Focusing light inside dynamic scattering media with millisecond digital optical phase conjugation.

Yan Liu,Junhui Shi,Yuecheng Shen,Cheng Ma,Lihong V Wang

doi:10.1364/optica.4.000280

Abstract

Wavefront shaping based on digital optical phase conjugation (DOPC) focuses light through or inside scattering media, but the low speed of DOPC prevents it from being applied to thick, living biological tissue. Although a fast DOPC approach was recently developed, the reported single-shot wavefront measurement method does not work when the goal is to focus light inside, instead of through, highly scattering media. Here, using a ferroelectric liquid crystal based spatial light modulator, we develop a simpler but faster DOPC system that focuses light not only through, but also inside scattering media. By controlling 2.6 × 105 optical degrees of freedom, our system focused light through 3 mm thick moving chicken tissue, with a system latency of 3.0 ms. Using ultrasound-guided DOPC, along with a binary wavefront measurement method, our system focused light inside a scattering medium comprising moving tissue with a latency of 6.0 ms, which is one to two orders of magnitude shorter than those of previous digital wavefront shaping systems. Since the demonstrated speed approaches tissue decorrelation rates, this work is an important step toward in vivo deep-tissue non-invasive optical imaging, manipulation, and therapy.

Highlights

In opaque media, such as biological tissue, the heterogeneous refractive index distribution causes light to scatter, which makes the media look opaque and prevents us from focusing light deep inside the media to achieve optical imaging and manipulation [1,2]
digital micromirror device (DMD) have several limitations for this application: (a) They typically achieve binary-amplitude modulation, which results in a lower focusing contrast compared with that of phase modulations. (b) The optical fluence threshold causing DMDs to malfunction under pulsed laser illumination is usually lower than that of liquid crystal based spatial light modulators (SLMs) [41,42]. (c) The alignment of a DMDbased digital optical phase conjugation (DOPC) system is significantly complicated by the oblique reflection angle of the DMD [23]. (d) a loaded pattern can be displayed at ~23 kHz on a DMD, transferring a pattern from a PC or an field programmable gate array (FPGA) board to the DMD can take 1.6–4.5 ms [23,38,40], limiting the speed of a DOPC system
The theoretical PBR is calculated by N/(2πM), where N is the number of optical degrees of freedom, M is the number of speckle grains in the DOPC focus, and the factor of 2 is because that the opal diffuser nearly completely scrambles the polarization and our system phase conjugates only a single polarization of the sample light [52]

Summary

Introduction

In opaque media, such as biological tissue, the heterogeneous refractive index distribution causes light to scatter, which makes the media look opaque and prevents us from focusing light deep inside the media to achieve optical imaging and manipulation [1,2]. To focus light through or inside highly scattering media, various wavefront shaping approaches are being actively developed [3,4,5,6], including feedback-based wavefront shaping [7], transmission matrix measurement [8,9], and optical time reversal/optical phase conjugation (OPC) [10,11,12,13] Among these techniques, OPC is most promising for in vivo applications because it achieves the shortest average mode time [14] (the average operation time per degree of freedom) by determining the optimum wavefront globally instead of stepwise. DOPC has enabled light focusing through ex vivo chicken tissue and tissue-mimicking phantoms up to 9.6 cm thick [20]

Methods

Results

Conclusion