NBSIR 76-1010

Optical Materials Characterization

Albert Feldman, Deane Horowitz and Roy M. Waxier

Inorganic Materials Division

Institute for Materials Research

and

Irving H. Malitson and Marilyn J. Dodge

Optical Physics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

February 1976

Semi-Annual Technical Report

Period Covered: August 1, 1975 to January 31, 1976

ARPA Order No. 2620

Prepared for

Advanced Research Projects Agency

Arlington, Virginia 22209

NBSIR 76-1010

OPTICAL MATERIALS CHARACTERIZATION

Albert Feldman, Deane Horowitz and Roy M. Waxier

Inorganic Materials Division

institute for Materials Research

Irving H. Malitson and Marilyn J. Dodge

Optical Physics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

February 1 976

Semi-Annual Technical Report

Period Covered: August 1, 1975 to January 31, 1976

ARPA Order No. 2620

Prepared for

Advanced Research Projects Agency

Arlington, Virginia 22209

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson. S^cntary

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Jphnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

and

OPTICAL MATERIALS CHARACTERIZATION

Albert Feldman, Deane Horowitz and Roy M. Waxier

Inorganic Materials Division

Institute for Materials Research

and

Irving H. Malitson and Marilyn J. Dodge

Optical Physics Division

Institute for Basic Standards

ARPA Order No 2620

Program Code Number 4D10

Effective Date of Contract January 1, 1974

Contract Expiration Date December 31, 1976

Principal Investigator Albert Feldman

(301) 921-2840

The views and conclusions contained in this document are those of

the authors and should not be interpreted as necessarily representing

the official policies, either express or implied, of the Advanced Research

Projects Agency or the U. S. Government.
OPTICAL MATERIALS CHARACTERIZATION

Abstract

Refractive index and the photoelastic constants have been measured on

specimens of reactive atmosphere processed (RAP) KCl and KCl nominally doped

with 1% KI . The refractive indices were measured by the minimum deviation

method in the wavelength range 0.25 pm to 15.5 pm. Measurements were made

near 20 °C and near 34 °C on the RAP specimen and near 20° on the doped

specimen. Each set of experimental data was fitted to a three term

Sellmeier-type dispersion equation. The temperature coefficient of index

was then computed for the RAP KCl. The data for the different types of

KCl were compared to each other and to earlier reported data on commercial

KCl. The piezo-optical constants '^^^f '^^2 ^44 elasto-optical

constants P^]^' ^44 ^^^^ obtained at 10.6 ym for the RAP and doped

KCl. The measurements required the use of a modified Twyman-Green interferometer capable of measuring fringe shifts '^.002 of a wave. Within

experimental error the coefficients of the two materials agree. Negligible

dispersion was found for 'i-^-^i '^12' ^^'^ ^12 visible and

10.6 ym while a small dispersion was found for q and p...
Table of Contents

1. Technical Report Summary 1

1.1 Technical Problem 1

1.2 General Methodology 1

1.3 Technical Results 2

1.4 Department of Defense Implications 2

1.5 Implications for Further Research 2

2. Technical Report 3

2.1 Refractive Index of Reactive Atmosphere Processed (RAP)

Potassium Chloride and Potassium Chloride Doped with

Potassium Iodide - Marilyn J. Dodge and Irving H. Malitson .... 3

2.1.1 Introduction 3

2.1.2 Experimental Technique 3

2.1.3 Index Data 4

2.1.4 Temperature Coefficient of Index 6

2.1.5 Conclusions 8

2.1.6 Acknowledgement 8

2.1.7 References 9

2.2 Photoelastic Constants of Reactive Atmosphere Processed (RAP)

Potassium Chloride and Potassium Chloride Doped with Potassium

Iodide - Albert Feldman, Deane Horowitz and Roy M. Waxier 10

2.2.1 Introduction 10

2.2.2 Discussion 10

2.2.3 Results .12

2.2.4 Acknowledgement 13

2.2.5 References 13

3. Appendix - Specimen Specifications for Photoelasticity, dn/dT, Thermal

Expansion and Refractive Index Measiirements 17

i

OPTICAL MATERIALS CHARACTERIZATION

1. Technical Report Summary

1.1 Technical Problem

Windows subjected to high-average -power laser radiation will undergo

optical and mechanical distortion due to absorptive heating. If the distortion

becomes sufficiently severe, the windows become unusable. Theoretical calculations of optical distortion in laser windows depend on the following material

parameters: absorption coefficient, refractive index, change of index with

temperature, thermal expansion coefficient, stress-optical constants, elastic

compliances, specific heat, thermal conductivity and density. Our program

has been established to measure refractive indices, changes of index with

temperature, stress-optical constants, elastic compliances, and thermal

expansion coefficients of candidate infrared laser window materials.

1.2. General Methodology

Laboratory experiments are conducted for measuring refractive indices,

changes of index with temperature, stress-optical constants, elastic compliances,

and thermal expansion coefficients.

The refractive indices of prismatic specimens are measured on

precision spectrometers by using the method of minimum deviation. Two

spectrometers are used. One instrument, which uses glass optics, is used

for measuring refractive indices in the visible with an accuracy of several

parts in 10^. The other instrximent, which uses mirror optics, is used for

measuring refractive indices in the ultraviolet and the infrared to an

accuracy of several parts in 10^. Using both spectrometers we can measure

refractive indices over the spectral region 0.2 ym to 50 ym.

We measure the coefficient of linear thermal expansion, a, by a method

of Fizeau interferometry . The interferometer consists of a specially prepared

specimen which separates two flat plates. Interference fringes are observed

due to reflections from the plate surfaces in contact with the specimen. We

obtain a by measuring the shift of these interference fringes as a function

of temperature.

