非線性弱極化旋轉與強飽和吸收啟動1.6-2.0微米鎖模光纖雷射之色散管理研究

Dispersion Management of the 1.6-2.0 um Mode-Locked Fiber Laser Started with Nonlinear Weak Polarization Rotation and Strong Saturable Absorber

林俊諺(Chun-Yen Lin)

指導教授 ： 林恭如

國立臺灣大學/電機資訊學院/光電工程學研究所/碩士(2016年)

https://doi.org/10.6342/NTU201603474

摘要

在本論文中，我們首先使用非線性偏振旋轉啟動1.6微米被動鎖模摻鉺光纖雷射，並嵌入不同長度之色散補償光纖進行腔內色散管理。在未嵌入色散補償光纖，即操作摻鉺光纖雷射在負色散區時，光固子鎖模脈衝具有最窄405飛秒之脈衝寬的雷射輸出在高泵浦電流的條件下。在更進一步嵌入近30公尺之色散補償光纖後，即調整此摻鉺光纖雷射被操作在微負色散區，此時藉由調整偏振控制器可觀察到有典型光固子與另一似噪脈衝之兩種鎖模操作態的自由轉換與輸出。當摻鉺光纖雷射在典型光固子模式工作時，最窄220飛秒之壓縮鎖模脈衝的產生可以被獲得;而在似噪脈衝模式工作時，最窄109飛秒之鎖模雷射脈衝可以被觀察到。最後，於摻鉺光纖雷射嵌入約55公尺之色散補償光纖後，藉由適當調整腔內偏振，鎖模脈衝更可進一步在三種含耗散光固子、展寬脈衝及似噪脈衝之鎖模操作態間自由切換。當摻鉺光纖雷射在耗散光固子之輸出下，中心波長位於1567.8奈米之1.35皮秒的鎖模脈衝可以被觀察到;在展寬脈衝的工作模式下，75飛秒之中心波長為1583.8奈米的輸出脈衝可被獲得;而調整在似噪脈衝之鎖模運作下，1597.8奈米之中心波長的150飛秒鎖模脈衝可被觀察到。由於雷射腔內之非線性偏振旋轉，輸出波長可藉由偏振控制器的調整而改變，根據波長相依色散之關係，不同波長分別對應至不同色散，因此不同的鎖模波長會對應至不同的腔內色散，產生不同色散下的鎖模操作環境，進而產生不同鎖模運作模式的脈衝輸出。
接著，我們使用低溫電漿增強化學氣相沉積以製備鍺薄膜來作為飽和吸收體並啟動1.6微米被動鎖模摻鉺光纖雷射。在材料與結構的特徵分析上，場發射掃描式電子顯微鏡與拉曼光譜之分析顯示出此鍺薄膜為厚度約200奈米且具備高結晶性的結構。能量色散X光譜與選區繞射圖案之分析分別指出此鍺薄膜已嚴重氧化與具有兩微弱之繞射環，分別對應至(111)與(311)之晶向。場發射穿透式電子顯微鏡之高解析影像分析指出此鍺薄膜為近乎非晶結構但有少數奈米結晶的存在，奈米鍺晶體成長之優選方向為(111)晶向。在非線性光學特徵的表現上，鍺薄膜之自振幅調變係數約為0.016，優於文獻上所發表之碳黑與木炭材料的數值，說明其可用於被動鎖模光纖雷射的啟動上。在被動鎖模摻鉺光纖雷射的性能表現上，最窄之654.39飛秒的鎖模脈衝且中心波長約為1600奈米的特徵可以被觀察到。儘管鍺薄膜飽和吸收體在鎖模力量上的表現劣於碳黑與木炭飽和吸收體，但在其已嚴重氧化的情況下還能夠啟動被動鎖模，意味著此鍺薄膜飽和吸收體亦為另一種可用於啟動第一階段光纖雷射被動鎖模之優良材料。
最後，我們使用非線性偏振旋轉啟動2微米之摻銩光纖雷射鎖模脈衝輸出。當操作雷射在低泵浦功率時，波長位於1.95微米之1.059皮秒的鎖模脈衝可被觀察到。當操作雷射在高泵浦功率時，鎖模脈衝更進一步被壓縮至909飛秒。由於非線性偏振旋轉的固有特性，鎖模脈衝對外界環境的擾動敏感度與中心波長的調變範圍皆可藉由適當調整腔內偏振來分析與達成。藉由調整偏振控制器之波片，此雷射具有高達18度之四分之一波片的可鎖模容忍角度與22奈米的波長可調變範圍，說明了此雷射相對於環境的擾動較不敏感與在2微米生醫使用波段的高價值應用。

