1996 International Topical Meeting on Optical Computing, Sendai, Japan, April 21-26.

1

Nonlinear joint transform correlator with a multiple quantum well

photorefractive device

S. A. Boothroyd, A. David, S. Chang, P. Palacharla, J. Chrostowski

National Research Council
Institute for Information Technology Ottawa, Canada K1A 0R6

Abstract. A nonlinear joint transform correlator incorporating a GaAs/GaAlAs MQW device is described and demonstrated for face recognition. The correlator emphasizes high frequency components in the input and shows good tolerance to variation in facial expression.

We report here a joint transform correlator (JTC) that uses a semiconductor holographic element in the frequency plane and show results of its use in a real-time face recognition system. The holographic element is a GaAs/GaAlAs multiple quantum well (MQW) structure and is shown schematically in figure 1. The structure was made semi-insulating by ion implantation. Incident light used to write a grating, with photon energy higher than the band gap
energy, is absorbed generating
free carriers. These move under
the influence of an electric field
applied between the transparent
electrodes and act locally to
screen the applied field. The
dielectric layers allow a variation
in applied field across the
structure which follows the
incident light pattern. The
variation in the absorption
coefficient around the band edge
is shown in figure 2 for a range of
voltages applied across the MQW
layers. Illuminating the structure
at the band edge wavelength of ~ 852 nm detects the large difference in absorption between the no field and high field regions. This absorption variation corresponds to an index change of several parts in 100 [1]. Thus although the MQW element is only a few microns thick the induced index change is large enough to produce strong light driven diffraction effects. The MQW element offers a compact laser diode based nonlinear JTC with high spatial resolution [2,3]. A grating can be written and erased in ~ 10 ms giving the potential for up to 105

correlations per second [2].
We used a liquid crystal spatial
light modulator to input an amplitude modulated image to the JTC. The joint power spectrum and its carrier interference pattern at 830 nm contained a few mW of power at the MQW device for an average intensity of ~ 30 mW/cm2. A laser beam at 852 nm and ~ 100 mW was used to read-out the gratings. The saturation intensity in the

ITO transparent electrode

ITO transparent electrode

Y O
2 3

Y O
2 3

dielectric

dielectric

gold pad

h+

h+

e-

e-

sapphire substrate

well

barrier

10 nm GaAs

150 nm AlGaAs etch stop layer

155periods

.29 .71
3.5 nm Al Ga As

approx.3mm

Figure 1. Schematic of MQW photorefractive device.

3000

6000

9000

12000

15000

820 840 860 880

wavelength, nm

absorptioncoefficient,/cm

0 V

20 V

30 V

10 V

Figure 2. Absorption characteristics of the MQW.