The change of refractive index with temperature, dn/dT, is measured

by two methods. In the first method, we measure the refractive index with

the precision spectrometers at two temperatures, 20 °C and 30 °C, by

varying the temperature of the laboratory. This provides us with a measure

of dn/dT at room temperature. The second method may be used for measuring

dn/dT up to a temperature of 800 °C. We obtain dn/dT from a knowledge of

the expansion coefficient and by measuring the shift of Fizeau fringes in a

heated specimen as a function of temperature. The Fizeau fringes are due to

interferences between reflections from the front and back surfaces of the

specimens.

We measure stress-optical coefficients and elastic compliances using

a combination of Twyman-Green and Fizeau interferometers. From the shift of

fringes in specimens subjected to uniaxial or hydrostatic compression, we

obtain the necessary data for determining all the stress-optical constants and

1
elastic compliances. In materials with small stress-optical constants or

in materials that cannot withstand large stress, we measure the stress-optical

effect with a modified Twyman-Green interferometer, which has a sensitivity of

less than O.OlA at 10.6 ym. In this case we must know the elastic constants

of the material in order to calculate the stress-optical constants.

1. 3 Technical Results

Refractive index and the photoelastic constants have been measured

on specimens of reactive atmosphere processed (RAP) KCl and KCl nominally

doped with 1% KI . The refractive indices were measured by the minimijm

deviation method in the wavelength range 0.25 ym to 15.5 ym. Measurements

were made near 20 °C and near 34 °C on the RAP specimen and near 20° on the

doped specimen. Each set of experimental data was fitted to a three term

Sellmeier-type dispersion equation. The temperature coefficient of index

was then computed for the RAP KCl. The data for the different types of

KCl were compared to each other and to earlier reported data on commercial

KCl. (Section 2.1)

The piezo-optical constants q^, q]L2 '^44 ^^'^ elasto-optical

constants p^i' P44 '^^^^ obtained at 10.5 ym for the RAP and doped

KCl. The measurements required the use of a modified Twyman-Green interferometer capable of measuring fringe shifts '^.002 of a wave. Within

experimental error the coefficients of the two materials agree. Negligible

dispersion was found for <li2' Pll ^^'^ P12 between the visible and

10.6 ym while a small dispersion was found for q44 and P^^- (Section 2.2).

1.4 Department of Defense Implications

The Department of Defense is currently constructing high-power

infrared laser systems. Criteria are needed for determining the suitability

of different materials for use as windows in these systems. The measurements

we are performing provide data that laser system designers can use for

determining the optical performance of candidate window materials.

1 . 5 Implications for Further Research

Measurements of refractive index, change of index with temperature,

thermal expansion, stress-optical constants and elastic compliances will be

continued on candidate laser window materials. The wavelength of interest

will shift from 10.5 ym to 3.39 ym, which is within the wavelength range

of interest to designers of chemical laser systems (3-5 ym range)

Apparatus is being procured and assembly of equipment is currently

underway for the interferometric measurement of photoelastic constants and

dn/dT. A temperature control system is being planned for interferometric

measurement of dn/dT to cover at a minim\am the temperature range -100 °C to

+100 °C which includes the temperatures of interest to military system

designers.

We are currently awaiting shipment of specimens under the Laser

Window Validation Program. A list of specifications has been sent to

Dr. James Stanford at the Naval Weapons Center at China Lake. These

specifications are included as an Appendix (Section 3,).

2
2. Technical Report

2.1 Refractive Index of Reactive Atmosphere Processed (RAP) Potassium

Chloride and Potassium Chloride Doped with Potassium Iodide

Marilyn J. Dodge and Irving H. Malitson

2.1.1 Introduction

It has been established that optical distortion can occur in laser

windows subjected to high power laser radiations [1-5]-^. To predict the

amount of distortion, it is necessary to know the refractive index, n,

temperature coefficient of index, dn/dT, and the thermal-expansion and

stress-optical coefficients of candidate laser window materials. An

optical materials characterization program is [6] currently in progress

at NBS to determine these pertinent optical properties. The ref ractometry

laboratory at NBS is determining the index of refraction and dn/dT of

selected window materials over a limited temperature range.

Potassium chloride is one of the most promising candidates in the

search for potential window and auxiliary optical component materials

for use in high-power laser systems. It has useful transparency from

about 0.2 to 20 ym with 90% transmittance from 0.45 to 14 ym [7].

Recently, ultra pure KCl having very low absorption [8] has been developed

and a specimen grown at Naval Research Laboratory by reactive atmosphere

processing (RAP) was made available by Dr. Philip Kline for refractive

index and temperature coefficient studies. KCl doped with ions is also

being considered for laser windows. A specimen o^ KCl doped with 1.5%

KI grown at Honeywell Corporation Research Center was supplied for this

investigation by Dr. E. Bernal. A sample of single crystal KCl grown by

traditional techniques at Harshaw Chemical Corporation about 1966 has

been studied by this laboratory and preliminary data were previously

reported [9]. For comparison purposes with the results of this investigation on RAP and doped KCl the final data of the earlier study are also

presented.

2.1.2 Experimental Technique

The specimens were In prismatic form and were measured by means of

the minimum-deviation method using a precision spectrometer shown

schematically in figure 1[10]. In the ultraviolet, visible and near

infrared regions of the spectrum, the index was measured at known emission

wavelengths of mercury, cadmium, zinc and helium. Beyond 2.0 ym a globar was used for the radiant-energy source, and measurements were made

1. Figures in brackets indicate the literature references at the end of

this paper.

