Search

While propagating inside the strongly scattering biological tissue, photons lose their incident directions beyond one transport mean free path (TMFP, ∼ 1 millimeter (mm)), which makes it challenging to achieve optical focusing or clear imaging deep inside tissue. By manipulating many degrees of the incident optical wavefront, the latest optical wavefront engineering (WFE) technology compensates the wavefront distortions caused by the scattering media and thus is toward breaking this physical limit, bringing bright perspective to many applications deep inside tissue, e.g., high resolution functional/molecular imaging, optical excitation (optogenetics) and optical tweezers. However, inside the dynamic turbid media such as the biological tissue, the wavefront distortion is a fast and continuously changing process whose decorrelation rate is on timescales from milliseconds (ms) to microseconds (μ s), or even faster. This requires that the WFE technology should be capable of beating this rapid process. In this review, we discuss the major challenges faced by the WFE technology due to the fast decorrelation of dynamic turbid media such as living tissue when achieving light focusing/imaging and summarize the research progress achieved to date to overcome these challenges. [ABSTRACT FROM AUTHOR]

Focused and controllable optical delivery beyond the optical diffusion limit in biological tissue has been desired for long yet considered challenging. Digital optical phase conjugation (DOPC) has been proven promising to tackle this challenge. Its broad applications, however, have been hindered by the system's complexity and rigorous requirements, such as the optical beam quality, the pixel match between the wavefront sensor and wavefront modulator, as well as the flatness of the modulator's active region. In this paper, we present a plain yet reliable DOPC setup with an embedded four-phase, non-iterative approach that can rapidly compensate for the wavefront modulator's surface curvature, together with a non-phase-shifting in-line holography method for optical phase conjugation in the absence of an electro-optic modulator (EOM). In experiment, with the proposed setup the peak-to-background ratio (PBR) of optical focusing through a standard ground glass in experiment can be improved from 460 up to 23,000, while the full width at half maximum (FWHM) of the focal spot can be reduced from 50 down to 10 μm. The focusing efficiency, as measured by the value of PBR, reaches nearly 56.5% of the theoretical value. Such a plain yet efficient implementation, if further engineered, may potentially boost DOPC suitable for broader applications. [ABSTRACT FROM AUTHOR]