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10-1-1982

Dynamics of a Discharge-Excited CO₂ LasersChinh Dang

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Recommended CitationDang, Chinh, "Dynamics of a Discharge-Excited CO₂ Lasers" (1982). Open Access Dissertations and Theses. Paper 1534.http://digitalcommons.mcmaster.ca/opendissertations/1534

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DYNAMICS ,OF DISCHARGE-EXCITED CO2 lASERS

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CHINH liANG, B'.Sc., M.Sc." .'

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A Thesis

Submitted to the School of Graduate Studies

in Partial Fulfilment of the Requirements

"Y',.)for the Degree '

Doctor of P~ilosOR~, .

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McMaster University

October 19M2

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DOCTOR OF PHILOSOPHY(Physics)

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McMAST~ UNIVERSITYHamilto~, Ontario

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~namics of Discharg~-Excite~)C02Lasers

Chinh Dang, B.Sc. (Universi1;e d~ Sherbrooke)\ M.Sc. (McMaster University) .

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-"SUPERVISORS: Professor B.K. Garside, Professor J. Reid

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NUMBER OF pAGES: xiv. 132

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/ABSTRACT.. '

A tunable diode laser (TOLl 'operating in the region of 2150 to-1 - ,

2350 cm wavenumber is used in this work to investigate the dynamics

of level pOp'ulations in CO2 lasers. The wide tunability ~f'the TOL is

exploited in determfning the populations in any level of the CO2 molecule.

Thus, the vibrational populat3on distributions in a CW CO2 laser d1s- -/

charge under both lasing and n~n-lasi~g conditions are measured and~.,compared with the mode temperature model.

, .. .The relaxation processes associated with the upper and lower;)'

levels of CO2 las~rs are also investigated. In particular, the electron

excitation and de-excitation rat sOf ~he upper las,er }evel 'i~YPiC ',' •

: CO2, tlischargesare determined ~dir ctly from the experiment, and' ;\..,is rshown that electron de-~xcitation is responsible for the saturation of ~ ,

the V3 mode' temperature in e~ectric~d CO2 lasers at high dis-

charge currents, and imposes 'a~damental limitation on the gain attain-

able 'i n cb2 1asers.

The relaxation of the lower laser leve~ 'is complicated by t~e

occurrence of several competitive vibration-vibr~V-V) processes.

The TOL measurements enable us, for the irs~ time, to monitor separately

'the population in all levels of cone rn t~ the relaxation of the lower

laser level and to determine th rate constant of each of the available2 \.

decay channels. It is shown that the 02 °level plays an important role0' ...

in the relaxation of the lower laser, level./'

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ACKNOWLEDGEMENTS, '. '\

~/I would lik~"to'express my deep appreciation to my 'supervisors,

Dr. 'B.K. Garsidi:-~'nd Dr. J. Reid"jor their helpful'advitEe\thr.oughout(/. , "'-\

the course'~f thi s work. '

It is ~y pleasure to thank each of the following people:

Dr. J.S. C~ang for the loan'bf the storage scope used in the experiment;

Mr. R.K. Brimacombe fO~ ~is Ctiticai~~i~g. ~hiS ~anuscript; ~~d

Mrs. G. Wang for her excellent typing of this thesis., ~

At last, to my parents, a very special thank-you for everything.. that they have done and given 0&0 me, throughout my 1He.

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TABLE oF' CONTE~S '

PAGE

ABSTRACT·

ACKNOWLEDGEMENTS

CHAPTER

1. ,INTRODUCTION

• 2. ENERGY:TRANSFER PROCESSES IN CO2 LASERS

2.1 Introduction2.2 CO Laser Transitions2.3 El~ctron-Molecule Collisional Processes2.4 Molecule-Molecule Collisional Processes2.5 Radiation and Diffusion Ral~xation Processes2.6, Sunrnary .l:-- ," f ' " .'

