Wavelength dependence of the optical control of a HEMT oscillator.

The direct optical control, tuning, FM modulation, and injection locking of a 2-GHz HEMT oscillator using a semiconductor laser (lambda is equal to 0.85 Mym) is reported. An optical tuning range of 4.3 MHz and an optical injection locking range of more than 0.55 MHz at fundamental frequency were achieved. For the first time an experimental study relating the photoresponse mechanisms in the HEMT to the wavelength of the incident light is presented. It is shown that for the 0.85-Mym-wavelength illumination the photoconductive effect dominates the optical response of the device, while for 0. 633-Mym illumination the photovoltaic effect prevails (1). The photoconductive mechanism, which is smaller but faster, is more suitable for high frequency applications such as direct optical injection locking.