Beam-Scanning Nonlinear Optical Endoscopy

在這篇論文中，我們將介紹一架新一代的非線性光學內視鏡。這架內視鏡可以伸入生物體腔取得標本雙光子螢光和二倍頻的影像。和傳統內視鏡相較，它可以看到表面下的組織、也有更好的解析度。而和其他「新一代」內視鏡，譬如共軛焦內視鏡、光學同調斷層掃描內視鏡比起來，它也有較佳的穿透深度、比較不容易造成生物的光破壞、可以取得生物分子資訊等種種優點。現今非線性光學內視鏡的發展主要受限於缺乏可以有效激發非線性內視鏡訊號的光源。非線性光學顯微術通常使用飛秒脈衝雷射作為激發光源，但這些光源應用在內視鏡系統時，他們的超短雷射脈衝卻會被光纖內的色散、自相位調變等效應拉長，因而大幅降低訊號的激發效率。在我們設計的非線性光學內視鏡系統中，我們巧妙地選擇波長1230奈米的飛秒脈衝鉻貴橄欖石雷射為激發光源來根本解決因色散造成的脈衝拉長問題。 我們量測通過光纖後雷射脈衝的自相關軌跡和頻譜隨入射雷射光強度而變化的情形。自相位調變和微弱的色散作用使得鉻貴橄欖石雷射脈衝被略微拉長，但變長的幅度已遠小於波長800奈米的鈦寶石雷射脈衝在相似實驗條件下的變化。除此之外，和鈦寶石雷射相比，生物對鉻貴橄欖石雷射也有較好的耐受性。因此我們覺得鉻貴橄欖石雷射是個理想的生物活體內視鏡光源。 我們利用一條光纖束將雷射光導至樣品。這條光纖束內包含了上萬根各自獨立、可以導光的光纖，它們之間填塞了一些吸光物質，當雷射被導引進不同根光纖，遠端的光纖出口就會有相對應的光點照亮樣本、激發非線性訊號。同樣地，光纖束會收集產生的多光子螢光或倍頻訊號，並將它們導入光電倍增管，轉成電訊號後經電腦處理重建出一張二維影像。影像的解析度取決於光纖束中各光纖的相對距離，應用愈小愈密的光纖束可以提高解析度，但同時會拉長雷射脈衝、降低訊號強度，所以我們必需折衷各種考量才能挑出合適的光纖束。 完成非線性光學內視鏡的架設後，我們利用一些樣品測試它的表現。它可以得到螢光小球、葉肉組織和牛肉片裡結締組織的影像。未來加以發展，它能夠用來檢查受傷組織的修復狀況、偵測早期組織病變或是進行生物發育的研究。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 III 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 IV 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.Chapter 1. Introduction 1 Chapter 2. Principles of nonlinear optical endoscopy 7 2.1 Theory of nonlinear optics on second harmonic generation (SHG) and two-photon fluorescence (2PF) 7 2.1.1 Second harmonic generation 8 2.1.2 Two-photon fluorescence 10 2.2 Merits of nonlinear optical microscopy 13 2.2.1 Intrinsic optical sectioning 13 2.2.2 Deeper penetration depth 14 2.2.3 Reduced photodamage and photobleaching 15 2.3 Nonlinear fiber optics 16 2.3.1 Pulse-propagation equation 17 2.3.2 Group velocity dispersion (GVD) 20 2.3.3 Self-phase modulation (SPM) 23 Chapter 3. Nonlinear optical endoscope system 33 3.1 The compact femtosecond Cr:forsterite laser 33 3.2 The scanning unit, the fiber bundle and the imaging system 37 3.3 Discussion 39 3.3.1 The fiber bundle and the beam-scanning mechanism 39 3.3.2 Chromatic aberration 42 Chapter 4. Performance of the nonlinear optical endoscopy 47 4.1 Advantages of using femtosecond Cr:forsterite laser as the excitation 47 4.1.1 Greater penetration depth and less photodamage 47 4.1.2 Better transmitted-laser-pulse profile 49 4.2 The endoscopic 2PF and SHG images of the artificial and biological samples 58 4.2.1 The endoscopic 2PF image of the fluorescent microspheres 58 4.2.2 The endoscopic 2PF image of the fresh Rhaphidophora aurea (devil’s ivy) leaf 59 4.2.3 The endoscopic SHG image of the bovine tissue 61 Chapter 5. Summary 6