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AUTHOR (year of submission) "Full thesis title", University of Southampton, name of the University School or Department, PhD Thesis, pagination

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THE LASER BEAM HEATED EVAPORATION SOURCE

J. READ

A Thesis Submittea for the Degree of Doctor of Philosophy

of the

University of Southampton

Department of Electronics University of Southampton

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ACKNOWLBDGSMBNTS

The author wishes to express his thanks to Mr. K. G. Nichols, the project supervisor, for his advice and encouragement during the course of this project.

Thanks are due to the Paul Instrument Fund for a grant to finance the construction of the apparatus. The helpful suggestions of

Dr. 0. Simpson in his capacity as assessor for this grant are gratefully acknowledged.

The author would like to thank the University of Southampton for the research

facilities made available to him. In particular thanks are due to the technical staff of the

Electronics Department for their interest and help, especially to Mr. G. Newell and the staff of the Mechanical Workshop.

For the help received from Messrs. K. Taylor and J. Pring of the Mechanical Workshop A.8. W.B. , Portsdown, the author expresses his appreciation.

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snmARY

The InveBtigation reported in this Thesis was directed, at ascertaining the feasibility of using the focussed "beain of a laser to evaporate materials for the vacuum deposition of thin films.

The advantages of this evaporation source would "be:

1) Edge definition of films "better than 1 ^ with 'out-of-contact' masking technioues.

2) Avoidance of film contamination from the material of the source crucible.

The design and construction of the experimental apparatus are described. Results of measurements of the output characteristics of the laser are given.

A theoretical treatment of the heating effect of the laser beam is presented and compared with the results

obtained by other workers.

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damage produced In. metals "by the focuesed laser beam with the output parameters of the laser and the thermal characteristics of the material. Suggestions are made for further investigations.

Evaporation experiments were carried out using several different source materials. In general the deposited films were of poor quality, being marred by

globules of metal w h k h had been ejected in tha molten state from the source.

It was possible to obtain good quality films of

materials which had high vapour pressures at that melting point. Materials with low vapour preseures at their melting points were difficult to evaporate.

Under carefully controlled input conditions the source fractions as the theoretical disc-evaporation model. The evaporant is then distributed about the normal to the source irrespective of the angle of incidence of th^ laser beam.

The edge definition of deposited films was investigated and found to be limited to about 5 This was due to

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IM)T3X

CKAPTER 1 INTBODUCTIOR

1.1 G-eneral

1.2 Evaporation Sources 1.3 Masking Techniques 1.1). S&ge Definition 1.5 Oontamination

References

page

1

3 5 6

7 8

CH.4PT3R 2 G0N8ID33RATI0N3 GOVBRNING THE OHOICS OF LASER SYSTEM

2.1 General 9

2.2 Calculation of Heeting Effect 10

2.3 Estimates of AAiounts Vapourised and

Thickness of Films 12

References Ij

Tables I-IV 17 -

Hi-CH.APTER 3 DESCRIPTIOi: OF THE BXPERII^TAL APPARATUS

3.1 Introduction 18

3.2 The Vacuum System 19

3.3 The Laser System 21

3.4 Reflector Alignment 26

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CHAPTER 4 M3A8UR3MSNY OP TIM3 OUTPUT CHAEACT3RI8TIC5 OF THE LASER

Introduction 28

Threshold Energy 30

4^2 Output Energy

4.2.1 General j2

^^2.2 Construction of the Copper Cone

Calorimeter 53

h^2.3 Construction of the integrating

Photodiode Circuit j8

4*3 Pulse length and Mean Po^er Output of the Laser

4.3.1 Pulse Length 40

4.3.2 Mean Power Output 41

4.4 Peak Power, Duration & Separation of the Spikes

4.4.1 Peak power of the les 42

4.4.2 Duration of the Spikes in the

Output Pulse 47

4.4.3 Separation of the Output Spikes 4? 4.5 Angular Divergence of the Laser Beam

4.5.1 Intensity Distribution in the

Focal plane 48

4.5.2 Mepsurement of the Far-Field Pattern 50 4.5.3 L.T.A* Intensity Distribution in

in the Focal Plane of a Lens 54

References 57

CHAPTER 5 E OF THE F0CU38ED LAS^R BEAM. (L:r'% i''RE SURVEY)

5. 1 Machining and Welding 58

5. 2 Gratering Effects and Momentum Transfer 59

5. 3 Thermionic Emmission and plasma Effects 60

5. 4 Conclusions 62

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CHAPTER 6 THBORSTICAL DISCUSSION OF THE HEATING AND OTHER EFFECTS OF THE FOCUS BED LASER BEAM

Introduction 65

6.1 General Model, Three-Dimensional Heat

Flo?/ 66

6.2 One-Dimensional Model (Ready) 70

6.3 Derivation of One-Dimensional Model

from Flux Considerations 71

6.It- Comparison of predicted Surfpce

Ten^eratures 7^

6.5 Comparison of predicted Interior

Temperatures 77

6.6 Change of State 79

6.7 Applications of the Theory 82

6. 7 . 1 Time Taken for Melting to Occur 82

6.7.2 Ten^erature Rises at the Heated Spot 8I4.

6.7.^ Power Loss by Evaporation 87

6.7.4 Radiation Pressure and Recoil Pressure 88

6.8 Conclusions 92

References 92

CHAPTER 7 EXPERIMENTS ON THE EFFECTS OF THE j<X)CU88BD BEAM OF THE LASER

7.1 Introduction 94

7.2 Effect of Misfocussing of the laser beam 95 7.j Efficiency of Material Ablation by the

Laser 98

7.4 Depths of Craters Formed with the Laser Pulse

7.4.1 Effect of Variation of Laser Output

Parameters 101

7.4.2 Experiments with Filters to Attenuate

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7.5 Diameter of the Oraters j^ormea with the Laser Pulse

7.5.1 Experiments with zinc. Varying laser Output

7.5.2 Effect of Variation of Focal Length of Lens used

7.5.3 Experiments with Different Materials photographs of Graters Produced by the Laser Beam

7.6

7.7 Conclusions

References 110 112 117 120 122 122a CHAPTER 8

8.1

8.2

8.3

8.1+

8.5

8.6

8.7

8.8

EXPERIMENTS ON EVAPORATION USING TEB P0CU38ED LASER BEAM

Preliminary Experiments

Experiments with Zinc as Source Material

Distribution of Evaporated Material Condensation Phenomena

Experiments wit. ji/.crent Materials Experiments on Sdge Definition

123 129 133 137 139 iWi Mechanism of Ejection of Molten Material I4f

149 Use Of the Q-Swltoh Laser Beam for

Evaporation

CHAPTER CONCLUSIONS 151

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CEAPTKR 1

INTRODnCTION

1.1 GENERAL

With the advent of the LASER (acronym for Light

.Angplification hy the Stimulation of Emitted Radiation) in 1960 (Maiman, et al^) many potential uses of this new device were suggested. Those of interest in the micro-electronics field are:

a) Micro-welding of leads to thin films and integrated circnits.

b) Milling of thin films of materials.

0) Evaporation of materials for vacnum deposition.

Industry has concentrated much effort into utilizing the laser for micro-welding. The advantage of this

technique over the nearest competitive technique, electron-heam welding, is that it can be done in air. Electron-beam welding must be done under vacuim. Similarly,

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- 2

-tut this is preferable to the use of a vacuum system. A disadvantage is that the limit on the size of the focussed spot of the laser team is greater than the

similar limit for the electron-beam. This is a function of the wavelengths involved. However, in practice lens aberrations are found to limit the spot size of the

electron-beam to similar dimensions to those of the focussed laser beam. Both of these lailllng technigues are currently under investigation in this Department.

