Fabrication and characterization of periodically poled ferroelectrics

在以光纖為主的非線性應用中，週期性極化反轉鐵電材料之晶體光纖具有許多優勢，本論文中首先介紹利用雷射加熱基座長晶法(LHPG)結合外加電場即時監控(in-situ)極化反轉過程，製作週期性極化反轉之鈮酸鋰(PPLN)及鉭酸鋰(PPLT)晶體光纖。實際應用上，在摻氧化鋅之PPLN晶體光纖中，以自串接式(Self-cascaded)的倍頻(SHG)加和頻(SFG)來產生可調變波長之藍綠光輸出，可調範圍從471.3 nm至515 nm，具有超過40 nm的可調頻寬。 此外，為了達到低損耗之光波傳導，首次利用共抽絲雷射加熱基座長晶法來製造出具玻璃纖衣結構之鉭酸鋰(LT)晶體光纖，藉由折射率分布曲線的量測可將玻璃纖衣結構之晶體光纖分為兩類: 一種為步階式折射率光纖，另一種為漸變折射率光纖，目前兩種光纖結構皆為多模光纖，但是若藉由高精準度控制LHPG系統也可以製作出單模LT晶體光纖。 在材料分析與特性量測方面，光學同調斷層掃描術(OCT)首次被應用在非線性晶體之檢測。我們利用自製的摻鈰釔鋁石榴石雙纖衣晶體光纖所產生的寬頻光源，來架設超高解析度OCT系統，可以成功的檢測出在PPLN極化反轉邊界的折射率差值與邊界之均勻性，更進一步地，針對週期性極化反轉鐵電材料之波導元件，其複合結構、色散、以及微小的折射率對比度皆可利用其非破壞性檢測之特性來完成量測與分析。其中，OCT系統在鐵電材料中的縱深解析度為0.68 μm，橫向解析度為3.2 μm以及折射率對比之靈敏度為4×10-7。 量測結果中，針對摻氧化鎂共融比之週期性極化反轉鈮酸鋰晶體，可計算出其+z以及-z反轉區域之折射率差為4.2×10-4，對於鐵電材料極化反轉過程的品質為重要之指標。更進一步地，針對非尋常光(extraordinary ray)波長500 nm到750 nm間，常溫下之色散曲線也可求得。利用OCT系統的高空間解析度以及高折射率對比靈敏度之檢測技術，未來可應用有助於準相位匹配非線性之波導元件的發展，來達到波長轉換效率的增進，以及耦光的模態匹配來降低插入損耗。Periodically poled ferroelectric crystal fibers are expected to have advantages when using in fiber based nonlinear applications. The laser-heated pedestal growth (LHPG) method with the external electric field in-situ poling is used to fabricate periodically poled LiNbO3 (PPLN) and periodically poled LiTaO3 (PPLT) crystal fibers. In application, a novel self-cascaded first-order second-harmonic generation and third-order sum-frequency generation in a ZnO:PPLN crystal fiber was demonstrated for the generation of tunable blue–green light. The tuning range was more than 40 nm, from 471.3 to 515 nm. To facilitate wave propagation with low loss, glass-clad LT fibers were fabricated by a co-drawing LHPG method for the first time. The glass-clad fibers are classified into two different categories by the measurements of refractive index profiles. One is a step-index fiber and the other is a graded-index fiber. Both fiber types are multimode at present, but single-mode LT fibers could be fabricated with the high-precision control of the LHPG system. To characterize the periodically poled ferroelectrics, optical coherence tomography (OCT) technique was employed on the nonlinear medium for the first time. Using the Ce3+:YAG double-clad crystal fiber based ultrahigh resolution OCT system, the refractive index difference and uniformity of domain boundary a PPLN crystal were successfully examined. Furthermore, it is demonstrated that the complex structure, dispersion, and small index contrast of periodically poled ferroelectric waveguides can be non-invasively characterized. An axial resolution of 0.68 μm, an transversal resolution of 3.2 μm, and an index contrast sensitivity of 4×10-7 were achieved. The index difference between the +z and –z domains in a MgO-doped congruent LiNbO3 was estimated to be 4.2×10-4, which is an important indicator for the quality of the poled ferroelectrics. The dispersion of extraordinary wave of a 5 mol.% MgO-doped CLN was characterized from 500 to 750 nm at room temperature. The high spatial resolution and high index contrast sensitivity technique can facilitate the development of quasi-phased nonlinear waveguide devices for improving wavelength conversion efficiency as well as reducing insertion loss by mode-matched coupling