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Applied Optics

Many-bit optical data storage using stimulated echoes

Myung-Keun Kim, Ravinder Kachru

Published Jun 15, 1989• 13 citations
Publisher

Abstract

We demonstrate faithful storage and retrieval of twentyfive optical data pulses by backwards timulated echoesi n a Pr(3+):YAG crystal using an acoustooptically modulated ring laser.
DOI
10.1364/ao.28.002186

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Optics Letters•Apr 1, 1991•27 citations
We propose a novel approach to time-domain storage of serial optical data. In this approach, two identical impulse- equivalent sequences are used as write and read pulses to store and later retrieve optical information. These pulse sequences are both amplitude- and phase-modulated in such a way that their autocorrelations resemble, as closely as theoretically possible, the autocorrelation of a single large brief pulse to ensure faithful data retrieval. Such impulse-equivalent sequences are known to exist for all lengths, permitting the storage of large bandwidth data. In a proof-of-concept experiment, we have successfully stored a 42-microsecond long stream containing 420 bits of data in a 40-MHz spectral channel in a Eu3+:Y2SiO5 crystal. The stored data was repeatedly recalled for up to eight times with high signal fidelity, showing no measurable sidelobe-induced noise.
The successful storage and retrieval of 248-bit temporal optical data in Eu(3+):YAlO(3) by accumulated photon echoes is reported. Owing to the long storage time of this crystal, the input data can be written bit by bit by quickly varying the separation of two laser pulses, while the stored data are recalled simultaneously after some storage period, which extends up to a few hours. The maximum data length, previously thought to be limited by the optical dephasing time T(2), has been found to be four times shorter than T(2) and attributable to spectral diffusion.
The diffraction-limited spot size of a focused laser beam puts an upper boundary on the storage densities in conventional optical data storage. This can be overcome by storing multiple bits of data in each spatial location. In materials with an inhomogeneously broadened absorption line, such as crystals doped with rare-earth ions, frequency can be used as an additional addressing dimension. In some materials the number of spectral channels can be as high as 107. Spectral data storage can be done using coherent transient interactions between light and atoms and can be described in terms of photon echo processes. In this Master's Project the swept-carrier time-domain (SCTD) approach, a hybrid between frequency- and time-domain approaches, is used. An overview of the theory is given, followed by a description ofthe set-up and equipment. A maximum of 330 bits of data have been stored in one spatial location. The necessary conditions for successful storage and retrieval of data have been examined and suggestions are given on how to extend the results further.
We propose a novel approach to time-domain storage of serial optical data. In this approach, two identical impulse-equivalent sequences are used as write and read pulses to store and later retrieve optical information. These pulse sequences are both amplitude and phase modulated in such a way that their autocorrelations resemble, as closely as theoretically possible, the autocorrelation of a single large brief pulse to ensure faithful data retrieval. Such impulse-equivalent sequences are known to exist for all lengths, permitting the storage of large-bandwidth data. The advantages of this approach include modest laser power requirement, high signal fidelity, and encrypted recording.
Using a frequency-chirped ring dye laser, we have stored and recalled as much as 48 bits of information with the copropagating stimulated-echo geometry on the (3)H(4)-(1)D(2) transition in Pr(3+):YAlO(3). Since the copropagating excitation scheme has all the excitation pulses spatially overlapping, this technique will be useful to the practical implementation of stimulated-echo memory because there is no need for alignment of beams. Our experiments also demonstrate that frequency chirping of the excitation pulses dramatically improves not only pulse-shape reproduction by the echo pulse but also the shot-to-shot stability of the amplitude of the recalled data.