiring special ventilation for the apparatus.
The operative particles creating the laser effect in excimer lasers are short-lived molecules comprising a combination of noble gas atoms and halide gas atoms.
These molecules are formed in the afterglow of an electrical discharge or other suitable means of e~citation, so that the laser operates as a series of pulses, each pulse comprising electrical discharge followed by a formation of molecules, followed by de-e~citation and emission of laser radiation. These molecules have an unbound lower state which quickly , : - , . ~ .
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WO91~03850 pcT/Auso/o~o3 ~ ~ 0 ~ 6 -decays, enabling population inversion.
Another type of gas laser is the neutral dimer laser. Neutral dimer lasers involve the combination of a noble gas atom in the ground state and a further atom of the sam~ noble gas in an excited state to form a molecule.
They have been demonstrated for most noble gases in pulsed afterglow and molecular beam techniques. They have large tuning ranges but only operate in comparatively low energy conditions, limiting their efficiencies.
No Neutral ~elium Dimer laser has yet been demonstrated. It is thought that the lifetime of the molecule He2 is too short to enable lasing in afterglows or molecular beams. r It is an object of the present invention to provide a laser, the wavelength of which can be tuned over a broad range. A further object is to provide a laser which is capable of continuous operation. A further object is to provide a laser which is powerful and efficient.
According to the present invention, there is provided a laser which uses dimer ions of a noble gas as its gain medium.
The preferred laser e~citation means is an electrical discharge, and indeed any suitable means of excitation including microwave radiation and particle beam excitation may be used.
The noble gas used in the present invention may be any noble gas including heli~lm, argon, xenon or krypton.
It is preferred that the laser of the present invention be capable of operation at high temperatures, because it has been found that noble gas dimer ions form . -: - , . , . - -: ................ . .
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2 0 6 0 1 8 (3 PCT/~U9~ 03 more readily at high temperatures (activation energy ~ 1 eV~ which allow for a high degree of efficiency. To this end, it is preferred that the active medium be contained within a ceramic, graphite or other refractory material vessel which is capable of withstanding high temperatures, rather than a conventional water-cooled glass vessel.
The invention will now be described in more detail with particular reference to an embodiment which uses the helium dimer ion, although it is to be understood that the invention relates to the use of any noble gas dimer ion.
Figure 1 of the drawings is an energy level diagram showing a plot of energy against inter-nuclear separation for a helium atom and a He+ ion.
Figure 2 is a schematic representation of laser apparatus embodying the present invention.
It has been discovered that the afterglow of an electric discharge in a noble gas may give rise to the existence of noble gas dimers, consisting of two atoms of a noble gas, in an excited state which provides a suitable first state for a laser pulse emission as described above. ~he feasibility of a helium neutral dimer laser has been described in an article by Hill entitled "Ultraviolet Continua of Helium Molecules", published after the priority date of this specification in Physics Review A 198~, Volume 40, page 5004. Neutral dimer lasers have been demonstrated in ~enon, argon and neon.
However, such dimer lasers have not yet been commonly used because of practical difficulties such as providing a sufficient population of the dimer in the afterglow of the electrical discharge, requiring the use .

