FREQUENCY DETECTOR USING PHOTO-EMF EFFECT

We propose a novel non-contact system to measure frequencies of objects using the photo-EMF effect. The set-up consists of two phase-modulated interference patterns incidents on semiconductor quantum well sample, each one creating its own photo-EMF signal that generate an electric signal corresponding in amplitude and frequency to the vibration. One of the interference patterns is phase-modulated by a vibrating object under test of unknown frequency and amplitude, the other one is modulated by a reference phase modulator of known frequency and amplitude. The interference patterns produce its own photo-EMF signal oscillating at different frequencies, resulting in a beating signal. When the frequency of both phase-modulated interference patterns is equal a sharp peak on the lock-in is measured, whose width is determined by the inverse of the time constant of the lock-in.