Light Control for Non-Invasive and Minimally Invasive Imaging

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Caravaca Aguirre, Antonio Miguel
Multimode fibers; Photoacoustics; Scattering media; Wavefront shaping; Bioimaging and Biomedical Optics; Biomedical; Electromagnetics and Photonics
thesis / dissertation description
The deterministic nature of scattering in turbid media and the advances in light control have opened the possibility of wavefront optimization to pre-compensate for the effects of multiple scattering. A major motivation for the development of new techniques is imaging and sensing in biological materials. These techniques have the potential to overcome tissue scattering, which usually limits the imaging depth to less than a millimeter. However, the dynamic nature of biological materials complicates the process exhibiting decorrelation times on the millisecond timescale. In this Thesis we investigate new techniques for high speed and blind imaging through complex media.We investigate the problem of optical non-invasively imaging through turbid media without the use of the so-called memory effect, namely allowing for thick and highly scattering obsta- cles. A photoacoustic signal, consisting of the integration of photoacoustic emissions from multiple speckle grains, is used as optimization feedback for focusing through such a scattering material. We demonstrate three-dimensional photoacoustic imaging behind a scattering material by scanning an absorbing object through the optimized focus.We also study the benefits of combining laser speckle contrast imaging and wavefront coding. We implement a laser speckle contrast imaging system with extended depth of field for measuring flow rates in brain-mimicking microfluidic systems, resulting in more than a four-fold improvement in depth of field compared to traditional laser speckle contrast imaging.Besides non-invasive techniques for imaging, we explore minimally invasive approaches using a single thin multimode fiber. We demonstrate a micro-endoscope that uses a single multimode fiber and a spatial light modulator to collect and process fluorescent images. The system focuses light through the fiber at high-speed by means of a phase modulation system built around a digital micromirror device. The system is adapted to maintain a focus through a multimode fiber whileis bent. Additionally, we investigate the performance of a variety of multimode fibers based on the robustness of the calibration and the optical enhancement of the optical focus created using wavefront shaping.