NBS TECHNICAL NOTE 703

ARPA-NBS Program

of Research on High

Temperature Materials

and Laser Materials

Reporting Period

January 1 to June 30, 1971
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards1 was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and

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nil PllSc/UJ OF STANDARD

9 St UNITED STATES DEPARTMENT OF COMMERCE

Maurice H. Stans, Secretary NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Director

NBS TECHNICAL NOTE 703

ISSUED 1971

Nat. Bur. Stand. (U.S.), Tech. Note 703, 39 pages (Dec. 1971) CODEN: NBTNA

ARPA-NBS Program of Research

on High Temperature Materials

and Laser Materials

Reporting Period

January 1 to June 30, 1971

Edited by A. D. Franklin and H. S. Bennett

Inorganic Materials Division

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

Supported by the Advanced Research Projects Agency

of the Department of Defense

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C, 20402.

(Order by SD Catalog No. C 13.46:703). Price 50 cents.

TABLE OF CONTENTS

Page

1. Introduction 1

2. High Temperature Materials Program 2 2.1. Objectives and Summary of Program in High

Temperature Materials 2 2.1.1. Objectives 2 2.1.2. Summary of Progress to Date 3 2.2. Project Summaries in High Temperature Materials . . 4 2.2.1. Diffusion in Oxides 4 2.2.1.1. Crystal Growth from Vapor .... 4 2.2.1.2. Diffusion in Refractory Materials 6

3. Laser Materials Program 9 3.1. Introduction to Laser Materials 9 3.2. Objectives and Summary of NBS Research on Laser

Materials 9 3.2.1. Objectives 9 3.2.2. Summary of Progress to Date 11

3.3. Project Summaries in Laser Materials 12

3.3.1. Damage to Laser Materials 12

3.3.1.1. Laser Induced Damage Studies ... 12

III
ARPA-NBS PROGRAM OF RESEARCH ON HIGH TEMPERATURE

MATERIALS AND LASER MATERIALS

Work Performed at the National Bureau of Standards

Supported by the Advanced Research Projects Agency,

Department of Defense

(ARPA Order 373-62)

Reporting Period January 1 to June 30, 1971

Edited by

A. D. Franklin and H. S. B-ennett

Progress reports are given for projects on the growth of

ultra-pure A1_0„ crystals, the development of a mass spectrometerbased sectioning technique for measuring oxygen diffusion in oxides

and the development of a precision facility for measuring the

threshold energy in a laser beam producing damage in a transparent

substance.

Key words: A1_0„; crystal growth; damage threshold; glass; high

temperature materials; laser; oxides; oxygen diffusion; pure

materials; sapphire.

IV
1. INTRODUCTION

The National Bureau of Standards, with the support of the Advanced

Research Projects Agency (Office of Materials Sciences) of the Department of Defense, is carrying out a program of research on High Temperature and Laser Materials. The overall objective of this program is to provide some of the key elements needed by DoD in its pursuit of new

technology through research and development. These elements include:

i. The preparation and characterization of materials needed for

critical studies where exacting criteria of chemical purity,

crystalline perfection, etc. must be met if progress is to be made,

ii. The development of new measurement techniques where these

are required, and the acquisition of critically-needed data

on certain material properties, and:

iii. The elucidation of the basic mechanisms contributing to or limiting the use of materials under extreme conditions

(e.g. high temperatures, high intensity laser light, etc.).

A summary of the results achieved in the period January 1 to June 30,

1971, is given here.

Among the many areas of Defense Technology where advances of the

kind described above are needed, this Program has focussed upon two. Ceramics are used in many ways, and many problems in their use relate

ultimately to mass transport effects. Highly important improvements

could be made by developing a firmer understanding of and control over mass transport processes in oxides. Since NBS has had considerable

experience in this field, it is a natural one upon which to concentrate

a portion of this Program.

The other area of concentration was introduced into this Program

several years ago in response to an urgent need seen by the Sponsor. As

laser power has moved upward, laser beams, especially pulsed beams of

very short pulselength, have been found to damage the materials (ampli1
fier laser rods, windows, lenses, prisms, and other optical elements)

through which the beams must pass. Not only must a better understanding

be developed of the processes leading to the several types of damage so far observed, but also there is a need for more reproducible techniques

for studing these damage processes and for characterizing the response

of given materials with respect to them.

Corresponding to these two areas of concentration, this Report is

divided into two parts, High Temperature Materials and Laser Materials.

Each part will open with a brief summary of objectives and accomplishments, followed by detailed Project progress reports.

2. HIGH TEMPERATURE MATERIALS PROGRAM

2.1. Objectives and Summary of Program in High Temperature Materials

2.1.1. Objectives

The operation of rocket and jet engines involves exposing metals

at high temperatures to oxidizing conditions, and raises the problem

of oxidation protection of high temperature refractory metal structural

components. This same problem, currently for superalloy turbine blades,

is posed again by jet engines. The oxidation protection of metals

requires some form of barrier layer to prevent diffusion. This layer,

either applied or developed by the early stages of oxidation itself,

will most likely be an oxide, and will function to keep the metal and

oxygen apart. It can work only if the diffusion rates of oxygen and

metal atoms through the layer are small enough. Thus, the strategy for

the development of oxidation protection of otherwise susceptible metals

at high temperatures will depend in part on being able to control the

rates of diffusion, and therefore, in having data on the diffusion

rates themselves.

