Note: Descriptions are shown in the official language in which they were submitted.
~q~3~
PULSED MULTILINE C2 L~SER OSCILLATO~
APPARATUS A~D ~ETHOD
Field of the Invention
In accordance with the present invention, there is provided a gas
laser oscillator comprising an optical resonant cavity having therein
means for conta-lning a C02 lasing medium, means for causing a population
lnversion in the lasing medium, and means or produc-lng wavelength dependent
loss within the optical resonant cavity to provide a multiline output.
Background of the Invention
The carbon dioxide laser has been found to be by far the most efficient
gas laser and the most powerful continuously operating laser. Efficiencies
o~ about 20% and outputs of tens of kilowatts are possible ln existing
single line output carbon dioxlde laser~ which work ln the ln~rared at a
wavelength of 10.6 mLcrometers (~m). For use ln nuclear fusLon exper:Lments
presently exlstlng mode locked C2 lasers have produced powers of 10
watts in pulAes of 1 nanosecond (10-9 second) duration.
To obtain laser action in the inErared, energy levels whose separation
is comparatiyely small must be found. Suitable levels are found in molecules
which do not depend on excitation of electronic energy levels, but on the
quantiza~ion of the vibrational and rotational movements of the molecule.
These levels can be very efficiently excited.
The carbon dioxide laser actually uses two additional gases: nitrogen
and helium, whose roles will be discussed hereinbelow.
In order to appreciate the theory of operation of the carbon dioxide
laser it is necessary to discuss the energy levels of the carbon dioxide
molecule. The carbon dioxide molecule can be pictured as three atoms
which usually lie in a straight line, the outer atoms being oE oxygen with
a carbon atom in between. There are three possible modes of vibration, in
each case the center of gravity remains fixed:
(1) The oxygen atoms may oscillate at right angle to the straight
line, this being called the bending mode.
~.
(2) Each oxygen atom can vibrate in opposition to the other along
the straight line, this mode being called the symmetric mode.
(3) The two oxygen ztoms may vibrate about th~ central carbon atom
in such a way that they are each always moving in the same direction.
This mode is called the asymmetric mode.
~ ach po~slble quantum state ls labeled as follows: for the symmetric
mode by 100, 200, 300, etc.; for the bending mode by 010, 0209 030, etc.;
and for the asymmetric mode by 001, 002, 003, etc. Combinations of all
three modes are possible, for example, 231, but they need not concern us here.
In addition to these vibrational modes the molecules can rotate and
therefore quantized rotational energles are possible; a set O;e rotational
leVel8 i9 associated wlth each vlbratlonal level, these are lflbeled ln
order o;E lncreaslng energy by ~ va:lue~, each ~a~ue belng e:Lthcr 0 or a
positive integer.
Summary of the Invention
In accordance with the lnvention there ls provlded a laser oscillator
for producing a plurality of laser spectral lines useful for driving high
power laser amplifiers comprising, a conventional gas discharge laser, means
disposed within the optical cavity of said gas discharge laser for filtering
each of said plurality of laser spectral lines by distinct amounts in
accordance ~ith the angular orientation of said means for filtering, and
means for retaining said means for filtering at a predetermined angular
orientation such that said plurality of laser spectral lines have
approximately equal output power levels to rapidly extract energy from
excited rotational transitions of said high power laser ampliflers.
In a preferreA mode locked embodlment of the invention, mode locklng
is achieved by an acoustooptic modulator and wavelength dependent loss
is provided by a Fabry-Perot etalon filter. Both the modulator and the
etalon are disposed within the cav:Lty. The hlgh energy unlform electrical
discharge is produced by Rogowski electrodes operably connected to a high
voltage source.
-- 2 --
31 ~3E~0~7~
One object of the invention is to provide a multiline output from a
C2 oscillator.
Another object of the invention is to increase the eficiency of CO2
laser amplifiers.
Still another object of the present invention is to utilize a plurality
of the available P-branch and R-branch transitions within a CO2 laser
osclllator and ampli~ier.
