NBSIR 78-1473

Optical Materials

Characterization

Albert Feldman, Deane Horowitz, Roy M. Waxier and Marilyn J. Dodge

Ceramics, Glass and Solid State Science Division

Center for Materials Science

National Bureau of Standards

Washington, D C. 20234

May 1978

Period Covered: August 1 , 1 977 to January 31,1 978

ARPA Order No: 2620

Prepared for Advanced Research Project Agency

Arlington, Virginia 22209

k

««

NBSIR 78-1473

OPTICAL MATERIALS

CHARACTERIZATION

Albert Feldman, Deane Horowitz, Roy M. Waxier and Marilyn J. Dodge

Ceramics, Glass and Solid State Science Division

Center for Materials Science

National Bureau of Standards

Washington, D C. 20234

May 1978

Period Covered: August 1 , 1 977 to January 31,1 978

ARPA Order No: 2620

Prepared for Advanced Research Project Agency

Arlington, Virginia 22209

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

OPTICAL MATERIALS CHARACTERIZATION

Albert Feldman, Deane Horowitz, Roy M. Waxier, and Marilyn J. Dodge

Ceramics, Glass § Solid State Science Division

Center for Materials Science

ARPA Order No . . 2620

Program Code Number . . 4D10

Effective Date of Contract

Contract Expiration Date

Principal Investigator

(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 expressed or implied,

of the Advanced Research Projects Agency or the U.S. Government.

The results in this report are preliminary in nature and are subject to change.
1

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 Thermo-optic and Linear Thermal Expansion

Coefficients 3

2.2 Piezo-optic Constants 4

3. Acknowledgement 17
OPTICAL MATERIALS CHARACTERIZATION

Abstract

The piezo-optic constants of CaF OJ BaF 0 , and SrF 0 have been

measured at 0.6328 pm and 1.15 pm. The temperature dependence of

the refractive indices of CdF ?J MgF OJ and NaCl have been measured

at several wavelengths in the"" infrared by the method of Fizeau

interferometry. The linear thermal expansion coefficients of NaCl

and CdF 0 as a function of temperature have also been measured.

1

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 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 gefractive indices in the visible with an accuracy of

several parts in 10 . The other instrument, 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 urn 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. We can measure

a from -180 °C to 800 °C.

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 from 180 °C to 800 °C. We obtain dn/dT from a knowledge of the expansion coefficient and by measuring the shift of Fizeau
- 2 -

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 piezo-optic 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 con- stants and elastic compliances.

In materials with small piezo-optic constants or in materials that

cannot withstand large stresses, we use interferometers designed to

measure fractional fringe shifts. At 10.6 ym we use a modified TwymanGreen interferometer which has a sensitivity of 0.01A. At 632.8 run, we use a modified Dyson interferometer which has a sensitivity of 0.002A.

When using these interferometers to measure piezo-optic constants we must know the elastic constants of the material under test.

1 .

Technical Results

The temperature dependences of the thermo-optic coefficients of

CdF 0 , MgF^ and NaCl have been measured over the temperature range

-180 °C to 200 °C at discrete wavelengths in the infrared by the method

of Fizeau interferometry. The linear thermal expansion coefficients of

CdF ? and NaCl were also measured over the same temperature range.

(Section 2.1)

The piezo-optic constants q^, q^, and q, , of CaF n , BaF ? and SrF ? have been measured at 0.6328 ym and 1.15 ym. [Section 2

1" 2

)

1 .4 Department of Defense Implications

The Department of Defense is currently constructing high-power

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

We plan to measure the refractive indices of SrF^ and MgF 0 from the

ultraviolet into the infrared. Measurements of the tHermo-optic coefficients of LiF, NaF 0 , MgF 0 , CdF ?