This report is concerned with the use of the laser to evaporate materials for vacuum deposition. It is

believed that similar work is being done by only one other group (Smith & Turner^).

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3 -1.2 EVAPORATION 80URCS8

Conventionally, materials are evaporated from foil "boats or wire filaments constructed of refractory metals

(see, for example, Holland, 1963^). The most commonly used metals are Tungsten and Molytdenum. These vapour sources are heated directly "by passing electric currents through them of the order of 40 A - 60 A, to obtain the required temperature for the evaporation. It is

generally found that in order to obtain good quality films the temperature must he kept such that the vapour pressure of the evaporant is ahout 10^ Hg. Temperatures in excess of this cause sputtering of molten material. At vapour pressures helow lOp, deposition "becomes a lengthy process

and contamination from the residual gases in the vacuum chamber can "be a major pro'blem.

For most of these vapour sources evaporation takes place over a'bout 1 cm or upwards of the length of the

filament or hoat. As will "be shown later this can "be a disadvantage. Two types of vapour source have "been developed with effectively small areas of evaporation. These are:

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— Lj. —

Source (a) is simply a flat foil with a small indentation in the middle. The material to he evaporated is placed in this 'dimple' and heated in the normal way. The charge of evaporant used must he such that the molten material is confined to the indentation. Source (h)

allows for greater amounts of material to he evaporated. It consists of a tube of refractory metal with a hole of 100^^500^ diameter about half way along its length. This tube is filled with material to be evaporated and the ends are crimped. On heating the boat, evaporation then

effectively only takes place through the lOOy^SOOy, hole in the tube.

Electron bombardment heating is also widely used for evaporating materials. A small-sized evaporation source can be generated with this technique. However the entire electron gun and focussing optics have to be mounted

inside the vacuum system used. This leads to contamination problems as evaporated material is deposited inside the

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5

-1.3 MASKING TBGHNIQUS8

In order to define the circuit pattern laid down on the Guhstrate, it is necessary to prevent evaporant

impinging on certain parts of the substrate by a 'mask'. Masking techniques can he divided into two categories: a) 'In contact' masking hy photo-chemical methods, h) 'Out of contact' masking hy etched metal foil.

(See for example Manfield, 1963.^)

In method (a) photoresist techniques (Kodak^) are used to etch away portions of a copper film covering the whole substrate. The desired circuit material,

e.g. nichrome, is then deposited on the exposed parts of the substrate and on the 'in contact' copper mask. The underlying copper is then etched away together with the unwanted nichrome. This process is r&peated for each layer of material deposited in the circuit, using such 'in contact' mask materials and etchants as necessary to

prevent damage to the actual circuit materials.

In 'out of contact' masking photoresist techniques similar to those above are used to etch through metal foil, usually copper or molybdenum, to the required circuit

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- 6

-substrate during the vacuum evaporation and. prevents evaporant reaching the substrate where it is not wanted.

For 'in contact' masking the vacuum must be broken for processing between depositing each separate layer. The 'out of contact' masks can however be changed under vacuum and a whole circuit can be deposited in one pump-down of the vacuum system.

1.4 EDGE DEFINITION

The sharpness of the edges of the thin films when using 'in contact' masking are primarily determined by the degree of control which can be exercised over the etching process. In practice edges of better than 1 micron can be obtained.

For the case of 'out of contact' imasking the sharpness of the edges depends upon the geometric penumbral region as shown by Fig. 1.1. To a first

approximation this is given by ab =

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7 -AB = length of Bouroe

D = source to mask distance d = mask to suhstrate distance

Typically AB is 1 cm, D is 10 cm, d is about 0.5 mm say, giving ab equal to 50^, which is vastly Inferior to the 1^ of 'in contact' techniques. However a reduction in the source size to 20Dp. would give ab = ly with 'out of contact' masking.

It would seem therefore that by using a focussed laser beam to evaporate materials it should be possible to obtain sharply defined thin films while retaining the inherent advantages of out of contact imasking.

1.5 CONTAMINATION

A further advantage of the use of the laser beam is that the material to be evaporated forms its own crucible and nothing except tha source becomes heated inside the vacuum system. With boat and filament sources

contamination of the deposited films by the heater material 6

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- 8

-RBPSREN0E8

1. MAIMAN, T.M. et al, Stimulated Optical Emission in Fluorescent Solids. Phys. Rev. 123. 4.

Aug. 15, 1961. pp.1145-1157.

2. SMITH, H.M. & TUimBR, A.P., Vacuum Deposited Thin Films Using a Ruhy Laser. Appl. Opt. 1. Jan., 1965. pp.147-8.

3. HOLIAND, L., Vacuum Deposition of Thin Films. 5th 3d. 1963. Chapman & Hall. pp. 108-121. 4. MANFIBIjD, H. G. , The Uses of Photo-mechanical

Processes in the Manufacture of Micro-miniature Circuits. Elect. Eng. 426. Aug., 1963. pp. 5 20-5 24.

5. KODAK, Photosensitive Resist Techniques.

6. HEAVBBf, 0. 8. , The Contamination in Evaporated Films by the Material of the Source. Proc. Phys. 800.,

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ab a AB d

Source Mask Substrate

Fig.1.1. Formation of the Geometric Penumbral Region.

2d.2fe

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CHAPTER 2

CONSIDERATIONS GOVBENING THE OHOIGB OF LASER SYSTEM 2.1 GENERAL

At the inception of this project there were several types of laser systems available. These are listed. In Table I together with details of their moda of operation and their output characteristics. For comparison

Table II lists the types of laser presently available.

In choosing a laser system for this application the following output characteristics have to be considered: a) The output powers and energies available.

b) The beam divergence of the coherent light output. c) The pulse repetition frequency in the case of pulsed

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1 0

-2.2 OALCnLATION OP HEATING BFPBCT

Preliminary calculations were made on the heating effect of the focussed team for each type of laser. To do these calculations the size of the focussed spot and the intensity distribution across it must he known or assumed. As will be shown later the intensity

distribution can he approximated to by a Gaussian expression.

The spot size is determined as shown by Pig. 2.1. Here f is the focal length of the lens and 8 is the angle

at which the beam intensity falls to of its value along the axis. Assuming an aberration-free lens the radius d at which the intensity falls to of its peak value at the centre is given by fB.

(In the literature, beam divergence is sometimes defined as the 10 dB intensity angle, sometimes not defined.)

Analytical expressions for the temperature

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1 1

-a pulse of tot-al const-ant power is given by,

Po , Vp

tan-l 2{lf )(t)2

where X =

t = the time after the start of the pulse, and Y and k are the thermal diffusivity and conductivity. Using this result the peak temperature rises which could be obtained at the surface of Aluminium and Tantalum using

a 3 0 cm focal length lens were calculated for each type of laser. Table Ill(a) shows the relevant thermal

characteristics of Aluminium and Tantalum. Table Ill(b) shows the computed temperature rises with details of the assumed laser pulse shape and intensity.

It can be seen therefore that it is necessary to use a pulsed solid state laser for evaporation of metals.

Also it can be shown in a similar manner that even if a continuous laser of suitable output were available the effective area over which evaporation takes place would be large compared to the pulsed case.

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1 2

-Blots to a minimum it is necessary to use a pulsed beam. (Wells^, Brown^.)

