Deep-Tissue Applications
Targeted light-delivery through scattering tissue is strongly limited by light scrambling. However, the technological improvement in hardware, computational power and methodology in recent years made it possible to control light inside or behind scattering media, by shaping the wavefront using a spatial light modulator before the light enters the scattering media. The main hurdle is to determine an adequate mask that allows pre-scrambling the light, so that the desired light pattern is delivered to the region of interest after scrambling. These are for example iterative optimization of the wave front, measurement of the transmission matrix, and digital optical phase conjugation (DOPC). DOPC has the advantage that it does not require time-consuming iterations or calibration measurements, but instead enables direct shaping with a single measurement. Commonly guide stars are used to probe the light scrambling. The phase of the guide star light is recorded by quantitative phase measurements, e.g. performed by digital holography, and a phase mask of the phase conjugate is displayed on the spatial light modulator. This approach enables to time reverse the scrambling effects and to recreate the guide star. We applied DOPC for example, to focus light through 400 μm thick part of a mouse skull, or to control the light field through multi-mode fibers (that basically act as scattering tissue).
a) Guide star. b) Speckled output through mouse skull without DOPC. c) With DOPC the guide star is recreated.
While without DOPC strong scattering is observable (Figure, b), DOPC allows focusing through mouse skull with high quality (Figure, c). Such an approach is e.g. important for the optogenetic in-vivo stimulation. We furthermore work on the development of guide star techniques, to enhance the applicability of DOPC and wavefront shaping techniques.
A wavefront shaping system consisting of an SLM and e.g. a guide star concept have to be optimized for each application separately.
N. Koukourakis, B. Fregin, J. König, L. Büttner, J. Czarske “Wavefront shaping for imaging based flow field measurements through distortions using a Fresnel guide star”, Opt. Express 24(19), 22074-22087 (2016).
J. Czarske, D. Haufe, N. Koukourakis, L. Büttner “Transmission of independent signals through a multimode fiber using digital optical phase conjugation”, Opt. Express 24(13), 15128-15136 (2016).
S. Rothe, H. Radner, N. Koukourakis, J. Czarske, “Transmission matrix measurement of multimode optical fibers by mode-selective excitation using one spatial light modulator”, Issue on Liquid Crystal on Silicon Devices: Modeling and Advanced Spatial Light Modulation Applications; Applied Sciences, 2019