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A parametric study of a transverse gas flow TEA CO2 laser Laidley, Thomas Edward 1973

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A PARAMETRIC STUDY OF A TRANSVERSE GAS FLOW TEA C02 LASER by THOMAS E. LAIDLEY B.Sc., M e m o r i a l U n i v e r s i t y o f N e w f o u n d l a n d , 1971 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n t h e D e p a r t m e n t o f PHYSICS We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA J u n e , 1973 In presenting t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree that permission f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . I t i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission. Department of The U n i v e r s i t y of B r i t i s h Columbia Vancouver 8, Canada Date ABSTRACT A transverse gas flow TEA C02 laser has been constructed and the use of a perforated hollow rod to serve simultaneously as the cathode of the discharge and the gas i n l e t vessel was successfully demonstrated. The e l e c t r i c a l e f f i c i e n c y of the laser is t y p i c a l l y 3% with peak powers of 40 kw being emitted on the P(20), P(18) and P(16) lines of the C02 spectrum. A parametric study of peak power, gas pressure, gas com-po s i t i o n , time delay of the laser pulse and the i n t e r -dependence of these quantities was undertaken. TABLE OF CONTENTS Page ABSTRACT i i LIST OF TABLES v LIST OF FIGURES vi ACKNOWLEDGEMENTS , v i i i Chapter 1 INTRODUCTION 1 1.1 Properties of C02 Lasers 1 1.2 Arrangement of Thesis 5 2 ELECTRODE DESIGN AND LASER CONSTRUCTION . . . . 7 2.1 Electrode Design 7 2.2 Laser Construction . . . . . 13 2.2.1 Reaction tube 13 2.2.2 Electrodes 14 2.2.3 Gas supply and extraction 17 2.2.4 Laser cavity 18 3 CARBON DIOXIDE LASERS 21 3.1 Vibrational Energy Levels of C02 21 i i i Chapter Page 3.2 Excitation and Inversion in C02 24 3.3 Present State of the Art 28 4 CHARACTERISTICS OF LASER 31 4.1 Aligning C02 Laser 31 4.2 Operating Conditions 34 4.2.1 The e l e c t r i c a l system . 34 4.2.2 Effect of gases and pressure. . . . 36 4.3 Properties of System 40 4.3.1 Power decay in a closed system. . . 40 4.3.2 E l e c t r i c a l e f f i c i e n c y 42 4.3.3 Time delay of pulse 45 4.3.4 Spectral analysis 48 5 CHEMICAL LASER. . . 50 6 CONCLUSIONS AND FUTURE IMPROVEMENTS 55 REFERENCES 58 APPENDICES A INFRARED DETECTORS 62 B SPECTRAL ANALYSIS 74 i v LIST OF TABLES Tab! e Page I Induced Vibrational Relaxation Rates of the C02 v 3 and v 2 Modes in the Presence of Other Gases at 300°K 26 II Gas Mixtures and Operating Conditions for Chemical Laser 51 A-l Characteristics of Infrared Detectors 67 B-I Spectral Lines Used for Spectrometer Calibrat i o n 77 B-II Raw Data from Spectral Analysis 79 B-IH I d e n t i f i c a t i o n of C02 Lines . . . 81 B-IV C02 Laser Transistions 82 v LIST OF FIGURES Figure Page 1. Gas Flow in C02 Lasers 4 2. Cross-Section Through C y l i n d r i c a l Test Chamber and Schematic View of Discharge C i r c u i t 8 3. Spark Channels in Text Chamber 10 4. Multiframe Exposure to Test Reproducibility o f Discharge 13 5. Brewester Angle Mount and Gas Inlet Coupling 15 6. Cross-Sectional View of Reaction Tube 16 7. E l e c t r i c a l System 17 8. Vacuum System 19 9. Vibrational Modes of C02 21 10. Energy Level Diagram for C02 and N2 (showing only those vibrational l e v e l s important for C02 lasers) 23 11. Rotation-Vibration Transistions 24 12. Alignment System 31 vi Figure Page 13. Multiple Lasing with Low Net Round T r i p G a i n 33 14. C02 Pulse Shape Parameters 33 15. Spark Channels and Laser Pulses 35 16. C02 Laser Pulse with Gas Additives 37 17. Peak Power vs. Total Pressure 39 18. Power Decay in Closed System 41 19. E l e c t r i c a l E f f i c i e n c y Curve 44 20. Time Delay vs. Total Pressure. . . 46 21. Modulation of C02 Laser Pulse 48 A-l Photoconductive Detectors 64 A-2 Au-Ge Detector C i r c u i t ; Output via 50ft Cable 68 A-3 Response of Infrared Detectors to a C02 Laser Pulse 69 A-4 Au:Ge Power Detector Output 70 A-5 Near Fiel d Radiation Pattern of Laser 72 B-l Spectrometer Calibration Curve 76 B-2 Spectral Analysis Arrangement 78 vi i ACKNOWLEDGEMENTS I would l i k e to take this opportunity to thank Dr. Boye Ahlborn for suggesting and patient l y supervising the torturous course of this work. On more than one occasion my sanity has been preserved by Dr. Shigeo Mikoshiba and other members of the Plasma Physics group and to these people I would l i k e to extend my heartiest thanks. Many thanks also go out to Mr. Dick Haines for his workshop in s t r u c t i o n and assistance. I would also l i k e to thank Frances for putting up with me when things went bad, footing those large telephone b i l l s and not carrying out her threat of divorce. The f i n a l preparation of this thesis was com-pleted with Dale Stevenson at the drafting table and Shari Haller at the typewriter keyboard. Financial assistance from the National Research Council is g r a t e f u l l y acknowledged. This work is supported by a grant from the Atomic Energy Board of Canada. v i i i Chapter 1 INTRODUCTION This thesis describes a parametric study of a C 0 2 laser carried out to acquire knowledge and experience in the operation of molecular lasers and infrared detectors, not previously studied in this laboratory. In order to obtain the highest f l e x i b i l i t y in the application of such lasers for future use, i t was the aim to design a laser cavity which would allow the study of e l e c t r i c a l as well as chemical activation and approach with modest means as high average laser powers as possible. 1.1 Properties of C02 Lasers Infrared lasers have attracted strong in t e r e s t in communications, plasma physics and spectroscopy for several reasons: The e f f i c i e n c y is high (theoretical maximum of 41% ) because the energy of the upper laser * Quantum e f f i c i e n c y of 00°l-02°0 t r a n s i s t i o n i s 41%. 1 2 level is only ~ 1/3 ev above the ground l e v e l . The energy difference between the upper laser level (00°1) and the lower laser level (02°0) is also rather small so the wave-length (10.6u) f a l l s in the 8-13u region where there is an atmospheric window making the C02 laser a t t r a c t i v e for communications purposes. The long wavelength of infrared radiation is also desirable and bene f i c i a l for inter-ferometry in laser scattering experiments and the produc-tion of plasmas. Laser emission from C02 has been obtained in both continuous wave (cw) and pulsed operation. The popu-l a t i o n inversion necessary for laser action can be obtained by electron impact ( e l e c t r i c a l discharges), supersonic expansion (Laval nozzles) and resonant energy transfer (N2 or DF). For the case of electron impact the parameter E/N governs the e f f i c i e n c y of the laser [1,2]. E/N is porportional to the energy electrons acquire, on the average, between two c o l l i s i o n s in an e l e c t r i c f i e l d E, with neutral p a r t i c l e density N. In theory E/N may be optimized for any e l e c t r i c a l l y excited C02 laser but in practice this has only been achieved in TEA (Transversely Exited at Atmospheric Pressures) l a s e r s . Such a TEA laser configuration consists of a multispark discharge at right 3 angles to the laser resonator a x i s , as w i l l be described l a t e r in more d e t a i l . This type of laser has been used to obtain laser action in various chemicals by chemical reactions [ 4 ] - [ 7 ] . The TEA laser stands in another respect at the end of the present development of C0 2 l a s e r s . The f i r s t C0 2 lasers operated at pressures of several torr and used r . f . or a.c. excitation between electrodes at opposite ends of a glow discharge tube. If the laser had a flowing gas system the input and exhaust ports were invariably at opposite ends of the tube so the gas flow was l o n g i t u d i n a l , Figure 1 (a). For low capacity pumping systems this arrangement had two d e f i n i t e disadvan-tages: Dissociation products formed in the discharge were not removed quickly enough [ 8 , 9 ] and the heating of the gas i t s e l f reduces the gain of the laser [ 1 0 ] . TEA arrangements combine s i m p l i c i t y in construc-tion and operation and the f l e x i b i l i t y of being able to obtain e l e c t r i c a l as well as chemical a c t i v a t i o n . However, in most devices with longitudinal flow the gas exchange rate is not very high and one cannot expect a high r e p e t i -tion rate or high average powers, Figure 1 (b). A device by Ahlborn et al. [ 7 ] with transverse gas flow through the ground electrode improves this s i t u a t i o n , but only at the cost of a rather complicated construction, Figure 1 ( c ) . F i g u r e 1. Gas F l o w i n C 0 2 L a s e r s . (a) l o n g i t u d i n a l f l o w , l o n g i t u d i n a l d i s c h a r g e (b) l o n g i t u d i n a l f l o w , t r a n s v e r s e d i s c h a r g e ( c ) t r a n s v e r s e f l o w , t r a n s v e r s e d i s c h a r g e (d) s i m p l i f i e d v e r s i o n o f ( c ) t e s t e d i n t h i s t h e s i s 5 The aim of this thesis was to design a TEA laser with transverse gas flow but with a greatly s i m p l i f i e d construction: eliminating the 200 gas i n l e t tubes of reference [ 7 ] and replacing them by a hollow ground bar with very small gas i n l e t holes opposite the pin electrodes, Figure 1 (d), the contention being that the exciting or activating transverse sparks would issue exactly from the holes in the ground electrode and hence guarantee that the replacement gas was flowing exactly into the region where i t was needed. 1.2 Arrangement of Thesis The organization of this thesis r e f l e c t s the purpose of the work: Test a new electrode configuration, design a laser and study i t s performance. Chapter 2 discusses preliminary experiments with hollow ground electrodes and gives the laser design derived from these i n i t i a l experiments. In order to appreciate the performance of this newly designed l a s e r , we have summarized in Chapter 3 some of the standard knowledge about C0 2 l a s e r s , which may be useful for someone just getting interested in this f i e l d and we try to show the l i m i t s of the present understanding of this l a s e r . 