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IEEE Lasers and Electro-Optics Society Annual Meeting (LEOS)

Signal correlator with programmable variable time delay based on optical coherent transients

K.D. Merkel, Zachary Cole, William Randall Babbitt

Published Jan 20, 2003• 0 citations
Publisher

Abstract

We have demonstrated a technique for programming variable time delays of a pattern waveform in a spectral grating. By frequency shifting the chirped programming pulses, correlation and delays over 450 ns were achieved without changing the programming sequence timing. The demonstrated bandwidth of 40 MHz only utilized /spl sim/0.2 % of the inhomogeneously broadened absorber's bandwidth. Thus, this technique has the potential to allow versatile, all-optical reception and processing of > 10 GHz signals in broadband array antenna beamforming.

DOI
10.1109/leos.1999.811853

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We have demonstrated a technique for programming variable time delays of a pattern waveform in a spectral grating. By frequency shifting the chirped programming pulses, correlation and delays over 450 ns were achieved without changing the programming sequence timing. The demonstrated bandwidth of 40 MHz only utilized /spl sim/0.2 % of the inhomogeneously broadened absorber's bandwidth. Thus, this technique has the potential to allow versatile, all-optical reception and processing of > 10 GHz signals in broadband array antenna beamforming.
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.
Summary form only given. A practical device creating a variable RF true-time delay (TTD) for signals on an optical carrier would be useful in array antennas. An optical coherent transient (OCT)-TTD device could provide high-resolution delays (/spl sim/1 ps) over a wide range (/spl sim/1 /spl mu/s), for high bandwidth signals (/spl sim/10 GHz) with currently available materials. In previously demonstrated OCT-TTD devices, two optical pulses separated in time and space program a delay in an absorbing medium as a spatial-spectral grating on an inhomogeneously broadened transition (IBT). In the paper, we experimentally demonstrate widely tunable OCT-TTD of a continuous optical data stream without changing the timing between the programming pulses.
Summary form only given. Accurate true-time delays are essential to correctly steer modern phased-array antennas. Optical coherent transient technology has the ability to store a variable true-time delay as a spatial-spectral holographic population grating in an inhomogeneously broadened absorber (IBA). Typically, two optical pulses program the delay. A third optical pulse or waveform applied to the IBA generates a delayed replica of itself. The delay is typically defined by /spl tau//sub 21/=t/sub 2/-t/sub 1/, the arrival time difference of the first two pulses. However, we find that this prediction for the delay of the emitted signal only holds for input optical pulse areas much less than pi. Experimental investigation and simulations were performed to observe the variation in arrival time of the output signal.
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).
Summary form only given. When phased array antenna systems operate with broadband (multiple gigahertz) signals, true time-delay devices are needed to solve the problem of beam squint. Optical coherent transient technology, also known as spatial-spectral holography, offers a unique way of using the space and frequency dimensions of inhomogeneously broadened absorbers to handle thousands of high-resolution delays in a compact volume. We demonstrated broadband true-time delay in a Tm/sup 3+/:YAG crystal, which offers delays up to a microsecond on signals having several tens of GHz bandwidth with sub-picosecond delay resolution.
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.