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UBC Theses and Dissertations

Miniature multiphoton endoscopy using frequency-doubled Er-doped fiber laser Huang, Lin


Multiphoton microscopy (MPM) is a non-invasive, high-resolution imaging tool for visualizing tissues and organs. MPM uses femtosecond laser pulses to excite nonlinear signals from tissues and is capable of inherent optical sectioning. MPM systems have been mostly implemented using free-space optics and microscope platforms. For clinical applications, a compact fiber-optic MPM endoscopy with a miniature probe is needed for in vivo imaging. In this thesis, I explore the potential of applying femtosecond fiber laser in MPM endoscopy. Femtosecond fiber laser has the advantages of compactness, robustness, and direct fiber-coupling. Challenges in developing the MPM endoscopy includes optimization of the laser source for highly efficient MPM excitation, managing femtosecond pulse delivery through optical fiber, and designing a miniature scanning probe. In this thesis, systematic design, optimization, and application of a miniature MPM endoscopy based on the frequency-doubled Er-doped fiber laser are conducted, and the challenges are addressed. An Er-doped fiber laser operating at 1580 nm wavelength is developed and then frequency-doubled into ~790 nm wavelength to excite intrinsic two-photon excitation fluorescence signal from tissues. The frequency-doubling unit is integrated into the distal end of the miniature probe which is implemented by a miniature scanner and objective. The Er-doped fiber laser is directly fiber-coupled into the probe, making the system compact and portable. Imaging speed of MPM endoscopy is critical for clinical applications. To increase the imaging speed, the laser is optimized to shorten its pulsewidth to 80 fs for increasing the multiphoton excitation efficiency. All-fiber dispersion compensation and pulse compression by single mode fiber are conducted. A fast MPM imaging speed at 4 frames/s is achieved. Furthermore, the MPM endoscopy is applied for simultaneous two-photon and three-photon imaging. The fundamental laser pulse at 1580 nm and its frequency-doubled pulse at 790 nm are used as a dual-wavelength excitation source. Simultaneous imaging of two-photon excitation fluorescence, second harmonic generation, and third harmonic generation are achieved to acquire complementary information from tissues. Label-free multimodal imaging is demonstrated for biological tissues. Through this study, the miniature MPM endoscopy using frequency-doubled Er-doped fiber laser is shown to have great potential for clinical applications.

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