|Wavefront shaping-empowered multimode fiber and its applications within and beyond biomedicine
|Lai, Puxiang (BME)
Image analysis -- Data processing
Optical fibers in medicine
Hong Kong Polytechnic University -- Dissertations
|Department of Biomedical Engineering
|, 137 pages : color illustrations
|Optical multimode fiber (MMF) is widely used in telecommunication and biomedicine. For example, benefitting from the small diameter (<100 μm) of optical fibers, single-fiber endoscopy has attracted a lot of attention for minimally invasive diagnosis and treatment. Without conventional bulky probes and/or heads, fiber-based endoscopy can be implanted into deep and complicated tissue regions, such as brain, with minimal invasion. Note that, however, information or optical pattern input to the MMF is scrambled due to mode dispersion within the fiber, resulting in a seemingly random speckle pattern at the output. This phenomenon has led to limited spatial resolution or selectivity for MMF-based applications, although high information capacity is supported. The development of wavefront shaping, a technique aiming for overcoming light scattering in complex media, opens up new venues for controlling light propagation through an MMF. With wavefront shaping, the output from the MMF can be manipulated by modulating the incident light with a spatial light modulator, empowering various applications that is otherwise impossible.
However, the practical applications for the combination of wavefront shaping and MMF are still under exploration. And some significant drawbacks, such as the influence of external perturbations, should be addressed. With this purpose, in this thesis, five wavefront shaping-empowered MMF applications have been explored and are presented in two categories. In the first part, three different wavefront shaping approaches for enhancing the control ability are developed with individual demonstrative applications. In the first application, transmission matrix measurement with Hadamard basis via binary amplitude modulation is used as an MMF twist sensor. The sensitivity and measurement range of the twist sensor can be adaptively controlled via wavefront shaping. In the second application, measuring transmission matrix by scanning the fiber proximal end is presented for programmable optical logic operators. By dividing the screen of the spatial modulator into multiple subregions and measuring the transmission matrices of each of them, a single MMF can be used for long-distance optical logic gating, such as basic functions (AND, OR and NOT), cascaded multiple logic operation, and multi-bit logic operation. In the third application, an example of iterative wavefront shaping optimization algorithm is developed for arbitrary image projection through a long and unstable MMF with high fidelity and quality.
In the second part of this thesis, the applications based on wavefront shaping-empowered MMF are extended to biomedicine. A highly integrated system that can support multiple functions, including precise optogenetics, fluorescence imaging, and photoacoustic imaging, is proposed. The capability of high spatiotemporal resolution optical delivery through MMF for precise neuron stimulation is first demonstrated. Such a capability can be extended to penetrate through highly scattering media like MMF itself and a mouse skull. Moreover, as the scanning of the optical focusing for excitation and sensing are controlled electronically by the spatial light modulator, bulky mechanical components can be removed; the probe is miniaturized as a single MMF, whose diameter is as small as tens of microns, being inherently compatible for investigation of complex tissue regions such as deep brain that no other probes can access with minimal invasion. Last but not the least, multimodal imaging functions, including fluorescence and photoacoustic imaging, are integrated into this compact system. The system may further be combined with optogenetics or phototherapies towards a holistic high-resolution imaging-therapy-stimulation platform for deep tissues.
In brief summary, this thesis has explored wavefront shaping-empowered multimode fibers and their applications within and beyond biomedicine, such as imaging, logic operation, and tissue stimulation. With further engineering, the technique may open up new venues to optically probe and treat biological tissues at depths with minimal invasion, which is desired by the community yet deemed highly challenging to date.
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