2. The use of company and brand names in this paper are for identification

purposes only and in no case does it imply recommendation or endorsement

by the National Bureau of Standards and it does not imply that the

materials used in this study are necessarily the best available.

3
at known absorption bands of water vapor, carbon dioxide, polystyrene

and 1,2-4 trlchlorobenzene. A series of narrow-band interference

filters was also used between 3.5 and 10.6 ym. A thermocouple with a cesium iodide window was used for the detector in the Infrared and a photocell was used In the ultraviolet below .25 ym. This spectrometer

has a scale which can be read to within one second of arc. Therefore,

the refractive l^dex of good optical material can be measured within a few parts In 10 over a wide wavelength range.

The refractive Index was determined from the ultraviolet to the

Infrared for all samples near 20°C and for the commercial and RAP specimens

near 30°C and 34°C respectively. Each set of experimental data was

fitted by a least square solution to a three-term Sellmeler-type dispersion

equation [11] of the form

o N A.A^

n^ - 1 = -J

2 2

X - X

2 The index of refraction Is represented by n, X Is the wavelength of

Interest, the X.'s are the calculated wavelengths of maximum absorption

and the A.'s ari^ the calculated oscillator strengths corresponding to

the absor'Jtlon bands. Primary emphasis is given to procuring a matheematlcal fit of the measured data useful for Interpolation. The X.'s

and the A.'s are not Intended to have any physical significance and are

influenced by the wavelength range covered by the experimental data.

2.1.3 Index Data

The constants calculated for each dispersion equation, the wavelength

range covered by the experimental data, the number of experimental

points, and the average absolute residual (the average difference between

the experimental values and the calculated values) are given in table 1.

TABLE 1: CONSTANTS FOR DISPERSION EQUATIONS FOR KCl

Harshaw RAP KI-Doped

19.40°C 29.50°C 20.20°C 33.80°C 19.90°C

*1 0. 74783561 0. 96002778 0.80902239 0. 82044813 0. 78085271

^2 0. 42626630 0. 21344321 0.36511458 0. 35248240 0. 39493953

*3 4. 6867104 9. 9867010 2.2323342 1. 6784518 2. 2662238

^1 0. 083633417 0. 096438975 0.088281620 0. 089075558 0. 086111530

^2 0. 15389998 0. 17244608 0.15737774 0. 15835015 0. 15600595

^3 95. 063422 137. 82107 65.870423 57. 529058 66. 360157

No of

Wavelengths 79 21 58 39 54

Wavelength

Range (ym)

0. 22-14.4 0. 28-14.4 0.25-15.5 0. 25-15.5 0. 25-15.5

Average

absolute

residual 2.3 6.8 2.9 3 .9 3.2

of index

X 10^

A
-I 1 1 1 1 I 

_ - -- - - _- < -

C/3 >- - QCO

u0 LU OCNl <LU <

CO

QC<

:r1 1 1 1 1 oooo2 E

CD><5

CO

ooo

LT)

OLD

oCNl

o

CD CO

XBQNI 3Ali3Vad3a

Q) U-l I—

1-1

O

oc 0) c

c

•H •po

m

C 0 •H

4-1 (U CO

CO iH 3

0) cr

CO > CD

»-l CO CO

CO

& OJ

C OJ •!—)

c

O CN r<

M-l •H 1 O CO CO CM

O

c

at er <c'~ r<

ex

o

•H

o p.

coj?;

w

iH

II •H •1—1

c

a n3

un CU II 0) 0) > 4-1 H

•H CO CO +J O

e 1 O CO CO O

CO COU CM

fr iH

CJ •H

M-i

C

<u

U

e 'C3 Pi

tH •H

ate

CM CO ar ul

cno

B

ao

O

u

•H

U iH CO

o

U

o 14-1

u

CU 0)

4J

H

(1) CO • O X•P X) CO (u a

•H

•H CO

14-1 o

(U +J

CO

>-l CO c ^ O 4-1

O

a

<u

o

4-1 QCc o & - o e

CO CO

>-i bC l-i CO d) •H Pe

CJO

•H

M

4-1

PO

QJa

CO

o

CP

OJ

o

P

•H iP P CO -H

CO CU

P

•H > C > -H CU

d)

U

T3 -H

CO

CUo

iH CD •H • O

00 0) •H }-l

CO

(U CD x: u

CO

CU .H bC

COI XI

CUc >^ O iH p E

CO <u

CO

CD

0) CO P

CO 0) P (X o o

S-I

a

CO

CO (U •H rH

CU

e

p

CD •H (U

a.

p

•H t3 E

0) p

COE

•HXo

S-I

CO

(U

ao

a

C CUa C CO

c

c

O CO (D •H a x P CD P

•H13 (U CO O H CO O -H o

CD

CU • J= 4= ^ P P CJ

00

c

C 0) 5

•H iH C (U >^

•H > iH

CO CO ^ P & S C <U •H CD CO d CO E CO CO

<Uc

•HE

S-I CU p

(U XJ

XIc

CO

CD

CO

COCo

•HP

•H

CDOa

CO (U

CU iH C CO •H O

iH CD 5
The stated average absolute residual shows the goodness of fit and

is indicative of the accuracy of the experimental data. The largest

residuals occur in regions where the largest uncertainties exist in

wavelength identifications. Absorption spectra induced by atmospheric

conditions superimposed on the absorption spectra being used for measurements, especially in the 5 to 8 ym range, and broad, less well defined

bands beyond 10 ym are the primary sources of these errors. Because of

the nature of the uncertainties their magnitude is not well known but ^n

error of 0.5% at 10.0 ym would result in an index residual of 18 x 10

As dn/dX increases, a small error in wavelength assignment will result

in larger index residuals. As a check on the individual Sellmeier

solutions each set of data was divided into odd and even data points,

that is, data points 1,3,5... and 2,4,6..., and new solutions for each

half were obtained. Their excellent agreement showed that the fitting

procedure was not sensitive to the Increased random error at the extreme

wavelengths. A complete repetition of the experiment using the same and

new KCl samples is needed to determine the components of variation

related to materials and also other sources of systematic error.