6 Chapter 4 begins with typical experimental " t r i c k s of the trade" and procedures to operate the laser and then gives the parametric study of the device operated as a transversely pulsed C0 2 l a s e r . Chapter 5 describes attempts to obtain chemical reactions in the cavity and Chapter 6 discusses conclusions and suggestions for the possible improvement of the device. * Many experimental d e t a i l s are contained in the appendices in order not to load the main body of the thesis with too much d e t a i l . which are standard knowledge but may save a future investigator much time. Chapter 2 ELECTRODE DESIGN AND LASER CONSTRUCTION 2 .1 Electrode Design It is attempted in this work to construct a TEA laser with transverse gas flow using the idea of in j e c t i n g the gas through holes in a hollow ground bar d i r e c t l y into the spark channels. To ascertain the f e a s i b i l i t y of this idea a study of the behaviour of spark channels created by a discharge between a chain of r e s i s t o r pins and a hollow ground rod with many holes was undertaken in a small scale experiment. For this purpose the test chamber shown in Figure 2 was constructed. The chamber was made e n t i r e l y of l u c i t e and enclosed an evacuated c y l i n d r i c a l region 7 cm high and 12 cm in diameter. On the top plate of the chamber in two staggered rows of 10 were place 20, 1 kfi each, r e s i s t o r s . The interelectrode distance, A (see Figure 2), was fixed at 0.5 cm for each ground rod constructed and the r e s i s t o r s were vacuum sealed using Apezion Q-compound. 7 F igure 2. C r o s s - S e c t i o n Through C y l i n d r i c a l Test Chamber and Schematic View of Discharge C i r c u i t . 9 The hollow ground bar to be tested was placed inside the chamber affixed to the bottom plate and aligned p a r a l l e l to the r e s i s t o r pins. The cathode-anode separation, d, was fixed at 2.54 cm for each ground rod tested. Twenty small holes were made in the top of the ground bar, one opposite each r e s i s t o r pin. The purpose of these holes was two-fold: to l o c a l i z e the arcs and i n j e c t the gas flow. Since a sharp edge has a small radius of curvature, large e l e c t r i c f i e l d strengths are p r e f e r e n t i a l l y created at the l i p of the holes and thus the discharge from the legs of the r e s i s t o r s would s t r i k e the edges of the holes, as shown in Figure 2. At the same time i t is possible to have a flow of gas, in this case a i r , from the atmosphere, through the holes in the hollow ground bar, into the spark channels afte r which i t is evacuated from the chamber through a hole in the top plate by a mechanical forepump. Photographs of the discharge were taken using a poloroid Land camera with a close-up lens and polaroid f i l m type 47, 3000 speed. The photos were q u a l i t a t i v e l y analyzed for uniformity and r e p r o d u c i b i l i t y of the d i s -charge pattern. In addition the r e p e t i t i o n rate was measured using a Rogowski c o i l and a type 551 Tektronix dual-beam o s c i l l o s c o p e . A photograph of a typical d i s -charge pattern is shown in Figure 3. F i g u r e 3- Spark C h a n n e l s i n T e s t Chamber a t 100 T o r r . Two d i f f e r e n t hollow grounding rods were tested. The f i r s t was constructed of 1/32" thick copper sheet, 10 cm long, which was bent and soldered into a traingular cross-section. The holes to serve as gas i n l e t s were made by simply puncturing the copper sheet with a n a i l , from the i n s i d e , so that the result i n g sharp edges were directed towards the r e s i s t o r pins. The second rod was a c y l i n d r i c a l copper pipe (3/4" i . d . , 7/8" o.d.), also 10 cm long, with 20 holes made with #65 d r i l l b i t (0.035"). The spark channels produced with these electrodes did indeed p r e f e r e n t i a l l y s t r i k e the l i p s of the holes in the ground bar but the spark channels had some tendency to inte r f e r e with one another. This was probably due to some i r r e g u l a r i t i e s in the spacing of the r e s i s t o r pins and the small inter-electrode distance, A, used. Since this interference was more noticeable with the more complicated t r i a n g u l a r l y shaped ground bar, a l l further observations made here were carried out with the c y l i n -d r i c a l l y shaped ground bar, which is shown in Figure 2. 11 The spark channels were photographed at pressures of .2, 60 and 100 t o r r , as measured by the Speedivac vacuum gauge, with and without the a i r flowing. There appeared to be no differences in the symmetry of the spark channels with the a i r s t a t i c and flowing at the pressures used. The ef f e c t of a i r pressure though was quite pronounced. At pressures of .2 t o r r the entire region between the cathode and anode is ionized and there are no d i s t i n c t spark channels. For pressures of 60 and 100 torr individual sparks are evident with those of 100 torr being more uniform. At higher pressures the number of spark channels decreases u n t i l there is a single arc and f i n a l l y no a i r breakdown at a l l . For these experiments we used the breakdown voltages of 14, 17 and 24 kv. The larger the voltage the higher the pressure attainable before the a i r ceases to break down. At 24 kv the highest pressure measured was 160 t o r r . The negative d.c. potential used to break down the a i r was applied to the r e s i s t o r chain and also to the hollow ground rod. However, this only affected the discharge pattern at low pressures. When the high voltage was applied to the r e s i s t o r chain each r e s i s t o r leg developed a corona whereas when the hollow rod was con-nected to the high voltage i t exhibited 3 to 4 hot spots on i t s surface. 12 The values of R and C used in the discharging c i r c u i t were also varied. Resistances of 10 and 20 Mfi and capacitances of 5 and 0.5 nF were used. These variations of R and C did not affect the spark channel patterns but changed the discharge r e p e t i t i o n rate. Since the spark gap was not externally triggered and the discharge time << the charging time, the r e p e t i t i o n rate is given by: f = 1 - RC £n S.G (2.1) where: V. - = breakdown voltage of spark gap. V0 = power supply voltage. The maximum re p e t i t i o n obtained was 260 pulses/ sec. The r e p r o d u c i b i l i t y of the discharge up to the highest r e p e t i t i o n rate was v e r i f i e d by rotating the camera quickly with the shutter open. This is a rather primitive smear camera technique but adequate for our purposes. The photograph shown in Figure 4 was obtained in this fashion. F i g u r e 4. M u l t i f r a m e E x p o s u r e t o T e s t R e p r o d u c i b i l i t y o f D i s c h a r g e . 2.2 L a s e r C o n s t r u c t i o n The p r e l i m i n a r y s t u d i e s o f m u l t i - s p a r k d i s c h a r g e s l e d to the f o l l o w i n g c o n c l u s i o n s : 1. Holes in the hollow ground bar l o c a l i z e d the spark channels q u i t e w e l l , with and without t r a n s v e r s e gas fIow. 2. The hoi low ground rod may be used as cathode or anode at p ressures above 60 t o r r . At lower pressures the bar developes only a few spots i f operated as the a node. 3. The d i s c h a r g e p a t t e r n is r e p r o d u c i b l e at r e p e t i t i o n r a t e s of at l e a s t 260 Hz. Encouraged by t h e s e r e s u l t s we d e s i g n e d a TEA l a s e r w i t h a p e r f o r a t e d h o l l o w bar e l e c t r o d e i n an o t h e r -w i s e s t a n d a r d arrangement. 2.2.1 R e a c t i o n t u b e . A l u c i t e tube (2.5" i . d . , 3.0" o.d.) 1.2 m l o n g was used to c o n t a i n the l a s e r ' s a c t i v e medium. F l a n g e s , 14 also made of l u c i t e , were cemented to each end of the tube. Potassium chloride ( K C £ ) windows (2" <j>) were placed on Brewster angle mounted supports at each end of the tube and were 0-ring sealed by the pressure d i f f e r e n t i a l between the atmosphere and the gas mixture in the tube. To feed gas into the tube a brass coupling was attached to one end (see Figure 5). To ensure mechanical s t a b i l i t y the reaction tube was secured to a 2.13 m long steel I-beam which was bolted to a 2.5 m long steel frame. This arrangement proved to be quite adequate. 2.2.2 Electrodes. The anode of the discharge is composed of 200-lkfi-lw Allan Bradley r e s i s t o r s (they l a s t longer!) connected in p a r a l l e l and mounted in two staggered rows of 100 on the top of the reaction tube. The r e s i s t o r legs protrude into the reaction tube through holes (#53 d r i l l b i t ) spaced 5 mm apart and are vacuum sealed with S i l a s t i c TRV cement and Apezion Q-compound. Using the results of our preliminary studies a copper rod (3/4" i . d . , 7/8" o.d.) 1.14 m long with 200 holes (0.01" <|>) at interva l s of 5 mm was used as the cathode. The rod was aligned p a r a l l e l to the r e s i s t o r s pins and adjusted so the r e s i s t o r legs were opposite the holes in F i g u r e 5. B r e w e s t e r A n g l e Mount and Gas I n l e t C o u p l i n g . cn the copper rod. The rod is e l e c t r i c a l l y connected to ground through the brass coupling used to feed gas into the reaction tube. The cathode-anode separation was fixed at d = 1 . cm. A cross-sectional view of the reaction tube is shown in Figure 6 . Figure 6. C r o s s - S e c t i o n a 1 View of Reaction Tube. 17 The c i r c u i t used to i n i t i a t e the discharge i s similar to the one used in our f e a s i b i l i t y studies and is shown in Figure 7. No external trigger is used for f i r i n g the spark gap. SPARK 25 M GAP Figure 7. Electrical System 2.2.3 Gas supply and ex t r a c t i o n. The flow rate of each gas that enters the reaction tube is found by making use of the P o i s e u i l l e e f f e c t . From i t s storage cylinder the gas flows through a c a p i l l a r y tube, 5 cm long, with the resulting pressure drop across the tube being measured by a U-shaped manometer with 76 cm long arms. Using c a p i l l a r y tubes of r a d i i 0.5, 1.0, 2.0 and 3.0 mm with n-butyl phathlate (p = 1.047 gm/cc) i t is possible to obtain flow rates from 10"" l/sec to 102 i/sec. 18 The flowmeter board constructed was comprised of four such manometers. From the flowmeter board the gases enter a mixing tank and then enters the reaction tube through the afore-mentioned brass coupling. A small piece of 1/2" polyflo connects the brass coupling to the copper tube used as cathode. The used gas is extracted through 20 exhaust ports (3/8" i.d.) spaced at interva l s of 4.8 cm along the topside of the l u c i t e tube (see Figure 6) by a mechanical forepump. An overall view of the vacuum system is shown in Figure 8. 