DC and pulse-light illuminated optical responses of microwave GaAs-MESFET oscillators

Experimental results of two kinds of optical effects, optical tuning and optical switching, of GaAs MESFET oscillators are presented. For optical tuning, the oscillation frequency decreases with optical illumination and the maximum tuning range depends principally on oscillator mode (common-source, common-drain, or common-gate), with 3.8 percent and 1.9 percent being achieved at S band and X band, respectively, with an optical power density of 0.5 mW/sq mm. The oscillator power output generally increases with optical illumination, the increase being around 1 to 2 dB at 0.5 mW/sq mm light intensity. For optical switching, power output switching from no oscillation to 7.5 mW was obtained at X band with illumination from a 2 mW laser diode. The optical response of microwave GaAs MESFET oscillators is attributed to the capture of holes by two kinds of hole traps in the Schottky-gate depletion region. The oscillation frequency and power output changes with optical illumination become appreciably reduced with increase of optical modulation rate, becoming 10 percent of the constant-illumination induced change at an optical modulation rate of 1 MHz and negligibly small at 10 MHz. The drain current bias still responds above 10 MHz, with a response amount about 0.5 mA attributed to the photoconductivity effect in the channel.