Dispersion Compensation for Nonlinear Optical Endoscopes

• Main Authors: Chih-ChunLin, 林治均
• Other Authors: Shean-Jen Chen
• Format: Others
• Language: en_US
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/16189944885213315038

碩士 === 國立成功大學 === 光電科學與工程學系 === 100 === The theory and various methods of dispersion compensation for nonlinear optical endoscopes have been investigated in this thesis. A one meter long double-clad fiber with 4 μm core diameter is used for femtosecond pulse delivery and fluorescence signal collection, which also introduces a large amount of dispersion and nonlinear effects. Since high peak intensity laser pulses are to be delivered at the fiber scanning head, dispersion compensation becomes a critical consideration. Gratings are a type of passive dispersion compensating element that are commonly used in fiber endoscopic systems; however, upon compensating second order dispersion, it generates a large amount of uncompensated third order dispersion, leaving room for improvement in the higher order dispersion region. In addition, the precise amount of dispersion and other pulse distorting effects are difficult to precisely identify, thus limiting the possibility for optimizing the pulse condition. Active dispersion compensation elements such as spatial light modulators and deformable mirrors have the advantage of optimizing pulse conditions by the aid of computer algorithms for self-correction, and also the capability of compensating second and higher order dispersion; but the compensating amount is not enough to fully cover the dispersion caused by the fiber. In this thesis, by utilizing a deformable mirror combined with gratings for compensating not only second order dispersion, but also other pulse distorting effects that were difficult or unable to compensate using passive components, improvement of multiphoton excitation efficiency could be achieved. Gratings are used to compensate the majority of second order dispersion, while the deformable mirror is used to compensate residual second order dispersion and other pulse distorting effects. To demonstrate the effect of deformable mirror corrections, two photon excited fluorescence photon counts are used as a feedback signal for the deformable mirror to find the shape that yields the highest photon count. Increased contrast from images of 10 μm fluorescence beads could be observed after the deformable mirror correction. Improvements in the average and maximum photon count are 64% and 22%, respectively.