Study of semiconductor electro-optic modulators for sensing extremely-low-frequency electrical signals

We investigated the characteristics of electro-optic modulators composed of ZnTe and CdTe. Their modulation depths below 100 kHz depend both on wavelength and input light power. The characteristics are explained by considering both the photoconduction effect and the Schottky effect. To keep the modulation depth constant below 100 kHz, the wavelength of the light should be long enough to suppress the photoconduction effect. Using the ZnTe electro-optic modulator, we developed a sensitive voltage sensor that has constant sensitivity from DC to 30 MHz. Clear electrocardiograms can be obtained with the sensor.