Wo91/03850 2 ~ 0 --8-- PCT/AU90/00403 of e~pensive and e~otic relativistic electron accelerators (Wrobel, Appl Phys. Let. Vol. 136, page 113, 1980).
The preS~nt invention results from the discovery of a new energy state in a noble gas dimer ion which is actually formed during the electric discharge, rather than in the afterglow. The presence of this new energy state was determined by the presen~ inventor's analysis of experimental results, such as those given in "New Vacuum -Ultraviolet Emission Continua of Helium produced in High-Pressure DischargesN, by R E Huffman, Y Tanaka and J
C Larrabee, published in the Journal of the Optical Society of America, Volume 52 No. 8 (August 1962) ~high energy pulse form); D Simon and K Rodgers, J. Appl. Phys.
37,2225 (1966) (continuous operation); and Y Tanaka, A
Jursa and F LeBlanc, J. Opt. Soc. Am. 8,304 (1958) ~low energy continuous operations).
In the energy level diagram of Figure 1, the higher energy state, having 2nU symmetry and referred to in this description and for the purposes of the claims as the C nu state, is the newly discovered energy state.
T2he lower energy state is labelled A2~g. The c n U state is bound when the inter-nuclear separation is between 0.4 and 1.2 ~ so that in this region a comparatively stable molecule is formed. The c2nu state represents the interaction of an He+ ion with a helium atom excited to the 23P level. Upon absorbing a photon with wavelength between 2500 and 10000 R, the dimer ion decays to the A2g+ state, emitting 2 photons having wavelengths identical to that of the incident photon. As the A2~g+ state is an unbound state, .. .
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the He+ molecule rapidly decays into individual He~
and He atoms, thus becoming transparent to the photons.
The symbolic representation of the process is as follows:

1. ~e~ t ~e(23P) + He -~ He2(C ~u) + He.

2. He2(C2nu) + nv(2sO0-lOOOOA) ~ He2(~2~g) + (n+l)v(2500-10000~)

3. He2(A2~g) ~ He + He + K.E.

As can be seen from equation 1, the formation of the excited dimer ion occurs in the presence of another helium atom (any ion, electron or neutral or multiple particle which can remove momentum) which absorbs momentum, lS allowing the dimer ion to settle in the bound state.
Equation 2 shows the stimulated emission of the excited helium dimer ion from the C2nu state to the A2~g+ state in the presence of photon radiation having wavelength between 2500 and 10000~ with the accompanying emission of a further identical photon.
Equation 3 shows the rapid decay of the A2~g state into He+ and He atoms.
This reaction is particularly favoured in high pressure high current discharges (arcs). In arcs the temperatures of all particles approach the same value, and the temperature of the gas increases to many thousands of degrees. This provides the particles with the activation energy necessary to produce the c2nu molecule. The reaction may be seen in overview as:

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O91/03850 I PCT/AU90/0~03 s~ 10~

4. He + He(23P) ~ He ~ He+ + He + He + v.

In other words, the Helium ions in the high temperature conditions of an arc act as a catalyst to liberate the energy trapped in the excited states of Helium. This process is highly efficient and operates at high power densities.
The transitions described above provide the explanation for the continuous emission spectrum observed in the range 2500 to 10000~ for helium, as described in the article by Huffman, Tanaka and Larrabee referred to above. It will be appreciated by those familiar with lasers that this is a considerably broader emission spectrum within which the laser is tuneable than any previous laser emission spectrum, so that access may now be had to regions of the spectrum that could previously only be accessed by non-linear light-mixing techniques which were very inefficient (less than 0.1% efficient).
Dyed liquids and solids can have broad ranges but can only be pumped with limited power, mostly using light as the excitation source. The present invention, on the other hand, provides a laser which may achieve greater than 50%
efficiency. This may be seen by the fact that over 50% of the light given off in Huffman's experiment is continuum, the remainder being line. As the present invention uses the molecule which produces the continuum, it can use more than 50% of the available light.
Further, by a process of radiationless symmetry transition wavelengths around 300-600 ~ may be achieved using the X2~u ground state of He2.

