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Optics Letters

Data compression in frequency-selective materials using frequency-swept excitation pulses

Felix R. Graf, Bernd H. Plagemann, Eric S. Maniloff, Stefan B. Altner, Alois Renn, Urs P. Wild

Published Feb 15, 1996• 12 citations
Publisher

Abstract

We demonstrate the temporal compression of photon echoes in frequency-selective materials by application of frequency-swept excitation pulses. Experimental results in Pr(3+):Y(2)SiO(5) for two- and three-pulse photon echoes are presented and compared with theory. A possible application to temporal reduction of optical data streams is shown.

DOI
10.1364/ol.21.000284

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Ryuzi Yano, Naoshi Uesugi

We demonstrated partial erasing of temporal optical data on a picosecond time scale by use of accumulated photon echoes. In a photon-echo memory the temporal data at time tau are stored as a frequency grating of period 1/tau in the absorption spectrum of a material. Partial erasing of the temporal data is achieved by irradiation of the material with a pulse pair at time separation tau , but the phase of one of the pulses is shifted 180 degrees . This process does not produce a frequency grating, and no echo is emitted. The experiment was performed with a dye-doped polymer film. The 180 degrees phase shift was obtained by use of a half-wave plate.

Optics Letters•Dec 1, 1999•5 citations
We demonstrate the temporal compression of photon echoes in frequency-selective materials by application of frequency-swept excitation pulses. Experimental results in Pr(3+):Y(2)SiO(5) for two- and three-pulse photon echoes are presented and compared with theory. A possible application to temporal reduction of optical data streams is shown.
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).
In recent years possible applications of the photon echo process in optical processing, data storage systems, bit-rate conversion of optical pulses, time compression of optical pulses etc. have been investigated.Much of this research aim at replacing electrical components in the framework of today's information networks with components that are entirely based on optics.It is likely that we in the future will see more and more of the electrical components linked to a fibre network being replaced by all optical components, e.g.routers and bit-rate converters.The concept of temporal compression of optical pulses, which is the subject of this thesis, could be used to increase the transmission rate through optical fibres and thereby use the full capacity of today's fibres.It should also be mentioned that more basic components, e.g.XOR gates, are studied in this research field and these components could possibly be used in future concepts of a "light-computer".In this thesis a theoretical model of time compression of both single optical pulses and optical pulse sequences using photon echoes is presented.A full analytical treatment was done for gaussian shaped pulses as a first step of building a numerical model.An analytic approximate approach was also used for both square pulses and pulse sequences.A theoretical understanding of the duration, the intensity spectrum and peak intensity of the compressed signals was reached.It is shown that the intensity spectrum of a compressed pulse sequence containing N bits is completely analogous to the pattern observed in the well-known Fraunhofer diffraction experiment with N slits.A numerical model was built and used to evaluate how non-linearities and noise affect the degree of compression.In addition theoretical models were developed to explain the effects of non-linearities and noise and the hope is that these might be used as guidelines on how to design future experiments in order to achieve highest possible degree of compression.Some of the results presented in this thesis is compared with experimental data obtained in the research group during the time this work was going on.
Single pulse compression and multi-bit pulse compression using the photon echo process was investigated.A single pulse could be compressed by a factor of 454 from 10 microseconds to 22 nanoseconds.A 1 O-bits pulse train was compressed by a factor of 92 from 20 microseconds in total length to 212 nanoseconds.The compression factors obtained during this work were an order of magnitude better than those published previously both for single pulse and multi-bit pulse train compression.Observation of the side-band under the envelope of the photon echo signal in multi-bit pulse compression indicates that the bit information is retained in the compression.Some improvements were achieved by optimising the experimental setup during the work.The experimental results were discussed theoretically and were good in agreement with theoretical calculations.The space-time duality of photon echo compression was also analysed in this thesis.The storage material used in the experiment is a Y AG crystal doped with the trivalent rare earth thulium ion with a concentration at 0.1-at.%.The crystal was kept at 4K during the experiment.The absorption, an important characteristic of the crystal was initially measured.The maximum absorption was about 40%, in the range between 7931.50A and 7931.60A with a full width ofhalfmaximum of3.5 A, which corresponds to 19 GHz.A homemade external cavity diode laser equipped with an intra-high coherency and ultrahigh-speed frequency chirping properties was used in the experiments.This system with an intra-cavity electro-optic crystal (EOC) was employed for the optical data compression because of its advantages of simplicity, high quantum efficiency, low cost, and compact size, which is necessary for practical applications in the future.Some critical properties for this laser were investigated.The laser line-width was measured to be 350 KHz over a 200 microseconds duration using the spectral hole burning method and a voltage-chirping rate of about 7 MHz/V was determined by calculations and experimental measurements.
It is predicted that linearly chirped optical excitation pulses give rise to photon echoes whose shortness is limited only by the total material bandwidth that contributes to the echo signal. The photon echo process acts as a frequency-dependent optical delay line whose dispersion is determined by the chirp rate of the excitation pulses rather than by their bandwidth. Consequently, long and short pulses can be compressed with equal facility without complicated alignment procedure. Accumulated echoes have similar properties and can be used with trains of weak excitation pulses.
We demonstrated partial erasing of temporal optical data on a picosecond time scale by use of accumulated photon echoes. In a photon-echo memory the temporal data at time tau are stored as a frequency grating of period 1/tau in the absorption spectrum of a material. Partial erasing of the temporal data is achieved by irradiation of the material with a pulse pair at time separation tau , but the phase of one of the pulses is shifted 180 degrees . This process does not produce a frequency grating, and no echo is emitted. The experiment was performed with a dye-doped polymer film. The 180 degrees phase shift was obtained by use of a half-wave plate.