2.2.4 Laser c a v i t y . The 2 m long laser c a v i t y , discussed in more detail in Section 4.1 (see Figure 12), consists of a gold coated concave mirror (100%, R = 10 m) and a Germanium f l a t with an experimentally measured transmissivity of 8% at 10.6u. Both mirrors forming the resonator cavity are held in Lansing mounts (3 min of arc adjustments) and have apertures of 2.5 cm. Beam waist calculations indicate that more than 99% of the radiation incident on the Germanium f l a t f a l l s within i t s aperature. C02 >  N 2 > He HYVAC 14 CAPILLARIES 9 9 r MIXING TANK FLOWMETER BOARD F i g u r e 0-100 torr SPEEDIVAC 0-760 torr SPEEDIVAC .20 P0LYFL0 GAS EXHAUST TUBES 3/8"I.D. LASER 200 HOLES 0.01 D. Vacuum System. 20 Before going into d e t a i l s of the c h a r a c t e r i s t i c s of our laser i t is useful to summarize some of the standard knowledge of C02 l a s e r s . A reader who is already f a m i l i a r with this background information can skip the next chapter and turn to Chapter 4 where the experimental results of the transverse flow transverse discharge laser are pre-sented and discussed. Chapter 3 CARBON DIOXIDE LASERS 3.1 Vibrational Energy Levels of CQ2 Carbon dioxide i s a l i n e a r , symmetric, triatomic molecule, which can vibrate in three d i f f e r e n t modes: the longitudinal symmetric strech mode (vi) (Figure 9 ( a ) ) , the bending or deformational mode (v 2) (Figure 9 ( b ) ) , and the asymmetric strech mode (v 3) (Figure 9 ( c ) ) . The S Y M M E T R I C (a) Wffl { ) f l ? S T R E T C H (b) _ B E N D I N G (0,l£\0) •4fe A S Y M M E T R I C ( J ^ S T R E T C H (0,0,2/^ } F i g u r e 9. V i b r a t i o n a l Modes o f C0 2 21 molecule can vibrate in more than one mode at the same time and possess more than one quantum of vibrational energy in each mode. The vibrational levels are normally designated by four numbers representing the number of vibrational quanta of each mode associated with that level and written in the order ( v i , v i , v 3 ) , where £ represents the number of quanta of angular momentum associated with the bending mode. The C02 energy levels of prime importance in laser action are shown in Figure 10 with th e i r spectro-scopic c l a s s i f i c a t i o n s . Rotation-vibration t r a n s i s t i o n s from the (00°1) vibrational level to the (10°0) level form the 9.6u* band and those from (00°1) to (02°0) the 10.6u band, using the notation of [21]. The l i n e s in a r o t a t i o n a l - v i b r a t i o n a l spectrum are designated by P ( J ) , Q(J) and R(J), where J is the rotational quantum number of the 1ower level of the t r a n s i s t i o n . The P, Q and R branches are destinquished from each other by the rotational t r a n s i s t i o n selection rule AJ = -1, 0 and +1 respectively (see Figure 11). For a l i n e a r , symmetric molecule l i k e C02 the rotation-vibration spectrum is greatly s i m p l i f i e d . There are no This band arises due to mixing of the (10°0) and (02°0) states caused by Fermi resonance. CARBON DIOXIDE NITROGEN 1 0 . E n e r g y L e v e l D i a g r a m f o r CO, M t u . o n l y t h o s e v i b r a t i o n a l l e v e l V ? 2 ( S H O W ? N 9 C 0 2 l a s e r s ) . d C , o n a l ' e v e l s i m p o r t a n t f o r V O — cvi or Be cc o — c\j ro o o o o 3 OJ ro o_ SL o_ 1 ) 1 ( 1 3 2 I 0 I 0 F i g u r e 11. R o t a t i o n - V i b r a t i o n T r a n s i s t i o n s . Q-branch transistions since both the upper and lower laser levels have V = 0 ( i . e . I = Si' = 0), where ll is the component of electr o n i c o r b i t a l angular momentum along the internuclear a x i s . Alternate lines in the spectrum of the y - Ia bands of C02 are also missing because of C02's symmetry [ 1 1 ] . 3.2 Excitation and Inversion in C02 A great deal of the early history of C02 lasers can be attributed to C.K.N. Patel . He was the f i r s t to 25 observe laser emission from C02 (1964 [12], from C02-N2 mixtures (1964 [13] and C02-N2-He mixtures (1965) [14]. The l a s t result was simultaneously obtained by Moeller and Rigden [15]. Patel o r i g i n a l l y believed [16] that C02 was dissociated into CO and 0 2 with the subsequent recombination leaving C02 v i b r a t i o n a l l y excited. Sovolev and Sobovnikov [17] disagreed with Patel and suggested that the CO formed by the d i s s o c i a t i o n of C02 was e l e c t r o n i c a l l y excited and transferred this energy to C02 by c o l l i s i o n s . However i t has since been shown by Boness and Schulz (1968) [18] and v e r i f i e d in laser experiments by Cheo (1967) [19] and McKnight (1969) [20] that the dominant excitation mechanism is d i r e c t electron impact, symbolized by: fast e" + C02 -»• slow e" + C02* . Direct electron impact is a rapid process which inverts the population densities because the electron impact excitation cross-section is larger for the upper ( o p t i -c a l l y allowed) laser level than the lower ( o p t i c a l l y forbidden) one [20]. The increase in e f f i c i e n c y and larger powers obtained by adding d i f f e r e n t gases to C02 can, in part, 26 be q u a l i t a t i v e l y explained by the data shown in Table i . In the T a b l e I Induced V i b r a t i o n a l R e l a x a t i o n Rates of the C0 2 V 3 and v 2 Modes i n the Prese n c e of Other Gases a t 300°K [213 Gas v 3 - v 2 ( s e c- 1- t o r r- 1) v 2 ground s t a t e ( s e c- 1 - t o r r- 1) C0 2 365 ± 15 200 ± 10 Gas N 2 1 1 0 + 5 - 40 Add i t i ves He < 85 4 ± .8 x 103 H 20 3.3 ± .9 x 1 01* 10s - 106 D i s soc i a t i on CO 1 93 4 x 1 03 P r o d u c t s 0 2 110 + 5 ~ 40 presence of other gases the populations of the upper ( v 3 ) and lower ( v 2 ) laser levels are modified by c o l l i s i o n induced vibrational r e l a x a t i o n . The main e f f e c t of these c o l l i s i o n s is to depopulate the v 2 level more rapidly than the v 3 l e v e l , thus aiding the formation of the population inversion necessary for laser emission. However the effects of N2 and He, the two main gas a d d i t i v e s , are not limited to c o l l i s i o n a l relaxation alone. 27 Nitrogen is a homonuclear molecule and therefore has a zero dipole moment in the ground s t a t e . Thus ra d i a -tive rotation-vibration transistions.are s t r i c t l y f o r -bidden and the v i b r a t i o n a l l y excited levels of N2 are very long l i v e d (m sees.) [11]. The total electron impact excitation cross-section for N2 is very large (3.8 x IO"1 6 cm2) [21] and since the 00°1 level of C02 is only 20 cm-1 above the v = 1 level of N there is an e f f i c i e n t near resonant transfer of energy from the v i b r a t i o n a l l y excited N2 to C02 by c o l l i s i o n s . Cw C02 lasers are more e f f i c i e n t because of this N2 storage mechanism while in pulsed C02 lasers the presence of N2 usually creates a long t a i l in the laser pulse shape by repopulating the depleted 00°1 level of C0 2. He, on the other hand, besides profoundly affec t i n g the population of the v 2 level also lowers the mean gas kinetic temperature of the discharge because of i t s high thermal conductivity. Thus the gain of the laser is increased [10] and the electron energy d i s t r i b u -tion is also affected. The importance of the electron energy d i s t r i b u t i o n was t h e o r e t i c a l l y demonstrated by Nighan and Bennett (1969) [1 ] . Using experimentally obtained cross-sections and computer solutions of the electron energy d i s t r i b u t i o n 28 from the Boltzman equation for s p a t i a l l y uniform steady states [2] they solved the electron k i n e t i c equation numerically as a function of E/N. Their results show that the electron energy d i s t r i b u t i o n is non-Maxwel1ian. Of even more importance are the i r results for the f r a c t i o n a l power transferred from the electrons to the vibrational and e l e c t r o n i c levels of C02 and N 2, as a function of . 0 0 — — 2 I 3/2 E/N and u (the average electron energy: u = -5- u f(u)du). For an E/N value of 10~16v-cm2 up to 65% of the electron energy goes d i r e c t l y into the ( 0 0°1 ) level of C0 2*. But for an E/N of 10"15v-cm2 this figure is only about 20% with most of the electron energy being used to excite the electronic levels of C02 and N 2. The more e f f i c i e n t the energy transfer is the more non-Maxwellian the electron energy d i s t r i b u t i o n becomes, f a l l i n g o f f faster at higher energies than the Druyvesteyn d i s t r i b u t i o n . 3.3 Present State of the Art In spite of this basic understanding of the molec-ular processes in C02 lasers i t is however not possible to * For a TEA laser at atmospheric pressure with a cathode-anode separation of 2.54 cm and an applied voltage of 20 kv, E/N = 3.2 x 10~1 6v-cm2. 