Diffusion in oxides is important to other classes of DoD problems.

Thus improvements in the reliability and efficiency of military

communication, computer, and radar circuitry could be made if ceramic

magnetic and ferroelectric components could be made more reproducible-
and reliable. Because such important properties as initial susceptibility and coercive force of ferrites, and dielectric and piezoelectric properties of ceramic f erroelectrics, are very sensitive to the

microstructural features (porosity, grain size) controlled by sintering,

the strategy for improvement here requires control over sintering in

these ceramics. The same is true of ballistic response of ceramic armor.

The development of transparent armor requires new techniques for the

suppression of grain growth during otherwise complete sintering of

inherently transparent ceramics like A1_0 .

Since sintering in ceramics is mainly brought about by diffusion

processes, the development of successful, long-range strategies to meet

these DoD needs rests in part on knowledge of the rates of diffusion

of metal and oxygen atoms in oxides.

Reliable techniques for measuring oxygen diffusion rates are not

yet available, and therefore reliable data for oxygen diffusion does not

exist for any save a few oxides. Thus one objective of this program is

to develop a reliable technique for measuring the rate of diffusion of

oxygen in oxides

Further, diffusion rates are notoriously sensitive to impurities,

and strategies for control require knowledge of the basic rate in the

pure material and of the way various classes of impurities influence it.

This in turn requires very pure crystals for study. To date, almost no oxide crystals have been prepared pure enough to reveal the intrinsic

diffusion rates. The second objective in this area is to prepare

intrinsically pure crystals of those materials for which mass transport

data are sorely needed. The first candidate has been A1„0„, an important

constituent in many ceramics, in particular for ceramic armor.

2.1.2. Summary of Progress to Date

In previous work a vapor deposition technique had been perfected

for the growth of A1„0_ single crystals, and the resulting crystals
shown to be very pure. Sources of residual impurity were shown to be

mainly the metal parts of the growth apparatus exposed to hot gases

and the starting Al metal.

A revised equipment has been constructed and will be used, together

with the purest obtainable Al metal to attempt the growth of ultrapure A1„0 crystals for intrinsic mass transport measurements.

A key problem in developing the equipment for measurement of the

oxygen diffusion coefficient in oxides has been the conversion of the

oxide sections to a form suitable for mass spectrometer analysis.

During the past period an apparatus for the chemical decomposition of

the sections by BrF has been built and is being tested. The reaction

has been mainly successful, although the yields of oxygen for the mass spectrometer analysis have been lower than anticipated and desired.

The explanation may lie in contamination of the starting BrF,.. Additional" work will be done during this next period to maximize this

yield, and it is also planned to begin diffusion studies on MgO

crystals.

2.2. Project Summaries in High Temperature Materials

2.2.1. Diffusion in Oxides

2.2.1.1. CRYSTAL GROWTH FROM VAPOR

H. S. Parker and C. A. Harding

Inorganic Materials Division

Institute for Materials Research

The objectives of the program are, first, the growth of aluminum

oxide mono- and bicrystals of sufficient physical perfection and chemical purity for use as research materials, second, the reduction of both

cation and anion impurities to sufficiently low levels to permit meaningful property measurements at the intrinsic level and third, the extension

of the technique to other materials of interest.

During the present reporting period, the growth of high purity mono- and bicrystals has continued, in order to supply additional
specimens to other ARPA investigators concerned with mass transport

in oxides. Relocation of the growth apparatus to a different laboratory caused some interruption to the experimental work. An additional

modification to the growth apparatus has been the inclusion of an oilwetted mechanical filter between the liquid nitrogen cold trap and the vacuum pump to prevent amorphous alumina powder remaining in the gas

stream from reaching the pump. This should materially extend the life

of the pump.

The components for power and temperature control of the new system

described in previous reports have been received and wired and the new

system will be in operation within two weeks. It is expected that the

combination of the new growth system with a minimum of exposed metal

parts to hot gases and the use of 99.9999 per cent pure aluminum metal

source material will produce specimens of the highest purity attainable

by this technique.

A recalculation of the magnetic susceptibility of our specimens has

i i mVg. 1 shown an error in the previously reported value of 0.318 ± 0.002 x 10 l2

1 NBS-ARPA Report for the period January - June 1970, A. D. Franklin and H. S. Bennett, Eds., NBS Report No. 10 353, p. 9.
The corrected value is 0.360 ± 0.002 x 10 12 m

3 /g, in much better

agreement with the theoretical value of 0.38 x 10 2 m

3 /g, as would

be expected for material of this purity.

During the next reporting period it "is planned to complete the

growth and characterization of high purity single crystals in the new

apparatus and complete the preparation of a publication describing the

technique and results

.

