Multifrequency paratellurite acoustooptic modulators

M. G. Gazalet, S. Carlier, J. P. Picault, G. Waxin, and C. Bruneel

A different acoustooptic configuration using Yano's paratellurite crystal cut is proposed in this paper. Bothconfigurations are compared in terms of relative bandwidth, efficiency, and potential applications. Experi-mental results are also reported.

Paratellurite (TeO2), presented first in 1968 by Arltand Schweppel is a very attractive acoustooptic mate-rial, mainly due to the existence of a very slow shearacoustic wave in the (110) direction.l 2 The first at-tempt to realize a TeO2 deflector with tangential phasematching3' 4 in a symmetrical configuration led to a dipin the efficiency vs frequency curve due to midbanddegeneracy. 5 In 1975, Yano et al.

6 proposed a newnonsymmetrical crystalline cut, which is free from thetwo main disadvantages of the previous symmetricalcut, i.e., the dip and the need for circular polarizationof the incident light. But it conserves the principaladvantage of the TeO2 , i.e., the very high figure ofmerit (1200 X 10-15 m3 /kg)7 for the slow acousticshear wave propagating along the (110) direction andpolarized in the (110) direction and the use of tangen-tial phase matching. This allows the realization of alow-frequency large-bandwidth device. Since the rel-ative bandwidth is very large, the main practical diffi-culty is to obtain a good transducer matching to agenerator over the whole bandwidth.

We propose and discuss a different use of Yano'scut,'6 which leads to the same acoustooptic bandwidthat a higher central frequency. The relative bandwidthis therefore smaller, the transducer matching is madeeasier, and the overall device bandwidth is no longerlimited by the transducer. This leads however to anincrease in the acoustic attenuation due to the higherfrequencies involved. The proposed configuration istherefore of limited interest as an acoustooptic deflec-tor; however, it may be used as a multifrequency acous-

J. P. Picault is with Automates et Automatismes, 19 rue de Paris,78460 Chevreuse, France; the other authors are with Universite deValenciennes, Laboratoire Opto-Acousto-Electronique U.A. No.832CNRS, 59326 Valenciennes CEDEX, France.

Received 13 June 1985.0003-6935/85/244435-04$02.00/0.

) 1985 Optical Society of America.

tooptic modulator8 9 since the transit time is usuallyvery small in such devices and the acoustic attenuationthen has only negligible effects. Experimental resultswill also be presented and compared to the theoreticalpredictions.

The basic idea is to use a different incidence anglefor the input light to the acoustooptic device, with boththe acoustic and diffracted light wave vectors remain-ing in the same directions as in the Yano configuration.This is shown in Fig. 1. Figure 1(a) shows the Yanointeraction diagram and Fig. 1(b) the proposed dia-gram. In both cases ki, kd, and K represent the opticalincident and diffracted and the acoustic wave vectors,respectively; Ha is the angle between the acoustic wavevector and the (110) axis.

In both configurations, the theoretical acoustic fre-quency bandwidth Af remains identical. If one con-siders only one diffracted beam, this is given by

Af _ 2v OW , AO X (1)

where v is the acoustic velocity, n, is the ordinaryrefractive index, Xv is the optical wavelength in vacu-um, W is the transducer width in the light propagationdirection, AO,0 and A0 are the allowed maximum phasemismatches'0 at extreme and central frequencies, re-spectively. In what follows, we shall assume AO, q500.799-x for a 3-dB bandwidth with high efficiency inter-action and A/0, 0.3237r for a 10% dip at the centralfrequency due to phase mismatch.

The central frequencies f+ and f have differentvalues for the two interactions. If optical activity isneglected, these can be approximated to

Ufle 2 2 (l e n a)\f ___ Xn ~ne n. 1 + n 0aJ (2)

where n, is the extraordinary refractive index, and the+ or - suffixes refer to, respectively, the proposed andYano configurations.

The exact values for f±, taking gyrotropic effects intoaccount, were also calculated numerically. These re-

15 December 1985 / Vol. 24, No. 24 / APPLIED OPTICS 4435

001MHz

K

a 110

b 110

Fig. 1. Interaction wave vector diagrams: (a) Yano configuration;(b) proposed configuration.

sults are shown in Fig. 2 together with the approximatevalues given by Eq. (2). (A 0.6328-Am wavelength wasassumed in calculations.) The results are in goodagreement for values of Ga larger than a certain valueGa > 3° or (a > 6° for + and - cases, respectively, i.e.,when the optical wave vectors are far enough from theoptical axis.