One advantage of the present invention is that in accordance therewith,
an increased output efficiency is obtainable from a CO2 laser amplifier.
Another advantage of the present invention is that in accordance
therewith multiline output is achievable Erom a C2 laser osc~llator.
Still another advantage of the present invention ls that a plural:lty
of P-branch transit~ons are utlllzable to provlde a multl:Llne OUtp~lt from
a CO2 lnser osclllator.
Other ob~ects and advantages of ~he present lnventlon wlll be evident
to those skilled in the art from the following description with reference
to the appended drawings wherein like numbers denote like parts and
wherein:
Figure 1 is a diagram of carbon dioxide laser energy levels;
Figure 2 is a typical gain curve of a CO2 laser output for P(18) and
P(20~ P-branch transitions;
Figu}e 3 is a schematic showing of a pulsed multiline CO2 laser
oscillator in accordance with a preferred embodiment of the invention;
Figure 4 is a graphical showing of laser oscillator output intensity as
a function of etalon angle of incidence for several laser transitionæ; and
~igure 5 is a diagrammatic representation of laser output power as a
function of wavelength for transltions in the 00l-10O vibrational band.
Detailed Description of the Invention
Figure l shows the sets of energy levels associated with each mode of
vibration together with a set oE rotational levels for the 001 and 100 modes
on a much expanded scale. The ground state and the first excited state of
-- 3 --
the nitrogen molecule are also s~own As only two atoms are involved, the
nitrogen molecule can have only one vibrational mode.
The mechanism of laser action is as follows: direct electronic excitation
of the nitrogen molecule into its one state by a collision of the first kind.
Thls process is represented by the followlng equa~ion:
el -~ N2 = N2 ~ e2 (1)
A collision of the second kind with a carbon dioxide molecule in the
ground state with excitation to the 001 state is symbolically described
as follows:
N*2 ~ C2 ~ N2 ~ C2 (001) (2)
This takes place because, as can be seen from the energy level dlagraml the
two energy level values almost coincLde. The 100 vlbrationa:l ~tate i9 of
much lower energy and 50 cannot be populated by thl~ proce~a.
The population o-~ the 001 levels now exceeds the population of the 100
levels and 80 the population inversion condition Eor laser action to take
place between these levels has been achieved. However, two points must be
born in mind. First, a transition from the 001 level to the 100 level must
obey a selection rule which states that J can only change by ~ 1. Thus,
if J equals 10 for a particular level, then only the transitions from J = 9
to J = 10 and J = 11 to J = 10 are permitted. If J changes by +1, the
transition is called a P-branch transition and if J changes by -1 it is
called an R-branch transition. For example, a transition from J = 9 to
J = 10 is called P~10) and a transition from J = 11 to J = 10 is called
R(10). Second, the population o~ the rotational levels of the 001 state will
have a Bolt~mann distribution, so, after taking degeneracy into account, the
effective population of J - 11 level, or instance, will be less than J = 9
level. The result of this is that P-branch transitions dominate because it
so happens that a particular P-branch level will ~ill up tln order to restore
equilibrium) by depletion of the population of the R-branch above it more
quickly than the R-branch level population decays by spontaneous emission to
the lower laser level. Wavelengths associated with the most powerful
-- 4 --
transitions of the carbon dioxide laser at nor~al operating temperatures
are P(18) - 10.57 ~m, P(20) - 10.59 ~m, P(22) - 10.61 ~m. The separation
between each transition is about 55 GHz.
Each gain curve corresponding to a P-branch transition has a line width
of about 50 M~lz. In comparision with other gas lasers this is a narrow
Doppler wldth that come3 about because the wavelength is some 20 times as
long and the mass of the molecule is greater than that of most atoms. The
sum of the areas under each gain curve in Figure 2 is proportional to the
population inversion between the 001 and the 100 levels and hence proportional
to the lntensity of the output. These areas are not Ln fact equal and so
it happens that because of the relative J-level populations, the area under
the P(20) gain curve is largest. The axial mode separatlon Eor a 100-cm-long
cavity illustrated is about 150 ~lz. ~L~ure 2 shows the P~:L8) and P(20)
gain curves and the axlal mode ~pacln~.