2.2.1.2. DIFFUSION IN REFRACTORY MATERIALS

A. L. Dragoo

Inorganic Materials Division

Institute for Materials Research

The objectives of this program are to measure and to interpret

diffusion of oxygen in single crystal oxides. Experimentally these

two objectives consist of (1) extending the sensitivity of present

techniques so that the distribution in a single crystal sample

can be measured, (2) comparing the results of these concentration

profile measurements with measurements of the rate of exchange of

between the gas phase and the oxide crystal, (3) determining the

dependence of the diffusion rate on temperature, oxygen partial

pressure, impurities in the solid, etc. and (4) obtaining reliable

diffusion and exchange coefficients. Oxygen self-diffusion in oxides

provides a tool for uncovering the type of point defects involved in

ionic migration in oxides and the energies required for the formation

and motion of these defects. Since oxygen diffusion can be the con- trolling step in the solid state reaction and sintering of oxides, the

diffusion coefficients are needed for interpretation and estimation

of these rates.

One of our experimental problems is to determine the distribution in oxide crystals after a diffusion anneal. To do this we are endeavoring to couple sectioning, i.e., removal of layers of about
1 ym in thickness from the crystals, to analysis of the " 18 o/ 16 o" ratio with a mass spectrometer. The chemical reaction which provides

this coupling is the most difficult step in the procedure. Preliminary

results from the reaction of BrF,. with MgO, A1„0 and TiO have

encouraged us to believe that at last we may have the chemical tool

which will accomplish this coupling.

The free energies for the reactions of BrF with A1„0„, MgO

and TiO are favorable even at room temperature (1)

AG ° 298.15' kJ

Oxide

A1

2 3 -603

MgO -151

TiO„ -426

The products are assumed to be BrF , 0„ and the fluorides of the

respective metals. BrF. also will react with the oxides.

Ruff and Menzel(2) were the first researchers to report on the

reaction of BrF with a number of oxides. Since that time geochemists

(3) have developed procedures for its use in oxygen isotopic analysis

of water and of silicate rocks.

Our experimental arrangement is detailed schematically in the

accompanying drawing. The "Metal System" is used to purify and to store the BrF , to react the BrF with oxide samples in the monel

reaction tubes and to separate the toxic gaseous fluoride products

from the oxygen in a chain of three liquid nitrogen-cooled coldtraps

,

The "Glass System" is used to collect the oxygen, to measure

the quantity of oxygen obtained, to convert the to C0„ using a hot carbon filament and to collect the C0_ for analysis with the
rH8HH» E

CO

Q_

X

I

CO

co
mass spectrometer. Carbon dioxide is more desirable for isotope

ratio measurements than is oxygen.

We estimate the yields from our initial experiments to be

20 - 30% for MgO

60% for Al

65% for Ti0

2

.

In current experiments we are measuring the amounts of reactants and

products with greater care, purifying tl the influence of KF on the reaction(4)

.

products with greater care, purifying the BrF and investigating

References

1. JANAF Tables, Dow Chemical Company, Thermal Research Laboratory.

2. 0. Ruff and W. Menzel, Z. anorg. allg. Chem., 202, 49 (1931).

3. J. R. 0'Neil and S. Epstein, J. Geophys. Res., TL, 4955 (1966).

4. I. Sheft, A. F. Martin, and J. J. Katz, J. Amer. Chem. Soc,

J78, 1557 (1956).

3. LASER MATERIALS PROGRAM

3.1. Introduction to Laser Materials

The National Bureau of Standards, with support from the Advanced

Research Projects Agency (Office of Materials Sciences) of the Department of Defense, is performing research on laser materials. This

research includes studies to characterize laser materials and to define and evaluate damage measurements in laser materials. A summary

of the results achieved during the reporting period is given here.

3.2. Objectives and Summary of NBS Research on Laser Materials

3.2.1. Objectives

Past experience in the high-power laser field indicates a need

to improve the reproducibility and precision of measurements concerning

damage and degradation in laser materials. As in other fields, this

requires an understanding of the important material and instrumental
characteristics and their influence on the results of the measurement.

Such a requirement for laser damage measurements means having a wellcharacterized laser beam in the sense that we know and can vary the

intensity as a function of position, time, and frequency for each

pulse.

To this end, we have constructed and are putting into operation a neodymium-doped glass laser system as the fundamental measurement tool.

We are taking great care in characterizing and controlling the beam.

The objectives of this program then include:

1. To study the measurement of the threshold for surface

and for self-focussing damage, the statistical nature of the measurement, and the influence on it of the beam characteristics; and

2. To study on a microscopic-physical and chemical basis

the nature of surface and intrinsic damage in various materials. These

studies will probably include:

a. The influence of electrostrictive and Kerr effects on

self -focussing and intrinsic damage; and

b. The effect of surface preparation and history on sur- face damage.

Even though the failure of laser materials due to opticallyinduced damage still hinders somewhat the advance in high power

laser technology, a significant change in the approach to the problem

occurred this past year. Whereas up to a year ago researchers con- sidered that particulate damage was the major problem, they now

consider that intrinsic mechanisms (e.g., self-focussing, surface

damage, and plasma formation) should receive the greatest attention.