29 predict the quantitative performance of a C02 l a s e r . A quantitative analysis would require a knowledge of: 1) what is precisely the electron energy d i s t r i b u t i o n in a pulsed discharge as a function of pressure, gas composition and current density, 2) how energy is transferred from electrons in a pulsed discharge to the vibrational levels of C02 and N 2, 3) how this vibrational energy relaxes because of c o l l i s i o n s of the gas p a r t i c l e s and, 4) how coherent radiation interacts with the excited population of C0 2. To-date no experimental measurement of the electron energy d i s t r i b u t i o n exists and i t s non-Maxwel1ian form makes the use of the concept of mean electron "temperature" questionable. The narrow amplification bandwidth of the C02 lasers (see Section 4.3.3) does not make i t suitable for mode-locking because of the small number of longitudinal modes that can be f i t t e d within the gain curve. Never-thele s s , mode-locking has been attempted and achieved with active and passive optical elements. Pulse widths as short as 1 nsec with peak power of ~ 1 Mw have been reported (1972) [22]. To increase the laser output one has to increase the number of electrons, nfil in the discharge. The most obvious way to do t h i s , namely to increase the current, 30 does not work, since increased current density w i l l lead to a higher gas temperature in the discharge, which increases the deactivation processes. Two other methods for i n -creasing n g however have been successful. F i r s t l y , Richardson et al. (1973) [23] at NRC have used elaborate techniques to preionize the gases and have obtained an output of 300 joules at several gigawatts. Daugherty (1973) [24] has used electron beams to i n j e c t electrons into the active medium and obtained 2000 joules with almost 100 Mw's of power. The lack of a comprehensive theory necessitates the extensive investigation of any new C02 laser with d i f f e r e n t features. To this end one should vary as many parameters as possible in order to understand the properties of the new device. The experimental studies of our laser are presented in the next chapter. Chapter 4 CHARACTERISTICS OF LASER 4.1 Aligning CQ2 Laser Any laser work starts with the f r u s t r a t i n g operation of alignment. Since the wavelength of radiation emitted by a C02 laser is in the infrared some of the optical components used are opaque to radiation in the v i s i b l e spectrum. Thus to align the laser cavity i t is necessary to introduce the alignment beam used in a round-about way. The set-up is shown in Figure 12. 100% GOLD-COATED MIRROR K C I * PLATE* Au=Ge I I i l i | I I I I l i R = IOm ^ - i O / DETECTOR GERMANIUM FLAT 8 % TRANSMISSION SILVERED MIRROR Figure 12. Alignment System, Removed after i n i t i a l alignment. 31 32 One should not be surprised i f , after careful alignment, no laser action is observed. This is due to the change in the r e f r a c t i v e index of the laser medium because of the heating of the gas and the pulsed discharge. Such an e f f e c t has been observed in low pressure C02 l a s e r s , He-Ne lasers and most recently by Fortin et al. (1971) [25] in a h e l i c a l TEA C02 laser where a diverging lens e f f e c t was attributed to each pitch of the h e l i x . This e f f e c t causes the alignment beam to "jump" on the surface of the mirrors ~ 0.5 cm each time the discharge occurs. Thus for alignment purposes i t is better to note * the position of the beam while the discharge is on. Once the laser has been aligned the KC1 plate is removed because i t i s a source of loss in the cavity and affects the pulse shape and delay time. Quite frequently multiple pulsing occurs with some pulses being emitted over 20 usee after the i n i t i a t i o n of the discharge (see Figure 13). This behaviour is in general the case i f the round t r i p gain is small, which may be caused by operating at low gas pressures, a s l i g h t misalignment of cavity mirrors or losses in the c a v i t y . These points are discussed further in Section 4.3.3. * Different beam displacements due to difference in index of r e f r a c t i o n for 10.6y (C02) and 6328A (He-Ne) may be neglected. 5.68 k w / d i v 5 u s e c / d i v F i g u r e 13. M u l t i p l e L a s i n g w i t h Low Net Round T r i p G a i n . The mechanical r i g i d i t y o f the l a s e r was such t h a t once the l a s e r was a l i g n e d o n l y s l i g h t a d j u s t m e n t s were n e c e s s a r y f o r s u c c e s s f u l o p e r a t i o n on a day-to-day b a s i s . The usual C0 2 p u l s e shape o b t a i n e d i s shown i n F i g u r e 14 w i t h the p u l s e shape parameter measured i n t h i s or i 1 1 > p PP = T = PEAK POWER PULSE WIDTH AT HALF POWER n - TIME DELAY V * T 0'* 1 | | H y ' " 2 3 ' ^ 5 6 7 8 7* pA TIME (microseconds) F i g u r e 14. C 0 2 P u l s e Shape P a r a m e t e r s . 34 chapter as indicated. Since no external trigger was used to f i r e the spark gap no great e f f o r t was made to e l i m i -nate the e l e c t r i c a l noise. On the contrary, the noise was used to trigger the oscilloscope so that x^ measures the elapsed time between the start of the current pulse and the beginning of the laser pulse. 4.2 Operating Conditions 4.2.1 The e l e c t r i c a l system. Of course the f i r s t question to be asked i s : Does the hollow ground rod actually work? The answer, found after several weeks of f r u i t l e s s adjustment, is yes. How well i t works though depends upon the interelectrode distance, A, used, as can be seen in Figure 15. Using a l l 200 r e s i s t o r s creates a discharge pattern that shows the sparks channels i n t e r f e r i n g and coupling with one another. By disconnecting one chain of r e s i s t o r pins the interelectrode distance is doubled and the discharge pattern is quite uniform. However, the power from the 200 r e s i s t o r s is larger than that with only 100 r e s i s t o r s . Also s i g n i f i c a n t is the reduced height of the t a i l of the C02 pulse. This means the energy of the pulse is less (but not by a factor 35 Spa rk C h a n n e l s L a s e r P u l s e s 200 r e s i s t o r s A = 0.5 cm 100 r e s i s t o r s A = 1.0 cm P = 140 t o r r 11.4 kw/div 1 y s e c / d i v C0 2:N 2:He = 2:1:10 F i g u r e 15. S p a r k C h a n n e l s and L a s e r P u l s e s . o f 2) because l e s s N 2 i s bei n g e x c i t e d . With a l l 200 r e s i s t o r s i n use more volume of the gas i s e x c i t e d and thus the energy c o n t e n t of the p u l s e i s l a r g e r . For t h i s reason the l a s e r was o p e r a t e d u s i n g 200 r e s i s t o r s . 36 Throughout the experiment r e p e t i t i o n rates of 1 pulse/sec were used. By varying the spark gap setting and the power supply voltage i t was v e r i f i e d that the repe-t i t i o n rate agreed with equation 2.1 (Section 2.1). The choice of 1 Hz was necessitated by the need to apply a reasonable voltage to the discharge and avoid excessive overheating of the 1 watt r e s i s t o r s used as the anode. In addition there was the very human problem of e l e c t r i c a l noise from the discharge and spark gap which brought frequent (1 Hz ?) complaints from fellow students and was eventually solved by simply setting the spark gap at a lower voltage, namely 16 kv. 4.2.2 E f f e c t of gases and pressure. As explained in Section 3.2, the presence of other gases a l t e r s the power and energy output of a C02 l a s e r . The oscillograms shown in Figure 16 v e r i f y these statements. With no gas additives (Figure 16(a)) the laser pulse shows a very fast r i s e and decay time. The rapid risetime is attributed by Beaulieu (1970) [ 3 ] to the fast excitation caused by the short current pulse. This causes a rapid build up in gain which creates a gain-switching s i t u a t i o n giving r i s e to a giant pulse. 37 (a) (b) (c) co 2 N2 He P r e s s u r e ( t o r r ) kw d i v 1 0 0 70 5.68 2 1 0 1 00 5.68 1 0 5 1 20 11 A F i g u r e 16. C0 2 L a s e r P u l s e The a d d i t i o n of N2 does much but i n c r e a s e s the energy of the p u l s e as e v i d e n t by the t a i l i n the C0 2 p u l s e ( F i g u r e 1 6 ( b ) ) . When He i s added to C0 2 ( F i g u r e 1 6 ( c ) ) the peak power i s alm o s t doubled but the p u l s e shape i s the same as w i t h o n l y C0 2 p r e s e n t . O s c i l l o g r a m s w i t h a l l t h r e e gases p r e s e n t are shown i n F i g u r e 15 and e x h i b i t the same p u l s e shape as t h a t o f C0 2 and N 2 a l o n e but w i t h the peak powers o f C0 2 and He. 38 The volume ra t i o of gases used throughout most of the experiment were 10 parts He:l part N2:2 parts C0 2. This r a t i o i s typical of the values cited in the l i t e r a t u r e for C02 l a s e r s . Some work was done on the e f f e c t of i n -creasing the He content in the gas mixture and i t was found that for ratios up to 25:1:2 the peak power was increased ~ 30%. This larger gas r a t i o was used when obtaining the spectrum of the laser (see Appendix B). Although more He gives more power, He is expensive! Enough said about that! The variation of peak power with gas pressure is shown in Figure 17. In this graph, as in all others, the error bars represent the rms error of five shots each. The curve has an approximate parabolic shape which is due to several e f f e c t s . The i n i t i a l increase in power with pressure is created by an increase in the energy per unit volume. If no other effects were important the power would increase l i n e a r l y with pressure. However, de-excitation mechanisms in the gas mixture are due to c o l l i s i o n s and the c o l l i s i o n a l relaxation rates also i n -crease l i n e a r l y with pressure [26]. Thus the inversion i t s e l f i s reduced by de-activating c o l l i s i o n s at high pressures. Also above 5.2 torr the l i n e width of C02 is c o l l i s i o n broadened [26] and this homogeneous broaden-ing increases the threshold for lasing and thereby reduces the extractable laser power. 40 A similar graph was also obtained with gas ratios of 15:1:2. This graph had the same parabolic shape but the optimum pressure occurred at 150 torr instead of the 140 torr with the 10:1:2 gas mixture. This is reasonable since He clears out the lower laser level by c o l l i s i o n s (see Table 1) and thus increases the population inversion even though the threshold increases with the increasing band width. Using these results the laser was operated at a pressure of 140 to r r (10:1:2 gas mixture) when studying other properties of the l a s e r . 4. 