Improving the axial resolution in time-reversed ultrasonically encoded (TRUE) optical focusing with dual ultrasonic waves
Focusing light inside highly scattering media beyond the ballistic regime is a challenging task in biomedical optical imaging, manipulation, and therapy. This challenge can be overcome by time reversing ultrasonically encoded (TRUE) diffuse light to the ultrasonic focus inside a turbid medium. In TRUE optical focusing, a photorefractive crystal or polymer is used as the phase conjugate mirror for optical time reversal. Accordingly, a relatively long ultrasound burst, whose duration matches the response time of the photorefractive material, is used to encode the diffuse light. With this long ultrasound burst, the resolution of the TRUE focus along the acoustic axis is poor. In this work, we used two transducers, emitting two intersecting ultrasound beams at 3.4 MHz and 3.6 MHz respectively, to modulate the diffuse light within their intersection volume at the beat frequency. We show that light encoded at the beat frequency can be time-reversed and converge to the intersection volume. Experimentally, TRUE focusing with an acoustic axial resolution of ~1.1 mm was demonstrated inside turbid media, agreeing with the theoretical estimation.
Additional Information© 2014 Society of Photo-Optical Instrumentation Engineers. The authors thank Yan Liu for helpful discussions on resolution. This work was conducted at Washington University in Saint Louis, and sponsored in part by National Academies Keck Futures Initiative grant IS 13, and National Institutes of Health grants DP1 EB016986 (NIH Director's Pioneer Award) and U54 CA136398.
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