In tangential phase matching, the central frequencyand the bandwidth are independent of each other [thisis also clear from Eqs. (1) and (2)]. The importantdesign parameter for the acoustooptic device is usuallythe bandwidth which can be adjusted by a properchoice of the transducer width. One must then choosethe central frequency for easy transducer matching inthe whole bandwidth, and this value is controlled bythe crystalline cut (angle Ga). From Fig. 2 it is seenthat two different angles Ga lead to the same operatingcentral frequency according to the retained configura-tion. The solution Ga corresponding to the plus sign(+ configuration) is smaller than the other and is to bepreferred for efficiency considerations. The increaseof the acoustic velocity v with this angle Ga (Ref. 2)makes the figure of merit M2 a rapidly decreasingfunction of a. Figure 3 shows the behavior of M 2relative to its maximum value (1200 X 10-15 sec3/kg atGa = 00) vs 0 a. It appears that, for a maximum loss of10% in figure of merit, i.e., a 10% increase in acousticdriving power to maintain the same efficiency, theangle Ga should not exceed 5°.

For this upper limit Ga = 5, the central operatingfrequencies at a 0.6328-gum optical wavelength are

f = 113.9 MHz and f = 56.6 MHz.

At the same time, the acoustic frequency bandwidth is

Af =-MHz,1W

where W is expressed in millimeters.For a practical value of W lying between 1 and 2 mm,

i.e., Af lying between 58 and 82 MHz, it is obvious thatit will be much easier to handle with the + interactionrather than the - interaction, since the electricalmatching will be easier in the first case.

Another slight advantage is the better optical cou-pling of a linearly polarized incident light to the ellipti-

Fig. 2. Central operating frequencies vs acoustical angle Oa:...... + configuration neglecting optical activity; -, - configura-

tion neglecting optical activity; - - -- -, + configuration withoptical activity; --- --configuration with optical activity.

Fig. 3. Reduced figure of merit M 2/M2 max vs acoustical angle 00.

cally polarized eigenmode of propagation inside thecrystal. This is optimal when the incident light polar-ization is parallel to the major axis of the ellipse. Thefraction X of the incident light intensity coupled to theeigenmode is calculated as a function of (3a for both +and - interactions and is presented in Fig. 4 for X =

4436 APPLIED OPTICS / Vol. 24, No. 24 / 15 December 1985

- b

E f f ic iency

0.85-

0 J

Fig. 4. Coupling factor of incident light intensity vs acousticalangle 0a: -, -configuration; ---- -, + configuration.

Efficiency

a

0.5-1

0

0 4 I I I I 1.6

Fig. 5. Numerical results. Efficiency vs reduced frequency: (a) -

configuration central frequency is 55 MHz; (b) + configurationcentral frequency is 110.8 MHz.

8'4 150Fig. 6. Experimental result; efficiency vs acoustical frequency.

0.6328 im. For Ga = 50, X_ = 0.94 and X+ = 0.99, i.e.,the use of the + interaction results, in this case, in aslight increase (5%) of the useful light intensity.

A further difference between the two configurationsis that the second-order diffracted light beam appearsin the low frequency limit for the + interaction and inthe upper limit for the - interaction, the central fre-quency for this dip being the same in both cases: (f+ +f-)/2.

A classical numerical computation, based on cou-pled wave mode analysis and considered five orders(for -1 to +3), has shown that, for W = 1 mm, onlyorders 0, 1, and 2 may have significant values. Thenumerical results for first-order diffraction efficiencyvs reduced frequency are shown in Figs. 5(a) and (b)for, respectively, the - and + configurations of acous-tooptic interaction. The central frequencies are 55and 110.8 MHz for, respectively, the - and + interac-tions.

It is clear that one may not take advantage of thewhole frequency bandwidth for the - interaction sincethe reduced frequency bandwidth Af/f0 is larger thanunity.

Since the two configurations, - and +, cannot beobtained with the same transducer thickness, we haveonly verified experimentally the bandwidth of the +interaction. Figure 6 shows the experimental resultsfor a 0.6-mm light beam diameter inside the crystalwith a W = 1-mm transducer width. The experimen-tal bandwidth is 66 MHz and is in good agreement withthe theoretical predictions (68 MHz) of the numericalcalculations of Fig. 5(b).

The acoustooptic configuration proposed here is notto be preferred to Yano's for laser beam deflection,owing to the higher acoustic attenuation. It is howev-er a good choice for multifrequency modulating pur-poses. The central frequency is higher than Yano's sothe device bandwidth may be enhanced by reducingthe transducer width W without being limited by elec-trical matching problems. Another slight advantageis due to the higher incidence angle: the incidentlinearly polarized beam is more efficiently coupled tothe elliptically polarized eigenmode of light inside thecrystal (lower ellipticity). The experimental resultsshow the feasibility of this configuration and confirmthe theoretical predictions.

