Multimode fiber optical imaging using wavefront control

Optical microscopy enables us to observe at high resolution and with a large field of view objects otherwise invisible for the naked eye. This is why it is becoming a fundamental tool in biology and in diagnostics, where observation at cellular level can tell about the health status of a tissue. At visible wavelengths, light undergoes scattering when propagating through tissues, limiting the imaging depth to the first cellular layers of a biological sample. Endoscopic approaches allow the bypass of the scattering layers and move the accessible volume centimeters inside tissues, ideally keeping large field of view and high-resolution capabilities. In this thesis we explore the combination of wavefront control and ultrathin multimode optical fibers to develop a new family of endoscopes able to generate images with micrometric resolution being, at the same time, minimally invasive. We demonstrate that a variety of imaging techniques can be implemented in optical fiber-based endoscopy, maximizing the amount of information that can be collected through a few hundreds of micrometers-thick probe. Firstly, we describe digital phase conjugation (DPC), which is the utilized wavefront shaping technique to convert a multimode optical fiber in an ultrathin endoscope. The use of DPC in synergy with multimode optical fibers resulted in ultrathin probes able to perform fluorescence imaging, photoacoustic imaging, and multiphoton imaging. Moreover, we show that using saturation excitation of fluorescent samples we can beat the resolution limit imposed by the numerical aperture of the optical fiber and obtain optical sectioning. In the first part of the thesis we describe which steps have been performed towards the implementation of these techniques. In the second part, we exploit coherent fiber bundles as imaging probes. This kind of fiber is typically utilized in endoscopy, but their resolution is limited by the distance between the fiber’s cores. Here we show two techniques - one based on DPC and a second based on statistical properties of speckle patterns - to obtain images with a better resolution than the one imposed by the core-to-core spacing, both relying on a unique property of multicore fibers: the memory effect. We also accomplished complex pattern transmission through multicore fibers with high resolution. Overall, the presented work provides new insights in imaging through optical fibers based on wavefront shaping techniques, opening up pathways for addressing current issues in this research field.