The refractive index was calculated at regular wavelength intervals

for each case using the fitted parameters given in table 1. Figure 2

shows the index of the Harshaw sample near 20°C plotted as a function of

the wavelength. The index values range from 1.64588 at 0.22 ym to

1.43670 at 14.4 ym.

A graphical comparison of the refractive index near 20°C of the RAP

and Kl-doped samples is made with the Harshaw sample in figure 3. The

index of refraction of^the RAP sample differs from the Harshaw sample by

approximately +3 x 10 fr^m 0.3 to 9.6 ym. Below 0.3 ym the difference

rises sharply to +19 x 10 at 0.25 ym, and drops sharply beyond 9.6 ym

to -68 X 10 at 14.4 ym.

The index values of the Kl-doped sample are higher than those of

the Harshgw specimen by about 136 x 10 at 0.25 ym, drops off to a +60 X 10 at 0.8 ym, remains fairly constant to 8.0 ym, then drops to

-13 X 10~ at 14.4 ym.

2.1.4 Temperature Coefficient of Index

The calculated values of index at two temperatures for the

Harshaw and RAP KCl specimens were used to determine An/AT (°C)

The results are plotted as a function of wavelength for each specimen in

figure 4. The solid lines represent the An/AT values calculated from

the fitted index values using the parameters at the respective temperatures

given in table 1. The dashed lines indicate An/AT calculated from

experimental values at specific wavelengths as indicated by the data

points on the curves.

6
1—I—

<

CO

GC<

CEo

— <ll<I

J I L

T

—I—I—

a.<

CMI I

<J|<I

J 1 I I I I E

a.

j3o) gOl

X

cH CU

CO 1 o

CO

CO 'V ---^ M-l '—

01 QJ 0)

U

C 4= j:: 0 • u

O OD O CD •H 4-J CO CO o o

nj > I—

O CO 14H X

'"^

O •x) 1-1 CM

(U TJ

S < 3 CJ

QJ I—

O •H CO 0) 4J

o 14-I T3 CO I—

x: CO

o

C

o U-l

O

B 4-1 • 1 )-i • •H w o CO o <U CU 4J D CN ^1

14-1

u 4-1 14-1

c

C

S 3 O •H 0) CO OJ CU 4-1 0 tn t3 CO 4-1 O CO

4-1 QJ CO <U

u

d 4-J 4-1 OJ

uo Q) CU (U •H o • CO U a

•H t-i 4-1 iH (U 4-J 3 B O CO ^—

CO 4J

u 4J CU •H 60 U CO 4-1 CO 4-1 O > rH CO 5-1 M-l CO (1) • o; c

QJ '—

fc^d

o p. CO ex CO

O

o

B 4-J

e CU

a rH rH (D CO CO CU

E 4-1 CO UH 1 4-1 4J 4-J OJ 4-1 o /-^ c CO C CO CJ O •H 3 cu >, 4-1 O & c CO CO

4-1 iH 5-1 4-1

e <u 3 O CD E CO > H 4-1 CO B • (U CO Q) <1 CO 4J o CO CU CU

(U CO

u rH rH

E txC

c CO <4-l

u aD

0)

C

o <1 dJ

o

BB

H

o •H

D •n MH CO CO

CO ^—

4J rH 4-1 0) CO CO

d

c CO •H 4-1 CO

o

S rH 0) 3 CO •H

& P-I S3- •H CO CO iH CO

4-i rd 0)

X CO 3 a CO »-l (U (1) CJ < CO c >-l P-: T5 C rH ^4 CU •H 3 CO C •H CO CO

ft. 14-1 PS •H CJ < 4-1

CMI toI I

QJ

4-> O CO

(-1

X CO

CU CC ICU s >

CO CO

4Ju

X

c

•H

•H -H

O rHIo > B 3 0) yi •H o 3 QJ

4-1 14H

c

N

O 14H •H

CO CO QJ >H CD 0) J3

14H QJ CD 4-1 Q) iH CO H a ?^ 0 C CO o •H CO O

CM QJ Q)

CO rH rH 4-1 <U u Vj CO C a CO a 0) c Q) CD CD

CU X) 3 QJ

V-i QJ

o

U

QJ CX rH •H

a

14H

o CJ) 0 QJ 4H 5-1 •H 1 4-1 M rH •H CD >^ CO 5-1 •H

4-1 CO 'O CD aC rH

Hc

o CJ

CO

cr rH

CO CJ

t)C Ph 3

•H <; -H

fj-i P2 CD

X, CO

4-1 4=

CjO CO 3 J-i QJ CO rH ac 7
The maximum differenge between the two curves for An/AT of Harshaw

KCl is eight parts in 10 between 10 and 14 ym, where the wavelength

values corresponding to the experimental data have greater uncertainties.

The curve representing the values based on the experimental data is of

the sime general shape ns a curve based on published values by Paschon

in 1908[12]. This curve also has the form which mlglit be predicted from

thermo-optic behavior of solids [13]. The negative An/AT indicates that

a decrease in the density of the material is the primary factor effecting

the change in index with an increase in temperature. The fact that

An/AT becomes less negative near the UV and IR primary absorption edges

indicates a shift of these edges towards longer wavelengths with increasing

temperature

.