The recent symposium at Boulder rather dramatically illustrated

this change. Speakers at this Third Symposium on Damage in Laser

Materials (May 1971) mentioned only briefly the particulate (extrinsic)

damage problem in Nd-doped glass, a problem of great concern just one year ago. This reflects more a success in technology than an advance in science. It is now possible to make glass with

10
so few particles that they are apparently no longer the primary source

of damage. Surface damage, the dynamics of the processes leading to surface damage, the statistical aspects of damage and degradation

processes, and the damage to laser materials for high power applications at 10.6 ]im have become now some of the foremost problems to occupy the efforts of researchers. This latter group of problems

is associated more with the intrinsic damage processes than with the

extrinsic damage processes due to impurities whose concentrations

may be reduced to acceptable levels.

Past experience suggests that the intrinsic damage problems will

not yield as easily to the technological approach as did the particulate

problem. We may solve the latter by removing the particles, but in

order to ameliorate the intrinsic damage mechanisms, we very probably

must first develop the science of intrinsic damage; that is, we must understand the damage processes and their relations to material

characteristics. This development of the science underlying the

intrinsic damage problem will require us to make proper, well-controlled

measurements 9f the damage threshold for each mechanism as we

seek correlations with changes among material compositions and

treatments. These are the reasons why the NBS program has adopted the

objective outlined above.

3.2.2. Summary of Progress to Date

A theoretical model for studying the thermal stresses produced

by inclusions in laser materials has been developed. A paper

containing the numerical predictions of this model was published

during the period covered by this report

.

i. H. S. Bennett, Absorbing Centers in Laser Materials, J. of Applied Physics, 42, 619 (1971).

The results of past projects on the characterization of laser

materials also were published during this period. These projects

have been terminated.

ii. R. M. Waxier, Laser Glass Composition and the Possibility

of Eliminating Electrostrictive Effects, IEEE J. of Quantum Electronics,

11
Q. E. 7 , No. 4, 166 (1971).

iii. R. M. Waxier, et al. , Optical and Mechanical Properties

of Some Nd-doped Laser Glasses, NBS J. of Res., 75A, No. 3, 163 (1971).

A commercial Nd-glass laser is being modified for precise singlemode operation. Its oscillator section now operates satisfactorily,

and its performance characteristics are being determined with precision.

In addition, a Twyman-Green interferometer and a polariscope have

been constructed. They are used to test the optical properties and

quality of all optical components in the laser.

The modified oscillator has been used for preliminary studies

on the intrinsic damage associated with self-focussing in borosilicate

crown glass.

The data from such studies agree reasonably well with the data

of others and include values of wavelengths and pulse widths which

other researchers have not reported. However, the present theoretical

models to interpret these data need many improvements. Their numerical

predictions do not agree well with the measurements.

During the next period it is planned to improve the two amplifier

sections of the laser, to continue in more detail the measurements on damage threshold, and to examine on a microscopic-physical and chemical

basis the several damage mechanisms.

3.3. Project Summaries in Laser Materials

3.3.1. Damage to Laser Materials

3.3.1.1. Laser Induced Damage Studies

Albert Feldman, Roy Waxier, and Dean Horowitz

Inorganic Materials Division

Institute for Materials Research

I . Objectives

12
The overall objectives for the laser materials program are discussed in Section 3.2.1. Briefly, they are to measure thresholds for

surface and intrinsic bulk damage and to examine on a physical and

chemical basis the nature of such damage. The objectives for the

present reporting period are more limited. They are first to operate the oscillator section of our laser in a well-controlled,

single-mode (T E M ) and second to study the mechanism of selfoo focussing and the intrinsic damage which may be produced by selffocussing.

II

.

Approach

Self-focussing is a precursor to damage from high power laser

beams. Preliminary measurements of the critical power for selffocussing have been made in one material and compared with existing

data and theories. A method was found for comparing data obtained

with lasers using different wavelengths and pulse widths. When our data are scaled, they agree favorably with previous measurements.

Further measurements are planned at increased power levels by use

of two amplifiers. Higher power levels are necessary for testing

current theories of self-focussing.

III. Laser System

The major part of this report period has been concerned with the

construction of a Nd:glass laser test facility. This involved ordering replacement parts for the laser system, setting up a Twyman-Green

interferometer for the testing of all optical components, and performance tests on the laser oscillator.

The original laser system has been described previously (1) . The

oscillator has been modified to produce a beam in the TEM mode by

oo using apertures up to 3.15mm in diameter. The oscillator rod is 12.7 mm in diameter by 177.8 mm long and has a 6° - 6° window configuration.

13
The unpumped wavefront distortion is less than A/10. (All measurements

were made with a helium-neon laser operating on the 632.8 nm line.) The

output reflector is a 45% dielectric mirror. Q-switching is presently

done with a Pockels cell. The two amplifier rods are 19.1 x 304.8 mm

and 19.9 x 215.9 mm respectively and have 6° - 6° window conf irgurations

.

The wavelength distortion in each rod is less than A/4. All rods are

pumped with helical flash lamps.

