Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~091/038~0 PCT/AU~/~03
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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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WO91/03850 PCT/AU90/0~03
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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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wosl/03850 ~ ~r3~ ~Q ~ PCT/AU90/00403
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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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WO91/03850 _ 5 _ 2 ~ PCT/AU90/~03
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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
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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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WO91/03850 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
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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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wos~/0~8-0 2 U C O ~A~0~03
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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
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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
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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,
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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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