主動式晶體光纖之光子元件

Active Crystal Fiber Based Photonic Devices

賴建智(Chien-Chih Lai)

指導教授 ： 黃升龍

國立臺灣大學/電機資訊學院/光電工程學研究所/博士(2010年)

https://doi.org/10.6342/NTU.2010.10487

摘要

由於過渡金屬具有未屏蔽電子組態，故將其摻入晶體作為雷射增益介質可獲得極寬頻之發光特性。在700-1000 nm波段，鈦藍寶石(Ti3+:sapphire)雷射已廣泛應用於寬頻可調雷射及鎖模雷射上。但在1200-1600 nm之光通訊波段，因摻Cr4+濃度低及熱效應問題，使得摻Cr4+雷射在此光通訊波段上進展受限。於本論文中，我們以共同提拉雷射加熱基座生長(codrawing laser-heated pedestal growth)法生長出具雙纖衣(double-clad)結構之摻鉻釔鋁石榴石(Cr4+:YAG)晶體光纖(crystal fiber)。並首次研製出室溫下具世界紀錄最高之斜率效率(6.9%)及最低之激發閥值(96 mW)之摻鉻雷射，其中激發閥值相較文獻上塊材式之摻鉻釔鋁石榴石雷射低超過1個數量級以上。此外搭配銅鋁合金包覆，此晶體光纖雷射可進一步提供纖衣激發(cladding pump)及更佳散熱效率。此具有極高效率及極低激發閥值之雙纖衣晶體光纖雷射在未來將相當有潛力達成通訊用波長可調光纖雷射。
此外，目前現有摻釔光纖雷射其光纖長度長達數米至數十米。對於輸出功率為瓦級之應用，尤其當需要單縱模雷射輸出時，此極長的光纖長度則顯得不實際。於本論文中，我們首次以共同提拉雷射加熱基座生長法生長出長度僅7 mm之摻釔釔鋁石榴石-玻璃(Yb3+:YAG-silica)光纖雷射，其輸出功率可達1 W/cm。此短長度光纖雷射於室溫下具有世界紀錄最高斜率效率(76.3%)及最低激發閥值(25 mW)，極適合與矽基平面元件整合。
在奈米尺度光學與微結構分析上，近場掃描式光學顯微術(near-field scanning optical microscopy)和高解析穿透式電子顯微術(high-resolution transmission electron microscopy)此兩種技術扮演極重要角色。本論文首先針對具異質結構(heterostructure)、高硬度及極脆弱之摻鉻釔鋁石榴石雙纖衣晶體光纖，成功製作高解析穿透式電子顯微鏡之試片，並藉此試片首度以具高空間分辨率之近場掃描式光學顯微鏡解析出位於內層纖衣(inner cladding)之奈米結晶顆粒其近場光譜特性，並搭配高解析穿透式電子顯微鏡於微結構上作分析比較。此外，本論文亦藉由量測其雙纖衣晶體光纖纖心(core)之近場光譜特性，首次分析生長後之應力變化(strain)分佈與螢光關係。以期能藉由生長參數調控纖心內應力，進而提升摻鉻釔鋁石榴石雙纖衣晶體光纖所研製之通訊用主動式光子元件效率。

關鍵字

摻鉻 ； 摻釔 ； 釔鋁石榴石 ； 光纖雷射 ； 近場掃描式光學顯微術 ； 高解析穿透式電子顯微術

並列摘要

Transition-metal ion doped laser gain media have broadband nature because of the non-screened electronic configurations. In the 700-1000 nm wavelength range, Ti3+:sapphire lasers have been widely used as tunable and mode-locked lasers. However, in the 1200-1600 nm optical communication band, Cr4+ doped lasers have limited progress because of the low concentration of tetrahedrally positioned Cr4+ ions and the thermal problem. In this dissertation, using the codrawing laser-heated pedestal growth technique, we demonstrate the first room-temperature (RT), continuous-wave (CW) Cr4+:Y3Al5O12 (Cr4+:YAG) double-clad crystal fiber (DCF) laser with a 6.9% record-high slope efficiency and a 0.75-mW record-low lasing threshold, more than 500 times lower in threshold than any reported Cr4+:YAG lasers. With a Cu-Al alloy diffusion process, the DCF allows for cladding-pumped configuration with efficient heat removal. The realization of ultralow-threshold lasing with record-high slope efficiency makes broadband tunability of this DCF laser possible for future all-optical communication systems.
In addition, most existing Yb:fiber lasers have long cavities that are in several meters to tens of meters for hundreds to kilos watts of powers. For watt-level applications, such long length is not practical especially when a narrow-linewidth laser is required. In this study, we report the first demonstration of an ultracompact, high-efficiency, and low-threshold Yb3+:YAG-silica fiber laser with nearly 1 W/cm CW output at RT. To the best of our knowledge, this 7-mm-short Yb3+:YAG-silica fiber laser with record-low threshold down to 25 mW and record-high slope efficiency up to 76.3% is the shortest active fiber reported to date for any short-length fiber laser operated at RT, making it suitable for integration with Si-based devices.
For nanospectroscopic and nanostructural characterizations, near-field scanning optical microscopy (NSOM) and high-resolution transmission electron microscopy (HRTEM) techniques have played key roles. Here we have successfully prepared the NSOM and HRTEM specimens of Cr:YAG DCFs, which are heterostructure, hard, but fragile. The NSOM results were compared with those obtained by HRTEM. In this dissertation, for the first time, we show the systematical studies on the nanospectroscopy and nanostructure of Cr:YAG DCFs. Further, we also show the first direct evidence of the impact of strain fields on the optical properties of Cr:YAG. This new class of strain-adjustable Cr:YAG DCF opens up new opportunity to improve the performance of crystal fiber based photonic devices in all-optic fiber communications.

並列關鍵字

Cr-doped ； Yb-doped ； YAG ； fiber laser ； NSOM ； SNOM ； HRTEM

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