CA1038070A - Pulsed multiline co2 laser oscillator apparatus and method - Google Patents

Abstract

APPARATUS AND METHOD
Abstract of the Disclosure An apparatus and method for producing a multiline output from a CO2 laser comprising an optical resonant cavity containing gaseous CO2, means for producing a controlled electrical glow discharge within the gas, such as Rogowski profile electrodes connected to a high voltage source, preferably mode locking means such as an acoustooptic modulator, and means within the cavity for producing a wavelength dependent loss, such as a Fabry-Perot etalon filter. The apparatus and method disclosed in the specification greatly increase the efficiency of energy extraction from large CO2 laser amplifiers such as those contemplated for use in inducing nuclear fusion.
The means for producing wavelength dependent loss within the laser oscillator cavity lowers the net gain of the usually dominant P(20) transition enough to allow the P(16), P(18), P(22), and P(24) transitions to successfully compete for available upper state population. In prior art pulsed laser oscillators, only the P(20) transition reached laser threshold because of its anomalously high gain coefficient at the expense of the remainder of the nearby rotational transitions. Thus in prior art lasers, the P(20) line dominated the output in all gain switched and mode locked Transverse Excited Atmospheric (TEA) laser oscillators, including electron beam controlled devices.

Description

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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.

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(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.

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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 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 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 ~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 ~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 ~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 ~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 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, ~(~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 ~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.

Claims (3)

What is claimed is:

1. A laser oscillator for producing a plurality of laser spectral lines useful for driving high power laser amplifiers comprising:
a conventional CO2 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 with the angular orientation of said means for filtering;
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 amplifiers.

2. The laser oscillator of claim 1 wherein said means for filtering comprises a Fabry-Perot etalon.

3. A laser oscillator for producing at least three output laser spectral lines to extract energy from a high power laser amplifier comprising:
a conventional CO2 gas discharge laser;
a Fabry-Perot etalon disposed within the optical cavity of said laser;
means for retaining said Fabry-Perot etalon at a predetermined angular orientation to fix the intensity of said plurality of output laser spectral lines produced by said laser in accordance with the transparency of said etalon for each spectral line frequency at said predetermined angular orientation;
whereby said plurality of output laser spectral lines have approximately equal output power levels to extract energy in a rapid manner from at least three excited rotational trausitions in said laser amplifier.