3 Properties of System 4.3.1 Power decay in a closed system. The most important component in the vacuum system is the laser reaction tube which is made of l u c i t e . Since this vessel had over 200 holes d r i l l e d in i t for the r e s i s t o r pins, exhaust ports, e t c . , i t was not too surprising that the lowest vacuum (?) obtainable was 200 microns as measured with a McLeod gauge. To see what eff e c t the vacuum leaks had upon the laser output the vacuum pump was closed from the reaction tube and no fresh gas was supplied to the tube. Thus the graph shown in POWER (Normalized Units) Figure 18 was obtained. Since the vacuum leak was at a rate of 5 torr/min some gas was p e r i o d i c a l l y evacuated from the system to maintain a constant pressure of 140 t o r r . At a leak rate of 5 torr/min, after 50 min some 250 torr of a i r w i l l have leaked into the reaction tube making the p a r t i a l pressure of a i r some 60% of the total pressure in the tube. Even with this large amount of impurities in the reaction tube the laser s t i l l lases but only at 30% of the power i t started with i n i t i a l l y . 4.3.2 E l e c t r i c a l e f f i c i e n c y . The e l e c t r i c a l e f f i c i e n c y of a laser is defined as the r a t i o of the energy of the laser pulse to the e l e c t r i c a l energy input. In a r e s i s t i v e l y loaded TEA laser the e f f i c i e n c y is t y p i c a l l y 5% [28], To obtain this measurement i t is necessary to know the voltage, V, applied to the r e s i s t o r chain from the spark gap. The e l e c t r i c a l energy input is the energy stored by the capacitors which is ( i ) C V2. The I2R losses due to the r e s i s t i v e l y loaded anode are usually neglected. The voltage across the capacitor bank can be obtained by ca l i b r a t i n g either the spark gap or, because there is no externally applied trigger pulse to the spark gap, the power supply. Since the electrodes of the spark gap 43 deteriorate with use the l a t t e r of the two methods was used. The power supply was calibrated using a compensated Tekronix high voltage probe (30 kv max.). The energy content of the laser pulse was found by measuring the area under the curve of the power pulse shapes from the Au:Ge detector (see Appendix A). The e f f i c i e n c y curve obtained is shown in Figure 19. To interpret this graph i t is necessary to look at the work of Nighan and Bennett (1969) [1] and Nighan (1970) [2 ] . They found that the f r a c t i o n a l transfer of power to the elec t r o n i c and vibrational levels of C02 and N2 are depen-dent upon the value of E/N, where E is the e l e c t r i c f i e l d strength and N the total neutral p a r t i c l e density. The f r a c t i o n a l ionization of the gases is quite small, t y p i c a l l y 10"8 - I O - 6 , and may be ignored. To fi n d E ( i . e . E = v'/d) i t is necessary to know the net voltage, v', applied to the gas mixture. This is the voltage applied across the electrodes by the power supply minus the cathode f a l l and voltage drop across the anode r e s i s t o r s . These potential drops have been estimated by Lyon (1973) [26] to be less than 2 kv in a laser with a gas mixture si m i l a r to that used here. Thus for a cathode-anode separation d = 1.8 cm at a pressure of 140 torr the values of the input energy used represents E/N values in the range 9.2 x 10~16v-cm2 to 22.5 x 10"1 6v-cm2. From these considerations the ELECTRICAL INPUT ENERGY (joules) F i g u r e 19. E l e c t r i c a l E f f i c i e n c y C u r v e . 45 increase in e f f i c i e n c y is not explainable since the values of E/N used indicate that the average electron energy (3.5 - 4.5 e.v.) is so high that most of the C02 and N2 is e l e c t r o n i c a l l y excited. With increasing applied voltage ( i . e . energy) less C02 and N2 is d i r e c t l y excited v i b r a -t i o n a l ^ and therefore i t would be reasonable to assume the e f f i c i e n c y was decreased. These results i l l u s t r a t e that one s t i l l does not f u l l y understand a l l facets of C02 l a s e r s . 4.3.3 Time delay of pulse. The time delay, x^, between the beginning of the laser pulse and the s t a r t of the current pulse is governed by losses in the laser cavity and the gain [30]. This was already mentioned in Section 4.1 in connection with the multiple pulsing caused by the KC£ plate used during alignment and operating the laser at low pressures (< 30 t o r r ) . The graph in Figure 20 shows this time delay as a function of total gas pressure. If the time delay was due to c o l l i s i o n e f f e c t we would expect the delay to decrease with increasing pressure because the c o l l i s i o n a l relaxation rates are d i r e c t l y porportional to the total pressure. The influence F i g u r e 20. Time D e l a y v s . T o t a l P r e s s u r e . 47 of losses may be excluded also since the only additional losses introduced by increasing the pressure as absorption losses which are n e g l i g i b l e . This leaves only the effect of the gain. It has been found by Gerry et al. (1966) [27] that above 5.2 torr C02 is c o l l i s i o n broadened with a bandwidth increase of 6.5 MHz/torr. Therefore, as the pressure increases the gain curve gets larger and broader thus increasing the threshold for l a s i n g . With a cavity length of 2 m the frequency separation between axial modes is 75 MHz ( i . e . Af = c/2L). Thus, at a pressure of 20 torr there are at most two axial modes within the gain curve whereas at 240 torr there are 21 modes present. The increase in the time delay may be caused by coupling between these modes with one mode saying to the other "After you" and the othermode replying "No, after you!" It is also known by those more f a m i l i a r with * C02 lasers that the current pulse plays quite an important role with C02 lasers and is affected by the discharge which in turn is p a r t i a l l y determined by the pressure. Another interesting observation is that up to pressures 100 and 120 torr the C02 pulse is modulated. This modulation was much more pronounced in mixtures of just C02 and N2 and is shown in Figure 21. By r e f e r r i n g * Private communications with Dr. A.A. Offenberger. i 48 C0 2 : N 2 = 2:1 100 t o r r 5.68 k w / d l v 0 . 5 u s e c / d i v F i g u r e 21. M o d u l a t i o n o f C0 2 L a s e r P u l s e . to F i g u r e 20 one w i l l note t h a t i t i s around t h e s e p r e s -sures t h a t the d e l a y time no l o n g e r remains c o n s t a n t but s t a r t s to i n c r e a s e w i t h p r e s s u r e . T h i s s u g g e s t s t h a t these two phenomena are r e l a t e d . T h i s m o d u l a t i o n , w i t h p e r i o d T ~ 70 n s e c , i s not caused by mode- l o c k i n g e f f e c t s and attempts to e x p l a i n i t as l a s e r s p i k i n g by the S t a t z -De Mars e q u a t i o n have proven f u t i l e [ 1 4 ] . Once a g a i n we r e t u r n to the e f f e c t s of the c u r r e n t p u l s e . For a c i r c u i t c o n -t a i n i n g n o n - n e g l i g i b l e i n d u c t a n c e the c u r r e n t p u l s e w i l l r i n g and thus pump the l a s e r l e v e l p o s s i b l y c a u s i n g t h i s e f f e c t . However, the r i n g i n g f r e q u e n c y of the d i s c h a r g e which can a l s o be seen i n F i g u r e 21 appears to be too slow to p r o v i d e the p o s s i b l e e x p l a n a t i o n . Measurements of the h a l f - w i d t h of the l a s e r p u l s e , x , show no v a r i a t i o n w i t h p r e s s u r e . 4.3.4 S p e c t r a l a n a l y s i s . The spectrum of the l a s e r was i n v e s t i g a t e d w i t h a J a r r e l - A s h 0.5 m s p e c t r o m e t e r equipped w i t h a 148 l/mm 49 grating. The alignment of this instrument caused some d i f f i c u l t i e s which were f i n a l l y overcome by using a small piece of untreated steel which was s u f f i c i e n t l y heated by the laser pulses to evaporate the grease on i t s surface * and emitting a f a i n t , v i s i b l e l i g h t f l a s h . The procedure used and d i f f i c u l t i e s involved in obtaining the infrared spectrum are described in Appendix B. Only three P branch transistions were observed: the P(20), P(18) and P(16) l i n e s . These lines did not s h i f t and other lines did not appear when the pressure was varied from 40 to 200 torr and spark gap settings of 17.4 and 15 kv were used. The P(20) l i n e was by far the most intense l i n e and using Patel's [10] analysis of the gain c o e f f i c i e n t s for P-branch tra n s i s t i o n s i t is possible to determine the population r a t i o N00ol^N02o0 " T n i s w a s d o n e °y maxi-mizing the gain c o e f f i c i e n t with respect to J and solving f o r N001y'N020 w i t n J + 1 = 20 . The result obtained was ^001^020 = l'H> f °r a kinetic temperature of 300°K. Having successfully operated the laser configura-tion as a transversely p u l s e - i n i t i a t e d C02 laser we moved on to the more d i f f i c u l t task of obtaining laser emission by means of chemical pumping. The gas mixtures used and r e s u l t s obtained are discussed in the next chapter. * Alcohol cleaned surfaces did not show the same e f f e c t . Chapter 5 CHEMICAL LASER Laser emission by means of chemical pumping was f i r s t successfully accomplished by Kasper and Pimentel (1965) [30] in a flas h p h o t o l y t i c a l l y i n i t i a t e d hydrogen/ chlorine explosion. In the past three years transversely s p a r k - i n i t i a t e d chemical lasers have been operated using CO [ 4 ] , [7] and HF [ 5 ] , [31,33], produced in chemical re-actions up to pressures of one atmosphere. During this time gas mixtures containing hydrocarbons [33-35] have also produced laser action but only at very low pressures in discharge tubes. Attempts to obtain laser emission from molecules produced in chemical reactions were not s u c c e s s f u l . The gas mixtures and operating conditions used are shown in Table I I . To obtain maximum s e n s i t i v i t y with the Au:Ge detector a KC& lens (f = 9.4 cm) was used to ensure a l l radiation was focused on the detector element and no 50Q 50 51 T a b l e I I Gas M i x t u r e s and O p e r a t i n g C o n d i t i o n s f o r Chem i caI L a s e r Gas M i x t u r e Gas R a t i o s by Volume PT0TAL ( t o r r ) L a s e r Act ion #1 C 2H 2 + C0 2 + N 2 + He 4.5:2:1:10 30-50 Yes #2 C 2H 2 N 2 + He 1:25:60 5-25 No #3 C 2H 2 + He + Ai r+ 1:4.2:1.7 5-75 No #4 C 2H 2 + C0 2 + 0 2 <l:5:40* 45 No #5 C 2H 2 + He + 0 2 * < 1 : 1 : 5 5 No #6 C 2 H 2 + 0 2 * 1 :2 5-60 No #7 SF 6 + C 2H 2 + He 12:1:233 1 5-40 No #8 SF 6 + H 2+ 1.7:1 1 0-40 No #9 H 2 + C0 2 + He 1:1.7:8.6 50-80 Yes #10 SF 6 + C0 2 + He 1:2:10 40 No Laser e m i s s i o n has been r e p o r t e d in t h e s e m i x t u r e s . Gas r a t i o s by p a r t i a l p r e s s u r e s . 