Acoustooptic Devices and Applications

8/3/2019 Acoustooptic Devices and Applications

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2 IEEE TRANSACTIONS ON SONICS AND ULTRASONICS,OL. su-23, NO . 1, JANUARY 1976

1. Acoustooptic Devices and Applications

Abstruct-The theory, technology, and applications of bulk-waveacoustooptic devices are reviewed. A coupled wave analysis for theacoustooptic interact ion in an anisotropic medium is presented. Thebandwidth and angular aperture characteristics of acoustoopticdevices are discussed in terms of simple phase mismatch considera-tions. The present status of acoustooptic materials and transducertechnology s summarized. The characteristics and applications ofacoustooptic devices, including deflectors, modulators, and opticalfilters, are discussed.

I. INTRODUCTION

WHEN AN acoustic wave propagat'es in a transparentmaterial, t produces a periodic modulation of theindthx o f refract,ioE via the elastooptical effect. This pro-vides a moving phasc grating which may diffract portionsof an incident light beam into ne or more directions. Thisphcnomenon, known as acoustooptic diffraction, has ledto various optical devices, includingeflectors, modulators,filters, etc .

The basic theoryof acoustooptic interaction in isot'ropicmedia was well understood before the advent of t he laser.Discussion of early theoretical work can be found in Bornand Wolf [l] and in a review paper by Quate et al. [ 2 ] .I t was laserdevelopment that stimulated extensive re-search on t,hedeviceapplications of acoustooptic nter-actions.Theoryand echniques of acoustooptic deviceswere developed for the purpose of modulat ing and deflect-ing laserbeams. By 1967 several mportant esults onacoustooptic devices had been reported.These ncludethe works of Gordon [ 3 ] on the efficiency and bandwidthof acoustooptic deflectorsand modulators, by Iiorpel t d.[4] onacousticbeam teering,andby Dixon [ G ] onacoustooptic interaction in anisotropic media. Since thenthere has been a rapid progress of acoustooptic devices,due primarily o the development of superior acoustoopticmaterialsand efficient broadband ransducers. Variousacoustooptic devices have evolved with many applicationsin diverse fields.

In thispaper we shall review the principles and practiceof bulk wave acoustooptic devices. In th e ection to follow,we shall present a coupled wave analysis of the acousto-optic nteraction n an anisot>ropicmedium and discussthebandwidthandangularaperturecharacteristics ofacoustooptic devices, using simple phase mismatch con-siderations. In Section 111, the present status of acousto-

Manuscript received July 18, 1975.The author is wit.h t,he Applied Technology Division, ItekCorpor-ation, Sunnyvale, CA 94086.

X

z = o Z = LFig. 1. Geometry of acoustooptic nteraction.

optic materials is summarized. Section IV is a review otransducer technology. In Section V we describe, in somdetail, hree basicdevices: deflectors, modulators, an dfilters. Several areasof application are eviewed in SectionVI, and we briefly discuss the future outlook for acousto-optic devices in Section VII .11. THEORY OF ACOUSTOOPTIC INTERACTIONSCoupled Wave Equations

The basicgeometry of an acoustooptic interaction sdefined in Fig. 1 . Th e z-axis is chosen perpendicular to thboundary of the medium. An acoustic wave is propagatinin thezz plane and makin g an ngle ea from the z-axis. Anoptical beam is incident in the same plane with an angleeo from the z-axis.' Under certain conditions, diffractionof the incident optical beam into one or more diffractionorders may occur.

Acoustooptic diffraction can be viewed as a parametriinteraction [6]. Via the elastoopticeffect, the inciden

Angles are measured inside the medium.


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CHANQ: ACOUSMOPTIC INTERACTIONS, I 3optical wave mixes with the acoustic wave to generate anumber of polarizationwaves at he combination f r equencies. The polarizationwaves in urn will generateoptical radiation a t these new frequencies. Let the angularfrequency and wave vector of the incident optical wavebe denoted by WO and ko, respectively, and those of theacoustic wave by W, and k,. Th e polarization waves thenconsist of waves with angular frequencies W , = WO +muand wave vectors K,,, = ko + mk,, here m = f l , f 2 , - S .Thus the tota l electric field of the incident and diffractedoptical waves can be expanded in plane wavesas

E ( r , t ) = 3 C & . E , ( z j expj(w,t - KR1-y) C.C. ( 1 )where gm s a unit vector in the directionf the electric fieldof the mth wave and C.C.s the complex conjugate.

We choose a phenomenologicalapproachandassumeth at th e acoustically-induced polarization is proportionalto the product of electric field and the acoustic strain,

OD

--m

P ( r , t ) = c & . S ( r , t ) E ( r , t ) ( 2 )where X is a nonlinear susceptability ensor describing theelastooptic effect. S ( r , t j is the strain f the acoustic wave.We further assume that thecoustic strain takes the simpleplane wave form,

S ( r , t ) = ${RSexpj(o,t - k , - r ) + c.c.] (3)where 5 is a unit strain tensor or the acoustic wave and Sis assumed to be independent of r . Substituting (1)-(3)into Maxwells equat ion nd neglecting second orderterms, we obtain a set of differential equations, [ 7 ]

where X , = e ,. - an d cm = cos BC + wz( , / k a ) cos e,Ak , is approximately given by [ 7 ]Ak , = Km2 - wo / c )2k&,

+ 2772- os (e , + &) + m2-ak0

where n, is t,he indexf refraction for the mth wave. Physi-cally Ak , is the magnitude of the momentum mismatchbetween the polarizationwave and he freewaves ofmedium, i.e.,

Ak,,, = Km- k,,,= ka + mk, - k,,, (6)wherek, s the wavevector f the free waveof th e medium.The magnitude of km is equal to (wm/c)n,,,.

Equation (4) s the coupled wave equation describingthe i nteraction of optical and acoustic waves in an aniso-tropic medium, which when solved gives th e electric fieldof the optical waves in var ious diffraction orders. For anisotropic medium, it reduces to the familiar Raman-Nathequations [l].

Later we shall seek the solution of the coupled wave

equations (4) norder odete rmine he efficiency andbandwidth of acoustooptic devices. We must first relatethe acoustooptic tensor X n our formulation to the morefamiliar elastooptic coefficients. Traditionally, the subjectof the elastooptic effect is based on Pockels phenomeno-logical theory [S] which sta tes ha t he change of theinverse dielectric tensor A B i , caused by the acoustic waveis proportional to th e acoustic strain Skl

ABij = C PijklSkl (71where the p i j k l are the elmtoopticalcoefficients. From ( 2 )and (7 ) it can be shown

Xijkl = -ni*n?pijkl. (8)The elastooptical coefficientsP i j k l are generally assumed

to be symmetrical withespect to indices ij an d kl and canbe contracted o prs(r,s= 1 , . - 6 ) . Nelsonand Lax [S]have shown this is, in general, not necessarily true. Theypointed out th at th e Pockels formulation is ncompleteand that rotat ion ffects, arising from shear waves, shouldbe also included in strongly birefringent media. In placeof the acoustic strain,onemust use thedisplacementgradient ft s the more basic variables.

Another modification of the theory of e lastooptic effectis to include an indirect contribution due to electroopticeffect in a piezoelectric crystal [IO]. In most materials,th e piezoelectric effect is small, and the indi rect contribu-tionmaybe neglected. In LiNbOs,however, herearesignificant contributions to the effectivelastooptic coeffi-cients [lo]. In view of these modifications, one must bevery careful when he applies th e conventional formulationin the study of acoustooptics.Isotropic Diffruction

We shall refer to the coustooptic diffraction as isotropicwhen either the mediums isotropicor the process does notchange the polarization of the ight beam. This case sparticularly simple since the refractive indices of incidentand diffracted light are about equal.2 The wavevectors alllie approximately on one circle. Fig. 2 shows the wave-vector constructions for isotropic diffractions. Note t,hatthe momentum mismatch Ak is constrained t o be normalto the boundaryof th e medium.

For isotropic diffraction, n, = n o , the momentum mis-match given by (5) reduces to (0, is taken to be equal to90)Akm = rXam21~,OS eo nA2 cos Bo

where n is the refractive index, X0 is the free-space opticalwavelength, and A is the acoustic wavelength.

to the maU di5raction angles neglected.* The small change of refractive index f an extroaordinary ray due


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4 IEEE TRANSACTIONS ON SONICS AN D ULTRASONICS, JANUARY 19In (14) we have used the relation P, = 4pV3 S I2L

where P, is the acoustic power, p is the mass density, Vthe acousticwavevelocity, and H is the height of tacoustic beam.

Since 9 is usually small compared to ( A k ~ L / 2 ) 2significant error is introduced f II n (13) is approximatby

Il= I l psin? (A- (1where sinc ( x ) = sin ( T L ) ( ? ~ x ) ,nd*I l p = sin2$l 2 (1

is t he peak intensity of the diffracted light beam underexact momentum matching condition ( A k l = 0) .

To analyze the acoustic frequency response of the dfracted light intensity, we introduce a phase mismatch3

A k m L6, =-aThe frequency dependence of 6, can be rea,dily derivfrom (9) . It is convenient to normalize acoustic frequecies to a center frequency fo of wavelength 120, where t

To obta in exact momentum matching for light into the Bragg condition is satisfied. I n terms of the normalizFig. 2. Wavevectorconstruction fo r isotropicdiffractions:

first order requires th at Akl = 0, which occurs when the frequency, F = f/fo, the phase mismatch 6, is given byangle eo of the incident beam is equal to th e Bragg angleB B , be defined by I I,2LO6, = 2nho2 os Bo sin 6'0 m F =

X02nhsin OB =-

In general, multiple diffractiono higher orders mayoccur.If the interaction length L is sufficiently large, however,light. intensity in the high diffraction orders becomes neg-ligibly small due to the arge value of Ak,L. In the limit,only two modes, the zeroth and the first order,need to beconsidered. I n th is case (4) educes to

where p is th e effective elastooptic coefficient for the par-ticular mode of acoustooptic interaction. The diffractionis said to be n he Bragg regime when the two-modeapproximation holds.The question of validity of the Bragglimit approximation will be discussed later.Equations (11) and (12) admit simple analytic solu-tions. At z = L , he normalized intensi ty for the first orderdiffraction [11] is

sin2 ( V + (A k l L / 2 ) 2 )I1 = t t + ( A h L / 2 1 2 (13)where

where L o is a characteristic lengt'h

At the center frequency (F = 1 ) where the Bragg contion is satisfied, the phase mismatch for the second ordis equal to L j L O , and the int,ensity of the second orddiffracted light builds up as sinc2 ( L / L 0 ) which becomvanishing small as L >> iTJ0. Thus Lo is approximately tminimum interaction length to insure that the diffractioccurs in theHragg regime. In th eopposite case knownthe Raman-Nath regime, L


