非理想組成之矽鍺飽和吸收體於被動鎖模摻鉺光纖雷射之應用與研究

Investigation of nonstocihiometric SiGe saturable absorber for passively mode-locked erbium doped fiber laser

楊麒正(Chi-Cheng Yang)

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(2015年)

https://doi.org/10.6342/NTU.2015.10704

摘要

被動鎖模光纖雷射已被廣泛地應用於許多研究領域上，包括光通訊、生醫顯像、光譜學、非線性光學等。在此研究中，我們探討非理想組成比例矽鍺材料之飽和吸收特性，並且使用矽鍺飽和吸收體產生飛秒級被動鎖模摻鉺光纖雷射。
首先，我們使用電獎輔助化學氣相沉積法成長可調比例之矽鍺飽和吸收體，並以化學方法剝落矽鍺飽和吸收體使之附著於單模光纖接頭。此研究討論透過調整非化學計量的矽鍺組成比例，進而控制被動鎖模摻鉺光纖雷射中的脈衝輸出。將矽鍺的組成比例中的鍺從由75.4%調變到94.3%的時，其非線性調變深度將由17%增加至22%，由此可知在矽鍺飽和吸收體中增加鍺的濃度可優化其飽和吸收特性。將被動鎖模摻鉺光纖雷射操作在鎖模門檻區域時，調變矽鍺中鍺的組成比例為75.4%、90.4%和94.3% 的時，可分別產生脈衝寬為820、760和730飛秒的雷射輸出。當被動鎖模摻鉺光纖雷射操作高增益的情況下，由自相位調變效應與群速度延遲色散效應所引發的光固子壓縮機制主導光脈衝的形狀與輸出，藉此可分別將雷射脈衝寬壓縮至346、342和338飛秒。
接著，我們持續詳細探討鍺含量越多時矽鍺的飽和吸收特性，以及對被動鎖模摻鉺光纖雷射特性的影響。將矽鍺的組成比例由94.8%提升至100%可將矽鍺的能帶能量由1.05降至0.87電子伏特，同時可優化自振幅調變參數由8.830×10-4至8.891×10-4。將被動鎖模摻鉺光纖雷射操作在鎖模門檻區域時，隨著矽鍺的組成比例由94.8%提升至100%時，所有矽鍺飽和吸收體皆可產生~700飛秒的雷射輸出；當被動鎖模摻鉺光纖雷射操作高增益的情況下，隨著鍺的組成比例由94.8%提升至100%可以產生330到305飛秒之脈衝寬度輸出，脈衝序列的載波振福抖動可以從1.81%被抑制到0.89%。與矽鍺相比，純鍺擁有最大的自振福調變參數，可以將脈衝寬壓縮至305飛秒，此研究中發現增加矽鍺材料中的鍺含量可以調整自振福調變參數的大小來控制材料鎖模能力。
最後，我們用電漿輔助化學氣相沉積法成長鍺奈米粒子，觀察其飽和吸收特性，並與純鍺材料比較其飽和吸收特性。使用掃描式電子顯微鏡與原子力顯微鏡分析鍺奈米粒子的尺寸以及純鍺薄膜的表面粗糙度；使用拉曼光譜分析鍺奈米粒子與純鍺薄膜的和結構特性。在鍺薄膜中，透過減少成長沉積的時間從60秒減至15秒(厚度估計為40奈米至15奈米)可以將調變深度從52%增加至58%。在鍺奈米粒子中，透過減少成長沉積的時間從60秒減至15秒可以將調變深度從4.3%增加至5.5%。將被動鎖模摻鉺光纖雷射操作在鎖模門檻區域時，鍺飽和吸收體能產生脈衝寬度為707至700飛秒的雷射輸出。在系統操作在高增益的情況下，由自相位調變所引發的光固子壓縮機制可以產生300到290飛秒之脈衝寬度輸出。在以鍺奈米微粒當作飽和吸收體的反射式被動鎖模摻鉺光纖雷射中，由於反射式系統引起較高腔內損耗以及鍺奈米粒子所引起之散射損害，在系統操作在高增益的情況下，將成長沉積時間從60秒減至15秒時，鍺奈米粒子產生脈衝寬度為3950至3700飛秒的不完全鎖模脈衝寬度輸出。

