Coherence-gated ultrasound detection through turbid medium

Abstract

Summary form only given. Photorefractive multiple quantum wells (PRQW) can be used as adaptive beam combiners in laser-based ultrasound detection. We previously demonstrated adaptive homodyne laser-based megahertz ultrasound detection using a CW laser. in some applications of nondestructive detection, excitation and detection using femtosecond lasers are necessary. We demonstrate for the first time homodyne detection using a femtosecond laser and dynamic holography, and experimentally study the detection of extremely low intensity and highly distorted wavefronts through turbid media. In our laser-based ultrasound detection system, adaptive beam combing is performed through two-mixing in a PRQW that serves as an adaptive holographic film. The requirement for maximum homodyne detection is quadrature in relative phase between reference and signal beams. To achieve quadrature, the relative phase is adjusted by selecting both the center wavelength of the laser and the detection wavelength of a monochromator. Due to the constant phase between the beams, the PRQW device ensures quadrature even in the presence of low-frequency mechanical vibration and speckle.