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CHANG: ACOUSTUOPTIC INTERACTIONS, I 5

' t

y t

Fig. 3. Wavevectorconstruction for birefringent diffractions. a)General case of birefringent diffraction. b) Collinear acoustoopticinteraction. (c) 90"birefringent phase mat.ched diffraction.

tion tha t diffraction occurs in the Uragg regime may b ecome invalid . The intensit y of the diffracted light in thesecond ordercan becomeappreciablecompared to hefirstorder,particularly at he low-frequency end [13].From (1'7) it is seen tha t he phasemismatch or thesecond-order diffraction 8 2 approaches zero as F is near0.5. The interactionbandwidth s hus imited o oneoctave or less.Birefringent Diffraction

Dison [S] has pointed out that acoustooptic diffractionbecomes significantlydifferent romnormaldiffractionwhen it tak es placebetween an ordinary wave and anextraordinary wave i n an optically anisotropic medium.We shall refer to this type of diffraction as birefringentsince the refractive indices for the incident and diffracted

waves are different. The bandwidt,h characteristics of theinteraction become significantly modified as a result of t hebirefringence.Fig. 3(a) shows the momentumdiagramfor the birefringent diffraction in a uniaxial crystal whenthe optical wave vectors are normal to theoptic axis. For:L given acoustic wave direction, there exists two distinctacoustic frequencies that satisfy exact momentum n-ratch-ing conditions. The general case has tw o limiting cases asshown in Fig. 3(b) an d 3 (c), espectively. I'ig. 3 (b ) corm-sponds to th c case of a collinear interaction of opticalwaves and acousticwaves. Nobice that he momentu mmatching condition can be approximately satisfied for agivenacousticfrequencyovera wide range of incidentlight directions. This feature (i.e., large angular aperturecharacteristics) is of signiffcant importance to tjhe opera-tion of acoustoopt,ic filters [14].

In the other limiting case (Fig. 3(c ) , when the twosolutions of momentum matching become degenerate, thediffracted light wave vector is perpendicularo thecousticwave, and hemomentummatching conditioncan beapproximately sat,isfied for a given incident light directionover a broad range of acoustic frequencies [S], [l.?]. Thebandpass haracteristics of the birefringentdiffractionprocess can be determined from the phnsc mismatch func-tion. From (.5) we have

(20)Let

be the acoustic frequency a t which t he degeneracy occurs,and choose fd to he equal to the center frequency so. nterms of the normalized frequency F = f!/.fo, 20) reducesto (for e,, = 90")

where L O is given by (18) with n = 11". When

Near F = l t8he phase mismatch remainsmall for a broadrange of acoustic frequencies. Th e diffraction bandwidthof the interaction for this degenerate case can be easilydetermined. Again we let & = 0.45 in (21) and obtain

Thus a significant bandwidth mprovement canbe ob-tained with the birefringent phase matching.


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6 IEEE TRANSACMONS ON SONICS AND ULTRASONICS, JANUARY 19100

B

6

4

2

v]124al-0I 10

- 1 8Y

6

4

2

1

I I I I I

# 1 - - B I R EFI N GEN T PH A SE MA TC H I N G

\\

L / L " \ \'\\ \ \\ \\O R D I N A R Y PHASE MATCHING&.

I I I0 20% 40% 60%

L I L O

t C TA VE B A N D WI D TH

80% 100% FR A C TI ON A LA N D WI D THFig. 4. Interaction engthversus ractionalbandwidth forbire-fringent an d isotropic phase matched diffractions.

The ength atio L / L o for birefringent and isotropicphase matching is plotted in Fig. 4 as a function of frac-tional bandwidth. We notice that the interaction lengthadvantage of birefringent 90" phasematching smostsignificant for relatively narrow fractional bandwidth. ora given bandwidth, this amounts to an interaction lengthadvantage of 2jO/Aj, or about three times for octave frac-tional bandwidth.

The fractional bandwidth can be further broadened bya factor of fi f one chooses momentum matching a t twofrequencies and allows the efficiency at the center dip t'obe as low as 3 dB.'

111. ACOUSTOOPTICMATERIALSIn th is sectlion, we briefly summarize the present stateof the art of acoustooptic materials. We shall not discuss

the subject of selection of new acoustooptic materials. The

interested eader s eferred to he excellent papers bPinnow [l61 and by Uchida and Niizeki [17].

A good acoustooptic material must satisfy the ollowinconditions. It must be of good optical quality and available n easonably arge size for deviceapplications. Ishould have low optical and acoustic attenuations, and should have high acoustooptic figures of mcrit . The commonly used figures of merit are

(25

'D. L. Hecht, private communication.


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CHANG: ACOUSTOOPTICNTERACTIONS, I 7TABLE ISELECTEDCOUSTOOPTICATERIALS

Aco ust ic Wave Op ti ca l Wave Fig ure s of Elerit

O p t i c a lTransmiss ionM a t e r i a l (W )

F u s e dS i l i c a 0 . 2 - 4 . 5

LiNbOj 0.4-4 .5

Ti02Sr0,75Ba0.25Nb206.4-6DiamondPbMo04Te02

GaP

AS12se55Ge33As ,Se

GaAS

Tl,AsS4T13PSea

0.2-50.42-5.50 . 3 5 - 5

0.6-101-14

0.9-111 - 1 1

0.6-120.85-82-20

Densi tyP(dCrn3)

2.24.64

4.235. 43.526.956 . 0

4.134. 4

4.645.346. 26.315 . 3 3

At tenua t ionMode and Veloc i ty aVDirection (lOscm/sec)sec-ofr'

0Propaga t ion dB/

LL[100]

L[ 1001L[OOl]L[lOO]L[ODl]L [ O O I ]

S ( l l O ]

L[110]1.

LL[I I f l ]L(0011L(0101

5.966.57

8 . 0 35 . 5

17.53.634. 20. b2b. 322. 522.255.152.152.0

7 . 20. I

- -2 . 12 .65 . 56 . 3

1 7 . 95.X1 . 7

71) j- . .15.5

S

30

R e f ra c t i v e ,P o l a r i z a t i o n n d e xDi re c t i o n1.46

35oy 2 .2r o t .[OlO] 2.58i f 2. 311 2 . 4 11 2.39L 2.26

C I R . 2.26/ l 5 . 3 1L 2 .7/ l 2.89l / 3.37/ / 2.83l/ 3 . 0 911 4 . a ~

C( m )

0.63.;0 . 6 3 5

0 . 6 3 30 . 6 3 30.5890.6330.6330.6330.6331.060.6331.150.6351 . l 5

-M 1

1 .8 . 5

8.33 4 .i9. 6

14.617.613.175.354.4

2 0 4

l1 8IS-'ib b

L11111 5.506. 5

Th e figure of merit M,, [lS], [l9 1 relates the diffractionefficiency to the acoustic power for a given device geome-try. From (14) we have

9=5Mz(XoZH)-'LPa. (27)M is used when only efficiency is of primary concern(narrow-band device).

Besides efficiency, another important design parameteris bandwidth. From (19) it is seen that th e bandwidth isproportional to n V 2 . Thu s o optimize efficiency band-width product for a fixed power, the relevant figure ofmerit is then M 1 = A2,nV2, [S], [19]. From (14) and (19)we have

9 9Jf1(Xo3foAfH)-'Pa. (2s)Adl is the most commonly used figure for deflector designwhen the t,ransducer height H is constrained by fabrica-tion imits or electrical mpedance considerations. Whenthe transducer height is not constrained by these factors,but may be made as small as the optical beam size, thenAd3 = Men'lr should be used [20]. Let H W V /Af in (28)then we obtain

7M 9,143 ( X*3fO) -]Pa. (29)Finally, in the esign of wideband deflectors or modulatorswhere the power density is the limitring actor, M 4 =M,( T L V ~ ) ~s hen applicable.From (14) and (19) weobtain

17 E 16M4(Xo4f02Af2 ) - 'Pd (30 )where P d = P,/LH is the acoustic power density.

It is seen ha t the igures of merit M I , 1113, and 42 , can bedetermined rom . M 2 and t,hendex of refraction andacoustic velocity d at a of tjhe medium. The figure of meri t

can be determined from a measurement of diffractionefficiency (27) for a given acoustic power and aspect ratio.n ixon and Cohen [21] described a simple echnique ofdetermining the magnitudeof M ? relative to thatof fusedsilica. From the measurement of 3f2, ne can determine heelastooptic coefficients pi? for the parti cular acoustic andoptical modes involved. Thr sign of the p can be deter-mined by static measurement or by a refractive deflectiont,echnique described recently by Rirgelscn [22].

Table I list,s the figurrs of meritandotheracousticand ptical ropertics of someelectedcoustoopticmaterials. The table is based on da ta from Pinnow [23],Uchjda and Yiizeki [17], and the rrcent work on chal-cogenide crystals by Gottlieh et al. [24]. The coefficientao(dB,/rs-GHz*) is the acoustic attenuation er nitt ime at f = 1 GHz, assuming t,he acoustir att enua tion isproportional to p. Th e da taf M,,n , and V are tjaken from[ l 71 and [23], and the dataof ,Ifl, 1123, and M a are calcu-lated values using (23)-( 26). T he figurrs of merit listedare normalized relative o fused silica, whichas t h r f o l l o ~ -ing absolute value of figures of meri t:

M 1 = 7.83 X 10-~ [crn2sg-']M, = 1.51 X IO-'' [s3g-'J. V 3 = 1 .3 X lo-" [cms2g-']A f 4 = 4.06 X lo5 [crn*s-'g-'].


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8 IEEE TRANSACTIONS ON SONICS AND ULTRASONICS, JANUARY 197TABLE I1

PROPERTIESF LiNbO: TRANSDUCER,LECTRODESND BONDING AYEESLiNbO Transducers :3

Mode Or i e n t a t i on V ( l o 5 cm/sec) z = V V ( l o 5 g / s e c cm7)-K i-L 35 OY 0 . 49 39 7 . 4SS

3 4 . 8163OY 0. 6 23. 56 21.4

X 0 . 6 84 4 . 87 . 3

Electrodean dBondingLayers:

EPOXY 2 . 6. 86 _ _ 1 . 2 2. 3 4P h e n y le n z oat e. 6. 4 5 - - l . 82I n

_ _2 . 2 5 0 . 2 3

2.256. 4 8 0 . 91. 4 1 6AU 3. 24 6 2 . 5. 0 2

Ag 5 . 6 5 36 . 0. 025. 61

1 . 2A1

2 3 . 2. 16 .27 . 3. 02. 0 4 6 . 2

16 . 7_ _

cu 5 . 010 . 6 2 . 1 1 18 . 3 _ __ _

_ _IV. TRANSDUCER TECHSOLOGY

For most acoustooptic device applications it is requiredthat theransducer used should have large fractional band-widthwith low conversion loss. Presently, the bestapproach s the technique of bonding hin-pla te piezo-electric transducers. Such techniques have been recentlyreviewed by Meitzler C253 and by Sittig [ 2 6 ] .

