A wideband microwave light modulator

An experimental KDP light modulator with a 10% (half-m) bandwidth at 6 GHz is described. When arranged for amplitude modulation, a modulation depth of 40% was obtained with 10 watts of input power at band center. With a different optical arrangement, phase modulation is also possible. The modulator uses a ring-plane traveling wave circuit with cylindrical KDP crystals filling the space inside the rings. The crystals are used in the longitudinal mode, i.e., with the light beam along the optic axis, thus avoiding some of the thermal problems associated with transverse mode operation. The total length of crystals and circuit is 8 in. The crystal diameter is 0.156 in. The ring-plane circuit provides adequate cooling for the crystals, and operation at average rf levels of 10 watts is possible with only moderate distortion of the light beam.