Acousto‐optic scanning and interfering photon density waves for precise localization of an absorbing (or fluorescent) body in a turbid medium

In most optical methods proposed for imaging an absorbing object embedded in a turbid medium, data are collected using a single source and detector scanned mechanically across the surface of the medium. In our setup, we exploited destructive interference of diffusive photon density waves originating from two sources to localize an absorbing (or fluorescent) body in a scattering medium. A frequency‐domain instrumentation is described that scans several laser‐beam spots across the surface of a turbid medium using 1D (or 2D) acousto‐optical deflectors. An additional acousto‐optic deflector is used to establish arbitrary phase shifts for the interfering photon‐density waves. A destructive interference pattern was created to laterally localize an absorbing (or fluorescent) body in the reflection and transmission modes. In some experiments the destructive interference pattern was altered by modulating the individual beam intensities to improve sensitivity and ameliorate surface texture problems. The experimental results were retrieved from a gated intensified CCD camera at 246 MHz modulation frequency. Results indicate that less than a 1 mm displacement of a small object embedded 10 mm in a medium with optical characteristics similar to bloodless skin tissue can be detected.