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DATE: 11 j 1993

DEPARTMENT OF ELECTRICAL ENGINEERING

THE CATHOLIC UNIVERSITY OF AMERICA

FINAL REPORT

GRANT NUMBER - NSG-5111

PROPERTIES OF MATERIAL IN THE SOB MI LI... I METER WAVE REGION

(I n s t r i..i m e n1 a 11 o n a n d M e a s u r e in e n t o f I n d e x o f R e f r;;; c t i o (>

1

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P r i n c :i. P a 1 1 n v & s t :i. si a t o r" s

J * I... a I ]. y a i"i d R > M e :i.;;; 1 e r

ugust 1977 - August 1980
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PROPERTIES OF MATERIAL IN THE SUBMILLIMETER WAVE REGION

.The "Properties of Materials in the Submillimeter Wave

Region" study was initiated at Catholic University* Electrical

Engineering department to instrument a sytem and to make

measurements of the complex Index of Refraction in the

wavelength region between 0»1 to 1.0 millimeters* While

refractive index data is available for a number of solids and

liauids there stil exists a need for an additional systematic

study of dielectric properties to add to the existing data* to

consider the accuracy of the existing data* and to extend

measurements in this wavelength region for other selected

materials* The materials chosen for consideration would be

those with useful thermal* mechanical* and electrical

characteristics. The data is necessary for development of

optical components which* for example* include beamsplitters*

attenuators* lenses* grids* all useful for development of

instrumentation in this relatively unexploited portion of the

spectrum.

At the outset of this study a literature search was made to

determine the extent of the index data in the region between 3

mm and 50 microns. Attention was also given in this review to

the type of instrumentation and the precision. The more recent

method of index measurement in this wavelength region is

typically by use of spacial interference patterns familiar in
Page 3

optics* Both the Michelson and Fabret Perot Interferometer have

been popular choices. In gathering the information it was found

that until the middle 70's instruments for dielectric

measurements in. the millimeter wavelength region mostly used

methods based on the Prism or the Interferometer* including

Fourier transform spectroscopy (FTS) and later dispersive

fourier transform spectrocopy (DFTS). This instrumentation

covered a wavelength range from 50 to 2000 microns.

The FTS instrument system Generally includes a 'black body"

source and depending on the wavelength region a cooled detector

as the major components in a Michelson Interferometer. Even

though the energy available from black body source is weak in

the submillimeter regioni with the addition of the cooled

detectort precision measurements are possible. The important

feature of this FTS method is that it permits data collection

over a continuous wavelength rantfe. Analysis of the data is

accomplished in the following manner. The detected output of

the two path interferometer without sample is fourier

transformed to give a spectral distribution of the instrument.

This spe?ctral distribution to be studied is then modified by

introduction of the sample into one of the two paths. Both

transmission and reflection methods are used. The detected

signal from sample insertion contains modified spectral

contributions as the signal strength v"(x) at the detector . varies

with the variations of path length.. The transform from the

space to the freauenca domain* Stone (13)y is given baJ
Page 4

In order to determine CT) which is directly related to

the index of refraction* 3 relation between V(x)r the

voltage output* of the detector and F. (x) must be determined. o

This is complicated because of asymmetries of the instrument

between the functions v"<x> and F(x) with reference to zero path

difference* In such cases the extraction of EI (v) reauires o

extensive calculations* Generally the instrumentation is

computer controlled which allows for both automatic data

acauisition as well as analysis*

The recent availability of the laser source with its

relatively high power and monochromatic output in. the

submillimeter wave length region provides for the possibility of

precision measurement at discrete wavelengths. Since the signal

to noise ratio with this method for the longer wavelengthr is

high* this data can also be used to check the accuracy of the

broadband FTS data. Stone (13)? at UK National Physical

Laboratory NPL using a Mach-Zehnder interferometer configuration

with laser source were able to make index measurements on both

"weak and strong' absorbing materials. Depending on the type of

material either transmission or reflection methods are adopted

to measure both the real and imaginary parts of the dielectric

permativity. With the availability of a calibrated attenuater

(rotating grid type discussed later) loss measurements are

considerably facilitated.
Page 5

The Mach Zehnder interferometer is an amplitude splitting

two ( beam device containing two beamsplitters (see figure 1).

