Reverse-Biased Gallium Phosphide Diodes as High-Frequency Light Modulators

Reverse-biased gallium phosphide diodes have been successfully operated as high-frequency intensity and phase modulators of light at room temperature. The high-frequency modulation is fundamentally limited by the series resistance and the capacitance of the diodes. Cutoff frequencies as high as 7 GHz have been measured. Diode capacitance, typically less than 7 pF at the operating point, and driver impedance determine the broad-band characteristic. Intensity modulation measurements have been performed up to 500 MHz. The power consumption per MHz bandwidth for 40% intensity modulation was 2.7 mW for a diode only 0.7 mm long. A diode of 1.5 mm length used as a phase modulator needed only 1.5 mW of power per MHz bandwidth for a modulation index of 1 rad at a wavelength of 0.633 μ. This latter value corresponds to an intensity modulation depth of over 80%.