Frequency-shift keyed generation with an optically controlled GaAs field-effect transistor in a surface-acoustic-wave oscillator circuit

Abstract

A prototype circuit for potential optical integration was developed and evaluated for the generation of frequency-shift keyed (FSK) signals by simultaneously exploiting surface-acoustic wave (SAW) technology and the optical response of the GaAs field-effect transistor (FET).A SAW delay line to be utilized in the feedback path of an oscillator circuit was designed for operation at the fundamental frequency of 111 MHz and operated at its 12th harmonic (1.33 GHz). A low-noise FET was incorporated into the circuit in series with the SAW delay, and a low-power He:Ne laser was focussed through an optical modulator onto the FET structure. The gate bias, which controls the phase shift in the feedback loop of the oscillator, was applied via the open-circuit photovoltage induced at the Schottky-barrier junction of the FET.The optical modulator was driven by a pseudorandom bit generator to obtain FSK generation by the oscillator circuit. Results of the frequency spectrum of the oscillator signal as a function of bit rate and light intensity on the FET were obtained. The successful performance of the circuit indicated that integration of all these elements on GaAs would yield a useful device for FSK signal generation.Un prototype de circuit pour intégration optique potentielle a été développé et évalué pour la génération de signaux FSK (frequency-shift keyed), en exploitant simultanéement la technologie des ondes acoustiques de surface et la réponse optique du transistor GaAs à effet de champ.