excitation of surface electromagnetic waves on water

Excitation of surface electromagnetic waves on water

A. K. Singh, C. A. Goben, M. Davarpanah, and J. L. Boone

Excitation of surface electromagnetic waves (SEW) on water was studied using optical coupling techniques

at microwave frequencies. Excitation of SEW was also achieved using direct horn antenna coupling. The

transmitted SEW power was increased by adding acid and salt to water. The horn antenna gave the maxi-

mum excitation efficiency 70%. It was increased to 75% by collimating the electromagnetic beam in the ver-

tical direction. Excitation efficiency for the prism (00 pitch angle) and grating couplers were 15.2% and

10.5% respectively. By changing the prism coupler pitch angle to +36°, its excitation efficiency was in-

creased to 82%.

1. Introduction

A surface electromagnetic wave (SEW) is a wavethat propagates along an interface between two differ-ent media without radiation 1 with exponentiallydecaying evanescent fields on both sides of the interface.This paper reports primarily on the investigation of thepossibility of using some optical and microwave SEWcoupling techniques for coupling microwave SEW ontoa water surface. Although coupling was achieved withthe grating, the prism,1 8' 4 and direct horn antennacoupling' 3' 1 4 techniques, the coupling efficiency wasconsiderably lower than those reported for SEW onaluminum strips.' 4

These experiments of SEW on water were conductedat 8.1 GHz and 9.0 GHz. By using such a high fre-quency of operation an available water tank 9.8 X 1.04m2 could be used as a model for the open sea. Anotheradvantage is the use of antennas and couplers which canbe handled easily.

It can be shown that the attenuation constant alongthe air-water interface is given by'

2[,~\ ( 1)+22"2Kwhere V is free space velocity of signal, fo and El are

The authors are with University of Missouri-Rolla, Department of

Electrical Engineering and Engineering Research Laboratory, Rolla,

Missouri 65401.Received 13 October 1977.0003-6935/78/1101-3459$0.50/0.© 1978 Optical Society of America.

permittivities of free space and water, al is the con-ductivity of water, Atl is the permeability of water, andco is the SEW angular frequency in rad/sec. Thisfunction yields a decreasing attenuation with increasingconductivity, which decreases more slowly at higherconductivities. The value of El for both fresh water andsea water is approximately 79eO in the x -band.

The theoretical attenuation constant at 9 GHz is 1.75dB/m. The value obtained experimentally was 1.5dB/m. The theoretical attenuation constant at apractical operating frequency of 9 MHz on the oceanwould be 0.0026 dB/m.

Experiments were conducted to excite SEW on water.The excitation was effected by changing various couplerparameters as mentioned below, the most notablechange concerning the height of the coupler above thewater surface.

Excitation efficiency is defined as the fraction ofpower coupled as a SEW to the total power fed to thetransmitting antenna.

II. Experiments

The experiments were conducted in a tank 9.80 X 1.04m 2 . The water in the tank was 0.50 m deep as shown inFig. 1. In each case the launcher and receiver wereseparated by absorbing materials to prevent any strayradiation from the source reaching the receiver. A highfrequency of operation (8.1 GHz and 9 GHz) was chosenso that the experiment could be conducted in the 9.80-mlong tank.

A. Acid and Salt Experiments

The acid and salt experiments were carried out at 8.1GHz. The launcher was a horn antenna pointed downat 5° relative to the water surface. This value is close

1 November 1978 / Vol. 17, No. 21 / APPLIED OPTICS 3459

GAP HEIGHT

SEW COUPLERITRANSMITTERI

.19

SHIELD

SEW

-_ V I+ r

SHIELD

B

MATCHEDTERMINATION

GAP HEIGHT

SEW COUPLER(RECEIVER) l

9

Fig. 1. The general experiment setup used for each experiment.Two shields were used to minimize direct coupling. A termination

was used to minimize reflection from the end of the tank.

