利用雙波長飛秒脈衝激發光子晶體光纖產生紫外超連續光譜之數值分析

Abstract

為了使光子晶體光纖產生的光譜有效的延伸到紫外光的波段，在這篇論文中，我們利用飛秒鈦藍寶石脈衝雷射產生的基頻以及倍頻光同時打入光子晶體光纖中，做了數值模擬以及實驗觀察，研究這兩道光脈衝在時空上重疊時所產生的交叉相位調變作用，使得產生的光頻譜有效地往短波長延伸。在數值模擬中，我們利用分離步驟法去求解藕合非線性薛丁格方程，發現利用790 nm 以及 395 nm 的光脈衝同時導入光子晶體光纖NL-PM 760中，在短波長的部分產生的光譜最連續且平坦，此效應會隨著兩道光脈衝峰值功率的增加變得更加明顯。但是在實驗觀察中，紫外光的光頻譜展開並沒有理論預測的明顯，推論這是因為短波長的光在光子晶體光纖中有嚴重的損耗，或是因為倍頻光影響了光的耦合條件，使得輸出的超連續光譜不如理論預測。

In order to make the further extension of the generated spectrum to the near UV part, we do the simulation and the experimental investigation on the supercontinuum generation using the fundamental and its second harmonic femtosecond pulses from a Ti:sapphire laser into the photonic crystal fiber simultaneously. When these two color pulses are spatially and temporally overlap in the photonic crystal fiber, the cross phase modulation will extend the spectrum toward short wavelength. In the simulation, we use the Split-Step Fourier Method to solve the coupled nonlinear Schrödinger equation. The most continuum and flattened spectrum on the UV part can be generated by using the dual wavelengths of 790 nm and 395 nm. The spectra become more and more continuous and flattened as the input peak power of these two pulse increase. Nevertheless, in the experiment, the spectrum on the UV part does not become broadened as expected in our simulation results. It might be due to the seriously loss on the UV part or the change of coupling efficiency resulted from thermal expansion by the launch of the second harmonic pulse to change the core size and the value of numerical aperture.