, NaCl, Al o 0_, CaF^, BaF ? , SrF,,, KC1, and

KBr are planned for the wavelengths 458 nfh and 350 nm. “Piezo r optic

coefficient measurements are planned for Si0 9 , CaF ? and A1.,0_ at 350 nm. All this work cannot be done by September 307 19787 however, we will do as much as possible under the constraints of the funding available.
3

2. Technical Report

2 . 1 Thermo-optic and Linear Thermal Expansion Coefficients

In addition to the previously reported [1] dn/dT data on CaF , BaF 0 , KBr(RAP), KCl(RAP), LiF, NaF, SrF ?

, ZnS(CVD), and ZnSe(CVD), dn/dT

was measured on single crystals of CdF 0 7 and NaCl

.

Figure 1 shows a plot of dn/dT as a function of temperature for

CdF^ at the two helium-neon laser wavelengths, 0.6328 pm and 3.39 pm, for

the temperature range, -180 °C to 200 °C. The solid line curve represents

a least squares, third order polynomial fit to the .6328 pm dn/dT data.

Table 1 presents the results of this fit and a similar third order

polynomial fit at 3.39 pm in a tabulated form with 20 °C temperature

intervals. The errors in the table are the standard deviation of the

experimental data to the least squares fit.

dn/dT as a function of temperature for NaCl is shown in Figure 2 for the three wavelengths, 0.6328 pm, 1.15 pm, and 3.39 pm. The results

of a least squares third order polynomial fits for each wavelength are presented in Table 2.

MgF 0 , which is an anisotropic crystal, was measured at 0.6328 pm and

3.39 ym with the electric field parallel to the c-axis to get dn /dT, and

with the electric field perpendicular to the c-axis to get dn /d¥.

The birefringence as a function of temperature, d(n -n )/dT, was also

measured at 0.6328 pm. The birefringence as a function of temperature

was too small to be measured at 3.39 pm. The upper set of points in

Figure 3 shows dn /dT as a function of temperature at 0.6328 pm and

3.39 pm, the midd?e set of points shows dn /dT as a function of temperature

at 0.6328 pm and 3.39 pm, and the lower set of points shows the experimentally

measured birefringence, d(n -n )/dT, at 0.6328 pm. The straight lines

represent a linear least squares fit to the 0.6328 pm data in which there

was the constraint that the difference in the dn /dT and dn /dT fits

equals the birefringence fit, d(n -n )/dT. Table 3 shows tfie tabulated

results for MgF,,.

The refractive indices, the specimen thicknesses, and references to

the thermal expansion coefficient, all of which were used in the computation

of the results for each of the materials, are given in Table 4.

The linear thermal expansion coefficients of NaCl and CdF 0 were

also measured. Figure 4 gives the thermal expansion of NaCl in which

the triangles are the experimental results and the circles are the AIP

handbook values. The dashed line represents the fit to our data, which

was used to calculate dn/dT for NaCl. Figure 5 gives the thermal

expansion of CdF 0 as a function of temperature. Only one reference to

the thermal expansion of CdF 9 was found.
4

i

2 . 2 Piezo-optic Constants

The piezo-optic constants have been treated amply in the literature

[2] so that it is not necessary to describe them here. The rare-earth

fluorides are cubic belonging to the crystal class m3m and have three

piezo-optic coefficients, q , q^, and ^ 44

- These coefficients have

been evaluated at two wavelengths, 0.6328 urn and 1.15 pm by measuring

the changes in optical path length induced by compressive loading on specimens in the shape of rectangular prisms. Helium-neon laser sources were used at both wavelengths, and the optical path change was measured

interferometrically by noting the shift in interference fringes. The

fringes were detected by a silicon matrix vidicon camera and observed on

a television monitor. Determinations were made for CaF ?

, SrF 9 and BaF ?

.