The two beams formed by the first splitter propagate along

separate paths and are finally recombined at the second beam

splitter after which a resulting "fringe" pattern may be

detected. For the index measurement a sample of either fixed or

variable thickness is placed in one leg of the interferometer

resulting in fringe shift* Measurement of this shift leads

directly to determination of the index.

{

In somewhat more detail the measurements are made as

follows! attenuation measurements are made on samples of

varying thickness (liouids or solids) using one leg of the Mach

Zehnder Interferometer. This is accomplished by placing the

sample in series with calibrated attenuator both of which are

placed in one arm of the interferometer. The detected signal is

maintained at constant level by adjustment of the attenuator as

the thickness of the sample is varied. Uith the attenuator in

series* a rotation (^ of the center grid of the attenuator

allows the sample output power versus sample thickness* ts> to be

constant. The attenuation ^>» in the sample is related by

jkrx (cofc. 6-^ - d JJ. 4. e.crxv^bwvut

The angle ^ represents rotation of the center grid of the

attenuator with respect to the reference grids.
Selection of References from Literature Study

1. 'Determination of the Refractive Index of a Solid Using a

Far Infra-red Maser"* J« E. Chamberlain and H« A,

Gebbie* Nature* Vol. 206, May 8* 1965* p. 602.

2. 'Power Transmittance of a 3 Mill. Mylar Sample'* E. E,

Russell and E, E. Bell* Infrared Physics? Vol. 6. 1966?

p. 82.

3. 'Panel Discussion on Submillimeter Wave Material Properties

and TechniQues" r J.J. Taub et.al.* Proceedings of the

Symposium on Submillimeter Uaves* New York 1970* P. 693.

4. "Subnii 11 imeter Wave Materials Properties and Techniaues!

Dielectric Constant* Loss Tahgerit* and Transmission

Coefficients of Some Common Materials to 2*000 GHz" A. P.

Sheppard et.al.* Proc. of the Symposium on Submillimeter

waves* 1970, p. 701.

5. "Submil1imeter Speetroscopy in the Liauid and Solid State

with a Tumable Optically Pumped Laser"* B. L. Beam and S.

Perkowits* Proceedings of the Symposium * 1977*

PP'. 158-9.

6. "Precise Dielectric Measurements of Low-Loss Materials at

Millimeter and Submillimeter Wavelengths"* G. W. Chantry

and M. N. Afsar. IEEE Transactions on MU) Theory and

Tech.* Vol. MTT-25* No. 6* June 1977* PP. 509-511.

7. "Dielectric Measurements in the Submillimeter Reaion and a

SuS3ested Interpretation of the Poley Absorbtion"* G. W.

Chantry. IEEE Trans. and MU Theory and Tech.* Vol.

MTT-25* No. 6* June 1977* PP. 496-500.

8. "Optical Properties of Sapphire in the Far Infrared"* E. V.

Loewensteinr Journal of Optical Society of Amer.* Vol. 51*

No. 1* Uan. 1961* PP. 108-112.

9. "Transmission of Germanium in the Far Infrared"* I. M.

Mills* Journal of Opt. Soc. Amer.* Vol. 45* No. 6* June

1955* kp. 489.

10. "The Reflection and Transmission of IR Materials? II

Bibliography"* D. E. McCarthy* APP!. Optics* Vol. 2 (6)

June 1963.

11. "A Quasi-Qptics Perturbation Technioue for Measuring

Dielectric Constants"* J. .E. DeSenford and P. D.

Coleman* Preceedin?j of the IEEE* Vol. 54(4)* April 1966* P.

520.
Page 8

12.

13.

14.

15.

16.

17.

18.

19,

20.

21,

•Dielectric Constants and Loss Tangent Measurements at 60

and 90 GHz Using the Fabry-Perot Interferometer'* C. A.

Balandis* The Microwave Journal* March 1971* p. 39.

'Electrical Standards of Measurement. Part 3!

Submi1limeter Wave Measurements and Standards'* N. Stone*

et.al.* IEEE Reviews* Volume 122* No. 10R, 1975* pg. 1054.

The Far Infrared Performance and Application of Free

Standing Grids Wound From Sum Siameter Tungsten Wire"* W.