60 r

50

I-- 402

30

2X

W3al 20

3W

a 10W

a:

0.0 % SALT CONCENTRATION

I I I I I I I0 1 2 3 4 5 6 7

HYDROCHLORIC ACID CONCENTRATION ( XIO3 %I

Fig. 2. Acid concentration in water vsSEW.

power transmitted by

to the Brewster's angle' value of 6.39° (see Sec. II.E).The reflection coefficient is at a minimum when anelectromagnetic wave traverses from one medium toanother, if it is incident at the Brewster's angle. Theheight of the launching horn antenna was adjusted,while maintaining the 50 orientation, until maximumcoupling was achieved. The receiver consisted of analuminum strip, immersed in the water at 10° to thehorizontal, and a horn antenna placed on this strip 30cm from the water-aluminum interface. The region ofthe aluminum strip from the water to the horn antennawas coated with a dielectric (3.2 mm thick, relativepermittivity 2.5) for improved coupling. Hydrochloricacid (36-38% solution) was added to the water until a66.23 X 10-4% solution was obtained. The receivedpower was read as the concentration of the acid waschanged. Salt was then added in increments of 50 lb(22.5 kg) until a total of 350 lb (157.5 kg) had beenadded. The final salt concentration in the water was2.8% by weight, which is approximately equal to seawater salt concentration. The received power was readas the salt concentration was varied. Figures 2 and 3show the resulting curves. As the fresh water aciditychanged from approximately zero to 66.23 X 10-4% So-lution, the received SEW power increased by 30%.When the salinity of the fresh water was increased fromapproximately zero to 2.8% by weight, the received SEWpower increased by 719%. The final salt concentrationwas approximately equal to the sea water salt concen-tration. Conductivity measurements were taken sep-

0.

- 0.

E 0.

9

8-

6-

5

4-

3

2

0.

0.

0.

0.

0.

400

I-

. 300

. 200

3W

i 10 0

. 1 2 3 4 5 6 7

ACIDITY I x I0-2 % )

Fig. 4. Conductivity of water-acid solution vs acid concentrationin water.

6

I-

WATER CONTAINS 6.63 X I03 % HCI

0 0.5 1.0 1.5 2.0 2.5 3.0SALT CONCENTRATION IN WATER BY WEIGHT II

Fig. 3. Salt concentration in water vs power transmitted by SEW.

6-

4

2

0 0 0.75 1.50 2.25 3.00

SALINITY 1% BY WEIGHT )

Fig. 5. Conductivity of water-salt solution vs salt concentration byweight.

3460 APPLIED OPTICS / Vol. 17, No. 21 / 1 November 1978

0.5,,

J0.TIm

t-l ' - - - I- He

< -I 9.80m r

>

SHIELDMATCHED

SHIELD TERMINATION

Fig. 6. Experimental arrangement for the measurement of excitationefficiency of the grating coupler.

12

10

08

06

24

0 I 2 3 4 5 6 7

COUPLER HEIGHT / WAVELENGTH

Fig. 7. Grating coupler excitation efficiency vs (gap height)/

(wavelength). The grating structure consisted of 10 bars of 1.27 cmdiam spaced 5.5 cm apart.

arately adding salt and acid to the new samples of freshwater. The plots are shown in Figs. 4 and 5. When acidis added to fresh water, its conductivity rises rapidly,but as the concentration increases the conductivity risesmore slowly, reaching a final value of 0.83 mho/m. Thenonlinearity is probably due to some of the acid reactingwith the chemicals in the water. By adding salt, theconductivity of the water increases linearly to a finalvalue of 6.7 mho/m. It was necessary to take thesemeasurements because only the conductivity of thewater is affected by adding acid or salt, which in turnaffects the SEW propagation on it, and the experimentsgave a quantitative value of the change.

B. Grating Coupling Technique Experiments

The experimental arrangement is shown in Fig. 6.The transmitting parabolic dish antenna has a diameterof 45.72 cm. It is linearly polarized with a 3-dB beam-width of 6°. The grating structure consists of 10 hollowaluminum bars 1.27 cm in diameter placed 5.5 cm apart.The angle of illumination of the microwave is 19.5°,which corresponds to 01, where 14

sinOm = 1 - [mX/d], m = 0, 1, 2,.... (2)

and 0m is the angle of incidence for a mode of order m,

d is the grating spacing, and X is the free space Wave-length (3.3 cm). As the electromagnetic beam passesthrough the grating coupler it obtains an additionalphase modulation. The center of the middle bar of thegrating was 1 m from the dish antenna; thus the gratingwas in the near field of the antenna. Excitation effi-ciency was calculated as a function of the height of thegrating bars above the water surface. The entire elec-tromagnetic radiation from the dish antenna was usedto illuminate the grating structure so that all the powerthat is fed to the transmitting dish antenna is availablefor coupling. For this reason the grating coupler exci-tation efficiency is defined as the ratio of the powercoupled to the SEW to the power fed to the transmittingdish antenna. The plot of excitation efficiency vscoupler height is shown in Fig. 7. Peaks are obtaineddue to standing waves that are sustained between thegrating structure and the water surface. The peaksoccur at an average interval of 0.55 wavelengths. If theheight of the coupler at the peak points is given by Hp,it can be expressed as