2.3 ESTIMATES OF AMOUNTS VAPORIZED AND THICKNB88B3 OP FILMS Tatle IV gives an order of magnitude evaluation of the amounts of material which could be evaporated per laser pulse.

The first two columns show the amounts in gm. and in cc. of different materials which could be vaporized by 5 joules assuming all of the energy goes into evaporation. The third and fourth columns show the thicknesses of films which could be produced by these amounts of material with

source to substrate distances of 5 cm respectively.

These thickness estimates were based on the assumption of uniform evaporation over the half sphere and unity

condensation coefficient.

It is apparent that a large number of 5 joule pulses would be required to produce a film of sufficient

thickness for micro-electronic applications.

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13

-to conduction losses. Correspondingly more pulses would therefore he required to produce a film of given thickness.

ESFBOmOBS

1. PITTAWAY, L.G., The Temperature Distributions in Thin Foil and Semi-Infinite Targets Bombarded hy an

Electron Beam. Mullard Research Lat. Report No.463. Aug., 1963.

2. WBLL8, 0. C. , Calculation of the Heat-Affected Zone During Pulsed Electron-Beam Machining. I.E.E.EL Trans. Electron Devices ^ (1965) p.224.

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-

-TABLE I

Laser Systems (December 1963)

Type of Laser

Solid State Gas

e.g. He-Ne Mixture Semiconductor Type of Laser e.g. Ruby Gas e.g. He-Ne Mixture Junction Diode Type of Laser e.g. Ruby Gas e.g. He-Ne

Mixture e.g. Ga.As. diode

Szoitatlon Optical pumping by flash tube

a. c. or a. c. discharge in gas Direct electrical excitation a) Oontinuous 20 mW t) pulsed Output a) Normal.

5-10 joules in 500

pulse h) Qrswitch.

1-2 joules in 50 nS pulse Continuous 20 mW Direct electrical excitation a) Oontinuous 20 mW t) pulsed Beam Divergence Efficiency 0/P Wavelength 2-5 mrad ^—2 ^ O'l # 0.1-0.2 mrad 1-2 ^ 1-5 degrees !

" f 20-^0 ^ Beam Divergence Efficiency 0/P Wavelength

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15 -TABLE II

Laser SystemB (July 1966)

Type Solid State

e.g. Ruby

ExcitationiQptioal pumping |ty flash tube

Output Beam Divergence Efficiency Wavelength a) Normal. 1,000 j in 500 pulse 60 p.p.m. h) Q-switch.

10 joules In 30 n8 pulse c) c.w. 10 watt

2-5 mrad 1-2 # O'l # O'Ol % 0'6945 7"

Gas ISemiconductor e.g. CO2-N2I G'S" Ga.As. e.g. He-He ' diode

Electrical (Direct discharge ln|electrical the gas lezcitation

132 watt continuous 300 watt peak in 1 p8 pulses at' 1 Ec/s

a) continuous

100 mW

b) 10 p3 pulses at 1 Ec/s. Mean power

about Iwatt

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— l6 — TABLE Ill(a)

Thermal and. Vapoiir Pressure Data for Aluminium and Tantalum

Aluminium Themal Conductivity (k) 2'30 Thermal Diffu^vity Boiling Point

( ° G )

Temperature

( ° o )

for Pp = 10 Aluminium

Themal Conductivity

(k)

2'30 0-97 2,736 1,465

Tantalum 0*54 0*22 5,510 3,040

- Juuie um.( y - omr sec"^

sec"^ deg'^0

TABLE

HeatinK Effect of Different Types of Laser

Ruty Laser

Pulse Oharacteristios Peak Temperature Rise (°C) Ruty

Laser

5 joules in 500 u8 t.a. 5 mrad. '

M Ta

6'45 X 10^ I'Ol z 10^ (at end. of pulse) Gas

Laser

20 mW continuous b.dU 0'2 mrad

17-5 (after 1 sec) DioAe

Laser

20 m%7 continuous t.d. 1 dagree

0'05 1*04

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17 -TABLE IV

EstimateG of Amounts of Material which Gould "be Vaporized with 5 Moules

and Resultant Film Thicknesses

Amount Vaporized Thickness of Film

Material 1, in A.U.

in gm. (xlO^) in c.c. (zlO^) at 5 cm at 10 an

Aluminiimi 3-59 13-3 85 21.3

Antimony 24-2 36-7 23^ 585

Chromium 6' 22 8.08 52 13

Gold 17-3 8.92 57 14.2

Silicon 2.11+ 8.92 57 14-2

Silver l6' 8 16.2 103 26

Tantalum 10" 1|. 6.25 40 10

Zinc

1

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1 8

-CHAPTER 5

D38GRIPTI0N OP THB BXPERIMBNTAL APPARATUS 3.1 In view of the accurate alignment needed for

optical systems using lasers it was decided to mount all the optical components on a rigid bench. The laser is external to the vacuum system and to prevent relative movement between the laser and the vacuum system both parts were mounted on a stainless steel baseplate. A line diagram of this part of the system is shown in

Pig. 3*1' Figure 3.2 is a photograph of the arrangement,

On the left a ^ metre optical bench holds the laser cavity with the external reflector mountings. The

vacuum chamber on the right makes a vacuum seal to the baseplate by a metal flange holding an '0' ring.

The stainless steel plate is mounted on a wooden framework with anti-vibrmtion mountings. This is to prevent mechanical shocks from other parts of the building misaligning the optical system.

Fig. 3.3 shows the overall arrangement of the

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Rotary seal.1.

Laser

O p t i c a l

w i n d o w

S u b s t r a t e

Rotaiy seal.2.

R, A R j . Reflectors

[image:30.595.84.569.57.815.2]

Fig 3.1. Line Diagram of Laser Evaporation System.

[image:30.595.98.521.448.771.2]
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FIGUES 3.3

Photograph of complete apparatus, showing; A - Vacuum chamber,

B - Focussing adjustment. Rotary Seal 2, C - Source drive, Rotary seal 1,

D - Optical window, S - Laser cavity, F - Laser reflector, Q - Laser power supply,

H - One of the two boxes of capacitors,

I - lonisation guage amplifier and power supply

J - Automatic trigger unit for operation of the laser at predetermined intervals,

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i

m

[image:32.842.69.758.85.581.2]

DANCER HIGH VOLTAGE

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— 19 — 3.2 THE VACUUM 8YST3M

The vacuum chamber is pumped in the conventional manner. A small rotary backing pump (Edwards High Vacuum 1550) is used to take the pressure down to about 10"2 torr. A two-inch oil diffusion pump (B.H.V. P 203) is then used to take the pressure down to about 10"^ torr, at which pressure most of the evaporation experiments

were performed. To isolate the vacuum chamber from the pump system when required a water-cooled baffle valve

(B.H. V. H5L2) is mounted between the two. This also serves to reduce the backstreaming rate during pumping and so keep the vacuum chamber free of pump-oil.

The pressure in the vacuum chamber was measured with an ionization gauge (B.H.V. 1G-3H) mounted in the

baseplate. The circuits for the ionization gauge are shown in Pigs. 3.^(a) and (b). The amplifier and power supply were designed and constructed as part of this work.

With this vacuum system it took 2^ to 3 hours to reach the working pressure of 10"^ torr.

The vacuum chamber itself is a 12 inch diameter

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am VOLT*

(v) — I

MAX?

Fig.3.4.a. Ionization Gauge Amplifier Circuit.

IG-3H

IM.

-vW^ ®

VARIAC

fOOK. " W V ^

lO K.

