時間閘-FROG：研究鎖模超快雷射光脈衝電場起動之新技術

Abstract

我們建立了一套可以量測鎖模超快雷射光脈衝電場起動時包含相位以及振幅的資訊的系統，這套系統包含了二倍頻式頻率解析光閘( Frequency Resolved Optical Gating, FROG ) 利用量測不同時間延遲下干涉式自相干儀的功率頻譜所得到的頻率解析光閘影像代入相位重建之演算法程式中重建光脈衝電場的振幅與相位，以及由兩個鎖相的光學斬波器所組成的時間閘系統，量測不同起動時間下光脈衝的振幅與相位。 我們同時也針對Haus所提出的鎖模主方程式，配合我們實驗室自製的鈦藍寶石條件，進行理論上的分析，以及能對脈衝形成動力學有更進一步的瞭解。 在實驗過程中我們發現脈衝的振幅與相位在起動初期有很明顯的不對稱的現象，我們推測主要的原因應該是當脈衝寬度小於飽和式布拉格反射鏡之載子生命期時，飽和式布拉格反射鏡之遲緩（relaxation）效應所造成的。

We built a system that can measure the amplitude and the phase information of the optical pulse during the buildup process of a mode-locked ultrafast laser. Two main parts constitute the system. One is the Frequency Resolved Optical Gating in SHG configuration. The power spectrum of the autocorrelatior signal with different delay, called a FROG trace, is measured and input a LabVIEW program with phase retrieval algorithm to obtain the amplitude and phase information of the laser pulse. The other is a time gating system that the composed of two phase-locked optical choppers. By adjusting the relative phase of these two choppers, we can measure the optical pulse at different buildup time. We also proceed the theoretical analysis based on the master equation of mode-locked proposed by H. A. Haus to gain further insight of the buildup dymamics. We found that the amplitude profile and phase of the pulse in the early stage of the buildup process have significant asymmetry. We postulate that the reason is due to the relaxation effect of the saturable Bragg reflector when the pulsewidth is smaller than its carrier lifetime.