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Proceedings Paper

Bit-efficient sub-millisecond wavefront measurement using a lock-in camera for time-reversal based optical focusing inside scattering media (Conference Presentation)

Paper Abstract

Optical focusing plays a central role in biomedical optical imaging, manipulation, and therapy. However, in scattering media, direct optical focusing becomes infeasible beyond ~10 mean free paths. To break this limit, time-reversed ultrasonically encoded (TRUE) optical focusing phase-conjugates ultrasonically tagged diffuse light back to the ultrasonic focus, thus forming a focus deep inside scattering media. In previous works, the speed of wavefront measurement was limited by the low frame rate of the camera used to record the four images required for phase-shifting holography. Moreover, most of the bits of a pixel value were used to represent an informationless background caused by the large amount of untagged light, increasing the amount of data to transfer and necessitating the use of costly high-resolution analog-to-digital converters (ADCs). Here, we developed a digital TRUE focusing system based on a lock-in camera (300×300 pixels), in which each pixel performs analog lock-in detection on chip. Since only the information of the signal, not that of the background, is digitized, the lock-in camera reduces the amount of data to transfer, and enables the use of cheap low-resolution ADCs. Using this lock-in camera, we were able to measure the wavefront of ultrasonically tagged light in less than 0.3 ms, and to achieve TRUE focusing in between two ground glass diffusers. Even when the signal-to-background ratio dropped to 6.32×10^-4, a phase sensitivity as low as 0.51 rad could still be realized, which is more than enough for digital optical phase conjugation.

Paper Details

Date Published: 27 April 2016
PDF: 1 pages
Proc. SPIE 9717, Adaptive Optics and Wavefront Control for Biological Systems II, 97171J (27 April 2016); doi: 10.1117/12.2211936
Show Author Affiliations
Yan Liu, Washington Univ. in St. Louis (United States)
Cheng Ma, Washington Univ. in St. Louis (United States)
Yuecheng Shen, Washington Univ. in St. Louis (United States)
Lihong V. Wang, Washington Univ. in St. Louis (United States)

Published in SPIE Proceedings Vol. 9717:
Adaptive Optics and Wavefront Control for Biological Systems II
Thomas G. Bifano; Joel Kubby; Sylvain Gigan, Editor(s)

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