Phase control algorithms for focusing light through turbid media

Abstract

Light propagation in materials with microscopic inhomogeneities is affected by scattering. In scattering materials, such as powders, disordered metamaterials or biological tissue, multiple scattering on sub-wavelength particles makes light diffuse. Recently, we showed that it is possible to construct a wavefront that focuses through a solid, strongly scattering object. The focusing wavefront uniquely matches a certain configuration of the particles in the medium. To focus light through a turbid liquid or living tissue, it is necessary to dynamically adjust the wavefront as the particles in the medium move. Here we present three algorithms for constructing a wavefront that focuses through a scattering medium. We analyze the dynamic behavior of these algorithms and compare their sensitivity to measurement noise. The algorithms are compared both experimentally and using numerical simulations. The results are in good agreement with an intuitive model, which may be used to develop dynamic diffusion compensators with applications in, for example, light delivery in human tissue.Comment: 28 pages double spaced, 9 figure

Enhanced coupling of light into a turbid medium through microscopic interface engineering

“…An optimum wavefront of the incident light is determined via iterative search algorithms (24,25) or measurement of the transmission/reflection matrix (26) to promote coupling into these modes. In practice, the sample must remain static for the entire measurement procedure, which may require seconds to minutes of time to complete.…”

Wavefront Shaping and Its Application to Enhance Photoacoustic Imaging

“…As one can imagine, in order to achieve a high R, a large N and/or small M are required: the former is determined by the technical specification of the wavefront modulator, and the latter by the sensitivity zone of the feedback signal with respect to the speckle grain size. That said, a large N usually requires a longer optimization process [43], which is also significantly affected by the refreshing rate of the wavefront modulator, as well as the speed of data transfer and processing between the sensor, the wavefront modulator, and the processor (usually a personal computer [56]). Table 1 lists three types of commercial wavefront modulators, i.e., LCoS-type spatial light modulator (SLM), deformable mirror (DM), and digital micromirror device (DMD), that have been popularly used in wavefront shaping.…”

“…However, recently, researchers began to notice that the seemingly random scattering events and the resultant speckles are actually deterministic within a certain temporal window [37,38], and it is possible to reverse [39][40][41] or compensate for [42] the scattering-induced phase scrambling. To do so, researchers have developed several wavefront shaping (sometimes also referred to wavefront engineering) techniques, such as iterative wavefront optimization [23][24][25][26]28,[42][43][44][45][46][47][48][49][50][51], measuring the transmission matrix of the scattering medium [21,22,[52][53][54][55][56], and optical time reversal via phase conjugation [39,40,[57][58][59][60][61][62][63][64][65]. Nevertheless, the goals of these implementations are identical, i.e., to make light wavelets traveling along different optical paths interfere coherently at a region of interest (ROI) and form a bright optical spot (focus) out of the much darker background.…”

Investigation of the methods for optical wavefront parameter manipulation

“…There are many algorithms for segment selection, for example, line-to-line processing of individual SLM segments, binary search, phase mask method, etc. Additional information about these algorithms can be found in [13].…”