H, = (0.2 + 0.55n)X, n = 0, 1, 2,.... (3)

where n is the number of the peak, and X is the freespace wavelength. The peaks continue to increase until

-22,

E -32

W -42

W: -52

FREQUENCY ( GHZ)

Fig. 8. Transmitted SEW power vs swept frequency for a gratingcoupler height of 13.3 cm.

-22 GRATING COUPLER GAP HEIGHT 170cm

E-32

0:

w -52

w, -62

8.2 10.3 124

FREQUENCY (GHz)

Fig. 9. Transmitted SEW power vs swept frequency for a gratingcoupler height of 17 cm.


a

HORN ANTENNA

a5.7cm -

\G6cm

16.4 cm | XJ .S W

/ SURFACE OFTHE WATER GAP HEIGHT

Fig. 10. The horn antenna is taped at the back of the prism so thatthe center line of the horn antenna intercepts one wavelength backfrom the right angled corner of the prism. Most of the electromag-netic wave that comes from the horn antenna then hits the bottom

of the prism at the critical angle.

a maximum is reached at 4.5 and again at 5.1 wave-lengths. The maximum excitation efficiency obtainedfor the grating coupler was 10.6%.

Figures 8 and 9 show the SEW power vs frequencydiagrams for two different coupler heights. When thegap height was 13.3 cm, maximum coupling wasachieved at 8.72 GHz, 9.58 GHz, and 10.71 GHz. Whenthe gap height was changed to 17 cm, maximum cou-pling was achieved at 8.60 GHz, 9.33 GHz, and 10.60GHz. It is seen that the frequency at which maximumcoupling occurs has shifted by changing the gap heightproving that the gap height for maximum coupling isfrequency dependent. Both these plots were obtainedat a fixed value of Am equal to 19.5°. This value was notthe optimum value as the frequency was swept. Theeffect of this is to lower the coupling and can be elimi-nated when the two figures are compared.

C. Excitation Efficiency vs Prism Coupler HeightThe setup for this experiment was similar to that for

the grating coupler except that the dish antenna gratingsystem was replaced by a prism-horn antenna sys-tem8 13" 4 (Fig. 10). The prism was so shaped that ahorn antenna taped to it would result in most of theelectromagnetic radiation hitting the base of the prismat the critical angle.8 ,13, 4 The importance of the criticalangle in the microwave frequency range is explainedbelow.

At the boundary of a dielectric-air medium with indexof refraction np > 1, electromagnetic energy can be to-tally reflected. When conditions for total reflection aresatisfied, there exists an evanescent wave propagatingalong the surface and decaying exponentially on bothsides away from the surface. 6 The wave vector k ofthis evanescent wave is increased compared to the freespace wave vector according to the equation6

k = np(w/c) sina, (4)

where k, is the velocity of light in vacuum, and a is theangle of incidence at the base of the prism of the elec-tromagnetic beam. In the microwave frequency range' 3

ks w/c. Therefore,

np = 1/(sina) (5)

in the microwave region. This means that the elec-tromagnetic beam hitting the bottom of the prism mustbe at the critical angle in order that a surface electro-magnetic wave be excited. When this condition issatisfied, the evanescent field in the air gap between theprism and the water surface travels with the same phasevelocity as the SEW mode which is excited." Theevanescent field of the prism is exponential in natureextending downward below the prism. The SEW modehas an exponential tail extending upward above thewater surface. These two fields that overlap are com-mon to the prism and the water surface and constitutethe coupling between them.' 2 Davarpanah8,13,14 hasshown that the center of the horn antenna should point1 wavelength away from the edge of the prism formaximum coupling efficiency. The prism is made ofsoft polystyrene whose dielectric constant is 2.25. Themouth of the horn antenna was 5.5 X 7.5 cm2 and couldbe easily taped at the back of the prism, which is 10.15cm wide.

The first part of the experiment with the prism cou-pler consisted of measurements of excitation efficienciesas a function of the height of the prism coupler abovethe water surface. The prism base was kept parallel tothe water surface, and the height was measured from theprism base to the water surface. The result is plottedin Fig. 11. A maximum excitation efficiency of 15.2%was achieved for a prism coupler height of 4.5 wave-lengths. The coupling is negligible for coupler heightsbelow 3 wavelengths and above 6 wavelengths.

