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Conference on Lasers and Electro-Optics

Steady-state accumulated complex spectral gratings for correlation signal processing

Nathan L. Seldomridge, R.K. Mohan, William Randall Babbitt, K.D. Merkel

Published Jan 1, 2001• 0 citations
Publisher

Abstract

Summary form only given. Complex spectral gratings, accumulated to nearly steady-state in a short-term persistent inhomogeneously broadened optical transition, have been demonstrated by repeated application of a pair of frequency stabilized and low power-optical pulses (i.e. continuous programming). Provided the grating is not saturated, it can perform continuous correlation of an amplitude and/or phase modulated optical waveform at multi-gigahertz rates with large time-bandwidth products. The present work demonstrates continuous programming of a complex spectral grating, representing a 30-bit pattern and 4 microsecond time delay, to and beyond steady state. Grating accumulation and maintenance is observed by applying a probe pulse (identical to the pattern) after each programming pulse pair. Each probe stimulates an optical coherent transient emitted signal that represents the correlation of the pattern and probe. The probe was non-destructive to the grating, and consequently did not interfere with programming. The frequency reference was a transient, regenerating spectral hole created by the optical carrier in the same inhomogeneously broadened transition.

DOI
10.1109/cleo.2001.947935

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Summary form only given. Complex spectral gratings, accumulated to nearly steady-state in a short-term persistent inhomogeneously broadened optical transition, have been demonstrated by repeated application of a pair of frequency stabilized and low power-optical pulses (i.e. continuous programming). Provided the grating is not saturated, it can perform continuous correlation of an amplitude and/or phase modulated optical waveform at multi-gigahertz rates with large time-bandwidth products. The present work demonstrates continuous programming of a complex spectral grating, representing a 30-bit pattern and 4 microsecond time delay, to and beyond steady state. Grating accumulation and maintenance is observed by applying a probe pulse (identical to the pattern) after each programming pulse pair. Each probe stimulates an optical coherent transient emitted signal that represents the correlation of the pattern and probe. The probe was non-destructive to the grating, and consequently did not interfere with programming. The frequency reference was a transient, regenerating spectral hole created by the optical carrier in the same inhomogeneously broadened transition.
A complex spectral grating is accumulated by repeated application of a pair of low-power optical programming pulses to a short-term persistent inhomogeneously broadened transition in Tm:YAG at 4.5 K and then probed to investigate the buildup dynamics. The necessary frequency stability is obtained by locking a cw Ti:sapphire laser to a regenerating transient spectral hole in the same transition. Grating accumulation is demonstrated for both a periodic spectral grating, representing a true-time delay, and a complex spectral grating, permitting correlation-based pattern recognition. This work is a step toward demonstrating an optical coherent transient continuously programmed continuous processor.
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.
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.
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.
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.
A novel technique for programming broadband true-time delays that uses two frequency-offset temporally overlapped linear frequency-chirped pulses to produce periodic spectral gratings in an inhomogeneously broadened absorber is presented. Advantages of this technique include its ability to use chirped pulses that are longer than the coherence time of the crystal, less stringent laser frequency-stability requirements for grating accumulation, lower power requirements, a simplified system design, and the ability to tune broadband (multigigahertz) delays over a wide dynamic range (picoseconds to microseconds).