The specimens, which were obtained commercially, had been precision

ground to the approximate dimensions, 38mm x 13mm x 13mm. The method of

mounting and loading the specimens has been described earlier [3] . Two

specimens of each material were fabricated. In the first specimen, the

longest dimension was parallel to the < 001 > crystallographic direction

and the light was propagated parallel to the <010> direction. In the

second specimen the longest dimension was parallel to the < 1 1

> direction

and the light was propagated along the < 11

> direction.

Two opposite long faces of each prism had been polished sufficiently

flat and parallel so that about six localized, Fizeau-type interference

fringes could be observed across the face when illuminated with collimated

monochromatic light. At the wavelength, 0.6328 pm, q^ and q. ? were

determined by measuring the shift in these Fizeau fringes witn load on the <001> specimens. The coefficient q^ was obtained from stress

birefringence measurements on a <111> specimen. The optical set-up and

equations relating the changes in refractive index with stress have been

presented elsewhere [3-6]

At 1.15 ym, the use of a <001> prism for the determination of

q ^ and q^ ? was found to be inadequate because of the small shift in interference

fringes with load; instead, the <111> prism was used. Measurements were made of the shift in interference fringes for light polarized both

vertically and horizontally, q and q^ were then evaluated by solving

simultaneously the two equations

An

i = n0

3

^n + 2t4 2

+ 2<W I

(1)

and

An_ = n 3 , _ ^ p

2 o (q n

+ 2 q12 - q44) L

(2)

o where An and An 0 are respectively, the refractive index changes for

light polarized vertically and horizontally, n is the initial refractive

index, and P is the applied stress. To determine q., and q ^ 9 from the

above equations, it is necessary to know q , and this value“was found

by measuring the stress induced birefringence in the < 11

> specimen.
5

'

The results of the study are presented in Table 5. The data

indicate that there is little dispersion between the determinations at

3.6328 ym and 1.15 ym. For comparison, data from the literature [7]

caken at 0.6328 ym are also presented. Except for and q^ 9 in CaF-,,

the disagreement of our data with the data in the literature is significant.

,Ye suspect that many of the deviations observed may be due to erroneous

values of the elastic constants in the literature. These constants are

jsed in the analysis of the interferometric data in order to obtain the

piezo-optic coefficients. They are also used in the conversion of

elasto-optic coefficients to piezo-optic coefficients.

An example of the difficulty with the elastic compliances arose in

the measurement of the piezo-optic constants of SrF

?

. It was found that

the coefficients q.^ and q^ 9 obtained on the <001> specimen differed

from the values obtained on“the <111> specimen. The discrepancy was

resolved by the performance of measurements on a Twyman-Green interferometer

[8] in addition to the Fizeau interferometer measurements. Both sets of

measurements permitted us to calculate elastic compliance components

which differed from values in the literature. These values will be

presented in a future report.
6

References

1. A. Feldman, D. Horowitz, R. M. Waxier, M. J. Dodge, and

W. K. Gladden, Optical Materials Characterization , National

Bureau of Standards International Report, NBSIR 77-1304

(August, 1977)

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

Press, London, 1957), pp. 243-254.

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

4. A. Feldman, R. M. Waxier, and D. Horowitz, Optical Properties

of Highly Transparent Solids , Ed. by S. S. Mitra and B. Bendow

(Plenum Publishing Corp. , New York, 1975), pp. 517-525.

5. R. M. Waxier and E. N. Farabaugh, J. Res., NBS 74A, 215 (1970).

6. A. Feldman, Electro-optical Systems Design, 8_, 36 (1976) .

7. S. K. Dickinson, Infrared Laser Window Materials Property Data

for ZnSe, KC1, NaCl, CaF n , SrF?

, BaF ?, (AFCRL-TR-75-0318 Physical

Sciences Research Papers, No. 635, Solid State Sciences Laboratory,

Projects 5620,3326, Air Force Cambridge Research Laboratories,

L. G. Hanscom Field, Bedford, Massachusetts 01730, June 6, 1975),

pp. 147-194.

