Focusing through dynamic tissue with millisecond digital optical phase conjugation.

Daifa Wang,Mooseok Jang,Edward Haojiang Zhou,Haowen Ruan,Joshua Brake,Changhuei Yang

doi:10.1364/optica.2.000728

Abstract

Digital optical phase conjugation (DOPC) is a new technique employed in wavefront shaping and phase conjugation for focusing light through or within scattering media such as biological tissues. DOPC is particularly attractive as it intrinsically achieves a high fluence reflectivity in comparison to nonlinear optical approaches. However, the slow refresh rate of liquid crystal spatial light modulators and limitations imposed by computer data transfer speeds have thus far made it difficult for DOPC to achieve a playback latency of shorter than ~200 ms and, therefore, prevented DOPC from being practically applied to thick living samples. In this paper, we report a novel DOPC system that is capable of 5.3 ms playback latency. This speed improvement of almost 2 orders of magnitude is achieved by using a digital micromirror device, field programmable gate array (FPGA) processing, and a single-shot binary phase retrieval technique. With this system, we are able to focus through 2.3 mm living mouse skin with blood flowing through it (decorrelation time ~30 ms) and demonstrate that the focus can be maintained indefinitely-an important technological milestone that has not been previously reported, to the best of our knowledge.

Highlights

Focusing light through tissues has long been a challenge for biomedical optics
The primary goal of this paper is to show that the use of a highspeed digital mirror device (DMD) and field programmable gate array (FPGA) data processing allows Digital optical phase conjugation (DOPC) to achieve high response speeds, as well
Following a similar derivation in Refs. [13,25], we find the theoretical peak-to-background ratio (PBR) for DMD-based DOPC to be where M is the number of modes in the focus, and N is the number of controllable modes on the DMD

Summary

Introduction

Focusing light through tissues has long been a challenge for biomedical optics. The turbid nature of tissues strongly scatters light and hinders the formation of a sharp focus. Different strategies have been developed to realize this process including iterative wavefront optimization [1,3,4,5], transmission matrix measurement [6,7,8], and optical phase conjugation (OPC) [9,10,11]. OPC implements the corrected wavefront by recording the scattered light field globally and playing back the conjugate light field by a phase conjugate mirror (PCM) without time-consuming iterations. Since the process of elastic light scattering is time symmetric, by playing a conjugate version of the scattered wavefront back through the scattering medium, the conjugate input wavefront can be recovered

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Conclusion