3. VIBRATIONAL POPULA1ION DISTRIBUTIONS IN CO2 GLOW, DISCHARGES" ' ,

3.13.23.3

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IntroductionExperimental Approac~

Theoretical Models3.3.~ Boltimann Distribution3.3.2 Treanor.DistributionMeasurements in a' .CW CO2 Las~r AmPl(fier)3.4.1 Experimental Technique (3.4.2 Vibrational Distributions .3.4.3 CO? Dissociation3.4.4 Conclusion

-,Measurements 1n a CW CO2 Laser Oscillator3.5.1 Experimental Apparatus3.5.2 Vibrational Distributions •3.5.3 Small-Signal Gain and Saturated G~in

3.5.4 Vibrational Temperatur~ of CO 'Summary

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TABLE OF CONTENTS (Cont'd)- ,

CHAPTER PAGE

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Relaxation Rates ~~,.764.6 , Relaxation Rates of the v3 Mode in a Discharge

4.6.1 CO2:He Mixt~re

4.6.2 C02:~:He Mlxture

4.7 Summary ,

4. VIBRATION~ RELAXATION OF THE CO2 UPPER LASER LEVEL

4.1 Introduction4.2 Relaxation Model for the Upper Laser Level4.3 Relationships Between Relaxation Rates and Electron

Excitation and De-Excitation'Rates4.4 Experimental Apparatus ,f

4.5 Relaxation Rates of the v1 Mode in Pure CO2 and inCO~:N2:He Mixtures in Thermal Equilibrium

4.5.1 Pure CO2 . '~.5.2 CO,:N,: He Mlxtures 04.5.3 Temper~ture Dependence of 00 1

...5. " VIBRATIONAL RELAXATION .OF THE COol LOWER LASER,' LEVELS

~.1 Introduction5.2 Experimental Apparatus5.3, Relaxation Rate Measurements in the vl and v2 Modes

of CO5.4 Six-L~Ye1 Kinetic Model5.5 Determination of the V-V Rate Constants5.6 Discussion and Conclusions'5.7 Summary

6. CONCLUSIONS

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APPENDIX

A. RELATIONSHIp· BETWEEN ABSORPTION COEFFICIENT ANBJVtBRATIONALLEVEL POPULATIONS 123 ,­CURRENT DENSITY DISTRIBUTION IN A CYLINDRIGAL DISCHARGETUBE ' 126

REFE ENCES 128

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LIST OF FIGURES

FIGURE PAGE

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2.1 Detailed transition diagram of laser oscillationin the 10.4 pm and 9.4 urn regular laser bands of CO2, 7

2.2 Summary of the collisional relaxation rates in a laserdischarge of,lO% CO?:lO% N2 :80% He mixture~at 20'Torrwith an electron density oT 1010OR~3. The solid ~arrowed lines indicate molecule-molecule collisionalprocesses whereas the dashed arrowed lines representthe electron-molecule collisional processes. Alsoshown are the regular and sequence 10 urn laser transi-tions.' 11

3.1 Simplified vibrational energy level diagram of CO2showing the three fundamental modes with theirassociated mode temperatures. Also show~ are some ,typical transition bands in the 2300 cm- region whichare probed with the tunable diode laser. 21. '

3.2 Photograph of the mounting of the tunable diode laserinside the cold head chamber. 26

3.3 Schematic diagram'of the apparatus. 27.;..

3.4 Typical TDl 'scans taken with the laser beam focussedthrough the discharge tube. The upper scan is takenwith no discharge current, while the lower scancorresponds to a current of 10 rnA in a 10% CO~:38%

N~: 52% He mixture at 15 Torr. The more impo tanta sorption lines are labelled beneath the traces. 29

3.5 Vibrational population distributions in the v3

modeof CO at discharge currents of 5 and 25 mAo Datapoint& are experimental measurements made with theTDl, while the solid lines are calculated Treanordistributions. The dashed line represents a Boltzmanndistribution at 1600 K, and appears as' a straight

. line in this semi-log plot. 32

3.6 Repeat of Fig. 3.5 for*a 2% CO2:20% N2:78% He mixture.Note the increase in T3 which occurs as the CO2 34 'content is decreased from 10% in Fig. 3.5.,

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LIST OF FIGURES (Cont'd) ..