G. Chambers* Infrared Physics* Vol. 19* 1979* PP.

437-442.

"A Mach-Zehnder Interferometer for the Measurement of the

Complex Permittivity of Lossy Dielectrics at Submillimeter

Wave "lengths' * J, E, Allnutt and J. A. Staniforth*

Journal of Physics E. Scientific Instruments* Vol. 4*

1971* P. 730.

'A Far Infrared Reflection Interferometer'* J, R. Bi rch*

Infrared Physics* Vol. 29.

'Self Calibrating Grid Attenuator for use at Submi 1 limeter

Wave Lengths'* J. Stanforth and J. Allnutt.* Proc. IEE*

Vol. 118* No. 2* 1971* PS. 343.

"High Precision Dielectric Measurements on Liquids and

Solids;' at Millimeter and Submil 1 imeter Wavelengths'* M.

Afsar* J. Chamberlain and G. Chontry* IEEE Trans. on

Instrumentation and Measurement* Vol. IM25* No. 4*

December 1976* pg. 290,

"Submil limeter Wave Dielectric Measurements on Absorbing

Matherials"* J. Chamberlain* et.al.* IEEE Transactions oh

Instrumentation and Measurements* Vol. IM-23* No, 4* Dec.

1974.

"Investigation of Solid Materials for Submi llimeter

Wavelengths'* V. N. Aeshechkin* et.al,* Institute for

Radio Engineering and Elect ronicsf Academy of Science of

USSR* Moscow. Translated from E'ribora i lekmka

No. 4* PP. 150* July-August 1971.

'A Far Infrared Reflection Interprometer* *

Infrared Physics* Vol. 12* 1972* pg. 29.

J. Birch*
Paae 9

22. The Luminosity of Spectrometers with Prisims Gratings or

Fabry-Perot Etlons"* P» Jacauinot* Journal of Optical

Society of America* Vol. 44* No» 10* 1954* P3. 761.

23» Interfererometry'* W. H» Steele* Cambridge University

Press* 1967.

2A\ 'Principles of Optics'* M» Born and E» Wolf* PerSamon

Press* 1975* Fifth Edition.

25. "The Far-Infrared Performance and Application of

Free-Standing Grids Wound From 5 Microns Diameter Tungsten

Wire'* C. Mok* W. Chambers* T. Parke and A. Coltley*

Infrared Physics* Vol. 19* PP. 437* 1979.

26. 'Optical Constants of Far Infrared Materials 21 Crystaline

Solids'* E. Loewenstein* D. Smith* R. Morgan* Applied

Optics* Vol. 12f # 2* 1973. •}

27. 'Measurement of the Refractive Indices of Dielectrics at

Millimeter Wavelengths'* G. Koslov* Translated from Pribora

i Tekhnika Eksperimenta* 4* pp 152-154* July/Aua 1971.
10

Listind of available real part - Table I and Loss Tangent

part - Table II of dielectric constants from the literature

study*

TABLE I - REAL PART OF THE DIEELECTRIC CONSTANTS

MATERIAL/FREG<

Fiberglass

Plexiglass

Polyethylene

Polyethylene*

Polystyrene

Ebonite

pyrex

Rexolite

Teflon

Del r in

Fused silica

Dynasil

Ti02 ceramic

a

230

GHZ

b

343

GHZ

2.54

2.06 2.09

3.88

88,36

c

250/450

GHZ

2

2

•?

,61

,31

.57

4

2

2

2

4

.34

.62

.27

.48

.46

1.97

c

850

GHZ

d

353

GHZ

c

IT 300

GHZ

c

2»000

GHZ

2.333 2.359 2.335 2.34

2.298

2.520 2.528 2.531

1.665

2.525 2.520 2.52

2.042 2.033 2.033 2.05

2,80 2.78 2.80

3.28 3.27 3.31

3.85

a - Meriakrir V.v". and Ushatkinr E,F,r "Measurement of the

Refractive Index at Submil1imeter Wavelengths"» Institute of

Radio Engineering and Electronics* Academy of Sciences of the

USSR? Moscowr translated from Pribory i Tekhnika Eksperimenta

No. If PP 143-145» March-April 1973.

b - Desdenf ord* J.E. and Colemanr P.D.» "A Quasi-Qptics

Perturbation Technioue for Measuring Dielectric Constants"r IEEE

Froc. 54» No. 4» April 1.966? PP 520-522.

c - Breedenr K.H.