The problemareas in he design and abrication oftransducers include: the development of new transducermaterials with large electromechanical coupling constantand low dissipation loss, the improvement of bondingtechniques and reductionof transducer thickness for high-frequency operations, and suitable design of t he electricalcircuit for broadband impedance matchingof transducers.Transducer Materials

Recently, a variety of new piezoelectric transducermaterialshas beenavailable.Acompilation of da ta ofthese materials can be found in a paper by Meitzler [%l.Among thesematerials,LiYbO, sprobably the mostwidely used in practice. The essential propertiesof LiW)03C%] are summarized in Table 11. Also listed in the tableare the properties of several elcctrodo and bonding layers.The data [ 2 5 ] , [a71 on the mechanical mpedance andacoustic attenuation of the electrode and bonding materia lsare useful in the determinationof transducer performance.Technology of Transducer Boding

The simplest echniquc to bonda transducer oanacoustoopticmedium s ,he use of an organicadhesivebonduchspoxy. The extremely low mechanicalimpedance of epoxy (Table 11) is highly mismatched oLiNbO, and other commonly used acoustooptic materials.

Hence, the thickness of the epoxy layer must be kept tosmall fraction of an acoustic wavelength. We have foundthat by careful control of contaminat ion, uniform epoxlayers about 0.1 pm thick could be obtained without toomuch difficulty. Intermediate metallic layers withelectemechanical impedance could be used in theepoxy bondinfor broadband impedance matching purposes. Satisfactoand reproducible esultshave been obtainedup t o 15MHz using epoxy bonding. At higher frequencies, ot,herbondingechniques such s cold-weld bondingusingme-tallic bonding layers) provide the best result,s.

ilmong these metallic layers, indium is the most commonly used [ 2 8 ] . Excellentbondingcan be repetitivelyobtained using low bonding pressure (=l000 psi) for a fewseconds. The impedance of indium is reasonably close ttha t of most substrates used. The major disadvantage oindium, as seen from Table 11, is its rather large acoust,iattenuation athigh frequencies.

The at't enuation of gold is very low and should be suitable for requencies above the 1-GHz range. Gold bonding&h good results have been reported [ 2 7 ] , C291. However, since the mechanical impedance of gold is extremelyhigh, it will be mismatchedto most acoustooptic material

Recently we have successfully accomplished the use oaluminum as the bonding medium i n our laboratory. Themechanical mpedance of aluminum is close to ha t oindium. However, the acoustic att'enuation of aluminumis only slightly higher than t'hat f gold. The high electricaconductivity of aluminum is another advantage for highfrequencyapplications.Transducerconversion loss lesthan l dB has been obtained a t 350 MHz.

The thickness of the bonded ransducershas tobereduced to a final dimensioncorresponding to he fre


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CEANO: ACOUSMOPTIC INTERACTIONS, I 9

0.6 0.7 0.8 0.9 1 o 1.l 1. 2 1. 3 1.4NO RM AL IZ E D FREQUENCY

Fig. 5. Frequency dependence of transducer mismatch loss with shunt inductance tuning.

quency of operation. Mechanical lapping generally givessatisfactoryresultsforreducing hickness to about 3-6microns. To furthereduce theransducerhickness,techniques of sputter etching [27],C301 and ion milling[291, C311 have been developed. Recently, Huang et al.C291 have used the ion milling technique to reduce thethickness of a gold-bonded shear-wave t ransducer to aslow- as 0.25 micrometer, and achieved less than 20 dB one-way-loss operationup o 11 GHz. Applying thesametechnique to anndium-gold bonded ransducer, Stevensonand Hanak [31] have obtained similar results and haveoperated the transducer up to5 GHz. They reported thevery impressive result of achieving a one-way transducerconversion loss of less than 1 dB at an operatingrequencyof 1 GHz.Transducer Impedance Matching

One of the recurringproblems in he fabrication ofacoustoopticdeviceshas been the design of transducerimpedancematchingnetworks.Since thedielectric con-stant of LiNhOs is very high, the impedanceevel of trans-ducers for most devices operated above100 MH z is of th eorder of a few ohms.Because of t,his, the parasitic n-ductance of the connecting wires can domina te the t rans-ducer impedance even with a series induct,ance of a fewnanohenries. Th is will raise loadedQ of the transducer andeffectively lower the bandwidth . o reduce the capacitanceand raise the impedance level,one practice has been o usemult,iple series-connec,ted transducers .

The design of transducersgenerally star ts with thedetermination of transducer requency response. Based

on Masonsequivalentcircuit,Sittig C321 andMeitzlerand SittigC331 have analyzed therequency characteristicof multilayer t,ransducers including the effect of t,op elec-trode, bonding, and intermediate layers. For given trans-ducer configurat,ion with chosen design parameters (mat ,e-rial data, layer thickness, et c.) , the transducer frequencyresponse can be determined with digital computations.Bytaking nto account he effec t of series inductance, thecalculated results generally agree with measured results.Various impedance matching networks based on filtersynthesishave been developed [34]. Due o he largevalue of the electromechanicalouplingonst,ant ofLiNbO, transducers,owever,roadbandmpedancematching is obtainablewithouthe use of elaboratematching techniques. Th e simplest, method is t)o resonatethe transducer with a shunt inductance and transform thesource impedance level by using either ferrite transformers,quarter-wave transmission lines, or LC ladder filters [%l.

Calculations were made using the method of shunt in-ductance tuning or the case of thin bonding, where oneassumes t he effects of internmcdiate layers and top electrodecan be neglected. Fig. 5 shows th e transducer mismatchloss 3fL as a function of the normalized frequency for afew choices of normalized nlechanical impedance of th emedium zd = &(medium) / Z o LiNbOa) The source im-pedance is chosen tobeequal o 2 .5 X C , where X,l / ( 2 ~ f ~ C ~ )s the reactance of t he clamped capacitance atthe transducer resonance.

We have found that transducer tuning with series in-ductancegenerally esults in reduced ractionalband-widt,h. Thus it isessential to minimize series impedance in


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10the construction of t he coupling circuit for high- frequenqdevices.

V. BASIC ACOUSTOOPTIC DEVICESSince its discovery,acoustooptic nteractionhas been

used to perform various optical beam control functions.In th is ection we shall discuss the operation of three typesof basic acoustoopt'ic devices. Thme devices are charac-terized by the three dis tinct egimes of interact ion geome-tr y depending on the parameter a = (68,/68,), the ratioof divergence angles of the optical beam, and the cousticbeam. In the limit a


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CEANO: ACOUSTOOPTIC INTERACTIONS, 11

first order diffraction for the beam steering case becomes,from ( 17 ) ,

- _LI. " ( F - l ) ( F - - ? ) (3.5)

We thus arrivea t similar results as the ase of birefringent90" phasematching (21). When S = L,, the bandwidthbroadening due o bcam steering is also givenby ( 2 2 ) .Although the effect of beam steering on first-order dif-fraction is similar to that of birefringent phase matching,the effect on second-order diffraction is quitedifferent.Th e acoustic deflector beam steering provides additionaldiscrimination against second-order diffraction. Thus thebeam steering technique is more effective when the opera-tion of an acoustooptic deflector over more than octavefractionalbandwidt'h sdesired. The analysisdescribedhere is only an approximation tha t is good when th e num-ber of transducers in the array is large. Pinnow C371 usedthe radiationpattern concept, and carried out amoreaccurate analysis.

In first-order beam steering, exact momentum matchingis maintained a t only two distinct frequencies, i.e., F = 1and F = L O / s n (3.7).By using large arrays of transducerswhere the phases of each ransducerelementare ndi-viduallyvaried, Couqin et a l. [l31 have extended thebeam steering technique o near perfect momentum match-ingover the hand . The price to pay in tha t case is, ofcourse, t'he complexity of the design.A differentapproach to reducc the power or powerdensity is the use of 90" phase matching birefringent dif-fraction (Section 11) [ S ] , [lS]. The centerfrequency f oof the deflector must he chosen to he close to

where An is the hircfringence. This frequc.ncy is, in general,vcryhigh. The first such type of birefringentdeflectormadeby Ilean et al. [l.',] used the uniaxialcrystal ofsapphire. The value of f o is 1.56 GHz, a frequency too highfor many applications.

One mportant birefringent deflector is he TeOz de-flector using th e shear wave propagating the [l101 axis.The originalonfiguration, proposed f)y UchidandOhmachi [39], utilized the anomalously low shear wavevelocity andhigh figure of merit n TcOz [39], [ M ] .Warner et al. C411 recognized th at he circularbire-fringence in TeOz can be used for broadband operation ina 90" phasematchingconfiguration.Since the circularbirefringence is small, fd occurs below 100 J'IHz for visiblelight; e.g., fd is equal to 37.4, 63.3, and 85 MHz for th ewavelengths of 632.8 nm, 4S-58nm, and 441.6 nm, respec-tively. For a fractional bandwidth of 50 percent, Warneret al. [41] demonstrated an int,craction length advantage

of 8, or equivalent to a reduction of power density by afac tor of 64 compared to isotropic deflectors.

Th e use of beam steering and birefringent phase match-ing t,echniques angreatly educe ,he coustic powerdensit,y, and thus the bandwidth limitationf acoustoopticdeflectors will beduemainly t o the maximum powerallowed.

Besides bandwid th, hereareother actors hatcanlimit thc deflector resolution. In some applications wherethe speed of th e deflector is not a major consideration, theresolution is limited by either the acoustic attenua tion a tthe high requency or by the spatial constraint on theoptical aperture. For most crystalline solids, the acousticattenuation is proportional t o f. f we allow a maximumattenuat ion of S(dF3), the maximum allowed value ofacoustic transit time is T , ~ ~~ / ~ y ~ f , , , . , 2 . Thus, when t,heacoustic attenuation is the limiting factor, the deflectorresolution is given b y

(371which reduces to X,,, = 1/ (a0Af ) = ( T / ( Y ~ ) ~ ' ~f weassume d: = 4 dB, E = 1 , and fmax = 2Af (octave band-width).