IV. Testing of Optical Quality

A. Twyman-Green Interferometer

A Twyman-Green interferometer has been set up in order to test the optical quality of all optical components in the system. This

gives us the ability to measure flatness and wavefront distortion to within A/20. Figure 1 shows a schematic diagram of the interferometer.

The light source is a two milliwatt helium-neon laser in the TEM mode,

oo and is polarized vertically. The power is adequate for measuring

wavefront distortion in our longest rods (304.8 mm).

We find that we achieve the best fringe visibility when

the optical path lengths in the two arms are equal or when the difference

between the optical paths in each arm is an integer multiple of the

length of the helium-neon laser (about 43 cm). However, we find that

fringe patterns are discernible even when this optical path difference

varies by as much as 10 cm. from a given integer multiple. Hence, wide

variance in the difference between optical path lengths is tolerable.

B. Effect of Pumping on Oscillator Rod

We performed tests on the oscillator rod while mounted in

the head to determine the uniformity of pumping. After carefully

aligning the flashlamp to be concentric with the laser rod, we inserted

the head in one arm of the Twyman-Green interferometer. The fringe

patterns were photographed at less than one second and at several

successive times greater than one second after the flash lamp was

fired. These are shown in figure 2. The fringe pattern immediately

14
o

CAMERA

EXPANSION TELESCOPE

EXPANSION OPTICS

Figure 1. Twyman-Green Interferometer

15
(a) (b)

(c) (d)

(e)

Figure 2. Twyman -Green interferograms of oscillator rod after pumping,

(a) Before pumping (b) Less than one second after pumping

(c) After 10 seconds (d) 20 seconds (e) ^fO seconds.

16
after the flash is cylindrically symmetric, indicating cylindrically

symmetric pumping. The decreased size of the image shows that the rod

is acting like a positive lens. From figures 2 and 3 and from other

observations we judge that maximum distortion in the rod takes place

about 5 seconds after the flash. Essentially all distortion in the rod

disappears after 40 seconds.

In figure 2c, we observe that the fringe pattern has a two fold symmetry with the fringes disappearing at about 45°. This

effect is caused by cylindrical stress in the rod, and the

polarization of light. In the 12 and 6 o'clock positions the light

is polarized in the radial stress direction, while in the 3 and 9 o'clock positions the polarization is in the tangential stress direction^) . Thus if any stress-induced birefringence is present we would

expect -to see the two fold fringe pattern.

The presence of pump-induced stress in the rod was observed by blanking off the reference arm of the interferometer

and inserting an analyzer in crossed position before the camera. The result is a double pass polariscope. Figure 3 shows the birefringence produced by pumping. The pattern again shows cylindrical

symmetry. The cross in the pattern occurs where the principal

stress axes are parallel to the light polarization.

The temporal development of the stresses in the rod agrees

with that calculated for stresses in a uniformly heated cylinder suddenly plunged into a constant temperature bath (3). These calculations

were made on the basis of the measured parameters of the neodymiumdoped glass (4). The fact that it takes a relatively long time

(5 seconds) for the maximum stresses to develop indicates that the

photograph immediately after the flash is fairly close to that which

would appear during the flash. Welling and Bickart(5) have taken

high speed photographs which show that stresses are set up during

the flash. These stresses are due almost entirely to thermal gradients

caused by heating as a function of radius in the rod.

17
(a)

(b) (c)

Figure 3- Photographs of oscillator in double pass polariscope after

pumping, (a) Less than one second after pumping (b) After

5 seconds (c) After 10 seconds.

18
C. Pockels Cell Aperture

The oscillator Pockels cell was placed in the polariscope to examine its aperture as a function of applied voltage. Figure 4 shows a photograph of the aperture. Note that the aperture is not in the center

of the cell. The maximum size of the aperture is 4.7 mm in diameter.

The Pockels cell was placed in the oscillator so that the aperture was

centered along the desired beam axis.

V. Oscillator Performance

Tests were made on the oscillator to determine total energy output,

cross sectional energy density distribution, time evolution of pulse, and

pulse reproducibility. As presently set up, all tests could not be made

simultaneously. Figure 5 shows a schematic representation of the test apparatus. All tests were performed under constant oscillator pumping

conditions.

A. Pulse Energy Measurements

The total energy per pulse was measured using a thermopile

energy meter directly in the path of the laser beam. The manufacturer

claims an accuracy of ±5% for the energy meter. Using an aperture of

2.44 mm and a pump energy of 3.53 kJ, we obtained an output energy of

99 mJ with no deviation larger than ±3%.

B. Cross-Sectional Energy Distribution

The cross-sectional energy distribution in the laser beam,

or beam profile, is the energy H per unit area falling on a surface

perpendicular to the beam direction as a function of distance from the

beam axis. The beam profile of our laser beam is obtained from

burn patterns on developed unexposed black (Polaroid) 2 film. The

output of the laser is maintained at a constant pulse energy but is

attenuated from pulse to pulse by interposing calibrated attenuation

filters between the oscillator and the film. Figure 6 shows the set

of burn patterns used for determining the beam profile. The radius of

Commercial materials are identified in this paper to specify the

particular substance on which the data was obtained. In no instance does

such identification imply recommendation or endorsement by the NBS or that the material identified is necessarily the best for the purpose.