- w v 230VL

50 c/m

^ I.G.AMPlimEA

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- 2 0

-moke vacuum tight joints against the "baseplate and the demountable lid.

An optically flat glass window, let into the side of the vacuum chamber, admits the light beam from the laser. There are two rotary vacuum seals in the sides of the chamber which were used to focus the lens during setting-up and to move the target during evaporation experiments.

Inside the vacuum chamber and mounted on the baseplate is a 25 cm optical bench. This is in line with the optical window and with the % metre bench which holds the laser system. A lens holder is mounted on this bench to hold the focussing lens. The shaft of rotary seal 2 (see Fig. 3'1) connects this lens holder to a micrometer outside the vacuum chamber which was used to adjust the position of the lens.

As will be described later the evaporation source for most of these experiments was in the form of a cylinder of the material to be evaporated. This was mounted on the shaft of rotary seal 1. In order to

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2 1

-rotated through a screw thread by a 1 rev. per hour clock motor. Thle device is ehowa in Pig. 3.5(a). successive pulses from the laser therefore describe a helix on the source.

There are also several vacuum 'lead-throughs'

mounted in the baseplate of the vacuum chamber. These were used for thermocouple wires and heaters as described

in the experimental work.

The substrates for collecting the evaporated material were held in position by various jigs as

demanded by the experiment. Pig. 3«5(b) shows the

interior of the vacuum chamber as set up for an Antimony evaporation.

5.3 TEE LASER SYSTEM

The considerations which governed the choice of laser system have already been discussed in Ohapter 2.

The active element utilized was a ^ inch long by § inch diameter ruby rod with 0*04% chromium doping

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Fig.5.5%. Motor drive arrangement for moving source

Fig.3'5t. Inside of vacuum chamber set up for an evaporation of Antimony with beam at normal incidence.

A - Lens holder,

B - Substrate balder with liquid nitrogen cooling.

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FIG.aSa.

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- 2 2

-arc. The sides of the ruby are polished and the ends have anti-reflection coatings for the ruby laser

wavelength of 6943 Angstroms,

The crystal axis of the ruby is at 90 to the optic axis. The laser output of the ruby is plane polarized light polarized at right angles to the optic and crystal

Basic principles of laser operation will not be discussed here and reference is made to Lengyel^ and

, 2 .

Bxcltetlon of the ruby is by optical pumping and this is accomplished using the intense flash of light accompanying the rapid discharge of electrical energy

The flashtubes used were 5 inch linear, Xenon filled tubes (Thermal Syndicate Ltd.). They have a maximum rating of about 1,000 joules at 2"5 kV.

It is necessary to concentrate as much of the light output from the flashtube as possible onto the ruby.

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Foci of the Ellipse — X.

-SOM-Fig.3y6ba. Cross-section of the Elliptical Cavity.

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23

-was used. Fig. 3.6(a) shows a cross-section of the elliptical-cylindrical cavity and Fig. 3.6(t) shows a view of the actual cavity with one side wall of the ellipse removed. The flashtube lies between one focus and the cavity wall with the ruby between the other focus and the wall.

The cavity was machined from a solid piece of aluminium and has provision for water-cooling.

The energy supply for the flashtube comprises a bank of paper-dielectric capacitors ('Hikonal', T.O.C.) of 400 in all which can be charged up to 2*5 kV from the power supply. The circuit diagram of the latter is

shown in Pig. 3.?' This supply also provides the pulse to trigger the flashtube discharge.

The trigger output from the supply is fed to a small ignition coil mounted on the end of the laser

cavity via 80^^ co-axial cable. Th^ voltage pulse from tha ignition coil is coupled to the flashtube by a fine wire wound round the flashtube. This ionizes the gas inside the flashtube and the energy stored in the

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N L 60MA OO K SOOK. lOO K.

-a6M 500K

-IPKV

R R

26 KY S/V\^-~^SK2.

TPKBEE

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— 2^ —

A general circuit diagram of the arrangement is shown in ?ig. 3*8(a)'

As the flashtuhe is mounted close to the wall of the laser cavity it was found necessary to enclose the

flashtube in another tube of clear fused-quartz to

insulate the trigger wire. The presence of the quartz tube did not alter the threshold energy by any

detectable amount.

The resistance of the discharge path of the

flashtube is of the order of 0*7 ohm. It is necessary to place inductors in series with the capacitors and flashtube to prevent the capacitors discharging too rapidly and shattering the glass envelope of the tube. The manufacturers recommended an inductance value of 200 for this purpose.

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aoo^n

K)O^M.

0000 I U

Trlgyer coll tOOuF.

S, & Sj . Discharge

switches

From Moger

on power

«upply.

Fig.3.8.a. Electrical Circuit of the Laser system.

2f

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25

-The pulse repetition frequency of the system was found to he limited hy the heating of the flashtube. At discharge energies of up to 800 joules it vms possible to operate tha system once every two minutes without any decrease in the efficiency as measured hy th^ output from the laser. For input energies between 800 joules and 1,200 joules intervals between 3 minutes (800 j) and 10 minutes (1,200 j) were allowed for cooling of the flashtube.

All connections between the power supply, capacitor banks and laser were made with Amphenol Heavy Duty

Co-axial lead.

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- 2 6

-The reflectors used in this system were multi-layer dielectric coated optical flats with 99% and 75%

reflectivity at 69^3 A.U. (Orubh Parsons Co.). The optical flats used for these were 1 inch diameter hy § inch thick. They were mounted in holders (see Fig. which have micrometer adjustments to allow for accurate

alignment of the reflector.

3.4 REELECTOR ALIGNMENT

The reflectors need to be aligned parallel to each other to within a few minutes of arc (see Section 4#1). An auto-collimater may be used for this. However it was found that sufficient accuracy could he obtained with the system shown in Pig. 3.8(b). With the aperture in the focal plane of the lens the emergent beam is collimated with a divergence angle of the order of ^^f radians as

shown. The system is placed in line with the laser and reflectors. The reflected images of the aperture back on itself from each mirror are then made to coincide with each other.

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27

-from the aperture can. he seen. These are brought into line with the direct image by suitable adjustment of the micrometers.

For optimum performance the mirrors aligned with the end faces of the ruby. This should not be necessary and suggests that the end faces of the ruby

are not auite parallel.

REPBRBNCE8

1. LBNGYSL, B. A. , Lasers. Wiley * Sons, Inc. New York. Ist Ed. 1962.

2. HBAVEN8, 0.S., Optical Masers. Methuen Monograph. Methuen A Co. Lts. London. 1964.

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- 2 8

-CHAPTBE 4

MBASUBrnMRNT OP THE OUTPUT CHARACT%BI8TI08 OP THE LASER

The iatenBlty,of the output pulse the ruby laser, in time, consiste of a series of short pulses of about 1 ^8 duration. A typical oscilloscope trace of the output from a photodiode placed in the laser beam is shown in Fig. 4.1. The short pulses will be referred to as'spikes' to avoid confusion with the output 'pulse' -the total laser output.

These spikes are random la intensity, duration and separation. Figures 4.2, 4.3 and 4.4 show parts of the laser pulse output on expanded time scales.

In order to understand the effects of the focussed laser beam it is therefore necessary to know several parameters of the output pulse from the laser. The following characteristics were thought to specify the output for these purposes:

a) Threshold Energy - the input energy to the flashtube at which laser action first takes place.

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PHOTOGRAPHS OJf LASER OUTPT^r WAVEFORM

Fig.4.1. Entire Pulse. Voltage l'80kv. Horizontal Scale 50 ^8/cm.

