Graduate Thesis Or Dissertation
Robust, Fast and High Resolution Multimode Fiber Endoscopes Public Deposited
Current optical imaging techniques to image inside biological matter are limited in penetration to a depth of about a millimeter due to the highly scattering nature of tissue. Multimode fibers (MMF), with their small footprint, high resolution, and efficient light collection make excellent candidates for minimally invasive endoscopes that can potentially go much deeper inside tissue. Light propagating through an MMF however experiences modal dispersion and inter-modal coupling, leading to a random speckle pattern on the other end. Imaging through them requires a means of controlling the illumination on the object and measuring the return signal from the object for many such controlled illuminations. Wavefront shaping enables this control by employing an interferometric calibration of the fiber’s input-output relationship or transmission matrix.
In this thesis, we address three main challenges in MMF imaging- robustness, speed, and resolution. We first present a technique to improve the MMF imaging speed and simplify the calibration process by employing the naturally occurring speckle patterns at the MMF output for scanning the object. By combining the return signals for different speckle illuminations with a reconstruction algorithm, the object can be recovered using fewer measurements from a simpler and more robust system.
Secondly, we demonstrate high-speed wavefront shaping using a one-dimensional modulator operating at 350 kHz, known as a grating light valve (GLV). We characterize the wavefront shaping performance of the modulator, present an optimal optical configuration to maximize its performance, and show record speed of focusing light through an MMF using it, hence paving the way to faster MMF imaging.
Furthermore, we demonstrate mode control through an MMF with more than 7000 modes. With the achieved mode tunability, we can select a smaller subset of modes to create focal spots at the fiber output and characterize the bend sensitivity of different mode groups within the fiber mode set. We show that certain modes of the fiber are more resilient to bending than others.
Finally, we present a technique to achieve the optical sectioning and resolution gain of confocal imaging, while retaining a high signal-to-noise ratio. The technique generalizes the principles of image scanning microscopy to complex media and enables a practical solution to achieve optical sectioning for imaging 3-D samples with high resolution by employing multiple virtual pinholes to collect the back-scattered light from the endoscope.
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