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Applied Optics

Optimization of time-domain storage density in the presence of excitation-induced spectral diffusion

Xiao A. Shen, Ravinder Kachru

Published Sep 10, 1997• 2 citations
Publisher

Abstract

We present a technique to reduce the loss of storage capacity in time-domain optical memory in the presence of excitation-induced spectral diffusion. We applied this technique to a Eu(3+):Y(2) SiO(5)-based memory that utilizes frequency chirp for recording-retrieval and successfully stored 860 bits of data in a 40-MHz data channel. In comparison with the conventional chirp method under identical experimental conditions, the present technique offers an increase of a factor of ~2 in storage capacity with no measurable loss of signal fidelity. The results are interpreted in terms of the phase error accumulated during dephasing-rephasing and spectral diffusion on excited-state relaxation.

DOI
10.1364/ao.36.006692

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We present a technique to reduce the loss of storage capacity in time-domain optical memory in the presence of excitation-induced spectral diffusion. We applied this technique to a Eu(3+):Y(2) SiO(5)-based memory that utilizes frequency chirp for recording-retrieval and successfully stored 860 bits of data in a 40-MHz data channel. In comparison with the conventional chirp method under identical experimental conditions, the present technique offers an increase of a factor of ~2 in storage capacity with no measurable loss of signal fidelity. The results are interpreted in terms of the phase error accumulated during dephasing-rephasing and spectral diffusion on excited-state relaxation.
We report a novel technique for storing digital optical data in coherent time-domain optical memory to circumvent the problem of coherent saturation. In this technique, the carrier frequency of a data stream to be stored is slowly chirped so its power is spread over the data bandwidth for maximum data storage. In a proof-of-concept experiment, a total of 500 bits of data were stored at a rate of 20 Mbits/s in a 45-MHz-wide channel within the inhomogeneous line of the (7)F(0)-(5)D(l) transition in Eu(3+):Y(2)SiO(5). The result suggests a minimum storage density of ~1.5 Gbits/cm(3) for this transition. Issues related to the storage of frequency-chirped data, such as the optimum chirp width and the saturation owing to peak-sidelobe interference, are also discussed.
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.
We demonstrate the impulse-equivalent approach [Opt. Lett. 21, 833 (1996)] to time-domain storage of optical data. Using the 128-bit real Huffman-Ackroyd code as the write-read sequences, we have successfully stored a 42-micros-long stream containing 420 bits of data in a 40-MHz spectral channel in a Eu(3+):Y(2)SiO(5) crystal. The stored data were repeatedly recalled as many as eight times with high signal f idelity, showing no measurable sidelobe-induced noise. Data retrieval with the use of mismatched write-read pulses was also examined, and implications of the results for data encryption are discussed.
We describe a practical approach to image storage in a coherent time-domain optical memory that can be readily implemented with existing technologies. In this approach, two-dimensional images are stored spectroholographically in narrow ( less, similar 1-MHz) frequency channels of a time-domain storage material by use of a low-power laser, with one image per channel. Advantages of this approach include fast single-frame recording time, variable playback speeds, and random frame access. Experimental results demonstrating the use of this approach for high-speed, long-term image storage in Eu(3+):Y(2)SiO(5) are presented.
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.
Swept-carrier time-domain optical memory [Opt. Lett. 17, 535 (1992)] has been proposed as a means of storing optical data streams in inhomogeneously broadened spectral recording materials. The process provides for full utilization of the material storage bandwidth regardless of the data rate and permits storage of data sequences with durations exceeding the material dephasing time. We report what we believe is the first experimental demonstration of this storage process. Our experiments, implemented with frequency-swept diode lasers and Tm(3+):YAG storage material, demonstrate single-event storage of 98-bit data streams whose duration is an order of magnitude longer than the material dephasing time.