It ls apparent from Figure 2 that where a cavity 1 meter in length ls
used, only one axial mode can oscillate under a gain curve at any given
time. If a much longer cavity were to be used, the modes would be closer
together and so several would oscillate. In any case, the axial mode which
experiences the greatest gain will tend to grow in intensity at the expense
of the others.
For a short cavity, where only one mode oscillates, the change in
cavity length due to instabilities will cause the output power to fluctuate.
If the laser is tuned so that the axial mode frequency is at the center, for
example, of the P(20) gain curve, then a gradual reduction in power will be
observed as the axial mode frequency drifts. If the next mode peaks at
P(18) or P(22) it will take over, so not only does the power Eluctuate, but
a frequency fluctuation is also obtained. On the other hand, for the case
of a ten meter cavity wi~h a corresponding mode separation of fifteen M~z,
several modes will be present under each gain curve, and so the P branch
with a maximum gain always oscillates because one axial mode will always be
present under the Doppler gain curve. An analogous situation exists among
-- 5 --
~q~3~Q~
the allowed rotational transitions o~ the CO2 ~olecule ~hich limits the
efficiency o~ energy extraction from prior art lasers. ~hichever rotational
transition experiences the highest gain will tend to grow in intensity
at the expense o the others. This happens because the line which starts
to oscillate initially depletes the population of the appropriate 001
level and, as e~plained above, it so happens that the relaxation rate into
such a depleted level from other J levels associated with the same
vibrational level (in order to restore a Boltzmann distribution) is much
~aster than the spontaneous decay rate rom any J level to a lower vibrational
level. Hence, the :Lnversion between other levels tends to feed into the
irst. The gaLn proiles will uniormly decrease to~ether and :Lt Eo:llows
thereore that the P-branch tran~lt:Lon~ are eEEectlvely homo~eneously
broadened.
The helium i9 efective in increasing the thermal conduction to the
walls o the tube, indirectly depleting the population of the lower laser
level 100 which is linked through resonant collisions with the 020 and the
010 levels, the latter level being directly depleted by the helium and by
"cooling" the 001 rotational levels which results in the available population
~:;
being more heavily distributed among the upper lasing levels.
; 20 Lasers considered ideal for inducing fusion reactions must have the
property that their total stored energy be released in pulses of one nanosecond
or less. In the case of prior art C02 lasers, the detailed dynamics of
the excited molecular species, as indicated above, severely limits the
amount o energy that can be extracted on a nanosecond time scale. The
origin oE this limltation stems Erom t'he Einite thermallæat:Lon rate between
excited rotational energy levels of the C02 molecule.
As noted above, the energy stored in the excited C02 laser mixture is
present in many excited rotational levels, 'but the typical oscillator that
is used to drive large amplifiers has its output spectrum primarily composed
o the P(20) rotational transition oE the 001 to 100 vibrational band or
reasons stated above. Thus, on a time scale small with respect to the
-- 6 --
~L~3~
thermalization time for the rotational levels~ energy will be extracted
from only the P(20) transition, J = 21 (001) to J = 20 (100), since there
is no time for the excited state popula~ion to redistribute itself from the
other nearby J levels of the 001 state and repopulate the upper level
~J = 21) of the P(20) transition. This situation is unacceptable for laser
induced fusion because the eficiency of energy extraction must be considered
as part of the overall energy 'balance when the feasibility of the entire
laser-induced fusion process is evaluated.
