DYNAMICS,OF DISCHARGE-EXCITED CO 2 lasers ~~.~.~,-

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1 " I / J.;, ",' '....~.' ';I: DYNAMICS,OF DISCHARGE-EXCITED CO 2 lasers \ '. " CHINH BY liang, B'.Sc., M.Sc. ".' ~~.~.~,",- '~ ~ ; /' '\. \ -l.~ / \ -'( ',--_./, A Thesis Submitted to the School of Graduate Studies in Partial Fulfilment of the Requirements "Y',.)for the Degree Doctor of P~ilosOR~,. ' -, ',J McMaster University October 19M2

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3 ..../ ~ DOCTOR OF PHILOSOPHY (Physics) 1. ~.. ). TITLE: ~namics (.~ p~ ~ of Discharg~-Excite~)C02Lasers McMAST~ UNIVERSITY Hamilto~, Ontario AUTHOR: Chinh Dang, B.Sc. (Universi1;e d~ Sherbrooke) \ M.Sc. (McMaster University). -" SUPERVISORS: Professor B.K. Garside, Professor J. Reid \ NUMBER OF pages: xiv. 132 ) \ ---. \.... / i i 1!,

4 ---~.., 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 CO 2 lasers. The wide tunability ~f'the TOL is exploited in determfning the populations in any level of the CO 2 molecule. Thus, the vibrational populat3on distributions in a CW CO 2 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 CO 2 las~rs are also investigated. In particular, the electron excitation and de-excitation rat sof ~he upper las,er }evel 'i~ypic ',' r : CO 2, tlischargesare determined ~dir ctly from the experiment, and' ;\..,is shown that electron de-~xcitation is responsible for the saturation of ~, the V3 mode' temperature in e~ectric~d CO 2 lasers at high discharge currents, and imposes 'a~damental limitation on the gain attainable 'i n cb 2 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 available 2 \. decay channels. It is shown that the 02 level plays an important role in the relaxation of the lower laser, level. iii I 0' "

5 .;r-- (J 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 Dr. J.S. C~ang pleasure to thank each of the following people: 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. J. > iv "

6 .. ABSTRACT ACKNOWLEDGEMENTS CHAPTER TABLE of' CONTE~S 1.,INTRODUCTION 2. ENERGY:TRANSFER PROCESSES IN CO 2 LASERS 2.1 Introduction 2.2 CO Laser Transitions 2.3 El~ctron-Molecule Collisional Processes 2.4 Molecule-Molecule Collisional Processes 2.5 Radiation and Diffusion Ral~xation Processes 2.6, Sunrnary.l:--," f ' ".' 3. VIBRATIONAL POPULA1ION DISTRIBUTIONS IN CO 2 GLOW, DISCHARGES" ', , j Introduction Experimental Approac~ Theoretical Models,. 3.3.~ Boltimann Distribution Treanor.Distribution Measurements in a'.cw CO 2 Las~r AmPl(fier) Experimental Technique ( Vibrational Distributions CO? Dissociation Conclusion,Measurements 1n a CW CO - 2 Laser Oscillator Experimental Apparatus Vibrational Distributions Small-Signal Gain and Saturated G~in Vibrational Temperatur~ of CO ' Summary, v.. '. ' PAGE ii i iv "' -----, ":-- l.

7 -'0 TABLE OF CONTENTS (Cont'd) -, CHAPTER 4. VIBRATION~ RELAXATION OF THE CO 2 UPPER LASER LEVEL 4.1 Introduction 4.2 Relaxation Model for the Upper Laser Level 4.3 Relationships Between Relaxation Rates and Electron Excitation and De-Excitation'Rates 4.4 Experimental Apparatus,f 4.5 Relaxation Rates of the v1 Mode in Pure CO 2 and in CO~:N2:He Mixtures in Thermal Equilibrium Pure CO 2. ' ~.5.2 CO,:N,: He Mlxtures Temper~ture Dependence of 00 1 PAGE, Relaxation Rates ~~ 4.6, Relaxation Rates of the v 3 Mode in a Discharge CO 2 :He Mixt~re C02:~:He Mlxture 4.7 Summary, " VIBRATIONAL RELAXATION.OF THE COol LOWER LASER,' LEVELS ~.1 Introduction 5.2 Experimental Apparatus 5.3, Relaxation Rate Measurements in the v l and v 2 Modes of CO 5.4 Six-L~Ye1 Kinetic Model 5.5 Determination of the V-V Rate Constants 5.6 Discussion and Conclusions' 5.7 Summary 6. CONCLUSIONS,. " APPENDIX A. RELATIONSHIp BETWEEN ABSORPTION COEFFICIENT ANBJVtBRATIONAL LEVEL POPULATIONS 123, CURRENT DENSITY DISTRIBUTION IN A CYLINDRIGAL DISCHARGE TUBE ' 126 REFE ENCES 128 vi