19
Figure k. Pockels cell as seen in double pass polariscope with 8 KV applied using the helium-neon 632.8 nm line. The aperture

is h.~] mm

.

20
(0E oc

UJ 4- Q 1) CC Q- O l_ O O UJIT

Q. E

a. ra < -v

21
Figure 6. Burn patterns on developed unexposed (Polaroid) film used

for determining beam profile at position of focusing lens

22
the outermost burn ring for each pattern is plotted as a function of

beam energy. The result is fitted to a Gaussian

2 H = H

q exp ( £j) .

, a

The constant H is obtained in units of J/cm from

o

2 E = 2ttHo /"exp ( £-) rdr , a

where E is the total pulse energy (99mJ) . Figure 7 shows a plot of the

beam profile together with the Gaussian which is fitted to the data

by eye. The profile, which is measured at a distance of 482 cm from

the oscillator aperture, has a 1/e point radius, a, of 1.81 mm.

It is important to know the energy distribution at the aperture

in the oscillator. For this purpose it is useful to assume that the

profile is indeed Gaussian. A Gaussian beam has the property of maintaining a Gaussian shape throughout all of space. At one position it

has a minimum width which is called the beam waist. Using the results

of Boyd and Gordon (6) we obtain a beam waist diameter (1/e point in the

beam profile) of 0.93 mm which is assumed to occur at the oscillator

aperture (we neglect the amplification of the oscillator rod in the

calculation). The ratio of the aperture diameter (2.44 mm) to the

beam waist diameter is 2.62. At the aperture radius, therefore, the

intensity of a Gaussian falls to 0.1% of its peak value and contains

99.9% of the total beam energy.

C. Pulse Time Evolution

In order to measure the laser pulse time evolution, three

percent of the beam is reflected from a fused silica beam splitter into

a high speed biplanar photodiode with a rise time of less than 0.3 nsec.

23
<

UJ

cr<

rrUl

Q.g 0.4

UJz

UJ

2.0

RADIUS (mm)

3.0

Figure 7- Energy per unit area as a function of distance from laser beam axis as measured at the focusing lens. The solid line

is a gaussian fit to the data. 24
The signal is displayed on a high speed oscilloscope

Figure 8 is a photograph of the oscilloscope trace. The ripples on the pulse are due to the beating of adjacent longitudinal modes of the

oscillator, whose optical path length is about 86 cm. The pulse width

is 20 nsec at half the maximum power. By integrating the pulse area using a planimeter, and by knowing the time base and total energy, we obtain a value for the peak power of 4.1 x 10 watts.

VI . Preliminary Self-Focussing and Damage Test

Preliminary measurements of self-focussing and damage in borosilicate crown glass (BSC 517) are made by focussing the laser beam

into the sample. The test apparatus is shown in Fig. 5. Table 1 gives all the relevant beam parameters as they are obtained in section V.

A. Test Procedure

The focal point of the focussing lens whose length is

nominally 152 mm is determined to within 1.25 mm by taking burn

patterns on a special high optical density glass (7) (Schott NG 1 glass) . The sample of BSC glass, a rectangular prism 38 x 38 x 76 mm, is placed in the beam so that the square front face was 25.4 mm

in front of the focus. Thus, the focus in the glass, whose refractive

index is 1.507 at 1.06 (Jm, is 38.2 mm from the entrance face.

The laser is fired into the glass and a damage track is

observed. The laser beam is subsequently attenuated from shot to shot, until no damage is observed.

B. Results

Figure 9 is the picture of a damage track. The downstream

end is predominantly a narrow filament while the upstream end shows

catastrophic damage. An end view of the track shows that the fractures

are planes intersecting on the track axis . These results are similar

to the damage observed by others. (8)25
TABLE I BEAM PARAMETERS

GAUSSIAN BEAM PROFILE

Wavelength 1.06 ym

Aperture Diameter in Oscillator 2.44 mm

Beam Energy 099 J Pulse Width (Full Width Half Maximum) ....... 20 nsec Peak Power 4.1 x 10 6 W

Aperture to Lens Distance 482 cm Lens Focal Length 152 mm

At Focussing At Lens

Lens Focus

Peak Energy Per Unit Area 0.95 14,600

(J/ cm

2

) Peak Intensity 4.0 x 10 7 6.1 x 10 11

(W/cm 2

) Beam Radius 1.81 mm 14.6 ym

1/e Energy Per Unit Area

26
10 nsec

Figure 8. Oscilloscope trace of the time evolution of the laser

pul se.

27
(a) (b)

4 mm

(c)

Figure 9- Damage track in B5C 517 glass.

(a) Laser beam direction is from right to left.

(b) End view of damage track.

(c) Track as seen through crossed polaroids. The pattern

observed is indicative of strain associated with the

damage sites.

28
In addition we have a view of the track observed in crossed

polaroids. Strain-induced birefringence patterns are observed, with

the greatest strains associated with the largest damage sites. The

presence of the strains suggests that the glass is heated rapidly to

a high temperature and then is cooled rapidly.

