Optically activated p-n-p-n light-emitting diodes (LA-LED) were demonstrated by Copeland et al.1 Similar to a p-n-p-n thyristor, the diode has an s-type I-V curve and can be operated as a bistable switch by applying the proper external load and bias voltage. A certain amount of light (input light) produces a photo current in the reverse-biased p-n junction and switches the device from the off state to the on state (see Fig. 1), causing one of the forward-biased junctions to emit light at substantially higher intensity than is required at the input. In this way, the device can be used as an optical repeater for optical data links, providing both a reshaped and amplified optical output and an electrical signal compatible with standard digital logic.
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