8. N. Born and E. Wolf, Principles of Optics (Pergamon Press, 1970),

p. 303.
7

Table 1. dn/dT of CdF

2 (lO' 5 ^

1

)

Temperature

Wavelength (pm)

(°C) 0. 6328 a

3 . 39b

-180 - 0.56 - 0.53

-160 - 0.64 - 0.64

-140 - 0.72 - 0.73

-120 - 0.78 - 0.81

-100 - 0.84 - 0.87

- 80 - 0.89 - 0.93

- 60 - 0.93 - 0.98

- 40 - 0.97 - 1.02

- 20

0

20

40

- 1.01

- 1.04

- 1.07

- 1.10

- 1.05

- 1.08

- 1.11

- 1.14

60 - 1.13 - 1.17

80 - 1.16 - 1.20

100 - 1.19 - 1.23

120 - 1.23 - 1.27

140 - 1.27 - 1.31

160 - 1.31 - 1.36

180 - 1.37 - 1.42

200 - 1.43 - 1.49

tandard deviation from a third degree polynomial fit is 0.02

tandard deviation from a third degree polynomial fit is 0.04
- 8 -

Table 2. dn/dT of NaCl (10" 5 K

_1

)

Wavelength (ym)

Temperature o (°C) .6328 1.15 3.39

a

-180 -2.16 -2.22 -2.24

-160 -2.40 00

c\| 1 -2.49

-140 -2.61 -2.70 -2.70

-120 -2.79 -2.89 -2.89

-100 -2.96 -3.06 -3.05

- 80 -3.09 -3.20 -3.19

- 60 -3.21 -3.32 -3.31

- 40 -3.32 -3.42 -3.41

- 20 -3.40 -3.51 -3.49

0 -3.48 -3.58 -3.57

20 -3.54 -3.64 -3 . 63

40 -3.50 1 C

/-1 o -3.68

60 -3.65 -3.74 -3.73

80 -3.69 -3.79 -3.78

100 -3.74 -3.83 -3.83

120 -3.78 -3.88 -3.88

140 -3.83 -3.93 -3.94

160 -3.88 -3.99 -4.01

180 -3.94 -4.06 -4.09

200 -4.01 -4.14 -4.18

£ Standard deviation from a third degree polynomial fit is 0.04
9

Table 3. dn/dT of MgF 0 C10~

6 K

-1

)

Wavelength (pm)

Temperature

C°c) 0 . 6328 a

3. 39

b

dn / dT

e dn /dT0

dn /dT

e dn /dT

0

-180 1.65 2.23 1.5 2.0

-160 1.54 2.12 1.4 2.0

-140 1.44 2.01 1.3 1.9

-120 1.33 1.90 1.2 1.8

-100 1.22 1.79 1.2 1.7

- 80 1.12 1.68 1.1 1.6

- 60 1.01 1.57 1.0 1.5

- 40 0.90 1.46 0.9 1.4

- 20 0.80 1.35 0.8 1.3

0 0.69 1.24 0.7 1.2

20 0.58 1.12 0.6 1.1

40 0.48 1.01 0.5 1.0

60 0.37 0.90 0.4 1.0

80 0.27 0.79 0.3 0.9

100 0.16 0.68 0.2 0.8

120 0.05 0.57 0.1 0.7

140 -0.05 0.46 0 0.6

160 -0.16 0.35 -0.1 0.5

180 -0.27 0.24 -0.2 0.4

200 -0.37 0.13 -0.3 0.3

Standard deviation to be determined

^Standard deviation from a linear fit is 0.2
- 10 -

Table 4. Data used in Computation of dn/dT

Material

Refractive index. n

0

t(mm) a

632.8 nm 1.15 nm 3.39 ym

CdF 9 1.5735

a 1.54

b 7.33 c

MgF

2 ne

1 . 3887d

1.384

s 1.369® 13.40 f

MgF

2 n

Q

1 . 3770d

1 . 373 e 1.358 6 13.40 f

NaCl 1.542 s 1.5305 s 1.5235? 14.08 h

a

B. Krukoska-Fulde, T. Niemyski, J. Crystal Growth 1, 183-6 (1967).

Estimated.

c See Figure 5.

b

A. Duncanson, R. W. H. Stevenson, Proc. Phys. Soc. (London) 72_,

1001 (1958).