Figure 10 shows the end view of a damage track between

crossed polaroids. A cross is observed, as mentioned in section IV B,

in directions where the principal stress axes are parallel to the polarizer and analyzer. The second picture (10b) represents a rotation of

the polarizers by 45°. If the stress field were radially symmetric,

then the stress patterns in both pictures would possess four fold

symmetry and would be identical. A slight two fold asymmetry is

observed. This could be either an experimental artifact or else could

be associated with the polarization of the laser whose beam is polarized predominantly in the horizontal plane. Kerr (9) has shown that

one might expect to find effects due to linear polarization in electrostrictive self-focussing because of anisotropy in the electrostrictive

forces.

Figure 11 shows the positions of the damage tracks with

reference to the front face of the sample and with reference to the

expected position of the lens focus. Also given are the lengths of

the tracks and the peak power of the laser pulse used to produce the

damage. It is known that all tracks start downstream(lO) . Notice

that the lower energy tracks are the farthest downstream. From the

theory of self-focussing and from results obtained previously(lO) , one expects that all damage tracks would start closer to the lens focus

for the beam configuration used than was observed. The effect

observed is not understood, and will require additional investigation.

29
4 mm

(a) (b)

Figure 10. End view of damage tracks in BSC 517 glass between

crossed polaroids.

(a) Polarizer parallel to polarization direction of

damaging radiation.

(b) Polarizers rotated ^5°.

30
£E o o < z

q: uj H -J

Id

(0JD<u

Q.

O

0.

<J> 0> CO sr 0>

O

N;

q

fO • CO 6 CVJ CM CM *'

<t GO lO

CM ID Oi in __

_ . * • • • CVJ ro *

CO <n <fr ro CM

—

j__n^_-,-_ _t_ =_ = m-x

UJ< O21 CO r-O

CC LJ

uj or < Q

CD

CM

4-1 CD CD c o X •— c CD 1

<u Ul !_ 3 x

0) CD 4- W x: ^^ 01 !_ cn E

!_ 0) nj

w 3 X) c ^o -C O 01

4-> CL

— •— JZ IM 3 01 JZ h- cnc 1 oo +J —

10 in Ul ro 01 i/l 3 O

!_ _c

U

en fO 4-1 o < coh- Q: r^ c c

cu 01 01 LTV > r -C t- o Ol-Y

4-> z go u 4- o CO o

1/1 03 o

IT c — 4-J in U.

•~' <

01

l/l -C .y • 4-J 4- 2 o

CD

01

X

_ o 4-» a. c

03

03C or (D u.

CDcnin §

<C 4- 03

C UJ E o s_ o CO x> 0)

4-J 01 -C z 4- o

TO

U

4-J < o 14- 03 in H c 01

CD c CO o o 0) CD ^M •— c u </) Q

4-» ID

!_ 3X

CD1_

CO 4-J o Q_ o C I_ 0) D_ 01 o_ !_

CD

31
An attempt is made to fit the limited data to current

theories of self-focussing and to earlier data. A self-focussing

length, z, is calculated from the data of figure 11 using the single

lens equation(ll) _ 1 1 _- z- = z

f " - z~ , where z is the distance of the upstream 'end of the damage track from the

sample entrance face and z is the threshold self-focussing length for a focussed laser beam which is taken to be the distance from the downstream

end of the lowest power track to the entrance face. The inverse selffocussing lengths were plotted as a function of input power. A value for

the critical power for self-focussing, P =1.18 MW, was obtained by

-1 c extrapolating empirically to z = o. The inverse normalized selffocussing lengths (z*) are plotted in figure 12 together with the

theoretically calculated curve of Dawes and Marburger (12) (our P is

equivalent to their P„) where z* = z/2ka 2 and k = 2irn /A; n is the reL o o tractive index in the medium; A is the wavelength of the laser beam in

air; a is the radius of the 1/e intensity point at the sample face. The

data qualitatively follow the theory, although they do not quite fit the

theoretical curve. The theory assumes that the refractive index is

o n = n + n2E

o whereas if electrostrictive or thermal self-focussing is important the

situation would be more complicated.

Figure 13 shows the threshold power for self-focussing in

BSC glass as a function of the laser beam radius at the focus of a lens,

where the beam radius a is calculated neglecting self-focussing effects.