Previous to the experimental measurements and theoretical investigation
by the inventor herein, this serious limitatlon to the short pulse efficiency
of C2 lasers was simply not appreciated. Work of Cheo and ~brams, ~pplied
Physics Letters 14, 47 ~1969) indicated that the rotational relaxatLon
time was .2 nanoseconds leading to a generally held opLnion thnt Eor one
nanosecond pul~es, one was still ~ully utillzlng all avallable stored energy
ln the excited C02-N2-He mixture. Measurements show that only a few
rotational levels were thermalizing and thus contributing to the energy
extraction of the nanosecond time scale. To remedy this situation, the
multiline C02 oscillator of the invention having an output spectrum
containing the P(18), P(20), and P(22) transitions at approximately equal
intensity was developed. This oscillator separately but simultaneously
extracts the energy stored in at least three--P(18), P(20) and P(22)--of the
excited rotational transitions in the C02 laser amplifier. Thus, the
multiline oscillator remedies the serious deficiency in the efficiency of
energy extraction from kilojoule amplifier systems such as those used to
initiate laser fusion reactions.
The term "cavLty" as used herein means not only one that could 'be
defined by walls, but also one that is not defined by walls or the like,
since in certain cases walls are not essential in practicing the invention.
~ "discharge~' as used herein i8, in an :Lonlzed medium, the flow of
current under the influence of a sustainer electric field or fields. While
the use of dc voltages with intracavity electrodes is primarily described
-- 7 --
~3!3~0~
herein, in accordance with the invention, one may provide a sustainer field
with radiofrequency electro-magnetic fields, inductive electrode structures,
capacitive electrode structures, movements of an electrically conductive
medium in the presence of an applied magnetic field, and the introduction
of laser energy into the working cavity.
However, at the present t-ime as known in the art, electrical discharge
excitation 16 the most efficient technique for pumping a gaseous lasing
medium. In an electrical discharge, the lasing gas is both directly
excited by electron collision and excited by resonant energy transfer from
a second gas excited by electron collis-Lon.
Reference is now made to Flgure 3, whereln the preEerred embodlment
of the pulsed mult:Lllne C02 laser osclllator of the ln~ent:lon 1B schematlcnl:l~
represented. The osclllator 10 compr.Lscs a cavlty 12 havlng n Brew~ter angle
wlndow 14 and a 90% to 98%, preEernbly about 95%, reElectivity output coupler
or mirror 16. The output coupler 16 and the Brewster window 14 are employed
for the usual purpose, i.e., to provide usable laser output and a gas seal
at the end of the cavity as well as a no-reflection lnterface with the
acoustooptic modulator and Fabry-Perot etalon, respectively. A substantially
100% reflector 18 is disposed at the other end of the cavity.
Parallelism of the mirrors 18 and 16 is a rigorous geometric requirement
in low gain lasers. This is because in low gain lasers, if the mirrors are
not precisely parallel, the light rays that build up in the cavity will tend
to digress further and further toward the edges of the mirrors as they are
reflected back and forth between the mirrors, and finally the rays will be
directed out of the cavity altogether~ I~t is essential that any deviation
from parallelism be so small that the coherent photon streams will reElect
back and forth a sufficiently large number of times to build up the required
~ intensity for laser action.
; The mlrrors ].6 and 18 may be simply polished metal or they may be
silvered or dielectric coated so that they behave as mlrrors which reflect
photons coming toward them Erom the interior of the cavity 12. The above
-- 8 --
~3~7C~
described structure, whether the mirrors are within or outside the container,
is called an optical cavity. In oscillators, it is called an optical
resonant cavity because the spacing distance between the two mirrors is
adjusted such that it i6 an integral number of half wavelengths long, thereby
providing reflected energy of the correct phase to produce the required
constructive wave interEerence.
Pumping is preferably brought about by an electrical discharge through
the Rogowski profile electrodes 20 and 22 charged to a high voltage by a
two-stage Marx generator 24 and a 20 kilovolt dc source 26. The Rogowski
profile i9, of course, well known to those skilled in the art. A preioni~ation
electrode 28 charged through capacltors 31 and 32 is preerably utlllzed.
