利用週期性極化反轉之非線性鈮酸鋰晶體致模態耦合現象以塑形泵浦結構光特性

Abstract

本篇論文分為三個部分。第一部分說明鈮酸鋰晶體以及準相位匹配的背景概念。第二部分介紹週期性極化反轉結構的製程流程、光罩設計的參數，以及取樣晶體內建電場與矯頑電場的方法。第三部分則著重於光學量測，研究入射光源與所設計光罩結構之間的關係。其中，該結構包含一個在單一週期內特定寬度的空白區域（該區域不具備週期性極化反轉結構），其週期大小為16.634μm。本研究的核心關注點在於泵浦光的模態變化，以了解入射光源在非線性光學頻率轉換過程中的影響。我們利用自製的影像系統來拍攝晶體內部的模態分布，實驗結果顯示，隨著光在晶體內部的傳播距離變化，模態會發生變化，並出現類似波導耦合的效應。我們針對四個參數進行分析，以確認影響模態變化的因素：入射模態的能量分佈、模態的發散角度、實驗操作溫度以及晶體厚度。實驗結果表明，操作溫度必須高於 95°C，且發散角度必須足夠大，以確保模態耦合現象不受干擾。此外，數據顯示泵浦光在樣品結構中可根據入射特定區域的能量大小決定該區的耦合效率。透過對這些相關參數的比較與分析，我們能夠更深入理解泵浦光在樣品結構中的運作條件，進而建立對泵浦光的基礎認識，這將有助於未來對非線性光學三波混成轉換過程中的結構光研究。

This thesis is divided into three parts. The first part explains the background concepts of lithium niobate crystals and quasi-phase matching are introduced. The second part describes the fabrication process of the periodically poled structure, the parameters for photomask design, and the methods used to measure the built-in electric field and coercive field of the sampled crystal. The third part focuses on optical measurements, examining the relationship between the incident light source and the designed photomask structure. Notably, this structure includes a blank region of a specific width within a single period (which does not contain a periodically poled structure), with a period size of 16.634 μm. The study emphasizes the modal variations of the pump light to understand the influence of the incident light source on frequency conversion in nonlinear optics. A self-developed imaging system was used to capture the internal mode distribution within the crystal. Experimental results show that as light propagates through the crystal, the mode evolves and exhibits an effect similar to waveguide coupling. Four parameters were analyzed to determine the factors influencing mode variations: the energy distribution of the incident mode, the mode divergence angle, the experimental operating temperature, and the crystal thickness. The experimental results suggest that the operating temperature must be greater than 95°C and that the divergence angle must be sufficiently large to ensure that the mode coupling phenomenon remains unaffected. Additionally, the data show that the coupling efficiency in a specific region of the sample structure can be determined by the amount of pump light energy incident on that region. By analyzing these parameters, this study provides insights into the operational behavior of pump light within the structured crystal, laying a foundation for future research on structured light in three-wave mixing processes in nonlinear optics.