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... ' ' ' ' ~ ' -' ~ ', .' WO91/03850 2~ ~0~ ~ ~ C~/AU90/~03 As the He2 molecule in the c2nu state is produced in very high power output c~nditions, the laser of the present invention is capable of converting Yery large quantities of energy into laser radiation.
Excimers, on the other hand, deteriorate very quic~ly when the energy input is increased significantly.
It is preferred that the pressure of the noble gas which forms the gain medium for the laser be approximately 2 atmosphere. The pressure may be any other suitable amount, but should in any event be greater than 200 Torr in order for the invention to work in an appropriate manner. It is further preferred that there be provided appropriate valves gauges and vacuum pump to ensure the desired pressure and purity of noble gas.
l~ The preferred means for exciting the gain medium comprises a cathode and an anode one of which is connected to a capacitor which sends an electrical discharge through the active medium once the capacitor has reached the breakdown voltage of the active medium. Other suitable means for exciting include microwave excitation means, particle beam e~citation means, flashlamp excitation means, and Tesla coils.
~ ecause the reactions which cause the present invention to work occur during the excitation of the noble gas, rather than in the afterglow, the laser of the present invention may be subjected to continuous excitation and may provide a continuous laser pulse, provided that a sufficiently large quantity of power is available for excitation. Alternatively, the laser of the present invention may be operated in pulses by means of a WO91/03850 PCT/AU~/0~03 2~
capacitor of approximately l microfarad, charged to approximately 30 kV, although these f igures are arbitary.
Ideally, the capacitor discharges along transmission lines, designed to have no impedance mismatches, when the voltage reaches such a value that an electrical discharge occurs between anode and cathode.
As an optional feature, there may be provided mean~
for tuning the laser of the present invention, taking advantage of the broad spectrum of the noble gas dimer ion so that the laser may be tuned to operate at any wavelength within that spectrum. One suitable means for tuning is the use as incident radiation of a laser of a set wavelength within the noble gas dimer ion spectrum.
The laser of the present invention will then adopt the lS wavelength of the input laser and amplify the incident li~ht.
Another suitable means for tuning involves the use of filters and di~fraction gratings, such as lithrow mount diffraction gratings. These may be placed according to known techni~ues between a mirror and the gain medium in order to feed the desired wavelength back into the active medium, ensuring that the desired wavelength becomes the one which is amplified.
If no means for tuning the laser other than the use of mirrors is used, or the active length is sufficiently long that no mirrors are required, the laser will operate at its natural wavelength o approximately 6050 R, as evidenced by calculation and the observation of gain narrowed emission seen in Huffman's experiment.
According to the embodiment represented in Figure 3, . ., : :. .. ; ,;: . ~ .. .

WO91/038~0 a noble gas such as helium is contained at a pressure of approximately 2 atmosphere within refractory discharge guide c. Refractory discharge guide c is bounded at one end by polished aluminium block mirror a and at the other end by partially tran~mitting mirror e.
Arranged at separate ends of refractory annular discharge guide c are cathode b and anode d. Anode d is electrically grounded, while cathode b is electrically connected to pulsed capacitor f. Pulsed capacitor f is in turn connected both to ground and via limiting resistor g to 30 kilovolt DC power supply h.
As will be seen from the foregoing, the present invention provides a laser which may be tuned within a broader range of wavelengths than any previous laser. It further provides a more efficient laser and a laser capable of continuous operation.
It is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit and ambit of the invention.

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Claims (10)

CLAIMS:

1. A laser which uses dimer ions of a noble gas as its gain medium.

2. A laser according to claim 1 wherein the gain medium is Helium dimer ions.

3. A laser according to claim 2 wherein the radiation emitted by the laser results predominantly from the transition of He? molecules from the C2Hu energy state (as hereinbefore defined) to a lower energy state.

4. A laser according to claim 1 wherein the gain medium is Neon Argon or Xenon dimer ions.

5. A laser according to any one of claims 1 to 4 further including means for tuning the wavelength of the laser radiation.

6. A laser according to any one of claims 1 to 5 wherein the gain medium is housed within a chamber constructed from a refractory material.

7. A laser according to claim 6 wherein the refractory material is a ceramic or graphite.

8. A laser according to any one of claims 1 to 7 wherein excitation of the gain medium occurs by means of periodic electrical discharge between two electrodes, with energy being accumulated in one or more capacitors between each discharge.

9. A laser according to any one of claims 1 to 7 wherein excitation of the gain medium occurs by means of continuous electrical discharge.

10. A laser substantially as herein described with reference to Figure 1 and/or Figure 2.