The An/AT curves based on fitted and experimental values for the

RAP KCl have essentially the same^shape. Again the maximum difference

between the two curves is 8 x 10 between 12 and 14 ym. Although the

rise in An/AT between 4 ym and 9 or 10 ym followed by the sharp drop in

An/AT shown by both of these curves is not what would be expected, the

very careful analysis of the data described earlier and the fact that

the two curves both show the same trend makes these findings difficult

to dispute at this time.

2.1.5 Conclusions

This study indicates that KCl grown by traditional techniques and

that grown by the new reactive atmosphere process have about the same

index of refraction between 0.3 and 10.0 ym with the absolute differences

in index increasing on either side of this range. The addition of the

1.5% dopant of KI, however, increased the index generally by about

60 X 10 in the visible and infrared to about 9 ym when it starts

decreasing, and becomes about equal to the Harshaw sample at 14 ym.

All of the data presented in this paper are for these particular

samples and under the specific conditions stated. Caution should be

used before applying any of these values to a sample of KCl of unknown

origin.

Because of the lack of published data to support or dispute the

An/AT curves which have been presented here, repeated measurements on

the same samples are scheduled. Index and temperature coefficient

determinations on other samples of KCl of the same types as those presented

here might be indicated to determine if these findings are typical or

peculiar to the samples in this investigation.

2.1.6 Acknowledgement

The authors express their sincere appreciation to Mr. Joseph M.

Cameron, Chief, Office of Measurement Services at NBS for his careful

analysis of the index and An/AT data and his helpful comments and

suggestions

.

8
2.1.7 References

[I] M. Sparks, J. Appl. Phys. 42, 5029 (1971).

[2] J. R. Jasperse and P. D. Gianino, J. Appl. Phys. 43, lf)86 (1972).

[3] B. Bendow, J. R. Jasperse, and P. D. Gianino, Opt. Commun. \ 98

(1972).

[4] B. Bendow, P. D. Glanlno, A. Hordvik, and L. H. Skolnik, Opt.

Coramun. I, 219 (1973).

[5] B. Bendow and P. D. Gianino, Appl. Phys. 2, 1 (1973).

[6] A. Feldman, I. Malitson, D. Horowitz, R. M. Waxier and M. Dodge,

Laser Induced Damage in Optical Materials: 1974, NBS Special Pub.

414, 141 (1974).

[7] Harshaw Optical Crystals, The Harshaw Chemical Co. (1967).

[8] Dr. Philip Kline, Naval Research Laboratory, private communication.

[9] See Reference 6

[10] W. S. Rodney and R. J. Spindler, J. Res. Nat. Bur. Stds. (U.S.),

51, 123 (1953).

[II] L. E. Sutton and 0. N. Stavroudis, J. Opt. Soc. Am. 5]^, 901 (1961).

[12] A. Smakula, Office of Technical Services; U.S. Dept. of Commerce

(October 1952) , p. 66.

[13] S. Ramaseshan, K. Vedam, R. Krishnan, in Progress in Crystal Physics , Vol. I, edited by R. Krishnan (Interscience Publishers, New York,

London, 1960), p. 139.

9
2.2 Photoelastic Constants of Reactive Atmosphere Processed (RAP)

Potassium Chloride and Potassium Chloride Doped With Potassium

Iodide - Albert Feldman, Deane Horowitz and Roy M. Waxier

2.2.1 Introduction

The measurement of piezo-optical constants of materials in the infrared

is part of a program of optical materials characterization. The properties

measured in this program are, refractive index, temperature coefficient of

index, thermal expansion coefficient, photo-elastic constants, and elastic

constants. These parameters are necessary for evaluating the performance of

high-power infrared laser windows subjected to intense laser radiation.

Absorption of a relatively small fraction of this radiation can produce

temperature and stress gradients of sufficient magnitude to produce intolerable

optical distortion and hence to render a window useless.1'2

In this paper we present the photoelastic constants of reactive atmosphere

processed (RAP) KCl and KCl doped with KI . We measured the constants at 10.6 ym

using a Soleil-Babinet type compensator and a modified Twyman-Green interferometer. The precision of measurement required in order to obtain reasonably

precise values for the coefficients was '^'.002 of a fringe shift at 10.6 ym

because of the inability of the specimens to withstand large stresses.^

2.2.2 Discussion

The definition of the pi^zo-optical coefficients has been discussed

extensively in the literature and need not be discussed here. There are

three independent coefficients, '^^^t '^^2' *^44' ^'^^ cubic materials of

class m3m to which KCl belongs. The coefficients q^^ and q - q^^ -

q^^^ describe stress-induced birefringence in a material, whereas qij^ and q ^ individually are needed to describe the absolute change of refractive

index due to stress.

The stress-induced birefringence is measured most easily and with the

greatest precision with a de Senarmont compensator and we have successfully

used this apparatus in the visible on the two types of KCl.^ However, in

the infrared at 10.6 ym this apparatus is not readily available; hence, we

use a Babinet-Soleil type compensator. The experimental arrangement is

shown in Figure 5. The variable wave plate (VWP) is a specimen of singlecrystal Ge in the form of a rectangular prism 12 x 12 x 36 mm with the [100]

axis along the long dimension. The specimen is placed in a screw clamp

stressing apparatus^ and stress is applied along the [100] axis at an angle

of 45° with respect to the vertical in a plane perpendicular to the beam

axis. The purpose of the VWP is to appropriately polarize the beam in

order to maximize the intensity transmitted through the wire-grid polarizer,

which is also oriented at 45° with respect to the vertical in the plane

perpendicular to the beam axis.