52 terminator was used with the detector. These two steps would enable detection of powers as low as 50 mw although d i s t o r t i n g the power pulse shape because of the integration due to the large RC time constant of the BNC c a b l e - o s c i l l o -scope input impedance network (see Figure A-4). When attempting to detect laser emission from HF (mixtures #7 and #8) e l e c t r i c a l noise was a problem. The HF laser pulse is created ~ 100 nsec after the i n i t i a t i o n of the discharge which would bury i t amid the e l e c t r i c a l noise from the discharge. After unsatisfactory attempts to reduce the noise a delay cable (3 usee) was used to delay the signal from the detector thus sidestepping the noise problem. The gas mixtures of primary interest are #3, #6 and #8. These mixtures have previously been reported to produce laser emissions from CO and C0 2, CO and HF molecules r e s p e c t i v e l y . The other gas mixtures were used to study what ef f e c t the gases of the primary mixtures ( i . e . #'s 3, 6 and 8) had on a laser mixture which is known to work in our l a s e r . In short, the conclusions and observations of these studies are: The v i o l e n t n a t u r e of C 2H 2 + 0 2 r e a c t i o n s r e -q u i r e s the use of low o p e r a t i n g p r e s s u r e s . The C 2H 2 + 0 2 m i x t u r e produced f l a m e - l i k e r e a c t i o n s , which are known t o 53 s u p p r e s s l a s e r a c t i o n , a t p r e s s u r e s i n e x c e s s o f 50 t o r r . U s i n g low p r e s s u r e s , h o w e v e r , i s d e t e r i m e n t a I t o a t r a n s -v e r s e l y s p a r k - i n i t i a t e d l a s e r b e c a u s e o f t h e r e s u l t i n g n o n - u n i f o r m i t y o f t h e d i s c h a r g e p a t t e r n . L a s e r a c t i o n i n t h e m i x t u r e s C2H2 + CO2 + N2 + He and H 2 + C 0 2 + He c a n be a t t r i b u t e d t o e l e c t r i c a l l y e x c i t e d CO2. T h e s e m i x t u r e s p r o d u c e d m u l t i p l e p u l s e s as so o n as H 2 and C 2 H 2 were i n t r o d u c e d i n t o t h e a c t i v e medium. T h i s t y p e o f b e h a v i o u r has a l r e a d y been d i s c u s s e d i n C h a p t e r 4 ( s e e F i g u r e 13) and i n d i c a t e s t h a t C 2 H 2 and H 2 d e c r e a s e t h e g a i n o f t h e l a s e r . L a s e r e m i s s i o n f r o m C 0 2 s t o p p e d i m m e d i a t e l y when SF6 and 0 2 e n t e r e d t h e r e a c t i o n t u b e . T h i s i s n ' t t o o s u r p r i s i n g s i n c e SFe q u e n c h e s e l e c t r o n s a v a i l a b l e f o r C 0 2 and N 2 e x c i t a t i o n and t h e n adds " i n s u l t t o i n j u r y " by r e a d i l y a b s o r b i n g what C 0 2 r a d i a t i o n i s e m i t t e d . F o r t h e s e r e a s o n s SF6 has been used t o q u e n c h a r c s i n c i r c u i t b r e a k e r s and a s a s a t u r a b l e a b s o r b e r f o r Q - s w i t c h i n g o f C 0 2 l a s e r s . S t u d i e s on t h e d i s s o c i a t i o n p r o d u c t s o f C 0 2 i n d i c a t e t h a t 0 2 i s d e l e t e r i o u s t o l a s e r e m i s s i o n b e c a u s e i t d e s t r o y s t h e p o p u l a t i o n i n v e r s i o n by d e p l e t i n g t h e u p p e r l a s e r l e v e l ( s e e T a b l e I ) . 54 Although laser emission by chemical pumping was not detected this does not necessarily mean that the laser reaction tube is inadequate for the task. In Chapter 6 improvements that may help to achieve an operating chemical laser are described and possible experiments for the future are discussed. Chapter 6 CONCLUSIONS AND FUTURE IMPROVEMENTS A transverse gas flow, transversely pulsed laser has been constructed and operated as an e l e c t r i c a l l y excited CO2 l a s e r . The innovation of using a hollow ground rod for transverse gal flow and discharge cathode was successfully demonstrated. The e f f i c i e n c y of the laser is comparable to other r e s i s t i v e l y loaded TEA C02 lasers although the powers obtained are not as large. Some measurements of the c h a r a c t e r i s t i c s of the laser pulse and laser system were not explainable by present theories of C02 l a s e r s . To improve the performance of the laser the interelectrode separation, A, should be fixed between 0.5 cm and 1.0 cm, large enough to prevent coupling of the spark channels and thus maximizing the excitation volume used. The e f f i c i e n c y and power output may be im-proved by optimizing E/N according to Nighan and Bennett's c r i t e r i a . This would require larger voltages but would 55 56 also increase the optimum pressure thus reducing problems with the vacuum system. Experimental results by Mikoshiba and Ahlborn (1973) [36] indicate that the power may be increased by optimizing the laser cavity using a variable focal length gold-coated mirror. Some rule of thumb guides for o p t i -mizing the transmissivity of the output mirror are given in an analysis by Meneely (1965) [37] for high power C02 l a s e r s . For operation as an e l e c t r i c a l l y excited C02 laser the Brewster angle mounted KCl windows should be removed and the mirrors forming the resonator cavity mounted d i r e c t l y on the reaction tube. These KC£ windows are a source of loss in the cavity but should be used for chemical laser operation because of the possible damage to good quality mirrors by the chemicals formed. Another small item of possible value is a mixing tank with f a c i l i t i e s for pre-cooling of the gas mixture. This would lower the gas kinetic temperature and thus increase the laser's gain. F a i l i n g to find laser action with chemical pumping, gain measurements would be useful to ascertain i f any v i b r a t i o n a l l y excited molecules are produced in the discharge. 57 The use of transverse gas flow makes this laser design suitable for studying the effect of gas replacement rates on the power output. This would require improving the vacuum system ( i . e . place anode r e s i s t o r s inside reaction tube) to eliminate impurities and constructing a fast r e p e t i t i o n rate high voltage supply. In conclusion this laser vessel has live d up to some but not a l l our expectations. Although no laser emissions were detected from chemical pumping processes the arrangement did produce adequate transverse discharges to allow operation as an e l e c t r i c a l l y excited C0 2 l a s e r , under various conditions of flow, pressure and gas mixture i t helped to esta b l i s h in this l a b , for the f i r s t time, the techniques of detection and measurement of C0 2 lasers and infrared r a d i a t i o n . REFERENCES Nighan, W.L. and J.H. Bennett. 1969. "Electron Energy Di s t r i b u t i o n Functions and Vibrational Excitation Rates in C02 Laser Mixtures." Appl. Phys. L e t t . , 14 (8):240-243. Nighan, W.L. 1970. "Electron Energy Distributions and C o l l i s i o n Rates in E l e c t r i c a l l y Excited N 2, CO and C02." Phys. Rev. A, 2 (5):1987-2000. Beaulieu, A.J. 1970. "Transversely Excited Atmospheric Pressure C02 Lasers." Appl. Phys. L e t t . , 16 (12):504-505. Jacobson, T.V. and G.H. Kimbell. 1970. "Transversely Spark-Initiated Chemical Lasers with High Pulse Energies." J . Appl. Phys., 41 (1 3):5210-5212. Pummer, H. and K.L. Kompa. 1972. "Investigation of 1-J Pulsed Discharge-Initiated HF Laser." Appl. Phys. L e t t . , 20 (9):356-357. Wood, O.R., G.E. Burkhardt, M.A. Pollack and T.J. Bridges. 1971. "High Pressure Laser Action in 13 Gases with Transverse Excitation." Appl. Phys. L e t t . , 1_8 ( 4 ) : 11 2-115. Ahlborn, B.A., P. Gensel and K.L. Kompa. 1972. "Transverse-Flow Transverse-Pulsed Chemical CO Laser." J . Appl. PHys., 43 (5):2487-2489. Smith, A.L.S. 1968. "The Effect of Gas Flow on the Composition and Power Output of a C02-He-N2 Laser." Phys. L e t t . , 27A (7):432-433. Smith, A.L.S. 1969. "Molecular Composition Changes in a Flow C02-N2-He-H2 Laser." B r i t . J . Appl. Phys., Ser 2 (2):1 1 29-1 134. 58 59 10. P a t e l , C.K.N. 1964. "Interpretation of C02 Optical Maser Experiments." Phys. Rev. L e t t . , 7_ (12): 588. 11. Herzberg, G. Spectra of Diatomic Molecules. D. van Nostrand Co. Inc., Princeton, 1961. 12. P a t e l , C.K.N., W.L. Faust and R.A. McFarlane. 1964. B u l l . Am. Phys. S o c , £ : 5 0 0 . 13. P a t e l , C.K.N. 1964. "Selective Excitation Through Vibrational Energy Transfer and Optical Maser Action in N2-C02." Phys. Rev. L e t t . , 1_3 (21): 617-619. 14. P a t e l , C.K.N., P.K. Tien and J.H. McFee. 1965. "CW High-Power C02-N2-He Laser." Appl. Phys. L e t t . , I (1 1 ): 290-292 . 15. Moeller, G. and J.D. Rigden. 1965. "High Power Laser Action in C02-He Mixtures." Appl. Phys. L e t t . , I (10):274-276. 16. P a t e l , C.K.W. 1964. "Continuous-Wave Laser Action on Vibrational-Rotational Transistions of C02." Phys. Rev., 1_36 (5a): Al 1 87-A11 93. 17. Sobolev, N.N. and V.V. Sokovikov. 1966. "A Mechan-ism Ensuring Level Population Inversion in C02 Lasers." Sov. Phys. - JEPT, 4:204-206. 18. Boness, M.J.W. and G.J. Schulz. 1968. "Vibrational Excitation of C02 by Electron Impact." PHys. Rev. L e t t . , 21 (5):1031-1034. 19. Cheo, P.K. 1967. "Effects of C0 2, He and N2 on the Lifetimes of the 00°1 and 10°0 C02 Laser Levels and Pulsed Gain at 10.6u." J . Appl. Phys., 38 (9):3563-3568. 20. McKnight, W.B. 1969. "Excitation Mechanisms in C02 Lasers." J . Appl. Phys., 40 (7):2810-2816. 60 21. Yariv, Annon. Advances in Quantum E l e c t r o n i c s . Wiley, New York, 1967. 22. Davis, D.T., D.L. Smith and J.S. Koval. 1972. "Generation of Single 1-ns Pulses at 10.6um." IEEE J . Quantum Electron., QE-8:846-848. 23. Richardson, M.C, A.J. Alcock, K. Leopold and P. Burtyn. 1973. "A 300-J Multigigawatt C02 Laser." IEEE J . Quantum E l e c t r o n i c s , QE-9 (2):236-243. 