利用FROG技術研究超快雷射特性

Abstract

我們發展一套ＳＶＤ演算法以對光脈衝電場進行特性分析，和以前使用的Conjugated-Gradient方法比起來，程式撰寫更容易且程式執行的速度加快，而在雜訊過濾上我們使用低通濾波器，以使演算的結果更好。實驗上我們使用色散補償脈衝來驗證程式實際上的可靠度，並獲得一不錯的結果，大致可以獲得脈衝的相位資訊。 利用這工具結合時間閘技術量測我們自製的鈦藍寶石雷射的啟動特性，我們發現在啟動後36微秒至42微秒期間脈衝的相位呈現比較不對稱的形狀，推測這跟飽和吸收作用而導致的脈衝前後緣不對稱，進而影響到脈衝經過增益晶體後受到的ＳＰＭ效果不一樣所引起的相位差。而且在這段時間內也觀察到模擬的自相關軌跡具有底座現象，所以還存在某些雜亂模使得雷射不是很穩定，所以疊代出來脈衝的寬度變化很大。

We developed our proceeding work on FROG technique. Using the SVD(Singular value decomposition)algorithm which is reported by D.J. Kane replaced the Conjugated-Gradient method. This technique made the program compact and run faster than the CG’s. Further, low-pass filtering lessens our experimental noise, which may smear the FROG trace significantly to cause inaccurate retrieved E-field and trace of pulses. Then, using the pre-compensation prism pair to prove our program accuracy and abtain an accurate result. The time-gated FROG technique incorporating the SVD algorithm to measure the build-up dynamics of our home- made passive mode-locked Ti-sapphire laser with a Strained Saturable Bragg Reflector (SSBR). There exist relaxation oscillation in the beginning of the build-up. The transit from initial mode-locking to steady state took about 20us.We find asymmetric phase shape of the E-field and there exist shoulder from 36us to 42us during this time. It may be the soliton mode locking mechanism to cause these result.