Noninvasive light focusing in scattering media using speckle variance optimization

Optical imaging deep inside scattering media remains a fundamental problem in bioimaging. While wavefront shaping has been shown to allow focusing of coherent light at depth, achieving it non-invasively remains a challenge. Various feedback mechanisms, in particular acoustic or nonlinear fluorescence-based, have been put forward for this purpose. Noninvasive focusing in depth on fluorescent objects with linear excitation is, however, still unresolved. Here we report a simple method for focusing inside a scattering medium in an epidetection geometry with a linear signal: optimizing the spatial variance of low-contrast speckle patterns emitted by a set of fluorescent sources. Experimentally, we demonstrate robust and efficient focusing of scattered light on a single source and show that this variance optimization method is formally equivalent to previous optimization strategies based on two-photon fluorescence. Our technique should generalize to a large variety of incoherent contrast mechanisms and holds interesting prospects for deep bioimaging. (c) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement