Qinetiq Limited

Michael B Allenson

David R Wight

Stephen Gerard Ayling

A light emitting device for generating at least one beam of output radiation from an input current of electrons comprises at least two p-n junction devices for converting the input current of electrons into photons, wherein the p-n junction devices are electrically connected in series such that the input impedance of the light emitting device is substantially equal to the sum of the individual impedance of the p-n junction devices. Hence the quantum efficiency of the light emitting device is substantially equal to the sum of the individual quantum efficiencies of the p-n junction devices. In a preferred embodiment, the light emitting device comprises a plurality of p-n junction devices connected in series such that the input impedance of the light emitting device is equal to 50 &OHgr; without the need for additional circuitry or impedance matching elements. The device may therefore have a 50 &OHgr; impedance over a broad frequency band, limited by the modulation frequency limit of the individual p-n junctions. Typically, the p-n junctions may be AlGaAs, AlGaInP, AlGaInAs or AlGaInAsp laser diode devices. The invention also relates to an optically coupled bipolar transistor device.

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Malvern, United Kingdom

Michael B Allenson