HORN ANTENNA

PRISM COUPLER

VjSEW-LSURFACE OF THE WATER

16

14

go 12

z 10

0

I-

R4-

2-

0 1 2 3 4 5 6 7GAP HEIGHT/ WAVELENGTH

Fig. 11. Prism coupler excitation efficiency vs (gap height)/(wave-length). Maximum efficiency achieved was 15.2%.


-60° -50° -40 -30°PRISM COUPLER PITCH ANGLE (DEGREES)

Fig. 12. Excitation efficiency of the prism coupler vs coupler pitchangle. Maximum efficiency achieved was 82% at a gap height of 4.5

wavelengths.

D. Excitation Efficiency vs Pitch Angle of PrismCoupler

The second part of the experiment with the prismcoupler consisted of excitation efficiencies as a functionof the pitch angle of the prism coupler. As pointed outby Ulrich,15 excitation efficiency can be increased byusing a nonuniform gap between the surface and theprism, if the incident beam fills the region between therectangular corner of the prism and the point where thenonuniformity starts.

The pitch angle was varied from +50° to -50°. Asignificant variation in the prism coupler excitationefficiency was observed by changing the pitch angle ofthe prism coupler. A maximum efficiency of 82% wasachieved for a pitch angle of 350 (as compared to anefficiency of 15.2% with 00 pitch angle) as shown in Fig.12. The pitch angle was varied by fixing the prismcoupler at a height of 4.5 wavelengths above the watersurface. This was found to be the optimum height forthe prism coupler (Sec. II.C). This excitation efficiencyis significant in that it represents the maximum cou-pling (82%) which could be achieved as compared withany other technique.

E. Excitation Efficiency vs Horn Antenna CouplerHeight

When a SEW is excited directly using a horn antenna,the reflection from the water surface must be minimizedso that more power is available to couple as a SEW.There will be no reflection when the electromagneticwaves from the horn antenna are incident on the watersurface at Brewster's angle for which an equation isderived below.

For a vertically polarized wave through air and inci-dent at any angle 4 on the horizontal flat surface ofwater, which is a homogeneous medium /,i, el, and ao,where El is the permittivity, 4,1 is the permeability, anda1 is the conductivity of water, EO is the permittivity offree space, the reflection coefficient p is given by'

(El/EO - jai/lwo) cos4 - [(El/fo - jail/wEo) - sin2i]1/2 (6)( -l/ O- jai/Wo) COSO + [(El/O - al/wE1) - sin2 ]'/ 2

The reflection coefficient is zero when

tan4 = [(i - j ) ]

Maximum coupling was achieved with the horn antennaby pointing it on the water surface at an angle of 5° withrespect to the horizontal. The real part of the theo-retical value calculated for was 6.39° (for = 6.7mho/m, El = 79, w = 2ir X 9 X 109 rad/sec).

A Zenneck wave consists of an inhomogeneous planewave outside the surface with the wavefront tilted for-ward to provide for flow of power into a lossy mediumbelow the surface.1 An ideal launcher must be infinitein extent in one dimension for only then would theZenneck wave be a complete solution to Maxwell'sequations. The X-band launcher is not ideal andcannot, therefore, give a pure surface wave. For thedirect horn coupler two experiments were conducted.

In the first experiment, an x -band horn antenna wasplaced with one edge resting on the water surface.Maximum coupling was achieved by tilting the hornantenna down 50 with respect to the horizontal. Whilemaintaining this 5° inclination the height of the hornantenna above the water surface was varied, and thetransmitted power was read. The plot is shown in Fig.13. It is observed that maximum coupling is achievedat coupler heights of 0 and 2 wavelengths. The maxi-mum excitation efficiency achieved was 70% for boththese gap heights.

F. Excitation Efficiency after Collimating the Beam

It was speculated that the excitation efficiency couldbe increased by collimating the microwave beam in the

HORN ANTENNA

R,~ _ AP HEIGHT

O4

' SURFACE OF THE WATER

80

70

1 2GAP HEIGHT/ WAVELENGTH

Fig. 13. Direct horn antenna coupler excitation efficiency vs (gapheight)/(wavelength). A maximum efficiency of 70% was achieved.


