光纖中謝倫可夫輻射之模擬分析

Abstract

光纖中的謝倫可夫輻射是一項具有高效率地短波長頻段轉換技術，其產生機制起源於高階光孤子分裂的概念。在本實驗室的硬體實驗中，我們產生了轉換效率高達40%，平均功率高達250 mW的謝倫可夫輻射，此高功率的謝倫可夫輻射已經被應用來產生雙光子螢光顯微鏡、倍頻顯微鏡及光學同調斷層掃描術，及本實驗室現在也正在作波長可調高頻頻域光子傳播技術，未來極有潛力作為受激拉曼顯微鏡之用。 為了將來工程上減少時間與金錢的消耗，並在理論方面上了解光纖中的謝倫可夫輻射的演化，本論文發展了採用有限時域差分法，來模擬在光子晶體光纖中，飛秒脈衝雷射隨距離傳輸的時域與頻域演化過程，發現脈衝在傳輸過程中，展現出其三個機制；一階光孤子的產生，光孤子的紅移以及短波長謝倫可夫輻射的脈衝激發。我們模擬出受到脈衝參數影響以及隨著傳輸距離的演變的謝倫可夫輻射特性，並且以實驗結果來驗證模擬方法的準確性，以期在做實驗前，就能用此模擬方法，在做實驗前，先預測謝倫可夫輻射的頻譜與時間特性。在本論文的最後，提出了可望增加謝倫可夫輻射的轉換效率的改進方法。

Fiber optic Cherenkov radiation (CR) is an efficient technique to convert a laser pulse to shorter wavelength range. The generation mechanism of Cherenkov radiation is mainly based on the concept of high-order soliton fission. In our experiment, we have demonstrated high wavelength conversion efficient (>40%) and average output power (>250 mW) Cherenkov radiation. This radiation has been utilized for multi-photon fluorescence microscopy, multi-harmonic generation microscopy, and optical coherence tomography. Also we have been doing frequency-domain photon migration. The multi-wavelength source shows great potential for stimulated Raman scattering microscopy in the future. In order to reduce the expense of time and money during experiment and to exactly understand the evolution of Cherenkov radiation in fibers, we use the finite difference time domain method to simulate the temporal and spectral evolution of femtosecond laser pulses inside photonic crystal fiber. Numerical simulations reveal three mechanisms during propagation: the emergence of fundamental soliton, the red-shift of soliton, and the Cherenkov radiation in short wavelength spectrum.We simulate the dependence of Cherenkov radiation on pump pulse parameters and its evolution during the propagation. The correctness of numerical method was verified by experimental results. We hope that we will predict the temporal and spectral property of Cherenkov radiation before the experiment by using the simulation software.In the end of thesis, we demonstrate the method to improve the efficiency of Cherenkov radiation.