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Vibrational pOPulafi~~ lstributions in the 'vland v2 modes of CO2 .at di scharge currents of .5 and 25 mAo Data points are experimental.measurements for the levels indicated in thefigure while the solid lines represent Boltzmanndistributions for given Tl '= T2 tempera:ures.

Vibrational pOP~lation distributions in the combina­tion levels Ollk, 020k and lOok with k =0 to'3at 25 mAo The mode temperatures as indicated inthe figure are dedu~d from the~combination levelsshown. A good agr~ment with T3 = 2250 K inFig. 3.5 is clear~ seen.

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Experimental values of T , T (= T ) and T as afunction of discharge cu~ren~. ThJ various measure­ments of T represent results calculated fromdifferent pairs of vibrational-rotational transi­tions in two different vibrational bands.

3.10 ~hematic diagram of the apparatus used to studyvibrational populations in the presence of a stronglaser field. The solid line indicates the pathof the tunable diode laser beam, whereas the dashedline represents the CO? laser beam. The CO2 lasercavity is formed by th~ grating G, and two highlyreflecting 10 ~m mJrrors, M] and M2.. The smallabsorber cell selectively aDsorbs the 10 ~m

radiation and prevents it from reaching the dqode~. ,

3.11 Typical TDL laser scans showing the effect of 10 ~m

laser radiation on the vibrational populations.lasing and no lasing refer; to the presence orabsence of a saturating HI ~m P(22) field with intensityof 560 W/cmz. M0E)0f (the absorpti on li nes are identi­fied at the bottom f the fi~ure. Lines A and Bareassociated with ~3.mode (upper 10 ~m laser level),while line C is coupled to the tower laser level. Theshort discharge tube contained a 9.2% CO2:1l%'N2 :79,8%He mixture at 19.4 Torr total pressure. The dTschargecurrent was 10 mAo

~.12 Vibrational populationJdistributions in the v3 mode of

CO for a discharg~-current of 25 mAo Lasing and nola~ing refer .0 the presence or absence of a saturating10 ~m P(22) field with intensity of 560 W/cmz. The dramaticreduction of the v3 mode level populations with lasingis clearly display~d.

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LIST OF FIGURES (Cont'd)..

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3.13 Vibrational population distributions in the v1 andv modes of CO at discharge current of 25 mAo Again,l~sing and no fasing refer to the presence or absenceof a saturating 10 pm P(22) field with intensity of560 W/cm 2 • Only a small increase of level populationsis observed upon lasing. 47

3.14 Repeat of Fig. 3.12 with a much lower gas'pressure fortwo cases: (a) no laser field is present; (b) a 10 pmP(22) field with intensity of 500 W/cm2 is present.Note that the upper laser level, 0001, has a populationsmaller than that expected from a Boltzmann distribution.

3.15 Repeat of Fig. 3.13 with a much lower 9as pressure forthree cases: (a) no laser field is present; (b) a 10 pmP(22) field with inten~ity of 500 W/cm2 is present;(c) a 9 pm P(22) field with intensity of lOB W/cm2 ispresent. Note that the lower laser levels, 1000 and0200, have anomalously hi9h pop~ under the 10 pmand 9 pm 1asings respectively. 50

3.16 Vibrational and rotaticna1 temperatures as a functionof discharge current in the presence of an intense10 pm field with intensity of 560 W/cm2 •

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3.1B:

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Vibrational and rotational temperatures as a function ofdischarge current in the absence of ~ laser field. Notethe saturation of T3 at high discharge current.-~

Calculated small signal gain, Yo' and satur ed gaO , Ys'as a function of discharge current using the redtemperatures displayed in Figs. 3.16 and 3.17. Alsoshown is the power density, P., extracted from the'short discharge due to stimu1~ted emission at 10 pm •.

Measured "values 'Of T::l and Tc; as ~'function of dischargecurrent with and witnout the pre ence of a 10 pm laserfield. Note the substantial cha ge in T::l with lasing;whereas T5 is only reduced by a ~ a11 amount.

Compari son between Tin a CO : CO :He mi xture and Tin •a CO2:N2:He mixture. 3 Note th~t the T3 temperature~ ina CO:N:H mixture are clearly higher than thosein a CO~:C~:H$ mixture for both lasing and no lasingconditions.

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