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Applied Optics

Coherent transient continuous optical processor

William Randall Babbitt, John A Bell

Published Mar 10, 1994• 43 citations
Publisher

Abstract

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.

DOI
10.1364/ao.33.001538

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Optics Letters•Oct 15, 1999•10 citations
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.
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.
This final report of contract F49620-91-C-0088 describes the development of the analytical tools required to evaluate optical coherent transient memory, triple-product correlator, and continuous signal processor systems. One key tool is a coherent transient simulator based on the optical Bloch equations. The coherent transient output signal that results from an arbitrary sequence of pulses, each having an arbitrary amplitude, duration, center frequency, chirp, phase, and timing can be calculated. The simulator was used to study excitation pulse saturation and it effects on the output signal efficiency and fidelity and to study frequency chirped reference pulse saturation. Analytical tools were developed to assess the effects of local heating and spatial crosstalk. A performance evaluation of a coherent transient continuous optical correlator is reported, along with new implementation techniques. The continuous processor's predicted time-bandwidth product, data bandwidth, and pattern storage density are 10000, 5 GHz, and 100000 patterns per square centimeter, respectively. A proof of concept demonstration of the continuous optical processor was performed in europium doped yttrium silicate in collaboration with IBM Almaden Research Center. Research into divalent ion doped crystals as photon gated materials for coherent transient systems was performed.
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.
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.
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.
A technique is proposed and experimentally demonstrated that achieves simultaneous optical pattern waveform storage and programmable time delay for continuous real-time signal processing by use of optical coherent transient technology. We achieve variable-time-delay and broadband signal processing by frequency shifting of two chirped programming pulses, the chirp rate of one being twice that of the other, without using brief reference pulses and without changing the timing of the programming sequence. We demonstrate the technique experimentally in Tm(3+): YAG at 5 K for 40-MHz chirps by performing temporal signal convolution with true-time delays that vary over a 250-ns range.