Coherence of digital phase conjugation for implementing time reversal in scattering media

Abstract

The strong light scattering in complex media, due to the highly inhomogeneous distributions of refractive indexes, is regarded as a fundamental impediment in numerous optical applications such as optical communications, biophotonics, and optical tweezer. Recently, many optical techniques based on the coherence of light source with long coherent length have been developed and widely used to suppress and control light scattering and propagation in complex media. Here, we propose and experimentally demonstrate the control and time reversal of only one part instead of all of light passing through complex media and different optical paths by combining digital phase conjugation and coherence gating based on partially coherent light source. Interference of reference and objective beams and corresponding phase maps are measured by the charge coupled device (CCD) and four-step phase-shift measuring technique only when the optical path difference between two beams is less than coherence length. Time reversal is achieved by spatial light modulator (SLM). In the experiment we further analyze the phase map and time reversal with different optical path differences and different coherence lengths of source. The experimental results demonstrate that for each optical path difference, the time reversal of only the part of light coming from the same scattering> and identical optical path is achieved by digital phase conjugation and coherent gating of broadband light source.