Optical detection of ultrasound using a microchip laser

Abstract

A novel ultrasound detector is presented which uses a microchip laser as the sensing element. An incident acoustic field modulates the optical cavity length of a microchip laser operating in continuous wave mode. The single longitudinal mode laser output wavelength is frequency modulated by the acoustic field. An optical slope filter is used to demodulate the FM signal and generate an intensity modulated optical signal which mimics the acoustic field present in the microchip laser. Parameters are derived for designing the microchip laser for maximum sensitivity and frequency bandwidth. An Nd:YAG microchip laser detecting 5 MHz ultrasound is predicted to have a shot noise limited nonresonantly enhanced sensitivity of 10−4 Pa*(mW/Hz)0.5, a linear dynamic range of 109, and a nearly flat frequency response from dc to slightly below the first acoustic resonance peak. Experimental verification of the sensor operation is presented and compares well with theory. The sensor had a monotonically increasing frequency response from 1 to 7 MHz, with a FM sensitivity of 1–2 MHz/kPa. A large sensitivity enhancement was observed at an acoustic resonance of 8 MHz.