Abstract

A self-contained, all-optical digital signal processing unit (processor) using photon echoes can be constructed assuming a number of basic operational concepts can be implemented. These concepts are Boolean operators, delay or memory units, interconnections between parallel or sequential calculating units and subsequent amplification of signals between each of these units. Implementation of a complete set of Boolean operators using photon echoes has earlier been experimentally demonstrated¹ and theoretically evaluated. The photon echo process has the inherent property that it can delay or memorise data sequences and data storage densities of Tbits/cm² have been predicted. Interconnections between calculating units can be implemented by steering the output from one unit into the next. However, the low efficiency of the photon echo process, which is of the order of 0.1%-1%, and additional losses in the system causes a need for amplification of the output signals before they enter the next unit. We have implemented such amplification of photon echoes using a Pr:ZBLAN fibre amplifier. This method of amplifying the echo gives a high fidelity with constant and linear amplification because the part of the wide band-width noise caused by amplified stimulated emission (ASE) that falls within the narrow band-width of the photon echo signal is small, thus a good signal-to-noise ratio is maintained². Additional losses in the set-up accounts for a decrease in the echo intensity which is of the order of 10. The amplification achieved with our present fibre amplifier system is roughly 45, and the upper limit is in our case set by ASE, which is a function of the numerical aperture (N.A.) of the fibre. We are presently working with a fibre with a smaller N.A. which implies a maximum achievable amplification a factor 7 larger. By reducing additional losses in the set-up and optimising the photon echo process-efficiency we aim to reach a level of amplification that compensates fully for all losses in the system thus making is possible to perform all-optical sequential operations on digital data using photon echoes. Our current achievements² as well as our work with the improved system will be discussed. Figure 1. Experimental set-up

© 1998 IEEE