具脊狀波導結構之準相位匹配綠光倍頻晶體研究

Abstract

本篇論文重點在製作出具脊狀波導之週期性極化反轉鈮酸鋰倍頻晶體(PPCLN:WG)，期望可達成微型化高效能綠光雷射之目標。主要分成波導與準相位匹配原理的介紹、晶體製程，以及光學量測。 製程部分，介紹質子交換溼式蝕刻法來製作脊狀結構，並採用較低溫的鎳擴散完成波導製程，且在第四章分析波導導光特性。接續介紹本實驗室建立的高電壓致極化反轉系統，詳細說明該架構與模型，並介紹完整製程流程與實驗結果。波導方面，成功製作出最深可達5.5um的脊狀結構，而鎳擴散在800℃、4小時與900℃、1小時皆有良好的傳導TM模態特性。週期極化反轉結構上，可在波導上製作出週期6.7um、晶體厚度0.5mm、有效長度6mm的晶體，寬度可隨波導寬度改變，成功製作出具脊狀波導之週期性極化反轉鈮酸鋰。 光學量測部分，首先藉由拋光端面作鎳擴散之定性分析並量測波導特性與觀察光場圖形。倍頻量測上分成兩個耦合系統介紹，最後量測出在1064nm基頻光源峰值強度25.5MW/cm2、平均功率6mW下，有效長度為6mm的倍頻波導有20%的外部轉換效率；而未做波導的bulk區域在峰值強度7.6MW/cm2、平均功率41mW下最高外部轉換效率為1.2%。

This thesis focus on the fabrication of periodically poled lithium niobate on ridge waveguides (PPCLN on WG). It includes three parts: an introduction to the theory of waveguides and quasi phase matching (QPM), followed by the fabrication techniques, and the results of optical characterization. As for the fabrication part, we use the method of proton exchange and wet etching to make a ridge structure, which is followed by nickel diffusion to accomplish the realization of the optical waveguide structure. We then apply the pulsed electric field poling technique to realize PPCLN on the ridge waveguides. A PPCLN-WG with a periodicity of 6.7um, with ridge length of 6mm of height of 5.5um can thus be accomplished. As for the optical characterization part: a Nd:YVO4 1064nm laser with 18.8ns pulse width was used as the pump source. We report an observation of 20% conversion efficiency for second harmonic green generation at a pump power of 6mW, equivalent to peak pump intensity of 25.5MW/cm2. In comparison, bulk PPCLN pumped by 41mW 1064nm laser only gives a conversion efficiency of 1.2% due to a weak pump intensity of 7.6 MW/cm2 and lack of optical confinement in the mixing of nonlinear waves.