The electrical discharge producing system, comprlslng voltage source 26,
generator 24, electrode~ 20, 22 and 28 are conventLonal :Ln naturc and p~ay
no part ln the lnventlon hereln. Thus no deta~led dlacussion oE them need
to be made hereln.
The pumplng or electrlcal dlscharge means brings about an electrical
discharge wlthin the laslng medium contained within the cavlty. The dlscharge
causes a population inversion among the desired energy states. In a small
fraction ot a second, spontaneous emission of photons from the gaseous
medium occurs. Most of the photons are lost to the medium but some travel
perpendicular to mirrors 16 ~nd 18 and are reflected back and forth many
times thereby. As these photons traverse the active medium, they stimulate
emission of photons from all atoms in the desired states which they encounter.
In this way the degree of light ampllflcation in the medium increases
extraordinarily. Because the photons produced by stimulated emission have
the same dlrection and phase as those whlch stimulate them, assumlng the
optical cavity of the laser medium is suitable, the electromagnetic radiation
field inside the cylinder or cavity is coherent.
In order to extract a useEul beam of thls coherent light from the
cavity, mirror 16 is made sllghtly transmissive. A portion oE the highly
intense beam leaks through the mirror and emerges with regularly spaced
_ 9 _
wave fronts. This is called the laser beam.
The laser oscillator in the preferred embodiment is preferably mode
locked for use in laser fusion applications. An acoustooptic modulator 30
for mode locking is provided. Acoustooptic modulator 30 is preferably a
germanlum acoustooptic modulator for actively mode locking the laser
oscillator. It wlll be appreciated by those skilled ln the art that mode
locking means are not to be limited to an acoustooptic modula~or. Other
mode locking means such as bleachable absorbers may also be utilized.
It is preferred that the active length of the device be on the order
of 60 cent-lmeters or larger and the ou~put coupling reflector 16 be
approximately 94 to about 98% re1ectlve. It has been found that these
cond:ltlons maximize the overall ~aln of the weaker laser ~rsm~:Lt:long. The
aystem should be exclted wlth an exc~tatLon den~ity Oe at :Le~t 300 Joulc~/
llter of the active volume.
In accordance with the invention, multiline operation is achieved with
the insertion of a means for producing wavelength dependent loss, such as
a sodium chloride Fabry-Perot etalon 36, within the laser cavity. Preferably,
the etalon is mounted approximately normal to the optical axis of the system
and is tilted by a micrometer dr-iven stage to facilitate variation of the
effective etalon thickness. The properties of a Fabry-Perot etalon are
well known to those skilled in the optical art so no discussion of the
theory of operation of the etalon is made herein.
A spectroscopic study of the output of an apparatus in accordance with
the invention showed that the appearance of particular lines was related
to ~hat particular wavelength having an optical path length, ln the sodium
chloride Fabry-Perot etalon, equal to an integral number of quarter
wavelengths. Thus, when the transmission of the etalon is maximum for some
particular wavelength, the probability of oscillation at that particular
wavelength is enhanced. Tf an opt-lcal wavelength does not satisfy the
quarter wave condition, a reflection loss out of the resonator cavity of up
-- 10 --
7~
to 15% is introduced into that particular transition. This loss is that of
a Fabry-Perot etalon possessing the Fresnel reflectivity (4% in the 10
micron wavelength region) of the sodium chloride surfaces.
The reason that multiline operation occurs when practicing the invention
is thought to be as ollows:
The gain coefficients of the various P- and R-branch transitions in
the m-lddle o~ the 10.6 ~ rotatlonal band of C02 are nearly identical
except for the P(20) transition. The Pt20) transition line has a gain
coefficient which is anomalously high, when compared to other transitions
of interest, by as much as 10%. This high gain coefficient causes the
P(20) llne to domlnate ln the output oE all galn switched T~ lasers. If,
in accordance wlth the invention, a wavelength dependent loss produclng
means, such as a Fabry-Perot etalon, 19 lnserted :lnto the osclllator cavlty,
the means serves to lower the net galn oE the uaually domlna~t P~20)
transitlon an amount sufflclent to allow one or more of the Ptl6), P(18),
P(22) or P(24) transltlons to successfully compete for available upper
state population.
