| Summary: | 碩士 === 國立臺灣大學 === 光電工程學研究所 === 93 === In this thesis, we present a beam-scanning nonlinear optical endoscope based on a Cr:forsterite laser, a fiber bundle, and micro aspheric lenses. This system is a new-generation endoscope which can take two-photon fluorescence and second harmonic generation images of biological samples and outperforms traditional endoscopes in several aspects. It has better resolution, sectioning ability and obtains sub-surface images. When compared with other “new-generation” endoscopy, like endoscopic optical coherence tomography and confocal endoscopy, the nonlinear optical endoscopy distinguishes itself by showing inherent three-dimensional sectioning, deeper penetration depth, reduced photodamage, and the ability to extract molecular information. Nonetheless, the temporal broadening of ultrashort pulses in fibers has greatly hampered the development of nonlinear optical endoscopy. Here we propose a solution to efficiently suppress the dispersion-induced pulse broadening rate as using a femtosecond Cr:forsterite laser at 1230 μm wavelength as the light source of our newly-designed nonlinear optical endoscope.
Firstly, we coupled the Cr:forsterite laser pulses through one core of the fiber bundle and measured the autocorrelation and spectrum of the transmitted pulse to characterize propagation of the laser pulses through the fiber pixel as a function of average transmitted power. It is demonstrated that those laser pulses are indeed
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virtually free from dispersion-caused broadening. On the other hand, for the femtosecond Ti:sapphire laser pulses with the similar initial condition, the transmitted pulses’ temporal widths were stretched considerably due to serious dispersion. Furthermore, the Cr:forsterite laser has been shown to penetrate deeper and to be less damaging inside the biological tissues. With all these merits, the Cr:forsterite laser is considered as a better illumination source for a nonlinear optical endoscope targeted at in vivo biological imaging than the Ti:sapphire laser.
We then characterize the scanning mechanism as well as the resolving power of our endoscope. The fiber bundle serves to deliver the laser pulses to the samples and provides a simple two-dimensional scanning mechanism. As the excitation laser beam being coupled sequentially into each image pixel, a light spot at the dismal fiber output then raster scans the sample. The fiber bundle will collect and deliver the nonlinear optical signals which finally will be detected by a PMT. The time-demodulating computation then reconstructs a two-dimensional image. Resolution of the endoscopic images is determined by the core spacing of the fiber bundle, while signal intensities are also heavily depended on core sizes. Thus, we have chosen the fiber bundle carefully to balance the quality of resolving power and signal intensities.
We put several samples, such as fluorescent microspheres, leaves, and bovine tissues
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under our endoscope. The endoscope is capable of taking two-photon fluorescence and second harmonic generation images that either locate the microsphere, or reveal the mesophyll cells, or map the distribution of bovine connective tissues. This novel nonlinear optical endoscope is a promising tool for in vivo biomedical imaging. It can perform optical biopsy on specimens without excision of tissue and image intact specimens within internal cavities of the body. Its applications might include diagnoses of tissue repair after injury, early-stage disease detection, and studies of developmental processes.... Thus, our nonlinear optical endoscope indeed shows good potential for future clinical use and biomedical research.
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