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Annual Meeting Optical Society of America

Pattern recognition by photon echoes

P. Talagala, M. K. Kim

Published Jan 1, 1991• 0 citations
Publisher

Abstract

We demonstrate, we believe for the first time, the capability of spatial pattern recognition by backward stimulated photon echo (BSPE) in a low temperature solid activated by rare earth ions. The BSPE experiment is done with three time-delayed input laser pulses. The first pulse (DATA) contains several characters in its beam profile, the second pulse (WRITE) is a plane wave, and the third (READ) has the search character. The echo pulse profile has peak intensity at the position of the search character. The experiment has a time scale of several tens of nanoseconds. Pattern recognition by BSPE is a result of the convolution/correlation property of holographic four-wave mixing geometry. In contrast to the photorefractive processes, however, the BSPE in rare earth-doped solids is a resonant coherent optical transient effect. By combining spatial and temporal holographic processes of BSPE, it would be possible to construct a high density, high speed optical processor of spatial and temporal data using BSPE.

DOI
10.1364/oam.1991.thl6

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We demonstrate, we believe for the first time, the capability of spatial pattern recognition by backward stimulated photon echo (BSPE) in a low temperature solid activated by rare earth ions. The BSPE experiment is done with three time-delayed input laser pulses. The first pulse (DATA) contains several characters in its beam profile, the second pulse (WRITE) is a plane wave, and the third (READ) has the search character. The echo pulse profile has peak intensity at the position of the search character. The experiment has a time scale of several tens of nanoseconds. Pattern recognition by BSPE is a result of the convolution/correlation property of holographic four-wave mixing geometry. In contrast to the photorefractive processes, however, the BSPE in rare earth-doped solids is a resonant coherent optical transient effect. By combining spatial and temporal holographic processes of BSPE, it would be possible to construct a high density, high speed optical processor of spatial and temporal data using BSPE.
Processing of two-dimensional images on a nanosecond time scale is demonstrated using the stimulated photon echoes in a rare-earth-doped crystal (0.1 at. % Pr(3+):LaF(3)). Two spatially encoded laser pulses (pictures) resonant with the (3)P(0)-(3)H(4) transition of Pr(3+) were stored by focusing the image pulses sequentially into the Pr(3+):LaF(3) crystal. The stored information is retrieved and processed by a third read pulse, generating the echo that is the spatial convolution or correlation of the input images. Application of this scheme to high-speed pattern recognition is discussed.
We demonstrate mixed binary multiplication of two or three numbers represented as spatial bit patterns by using a backward stimulated photon echo in a low-temperature solid activated by rare-earth ions. The photon-echo output image is the convolution-correlation of input images in a holographic arrangement, which can be interpreted as multiplication in a mixed binary format. The computation is extremely fast (~ 50 ns) and may be combined with temporal data processing for a versatile vector and matrix processor.
The phase conjugation property of backward-stimulated photon echo (BSE) has numerous potential applications in optical data storage and processing. We demonstrate mixed binary multiplication of spatially encoded bit patterns, by taking advantage of the fact that the geometry of BSE is analogous to that of conventional holography or four-wave mixing of nonlinear optics, where an object (data) and a reference (write) beams interfere to create a grating inside the medium, and another reference (read) beam is diffracted from that grating. When all three input beams contain structures, the output pattern is the convolution and correlation of the input patterns, which, with certain arrangements and interpretation, can be used for multiplication of binary numbers. The experiments were done by using a 477.8 nm transition of praseodymium ion-doped in single-crystal lanthanum trifluoride.
We present experimental results demonstrating the use of stimulated echoes for high-speed pattern recognition. One data image and two reference images (spatially encoded laser pulses) were stored and processed in a Pr(3+):LaF(3) crystal by focusing the pulses sequentially into the storage material. The processed information was later retrieved by a read pulse, generating two temporally separated echo images that are the spatial correlations between the data and reference images. Identification of the data image was accomplished by finding the echo image that showed the highest spatial correlation with the data image. The present processing speed is 30 ns per correlation operation.
Optical data storage has been interested in several decades because of the potential of fast access time and high-density storage. Recently volume holographic data storage technique has drawn much interest as new materials and improved techniques have developed.1 However, the large of hologram storage is restricted by low diffraction efficiency and mainly focused to read only memory because of the material's characteristic difficult to rewrite in short time interval. Another optical memory technique (photon echo optical memory)2 based on time-domain has developed since 1980's and made big progress for the area of real time image processing and dynamic random access memory. In this technique, data pulses interfere with timely separated write pulse (reference), and their interference pattern is written as population gratings. A read pulse (reference) retrieves each data pulse information as a four-wave mixing process. The write and read pulse separations are restricted by phase and population decay times of the material, respectively, and storage density is determined by the ratio of inhomogeneous to homogeneous linewidths. Even though photon echo technique has advantages of fast access time (≥ ns) and high-density (≥ Mbits/laser spot), its operating temperature is kept extremely low around liquid helium temperature. This is because that homogeneous decay time rapidly shortens as temperature increases due to phonon interactions, i.e., the ratio of inhomogeneous to homogeneous decay rates is ~ 1 at liquid nitrogen temperature (77 K) in a rare-earth doped solid, so that restricts its applications.