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Lund reports on atomic physics

Multi-Bit Data Storage Using Photon Echoes

Mattias Nilsson

Published Jan 1, 2000• 2 citations
OpenAlex

Abstract

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.

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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.

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Time-and-space-domain holography [1,2] by stimulated photon echoes [3] provides a means of storing temporal and spatial properties of an ultrafast-varying light signal and recalling its authentic replica at any later moment. The possibility of recording complex time shapes of light pulses, which also means time-domain optical data storage, is based on the ability of frequency-selective absorbers to store with high accuracy the temporal Fourier components of the incident light [1-3]. The time-domain frequency-selective optical storage of binary or analog data is an alternative approach to the concept of frequency-selective high-density data storage that provides a parallel ultrafast data storage and processing in the frequency domain. The shortest detail of the signal to be stored is limited by the reverse inhomogeneous bandwidth of the storing media which reaches 10-100 fs for frequency-selective materials while the duration of the signal is limited by phase relaxation time T2 (usually 10-3-10-6s).

Spectral Hole-Burning and Related Spectroscopies: Science and Applications•Jan 1, 1992•0 citations
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.
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.
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.
Spectral hole burning and photon echoes are the techniques for frequency-selective optical memories, which have the potential to increase areal data storage density by a factor 10**7. They also have the capability to perform in-memory data processing. In this thesis, several materials and techniques for optical memories in the frequency- and time domain have been investigated. In the frequency domain, ways to increase the storage density and the working temperature were studied using photon-gated spectral hole burning materials of the metal-tetrabenzoporphyrin derivatives. A selection rule for the electron acceptors in the donor-acceptor electron transfer systems was deduced. By doping the material with two different donors a maximum inhomogeneous line width of more than 30 THz was achieved. A polarization holographic technique was established for the formation and detection of spectral holes. Our results indicate that this technique can provide a better signal-to-noise ratio than that achieved by a conventional holographic technique, in particular for samples of poor optical quality. In the time domain, magnetic field-induced and the intensity-induced dephasing processes were studied in Pr3+ doped in a YAlO3 crystal and a Y2SiO5 crystal, respectively. As a step along the route to make photon echo based optical processing possible, the amplification of photon echo signals by the use of a fiber amplifier (Pr3+-doped ZBLAN) was demonstrated. A gain of 45 was achieved. The erasure of stored data using photon echoes has also been investigated. A technique to diagnose the phase and frequency stability of a light source by the photon echo erasure process was proposed and illustrated. An approach to bit-selective data erasure, that is free from laser phase and frequency fluctuations, is also suggested. A concept for an arbitrary shape pulse generator based on photon echoes and hole-burning was experimentally and theoretically studied. Finally, a comparison between the properties and capabilities of spectral hole burning and photon echoes was made.
Time-and-space-domain holography [1,2] by stimulated photon echoes [3] provides a means of storing temporal and spatial properties of an ultrafast-varying light signal and recalling its authentic replica at any later moment. The possibility of recording complex time shapes of light pulses, which also means time-domain optical data storage, is based on the ability of frequency-selective absorbers to store with high accuracy the temporal Fourier components of the incident light [1-3]. The time-domain frequency-selective optical storage of binary or analog data is an alternative approach to the concept of frequency-selective high-density data storage that provides a parallel ultrafast data storage and processing in the frequency domain. The shortest detail of the signal to be stored is limited by the reverse inhomogeneous bandwidth of the storing media which reaches 10-100 fs for frequency-selective materials while the duration of the signal is limited by phase relaxation time T2 (usually 10−9-10−6s).
Time-and-space-domain holography [1,2] by stimulated photon echoes [3] provides a means of storing temporal and spatial properties of an ultrafast-varying light signal and recalling its authentic replica at any later moment. The possibility of recording complex time shapes of light pulses, which also means time-domain optical data storage, is based on the ability of frequency-selective absorbers to store with high accuracy the temporal Fourier components of the incident light [1-3]. The time-domain frequency-selective optical storage of binary or analog data is an alternative approach to the concept of frequency-selective high-density data storage that provides a parallel ultrafast data storage and processing in the frequency domain. The shortest detail of the signal to be stored is limited by the reverse inhomogeneous bandwidth of the storing media which reaches 10-100 fs for frequency-selective materials while the duration of the signal is limited by phase relaxation time T2 (usually 10-3-10-6s).