1ILI

"II

0

19

"III-�iI

ZY

I

XI

70

s

60

I.0

0 50

X

40g

30

u-

< 20Z

Z

. 10

"I

Cot

10.3

HORN ANTENNA COLLIMATED BEAM

LENS

0 2 4 6 8GAP HEIGHT/ WAVELENGTH

10 12

Fig. 14. Direct horn antenna coupler excitation efficiency vs (gapheight)/(wavelength) with electromagnetic beam collimated in verticaldirection. Maximum excitation efficiency of 75% was achieved, which

is 5% higher than that obtained without collimating the beam.

vertical direction. Without collimating the microwavebeam most of the radiation from the horn antenna (withan 180 beamwidth) was pointed away from the sur-face.

In the second experiment, a lens arrangement con-sisting of five separate pieces of Plexiglass was placedin front of the horn antenna so that the horn antennaapex was at the focal point of the lens. The lens wascylindrical, and only vertical collimation was attempted.Having collimated the beam (beamwidth was reducedfrom 180 to 9) once again maximum coupling wasachieved by pointing the horn antenna toward the watersurface at 5 to the horizontal. The coupler then con-sisted of the horn antenna-lens system. By varying theheight of the coupler a maximum excitation efficiencyof 75% was achieved with a coupler height of 6.25wavelengths. An increase of 5% was thus achieved bycollimating the beam vertically. The plot is shown inFig. 14.

Ill. Discussions

It is observed that excitation efficiency increases withthe addition of acid and salt to the water. Equation (1)gives the attenuation constant of the SEW at highfrequencies (x -band) and shows it is proportional toconductivity. At low frequencies (MHz range) it isinversely proportional to conductivity (see Fig. 1).

For the grating coupling technique, the number ofgrating bars that can be accommodated depends on thesize of the dish antenna, which in turn determines the

size of the coupling aperture. The grating bars wereadjusted so that the end bars did not extend beyond thebeam from the dish antenna. Otherwise, the farthestbars would decouple part of the SEW energy.13

The direct horn antenna coupler gave a maximumexcitation efficiency of 70%. It was speculated that alarger excitation efficiency could be achieved by colli-mating the microwave beam in the vertical direction sothat more of the electromagnetic energy from the hornantenna would be available for coupling. (The antennaused had a beamwidth of 180.) After collimating thebeam in the vertical direction an additional 5% increasein the excitation efficiency was possible. By collimatingthe beam, the peak coupling was achieved for a gapheight of 6.25 wavelengths as compared to 0.0 and 2.0wavelengths without collimating the beam. Anothernotable difference in Figs. 13 and 14 is that of the fullwidth at half maxima increased from a mere 0.76wavelength to 10.3 wavelengths by collimating thebeam. The possible reason for this is the apparent in-crease in the aperture size of the coupler.

When the acid and salt experiments were performed,the power received by the receiving antenna was in-creased by coating the metal with dielectric. The phasevelocity of the SEW decreased from 3 X 108 m/sec to2.915 X 108 m/sec by coating the metal with dielectric.The phase velocity of the SEW on water was 3.02 X 108m/sec. Also, the reactance of the water was 3.7 w, of themetal 4.433 X 10-2 w, and of the metal coated with di-electric 90.95 . The increased reactance concentratesthe SEW power near the interface, and consequentlymore of the SEW power can be captured by the hornantenna.

The fact that different couplers gave different exci-tation efficiencies at different coupling heights showsthat coupling is a function of the properties of not onlythe surface on which SEW is coupled, but also of thecouplers.

IV. Conclusions

Coupling of electromagnetic energy to water in theform of a surface electromagnetic wave was achieved foreach of the three techniques of grating, prism, and directhorn antenna coupling. Thus optical excitation tech-niques of prism and grating coupling are successful inthe microwave frequency range for experiments onwater-air interfaces as they were for metallic-air in-terfaces. 8,13 ,14

The largest excitation efficiency (82%) was achievedby using the prism coupler and changing its pitch angleto +360 at the optimum coupling height, which was 4.5wavelengths. When the pitch angle was 0, the cou-pling was only 15.2%, which means coupling was in-creased by a little over five times just by changing thepitch angle of the prism coupler.

The grating coupler gave the smallest excitation ef-ficiency of 10.6% for optimum coupling heights of 4.7and 5.3 wavelengths.