0

H. H. Li, to be published.

J. S. Browder, S. S. Ballard, Appl. Optics 16 (12), 3214-7.

: s. S. Ballard, J. S. Browder, J. F. Ebersole, AIP Handbook,

Dwight E. Gray ed. (McGraw-Hill Book Co., 1972), pp. 6-12 to 6-57.

R. K. Kirby, T. A. Hahn, B. D. Rothrock, AIP Handbook,

Dwight E. Gray ed. (McGraw Hill Book Co., 1972), pp. 4-119 to 4-142

and Figure 4.
11

Table 5. Piezo-optic Constants of Three Alkaline Earth Fluorides

X = 0.6328 -pm X = 1.15 ym

NBS

a T

• b Literature NBS

a

CaF

2

q

il

-0.38±0.03 -0.41 -0.4010.06

q l2

1.0810.03 1.04 1.0910.06

(‘ q ll' q12') -1.4610.01 -1.45 -1.4910.02

q44

0.7110.01 0.84 0.7210.01

SrF 9

q ll

-0.6410.04 -0.58 -0.6310.05

ql 2 1.4510.04 1.77 1.5010.06

(' q ll" q

l2'

) -2.0810.01 -2.35 -2.1310.04

q44

0.6010.01 0.59 0.6210.02

BaF

2

q ll

-0.9910.03 -0.62 -0.9110.07

q i2

2.0710.04 2.31 2.1310.07

(q ir q

i 2

} -3.0610.01 -2.93 -3.0310.02

q44 0.9510.01 1.06 0.9510.01

aThe errors were calculated from the standard deviations of the

experimental data.

^Reference (6) , the data for SrF 0 were calculated from the values

of p . and s.. given in reference (61.

ij ij
12

(,_Mg_0l) iP/U P

Mg.

1

dn/dT

of

CdF

as

a

function

of

temperature.
13

dn/dl

of

NaCl

as

a

function

of

temperature.
14

C\J

( v Ji

9 .oi) ip/ u p

CNJ

i

Fig.

3

dn/dT

of

MgF,,

as

a

function

of

temperature.

The

upper
15

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00
17

3. Acknowledgement

We thank Ronald Munro for his assistance with the least squares fit

of the MgF

? data.

S-l 1 4 A (REV. 7-73)

U.s. DEPT. OF COMM.

bibliographic data

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR—78—1473 (ARPA)

2. Gov’t Accession

No.

TITLE AND SUBTITLE

OPTICAL MATERIALS CHARACTERIZATION

3. Recipient’s Accession No.

5. Publication Date

6. Performing Organization Code

author(S) Albert Feldman, Deane Horowitz, Roy M. Waxier,.

and Marilyn J. Dodge

8. Performing Organ. Report No.

PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 5650442

11. Contract/Grant No.

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

)

Advanced Research Projects Agency

Arlington, Virginia 22209

13. Type of Report & Period

Covered

14. Sponsoring Agency Code

SUPPLEMENTARY NOTES

ABSTRACT (A 200-word or less (actual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

The piezo-optic constants of CaF?

, BaF ?, and SrF9 have been measured at

0.6328 pm and 1.15 pm. The temperature dependence of the refractive indices

of CdF^, MgF^, and NaCl have been measured at several wavelengths in the

infrared by the method of Fizeau interferometry. The linear thermal expansion

coefficients of NaCl and CdF ? as a function of temperature are also presented.

'• 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)

MgF

2

; NaCl

Thermal coefficient of refractive index; thermo-optic constant; linear thermal

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