腔內非線性光學之波長轉換與斑圖形成

Abstract

利用非線性晶體的受激拉曼散射特性，可以有效率地實現新波長固態雷射，然而在實驗過程中，我們發現拉曼雷射效能總是受限於晶體的熱透鏡效應，尤其是二極體雷射端面激發的固態腔內拉曼雷射。本論文分為兩個部分。第一部分致力於改善晶體的熱透鏡效應，用以得到高效率的腔內非線性波長轉換。首先，我們藉由量測三種結構Nd:YVO4雷射晶體之等效熱透鏡焦距，比較其熱透鏡效應的強度，包括傳統晶體、單端複合晶體與雙端複合晶體。實驗結果證實:使用雙端接上2.1 mm YVO4的複合晶體可以最有效地改善1342 nm Nd:YVO4雷射之熱透鏡效應。之後，我們設計獨創性的雙端複合Nd:YVO4晶體，並首次使用複合晶體當自發拉曼晶體，有效提升Q-開關人眼安全拉曼雷射效能。我們又利用此雙端複合Nd:YVO4自發拉曼晶體與BBO腔內混頻晶體，實現高效率Q-開關黃綠光拉曼雷射。此外，我們首次使用釩酸鹽當拉曼晶體，將雷射晶體與拉曼晶體分開，提升了使用傳統晶體的1176 nm Q-開關Nd:YVO4自發拉曼雷射效能。 我們亦利用Stokes位移值小的KTP與KTA拉曼晶體，實現Q-開關多階拉曼雷射，並觀察到受激拉曼散射與圓錐形二倍頻轉換分別形成的「交叉」與「圓環」遠場斑圖。本論文第二部分係利用非線性轉換機制，研究斑圖形成。我們利用有延伸性缺陷之非線性晶體，可從Q-開關雷射加上腔內倍頻有效產生圓錐形二倍頻轉換。再利用光學技術量測近場模態，探討無序性波函數之空間分布結構，有趣的是，我們找到了波函數可以把混沌波與Bessel光束作連結。

Stimulated Raman scattering (SRS) has been regarded as an efficient and practical method of generating solid-state laser sources at new wavelengths. In our experiments, it is found that the thermal lensing effects are critical issues exacerbating the laser performance of diode-end-pumped solid-state intracavity Raman laser. The thesis is divided into two parts. The purpose of the first part is to improve the thermal effect and demonstrate high-efficiency intracavity nonlinear wavelength conversion. First the comparison of thermal lensing effects between conventional, single-end composite, and double-end composite Nd:YVO4 crystals is investigated by measuring the effective focal lengths of thermal lens. The experimental results confirm that using a composite Nd:YVO4 crystal with double 2.1-mm-long ends can efficiently improve the thermal lensing effect for 1342-nm transition. Then we design a original double-end composite Nd:YVO4 crystal and first employ a composite crystal as a self-Raman medium to improve the laser performance of Q-switched eye-safe Raman laser. A high-efficiency Q-switched yellow-green Raman laser with self-Raman frequency conversion in the double-end composite Nd:YVO4 crystal and intracavity sum-frequency generation in BBO crystal is also demonstrated. Moreover we use a new design of Q-switched Nd:YVO4 Raman laser with an intracavity vanadate as a Raman crystal to improve the laser performance of 1176-nm self-Raman laser with a conventional Nd:YVO4 crystal. In addition, KTP and KTA crystals with low value of Stokes shifts permit generation of multi-frequency radiation with cascade SRS. The “cross” and “ring” far-field wave patterns of SRS and conical second harmonic generation (SHG) are observed in a cascade Raman laser respectively. The second part of this thesis is intended as an investigation of pattern formation in a laser with nonlinear wavelength conversion. By using a nonlinear crystal with extended defects, the conical SHG is efficiently generated from a Q-switched laser with intracavity SHG. The near-field patterns of conical SHG can be used to explore the spatial structure of disordered wave functions. It is interesting that chaotic wave can be connected with Bessel beam by the wave functions.