Vertical Scale 3 ki,y/cm.

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BA m I m k kk » * # .# & * » * a ' ' km.**,

•MWUMiltlil!

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Fig.4.3. Pump voltage 1'30 kv. Horizontal Scale 5 u8/cm. Vertical Scale 3 kw/cm.

Pig.4.^, Taken near end of the laser pulse. Pump voltage 1*80 kv.

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FIG.4.3.

• • • • • — • •

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29

-c) Pulse length and. Mean Power Output in the pulse. d) Peak Power, duration and separation of the spikes

occurring within the pulse.

e) Angular Divergence of the output team.

The characteristics (h) to (e) vary over the range of input energies and have to he known over this range.

These parsmeters vary considerably even between

similar rubies operated under nearly identical conditions (see for example Kellington^).

Any alterations in the reflectivities of the mirrors used for the laser activity or in their position with

respect to each other also changes these output parameters. Further, some authors have noted changes in the output

team divergence of lasers caused hy the changes in the flashtube used. (Wsynant, et al)^ This was thought to be due to small changes in the position of the flashtuhe and consequent alteration of the pumping energy distribution in the laser rod. No significant changes in these

parameters were noted here with different flashtuhes of the same type.

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30 -4.1 THRESHOLD ENERGY

For laser action to take place in ruty at least half of the groTind-state population of the Chromium ions must he stimulated into upper energy levels (Maiman^). When

this condition obtains stimulated emission takes place with angplification rather than ahsorption.

For the laser to act as an oscillator there must he feedback in the system which, as described above, is achieved using reflectors. These may he concave as is often the case with gas lasers. In the present system plane mirrors were used to give smaller team divergence

and to utilize the whole volume of the ruhy.

The reflectors have to he aligned parallel to each other to within a few minutes of arc to give the required feedback. Any misalignment gives rise to an increase in threshold and a decrease in output efficiency above

threshold.

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31

-For this system the threshold was at ^.20 joule with 99^ and 75% reflectivity mirrors.

At first the threshold was at 400 joule. However several flashtube breakages during operation damaged the aluminium cavity causing a decrease in reflectivity. The threshold rose to 420 joule and remained reasonably constant.

Towards the end of this work the dielectric reflectors also became damaged, due to Q-switching experiments, and the threshold then rose to nearly 500 joule.

The degree of misalignment of the reflectors which could be tolerated was measured. This was done by

altering the angle of one mirror (the 99% reflector) with reference to the rest of the optical cavity by known

amounts. An auto-collimator with a calibrated eyepiece graticule was used for this purpose.

A 10% increase in threshold energy occurred at

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32 -4.2 OUTPUT ENSRGY

4^2.1 General

There are several ways of meaBuring the output energy from the laser. However no Instruments for the purpose were available in the Department when this work was started and a device was made up as part of the work.

The methods which have been used elsewhere are: a) Rat's nest calorimeter.

b) Liquid calorimeter.

c) Copper cone calorimeter.

d) Integrating Photodiode or Photomultiplier. These methods are detailed in a review paper on the

subject by Bateman\

The calibration of the calorimetric methods can be made absolute provided that any losses caused by the

reflectivity of the devices are known. In order to use an integrating photodiode it is necessary to calibrate the device against a calorimetric method. It was

decided that it was necessary to have both a calorimetric method and a direct electronic method of energy

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33

-Attempts were made to use thin films of silver with blackened surfaces to absorb the laser energy. The films were, however, damaged very easily by the

unfocussed beam. A further attempt to use indirectly heated thermistor beads as calorimeters was similarly frustrated.

It was therefore decided to use one of the conventional methods. In the case of the rat's nest calorimeter and the liquid calorimeter, reflection losses can be 20^ or more and are difficult to measure as the reflections may be diffuse in character. With the cone calorimeter the angle of the cone can be made such that the number of internal reflections is increased to the point where the total losses are negligible. An

instrument of this kind was therefore constructed it being considered the easiest to operate reliably.

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34

-Pig. 1, Batemann^, the total absorptivity was therefore estimated at tetter than

The two cones were matched for weight before and after their seams were soldered. The apex of each cone was indented slightly to take a small head-type thermistor (Mnllard Type VA 3102). These were then mounted using a small quantity of Araldite. This

ensured a good thermal contact between the copper cones and the thermistors while insulating them electrically. The two cones were then mounted side by side on a piece of perspez which formed the end piece of a metal cylinder. The balance controls, supply and output sockets for the circuit were mounted on insulating material in the other end of the cylinder.

It was decided to use the thermistor beads in a simple bridge circuit for measurement purposes. This circuit is shown in Pig. Figures 4^7(a) and (b) show the completed calorimeter.

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R1, R2. V A 3 1 0 2 T h e r m i s t o r s ( M u l l a r d ) .

R3, R4. 2 2 K . 1% hi - s t a b ,

R5 I K . , R6 lOO'.i.

2 2 K at 20°C.

FIG.4.5. Bridge Circuit for Calorimeter.

((.

^ +

IK. no

•X"

+ 9 VOLT SUPPLY

oa4?

3,300 pF

-5) OUTPUT TO

OSCILLOSCOPE

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35

-A &. c. supply of 9 volt was used for the bridge. The voltage between the points A and B was measured using

a Phillips d.o. milli-voltmeter type GM 6020. The reading of the milli-voltmeter was noted at 10 second intervals for about 3 minutes after the operation of the laser aad graphs were then plotted of the voltage.

The voltage output reached a maximum about 5 seconds after the laser was operated. During this time the heat energy was being distrlhuted by conduction in the cone until a uniform temperature was reached. The absorbed e n e r g y a l s o being conducted and radiated

away from the cone and the second part of tha curve, when the cone has settled down to an even temperature,

shows the rate at which this loss of energy or temperature was occurring.

The time constant of the decay of the temperature was found to be 66 seconds with the calorimeter in air. Extrapolation of this part of the curve back to t = 0 allowed for the loss of heat energy occurring during the initial period before a uniform temperature distribution was reached.

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[image:61.595.89.596.69.741.2]

FIG.4.7a.

The Copper Cone Calorimeter.

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36

-curves tack to t = 0 the logarithm of the meter reading was plotted against a linear time scale. . This gave a straight line for the decay of the voltage output with time.

The time-constant of 66 sec was found to he rather short for taking accurate meter readings. To Increase the time-constant the calorimeter was placed inside the vacuum system. The balance controls were brought outside the vacuum system and all electrical

connections were made using the electrodes in the base-plate of the vacuum system.

At a pressure of lO"^ torr the thermal time-constant was 82 sec and at 10 ^ torr it was l60 sec. The calorimeter was therefore used at 10 torr and

measurements of the voltage unbalance of th^^ bridge were noted for different output levels from the laser.

Calibration of the calorimeter was done as follows. The bridge circuit was connected up with

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^ 2 0

Meter Reading (mV)

FIG.4.8L Calibration Curve for the Calorimeter

^0 MO

Input Energy (o (ho FlnsMtul^o i-j

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37

-changes were made in one and. the voltage unbalance in the bridge noted using the milli-voltmeter.

Then, using the manufacturers data on the temperature coefficient of resistance (given as ±5^^ of the thermistors a graph was plotted of change in

temperature against change in resistance. This was found to he linear over the small range of temperatures likely to he experienced.

The thermal capacity of the cones used was known from the previous weighings, assuming values for the specific heats of copper and solder. A calibration curve for the calorimeter giving extrapolated meter

readings against energy input is shown in Pig. 4.8. The slope is 35 mj/mV.

