Research On High Contrast-to-Noise Ratio Multimode Fiber Confocal Imaging

As a new type of endoscope technology,multimode fiber confocal imaging technology have attracted widespread attention for its small footprint and high resolution.Since the invention of wavefront modulation devices,multimode fiber confocal imaging technology has been greatly developed.Almost all the previous related researches focus on the large field of view,high resolution and 3D imaging capability of multimode fiber confocal imaging but ignoring the contrast-to-noise-ratio(CNR)of multimode fiber confocal imaging.In fact,high CNR is indispensable for many real applications,including the observation of dynamic activity and fine stimulation of neurons,the observation of subcellular structures in vivo,and the detection of micro-cavity defects.In our research,different novel methods are proposed to enable high CNR in multimode fiber confocal imaging.The major innovations can be summarized as follows:(1)We proposed a new method to improve the CNR of multimode fiber confocal imaging by suppressing the background speckle generated by mode interference in the fiber effectively.When we use different fiber states to form a focus point at same position,background speckle around the focus point is different from each other,so the background speckle can be effectively suppressed by superposing the focus points at different fiber states.We also studied the relationship between the speckle pattern on the end surface of the fiber and the state of the fiber,finding that a small change in state of the fiber can make the speckle patterns significantly irrelevant.Then we verified the irrelevance of the background speckle through simulation and experiment.(2)We verified the proposed high CNR multimode fiber confocal imaging method through superposing images captured with different wavelengths in simulation and experiment.In simulation,100 images corresponding to 100 different wavelength states were superposed,achieving a 6-fold enhancement in CNR of image.In experiment,20 sets of wavelength states were superposed,achieving a 4-fold enhancement in CNR of image at 200 ?m field of view.(3)We verified the proposed high CNR multimode fiber confocal imaging method through superposing images with different fiber bending states in simulation and experiment.In simulation,50 images with different bending states were superposed,achieving a 4-fold enhancement in CNR of image.In experiment,10 sets of bending states were superposed,achieving a 3-fold enhancement in CNR of image at 200?m field of view.In summary,we proposed a new method about multimode fiber confocal imaging with high CNR.This method endows multimode fiber confocal endoscope imaging with large field of view,high CNR and high resolution,promoting the application of multimode fiber imaging in neuroscience,medical endoscopes,defect detection and other fields.