E'ig. 7 shows the estimated acoustooptic deflector esolu-tion versus access time foraselection of acoustoopticmaterials. The limits f performance are determined basedon the following assumptions: the aperture size is limitedto 5 centimeters, the maximum attenuation is limited to4 dB, and thebandwidth is limited to ~ l O O ( ~ ~ f l ) l ~ zHz.Fig. 7 shows that themaximum resolution of acoustoopticdeflectors is probably limited to a few thousand.

The tradeoff relation (34) between resolution an d speedis limited to first-order diffraction of a single deflector. Itis possible to increase the resolutionwithoutchange ofeither the bandwidth or the trans it time by using severaldeflect'ors in cascade 1421. High-orderdiffraction couldbe used to increase Ai', however, the correspondingdif-fraction efficiency is too low to be practical . One nter-esting approach is to utilize the second-order diffractionin a birefringent deflector.At fd , the first- and second-orderdiffraction are exactlydegeneratelyphase matched, en-abling higher diffraction efficiency into the second order.By utilizing this technique, Chang and Hecht c431 haverecentlydemonstrated th c operat ion of a second-orderbirefringent TeO, deflector andhaveobtained doubledresolutionmaintaininghighdiffraction efficiency. 1200spot was achieved with 25 microseconds access time and2.5 >[Hz bandwidth.diodulators

The acoustoopticnteractionhas also been used tomodulate light. Both amplitude and frequency modulatorscan be achieved [S], 1441. Gordon [3] showed that forproper modulator operation, the divergence of the opticalbeam should be about equal to th atof th c acoustic beam,i.e., a = 60,/60, X 1 . In order to m atch the Rragg condi-


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12 IEEE TRANSACTIONS ON SONICS AN D ULTRASONICS, JANUARY 197

5.000

2.000

1,000

2 5000+32

v)0& 200

100

50

20

10

-.-.-.-.. L I M I T B Y B A N D W I D T HL I M I T B Y A T T E N U A T I O NLIMIT B Y A P E R T U R E

1 I I 1 I 1 I I I0.1 0.2 0. 5 1.0 2.0 5.0 10 20 50 100

ACCESS TIME (prec)Fig. 7. Resolution versus access time for acoustooptic deflectors.

tionover he modulat,or bandwidth, heacousticbeamshould be made narrow, as in the casef deflectors. Unliketh e case of deflectors, however, the optical beam shouldalso have a divergence approximately equal to th atof th eacoust,ic beam, SO that thecarrier and the idebands in thediffracted light will mix collinearly at the de tec tor to ivethe intensity modulation. The actual value of the diver-gence ratio depends on the tradeoff between desired effi-ciency and modulation bandwidth.

The divergent optical beam can be obtained by simplefocusing optics. The diffract ion geometry for an acousto-optic modulator is shown in Fig. 8.In most applicationswe shall be concerned with an inc ident laser beam whichhas a Gaussian distribution with beam waistof diameterd. The corresponding optical beam divergence is

(38)

il DIFFRACTED

LENS - + z

I

TRANSDUCERFig. 8. Diffractiongeometry of acoustoopticmodulator.

The acoustic wave is assumed to be generated from a flattransducer of width L . The corresponding acoustic beamdivergence is

AI,8, = - (39)

where A is the acoustic wavelength.the divergence ratio a is given C451 byIn terms of the optical and acoustic bcam dimensions,

The choice of the interaction length I, in the modulatodesign depends on the choice of a , which requires a calclation of the diffract,ionof a Gaussian beam from a modulated acoustic wave. One way to accomplish this is tdecompose the Gaussian beam into plane aves and appthe planewavesolution (1Fi) as n a standardFourieranalysis. The simple ca.sc of amplitude nmdulat'ion waanalyzed by Gordon [S], and basedonhis nalysis,numericalcalculations were made. The results are summarized in Figs. 9 and 10. Fig. 9 shows the dependence othe modulation bandwidth on the divergence ratio a. A


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CHANG: ACOUSTOOPTIC INTERACTIONS, I13

0.7

0. 6

0.5f

0.4

0.:

0 :

I I I I l1 2 3 4 5

Ail,, OPTICALI,AtlaACOUSTIC)=

1 o

0. 8

RELATIVEDIFFRACTEDL I G H TINTENSITY

0 6

0.4

0. :

l I I I l1 2 3 4 5


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14 IEEE TRANSACTIONS ON SONICS AN D ULTRASONICS, JANUARY 197low values of a, the modulation bandwidth approaches thelimit

fm 0.75,~41)where T = d/V is he acoustic ransit ime across theoptical beam.As a increases, bandwidth st ar ts to ecrease,while the peak intensity of the diffracted beam star ts tobuild up as shown in Fig. 10. The proper choice of a thusdepends on the tradeoff between modulation bandwidthand peak diffraction efficiency. A plot of the product ofbandwidth and peak intensity as a function of a shows abroad maximum between1.5 and 2 . The modulation band-width in this region is approximately given by

fm W 0 . 6 5 / r . (42)Maydan [45] has treated in detail the important case ofpulsed modulators. His results show that in the optimumdesign, a z l.Fi, which yieldsa isetime (10-90%,) oft, Z 0.857.

The choice of a for the design of continuous wave orpulsed modulators amounts to LILO2 , C451 indicatingth at th e Bragg regime approximation is valid for modula-to r operations.One requirement in the design of acoustooptic modu-lators is that the diffracted beam and undiffracted beammust be well separated. For an adequate extinction ratio,th e Bragg angle should be a t least as large as the diver-gence of the optical beam. This ondition puts a minimumvalue on the center frequency. Equating the Bragg ngle

Xofoen =-n /and the diffraction angle of the Gaussian beam

4x0ae a =-m dit follows the lower limit of the acoustic frequency is givenby

S.m=-.Ti- (43)

We may combine ( 42 ) and (43) o determine a limitof modulator bandwidth of acoustooptic modulators:

fo 4fm (44)i.e., the modulation bandwidth is approximately equal to25 percent of the midband acoustic frequency. I n view ofth e present status of transducer technology, th e modula-tionbandwidth of acoustoopticmodulators sprobablylimited to several hundred MHz.In certainapplicationssuchas the laserdisplay [4]where the acoustoopticmodulator is used in asystemwhich scans a line at a uniform scan velocity, i t is possibleto use a much broader optical beam inhe modulator thanwould be allowed by the transit time limitations42).Thekey to his approach s the ingenious echnique of theScophony ightmodulator [46], [47]. In the ir work on

laser elevisiondisplay,Korpel et al . [4] adapted heScophony principle to achieve high efficiency without loof bandwidth due to finite beam size. The laser beam wachosen wide enough to illuminate several picture elemenin the modulator. The image of these picture elements,which trave ls across the beam a t sound velocity,was madstationary on the screen by directing the image throughhorizontal deflector. The informationhandwidth of t,hScophonymodulator n his case is not imitedby heacoustic transi t time and is thus equal to the in teractionbandwidth of the device. Longer interaction could be usefor improved efficiency if techniqucs such as beam steeror birefringent phase matching are used.Filters

In many respects the principle of acoustooptic diffraction is similar o tha tof a transmission diffraction grat,inOne naturally raises he question whether he acoustooptdeflector can also be used as a dispersive element n aopticalpectrometer. One obvious advantage of thacoustooptic grating is that the grating constant (wis equal t o the acoustic wavelength) can be electronicallchanged, thus providing a capability of rapidly scanninthe spectral region. In practice, however, acoustooptdeflectors utilizingsotropic diffractionhavenot beefound useful inoptical pectrometerapplicat.ions.Onereason s the relatively poor resolving power obtainabwith the acoustooptic deflectors. The resolving powR = Xo/AX s equal to the total numher of lines on t,grating, and for an acoustooptic deflector i t is thus approxmately equal t,o fr , where f s the acoustic frequency an7 is the acoustic transit time across the optical apcrtureThe acoustic frequency used has to he sufficiently high order to achieve even a modcrate resolving power. Thmaximum resolving power is thus limited by the acoustattenu ation at the high frequency end.A more fundamental reason why this approach is impractical s that he angularaperture associated witisotropic diffraction s proportional to the optical bandwidthand is exceedingly small. This s because n thisot,ropic diffraction, a changeof angle of t,hc incident ligwill introduceamomentummismatch seeFig. 2 ) . Tachieve a wide angular aperture, the momentum matchcondition with a fixed acoustic wavevector must be maitained for a large spread of incidence angles. As we havpreviously noted, thi s condition is approximately met inthe collinear acoustooptic interaction in birefringent crytals. At f m i n , the loci of t he incident and diffracted lighwavevectors are parallel, and t>o irst order in the changof incidence ngle, the momentummismatchs zcrHarris el al. first proposed C141 and then experimentallC481 demonstrated the operation of a collinear acousto-optic filter. LiNbO, was used as the medium, and tuningfrom 700 to 55 0 nm was obtained. The optical bandwidmas about 2 A . A later design using Ca3ioOc hada similtuning range, a resolution of S A, and a peak transmissioof 9.5 percent [49].


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CHANG: ACOUSTOOPTIC INTERACTIONS, I 15

I N C I D E N TL I G H T

z c lO L A R I Z E RACOUST ICT E R M I N A T I O N

AMPL IF IERRF POWER

T U N A B L E R FO S C I L L A T O R

Fig. 11 . Schematic of transmissive type collinear acoustooptic filter (after Harris et al . (49)).