8 ( " \" FIGURE LIST OF FIGURES PAGE 2.1 Detailed transition diagram of laser oscillation in the 10.4 pm and 9.4 urn regular laser bands of CO 2, Summary of the collisional relaxation rates in a laser discharge of,lo% CO?:lO% N 2 :80% He mixture~at 20'Torr with an electron density ot 1010OR~3. The solid ~ arrowed lines indicate molecule-molecule collisional processes whereas the dashed arrowed lines represent the electron-molecule collisional processes. Also shown are the regular and sequence 10 urn laser transitions.' Simplified vibrational energy level diagram of CO showing the three fundamental modes with their 2 associated mode temperatures. Also show~ are some, typical transition bands in the 2300 cm- region which are probed with the tunable diode. laser. 21 ' 3.2 Photograph of the mounting of the tunable diode laser inside the cold head chamber Schematic diagram'of the apparatus. 27.; Typical TDl 'scans taken with the laser beam focussed through the discharge tube. The upper scan is taken with no discharge current, while the lower scan corresponds to a current of 10 rna in a 10% CO~:38% N~: 52% He mixture at 15 Torr. The more impo tant a sorption lines are labelled beneath the traces Vibrational population distributions in the v 3 mode of CO at discharge currents of 5 and 25 mao Data point& are experimental measurements made with the TDl, while the solid lines are calculated Treanor distributions. The dashed line represents a Boltzmann distribution at 1600 K, and appears as' a straight. line in this semi-log plot Repeat of Fig. 3.5 for*a 2% CO 2 :20% N 2 :78% He mixture. Note the increase in T 3 which occurs as the CO 2 34 ' content is decreased from 10% in Fig. 3.5., vii 1_..

9 , I LIST OF FIGURES (Cont'd).. FIGURE PAGE / 3.7 " Vibrational populafi~~ lstributions in the 'v l and v 2 modes of CO 2.at di scharge currents of. 5 and 25 mao Data points are experimental. measurements for the levels indicated in the figure while the solid lines represent Boltzmann distributions for given T l '= T 2 tempera:ures. 36, Vibrational pop~lation distributions in the combination levels Ollk, 02 0 k and look with k =0 to'3 at 25 mao The mode temperatures as indicated in the figure are dedu~d from the~combination levels shown. A good agr~ment with T 3 = 2250 K in Fig. 3.5 is clear~ seen. Experimental values of T, T (= T ) and T as a function of discharge cu~ren~. ThJ various measurements of T represent results calculated from different pairs of vibrational-rotational transitions in two different vibrational bands ~hematic diagram of the apparatus used to study vibrational populations in the presence of a strong laser field. The solid line indicates the path of the tunable diode laser beam, whereas the dashed line represents the CO? laser beam. The CO 2 laser cavity is formed by th~ grating G, and two highly reflecting 10 ~m mjrrors, M] and M 2.. The small absorber cell selectively adsorbs the 10 ~m radiation and prevents it from reaching the dqode ~., 43 \ Typical TDL laser scans showing the effect of 10 ~m laser radiation on the vibrational populations. lasing and no lasing refer; to the presence or absence of a saturating HI ~m P(22) field with intensity of 560 W/cm z. M0E)0f (the absorpti on lines are identified at the bottom f the fi~ure. Lines A and Bare associated with ~3.mode (upper 10 ~m laser level), while line C is coupled to the tower laser level. The short discharge tube contained a 9.2% CO 2 :1l%'N 2 :79,8% He mixture at 19.4 Torr total pressure. The dtscharge current was 10 mao ~.12 Vibrational populationjdistributions in the v 3 mode of CO for a discharg~-current of 25 mao Lasing and no la~ing refer.0 the presence or absence of a saturating 10 ~m P(22) field with intensity of 560 W/cm z. The dramatic reduction of the v3 mode level populations with lasing is clearly display~d. 46 vi i i 45

10 FIGURE LIST OF FIGURES (Cont'd).. PAGE 3.13 Vibrational population distributions in the v1 and v modes of CO at discharge current of 25 mao Again, l~sing and no fasing refer to the presence or absence of a saturating 10 pm P(22) field with intensity of 560 W/cm 2 Only a small increase of level populations is observed upon lasing Repeat of Fig with a much lower gas'pressure for two cases: (a) no laser field is present; (b) a 10 pm P(22) field with intensity of 500 W/cm 2 is present. Note that the upper laser level, , has a population smaller than that expected from a Boltzmann distribution Repeat of Fig with a much lower 9as pressure for three cases: (a) no laser field is present; (b) a 10 pm P(22) field with inten~ity of 500 W/cm 2 is present; (c) a 9 pm P(22) field with intensity of lob W/cm 2 is present. Note that the lower laser levels, and , have anomalously hi9h pop~ under the 10 pm and 9 pm 1asings respectively Vibrational and rotaticna1 temperatures as a function of discharge current in the presence of an intense 10 pm field with intensity of 560 W/cm B : Vibrational and rotational temperatures as a function of discharge current in the absence of ~ laser field. Note the saturation of T 3 at high discharge current. -~ Calculated small signal gain, Yo' and satur ed gao, Ys' as a function of discharge current using the red temperatures displayed in Figs and Also shown 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 discharge current with and witnout the pre ence of a 10 pm laser field. 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 CO 2 :N 2 :He mixture. 3 Note th~t the T 3 temperature~ in a CO:N:H mixture are clearly higher than those in a CO~:C~:H$ mixture for both lasing and no lasing conditions. ix ] ' ".:{ I,

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HIGH SPEED INTERDIGITAL MSM PHOTODIODES. RICHARD JOHN SEYMOUR, B.Sc., M.Eng. A Thesis. Submitted to the School of Graduate Studies

HIGH SPEED INTERDIGITAL MSM PHOTODIODES '.S) By RICHARD JOHN SEYMOUR, B.Sc., M.Eng. A Thesis Submitted to the School of Graduate Studies in Partial Fulfillment of the Requirements for the degree Doctor