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

An optical coherent transient signal correlator with variable true time delay is experimentally demonstrated. Storage of a pattern waveform in a spectral-spatial population grating is achieved by interfering it with two chirped pulses instead of a single brief reference pulse. Correlative signal processing results when a data waveform probes that grating, stimulating an emitted coherent output signal after the appropriate time delay. The signal is the result of a six-wave mixing process that can be as efficient as the traditional four-wave mixing process using a single brief reference pulse. Frequency shifting of the two chirped pulses enables programming of a variable time delay in the grating without changing the timing of the programming pulses. This concept is experimentally demonstrated with variable time delays over 448 ns (offset by greater than 1 mu s) for the auto-correlation of a code with an experimentally limited bandwidth of 40 MHz. Processing bandwidths exceeding 10 GHz are achievable in materials such as Tm3+:YAG (0.1 at%). (C) 2000 Elsevier Science B.V. All rights reserved.