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The turbid nature of refractive index distribution, common for all living tissues, introduces severe aberrations to light propagation thereby severely compromising light-based observations using currently available non-invasive techniques. Numerous approaches of endoscopy, based mainly on fibre bundles or GRIN-lenses, allow imaging in extended depths of turbid tissues. Their footprint, however, causes profound mechanical damage to overlying regions and their imaging performance is very limited.
Optical systems have traditionally been understood as assemblies of components acting in a predefined and determinate manner. This notion is currently undergoing a transformation due to rapid advances in the technology and methods for spatial light modulation. Computer-controlled holographic modulators now facilitate the deployment of unusual and complex optical media with properties that bring unique advantages to biomedical applications.
Progress in this domain enabled a new generation of minimally invasive, high-resolution endoscopes by substitution of the Fourier-based image relays with a holographic control of light propagating through apparently randomizing multimode optical fibres.
The use of multimode fibres (MMFs) as ultranarrow endoscopes is a promising example of this concept, since it allows one to overcome the trade-off between the size of the optical element and the attainable resolution. The nature of light transport through MMFs leads to the transformation (or scrambling) of incident wavefronts into seemingly random speckle patterns. Adaptive optics provide a means for overcoming this signal degradation. A number of recently developed techniques in this domain have enabled the randomized output fields to be tailored into any desired distribution across the distal fibre facet or an arbitrarily remote plane. The most common form of laser scanning microscopy relies on the formation of diffraction-limited foci behind the fibre, combined with image reconstruction from fluorescence signals that are collected and guided backwards.
Digital micromirror devices (DMDs) have recently opened up a range of opportunities in this domain by increasing the achievable light modulation refresh rates by several orders of magnitude compared to well-established nematic liquid crystal-based devices. The foci behind a MMF can be scanned at several tens of kHz, thus acquiring images at speeds approaching video rates. Furthermore, it has also been shown that DMDs generate foci of higher quality compared to other modulators. Building on these advances, the focus of researchers is currently shifting towards implementations in biomedically relevant settings, including in vivo applications. I will review our fundamental and technological progression in this domain and introduce several applications of this concept in bio-medically relevant environments. I will present isotropic volumetric imaging modality based on advanced modes of light-sheet microscopy: by taking advantage of the cylindrical symmetry of the fibre, it is possible to facilitate the wavefront engineering methods for generation of both Bessel and structured Bessel beam plane illumination. Further, I will demonstrate the utilization of multimode fibres for imaging in living organism and present a new fibre-based geometry for deep tissue imaging in brain tissue of a living animal model. Lastly I will show the development and exploitation of highly specialised fibre probes for numerous advanced bio-photonics applications including high-resolution imaging and optical manipulation.
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