The figure is from the work of Kerr(9) and of Steinberg(13) . Also, the

data from this report are shown in the box on the figure. The solid

circles represent powers at which damage occurred and the open circles

represent powers at which no damage occurred. The X represents our threshold value. The solid curve is a theoretical fit from Kerr's

theory of electrostriction(9) . It is necessary to scale our data

because our wavelength (A = 1.06 urn) and pulse width (t = 20 nsec) differed from those of Steinberg (A = 694.3 nm, T = 55 nsec). The theory of

electrostrictive self-focussing (9) gives and expression for the selffocussing power threshold

32
2.5

_|CM

Figure 12. Self-focusing length in BSC glass as a function of beam

power. The solid curve is a theoretical curve derived

from Dawes and Marburger (Ref. 12). Assumptions made to obtain this plot are given in the text. The critical

powe r P = 1 . 1 8 MW

33
5000

1000

5

x

3

50

10

l—l l l I i

| B0R0SILICATE CROWN GLASS

I = 60 GW/cm 2

K = 937 kW —

vp = 235jim

J i i i i i i I i i i i i i i 10pm 100pm

LOW INTENSITY BEAM RADIUS a Imm

m

Figure 13° Track formation thresholds in BSC glass from the work of

Kerr and of Steinberg (Ref. 9, 13)= Also shown are data from the present work enclosed in the box. It was necessary to scale the data as the wavelength used and pulse

width differed from Steinberg's. The open circles rep- powers at which no damage occurred and the solid circles

represent powers at which damage occurred. The X represents the scaled value of the critical power as obtained

from Fig. 12.

34
,2 2 P =i-H- f (IZ)

c , 3nN 2 a

(p —) o o3p

where v is the velocity of sound in the medium, 9n/3p is the change

of refractive index with density, p is the density of the material,

TV and f(—) is a known function. Thus the value of P which we obtain

a c

o is scaled with wavelength and the value of a is scaled with the pulse

width. The wavelength dependence of p 9n/3p is not taken into account

as it is not known and is expected to be small. The result is that our data points compare favorably with Steinberg's. The result is important

because a method now exists for matching self-focussing data taken with

different wavelengths and different pulse widths.

VII. Acknowledgement

We wish to thank Captain Erlan Bliss of the Air Force Cambridge

Research Laboratory for his help in discussing the self-focussing

problem and for valuable suggestions to our program.

35
References

1. A. Feldman, R. Waxier, and D. Horowitz, NBS Technical Note 565, 41

(1971).

2. E. D. Williamson, J. Wash. Acad. Sci. 9_, 209 (1919); L. H. Adams

and E. D. Williamson, J. Frank, Inst. 190, 597-631; 835-870 (1920).

3. L. H. Adams and R. M. Waxier, Temperature-Induced Stresses in

Solids of Elementary Shape, NBS Monograph 2 (1960)

.

4. R. M. Waxier, G. W. Cleek, I. H. Malitson, M. J. Dodge, and T. A. Hahn, J. Res. Nat. Bur. Stand. (U.S.), 75A (Phys. and Chem.), 163

(1971).

5. H. Welling and C. J. Bickart, J. Opt. Soc. Amer. 5_6, 611 (1966).

6. G. D. Boyd and J. P. Gordon, Bell Sys . Tech. J. 40, 489 (1961).

7. W. Haller and N. N. Winogradoff, J. Am. Cer. Soc, 54, 314 (1971).

8. C. G. Young and R. F. Woodcock, "Laser Induced Damage in Glass,"

Damage in Laser Glass , ASTM STP469 American Society for Testing

and Materials, 1969 pp 84-99.

9. E. L. Kerr, Phys. Rev. to be published; E. L. Kerr, Perkin-Elmer

Report No. 9878 (1970). (Copies are available from the Defense

Documentation Center.)

10. C. R. Giuliano, Third ASTM Symposium on Damage in Laser Materials,

May 1971, to be published.

11. E. Bliss, private communication.

12. E. L. Dawes and J. H. Marburger, Phys. Rev. 179, 862 (1969).

13. G. N. Steinberg, Phys. Rev. to be published; Perkin Elmer Corporation Report No. PE-TR-7945 (1963), prepared by G. N. Steinberg.

•^O OU. S. GOVERNMENT PRINTING OFFICE : 1971—481-327/102
FORM NBS-H4A (1-71) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS TN 7032. Gov't Accession No.

3. Recipient's Accession No.

TITLE AND SUBTITLE ARPA-NBS Program of Research on High Temperature Materials

and Laser Materials, reporting period January 1 to June 30,

1971

5. Publication Date November, 1 971

6. Performing Organization Code

7. AUTHOR(S)

A. D. Franklin and H. S. Bennett (Edited by)

8. Performing Organization

9. PERFORMING ORGANIZATION NAME AND ADDRESS NATIONAL BUREAU OF STANDARDS DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 3000400

11. Contract/Grant No. ARPA Order 373-62

12. Sponsoring Organization Name and Address Office of Materials Sciences

Advanced Research Projects Agency

1400 Wilson Boulevard

Arlington, Virginia 22209

13. Type of Report & Period Covered Interim

Jan 1 to Jun 30, 1971

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant bibliography or literature survey, mention it here.) Progress reports are given for projects on the growth of ultra-pure AI2O3

crystals, the development of a mass spectrometer-based sectioning technique for measuring oxygen diffusion in oxides and the development of a precision facility for

measuring the threshold energy in a laser beam producing damage in a transparent

substance

.

17. KEY WORDS (Alphabetical order, separated by semicolons) AI2O3; crystal growth; damage threshold; glass; high temperature materials; laser;

oxides; oxygen diffusion; pure materials; sapphire.

18. AVAILABILITY STATEMENT

gg UNLIMITED.

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