Homodyne time-of-flight acousto-optic imaging for low-gain photodetector.

Acousto-optics imaging (AOI) is a hybrid imaging modality that is capable of mapping the light fluence rate in deep tissue by local ultrasound modulation of the diffused photons. Since the intensity of the modulated photons is relatively low, AOI systems often rely on high-gain photodetectors, e.g. photomultiplier tubes (PMTs), which limit scalability due to size and cost and may significantly increase the relative shot-noise in the detected signal due to low quantum yields or gain noise. In this work, we have developed a homodyne AOI scheme in which the modulated photons are amplified by interference with a reference beam, enabling their detection with a single low-gain photodetector in reflection-mode configuration. We experimentally demonstrate our approach with a silicon photodiode, achieving over a 4-fold improvement in SNR in comparison to a PMT-based setup. The increased SNR manifested in lower background noise level thus enabling deeper imaging depths. The use of a fiber-based configuration enables the integration of our scheme in a hand-held AOI probe.