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38

-Pig. 4.9 is the result of many determinations of the output energy. later in the course of this work a commercial instrument (T.R.G. thermopile 101 and energy meter 102) was available. The output energy was checked with this instrument and the results were found to agree to within ±2%.

4*2.3 Construction of Integrating .r^jto-Diode Circuit The photo-diode used during most of the work was a silicon device (Semiconductors Ltd. 80 1). For measurements on the internal intensity structure of the laser pulse, in time, this diode was simply used reverse-hiased with a 220A load resistor. The voltage across the load resistor was monitored on an oscilloscope.

The linearity of the output voltage with light intensity on the diode was checked with an ordinary tungsten bulb light source and found to he good up to 0*5 volt output which was the maximum obtainable with the experimental

arrangement. The linearity of the device response was further checked up to about 2 volts output using the integrator and filters.

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39

-ciroultG. However it was found that tha low input

impedance of the transistor made the decay time constant of the output voltage too short for measurements to he made on a meter. These circuits were therefore

discarded in favour of the simple R-0 integrating circuit shown in Pig. 6 (Bhawalkar^).

This circuit also obviates the bandwidth problems inherent in the active-device electronic

integrating circuits. The low forward resistance of the 0Ab7 diode allows an appreciable charge to collect on the capacitor during each of the spikes of the laser pulse. During the intervals between the spikes the high reverse resistance of the diode prevents the capacitor from

discharging too rapidly through the otherwise low load. The decay time-constant of the voltage on the capacitor was about 5 mS. Pig. 4.10 shows the output as displayed on an oscilloscope.

The circuit was tested by measuring the peak of the output voltage waveform of the Integrator on the

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FIGAIO.Oscilloscope Display of Integrator Output,

400 ifc 1200

Input Energy t o the Flashtube ( J o u l e )

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1+0

-o r i g i n sh-owing t h e l i n e a r i t y -o f t h e I n t e g r a t -o r w i t h i n p u t

e n e r g y . The p e a k v o l t a g e a c r o s s t h e l o a d was k e p t h e l o w

ahout 2 v o l t s d u r i n g t h i s e x p e r i m e n t t y p l a c i n g n e u t r a l

d e n s i t y f i l t e r s i n t h e t e a m . I n a l l t h e e x p e r i m e n t s

u s i n g t h e p h o t o - d i o d e , t h e l a s e r t e a m was i n t e r c e p t e d h y

a g r o u n d - g l a s s s c r e e n b e f o r e r e a c h i n g t h e d i o d e . T h i s

e n s u r e d t h a t t h e p h o t o - d i o d e sees a s p a t i a l l y i n t e g r a t e d

p i c t u r e o f t h e l a s e r o u t p u t .

The d i s t a n c e s b e t w e e n t h e l a s e r , t h e g r o u n d

-g l a s s s c r e e n and t h e p h o t o - d i o d e were c r i t i c a l and e x c e p t

f o r t h e e x p e r i m e n r c d e s c r i b e d i n S e c t i o n 4 . 4 . 1 o n l y

r e l a t i v e measurements were made w i t h t h e s y s t e m .

4 . 3 LENGTH AMD MEAN POWER OUTPUT OF TH3 LA8BR

4 . 3 . 1 P u l s e L e n g t h

The p e r i o d o f t i m e f o r w h i c h l a s e r o u t p u t

o c c u r r e d was f o u n d t o v a r y w i t h t h e I n p u t e n e r g y t o t h e

f l a s h t u b e . Measurement o f t h i s p u l s e l e n g t h c o n s i s t e d

s i m p l y o f p h o t o g r a p h i n g t h e t o t a l l a s e r o u t p u t

( a s F i g . 4 . 1 ) s e v e r a l t i m e s f o r d i f f e r e n t i n p u t l e v e l s .

The t i m e i n t e r v a l b e t w e e n t h e f i r s t and l a s t o f t h e

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41

-the oeolllosoope. The spread "between results at each energy level was less than 9% and the results are shown on the graph of Fig. 4.11.

The limiting of the period of the pulse is due to the length of the light pulse from the flashtube. This latter is about 650 jiS "between half-peak intensity points. Laser action did not start until ahout 200

after the start of the flashtuhe pulse - the initial energy being used to obtain threshold population

inversion in the ruhy. Thus the flashtuhe output was falling off after a further 450 and was not sufficient to maintain threshold inversion and laser action ceased.

4.3.2 ,ep% ^^'3ut

Using the results on the output energy of the laser and the time length of the output pulse the mean, time-averaged, power output of the laser was calculated

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8W

Input Energy t o t h e Fiashtube (Joule)

FIG.4.12. Mean Output Power in Laser Pulse.

i-f 100-.

IR # 75

Integrated lr#)ut ( v o l t - s e c % 1 0 ' )

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42

-4.4 PEAK POWER. DURATION AND SEPARATION OP THE SPIKES It can be seen from the photographs of Pigs. 4.1 to 4.4 that the spikes in the laser output exceed the mean power by several times, perhaps even by an order of

magnitude. A knowledge of the peak powe^^ available is necessary before deductions can be made about the action of the focussed laser beam.

4.4.1 Peak Power of the Spikes

The photodiode circuit of Fig. 4.6 w^s used to measure the integrated laser output. The signal at point A is proportionel to the intensity of the light beam Incident on the photodiode. If a calibration of

the voltage on the integrating capacitor in terms of the voltage at A is made it is then possible to calibrate, the signal at A in terms of power/voltage output using previously obtained results. This calibration of the integrating circuit was done as follows:

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43

-Intervals much longer than the decay time-constant of the integrating circuit. This interval was made to he

100 m8 and gave sufficient time for recovery of the

circuit between pulse trains. The output voltage of the integrating circuit was monitored on an oscillosoope for various input conditions. The Input at A was varied between 0*1 to 3"0 volts with pulses of ft^m 0*2 to 3'0 ^8 duration at intervals between 2 and 10 ^8.

A graph of the output voltage on the capacitor against the integrated input (in volt-seconda) is shown in Fig. 4.13' spread of results is w^a2 within the experimental accuracy of about 5% involved in measuring the peak integrator output voltage on the oscilloscope. Prom the slope of the graph of Fig. 4.13 the integrator output is approximately 125 mV per 100.lO"^ volt-sec input.

The output at A can now be calibrated. Consider the integrator output voltage for the laser output pulse corresponding to energy input to the

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44

-oorrespoadB to an input of ^25 % 10"^ = 192 z 10"^ volt-Bee.

The total energy output of the laser for 960 joule input is 578 milli-joule and the laser output lasts for 450 ^8. The mean power during the pulse is therefore 1'28 kwatt.

The mean v o l t a g e a t p o i n t A ( P i g . 4 . 6 ) t o g i v e t h e

19? z 10"^

measured output is p = 427 mV. The calibration 450 z lO'b

for the output across the photodiode load resistor for the arrangement used in this ezperiment was therefore 427

1.28 = 334 my/kW 0% alternatively, 3 kwatt/volt. The estimated accuracy of this result is ±10^^ This calibration is valid for any input from the laser. Although the ahove calculation was done for the 960 j input to the flashtuhe and using the known output

characteristics at this level, the same result is obtained (to within ±10%) using other input levels.

It is now possible to estimate tha peak powers of the spikes in the laser pulse to within the accuracy of the above result.

