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Optics Letters

Continuous coherent transient optical processing in a solid

M. Zhu, William Randall Babbitt, C. Michael Jefferson

Published Dec 15, 1995• 23 citations
Publisher

Abstract

We report what is to our knowledge the first experimental demonstration of a continuous coherent transient optical processor. A 13-bit pattern was stored as a spectral population grating in the Eu3+-ion ground state in a Eu3+:Y2SiO5 crystal. A 3120-bit data stream was processed continuously, yielding a cross-correlation signal that agreed well with theory. The data stream’s duration exceeded both the absorbing transition’s homogenous transverse dephasing time and upper-state lifetime. This experiment showed that by permanently storing the pattern as a population grating in the absorber’s ground state, coherent transient optical devices provide continuous, real-time data processing capability.

DOI
10.1364/ol.20.002514

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Coherent Transient Continuous Optical Processing in a Solid

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When a sequence of temporally modulated optical waveforms illuminate an inhomogeneously broadened absorbing medium, the resultant optical coherent transient output signal's represents the cross-correlation or convolution of the input temporal waveforms.1 The projected performance characteristics of coherent transient processors include data rates greater than 10 GHz, time-bandwidth products far in excess of 10,000, and the ability to fully process both amplitude and phase modulated waveforms.1,2 Previously it was assumed that the input data stream and pattern stream must both be shorter than the absorbing transition's homogeneous dephasing time and must both be reentered in order to process longer or multiple data streams.1 It has recently been proposed that patterns could be permanently stored in an inhomogeneously broadened solid and that input data streams of indefinite length could be continuously processed in real time without the need to reenter the input pattern.3,4 In this paper, we present a proof of concept demonstration of an optical coherent transient continuous correlator.

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Linear- and nonlinear-optical properties of Eu3+:Y2SiO5 have been investigated. All the energy levels of the7F0,1,2,… ground-state manifold and the 5D0,1,2 excited-state manifold were identified, and they were found to be doubly structured because of the two inequivalent optical sites. Two absorption lines of the 7F0–5D0 transition were studied in detail by using nonlinear spectroscopic techniques, photon-echo and stimulated-photon-echo methods. It was found by the photon-echo measurement that the optical dephasing time for one site is extremely long and is excitation-intensity dependent. With the smallest possible intensity we observed the longest ever dephasing time in a solid, 822 μs, or a 387-Hz homogeneous linewidth. From the stimulated-photon-echo measurement, the linewidth was found to be constant over a wide range of time scales, implying the absence of spectral diffusion in this crystal. These quite advantageous properties of this crystal were used for the demonstration of two novel types of time-domain optical data readout: time- and frequency-domain hybrid optical memory and spectrally programmed coherent transients.

Journal of the Optical Society of America B•Jun 1, 1992•59 citations
We report what is to our knowledge the first experimental demonstration of a continuous coherent transient optical processor. A 13-bit pattern was stored as a spectral population grating in the Eu3+-ion ground state in a Eu3+:Y2SiO5 crystal. A 3120-bit data stream was processed continuously, yielding a cross-correlation signal that agreed well with theory. The data stream’s duration exceeded both the absorbing transition’s homogenous transverse dephasing time and upper-state lifetime. This experiment showed that by permanently storing the pattern as a population grating in the absorber’s ground state, coherent transient optical devices provide continuous, real-time data processing capability.
When a sequence of temporally modulated optical waveforms illuminate an inhomogeneously broadened absorbing medium, the resultant optical coherent transient output signal's represents the cross-correlation or convolution of the input temporal waveforms.1 The projected performance characteristics of coherent transient processors include data rates greater than 10 GHz, time-bandwidth products far in excess of 10,000, and the ability to fully process both amplitude and phase modulated waveforms.1,2 Previously it was assumed that the input data stream and pattern stream must both be shorter than the absorbing transition's homogeneous dephasing time and must both be reentered in order to process longer or multiple data streams.1 It has recently been proposed that patterns could be permanently stored in an inhomogeneously broadened solid and that input data streams of indefinite length could be continuously processed in real time without the need to reenter the input pattern.3,4 In this paper, we present a proof of concept demonstration of an optical coherent transient continuous correlator.
After the absorption profile of an inhomogeneously broadened solid is programmed by two temporally modulated pulses (at least one encoded with a pattern), it can be gated to fix the ground-state spectral population distribution permanently. The subsequent illumination of the solid by an uninterrupted, temporally modulated optical beam results in a coherent transient output signal that represents the correlation of the signal with the stored pattern. Multiple patterns can be stored at different locations on the sample and accessed randomly, enabling fast reprogramming of the processor. A performance analysis of this optical signal processor predicts that real-time continuous processing is possible with a processor bandwidth that exceeds 5 GHz, a time-bandwidth product that exceeds 10(4), and a pattern storage density that exceeds 10(5) patterns per square centimeter.
The design issues for the technique of continuously programming a coherent transient spatial-spectral optical signal processor are discussed. The repeated application of two spatially distinct optical programming pulses to a non- persistent hole-burning material writes an accumulated, spatial-spectral population grating with low intensity optical pulses as compared to single shot programing. An optical data stream is introduced on a third beam, resulting in a processor output signal spatially distinct from all the input pulses. Programming and processing take place simultaneously, asynchronously and continuously. For accumulated gratings, the frequency stability of the optical source is an important consideration. Assuming a sufficiently stable optical source, simulations show that an accumulated (and maintained) grating in steady state, for both storage of a true-time delay and/or pattern waveform, can be highly efficient using currently available materials, on the order of that predicted for a perfect photon-gated device. An experimental demonstration of the continuous programming concept for true time delays programmed with chirped pulses is presented, showing the accumulation of the grating with low area pulses over time until it reaches steady state, for times longer than the persistence of the material.
A novel technique for continuously programming an optical coherent transient spatial-spectral signal processor is proposed. The repeated application of two spatially distinct optical programming pulses to a nonpersistent hole-burning material writes an accumulated spatial-spectral population grating. An optical data stream is introduced on a third beam, resulting in a processor output signal that is spatially distinct from all the input pulses. Programming and processing take place simultaneously, asynchronously, and continuously. In the case of true-time delays, the efficiency that is achievable with currently available materials is of the order of that predicted for a perfect photon-gated device.
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.
Linear- and nonlinear-optical properties of Eu3+:Y2SiO5 have been investigated. All the energy levels of the7F0,1,2,… ground-state manifold and the 5D0,1,2 excited-state manifold were identified, and they were found to be doubly structured because of the two inequivalent optical sites. Two absorption lines of the 7F0–5D0 transition were studied in detail by using nonlinear spectroscopic techniques, photon-echo and stimulated-photon-echo methods. It was found by the photon-echo measurement that the optical dephasing time for one site is extremely long and is excitation-intensity dependent. With the smallest possible intensity we observed the longest ever dephasing time in a solid, 822 μs, or a 387-Hz homogeneous linewidth. From the stimulated-photon-echo measurement, the linewidth was found to be constant over a wide range of time scales, implying the absence of spectral diffusion in this crystal. These quite advantageous properties of this crystal were used for the demonstration of two novel types of time-domain optical data readout: time- and frequency-domain hybrid optical memory and spectrally programmed coherent transients.