The compensator is the same as the VWP except that a calibrated precision

stressing frame is used with the stress along the vertical. The stressing

frame has been discussed in the literature.^ The Ge specimen in the frame is

calibrated by obtaining the shift of a specified number of fringes as a function of the applied force and by calculating the fringe shift per unit

applied force (An/AF) Ge

10
FOCUSING MIRROR

MIRROR'

VARIABLE

PYROELECTRIC

DETECTOR

WAVE PLATE \ A. i LASER 1

RADIATION 1 KA^

-HC> ' J \/beam

CHOPPER I

I

IBIpoLARi:

BEAM STOP

MIRROR

Figure 5. Apparatus for measuring stress-induced birefringence.

OSCILLOSCOPE

SIGNAL

LOCK- IN AMPLIFIER A C AMPLIFIER

0

REFERENCE

PYROELECTRIC

DETECTOR

FOCUSING

MIRROR

BEAM SPECIMEN

SPLITTER /

MIRROR

\

REFERENCE

SPECIMEN

MIRROR ON

PZT DRIVE

VARIABLE

WAVE PLATE

LASER

RADIATION

Figure 6. Apparatus for measuring absolute piezo-optic constants.

11
The unknown specimen is placed in a similar calibrated precision

stressing frame, with the stress applied along the vertical. The specimen

has the same dimensions as the VWP and the compensator. The retardation

in the specimen is measured as follows: Stress is applied to the compensator

until a first order null is obtained, which occurs at an applied force of

about 2700 N (600 lbs.). A small incremental stress is then applied to the

specimen AFjccl which produces a signal at the detector. The force on the

compensator is then either increased or decreased by an amount AFq^ until

the system is returned to a null and the stress readings are recorded. This

procedure is repeated until what we consider to be the maximum allowable

force is applied to the specimen. For the RAP KCl this force is 178 N

(40 lbs.) yielding a stress of 11 bars, and for the KI doped KCl this force

is 356 N (80 lbs.) yielding a stress of 21 bars. With these data and with

the Ge calibration, we obtain the stress birefringence constants for the KCl

from the equation

Af

Aw .An Ge (q = q,, - qno for [100] stress ^ = ^ W aF~

n Ge KCl (q = q^^ for [111] stress

where X is the wavelength, w is the specimen width, and n is the specimen

refractive index. Equation (1) is valid because AF varies linearly with

AF ,

. Ge , KCl

The method for measuring the absolute piezo-optic constants q^^ and

qj2 with a modified Twyman-Green interferometer has been described m

earlier publications^'^; consequently it will not be described here.

Slight modifications have been made to the apparatus, however, and these

are shown in Figure 6 which is a schematic of the apparatus. A VWP has

been added to permit orientation of the radiation polarization vector

either parallel or perpendicular to the stress axis of the specimen. A

wire-grid polarizer has been added to correct for errors in the setting

of the VWP and to insure that radiation of the proper polarization reaches

the detector. In addition, focusing optics have been added to the detection

system to permit detection of smaller radiation power. The use of lower power

diminishes the possibility of heating of optical elements due to residual

absorption which could produce drift in the interferometer. Thus, although

the CO2 laser used has an output power of about 4 watts, measurements are

usually made with an attenuated power of less than 500 mW.

The fringe shift per unit applied force An/AF is measured on a specimen

subjected to [001] stress for the radiation polarized parallel to the stress

axis and then for radiation polarized perpendicular to the stress axis. The

piezo-optical constants are obtained from the equation

Xw AN

2 AF

where e is the polarization vector.

2.2.3 Results

(2)

The piezo-optical constants of pure and doped single-crystal specimens

of KCl were measured at 10.6 ym. The four specimens are listed in Table 2

together with their crystallographic orientations. The constants we obtained

are listed at the bottom of Table 3 together with earlier reported data in the

12
-12 2 visible. In computing q-j^-^ and used a value of s-j^2 = -3.53 x 10 m /N

which is an average of many values presented in the literature.-'-'^ Earlier we

had reported preliminary stress-optical constant values at 10.6 pm^; however,

the data we present here have a greater precision. The errors given represent

the standard deviation of many measurements and the size of the errors corresponds to a fringe shift of approximately . 002 A at 10.6 pm. The values for

- ci that we present are data obtained independently from q and q, „•

An examination of Table 3 shows that earlier workers did not report

values for the coefficients q^^^ and ^.-^2' rather values for p^^ and

^12" '^^'^'^ report values for q - q and q^^. ' A comparison of our

data with the earlier work suggests that some dispersion exists in q^^

in going from the visible to 10.6 ym, whereas the dispersion in q^ - q^^

is negligible within the experimental error. Our data also indicate

that, within experimental error, the dispersions in q^^ and q^^ individually

are also negligible.

At the lower part of Table 4 we list the elasto-optic constants we have

calculated from the piezo-optical constants using the formula

p. = E q. . s., (3)

D -12 2 -12 2 where s^^ = 26.9 x 10 m /N and s^^ = 150.7 x 10 m /N. The ej^astic

compliance components are an average of values from a compilation which

contains a large variation in values. A comparison of our values with

values of other workers indicates agreement for some values, and disagreement

for others. There is little dispersion in p^^^ and P.j^2' '^^'^ some dispersion

"9 A A' 44

In the calculations we have used the same elastic constants for both

the doped and the RAP KCl because of lack of values for the undoped. On

this basis the measurements and calculations indicate that, within experimental

error, the piezo-optical constants of the two types of material are the

same.

2.2.4 Acknowledgement

We thank E. Bernal of Honeywell and P. Klein of the U. S. Naval Research

Laboratory for supplying materials for the specimens

.