A schematic of the collinear acoustooptic filter is shownin Fig. 11. Note that the incident optical beam and thediffracted optical bean1 have different polarizations andare separable from each other by the use of polarizers. Ata fixed RF frequency, only a narrow band of optical waveswill be diffractedand ransmitted hrough heoutputpolarizer. The center wavelength of the passband can bescanned by changing the frequency of t he R,P signal. Atthe center of the passband Ak l = 0, (6) for collinear inter-action reduces to k , = kl - , = ( % r h o ) ( n l - no),whichyields a relation between the center of t.he passbandX0 andthe acoustic frequency f:

=f&An

(45)where A n = I nl - 10 1 is the birefringence. Equation (45)is the tuning relation of t he collinear acoustooptic filter.The optical bandwidth and angular aperture characteris-tics of the collinear acoustooptic filter can be easily deter-mined romphasemismatch onsiderations. A Taylorseries expansion of phase mismatch for small wavelengthchange AA and angular deviations Atlo near he collinearinteraction (e, = 80 = 0) yields, from ( 2 0 ) ,

where b is the dispersive constant [14]. To determine th efilteresolution, we assume a collimat,ed light eam(A80 Oj , and let 1 61 I - .4.5n (46) o obtain

(47)Neglecting dispersion, b M 3 r A n , (45) and (47 ) show tha t

the resolving power of t he acoustooptic filter is equal to.fr, as one probably espected.The same phase mismatch results if the angular devia-

tion Atlo reaches the value(!!S>while AAz . The totalsolid acceptance angle external tothe medium C501 is

AQ M l% -nAn I, (48)In crystal quartz, thephase velocity and group velocity

of the shear xave polarizedalong the s-axis are non-collinear; i.e., he acoustic beamwalks off from the acousticwave front. Kusters t al . c511demonstrated thatacousto-optic filters with large angular aper ture could be obtainedwith the incident ightpropagatingcollinearlywith thegroup velocity. The collinearity of light and acousticgroup velocity hashe advantagcf maximizing interactionlengt,h and optimizing efficiency. Chang C521 described afilterconfiguration in which the lightbeam is chosencollinearwith the acoustic phasr vrlocity. The largeangular perture haracteristics rc etained,hut,helight beam soon walks out of the acoustic beam andesult,sin higher acoustic power. Yeverthelcss, th e walkoff con-figuration has t,he advantage of large tuning range sinremultiple transducers can he used. Tn the \valkoff quart,zfilter [X],uning of optical wavdength from 2.50 to 650nm was obtained.

Th e collincarity requirement limits the filter materialsto rather restricted classes of crystals. Somr crystals withvery high figures of meri t, e.g., TcO? , are excluded forcollinear filter application because of symmetry considcra-tions. For such materials, t,may be possihlc to oprrate thcfilter ina noncollinear configuration. However, hc angular


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16 IEEE TRANSACTIONS ON SONICS AND ULTRASONICS, JANUARY 19762 (OPTIC A X I S )

Fig. 12. Wavevectorconstruction for acoustooptic nteraction nnoncollinear filter (after Chang (53)).

aperture associatedwith the noncollinear interaction sgenerallyvery mall. To overcome thisdisadvantage,Chang [53j describedmethod t,o obta in wide-anglefilter operation in a noncollinear interact ion configuration.The method is based on the compensation of momentummismatch due to the angular change of incident light bythe angular change of birefringence of the extraord inarywave. As shown in Fig. 12, th e acoustic k-vector is chosenso tha t the tangen ts to the incident and diffracted lightwave vect,or oci are parallel. Thus, for a change in incidentlight direction, the moment'um matching is still approxi-matelymaintained. In th e describednoncollinear ilter,TeOz was used as the ilter medium. The filter was tunablefrom 700 to 450 nm wit,h a half-power bandwidth of40 1 t an f/ 4 aperture . Nearly 100% transmission wasobtainedwithadrive power about 120 milliwatts. Oneinterestingfeature of th e noncollinear filter is that hediffractedopticalbeam sspatiallyseparated rom theincident optical beam so tha t t he filter could be operatedwithout the use of polarizers. This angle of separation forthe TeOn filter is equal to 3.S".

The wavelength range of acoustooptic filters has beenextended to infrared regions of the spectrum.Recently,Feichtner et aE. [.M]reported the operation of a collinearfilter using TlsAsSc3 as the filter medium. Th e filter wastested at 3.39 and 3 .3 micrometers.

In concluding this section it is instructive to comparethe performance of acoustooptic iltersanddiffractivegra tings. The resolving power of the filter (47) is limitedby the available size of the filter medium. Considering aLiNhO, filter with an interaction leng th f 10 centimeters,(47) gives a calculated half-power bandwidth of 0.18 a tX = 0.5 micrometer. This corresponds to a esolvingpower of t he order of 38 000, number much ess than onecould achieve with a high-resolution grating. Although itsspectral resolving power is not argc , he acoustoopticfilter has an entendue advantage . ssuming equal aperture

size and resolving power, the large ngular perturecharacteristics makes the ent end ue of acoustooptic filterbetter than the grating by two ordersf magnitude.

VI .APPLICATIONSAcoustooptic devices can e used in a varie ty of applica-

tions. Here we shall discuss only a few of them that appemost practical or most promising in future developmentsDejlectors

One of t'he first system applications of a n acoustoopticdeflector was the horizontal deflection in a laserTV displadescribed by Korpel el al. [4j.This system used wa ter asthe interaction medium.eam steering was used to achievea 3-dB resolution of about 200 resolvable TV elements.Later developments included the increase of resolution byusing two deflectors n cascade [42]. In thewater cells, thebandwidth of these deflectorswas relatively low (16 MHz)and good resolution was achieved because of the use of arelatively long transi t ime .The development o f newacoustoopticmaterials uchasPbRlo04and TeOz hasmade it possible to increase the deflector bandwidth significantly.Pinnow et al . C551 constructed wo-stageX - Y deflector using PbMoOl as the deflector mediumThe bandwidth f each deflector was 80 RfHz. These highspeed deflectors provided fas t, random-access readout inan optical holographic memory system [56]. Gorog et aldescribed a TV rate laser scanner using a PbMoOI deflector C571 or a birefringent TeOz cell deflect'or C583 athe horizontal deflector. With pproximately 50 mWaverage drive power a t television horizontal scan ra te , thTeOz deflector had an average deflection efficiency o50%. The resolut,ion of the deflector was about 500 spot's

An interestingapplication of acoustooptic deflectorusing the multifrequency mode was described by Hrbekand Watson c591 in a high-speed laser lphanumericgenerator based on acoustooptic components. The systemincludes a first Bragg cell th at was driven by seven frequencies. Each signal can be individually turned on or offthus forming a vertical line of seven dots.,4 econd Braggcell acts as a horizontal scanner. In t his man ner , a 5 X 7matrix character was generated, with a writling speed inexcess of 100 000 characters per second. Thi s has application o hard-copy printingas well ascomputeroutputmicrofilm (COJI ) prin ting.Modulators

Acoustoopticmodulators have become increasinglypopular n ecentyearsdue t,o thei rmanyadvantagessuch as low drive power, high extinction atio, insensitivitto temperature changes, and simplicity in design and construction. The application of acoust'ooptic modulators inthe infrared is attractive due to the existence of severasuperior infrared acoustooptic materials. Design and construct ,ion of acoustooptic modulators for communication


16/21

CEANQ: ACOUSTOOPTIC INTERACTIONS, 17applications in the infrared were reported for 1.06 [So],3.39 [Sl], and 10.6 micrometers [Sl], [62].

Another ype of acoustoopticmodulator nvolves theuse of standing acoustic waves. Large modulation s attain-able a t low drive power due to the enhancement of dif-fraction efficiency caused by coustic resonance. Thestanding wave device is usable over a narrow band of fre-quenciesnear the aoousticresonance and s basically asingle frequency modulator. Tsai and Yao [SS] describedand demonstrated the se of standing wave modulators asmultiplexers and demultiplexers in an optical pulse-codemodulation syst,em.

The use of an acoustoopticmodulator nsidea asercavity has been another major application area. This isbecause the acoustooptic effect occurs in crystals of allclasses and amorphous solids, and acoustooptic mater ialswith excellent opticalqualities are easy to find.Theseintracavity applications include Q-switching [S4], modelocking [Ss], andcavitydumping [W]. Q-switching ofNd-YAG lasers [S71 has been of technical mportancedue to its applications to materia l scrib ing, resistor tr im-ming, nd circuit tching. In nearly ll coustoopticQ-switches used today , fused silica is employed primarilybecause of it s excellent optical quality andhigh thresholdfor optical damage. The acoustooptic modulator generallyoperates in the ndiffracted mode; i.e., whenhe R F poweris on, the acoustic diffraction of light introduces loss inthe cavi ty and uenches the laser oscillation, and when theRF is off, the laser Q-switches and emits high peak powerpulses.

The early design by Chesler et al . [S71 utilized longitudi-nal acoustic waves. I n fused silica, the elastooptic coeffi-cient p12 s larger than p l l , resulting in five times greaterdiffraction efficiency for light polarized perpendicular tothe propagation direction of the acoustic wave. The laserwill always choose to oscillate in hepolarization hatexperiences the minimum loss. Thus theQ-switch operatesin he polarizationwith the low diffraction efficiency.Because of this, shearwave has generally been used o thatthe operation of the Q-switch is insensitive to the st atefpolarization. Due to the ow figure of merit of th is mode,substantial RF power (greater than50 watts) is generallyrequired to hold off a laser with a moderate gain. Tech-niques have been developed to use the longitudinal modemore effectively by use of additional quarter-wave plates[SS], so that in a round trip both the ow-loss polarizationsta te and he high-loss polarization st ate are used andresul t in effective increase of total loss. We have chosen adifferent design th at uses a cascade of X and Y deflectors.Using Q-switches based on this scheme, high power YAGlasers have been completely quenched with acceptable Fdrive power.

I n mode-locking applications, a standing wave acousto-optic modulator is used inside the cavity and introducesloss modulation with a frequency tha t is equal to the dif-ference between axial mode oscillations. The loss modula-tion effectively locks he modes and results insequence of

short optical pulses. For mode locker applications, fusedsilica is generally used for the visible and Ge [S91 for10.6 pms. Acoustooptic modulators can also be used out-side the laser cavity for mode locking70], which has beenuseful in optical hetrodyne applications.

