Ultrasonically encoded wavefront shaping for focusing into random media

Abstract

Phase distortions due to scattering in random media restrict optical focusing beyond one transport mean free path. However, scattering can be compensated for by applying a correction to the illumination wavefront using spatial light modulators. One method of obtaining the wavefront correction is by iterative determination using an optimization algorithm. In the past, obtaining a feedback signal required either direct optical access to the target region, or invasive embedding of molecular probes within the random media. Here, we propose using ultrasonically encoded light as feedback to guide the optimization dynamically and non-invasively. In our proof-of-principle demonstration, diffuse light was refocused to the ultrasound focal zone, with a focus-to-background ratio of more than one order of magnitude after 600 iterations. With further improvements, especially in optimization speed, the proposed method should find broad applications in deep tissue optical imaging and therapy.

Figure 1. Experimental setup.

The incident beam on the first diffuser (D1), initially planar, is spatially tailored using a phase-only spatial light modulator (SLM). Five cycles from a focused 6 MHz ultrasonic transducer (UST) are sent through a clear gelatin medium (CM), modulating light within the acoustic focus. After the second diffuser (D2), the modulated beam is collected using an optical fiber bundle (OFB). The signal is measured using a photorefractive detection (PRC) system and a photodiode (PD), and its amplitude is subsequently used as feedback to optimize the pattern on the SLM. To visualize the focal spot, a bar of fluorescent quantum dots (QD) is embedded along the y-axis of the acoustic focal zone. The resulting fluorescence intensity is longpass filtered (LPF) and measured using a CCD camera.

Figure 2

(a) Improvement over the initial signal amplitude as the algorithm progresses. An improvement of 11 times in signal amplitude was obtained, which corresponds to a similar increase in light intensity within the acoustic focal zone. An order of magnitude improvement was achieved at 370 iterations, as indicated by the dotted line. (b) Measured ultrasonically encoded signal amplitude with the optimized pattern (red, solid line), and with an initial, randomized pattern (blue, dashed line) displayed on the SLM. The traces were averaged over 75 acquisitions.

Figure 3

Visualization of the optimized focal spot when uniform, randomized, and optimized phase patterns (a–c) were displayed.The captured CCD images of the fluorescent bar are given in (d–f). The color bars indicate the phase for (a–c) and the CCD intensity for (d–f). (g) The cross sectional intensity, as indicated by the white dotted lines in (d–f). An increase of an order of magnitude is seen using the optimized pattern, compared to both uniform and randomized patterns.