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— —

About ^5^ of the epikes had peak powers less than twice the mean power. A further had peak powers between 2 and ^ times the mean power and about 10%

b e t w e e n 4 - 6 t i m e s t h e mean p o w e r . The r e m a i n i n g 5% o f

t h e s p i k e s h a d p e a k p o w e r s g r e a t e r t h a n 6 t i m e s t h e mean

p o w e r .

The o u t p u t o f t h e l a s e r a t e n e r g y i n p u t s c l o s e

t o t h r e s h o l d was more u n i f o r m i n peak p o w e r o f t h e s p i k e s

t h a n t h a t a t t h e h i g h e n e r g y l e v e l s . The above f i g u r e s

o n l y g i v e an a p p r o x i m a t e e s t i m a t e o f t h e d i s t r i b u t i o n .

A t l o w p u m p i n g l e v e l s , t h e i n c i d e n c e o f s p i k e s o f 6 t o 8

t i m e s t h e mean power i s a few p e r c e n t l e s s t h a n

i n d i c a t e d . T h i s i s a f u n c t i o n o f t h e r a t e o f pumping o f

t h e r u b y b y t h e f l a s h t u b e . A t h i g h r a t e s o f p u m p i n g i t

i s t o be e x p e c t e d t h a t t h e r e w i l l be a g r e a t e r s p r e a d i n

t h e p e a k p o w e r s o f t h e s p i k e s .

T h e r e a r e two p o s s i b l e s o u r c e s o f e r r o r s i n t h e

above m e a s u r e m e n t s :

a) R i s e - t i m e e f f e c t s .

b) Non-linearity of the photodiode output.

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46

-then the peaks of the resultant waveform would he

attenuated. Allowing 100 pP for the diode capacitance and stray s thstime constant would he about 10 n8 which would not affect the waveform.

Also, the rise-time of the square-wave pulses used to calibrate the Integrator circuit exceeded the expected rise-times of the spikes. No decrease in the integrated output was observed when using input pulses of O'l ^8 compared with the output for longer pulses giving the same integrated input. Thus errors due to the finite rise-time of the circuit and oscilloscope were not thought to have affected the results.

The linearity of the photodiode was only

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— —

transmlBBion filter the relative integrator output taken

to be 80 z ^^0 = 200 mV. This is also a way of checking

the linearity of the response of the photodiode for load

voltages greater than 0-5 volt.

The ratio of the integrator outputs for the attenuated and unattenuated teams of the same energy was compared with the transmission of the filter. Within the limits of the experimental accuracy (an estimated 1^^ the output was linear up to 2 volts peak across the load. This can only serve as a rough guide however, as a 10^ reduction of the peak voltages of the largest spikes

would probably only cause a 1% reduction in the Integrator voltage.

4.4.2 Dur^ti^n of the Spikec jn the Output Pulse

The average duration of the spikes observed was about 1 Spike durations of between 0*5 and

1'5 were observed. No correlation between pulse height and pulse duration was noted.

4.4.3 Separation oi the Output Spikes

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48

-e x p l a i n -e d , b y t h -e d -e c r -e a s i n g p u m p - r a t -e a t t h -e -end o f t h -e

f l a s h t u b e p u l s e end hence t h e l o n g e r t i m e s n e c e s s a r y t o

a c h i e v e a p o p u l a t i o n i n v e r s i o n i n e x c e s s o f t h r e s h o l d

i n v e r s i o n . F o r t h e m a i n p a r t o f t h e l a s e r p u l s e t h e

s p i k e s a r e s e p a r a t e d h y i n t e r v a l s o f 10 and l e s s .

S e v e r a l o f t h e s p i k e s i n t h e p u l s e s o v e r l a p . I n g e n e r a l

t h e mean s e p a r a t i o n s r a n g e d f r o m a b o u t ^ t o 5 a t l o w

o u t p u t e n e r g i e s , j u s t above t h r e s h o l d , t o about 2 ^ 8 a t

t h e h i g h e s t e n e r g y l e v e l s ( 1 , 2 0 0 j o u l e i n p u t ) .

4 . 5 ANGULAR DIVEEGENCB OP THE LASER BEAM

4 . 5 . 1 I n t e n s i t y D i s t r i b u t i o n i n t h e F o c a l P l a n e

I d e a l l y t h e l a s e r o u t p u t w o u l d t e a p l a n e wave

c o h e r e n t a c r o s s t h e w h o l e end f a c e o f t h e r u b y . The

d i s t r i b u t i o n o f i n t e n s i t y i n t h e f o c a l p l a n e o f a l e n s i n

t h e s y s t e m w o u l d t h e n he g i v e n h y t h e A i r y p a t t e r n

( s e e f o r example L o n g h u r s t ^ ) . The a n g l e w h i c h d e f i n e s

t h i s d i s t r i b u t i o n w o u l d t h e n he t h e d i f f r a c t i o n a n g l e due

t o t h e a p e r t u r e o f t h e end o f t h e r o d o r t h a t due t o any

p a r t o f t h e f o c u s s i n g o p t i c s p r e s e n t i n g a s m a l l e r a p e r t u r e .

However, measurement o f t h e a n g u l a r d i v e r g e n c e

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49

-or three -orders of magnitude greater than that predicted hy these simple considerations.

The angle of the first minimum of the diffraction pattern due to a circular aperture of diameter 'a' Is given hy

1-22 >

e =

For ruby, the wavelength X of the emitted light is

69^3 A. U. The aperture formed hy the end of the rod has diameter 1*0 cm

.-.g = 1 0 - . g.kF , IG-S

From direct measurement it is found that 6 is of the order of 5 x 10"^ rad. This corre&ponds to an effective aperture diameter of

a = ^ 3 J 0 U

5 z 10"^^ 7

High-speed photography of the end face of a ruby laser shows that laser action does not take place across the whole diameter of the rod but In filaments along the length of the ruby. These filaments are typically of the order of 100-200 p in diameter.

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50

-in the focal plane of a lene can te predicted. All rays of light at e to the optic axis of the system will focus to points on a ring of radius f8 where f is the focal length of the lens. This gives what is known as the

'far-field pattern' of the laser.

4.5.2 Measurement of the Far-Fielc Pattern

The experimental arrangement consisted of a

35 mm camera, focussed at infinity, arranged to photograph the laser team. Neutral Density filters were used to attenuate the laser team to a level below th^ saturation of the film. A narrow-hand filter (Gruhhs Parsons Type 1, centred at 6944 A.U. with a 50 A.U. bandwidth) was placed directly in front of the camera lens in order to reduce the effect of the ambient light level in the laboratory to negligible proportions.

With the camera focussed at infinity the film is in the focal plane of the camera lens. The intensity distribution of the laser beam in the plane of the film is then the far-field pattern as described in 4.5.1.

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5 1

-For each pump voltage from 1*50 k ^ i n steps of 100 volt, to 2'50 kV a range of photographs of the beam were taken with varying attenuation in the beam.

The filters were of 2 mm thick Chance

Pilkington glass neutral density types ON 10 and ON 30 giving transmissions of 40% and 3^ respectively at 6943 A.U.

The films were then developed and fixed. A graph of the density of the photographic negative across each far-field pattern was then plotted using a micro-densitometer. The magnification used was x50 and the wedge range used was 2*3 d with a density of 0'13 d. per cm.

A typical result is shown in Pig. 4.14.

In order to extract information from this trace it is necessary to calibrate the response of the film to exposures of such short durations.