2.2.5 References

1. M. Sparks, J. Appl . Phys. 42^, 5029 (1971).

2. J. R. Jasperse and P. D. Gianino, J. Appl. Phys. 43^, 1686 (1972) ; B. Bendow

and P. D. Gianino, Appl. Phys. 2j 1 (1973) ; B. Bendow, P. D. Gianino,

A. Hordvik, and L. H. Skolnik, Optics Commun. 1_, 219 (1973) . 3. W. W. Wilkening, J. Friedman, and C. A. Pitha, in Third Conference on

High Power Infrared Laser Window Materials, 1973, ARCRL-TR-74-0085-I

.

4. J. F. Nye, Physical Properties of Crystals (Oxford University Press,

London, 1957), pp. 243-254.

5. A. Feldman, D. Horowitz, and R. M. Waxier, in Laser Induced Damage in

Optical Materials : 1975, to be published as a Special NBS Publication.

6. A. Feldman, D. Horowitz, and R. M. Waxier, in Third Conference on High

Power Infrared Laser Window Materials , 1973, AFCRL-TR-74-0085-I , p. 403.

7. A. Feldman and W. J. McKean, Rev. Sci. Instrum. 46, 1588 (1975)

.

13
8. A. Feldman, I. H. Malitson, D. Horowitz, R. M. Waxier and M. Dodge, in

Proceedings of the Fourth Annual Conference on Infrared Laser Window

Materials, 1974, p. 117.

9. A. Feldman, R. M. Waxier, and D. Horowitz, in Optical Properties of Highly

Transparent Solids, edited by S. S. Mitra and B. Bendow (Plenum Publishing

Corp., New York, 1975), p. 517.

10. R. V. G. Sundara Rao, K. Vedam, and R. S. Krishnan, in Progress in Crystal

Physics, Vol. I, edited by R. S. Krishnan { Interscience Publishers,

New York, London, 1960) , p. 89.

14
Table 2. Specimen Orientations

Specimen Orientation Constants Measured

Stress Propagation

1

- RAP KCl [001] [110] ^11' ^12' %1 - ^12

2 - RAP KCl [ill] [110] q44

3 - Kri(i% Ki nom) [001] [110] ^11' ^12' ^11 - ^12

4

- KCl Q% KI nom) [110] [001] q44

Table 3. Piezo-Optic Constants of KCl , -12 2 , (10 m /N)

ani ^12 %4 ^11 - ^12 Ref

.

.589 -4.22 1.67 a

.589 -4.42 1.66 b

.589 -4.94 1.47 c .480 1.42 d

. 589 -4.74 1.57 e

.633 1.81 f

.633 4.6 +.2 2.7 +.8 -3.9 +. 8 1.7 +.4 g

.544 -4.4 +. 2 1.9 +.4 g

* .633 4.6 +.2 2.8 +.2 -4.6 +. 2 1.9 +.2 g

* .644

10.6

10.6

-4.7 +.

-2.62

2 1.9 +.1

2.0

g

h

10.6 4.3 +.3 2.8 +.3 -3.4 +. 4 1.8 +.4

* 10.6 4.2 +.2 2.5 +.2 -3.6 +. 3 1.8 +.2

Nominally doped with 1% KI

F. Pockels, Lehrbuch der Kristalloptik (Teubner, Leipzig, 1906) , p. 480.

'shagavantam and Y. Krishna Murty, Proc. Indian Acad. Sci . A45, 399 (1957).

K. G. Bansigir and K. S. Iyengar, Proc. Phys. Soc. London 7 IB , 225 (1958)

.

^R. Srinivasan, Zeit. f . Physik 155 , 281 (1959)

.

K. V. Krishna Rao and V. G. Krishna Murty, Proc. Indian Acad. Sci. 64_, 24 (1965) .

Reference 3.

^Reference 5.

^C. S. Chen, J. P. Szcznesniak, and J. C. Corelli, J. Appl . Phys. 4£, 303 (1975).

C. A. Pitha and J. D. Friedman, in Proceedings of the Fourth Annual Conference

on Infrared Laser Window Materials , 1974, compiled by C. R. Andrews and

C. L. Strecker.

15
Table 4. Elasto-Optic Constants of KCl

^11 n^12 M4 ^12^^11 Ref

. 589 -0.0275 a

.589 0. 215 0.159 -0.024 -0.74 b

.589 0.246 0.192 -0.0298 -0.78 c

. 633 0 . 21 0. 15 -0. 026 -0.70

.644 -0.029

* .633 0.21 0.15 -0.031 -0.72

* .644 -0.031

10.6 0.20 0.15 -0.023 -0.76

*10.6 0.19 0.14 -0.024 -0.71

Nominally doped with 1% KI

^F. Pockels, Lehrbuch der Kristalloptik (Teubner, Leipzig, 1905) , p. 480.

K. S. Iyengar, Nature 176, 1119 (1955).

K. V. Krishna Rao and V. G. Krishna Murty, Proc. Indian Acad. Sci. 54^

24 (1966)

.

16
Appendix

Specimen Specifications for Photoelasticity , dn/dT,

Thermal Expansion, and Refractive Index Measurements.

.1

17
I. Photoelasticity Specimens

A. Specifications

1. Dimensions - Rectangular prism 1/2" x

(12 X 12 X 36 mm)

1/2" X 1-1/2

2. Polish two opposite 1/2" x 1-1/2" (12 X 36 mm) faces.

a. Flatness - One (1) fringe 0.6328 \im radiation when

viewed against an optical flat across 90%

of the central aperture area.

b. Parallelism* - Six (6) fringes of 0.6328 ym radiation

when viewing Fizeau interference between

both polished faces.

