Frequency-swept time-reversed ultrasonically encoded optical focusing.

A technique to rapidly scan an optical focus inside a turbid medium is attractive for various biomedical applications. Time-reversed ultrasonically encoded (TRUE) optical focusing has previously demonstrated light focusing into a turbid medium, using both analog and digital devices. Although the digital implementation can generate a focus with high energy, it has been time consuming to scan the TRUE focus inside a sample. Here, by sweeping the frequencies of both ultrasound and light, we demonstrate a multiplex recording of ultrasonically encoded wavefronts, accelerating the generation of multiple TRUE foci. Using this technique, we obtained a 2-D image of a fluorescent target centered inside a turbid sample having a thickness of 2.4 transport mean free paths.