Submil1imeter Wave

Journalr Nov. 1967»

and Sheppardr A,P.* "Millimeter and

Dielectric Measurements" IEEE Mic'rowave

pp 59-62.

d - Chidryalr E,E,r "Measurement of Refractive index of Solid

and Liouid Dielectrics at Submil1imeter Wavelength. Institute

of Radio enaineerin3 and Electronicsf Academy of Sciences of the

USSR* Moscow. Translated from Pribory i Tekhnika Eksperimentaf

No. 6f PP. 146-148* Nov.-Dec, 1974
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TABLE II - DATA* LOSS TANGENT (X1000)

MATERIAL/FREQ.

Fiberglass

Plexiglass

Polyethylene

Polystyrene

Ebonite

Pyrex

Rexolite

Teflon

Delrin

Fused Silica

Dynasl

400

GHZ

41

20

1.9

3

28

1

1

17

1,7

500

GHZ

41

20

1,7

20

2.7

600

GHZ

43

36

1.5

5

40

3

2

21

3.6

700

GHZ

1.4

23

5.1

800

GHZ

1.3

24

5.9

900

GHZ

1.3

24

6.2

1000

GHZ

1.3

7

5

2

6.4

Tio: ceramic

#Final Reportr "Millimeter and Submillimeter Wave Dielectric

Measurements with Interference Spectrometer"i A. McSweeney and

A. Sheppardf Project No, A-934? Georgia Institute of

Technology Engineering Experiment Station. Contract Number

991(13)» 14 April 1970,
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INSTRUMENTATION:

One of the main objectives of this grant was to develop

instrumentation for the measurement of complex dielectric

properties in the SMMW range. The plan agreed upon with

N.McAvoy (Technical officer for this contract) » for obtaining

the necessary equipment was that CUA would develop the

dielectric measurement instrument* purchasing the CO/2 pump

laser and miscellaneous eauipment thorough the contract and that

NASA would supply a SMMU laser Fig. 4a?b which was their

design. Mr. McAvoy would coordinate the SMMU laser effort

closely with CUA.

After reviewing the literature on dielectric measurements

in solids* we decided as Indicated in the preceding discussion

to use a Mach-Zender interferometer (Fig. 1 ) r Allnutt (15) with

modification by Stone (13) to measure the phase and loss

properties of the sample. The Phaser shifter? attenuator and

beam splitter were not commerically available components and

therefore had to be developed. The phase shifter (phase delay)

1 was constructed using pairs of mirrors mounted in a parallel "V*

arrangement on a translation stage with a differential

i

] micrometer drive capable of direct reading to 1/1 Or 000 inches.

The phase shifter was constructed and is shown in Fig, 2.

The calibrated attenuator was designed using a technique

described by» Stanforth (17)* which employes two fixed grids and

one rotating gridr Fig. 3. An initial design of the attenuator
1

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was assembled using 3 set of grids with 3 symmetries! 50 micron

period fabricated by J«Lsmb of NASA. The unit employs an ORIEL

rotating stage with micrometer drive which sllows for rotation

of the center grid. The 50 micron grid set wss the only one

available to us at the time and did not result in 3 workable

attenuator because of the large grid size. A final design of

the attenuator awaited fabrication of a desired 5 micron grid

spacing. J. Lamb was studing methods for producing grids of

this spscing for a number of other applications.

A SMMW laser was to be assembled at CUA. The major psrts

were supplied by NASA. Basically the laser w;;ts constructed

using a pyrex tube 1-1/2" in diameter with a metal input mirror

and a silicon output mirror forming the resonant cavity. The

input mirror is metal with gold plating having a 6.3 meter

radius of curvature? and a 4 millimeter off axis coupling hole.