* Alternate parallelism specification: wedge angle between polished

faces to be 25± 5 minutest

3. Grind all other surfaces so that the angles between all

adjacent surfaces to be 90° ± 1 min. of area. No noticeable

wedges, ridges or bumps shall be seen when testing angles

with a precision machinist's knife-edge square. (In a wedged

specimen this specification will hold for one polished face

and the four ground surfaces .

)

4. For single crystal specimens, orient the axes of the

specimen along specified crystallographic axes to within

one-half degree (1/2°).

5. Use grinding and polishing techniques that minimize adding

strain to the specimen.

6. Polish faces to be perpendicular to the growth or deposition

direction, if one exists and if compatible with above specifications .

B. Specimens Required

1. Amorphous or polycrystalline specimens - supply one parallel

and one wedged.)

2. Cubic material specimens - supply one parallel and one wedged

for each crystallographic orientation:

a. Long axis along [100] ; other faces arbitrary.

b. Long axis along [111]; other faces arbitrary. If not

available then an alternate form would be long axis

along [110] and short axes along [110] and [001] with

the [001] faces polished.

18
3. Other crystal classes - contact us for required specimen

orientations

.

4. If possible supply duplicates of above specimens.

19
II. Specimens for dn/dT and Thermal Expansion /

A. Specifications

1. Dimensions - Plate 1/2" dia. x 1/4" to 1/2" thick.

(12 mm dia. x 5 to 12 mm thick)

.

(Plate may be nominally 1/2" square

X 1/4" to 1/2" thick.)

2. Polish flat surfaces.

a. Flatness - One (1) fringe 0.6328 ym radiation when

viewed against an optical flat across

90% of the central aperture area.

b. Parallelism - Three (3) fringes of 0.6328 ym radiation

in a circular pattern when viewing

Fizeau interference between both polished

faces

.

3. Polished faces to be perpendicular to the growth or

deposition direction if one exists.

B. Specimens Required for dn/dT

1. Cubic*, polycrystalline , and amorphous materials - supply

two specimens.

* Cubic specimens require no particular crystallographic orientation.

2. Other crystal classes - contact us for required specimen

orientations.

C. Specimens for Thermal Expansion - Identical with dn/dT specimens

Two are required which may be prepared simultaneously with

dn/dT specimens

.

20
III. Refractive Index Specimens

A. Specifications

1. Triangular prism with

approximately 1" (2.5

2. The angle between the

given by the formula

6=2 sin""""

tv;o polished rectangular faces

cm) square.

two polished faces, 9 , to be

.866

within one (1) degree, where n is the maximum nominal

index value, but 0 is not to exceed 70°.

3. Polish the faces flat to 1/8 wave 0.633 pm light if

possible.

B. Specimens Required

1. Cubic*, polycrystalline and amorphous material - supply

one specimen.

* Cubic specimens require no particular crystallographic orientation.

2. Contact us for specimen requirements for non-cubic crystals

3. As a test for homogeneity, supply specimens from different

portions of a batch or from different batches.

/

USCOMM NBS-DC 21
NBi-l MA (REV. 7-73)

4. xn LI-; AND suivi nu:

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. i->uiu.i(;ation ok Ki-.i^ou r no. NBSIR 76-1010

2. Ciov't Accession

No.

Optical Materials Characterization

7. AUTHOR(S)

. Fe].dnian, D. Horowitz, R. Waxier, I. Malitson and M. Dodge

9. PERFORMINC. ORGAN IZATION NAMH AND ADORLSS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

2. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Advanced Research Projects Agency

Arlington, Virginia 22209

3. Recipient's Accession No.

5. Pub! icntion Hate

February 1976

6. Performing Organization ( ode

8. Performing Organ. Report No.

NBSIR 76-1010

10. Proiect/ lask/Work Unit No. 3130442

11. Contract /Grant No.

2620

13. Type of Report iSi i'eriod Covered

Semi-Annual Tech. Report

R-1-7S -t-n 1-71-76

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. It document includes a significant

bibliography or literature survey, mention it here.)

Refractive index and the photoelastic constants have been measured on specimens of reac

tive atmosphere processed (RAP) KCl and KCl nominally doped with 1% KI. The refractive

indices were measured by the minimum deviation method in the wavelength range 0.25 iim tc 15.5 ym. Measurements were made near 20 °C and near 34 °C on the RAP specimen and near

20 °C on the doped specimen. Each set of experimental data was fitted to a three term

Sellmeier-type dispersion equation. The temperature coefficient of index was then

computed for the RAP KCl. The data for the different types of KCl were compared to eac

other and to earlier reported data on commercial KCl. The piezo-optical constants . q^^

,

q and q and the elasto-optical constants p^^, p^^ and p^^ were obtained at 10.5 pm

for the R^P and doped KCl. The measurements required the. use of a modified TwymanGreen interferometer capable of measuring fringe shifts a,. 002 of a wave. Within

experimental error the coefficients of the two materials agree. Negligible dispersion

was found for q^, , q3^2'Pll ^12 visible and 10.6 ym while a small dispersion was found for q^^ and P^^-

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper

name; separated by semicolons) Birefringence; elastooptic constants; infrared-laser window

materials; interferometry ; KCl; photoelasticity ; piezo-optic constants; refractive

index; stress-optical constants; thermal coefficient of refractive index.

I

18. AVAILABILITY Unlimited

I

' For Official Distribution. Do Not Release to NTIS

r~] Order From Sup. of Doc, U.S. Government Printing Office

Washington, D.C. 20402, SO Cat. No. CI 3

Order From National Technical Information Service (NTIS)

Springfield, Virginia 22l')l

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIF.D

20. SIX URITV ( LASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

2b

22. P rice

$4.00 _i

USCOMM.DC 29042. P'',