In th e &-switching operation, the repetition r ate hasbeen limited by the time to repump,he population inver-sion. Cavity dumping, first eveloped by Maydan [SS], isone way to obtainoptical pulses a t repeti tion rates higherthan 100 kHz. nhe cavity-dumpingcheme,hortacoustic pulses (-a few nanoseconds) are fed t,o a fa stacoustooptic modulator hat performs as an outp ut ouplerin a laser cavity. The cavity s always kept above thresh-old, and the repetition rates only limitedby the switchingspeed of the modulator.Cavit,y umping rovides atechnique to obta.in nanosecond laser pulses withhighpeak powers. It has also been shown th at in th e cavity-dumping mode, average power close to th at of the CWlaser is possible [SS]. These superior performances makethecavitydumper useful inapplicationssuch as lasermachining andmage recordingn th in films [71].Recently, Johnson C721 described an efficient scheme forcoherentcavitydumping of mode-locked lasers. Sirnul-taneousmodeocking nd Q-switching has also beenreported [73].Filters

The tunable optical filter is the newest member of thnacoustooptic device family. As such, its application is yetat an early stage. Because of their unique characteristics,acoustooptic filters shoulde useful for a variety of applica-tions.Some of the salient eatures of th e acoustoopticfilter include: electronic tuning with fast scan rat,e, highoptical transmission,moderateesolving ower,argeangularaperture,capability of operating in sequential,random access, and multiwavelengthmodes, etc. Due to trecentdevelopment, of acoustooptic ilters in he ult,ra-violet and infrared regions, special purpose spectrometersusing hese ilters are expected toappear soon. Otherapplications include: t,uning of dye lasers [74], radiome-try, laser detection, and multispectral imaging. Recently,we have demonstrated color filtered imaging using a TeOznoncollinear acoustooptic filter. The filter ha,d an opticalpassband of 200 A, an external angular aperture of f/4,and was t,unable over the visible spectrum. 1Vit.h a filteraperture of 2 mm, image resolution ette r than 0 lines/mmwas obtained. The filter color can be switched in a periodof 5 microseconds. Figure 13shows a phot,ograph of theimage of resolut,ion test chart illuminated by a whit'e lightsource through t'he filter operating near..5 pm. The picturewasobtainedwithout the use of polarizers. The dif-fracted (green) image was spatially separated from undif-fracted (white) image. The angle of separation was equalto 6'. The drive power for the filter was about 200 mW.As a result of the low drive power, i t should be possible toconstructargeperture filters for improvedmageresolutions.


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18 IEEE TRANSACTIONS ON SONICS AND ULTRASONICS, JANUARY 197

Fig. 13. Images of resolution test chart hrough TeOz acoustooptic filter. Undiffractedwhite mage right)anddiffracted green image (Left) are separated by 6" .

LA SER B E A MX P A N D E R BRAGGI A N A M O R P H I C )E L LL E N S( F O U R I E RT R A N S F O R M ) P L A N EF R E Q U E N C Y

Z E R O O R D E RSTOPF R E Q U E N C Y 1F R E Q U E N C Y 2

E L E C T R O N I C f +I N P U TFig. 14. Basic apparatus of acoustoopticspectrumanalyzer.


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CHANG: ACOUSMOPTIC INTERACTIONS, I 19Signal Processors

Another important area of application may be classifiedas signal processing. Th is includes pulse compression C7.51,[76], opticalcorrelation C771 andspectrum analysis ofRF signals [78]. A thorough review of acoustoopticalsignal processors was given by Damon et al. [79].

The application of acoustooptics to the spectrum analy-sis of RF signals is of particularly practical importance. Inthe following we shall briefly describe the principles ofoperation and report some of t,he recent development$.

Fig. 14 shows the basic apparatus of a n acoustoopticspectrum analyzer, which displays t'he frequency spect,rumof an incoming RF signal applied to the electrical input.The display is obtained h?; the acoust'ooptic diffraction ofthe laser beam in the Bragg cell. A4nanamorphic beamexpander is used to fill the whole optical aperture of t8heBragg cell, while a ylindricalFourierransformensresolves the signal hcamx as diffractrd limited spots in thefrcyuency plane. Thr relative intensities of t he diffractedlight in the requency plane are proportional to the powerdensity spectrum in the input RF signal. The amplitudeand phase o f each signal beam is modulated according tothe modulation of the RF signals. The instantaneous RFspectrum o f the incoming signals re thusecorded. Typicaloutput modes arc direct aser beam display, ilm recording,and multichannel hotoelectronic detection,Thusheacoustooptic spectrum analyzer offers a unique techniquefor analyzing wideband signals in a no nxanning fashion.Fig. l5 shows a photograph of a compact, size acoustoopticspectrum a.nalyzc:r developed in our laboratory, habing aresolution o f 1 3ZHz in a bandn idth of 200 AlHz.

Experimental results nit h a 200 MHz handwidt>h Braggcell with l20 kHz resolution used in a spcctrum analyzerwas reported by Hecht [tjo].Fig. 16 shows a sample ofsimultaneous recording of two wideband signals that. arcasynchronouslywept and chopped. Hecht [ X l ] alsoanalyzed the cffcct of multifrequency signals applied to anacoustooptic deflector, including compression, cross m(JdU-lation, and intermodulation.

I3andwidths up tjo 700 JTHz have been achieved usingLiN1,03 as theaeoustooptic medium. In view of th e recentadvances in transducerabricationechnology, and-widths up t o several GHz mayeventuallybeachieved.This would be very significant in radarprocessing applira-tions.

Fig. 15. Photograph of acoustooptic spectrum analyzer.

VII . CONC1,USIOii- Fig. 16. Recording sample of RF requency spectrum functionIn t'his paper we have attempted to present a unifiedreview of the principles of bulk-wave acoustooptic devices.

The bandwidth and angularaperturecharacteristics ofthe devices are discussed in term sof t he simple considera-tions of phase mismatch of acoust,ooptic interactions. Thepresent status of acoustooptic materials, transducers, anddevice design, as well as areasof application, a re reviewed.We have estr icted our discussions on acoustoopt,icdevices based on diffraction of light by acoustic waves.Refractive deflection of light due to index gradients has

of time (after Hecht ( S O ) ) .


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20 IEEE TRANSACTIONS ON SONICS AND ULTRASONICS, JAKUARY 19also been used for device pplications [8%],XS]. Inparticular, we should ment,ion he traveling wave acousticlens approach described by Foster et al. [83]. This tech-nique has potential for significantly increasing th e resolu-tion of scanners.

In practical applications, acoustooptic devices haw t ocompetr: tvith mechanical and elcctroopticdevices. Theactual choice of t he approach depends on the specific re-quirements for the application. In general, the parametersof performance to be considered a re efficiency, speed, andresolution. The comparison of acoustooptic devices withmechanical and electrooptic devices is thus to be made nthe basis of speed and/or resolution, assuming a certainlevel of efficiency is attained.

For deflector applications, the performance of acousto-optic devices appears to sui t the middle range betweenmechanical and electroopticdevices.hlechanical devices(such as galvanometerdeflectors)generallyhave argwresolution, but heir speed is slower.Electrooptic d eflectors, on the other hand, have smaller resolution: tmtwith aster response. I n general,acoust,ooptic deflectorsare expected to provide a favorable approach, excclpt inhigh-resolution scanner applications where galvanometerdeflectors are probably preferred. An approximate bound-ary line between the two approaches may be set by theresolution of 1000 spots.

kor modulatorapplications, the acoustooptic devicesoffer theadvantages of low operating voltagc: and l o wdrive power, high extinction ratio , simple drsign, ruggedconstruction, nsemitivity o ernperaturc changes, andhigh afety actors. Thus acoustoopticmodulators aregenerally preferred toheir electrooptic counterpar tsexcept in those applications where very large bandwidthsare required. For modulation bandwidths more than ap-proximately 100 MHz , the basic imitation of acoustictransit time in ancoustooptic modulator makes the elec-trooptic modulator more attractive.

Acoustooptic filtersare expected to be useful in a varietyof app1icat)ions. In the opinion of t,his autllor, t8hey couldbe one of the no st, exciting areas of bulk-wave acousto-optice devicesfo r the next few years. In the areaof opticalspectrometers, the relatively low-rt:solution acoust>oopt,icfilters will be useful primarily for special purpose applica-tions such as fast scanning multichannel spectrometers.

The prospect of future mprovement of acoustoopt,icdevices is good, depending primarily on the developnlentof superior acoustooptic materials and efficient broadbandtmnsducers a t high frequencies. Pinnow [l61 has studicdin detail the propcrties of acoustooptic nlatwials, and heshows that thcrct is an empirical tradcoff r c la t ion h t w r w ~figure of merit and acoustic attenuation. This appea rs toset an ultirnatc limit, on t,heachievableperformance ofacoustoopticdevices. On the other hand, thcrc eems to hcno furldanlental imit. on the possilAc improvenlent o ftratLsducrxr perforrnancc.. As a rcsult of the recent progrrssof transducer cchnology, transducers with a convwsionloss of a few dB and an octave bandwidth operat,ing at,

sevmal (;Hz arc cxpectecl to beavailable in henearfuture. This \vi11 llavc. the effect of further improving tspeed o f acoustooptic devices.

ACKKOWLEDGhLENTThe autthor would like to thank Dr. D. L. Hecht f

many clarifying an d helpful discussions. Many thanksalso due I)r. C. H. Crumly for his critical reading of tmanuscript and or his rcmindcr o me ahout the copholight modulators. REXXRENCES

[ l ] h,1. Born and E. Wolf, Principles of Optics, ThirdEditionPergamon Press, New York, 1965, ch. 12.[2] C. F. Quate, C. D. W. Wilkinson, and 9; . Winslow, Intaction of Light and Microwave Sound, Proc. ZEEE, Vol.[3] E. I. Gordon, A Review of Acousto-OpticalDeflection ahfodulat.ionDevices, Proc. ZEEE, Vol. 54, Oct. 1966,[4 ] A,.Korpel, R. Atller, P. Desmares, and W. Watson, A TevlslonDisplayUsingAcousticDeflectionandModulation[5] R. W . Dixon,AcousticDiffraction of Light nAnisotropicCoherent Light, Proc. ZEEE Vol. 54, Oct. 1966, pp. 1429-143Media, Z E E E J . Quantum Electron. , Vol. QE-3, Feb. 19pp. 85-93.[6] N. Bloombergen, NonlinearOptics. Benjamin, New Yo[7] I. C. Chang. Coupled Wave Theory for Acousto-Optic Inter-1965, ch. 4.[8] J. F. Npe , PhysicalProperties o j Crystals. ClarendonPressactions in Anisotropic Media , to heublished.[9] D. F. Nelson and hl. I,ax, (New Symmetry for Acousto-OpOxford, England, 1967.Scattering, P h y s . Rev. L e t t . , Vol. 24, Feb. 1970, pp. 378-38Theory of Photoelastic nteraction, Phys . Rev., Vol. BApr. 1071, pp. 2778-2794.[l01 G. A. Couqln, Acousto-Optic Interactionsn PiezoelectCrvstals,uresented at t,he 1960 IEEE Ult.rasonicsSymp

Oct. 1965, pp. 1601-1623.

1391-1401.