15 December 1985 / Vol. 24, No. 24 / APPLIED OPTICS 4437

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References

1. G. Arlt and H. Schweppe, "Paratellurite, a New PiezoelectricMaterial," Solid State Commun., 6, 783 (1968).

2. Y.Ohmachi, N. Uchida, and N. Niizeki, "Acoustic Wave Propa-gation in TeO2 Single Crystal," J. Acoust. Soc. Am. 51, 164(1972).

3. R. W. Dixon, "Acoustic Diffraction of Light in Anisotropic Me-dia," IEEE J. Quantum Electron. QE-3, 85 (1967).

4. E. G. H. Lean, C. F. Quate, and H. J. Shaw, "Continuous Deflec-tion of Laser Beams," Appl. Phys. Lett. 10, 48 (1967).

5. A. W. Warner, D. L. White, and W. A. Bonner, "AcoustoopticLight Deflectors Using Optical Activity in Paratellurite," J.Appl. Phys. 43, 4489 (1972).

6. T. Yano, M. Kawabuchi, A. Fukumoto, and A. Watanabe, "TeO2Anisotropic Light Deflector Without Midband Degeneracy,"Appl. Phys. Lett. 26, 689 (1975).

7. T. Yano and A. Watanabe, "Acoustooptic Figure of Merit ofTeO2 for Circularly Polarized Light," J. Appl. Phys. 45, 1243(1974).

8. D. L. Hatch, T. Mannigel, J. Rieden, and M. Silver "WidebandMultifrequency Recording Using Acoustooptics," in 1973 Elec-trooptic Systems Design Conference Proceedings, New York,Sept. 18-20 1973, p 112-116; available from Ind. & Sci. Conf.Manage., Inc., Chicago, Ill., 1973.

9. D. L. Hetch, "Multifrequency Acoustooptic Diffraction," IEEETrans. Sonics Ultrason. SU-24, 7 (1977).

10. M. G. Gazalet, G. Waxin, J. M. Rouvaen, R. Torguet, and E.Bridoux, "Independent Acoustooptic Modulation of the TwoWavelengths of a Bichromatic Light Beam," Appl. Opt. 23, 674(1984).

Two mirrors were fabricated and oriented in a vertical plane, with no signs ofdeterioration after more than 6 months. When not in use, they are coveredwith a nontransparent plastic sheet. Their final weight was -140 kg each.

This work was done by Richard T. Schneider, Ulrich H. Kurzweg, and JohnD. Cox of the University of Florida for Langley Research Center. Refer toLAR-13139.

Camera for monitoring vegetationA video camera uses solid-state imaging devices and light filters to bring out

subtle spectral differences between healthy and stressed vegetation, differ-ences not readily detectable with infrared film cameras. The camera employstwo detector arrays. The images falling on them may come through a singlelens or two separate lenses, but in either case through two different opticalfilters (see Fig. 12). The same scene is thus imaged in two wavelength regions.

Il l

Fig. 11. Mirror is rotationally cast after it has been dynamicallybalanced on the drive shaft.

This combination is next placed onto a rotating system consisting of a heavystationary table (Fig. 11) and a rotating vertical shaft. The shaft is held bybearings and is driven by a variable-speed electric motor. The frame/foamcombination is dynamically balanced on top of the drive shaft and is set intorotation at a constant angular velocity, which is monitored electrooptically.The rotation rate can be kept constant to within 1% over periods as long as 8 h.The rotation period for the constructed mirrors was 10.4 sec, which is consis-tent with the formation of a 13.4-m focal-length parabolic surface made bypouring a liquid into the rotating parabolic dish.

The final parabolic mirror surface is fabricated by pouring liquid epoxy intothe spinning dish and letting the epoxy harden. Typical solidification timesare 5 h. The rotation is maintained for well over 8 h to ensure sufficienthardening. The final hardened surface is estimated to be within 1 mm of aperfect parabola over the entire mirror surface, with the exception of a fewlocalized indentations believed to be due to air bubbles trapped between thefiberglass and foam surface during fabrication. For proper solidification, therelative humidity has to be maintained at less than 70%. The last stage of thefabrication process involves placing strips of self-adhesive aluminized acrylicfilm onto the hardened epoxy surface. Since this surface has small curvature,there is no difficulty in applying 0.58-m wide strips of the film directly onto thesurface without wrinkling problems.

DETECTOR ARRAY 2

ONE LENS WITHBEAM SPLITTER

DETECTOR ARRAY 1 DETECTOR ARRAY 2

TWO LENSES WITHOUTBEAM SPLITTER

Fig. 12. Either of two optical systems can be used to implement thevideo camera.

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