Optoelectronic modulators for millimetre-waves

Optoelectronic modulation for mm-waves (60 to 90 GHz) has been realized at relatively low optical power levels with the advantage of using small diode lasers. Modulation is accomplished by using a dielectric image guide, made from a high resistive semiconductor, the electrical behavior of which is controlled by light waves. The simple structure has the potential for broad bandwidth, fast switching; and since bulk photo-conductivity is used, the devices have high power handling capability. A slow modulator using undoped Si provides blanking, AM modulation and leveling of mm-wave sweepers. On/off ratios of 35 dB are accompanied by an insertion loss of 1.5 dB. The fast modulator uses GaAs or gold-doped Si and is of interest for a pulse radar to switch a mm-wave source or to protect a sensitive receiver. Rise-times less than 100 ns and fall-times of a few hundred ns, accompanied by 8 dB modulation depth, were measured.