St,.-T,ouiS, f i ~ . 1111 P. Phariseau. On the Diffraction of IiehtbvProaessive, Supersonic Wa-ves, Proc.Zndian Acacl. L%., vol. 43A, O1956, pp. 165-170.[l21 W. R. Klein and R . D. Cook, Unified Approach to UltrasoLight Diffraction, I E E E Trans . Sonics Ul trason., Vol. SU-July 1967, p p . 123-134.[l31 G. A . Couqin, ,J. P. Griffin, and L. K. Anderson, Wide-BanAcoustn-Oof,ic lleflctorsUsine Acoustic h a m Steerin1141 S. E. Harris an d R.. W. $allace. Acousto-Ootlc TunaI E E E Trahs . Sonics Ultrason Vol. SIJ-17, Jan. 1970, pp. 344, _ _ , ~ ..~~...Filter, J . O p f .Soc. Am.,Vol. 59, J u i e 1969, p 744-747.[l51 E. G. 1. I,ean, C. F. Quate,and H. J. ha w, ContinuoDeflection of Imer Beams, A p p l . Ihys. Lett . , Vol. 10, Ja.1967, pp. 48-51.[l61 D. A . Pinnow, Guided Lines for the Selection of Acouto-OpMat.erials. I E B E J . Qunntwm Electron. Vol. QE-6. Aor. 19I _pp. 223-258.[l71 N . Uchitla and N. h-iizeki, .4cousto-Optic Deflection MateriandTechniques, Proc. ZEEE, Vol. 61, Aug. 1973, pp. 107[l81 T. %I. Smithand A . Korpel,Measurement of LighbSou1002.Interaction Efficiency in Solids, I E E E J . Quantum Electro(oorresp.), Vol. QE-1 , Sept. 1965, pp. 283-284.[l91 E. I. Gordon, Figures of Merit,forAcousto-OpticDeflection

resp.), Vol. QE-2, May 1966, pp. 104-105.andModulat,ionDevices, I E E E J . Quantum Electron. (c1201 R. W. Dixon,PhotoeLmtiaProperties of Selected Materiant1 Their Relevxnce,tfor Applications t,o Acoustic Light ModlatorsantiScanners, J . Appl .Phys . , Vol. 38, Dec. 1967, p

[21] R . W. Dixon ant1 M . G. Cohen, A New Techniq ue for Measuing i?Iagnit,utles of Photoe1ast)ic Tensors and its ApplicationLithiumNiobate, Appl .Phys . ,Let t . , Vol. 8, Apr. 1966, p[22] D.K. Riegelsen, ~JltrasonicTechnique for Measuring TAbsolute Signs o f Photoelastic Coefficient,s and its Applicatioto Fused Silica mid Cadmium Molybdate, A p p l . Phys. LettVol. 22,Mar. 1973, pp. 221-223.I231 D. A. Pinnow, Elast,o-O t,ical Materia ls, in CRC HandbookLasers, R. . Pressley, &d. Cleveland,Ohio: The ChemiRubber Co., 1971.[24] M . Gotklieb, T. ,J. Isaacs, J . D. Feichher, and G. W . Rolan

5149-5153.

205-207.


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CgANG: ACOUSTOOPTIC INTERACTIONS,

Aroust,o-OnticPronerties of SomeChalcoeenideCrvst,als.21

._.~~~.J . A p p l . P h i s . , Vol. 45, Dec. 1974,pp. 5145-5151.[25] A. H. Meitzler, Piezoelectric Transducer Materials and Tech-niques for UltrasonicDevicesOperatingAbove 100 MHz,in UEtrasonic Transducer Ma teria ls. O.E. Mattiat, Ed. Plenum,

I

New York, 1971.[26] E. K. Sitt ig, Design and Technology of Piezoelect .ric Trans-ducers for Frequencies Above 100 MHz, in Physical AcousticsVol. IX, W. P. Mason andR. N.Thurston,Eds. Academic[27] J. D. Larson, I l l , and D. K. Winslow, IJltrasonically WeldedPress, New York, 1972.PiezoelectricTransducers. Z E E ET r a n s . Sonics.. Ultra so n. .VoSU-18, Ju ly 1971,pp. 142-152.[28] E. K. Sittig and H. D. Cook, A Method for Preparing andBondingUltrasonicTransducersUsednHigh requencyDieit,al Delav Lines, P T O C . E E E , (Lett.) Vol. 56, Aug. 1968.-1375-13j6.129) g.C. HFabrication of Submicron LiNbO, Transducers for Microwaveuang, J . D. Knox, Z . Turski, R. Wargo, and . J. Hanak,

Feb. 1974, pp. 109-111.Acoustic-(Bulk) delay Lines, A p p l . P h y s .Le t t . , Vol. 24,[30] D. Beecham, Sputter Mmhining of Piezoelectric Transducers,[31] D. M.Stevensonand J. J. Hanak, Low Loss, BroadbandJ . A p p l . P h y s . , Vol. 40, Oct. 1969, pp. 43574361.MicrowaveUltrasonicDelayLinesUsing on-BeamMilledShear Wave Transducers,R C A R e v i e w , Vol. 35, Sept. 1974, pp.[32] E. K. Sittig, Effects of Bonding and Electrode Layers on theTransmission Parameters of Piezoelectric Transducers Used inUltrasonic Digital Delay Lines, Z E E E T r a n s . S o n i c s U l t r a s o n .,[33] A. H. Meitzler and E. 8.Sittig, Characterization of Piezoelec-Vol. SU-16, Jan. 1969, p. 2-10.

tri c Tra nsi ucer s IJsed in Ult,rasonic Devices Operating Above0.1GHz., J . A p p l . P h y s . , Vol. 40, Oct. 1969, pp. 4341-4352.[34] T. M. Reeder and D. K. Winslow, Characterist ics of Micro-waveAcoustic Transducers forVolume WaveExcitation,Z E E ETra n s . Micro wa ve ThemuTech. , Vol. MTT-17, Nov.

355-371.

1969, pp. 927-943;[35] G . L. Matthaei, Tahles of Chebyshev Impedanc e Transform -ingNetworks of Low-Pass FilterForm, Pro c .Z E E E , Aug.[36] J. Randolph and J. Morrison, Modulation Transfer Character-1964, pp. 939-963.ist ics of an Acousto-Opt!: Deflector , A p p l . Optics, Vol. 10,1971, pp. 1383-1385. Rayleigh-Eqnivalentesolution ofAcousto-OpticDeflectionCells A p p l . O p t i c s , Vol. 10, 1971,1371 D. A. Pinnow, Acousto-Optic Light Deflection: Design Con-sideration for First Order Beam Steering Transducers, Z E E E[38] E. K. Sittig, Elasto-Optic Light Modulation and Deflection,Tra ns . So nic s U l t ra so n. ,Vol. SIJ-18,Oct. 1971, pp. 209-214.in P T O U T ~ S Sn Ontics Vol. X. Wolf. Ed.. North-Holland Puhlish-

pp. 1453-1454.

ing Co. Amsterd am, 1972.[39] N. Uchida and Y. Ohmachi, Elastic and Phot,oelastic Proper-ties of TeOz Single Crystal , J . A p p l . P h y s . , Vol. 40,Nov. 1969,pp. 4692-4695.[40] T. Yano and A. Watanabe, AcoustEOptic Figure of Merit ofTeOl fo r CircularlyPolarizedLight, J . A p p l . P h y s . , Vol. 45,[41] A. W. Warner, D. L. White, and U. A. Ilonner, Acousto-OpticMar. 1974, pp. 1243-1245.Light DeflectorsUsingOptical Activityn aratellurit.e,[42] W. H. Watson arid R. Adler, Cascading Wide-Band Acousto-J . A p p l . P h y s ., Vol. 43, Nov. 1972, pp. 4489 449 5.Optic Deflectors, IE E E Conf. Laser Engineering and Applica-[43] I. C . Chang and D. L. Hecht.,DoublingAcousto-Optic De-tions, Washington D. C. lune 1969.flector Resolution Iltilizine Second Order Rirefrinecnt Diffrac-

, .

~ ~~~~ ~tion, A p p l . Phys . Le t t . ,v$. 27, Nov. 1975.144) R. W. Dixon and E. I. Gordon,Acoustic Light ModulatorsUsingOpticalHeterodyne Mixing, B . S . T . J . , Vol. 46, Feb.[45] D. Maydan, Acousto-Optical Pulse Modulators, J . Qua ntum1461 F. Okolicsanvi. The Wave-Slot. An OuticalTelevision Svs-

1967, pp. 367-389.E l e d r o n . , Vol. QE-6, Jan. 1970, pp. 15-24.. -, km , Wire le&Enq r . , Vol. 14,Oct.; 1937, p 527-536.[47] D, M. Robinson, The upersonic Light 8ontr ol nd it s Appliia-tion to Television with Spatial Reference to the ScophonyTele-vision Receiver, P r o c . I R E , Vol. 27, Aug. 1939, pp. 483486.[G] S. E. Harris, S. T. K . Nieh, and D. K. Winslow, ElectronicallyTuna ble Acousto-%tic Filter. A m . Phus . Le t t . . Vol. 15. Nov., L I1969, pp. 325-326.[49] S.E. Harris, S. T. K. Nieh,andR. S. Feigelson,CaMoO,Electronically Tunable Optical Filter, A p p l . P h y s . Lett., Vol.17,Sept. 1970, pp. 223-225.1501 S. T. K. Nieh and S. E. Harris, Aperture-Bandwidth Char-acteristics of Acousto-htic Filter. J . O D [ .Soc. A m . . Vol. 62.May 1972, pp. 672476.

[51] J. A. Kuste rs, D. A. Wilson, and D. L . Hammontf, OptimumCrystalOrientation forAcoustically Tuned Opt,ic T%lt,ers,J .Opt . S o c . A m . ,Vol. 64, Apr. 1974, pp. 434340.[52] I. C. Chang, Tunable Acousto-Oztic Filter IJtilizing AcousticBeam Walkoff inCrystalQuartz, A p p l . P h y s .Le t t . , Vol. 2 5 ,[53] I. C.Chang,NoncollinearAcousto-OpticFilterwithLargeAngular Aperture, A p p l . P h y s .Le t t . , Vol. 25, Oct. 1974, pp.370-372.[54] J. D.Feichtner, M. Gottlieb,and J. J. Conroy, A TunableCollinear AcousttrOpticFilt,er for t,he Intermed iate nfraredUsing Crystal TIAsSes, IE E E Conf. Laser Engineering and[55] D. A. Pinnow, I,. G. Von IJitert, A. W . Warner, and W. A.

Applications, Washington D.C.,May 1975.Bonner, Lead Molybdate:A Melt-Grown CryPtal With a HighA p p l . Phys . Le t t . , Vol. 15,Aug. 1969, pp, 83-86.Figure of MeritorAcousto-OpticDeviceApplications,[56] L. K. Anderson et al., An Experimental Read-only -IolographicOptical Memory, Sixtal Int. Quantum Electron. Co nf., Kyot,o,Japan, Sept. 1970.(571 I. Gorog, J. D. Knox, antl P. V . Goedertier, A Television-RateLaserScanner. I. GeneralConsiderations, R CA Rev . , Vol.