關鍵字

矽鍺 ； 飽和吸收體 ； 被動鎖模

並列摘要

Ultrafast pulse laser sources generated by passive mode-locking with saturable absorbers have immense impacts on optical telecommunication, biomedical imaging, spectroscopy and nonlinear-optics, etc. In this thesis, nonstoichiometric Si1-xGex saturable absorber with the composition ratio dependent saturable absorption is investigated to passively mode-lock the erbium-doped fiber laser (EDFL).
At first, the vaporized synthesis and chemical exfoliation of the nonstoichiometric Si1-xGex by plasma enhanced chemical vapor deposition (PECVD) with the composition ratio dependent saturable absorption is demonstrated to passively mode-lock the EDFL. The Si1-xGex with varied composition ratio (Ge/Ge+Si)) from 0.754 to 0.943 exhibits tunable nonlinear modulation depth from 17% to 22%, where the Si1-xGex with the highest Ge content performs the largest nonlinear modulation depth to be considered as a nice saturable absorber for passive mode-locking. When operating the EDFL at mode-locking threshold, the Si1-xGex with Ge/(Ge+Si) composition ratio of 0.754, 0.901 and 0.943 self-started the EDFL pulsation to produce the pulsewidth of 820, 760 and 730 fs. When operating the EDFL in the high gain region, the interaction of self-phase modulation (SPM) and group-delay dispersion (GDD) induced soliton compression dominates the re-pulsation of passively mode-locked EDFL, which further shrinks the EDFL pulsewidth from 346 to 338 fs.
Later on, the vaporized synthesis and chemical exfoliation of the Ge-rich Si1-xGex with the composition ratio dependent saturable absorption is analyzed. By increasing the composition ratio Ge/(Ge+Si) from 0.948 to 1, the optical bandgap energy decreases from 1.05 to 0.87 eV. The nonlinear self-amplitude modulation (SAM) coefficient of Ge-rich Si1-xGex is enhanced from 8.83x10-4 to 8.891x10-4 by increasing the Ge content. All of the Ge-rich Si1-xGex saturable absorber can self-start the EDFL pulsation to generate the EDFL pulsewidth of ~700 fs at nearly mode-locking threshold. At high-gain pumping power for Ge-rich Si1-xGex saturable absorbers with Ge/(Ge+Si) composition ratios enlarging from 0.948 to 1.000, the passively mode-locked EDFL can be soliton compressed to shorten the pulsewidth from 330 to 305 fs. The carrier amplitude jitter (CAJ) of the pulse-train further suppresses from 1.81 % to 0.89 %. The pure Ge film exhibits the largest SAM coefficient among all Ge-rich Si1-xGex samples, which shortens the pulsewidth to 305 fs. Different SAM coefficients of Ge-rich Si1-xGex with different Ge/(Ge+Si) ratios are observed and contributes to the varied mode-locking force.
Eventually, the Ge nanoparitlce grown on gold film by PECVD is synthesized to investigate its saturable absorption for passively mode-locked EDFL. The saturable absorption of Ge nanoparticle is compared with that of Ge film. Scanning electron microscopy (SEM) and atomic force microscope (AFM) are used to measure the size of Ge nanoparticle and the surface roughness of Ge film. Raman scattering spectroscopy is utilized to analyze the structural property of Ge nanoparticle. For the Ge films, decreasing the deposition time from 60 to 15s enlarges it nonlinear modulation depth from 52 to 58%. For the Ge nanoparticles, decreasing the deposition time from 60 to 15s enlarges it nonlinear modulation depth from 4.3 to 5.5%. The Ge saturable absorbers with different deposition time of 15, 30 and 60s self-start the EDFL pulsation to produce the pulsewidth of 700, 702 and 707 fs. By operating the EDFL at the high gain pumping condition, the pulsewidth of 290, 294 and 300 fs with the broadened spectral linewidth of 8.95, 8.83 and 8.58 nm for three samples is delivered due to the soliton compression. The performance of the reflection-type passively mode-locked EDFL with different Ge nanoparticles deposition time of 15, 30 and 60s operated under high gain situation of 249.6 mW shows the narrow-band optical spectrum with the broad pulsewidth of 3700, 3800 and 3950 fs which exhibits the incomplete passively mode-locking, which is induced by a large cavity loss induced by the reflection-type system and the scattering loss of Ge nanoparticle.

並列關鍵字

SiGe ； saturable absorber ； passive mode-locking

參考文獻

Harame D. L.; Meyerson B. S. The early history of IBM’s SiGe mixed signal technology. IEEE Trans. Electron Devices 2002, 48, 2555-2567.

[2] Subramanian V.; Saraswat, K. C. Optimization of silicon-germanium TFT’s through the control of amorphous precursor characteristics. IEEE Trans. Electron Devices 2002, 45, 1690-1695.

[3] Wu C.-L.; Su S.-P.; Lin G.-R. All-optical modulation based on silicon quantum dot doped SiOx:Si-QD waveguide. Laser Photon. Rev. 2014, 8, 766-776.

[4] Soref R. Mid-infrared photonics in silicon and germanium. Nat. Photonics 2010, 4, 495-497.

[5] Soref R. Group IV photonics for the mid infrared. Proc. SPIE, 2013, 8629, 862902.