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Optics Letters

248-Bit optical data storage in Eu^3+:YAlO_3 by accumulated photon echoes

Masaharu Mitsunaga, N. Uesugi

Published Feb 1, 1990• 65 citations
Publisher

Abstract

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.

DOI
10.1364/ol.15.000195

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The photon-echo memory[1], or the time-domain optical data storage, resembles the hole-burning memory [2] in that the inhomogeneous absorption line of a material is employed as a storage medium and the information is stored in it as a frequency-dependent absorption change with the resolution determined by the homogeneous width. Having the common memory capacity N given by the inhomogeneous-to-homogeneous width ratio, both types of memories share common problems like low-temperature operation, spectral diffusion and so on. Although a storage medium is common, storage techniques are definitely different for the two memories. The photon-echo memory utilizes the temporal interference effect between a reference pulse and a data pulse and their interference pattern is stored as a population grating in the frequency domain. In this sense this is a time-domain analog of holography where the spatial interference pattern between a reference beam and a data beam is stored in a medium as a population grating in the real space. Like the spatially diffracted beam from the read beam gives the information stored in the hologram, the “temporally diffracted pulse", or the photon echo, after the read pulse can give the information stored in the inhomogeneous distribution.

Spectral Hole-Burning and Related Spectroscopies: Science and Applications•Jan 1, 1991•1 citation
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.
We propose and demonstrate a novel type of frequency-selective optical memory that writes and reads the data in both the time domain and the frequency domain. Temporal 16-bit data were stored by accumulated-photon-echo bit-by-bit storage at 103 frequency addresses within the inhomogeneous line of the (7)F(0)-(5)D(0) transition of Eu(3+):Y(2)SiO(5), which yields a total memory capacity of 1.6 kbits in a single spot of 240-microm diameter. The keys to the success of this experiment are this material's long dephasing time and lack of spectral diffusion.
The backward stimulated echo has been considered as a novel high-speed high-density optical data storage method. But its potential application has been confined to transient dynamic memory because of relatively short lifetime of optical coherence inside low-temperature solids. Here we report the longest observed storage time of at least six hours in the 581.8 nm7F0-5D0 transition of eu 3+ -doped YAlO3 crystal at 2°K. The experiments are performed using YAG-pumped pulsed dye lasers, beamsplitters and optical delay lines to produce five DATA pulses, at 20 nsec intervals, and WRITE and READ pulses. The measured echo intensity as a function of storage time has exponential decay time of 3.8 hours. This very long storage time is due to the very small effective nuclear magnetic moments of Eu3+ ions, and is consistent with the long lifetime of spectral hole in this system. We also observed that the signal fidelity is not noticeably affected by the number of DATA pulses and the long storage time.
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.
Eu-doped yttrium orthosilicate (Eu(3+) : Y2SiO5) had been a subject being investigated for coherent time-domain optical memory and information processing applications since its ultraslow optical dephasing was discovered several years ago. In this crystal the weakly allowed (7)F0 - (5)D0 transition of europium ions exhibits a sufficient long dephasing time and no spectral difli.usion on a time scale of several hours at low temperature, thus an information pattern or data can be stored as a population grating in the ground state hyperfine levels. On the other hand, the study on photon-echo relaxation shows that the dephasing time T2 of Eu (3+) and other rare-earth ions doped YAG, YAlO3 strongly depends on the intensity of the excitation pulses. In Eu (3+) :YAlO3, an exponential decay of photon-echo with T2 = 53 microseconds if the excitation pulses are weak (5 vJ/pulse) was observed. However, when the excitation pulses are strong (80 pJ/pulse) they observed a much shortened T2 with a highly nonexponential decay pattern. The conclusion they derived is that the intensity-dependent dephasing rate effects are quite general, and it depends on how much the excitation intensity varies. In this paper we use transient grating formation technique showing that a temporal lattice distortion may only occur along crystal c axis, caused by EU (3+) excitation. At high excitation level the produced exciton in conduction band may also couple to the dynamical lattice relaxation process, giving rise to an apparently much shortened dephasing time.
The photon-echo memory[1], or the time-domain optical data storage, resembles the hole-burning memory [2] in that the inhomogeneous absorption line of a material is employed as a storage medium and the information is stored in it as a frequency-dependent absorption change with the resolution determined by the homogeneous width. Having the common memory capacity N given by the inhomogeneous-to-homogeneous width ratio, both types of memories share common problems like low-temperature operation, spectral diffusion and so on. Although a storage medium is common, storage techniques are definitely different for the two memories. The photon-echo memory utilizes the temporal interference effect between a reference pulse and a data pulse and their interference pattern is stored as a population grating in the frequency domain. In this sense this is a time-domain analog of holography where the spatial interference pattern between a reference beam and a data beam is stored in a medium as a population grating in the real space. Like the spatially diffracted beam from the read beam gives the information stored in the hologram, the “temporally diffracted pulse", or the photon echo, after the read pulse can give the information stored in the inhomogeneous distribution.