Implementation of a digital optical phase conjugation system and its application to study the robustness of turbidity suppression by phase conjugation

Meng Cui,Changhuei Yang

doi:10.1364/oe.18.003444

Abstract

In this work, we report a novel high capacity (number of degrees of freedom) open loop adaptive optics method, termed digital optical phase conjugation (DOPC), which provides a robust optoelectronic optical phase conjugation (OPC) solution. We showed that our prototype can phase conjugate light fields with ~3.9 x 10−3 degree accuracy over a range of ~3 degrees and can phase conjugate an input field through a relatively thick turbid medium (μsl ~13). Furthermore, we employed this system to show that the reversing of random scattering in turbid media by phase conjugation is surprisingly robust and accommodating of phase errors. An OPC wavefront with significant spatial phase errors (error uniformly distributed from – π/2 to π/2) can nevertheless allow OPC reconstruction through a scattering medium with ~40% of the efficiency achieved with phase error free OPC.

Highlights

Biological tissues are highly turbid media in the optical regime [1]
Comparing to conventional optical phase conjugation (OPC) methods that rely on nonlinear light-matter interactions, digital optical phase conjugation (DOPC) can work with both CW and pulsed laser systems of various wavelengths and power levels
One limitation of the DOPC method is that the update rate is determined by the speed of the wavefront measurement combined with the update rate of the SLM employed

Summary

Introduction

Biological tissues are highly turbid media in the optical regime [1]. The extensive scattering of light by tissue is a significant obstacle for deep-tissue optical imaging and optical sensing [2]. Several publications [3,4,5,6] have reported that it is experimentally possible to mitigate the effects of scattering by tailoring an input light field wavefront appropriately. Mosk’s group showed that it is possible to focus light through a scattering medium by modifying and optimizing the wavefront of an input light field with a spatial light modulator [3,5]. Our group showed that an optical phase conjugate (OPC) copy of an initial transmission through a biological sample can likewise undo the effects of the initial scattering [4,7,8]. Employing optical phase conjugation to suppress tissue turbidity is appealing because it requires the duplication of a transmission light field for which the phase at each point on the wavefront is sign-reversed. An OPC system that can work with various light sources of different wavelengths, coherence lengths, and power levels would be preferable

Methods

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Conclusion