24. Daugherty, J.D. 1972. "Electron Beam-Sustainer C02 Lasers." Presented at 7th Int. Quantum Electronics Conf., Montreal, Canada. 25. F o r t i n , R., M. Gravel and R. Tremblay. 1971. "Helical TEA-C02 Lasers." Can. J . Phys., 49:1783-1793. 26. Lyon, D.L. 1973. "Comparison of Theory and Experi-ment for a Transversely Excited High-Pressure C02 Laser." IEEE J . Quantum E l e c t r o n i c s , QE-9 (2):139-153. 27. Gerry, E.T. and D.A. Leonard. 1966. "Measurement of 10.6u C02 Laser Transistion P r o b a b i l i t y and Optical Broadening Cross Section." Appl. Phys. L e t t . , 8 (9):227-229. 28. Beaulieu, A.J. 1971. "High Peak Power Gas Lasers." Proc. of IEEE, 59 (4):667-674. 29. Garside, B.K., E.A. B a l l i k and J . Reid. 1972. "Pulse Delays in TEA C02 Lasers." J . Appl. Phys., 43 (5):2387-2390. 30. Kasper, J.V.V. and G.C. Pimentel. 1965. "HCl Chemical Laser." Phys. Rev. L e t t . , 14 (10):352-354. 31. Jacobson, T.V. and G.H. Kimbell. 1971. "Transversely Pulse I n i t i a t e d Chemical Lasers: Atmospheric Pressure Operation of an HF Laser." J . Appl. Phys., 42 (9):3402-3405. 61 32. Pummer, H. , W. B r e i t f e l d , H. Weoller, G. Klement and K.L. Kompa. 1973. "Parameter Study of a 10-J Hydrogen Fluoride Laser." Appl. Phys. L e t t . , 22 (7):319-320. 33. Barry, J.D. and W.E. Boney. 1971. "Laser Emission From He-Air-CHu and He-Air-C 3H 8 Mixtures." Appl. Phys. L e t t . , 1_8 (1):15-16. 34. Barry, J.D. and W.E. Boney. 1972. "CO Laser Action by C 2H 2 Oxidation." Appl. Phys. L e t t . , 20 (7): 243-244. 35. Boney, W.E., J.D. Barry and J.E. Brandelik. 1973. "CO and C02 Laser Action by Organic Molecule Oxidation." IEEE J . Quantum E l e c t r o n i c s , QE-9 (1):246-247. 36. Mikoshiba, S. and B.A. Ahlborn. 1973. "Laser Mirror with Variable Focal Length." Rev. S c i . I n s t r . , 44 (4):508-51 1 . 37. Meneely, C T . 1967. "Laser Mirror Transmissivity Optimization in High Power Optical C a v i t i e s . " Appl. Opt. , 6 (8):1434-1436. 38. Stanley, C.R. 1971. "Infrared Detectors for the Range 1.5-30um." Optics and Laser Tech., August, 1971:144-149. 39. Putley, E.H. 1971. "Infrared Applications of the Pyroelectric E f f e c t . " Optics and Laser Tech., August, 1971:150-156. 40. Gibson, A.F. and M.F. Kimmitt. 1972. "Photon Drag Detection." Laser Focus, August, 1972:26-8. 41. Siegman, A.E. An Introduction to Lasers and Masers. McGraw-Hill, 1971 . APPENDIX A INFRARED DETECTORS The detection of radiation in the intermediate infrared range T.5u to 30u [ 3 8 , 3 9 ] is limited by background noise, since at room temperature the peak of the black body radiation curve f a l l s near l O u . For intense laser radiation this l i m i t a t i o n may be bypassed but this also l i m i t s the number of useful detectors. There are three main types of infrared detectors: thermal, photoelectric and p y r o e l e c t r i c . Thermal Detectors Thermal detectors depend upon the heating pro-perties of the radiation to produce a temperature r i s e and hence involves a change in the bulk properties of the detector element. Although these detectors have a very wide spectral range they also have very slow response times and low power absorption c a p a b i l i t i e s . Thermocouples, thermopiles, Golay c e l l s and calorimeters have been used 62 63 to measure the power output of low power cw C0 2 lasers but are too slow for pulsed C0 2 l a s e r s . Thermal detectors are normally operated at room temperature. Photoelectric Detectors Photoelectric detectors are exclusively based on semiconductor materials and rely on a quantum i n t e r -action between the radiation and the detector. The radia-tion creates a voltage signal by either a photovoltaic or photoconductive e f f e c t . In detectors based on the photovoltaic e f f e c t the radiation modifies the junction ba r r i e r potential between di f f e r e n t i a l l y - d o p e d semiconductor materials. Two types of photoconductive detectors are in use: Intrinsic Vino to conductive Detectors With these undoped semiconductor materials the radiation is detected by the a l t e r a t i o n of the e l e c t r i c a l conductivity by d i r e c t excitation of electrons across the band gap, E g (see Figure A - l ( a ) ) . Radiation of energy less than Eg does not cause i n t r i n s i c absorption but since E q is a function of temperature i t is possible to decrease (a) INTRINSIC PHOTOCONDUCTIVE CONDUCTION 4 BAND hi/ VALENCE / BAND e ' CONDUCTION BAND (b) N-TYPE EXTRINSIC E n hi/ PHOTOCONDUCTIVE , 9 VALENCE BAND (c) P-TYPE EXTRINSIC PHOTOCONDUCTIVE CONDUCTION BAND h i / VALENCE BAND Figure A - l . Photoconductive Detectors. 65 Eg by c o o l i n g the d e t e c t o r element thus i n c r e a s i n g the d e t e c t o r ' s s p e c t r a l range. PbS and InSb are two examples of photoconduct ive d e t e c t o r m a t e r i a l s . Both are operated at room temperature w i th the l a t t e r a l s o being used at 77°K ( l i q u i d N 2 ) . E x t r i n s i c Photo conductive Detectors These d e t e c t o r s are almost e n t i r e l y made of doped Germanium. In n - type d e t e c t o r s (F igure A - l ( b ) ) a conduc-t i v i t y change occurs w i th e x c i t a t i o n of e l e c t r o n s from a donor l e v e l i n t o the conduct ion band. When the e x c i t a t i o n i s from the valence band i n t o an acceptor l e v e l w i t h i n the band gap (F igure A - l ( c ) ) we have a p - type d e t e c t o r . Some dopants produce l e v e l s which are c l o s e to the upper l i m i t of the va lence band and as thus f i l l e d by thermal e x c i t a t i o n at room temperature . I t i s t h e r e -f o r e necessary to " f r e e z e out" these l e v e l s to have an e f f e c t i v e d e t e c t o r and reduce the background n o i s e . Two temperatures are commonly used , 77°K ( l i q u i d N 2 ) and 4°K ( l i q u i d He) . 66 P y r o e l e c t r i c Detectors The p y r o e l e c t r i c d e t e c t o r i s composed of a f e r r o -e l e c t r i c m a t e r i a l which possesses a temperature dependent permanent e l e c t r i c p o l a r i z a t i o n . R a d i a t i o n absorbed by the d e t e c t o r i s converted i n t o heat which a l t e r s the l a t t i c e spac ing of the f e r r o e l e c t r i c . Below the Cur ie temperature a change in e l e c t r i c p o l a r i z a t i o n r e s u l t s from these l a t t i c e a l t e r a t i o n s . U n l i k e other i n f r a r e d d e t e c t o r s the vo l tage s i g n a l generated i s p o r p o r t i o n a l to the time r a t e of change of the temperature and thus the d e t e c t o r does not come i n t o e q u i l b r i u m wi th the r a d i a t i o n . P y r o e l e c t r i c dev ices are a l s o c a p a c i t i v e r a t h e r than r e s i s t i v e thus having e s s e n t i a l l y an i n f i n i t e e l e c t r i c a l f requency response . During the course of t h i s experiment th ree i n f r a r e d d e t e c t o r s were used: a go ld doped Germanium (Au:Ge) power d e t e c t o r , which i s a p - type i n t r i n s i c photo -conduct i ve d e t e c t o r , a p y r o e l e c t r i c power d e t e c t o r and a p y r o e l e c t r i c energy meter . Table A - i g i ves the p e r t i n e n t data f o r each d e t e c t o r . The d e t e c t o r elements w i th the necessary d e t e c t o r c i r c u i t s are prov ided f o r by the manufacturers i n the Gentec and Molec t ron p y r o e l e c t r i c d e t e c t o r s . However, f o r the Au:Ge d e t e c t o r i t i s necessary to c o n s t r u c t a Tab Ie A-I C h a r a c t e r i s t i c s of I n f r a r e d D e t e c t o r s D e t e c t o r D e t e c t o r Area (cm 2) Respons i v i t y Operat i ng Temperatu re R i s e t i me Recovery T i me Au :Ge * Mo 1ectr o n Gentec 3.14 x 10"2 1.0 x 10"2 3.6 1 .35 v/kw 1.5 v/kw t 8.1 v / j 0 u1e 77°K 293°K '293°K 30 nsec 50 nsec 5 msec 0.3 sec R e s p o n s i v i t y and r i s e t i m e are v a r i a b l e . t When t e r m i n a t e d w i t h I Mfl. 68 c i r c u i t f o r the d e t e c t o r (F igure A -2 ) and a dewar c o n f i g u -r a t i o n f o r c o o l i n g the d e t e c t o r e lement , which in our * design i s mounted i n a vacuum. A CaF 2 window i s used to couple r a d i a t i o n i n t o the d e t e c t o r . 100 K I J J F F i g u r e A - 2 . Au:Ge D e t e c t o r C i r c u i t ; O u t p u t v i a 50ft C a b l e . To compare the responses of the d e t e c t o r s to a C0 2 l a s e r pu lse a KCJl converging lens (f = 9 .7 cm, <)> = 2 cm) was used to focus r a d i a t i o n on the d e t e c t o r elements and mylar sheets were used as a t t e n u a t o r s to prevent damage to the d e t e c t o r s . F igure A - 3 shows t y p i c a l o s c i l l o g r a m s obta ined wi th each d e t e c t o r . The energy of the pulse i s found by o b t a i n i n g the peak vo l tage from the Gentec o s c i l l o g r a m and knowing the r e s i s t a n c e used to te rminate the c o - a x i a l cab le the * E x p e r i m e n t a l l y found to a t tenuate l a s e r pu lse by f a c t o r of 0 . 4 2 . 