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Applied Optics

Recording density limit of photon-echo optical storage with high-speed writing and reading

Stefan Kröll, Per Tidlund

Published Dec 10, 1993• 20 citations
Publisher

Abstract

The first analysis to our knowledge of the optical data storage density of photon-echo storage is presented. Mainly considering signal-to-noise ratio performance, we calculate the obtainable storage density for data storage and processing using photon echoes to be approximately 100 times the theoretical limit for conventional optical data storage. This limit is similar to that theoretically calculated for data storage by use of persistent spectral hole burning. For storage times longer than the upper-state lifetime the highest densities can, however, be obtained only if all the excited atoms decay, or are transferred, to a different state than that from which they were originally excited. The analysis is restricted to samples with low optical density, and it also assumes that for every data sequence, writing is performed only once. It is therefore not directly applicable to accumulated photon echoes. A significant feature of photon-echo storage and processing is its speed; e.g., addressing 1 kbyte/(spatialpoint) permits terahertzread and write speeds for transitions with transition probabilities as low as 1000 s(-1).

DOI
10.1364/ao.32.007233

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The first analysis to our knowledge of the optical data storage density of photon-echo storage is presented. Mainly considering signal-to-noise ratio performance, we calculate the obtainable storage density for data storage and processing using photon echoes to be approximately 100 times the theoretical limit for conventional optical data storage. This limit is similar to that theoretically calculated for data storage by use of persistent spectral hole burning. For storage times longer than the upper-state lifetime the highest densities can, however, be obtained only if all the excited atoms decay, or are transferred, to a different state than that from which they were originally excited. The analysis is restricted to samples with low optical density, and it also assumes that for every data sequence, writing is performed only once. It is therefore not directly applicable to accumulated photon echoes. A significant feature of photon-echo storage and processing is its speed; e.g., addressing 1 kbyte/(spatialpoint) permits terahertzread and write speeds for transitions with transition probabilities as low as 1000 s(-1).
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.
Persistent spectral hole-burning and photon echoes can provide insight into the dynamics of amorphous and crystalline materials on the molecular/atomic level. Part of the current interest in these methods, however, is also caused by the existence of potential applications in optical data storage and processing. At present it may be difficult to specify exactly at what applications these techniques should try to target but, based on current research efforts and results, it appears that persistent spectral hole burning is a possible candidate for high density, possibly long time, volumetric storage, while photon echoes have properties more attractive for short time storage or storage connected to processing applications.
The noise characteristics of the photon echo memory have been investigated. The photon echo memory has the ability to store many bits of information in a diffraction-limited spot, thereby dramatically increasing the storage density. The temporal Fourier transform of the input data sequence is written into the inhomogeneous absorption profile of the recording medium. Data are encoded by means of temporal modulation onto the waveform of a finite duration data beam. Individual bits are not localized to a specific spectral channel; instead, they are stored throughout a region of spectral-addressing space, In order to store and recall the input data accurately, the Fourier transform of the input data sequence must be narrower than the inhomogeneous bandwidth. In the photon echo memory mechanism, there are several factors affecting the system bit error rate such as finite-width write/read pulses, echo efficiency, shot noise, thermal noise, etc. The accuracy of the echo output depends on those system factors. In this paper we formulate a simple model of the photon echo system, and by analyzing this model we derive the relationship between the characteristics of the echo output signal and several factors such as the bandwidth of the system, echo efficiency, atom excited state population fluctuations, shot noise, and thermal noise.
To determine the complete set of required materials properties for a viable frequency-domain optical-storage system based on persistent spectral hole burning, we have performed analytic and computer simulations of the writing performance, signal-to-noise ratio, and number of reads that can be obtained from single-photon or monophotonic (no threshold) hole-burning mechanisms with realistic spot sizes and reading and writing times. We find that acceptable reading performance occurs only for materials with high quantum efficiencies, low absorption cross sections, and high solubilities. This work for the first time clearly defines the parameter space of single-photon materials that can be utilized in a frequency-domain optical-storage system.
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.
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.