An operable C02 laser oscillator may be utilized in practiclng the
invention. This includes both high pressure and low pressure, high power
and low power, mode locked, Q-switched, gain switched, and continuous, with
static gases or gas flowing C02 laser oscillators, having cavity geometry
suitable to the insertion of means for providing wavelength dependent loss
of a proper amount.
In an exemplary embodiment, a Lamberton-Pearson double discharge laser
was used having Bruce profile electrodes of 18 centimeters active length,
separated by 1.5 centimeters. The cavity compr:lsed an lnternally mounted
germanium output coupler of 3.0 meter radius of curvature and 98% reflectivity
separated by 84 centimeters from a totally reflecting, dielectric-multilayer
coated optlcal flat.
When operated ln the galn-switched mode with 7.0 joules input,
corresponding to an excitation loading of approxlmately 80 joules per liter,
-- 11 --
~(~3~
the laser exhibited a gain coefficient of 2.8% per cm with a gas mixture
consisting of 7:1:1, He:C02;N2, The multiline output pulses were of
75 + 5 millijoules energy and had a duration of 125 + 15 nanoseconds (full
width at half maximum) when measured with a fast response photon drag
detector.
A spectroscopic study of the device was made w-lth an Optical Engineering
~nc. Model 16a, 3/4-meter C02 laser spectrum analy~er.
Figure 4 schematically demonstrates the relative intensities of the
various laser transitions that appeared as the sodium chloride Fabry-Perot
etalon was rotated through an angular excursion of 5 degrees. The
periodicity of each transition in the output was determined to be related
to the respective transition having an optical path length~ Ln the etalon,
equal to an integral number oE quarter wavelengths. The re~ult~ are
summarized in the Table.
TABLE
TransitionR~16) P~16) P(20) P(24)
air (~ 10.27510.551 10.591 10.632
~ salt (~) 6.ô78 7.062 7.089 7.116
Qp (~) path
change in etalon
for transition
appearance 1.707 1.793 1.742 1.800
~ salt .248 .254 .246 .252
Hence the periodicity of laser oscillation appearance is shown to be 4.
When the transmission of the etalon was a maximum for some particular
wavelength, its probability of oscillating was enhanced. If the optical
path length does not satisEy the quarter wavelength condition, a reflection
loss out of the resonator of up to 15% is introduced into that particular
transition. As above stated, this loss is that of a E'abry-Perot etalon
possessing the Fresnel reflectivity (4% in the 10 ~ wavelength region) of
the salt surEaces. Since the gains of the various transltions are within
- 12 -
~3~
10% of one another, it is quite reasonable that the wavelength dependent
loss introduced by the tilted etalon could depress the highest gain transition,
P(20~5 and allow one with a lower overall gain, such as the P(16), when it
fulfills the quarter wave condition, to also oscillate. The appearance
o the R(16) llne instead o the P(18) line with which it competes in the
C2 level structure ls attributed to additlonal wavelength selectivity
introduced by the one non-Brewster angle salt window employed to allow
access to the intracavity region.
An accurate spectral measurement was made by ad~usting the angular
orientation of the etalon such that the R(16), P(16), and P(20) lines
oscillated simultaneously. The relative energies were measured by d:lrect:Lng
the .laser output through a Jarrel:L Ash ~odel 82-~20, 0.25 meter monochrometer
equipped with a pyroelectric detector. The OUtp.l~ spectr.lm Ls ~hown Ln
Figure 5.
The various eatures and advantages of the invention are thought to
be clear rom the foregoing description. However, various other Eeatures and
advantages not speciically enumerated will undoubtedly occur to those
versed in the art, as likewise will many variations and modifications o the
preferred embodiment illustrated, all of which may be achieved without
departing from the spirit and scope of the invention as defined by the
following claims.
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