The output mirror has a 6 meter radius with a reflecting coating

for 10.6 micron (C02)r and an anti reflecting <AR) coating for

the SMMW. The SMMW energy was coupled out of the cavity by

transmission through an on axis 4 mm circular aperture. The

assembly and alignment of the SMMW laser was closely coordinated

with N.McAvoy who participated in bringing this first laser to

the operational stage. Since initial study was to be in the

range of 100 micron wavelength Methyl Alcohol was selected for

the active medium. Methyl Alcohol has a strong SMMW line at 118

microns.
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When finally made operational the laser emitted a

sufficiently strong 118 micron signal. However* this ShMW

signal was auite unstable and very often would completely

disappear within one minute* After many adjustments to improve

the stability without substantial success* it was decided to

continue with another part of the instrumentation* i.e.*

assemble the Interferometer while continuing to look for a

solution to the stability problem. It would not be possible to

make measurements until the cause of the system instability was

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removed*

.

Subsequently* Mr. McAvoy recommended that a SMMW laser of

different design* which had recently been developed by his group

at NASA* be substituted for the original SMMW laser system.

This new system would perhaps provide improved stability. The

new SMMU laser is longer than the original* approximately 1.75

meters compared with .1 meter* provides more power and has an

improved cavity tuning adjustment. The tuning head of our

original laser was ouite difficult to adjust. The new laser was

subseauently assembled and put into operation but did not solve

the instability problem. It was then considered that the main

cause of instability may be in the CO/2 laser system. A

phase-lock system was obtained to provide additional

stabilisation to the CO/2 pump laser. Fig. 4a*b. are

photographs of the CO/2 laser and SMMW output tuning used. This

new eouipment was put into operation* however the stability of

Jj the SMMU) signal was not substantially improved although there

were some periods of several minutes when the SMMW signal
1'

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remained somewhat constant*

An attempt was made to measure the attenuation/length of

teflon at 118 micron without the precision attenuator since

arids were not available* The procedure used was to illuminate

sample rods of the same diameter of different lengths and

measure transmission loss* Assuming detector linearityr a plot

of the resulting attenuation versus sample length yields a curve

with a slope eaual to the attenuation constant of the material*

The dat^a obtained in this manner for teflon was not reliable*

due to laser instability* Measurements could not be made with

this system.

While the system could not be used for making index

measurements - rough studies on beamsplitters were also

attempted. It was desirable to have a beam splitters for the

Mach Zehnde interferometer that would divide the power

approximately eoually. Ule initiated a beamsplitter design using

grids which from the literature data appeared to have desirable

characteristics. NASA supplied us with two 50 micron grids and

one 10 micron grid on .002' Kapton for experimentation.

Transmission and reflection measurements were made on each grid

with the angle of incidence eaual to 45 degrees for both

perpendicular and parallel polarization conditions* The results

indicated that at 118 microns* the 50 micron grid splits the

power eoually regardless of polarization*
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In conclusion* it would be appropriate to review

chonologically the tnaJor problems encountered which lead to the

results obtained?

Contract was awarded September 1976*

Initial ordering of Optical Compontents - January 1977»

Initial ordering of components for SMMW Laser - March 1977.

CO/2 Laser arrived - June 1977,

Building components for Mach Zehender Interferometer

started and SMMW System in process of Assembly -

June 1978.

SMMW Laser assembled and aligned. Start UP using

1 methyl alcohol - no signal - March 1979.

Continued effort* and obtained first operative results

but with severe stability problem. Attempts at

measurements - June 1979.

Substitution of the 'new* SMMW Laser from NASA

Arrived and assembled - no improvement in stability

- June 1980,

Continuing attempts to find source of instability lead

to another review of entire system. Temperature

stability appeared to be a major problem - Aug. 1980,

Reauest for renewal without additional funding was approved

for the year starting August 1980 and the SMMW effort was

continued* Assembly of a temperature control system somewhat

improved the stability problem. It was decided that temperature

controlled refrigeration system would substantially improve the

stability and allow measurements. The contract terminated

August 31» 1980 and the effort continued without funding.

1
*..-.> \, I I •* /• f _.L_»n-_L \- n_ __.-. .-

Figure 1 - Mach-Zender Interferometer
T

1

1

1

1

1

I

1

1

1

1

1

T

1

1

1

1

Figure 3 - Variable Attenuator

Figure 2 - Phase Shifter
T

1

1

T3

1!

H

13

H

U

1

1

T!

1

H

1]

1

1

Figure 4a - CO Laser System

Figure 4b - SMMW Output Window and Tuning Head