關鍵字

摻鉺光纖 ； 摻銩光纖 ； 光纖雷射 ； 被動鎖模 ； 非線性偏振旋轉 ； 飽和吸收體 ； 色散管理

並列摘要

At the first stage, the ultrafast passively NPR mode-locked EDFL with and without intracavity dispersion management is experimentally demonstrated. The dispersion management of the EDFL is achieved by incorporating a different lengths of DCF into the EDFL cavity. The traditional nearly transform-limited mode-locked soliton pulsation without inserting the EDF exhibits a narrowest pulse duration of 405 fs, a widest 3-dB spectral linewidth of 6.7 nm, a repetition rate of 24.48 MHz, an average output power of 3.261 mW can be observed. After inserting the 29.2-m-long DCF into the EDFL cavity, the dispersion managed 220-fs mode-locked pulsewidth with an output power of 3.07 mW, a 12.3-nm spectral linewidth, and a repetition rate of 5.42 MHz under the operation of conventional soliton state. When detuning the PC, the other noise-like pulse operation can also be observed. The NPR mode-locked noise-like EDFL pulsation with a 1.68-mW output power has a narrowest pulsewidth of 109 fs and a 3-dB spectral linewidth of 25.1 nm is accomplished. Finally, when inserting the 55.4-m DCF into the EDFL cavity, the switched mode-locked stretched-pulse pulsewidth centered at 1583.8 nm possesses the narrowest pulsewidth of 75 fs and the widest spectral linewidth of 44.9 nm, respectively.
At the second stage, the nanocrystalline Ge thin film SA mode-locked EDFL is experimentally investigated. For material characterizations, the Ge thin film possesses a nearly 200-nm thickness and high crystallinity determined by FESEM and Raman spectrum. The severe oxidation problem and the primary growth orientation of (111) and (311) directions for the Ge thin film are indicated by the EDS and SAD analyses. The HRTEM images indicate that the crystal structure of the Ge thin film is nearly amorphous but a few nanocrystal Ge is existing in some area with (111) growth orientation. For passively mode-locked performances, the narrowest pulsewidth of 654.39 fs and broadest linewidth of 4.17 nm are achieved, respectively.
At the final stage, the wavelength switchable and sub-picosecond NPR mode-locked TDFL centered at 1950 nm is experimentally explored. The low pumped pulsewidth of 1.059 ps and the linewidth of 3.828 nm for the mode-locked TDFL soliton pulse can be observed. With the high pump power condition, the narrowest pulsewidth of 909 fs and a longest linewidth of 4.426 nm for the mode-locked TDFL pulsation are obtained, respectively. The mode-locked TDFL is relatively insensitive to the perturbations of environments indicated by such a large mode-lockable tolerance angle of up to 18o. The NPR mode-locked TDFL exhibits a wavelength tuning range of up to 22 nm by adjusting the intracavity polarization state is studied as well.

並列關鍵字

Erbium-doped fiber ； Thulium-doped fiber ； Fiber laser ； Passive mode locking ； Nonlinear polarization rotation ； Saturable absorber ； Dispersion management

參考文獻

[1] H. A. Haus, J. G. Fujimoto, and E. P. Ippen, 1991 “Structures for Additive Pulse Mode Locking,” J. Opt. Soc. Am. B, 8, 2068-2076.

[2] C. J. Chen, P. K. A. Wai, and C. R. Menyuk, 1992 “Soliton Fiber Ring Laser,” Opt. Lett., 17, 417-419.

[3] V. J. Matsas, T. P. Newson, and M. N. Zervas, 1992 “Self-Starting Passively Mode-Locked Fiber Ring Laser Exploting Nonlinear Polarization Switching,” Opt. Comm., 92, 61-66.

[4] K. Tamura, H. A. Haus, and E. P. Ippen, 1992 “Self-Starting Additive Pulse Mode-Locked Erbium Fiber Ring Laser,” Electron. Lett., 28, 2226-2228.

[5] K. Tamura, C. R. Doerr, L. E. Nelson, H. A. Haus, and E. P. Ippen, 1994 “Technique for Obtaining High-Energy Ultrashort Pulses from an Additive-Pulse Mode-Locked Erbium-Doped Fiber Ring Laser,” Opt. Lett., 19, 46-48.