An excitation efficiency of 70% was obtained for thedirect horn antenna coupling technique and was


. . . _ , ,

achieved with optimum height of coupling of 0.0 and 2.0wavelengths. This was increased to 75% by collimatingthe electromagnetic beam from the horn antenna in thevertical direction. The optimum height of coupling was6.25 wavelengths.

It is seen that the optimum height of coupling isseveral wavelengths in all cases. This appears to be inaccordance with McMullen's16 observation in the nearir and Davarpanah's8,1 3 14 observation in the microwaveregion that the height of the prism coupler above ametal surface could be surprisingly large (severalwavelengths). At frequencies below 100 MHz, thecouplers may have to be replaced beyond heights of 20m from the water surface and that may be impracti-cal.

The effect of addition of acid and salt to the freshwater was to change the excitation efficiency and theattenuation constant. The received power increasedby a little over six times by addition of salt to the water.The final concentration of salt was 2.8% by weight, avalue close to the sea water salt concentration. Thisconcentration of salt was selected because most of thepossible applications of SEW on water are foreseen asapplications on the ocean.

The effect of the hydrochloric acid was to increase thereceived power by about 30%. The final acidity of thewater, however, was very small, 66.23 X 10-4/ solu-tion.

These experiments demonstrated that SEW can beexcited on water by using the known methods of SEWexcitation on metals.' 4 After successfully exciting SEWon water, various possible applications have been in-vestigated.'7

We thank T. Van Doren, R. J. Bell, and A. J. Penicofor discussion of this work. We also thank M. Nour-eddine and A. Esker for their good work which made theexperiments possible.

References1. H. E. M. Barlow and J. Brown, Radio Surface Waves (Oxford U.

P., New York, 1962).2. D. Beaglehold, Phys. Rev. Lett. 22, 708 (1969).3. W. E. Anderson, R. W. Alexander, Jr., and R. J. Bell, Phys. Rev.

Lett. 27, 1057 (1971).4. N. Marschall, B. Fisher, and H. J. Queisser, Phys. Rev. Lett. 27,

95 (1971).5. R. H. Ritchie, Surf. Sci. 34,1 (1973).6. B. Fisher, N. Marschall, and H. J. Queisser, Surf. Sci. 34, 50

(1973).7. L. F. Teng, R. W. Alexander, Jr., R. J. Bell, and B. Fisher, Phys.

Status Solidi B 68, 513 (1975).8. M. Davarpanah, C. A. Goben, and R. J. Bell, Wave Electron. 3,

19 (1977).9. C. A. Angulo and W. S. C. Chang, IRE Trans. Antennas Propag.

AP-7, 359 (1959).10. A. F. Harvey, IRE Trans. Microwave Theory Tech. MTT-8, 30

(1960).11. P. K. Tien, R. Ulrich, and R. J. Martin, Appl. Phys. Lett. 14,291

(1969).12. P. K. Tien, Appl. Opt. 10, 2395 (1971).13. M. Davarpanah, "Excitation of Surface Electromagnetic Waves

at Microwave Frequencies Using Optical Techniques," Ph.D.Dissertation, U. Missouri-Rolla Library, Rolla (1975).

14. M. Davarpanah, C. A. Goben, D. L. Begley, and S. L. Griffith,Appl. Opt. 12, 3066 (1976).

15. R. Ulrich, J. Opt. Soc. Am. 61, 1467 (1971).16. J. D. McMullen, Solid State Commun. 17, 331 (1975).17. A. K. Singh, C. A. Goben, M. Davarpanah, and J. L. Boone, to be

published.

C-4-78 OPTICAL COMPUTING-A NATIONAL CONFERENCE AT VISEGRAD,HUNGARY by V.N. Smiley

The International Conference on Optical Computing in Re-search and Development was held in Visegrad, Hungary 4-9October 1977. The fact that this field is in an earlystate of evolution was brought out by the speculative andpreliminary nature of many papers. Hybrid systems com-bining digital or analog electronics and optical deviceswere emphasized in several papers and in a roundtable dis-cussion. A factor slowing the development of such systemsis that people from different disciplines are required tointegrate their ideas. The main subjects discussed in thereport are: laser graphic devices, holograms, hybrid imageprocessing, and biological applications. In addition, somecritical discussion of the general field of optical computingas well as some specific areas is included.

Copies of this ONRL Report may still be available from DefenseDocumentation Information Service, 5285 Port Royal Road, Springfield,Va. 22161.


excitation of surface electromagnetic waves on water

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