1581 I. Gorog, J. D. Knox , P. V. Goedertier, and I. Shidlovsky A33, Dec. 1972,pp. 623-666.TelevisionRat.eLaserScanner. 11. RecentDcvclopment,s,R C A R e v. , Vol. 33, Dec. 1972, pp . 667-673.[59] G. Hrbek an d W. Watson, A High Speed 1,aser AlphanumericSept. 1970.Generat,or, Electro-Optical System Design Conf., New York,[60] A. W. Warner and D. A . Pinnow,Miniature A4coust,o-Opt,icVol. &E-$],ec. 1973, p 1155-1157.Modu lato rs for Optical Communications. . Quantum Electron.,

[61] R. L. Abramsand&..A. Pinnow, EfficientAcousto-Opticmanium, Z E E E J . QuantumElectron. (Corresp.), Vol. QE-7,Modulator at 3.39 and 10.6 Microseconds n Cryst,allinc Ger-Mar. 1971, pp. 135-136.1621 I. C. Chang andG. Moradian, Frequency Modulat,etl Acousto-Optic bIoclulat.orsfor 10.6 pm 1,aser Comrnunicat,ions, Electro-1631 C. S. Tsai and S. K. Yao, Bragg Diffraction by Standing Ult,ra-Optics System Design Conf., San Fransisco, Nov.974.sonicWaves With Application tlo Opt,icalDemult,iplexing,J . A p p l . Phys. , Vol. 43, Dec. 1972, pp. 5081-5084. Intl Ek c lro nDm. Med ingTech.Dig es t , Washington, D. C. Dec. 1973, pp.22Q-222.1641 A. J. Demaria, R. Gagosx,antlG. Rarnard, Ultrasonic-Re-fraction Shut#ter for Optical Maser Oscillators,J . A p p l . P h y s . ,[65] L. E. Hargrove, R. J,. Fork, and M. A. Pollack,1,ocking ofVol. 34, Mar. 1963, pp. 453-456.He-Ne Imer Modesnduced hy Synchronousntracnvit,yModulation, A p p l . P h y s . Lef t . , Vol. 5 , .July 1964, pp. 4-5.[66] D. May dan , Fast Modulat,or for Ext.ract,ion of Int,ernal 1,ase.rPower, J . A p p l . P h y s . , Vol. 41, Mar. 1970, pp. 1552-15FiR.[67] R . B. Cheder, M. A. kar r , and J. E. Geusic, A n ExperimrntalandTheoretical St,udy of High RrprtitionRat.e &)-SwitchedNd:YAIG Lasers, Proc. Z E E R , Vol. 58, Dec. 1970, pp. 1899-1914.[68] M. G.Cohen,ASimpleHighlyEfficjentAcousto-Optic Q-Switch, IEEE Conf. Laser Engineermg ntl App lica tio~ ~s1691 0. It.Wood, R. l,. Abrams, and T. J. Rritleq Mode 1,ockingWashington, D. C. May, 3973.of a Transversely Excited Atmospheric Pressure COr L m c b r , A p p l . Phys . Le t t . , Vol. 17,Nov. 1970, pp. 376-37:;[70] I,. C.Foster, M. D.Ewy,andC. R. Crumlv, Laser ModeLocking by an External Doppler Cell, p p l . Phys. Lett . , Vol. C,,[ i l l D. Maydan, Mirromachining and Image Recording on ThinJan. 196.5, pp. 6-8.Films by Laser Reams R. S . T . J . , Vol. 50 , July-August, 1971,

Sept. 1974, pp. 323-324.

pp. 1%-1789.Dumping n a ModeLocked Laser, J. Q~mntumElectron.R. H.ohnson,Characteristics of Acoust,n-Optir: Cavity(corresp.)), Vol. QE9, Feh. 1973, pp. 2552.57.Simultaneous@Switching antl Modemckingn the C WD. 6.Kuizenga, D. W . hillion, T. Lund, antl A . E. Siegman,Nd:YAG Laser, Optics .Comrn..Vol. 9 , Nov. 1973, pp. 221-226.D. .J. Taylor, S. T. K. Neih, and T. W . Hansch,ElectronicTuning of a Dye Laser Using the Acousto-Opt,icFilter, A p p l .Phys. Z,eLt., Vo. 19,Oct. 1971, pp, 269-271.Compression Jsing Rragg Scat,tering hy Ult,rmonicWaves,M . B. Schulz, M . G. Holland, and 1, . Davis, ,Jr., Opl~ical PulseA p p l . Phys . Le t t . , Vol. 11,Oct,. 1967, pp. 237-?f0.J. H. Collins, E. G. H. Lean, and H. J. Phaw, Pnlse Compres-A p p l . Phys. Le t t . , Vol. I I , Oct. 1967, p t . 242-242.sion bv Bragg Diffraction of LightwithMicrowaveSound,M. P. Wenkoff and M. Katchkv. An ImurovedReatl-InTechnique For Opt,ical Delay T,ineCorrelat.ors,A p p l ie d O p f i c s ,Vol. 9,Jan. 1970, pp. 135-147.


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22 IEEE TRANSACTIONS ON SONICS ~ N D LTRASONICS, VOL. SW-23,NO. 1, JANUARY 1976[78]L. B. Lambert,Wide-Band, nstantaneous SpectrumAn- [81] D. L. Hecht,Acousto-OpticNonlinearities in Multi-Frequencyalyzers Em loyingDelay-LineLightModulators, I R E I n f . Acousto-OpticDiffraction, Tntl.Quantum ElectronConf.,Conv. Rec., fit. 6, Vol. 10, Mar. 1962, pp. 69-78.[79] R. W. Damon, W. T. Maloney, and D. H. McMahon, Ink:; [ 82 ] A. J. DeMaria and G. E. Danielson, Internal Laser ModulationSan Francisco, June 1974.

in Physical AcousticsVol. VII , W. P. Mason and R. N. Thurston,action of Light with Ultrasound: Phenomenand Applicat,ions, Vol. QE-2, July 1966, pp. 157-164.by AcousticLens-LikeEffects, I E E E J . Quantum Electron.[SO] D. L. Hecht, Broadband Acousto-Optic Spectrum Analyzers,Eds.,cademicress,ew York, 1970. (831 L. C. Foster, C . B.mmly, and R. L . Cohoon, A Highesolu-tion Linear Optical Scanner lls ing A Traveling Wave AcousticUltrasonicsSymp., Monterey, Calif. ,ov. 1973.ens, A p p l i e d Optics, Vol. 9, Sept. 1970, pp. 2154-2160.

II. Acoustooptic nteractions Between Guided Optical Wavesand Acoustic Surface Waves

RONAJ,l) V. SCHMIDT

Abskroct-Highly e5cient, largeandwidthcoustooptice-flectors have been demonstrated by using acoustic surface waves toWr ac t guided optical waves. The basic principlesf the acoustoopticinteractionbetween uided pticalwaves and acoustic urfacewavesarereviewed.Deviceparameters of thin-film acoustoopticdeflectors are discussed, and criteria are established for the effici-ency of the various thin-film deflectors. Current experimental resultsare reviewed.

T ERE HAS been a recent trend from bulk acousticand optical signal processing devices towards planardevices which respectively utilize acoustic surface wavesand guided optical waves. The mot iva tion for consideringplanar devices of this type is usually improved perform-ance, miniaturization, and the utilization of new dcvicetechniques which are unique to the planar configuration.For these reasons i t is natural to consider the acoustoopticinteraction bet,ween acousticsurfacewaves and guidedoptical waves for laser modulation and deflection. L7nceKuhn et al . [l] firstdemonstrated an acousticsurfacewave-guided optical wave deflector in 1970, a number ofdifferent deflector and modulatorconfigurabions have beenproposed and demonstrated . It is now clear th at planaracoustoopticdevicescanbemademuchmore efficientthan their bulk counterparts because diffraction limits onlong interaction length? and high-power density, presentwith bulk devices, are reduced for planar devices wherethe waves a re confined to the crystal surface. The planardevice also provides promise of greater transducer flexi-bility and more controlver phase-matching requirements.

Planar acoustooptic deflectors can be divided into twotypes. In thecase of the first, type the incident, light beamand thedeflected light beam are oth guided y the pticalwaveguide. Several versions of this ty pe of deflector have

Manuscript received July 18,1975.Theauthor is withBellTelephoneLaboratories,Holmdel, Nd07733.

been reportcd [1]--[11], These deflectors ar r analogous tbulk defl&ors. In t hccase o f the second type of deflectorwhich was first proposed by Chang [la], the acousticsurface wave deflects the incident guided light out of thewaveguide int.0 an unguided optical beam called a radia-tion modr. 1 3 y varying the frequency of the sound wave,the angle a t which the light beam escapes from the wave-guide j s changcd.This type of deflector has also beendemonstrated [lS].

In this paper we nil1 describe and compare the variousplanar acoustoopt,ic deflector configurations which haveh e n demonstmtrd. Also, the uniquefeatures of planaracoustooptic cleflectorswill be discussed. The complexnature of t,hc acoustic surface wave-guidcd opt(ica1waveintrracttion due to nonuniform optical and acoustic fielddist8rihut,ionsxi11 be investigated so tha t he variousplanar deflectors can he compared and simple guidelinescan he established for obtaining efficient interact,ions.

The acoustooptic nteraction between acoustic surfacemves and guided optical wave differ from that o f bulkdeflectors primarily because of t,he conlplicated straindistributions of t,he acoustic surface wave and because o fthe guidedwaveproperties of the optical wave. Theefficiency of an acoustic surface waveguided optical waveinteraction depends strongly on optical waveguidc prop-erties and the acoustic wavelength. In order to establishsome guidelines for an efficient interaction and to definean acoustooptic figure of merit for planar deflectors, it isnecessary to discuss the interaction in detail and reviewsome basic properties f acoustic surface waves and guidedoptical waves. We will primarily consider a simplr isotropicslah optical waveguide illustrated in Fig. 1 a) . It consistsof a substrate of index n,*, nd a waveguide of thicknessh a,nd index ?zJ. The cover index n, is taken to be that offree space, i.e., n., = 1.It is further assumed that subst rateand wavcguide form an isot,ropic homogeneous elasticsolid. With this simple model it will be possible to illus-

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Acoustooptic Devices and Applications