Cryogenic gallium phosphide acousto-opticdeflectors

Ian Fuss and Darryn Smart

We present measurements of the acoustic intensity in a gallium phosphide acousto-optic deflector for the0.6-1.3-GHz frequency range and the 8-295-K temperature range. The data show a significant increase inthe available time aperture of this deflector as a result of cryogenic cooling.

Introduction

In a previous publication' it was shown that cryogeniccooling can increase the sample space of a large-bandwidth acousto-optic (AO) deflector. This demon-stration was based on experimental data measuredwith a tellurium dioxide AO deflector. Similar mea-surements made with a gallium phosphide AO de-flector are presented in this paper.

Gallium Phosphide Deflector Construction

An AO deflector was constructed by using the LI[110]acoustic mode of GaP. This deflector was designed tofunction over the 1-300-K temperature range. De-sign considerations similar to those previously de-scribed in Ref. 1 were used.

The principal difference in the design is that thethermal expansion coefficients of GaP and the LiNbO3transducer differ significantly (see Table I). Hence itis significant that the transducer bond did not fail inthe 4-300-K temperature range.

Gallium Phosphide Deflector Data

The data measured for this deflector by the acousticintensity analyzer described in Ref. 1 decays exponen-tially as a function of the distance from the trans-ducer and hence can be characterized by an attenua-tion coefficient a. The shape of the intensity curveand the magnitude of a are independent of the appliedelectromagnetic power. These facts indicate that har-

The authors are with the Electronics Research Laboratory,Communications Division, Signal Processing Group, P.O. Box1600 (Salisbury), South Australia 5108, Australia.

Received 6 July 1990.0003-6935/91/314526-02$05.00/0.© 1991 Optical Society of America.

monic depletion is not a significant loss mechanism inthe deflector for the temperature and frequencyrange over which measurements were made.2

If harmonic depletion is ignored, the measuredattenuation coefficient a is given by

a = aC, + Uath, (1)

where a,, is the attenuation coefficient from acousticscattering by lattice defects and ath is the attenuationcoefficient for acoustic scattering by thermal pho-nons.1

Figure 1 is a smooth-curve plot of the attenuationcoefficient as a function of temperature for the acous-tic frequencies 0.7, 1.0, and 1.3 GHz. These datapermit a simple interpretation. Since a.. is indepen-dent of temperature, the temperature dependence ofa is due to ath. At temperatures below 40 K ex

dominates ath; that is, a arThis model provides an insight into the frequency

dependence ffn of the room-temperature acoustic at-tenuation data plotted in Fig. 2. It is common practiceto use the magnitude of the power n as a measure ofcrystal quality; a poor-quality material yields 1.2 <n < 1.4, and a high-quality material has n > 1.81.3The measured room-temperature acoustic attenua-tion data yield n = 1.52 ± 0.06. However, if the defectcomponent a,, plotted in Fig. 3, is subtracted fromthese data, a fit to the resulting data ath yields n =1.81 ± 0.09. Hence for this crystal the defects reducethe power n of the frequency dependence of a( f ) byadding a component aex ( f ) to a-th ( f)-

To specify the performance of an AO deflector it isuseful to specify a measure of the attenuation coeffi-cient a that is characteristic of the total frequencyband covered by the device. One such measure is themaximum value aLM across the band. The available

4526 APPLIED OPTICS / Vol. 30, No. 31 / 1 November 1991

Table I. Thermal Expansion Tensors E (parts in 10/K)

Material

Tensor GaP LiNbO,

El, 4.7 14.4E22 4.7 15.9E33 4.7 7.5

GaP L[110]

12-

10-

8-

6.

4.

010 100

Temperature (Kelvin)

Fig. 1. Temperature dependence of acoustic attenuation

GaP L[110]

Temperature = 295 K

14.

12.

10.

8.

6.

4.

0

24

cX

Cc th0

* 0* 0

00

0..

0.6 0.8 1.0

Frequencyf (GHz)Fig. 2. Total and thermal acoustic attenuation at room tempera-ture.

time aperture T of the deflector can then be specified:

RvaM

TrA =

Rif - < T

yaM

Rif - > T

VaM

GaP L110]

Temperature = 8 K

14'

12'

a 10'ma_ 8'

C:c0

V:2 '

n0.4 0.6 0.8 1.0

Frequency f (GHz)1.2 1.4

Fig. 3. Frequency dependence of acoustic attenuation at 8 K.

where v is the acoustic velocity and R is a measure ofthe required signal fidelity, e.g., for a 3-dB drop in

oao intensity, R = 3.' With this criterion the effectivetime aperture of the deflector is reduced to 0.5 [Lsby acoustic attenuation at room temperature, but at 8K the full aperture time of 3 ps is available. Thus thesample space of this deflector is increased by a factorof 6 by cryogenic cooling.

The frequency trend of the acoustic attenuationdata at 8 K, which is plotted in Fig. 3, fosters the hopethat larger-bandwidth GaP AO deflectors can beconstructed while a large 3-ps time aperture ismaintained.

o 0

Summary

In this paper we presented experimental measure-ments of the acoustic attenuation in a GaP AOdeflector. These results show that at acoustic frequen-cies of 1 GHz the time aperture of a GaP AOdeflector can be increased by a factor of 6 by cooling to-10K.

We thank the United States Government for sup-porting this program, Crystal Technology for con-structing the GaP deflector, Jo Cockayne for typing

1.2 1.4 the manuscript, and Evelyn Bartlett for preparingthe figures.

References1. I. Fuss, "Cryogenic large bandwidth acoustooptic deflectors,"

Appl. Opt. 26,1222-1225 (1987).2. G. Elston and P. Kellman, "The effects of acoustic nonlineari-

ties in acousto-optic signal processing systems," UltrasonicsSymp. Proc. 1, 449 (1983); R. N. Thurston, "Wave propagationin fluids and normal solids," in Physical Acoustics, 1, W. P.Mason, ed. (Academic, New York, 1964).

3. J. Rosenbaum, Bulk Acoustic Wave Theory and Devices (ArtechHouse, Boston, Mass., 1988).

1 November 1991 / Vol. 30, No. 31 / APPLIED OPTICS 4527

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