研究固態拉曼雷射中多波長、自鎖模與光渦之產生

Abstract

固態雷射已經被廣泛地應用在醫學治療、生物醫藥，以及遙控感應、測距等工業，甚至是國防等不同領域當中，在現今社會中已是十分重要的一門科學。但由於雷射晶體之能階特性關係，雷射輸出波長之可變性因而受到限制。因此，擴展固態雷射之輸出波段使其能應用於更多不同的領域中，成為現今雷射研究的一大主題。在過去的研究當中常利用二倍頻諧波轉換、光參數震盪等非線性光學來達成雷射輸出波長的轉換。此外，某些非線性晶體的受激拉曼散射特性也可有效地實現新波長之雷射輸出，此種雷射我們稱之為固態拉曼雷射。本博士論文將以受激拉曼散射作為波長轉換的基礎，來探討固態拉曼雷射中多波長、自鎖模與光渦之產生及現象。 在多波長固態拉曼雷射中，我們利用Stokes位移較小之KTP做為拉曼晶體，並配合三種不同高反射鍍膜的輸出鏡以產生多波長雷射輸出；此外我們也使用擴散鍵結異質複合晶體來實現雙波長固態拉曼雷射。在自鎖模的研究中，我們利用了平凹與單晶體兩種共振腔來探討自鎖模拉曼雷射中的脈衝時序。其中，我們進一步將單晶體自鎖模拉曼雷射放入低溫系統，實驗結果發現除了輸出功率有顯著的改善，更發現了當溫度低於125K時拉曼雷射可達成單一縱模之輸出。最後，我們利用兩種方式來產生位於拉曼波段之光渦。利用離軸激發的方式，我們可以產生各種在拉曼波段之高階橫向模態，接著再利用柱透鏡組將這些高階橫向模態進一步轉換得到光渦。另外，我們也使用圓環形的激發光源來有效且直接地產生在拉曼波段之光渦。

This thesis demonstrates various solid-state Raman lasers to study the topics of multi- wavelength emission, self-mode-locking (SML), and optical vortex. First of all, we utilize a Nd:YVO4 crystal as the gain medium and a KTP crystal as the intra-cavity Raman medium to realize continuous-wave multi-wavelength solid-state Raman lasers through the cascade stimulated-Raman-scattering (SRS) process. Besides, a new type of diffusion-bonded hetero- composite crystal is used to obtain dual-Stokes-wavelength emission. In the part of the SML, we employ plano-concave and monolithic resonators to investigate the temporal behaviors of the SML. For the plano-concave resonator, the complex spatial-temporal dynamics is experimentally observed and theoretically reconstructed to verify that the spatiotemporal dynamics arises from the total mode-locking of fundamental and high-order transverse modes. For the monolithic cavity, the phase-locked frequency comb can be significantly expanded through the SRS process according to the width of the Raman gain. Furthermore, we put this monolithic SML self-Raman laser into a cryogenic system to scale up the SRS output power. Experimental results reveal that not only the SRS output power can be improved from 0.78 W to 1.36 W with temperature decreasing from 285 K to 80 K but also the single-longitudinal- mode operation can be observed when temperature is lower than 125 K. Finally, we display two methods to generate SRS optical vortices. The first method is to convert high-order Hermite-Gaussian beams at the Stokes wavelength into the SRS Laguerre-Gaussian beams with the help of a π/2 cylindrical-lens mode converter and the second method is to directly pump the Raman-active medium with an optical vortex.