A wide-band microwave light modulator

An experimental potassium dihydrogen phosphate (KDP) light modulator with a 10 percent (half-m) bandwidth at 6 GHz is described. When arranged for amplitude modulation, a modulation depth of 40 percent 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 node operation. The circuit provides adequate cooling for the crystals and CW or pulsed operation at average RF levels of 10 watts is possible. Measurements indicate that the limitation in bandwidth is due mainly to the dispersion of the circuit and that with suitable modifications bandwidths of 20 percent are quite feasible.