For a given value of laser pump energy the densities of the film at the centres of the far-field pattern can be plotted as a function of the relative

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hfsec

tasao IBdii ic&ti

iar-ga

photographic negative.;

r • » I - • I t 1 L I :

Distaace along

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3 a

ro

10 10 10

Relative Exposure. terms of attenuation due to filters.)

FIG.4.15.Calibration Curve for ILFORD FP3 Film

C 2 0

trpiii Eneroy :o Tnc (^Inshlunr

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52

-typical plot is shown in Pig. 15 for the laser output at the pump voltage of 1'90 kV. It can. he seen that the response of the film is logarithmic and. given "by an

expression of the form

Density = A + B log^gtlntensity). d where A and B are constants. The constant B can he found directly from the graph to he 1*75 d/decade.

It is now possible to determine the distance below the peak value on the micro-densitometer trace at which the intensity falls to a given fraction of the peak value. If the peak value of the long-time-averaged

(l.T.A.) intensity is then the point at which the L.T.A. intensity is El can be found as follows:

e

let = A + 1'75& log^gl^ (1) and Dg = A + l'75d log^oEl (2) then, subtracting (2) from (l) gives

Di - Dg = 1.75d log^ge = 0'76d The density gradient of the wedge used for the micro-densitometer traces was 0'13 d per cm. The difference in density - Dg of 0*76 d therefore corresponds to

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53

-the peak of each trace and. measuring -the width of -the trace at this point. To find the actual radius of the far-field pattern these results have to he divided hy 2 z the lever ratio = 100. The lever ratio defines the magnification of distance "by the micro-densitometer. The focal length of the camera lens was 5*0 cm. Hence

the angular divergence of the laser team at which the Intensity falls to ^ of its axial value is given hy

radius at which intensity is of peak (%n)

w = 5*0

with the units of radians (since 6 = tan G for the range of angles encountered here).

The final results are plotted as a function of the input energy to the flashtute in Fig. 4.16.

The limiting of the beam divergence seen here has also been observed by Ohang and Kilcoyne^.

The increase in the beam divergence angle for pumping levels up to about twice threshold is due to the

(85)

-

-is therefore due to increasing energy out per mode and no increase in the angular divergence of the teem is to he expected.

4.5.3 L.T. A. Intensity Distribution in the Focal Plane of a lens In the above treatment for finding the beam

divergence no account has been taken of the time length of the laser pulse or the effect of this on the film response.

It is more correct to write the formula used as, Density = A + B log (Energy Density^d (1) To find the value of the constant A some assumption has to be made to give values for the energy density. It was tentatively assumed that a Gaussian Distribution would fit the actual Distribution. The equation for

such a distribution is 3 y

B = exp(- y s^) (2)

where B = Energy Density (watt

= Total Energy in the pulse (watt)

y = ^2 where d is the standard deviation of the distribution (cm)

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55

-Assuming this distribution of energy a value for the actual energy density at the centre of the focussed &pot can be calculated. For the output at 1*90 k*rpump

origin) = 5'6 z 10^ watt crn^ with a 5 cm focal length lens.

It can then be shown that A = 5'35 and the final equation for the film calibration is given by Density = 5'35 + 1'75 log^Q (Energy Density^d

The densities at the centres of the far-field patterns for the laser outputs at pump voltages of

2'00 kV and 2*30 kV were calculated. The calculations were based on the assumption of a Gaussian energy

distribution and the previously measured values of the laser output parameters. The theoretical curves fitted the actual plots, similar to Pig. ^*15, to within 5%.

It is now possible therefore to plot the distribution of energy In the focal plane of the lens using the formula

Density - A Energy Density = 10

(87)

— 56

-Figure 4.17 shows an expanded plot of the energy density distribution taken from the micro-densitometer trace of Pig. 4.14.

There appears to he no theoretical treatment in the literature of the distribution of energy across the focal spot for the multi-mode case of the ruhy laser.

Normalized curves of the Airy disc pattern and the Gaussian Distribution have teen drawn on Pig. 4.17 for comparison.

The equation for the Airy disc pattern is I(P) = 2 kaw (kaw^

2

Iq (3)

(see Born & Wolf^) 277

where k = ^

a = radius of diffracting aperture (an)

w = sine of the angle which the direction P makes with the optic axis

€ D - " F ^ 9

where D = wa

(88)

Observed

Gaussian.

J/ k r . rc .

(89)

57

-Neither distribution fits the experimental

curve very accurately. Several such plots were made for different micro-densitometer traces.

In the absence of any more detailed information on the intensity distribution it was decided to use the Gaussian approximation in further calculations.

BBPBR8N0E8

1. EELLINGTOM, 0. M. , & KATZMAN, M., Beam-Divergence and Far-Field Patterns of Rubies of Varying Optical

Quality. J. j^Mpl. Phys. ^6 9. 2910-2914. Sept. 1965.

2. WAYNAL'TT, R.W. , OUHLOM, J. H. , BASIL, I.T. & B.ALDWIN, G. D., Beam Divergence Measurement for Q-Switched Ruhy Lasers. Aop. Opt. 4 12. 1648-1650. Dec. 1965.

3. MAIMAN, T.H. , et al, Stimulated Optical Emission in Fluorescent Solids. Phy. Rev. 123 4. 1145-1157. Aug 15, 1951.

4. BATEMAN, P.J,, The Measurement of Laser Power and Total Energy. Conf. on Lasers and their

Applications, 29th Sept - Ist Oct, 1964. Paper 40.

5. EHAHfALKAR, D.D. , Private Oommunication.

6. LONGBHRST, R.8., Geometrical and Physical Optics. 2nd Bd. Longmans 1960. 209-213.

7. CHAMG, W. 8.0. , & KILCOYBE, N.R. , A Study of Partial Ooherence and its Application to the Oollimation of Pulsed Multimode Laser Radiation. App. Dot. 4 11. 1404-1411. Mov. 1965.

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58 -CHAPTER 5

BPFBGT8 OP TH3 P00U88BD LASER BEAM

This chapter will briefly review the literature on the effects of focussed solid-state laser beams on

materials, with particular reference to imetals.

5.1 MACHINING AND WELDING

The use of the laser beam as an industrial tool for machining and welding has been considered by several

authors. Namba and Kim^ have compared the use of electron beams and laser beams for drilling various

materials. They found that slightly smaller spot sizes could be obtained with electron beams than with the laser beam. They also calculated that the efficiency of

material removal was of the order of 50% for laser processing but only a few per cent for electron beam processing.

p

Osial et al^ have considered the use of the laser as a tool for welding and have made successful welds of wire to wire in many materials. They also successfully

bonded wire leads to silicon integrated circuits and welded the edge connectors of 'flat packs' to printed

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59

-Some attempts have teen made In the literature to calculate the parameters of the laser j^ulse needed to make successful welds In given materials. (See, for

example, Phllpott * Dillon-Harris^) In practice,

however, these parameters are normally found hy experiment for each different application.

5.2 CRATBRING BPF3GT8 AND MOMENTUM TRANSFER

Neumann^^ has imeasured the momenta of targets Irradiated with laser beams. The results show that although the laser beam itself has appreciable momentum the momentum due to the recoil pressure of evaporated or otherwise ejected material is several orders of magnitude greater than this. The subject of recoil pressure has been treated theoretically by Askar'yan and M o r o z ^ .

High-speed photographs of the ejected material have been taken by Ready^ and by Harris^. The former were

Figure

Fig 3.1. Line Diagram of Laser Evaporation System.
FIG. 3.3.
FIG.4.7a.
Fig. 7.15. Crater in Copper. 0,7 Joule pulse. 1.25" lens.
+7

References

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