69 (a) G e n t e c Energy M e t e r (b) Au:G e Powe r D e t e c t o r (c) M o l e c t r o n Power D e t e c t o r 0.05 v / d i v 10 msec/d i v 2 v / d i v 0.5 u s e c / d i v 0.2 v / d i v 0.5 y s e c / d i v F i g u r e A-3• Response o f I n f r a r e d D e t e c t o r s to a C0 2 L a s e r P u l s e 70 manufacturer's c a l i b r a t i o n curve gives the number of volts/ j o u l e . Terminating the cable with the 1 Mft input impedance of the oscilloscope gives a c a l i b r a t i o n figure of 8.1 v/joule. When using the power detectors i t is necessary to terminate the cable with a 50ft terminator. Otherwise the RC time constant of the cable and oscilloscope input impedance network is of the order of 50 usee and the oscilloscope would display the time integration of the power detector's output, which is the energy (see Figure A-4) . a) Output w i t h 50ft t e r m i n a t o r . b) Output with, no t e r m i n a t o r F i g u r e A-**. Au:Ge Power D e t e c t o r O u t p u t . Comparison of the pulse shapes obtained with the Au:Ge and Molectron power detectors show that the pyroelectric Molectron detector does not give as much detail of the pulse shape as the photoconductive Au:Ge detector. The fast risetime of the pulse is evident in both oscillograms but the Molectron gives increase thus 71 c u t t i n g o f f the t a i l of the l a s e r p u l s e . Therefore i t i s not p o s s i b l e to make es t imates of the energy of the l a s e r pu lse us ing the Molect ron d e t e c t o r . The c a l i b r a t i o n of the d e t e c t o r s was s e l f - c o n - s i s t e n t l y v e r i f i e d us ing a 2 w cw C0 2 l a s e r . The r a d i a t i o n was chopped at a f requency of 50 Hz and focused on the d e t e c t o r elements us ing the KC£ l e n s . Assuming t h a t c a l i -b r a t i o n of the Gentec energy meter was c o r r e c t , the c a l -c u l a t e d r e s p o n s i v i t i e s of the Au:Ge and Molect ron d e t e c t o r s agreed wi th the m a n u f a c t u r e r ' s va lues to w i t h i n 10%. The d e t e c t o r used throughout the experiment was the l i q u i d n i t r o g e n cooled Au:Ge d e t e c t o r . Exper imental r e s u l t s have shown t h a t the s h o t - t o - s h o t power v a r i a t i o n i s the same wi th and w i thout the KCJt l e n s . T h e r e f o r e , because the KCZ lens i s hydroscop ic and d e t e r i o r a t e s when exposed to the atmosphere, i t was decided to make power measurements w i thout the l e n s . When us ing the lens the peak power i s i nc reased by a f a c t o r of 3 . 2 2 . We are f a i r l y c e r t a i n tha t the lens (2 cm diameter ) would cover the important par ts of the c r o s s - s e c t i o n of the l a s e r tube having measured the near f i e l d r a d i a t i o n p a t t e r n of the l a s e r (see F i g u r e A - 5 ) . Using the value of 1.35 v/kw f o r the r e s p o n s i v i t y and t a k i n g i n t o account the a t t e n u a t i o n by the CaF 2 window 2.5 2.3 2.1 1.9 1.7 1.5 1.3 I.I 0.9 X - A X I S (cm) F i g u r e A - 5 . Near F i e l d R a d i a t i o n P a t t e r n . ro and the power i n c r e a s e i f the lens was present a c a l i b r a -t i o n f i g u r e of 5.68 kw/v was used to c a l c u l a t e the peak powers w i th the Au:Ge d e t e c t o r . APPENDIX B SPECTRAL ANALYSIS The s p e c t r a l output of the l a s e r was analyzed using a J a r r e l - A s h 0.5 m Ebert spectrometer (Model 82-010). The g r a t i n g used was blazed f o r 5.0u at 21.6° w i th a r u l e d area 52 x 52 mm1s and had 148 Z/mm. With t h i s g r a t i n g o the l i n e a r d i s p e r s i o n at the e x i t s l i t was 128 A/mm. The entrance and e x i t s l i t s were both set at 400u s l i t w i d t h . The manual d r i v e of the spectrometer was o r i g i n a l l y c a l i b r a t e d wi th a g r a t i n g of 1180 l/mm. Th is corresponds to a d i a l m u l t i p l i c a t i o n f a c t o r of 1.0 and o a maximum d i a l reading of 8600 A. With the 148 l/mm g r a t i n g the maximum measureable wavelength was 6.85u which f a l l s shor t of the 10.6u reg ion necessary to study C02 r a d i a t i o n . I t was t h e r e f o r e necessary to r e l e a s e the g r a t i n g ho lder from the g r a t i n g p i v o t mount and r o t a t e the g r a t i n g by hand. By f i r s t observ ing the zero order of the spectrum of the c a l i b r a t i n g l i g h t source and count ing the orders of the spec t ra as the g r a t i n g was r o t a t e d there 74 75 was no d i f f i c u l t y i n p o s i t i o n i n g the g r a t i n g f o r use in the 10.6u r e g i o n . The spectrometer was c a l i b r a t e d us ing three s o u r c e s . A He G e i s s l e r t u b e , a He-Ne al ignment l a s e r and a mecury (Hg) lamp. The mecury lamp conta ined a sodium impur i t y and i t s spectrum e x h i b i t e d the sodium D doublet besides the g reen , y e l l o w and v i o l e t Hg l i n e s . When working i n the i n f r a r e d care must be taken when o b t a i n i n g measurements of wavelength s i n c e the v a r i a t i o n of wavelength due to the changes in the r e f r a c t i v e index o of a i r i s n o n - n e g l i g i b l e . I t i s of the order of 29 A f o r 10.6u. A l l wavelengths used w i th the c a l i b r a t i o n are those measured i n a i r . Table B-I shows the l i n e s used f o r c a l i b r a t i n g the spec t rometer . A s m a l l e r v e r s i o n of a t y p i c a l c a l i b r a t i o n cruve i s shown i n F igure B - l . For wavelengths g r e a t e r than ~ 10.6u the graph d i s p l a y s a n o n - l i n e a r i t y . Th is i s not s u r p r i s i n g because of the unusua l l y l a r g e angle the g r a t i n g makes to the i n c i d e n t r a d i a t i o n . The procedure f o r c o r r e c t i n g n o n - l i n e a r i t i e s of the s ine bar d r i v e was c a r r i e d out but there was no n o t i c e a b l e change and the n o n - l i n e a r i t y remained. The c a l i b r a t i o n data was a l s o analyzed by com-puter and l e a s t squares f i t s were o b t a i n e d . A l i n e a r 3350 3530 3710 3890 4070 4250 SPECTROMETER DIAL SETTING Figure B-l. Spectrometer Calibration Curve. Table B-I S p e c t r a l Lines Used f o r Spectrometer C a l i b r a t i o n Source X Order nX Colour o o (A) Used (A) Hg lamp 5461 1 9 103,759 Green Hg 1 amp 5790 1 8 104,220 Yellow Hg lamp 4358 24 104,592 B 1 ue Ge i ss1er tube 5875.6 1 8 105,761 Ye 1 1ow He-Ne l a s e r 6328 1 7 1 07,576 Red Hg 1 amp 5461 20 1 09,220 Green l e a s t squares f i t was done using on ly the f i r s t four p o i n t s of the c a l i b r a t i o n d a t a , wi th the r e s u l t shown below: Y = (86896 ± 12) + (5.1289 ± .003)X where, o Y = wavelength in A X = d i a l s e t t i n g of spec t romete r . us ing a l l A quad r a t i c l e a s t squares s i x data p o i n t s , w i th the f i t was a l s o t r i e d r e s u l t b e i n g , Y = (84876 ± 55) + (6.2414 ± .029)X - (0 .15 x 10""* ± . 0 0 0 ) X 2 For o b t a i n i n g the s p e c t r a l output the apparatus was a l i g n e d as shown in F igure B - 2 . The spectrometer SPECTROMETER Figure B-2. S p e c t r a l A n a l y s i s Arrangement. al ignment was g r e a t l y f a c i l i t a t e d by us ing a smal l p iece of unt reated s t e e l which developed v i s i b l e f l a s h e s of l i g h t when h i t by the l a s e r r a d i a t i o n . Knur led brass was a l s o found to be e f f e c t i v e in a l i g n i n g the spec t rometer . 79 To compensate f o r the l o s s of power due to absorp -t i o n and s c a t t e r i n g the l a s e r power was inc reased by doubl ing the amount of hel ium in the gas mix ture (see Sec t ion 4 . 2 . 2 ) so the volume r a t i o of He:N : C 0 2 was 2 0 : 1 : 2 . When i d e n t i f y i n g l i n e s the spectrometer d i a l was ad justed to o b t a i n maximum peak power. A l l l i n e s were o measureable to an accuracy of ± 1 d i a l d i g i t or ~ ± 5 A. The r e s u l t s obta ined are shown in Table B-II. T a b l e B-II Raw Data from S p e c t r a l A n a l y s i s Mode of Measu rement Peak Vo1tage D i a l S e t t i ng Width o f * L i n e (A*) 2 s l i t s + lens e x i t s l i t + l e n s 1 .2 v 1 .2 v O . I 5 v 0.02 v 3706 3706 . 3669 3633 45 45 5 5 No s l i t s + lens Range of v a l u e s 3545-3771, but not peaks d i s c e r n i b l e . Width of l i n e i s range over which any power o u t p u t i s o b t a i n e d . I t i s not the h a l f power w i d t h . The r e s u l t s obta ined wi th both s l i t s and the lens are cons idered v a l i d d a t a . The data obta ined wi th the entrance s l i t removed i s a l s o cons idered v a l i d because there was no n o t i c e a b l e i n c r e a s e i n the peak v o l t a g e at the 3706 d i a l s e t t i n g . This i n d i c a t e s that the KC£ lens 80 i s e f f e c t i v e l y focused on the entrance s l i t . Thus the lens acts as i f i t was the entrance s l i t by c o l l i m a t i n g the l a s e r beam. The data taken wi th no s l i t s g i ves an i n d i c a t i o n of the p o s s i b l e range of C0 2 l i n e s l a s i n g , but should be taken wi th a g r a i n of s a l t . The e x i t s l i t in t h i s case i s e f f e c t i v e l y the diameter of the Au:Ge d e t e c t o r element which i s 2 mm and thus the p o s s i b l e number of o o l i n e s may be measured to w i t h i n ± 256 A (2 mm x 128 A/mm). The r e s u l t s obta ined w i th no entrance s l i t are shown in Table B - i n . I n d e n t i f i c a t i o n of l i n e s i s based upon comparison wi th s tandard C0 2 spec t ra (see Table B - i v ) . From the c a l i b r a t i o n graph the data taken wi th no s l i t s i n d i c a t e s the l i n e P(12) -»• P(22) are a l s o p r e s e n t . The spectrometer was a l s o scanned in the 9.6u reg ion but no l i n e s were d e t e c t e d . T a b l e B - I I I I d e n t i f i c a t i o n o f C 0 2 L i n e s c o 2 G r a p h L i n e a r F i t Q u a d r a t i c F i t L i ne Y (A ) Y - P ( J ) Y CA) Y - P ( J ) Y CA) Y - P ( J ) P ( 2 0 ) 1 0 5 , 9 4 0 - 1 1 0 5 , 9 0 4 ± 16 - 3 7 1 0 5 , 8 7 8 ± 1 2 1 - 6 3 P ( 1 8) 1 0 5 , 748 + 6 1 0 5 , 7 1 4 ± 16 - 2 8 1 0 5 , 6 9 0 ± 120 - 5 2 P ( 1 6 ) 1 0 5 , 5 5 5 + 8 1 0 5 , 5 2 9 ± 16 - 1 8 1 0 5 , 5 0 6 ± 1 1 9 - 4 1 82 Table B - I V C0 2 Laser T r a n s i s t i o n s Wavelengths in vacuum from Patel CI6] and c o r -r e c t i o n s f o r a i r c a l c u l a t e d from C.R.C. Rubber B i b l e (50 Ed.), p. E-233 A l l wavelengths in angstroms. 0 0 ° I - 0 2 ° 0 Band 00°I - I0°0 Trans i s t ion Wave length C o r r e c t i on Wave 1ength in Vacuum Due to A i r in A i r P ( 1 2 ) 105,135 28.66 105,163.7 P( 1 4) 105,326 28.71 105,354.7 PC 1 6) 105,518 28.77 105,546.7 •PC 18) 105,713 28.82 105,741.8 P C 2 0 ) 105,912 28.88 105,940.8 P C 2 2 ) 106,1 18 28.93 106,146.9 P C 2 4 ) 106,324 28. 99 106, 352. 9 P C 2 6 ) 106,534 29 .04 106,563.0 P C 2 8 ) 106,748 29. 1 0 1 06,777.1 P C 3 0 ) 106.965 29. 1 6 106,994.1 P C 3 2 ) 107,194 29.22 107,223.2 P C 3 4 ) 107,415 29.28 107,444.3 P C 3 6 ) 107,648 29.35 107,677.3 P C 3 8 ) 107,880 29 .4 1 107,909.4 P C 2 2 ) 95,691 26.09 95,717.1 P C 2 4 ) 95,862 26. 1 4 95,888. 1 P C 2 6 ) 96,063 26. 1 9 96,089.2 . P C 2 8 ) 96,211 26.23 96,237.2 P C 3 0 ) 96,391 26.28 96.417.3 P (3,2) 96,576 26.33 96,602.3 P C 3 4 ) 96,762 26. 38 96,788.4 

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