探索與偏振相關脈動石墨烯與二硫化鉬吸收體摻鉺光纖雷射鎖模

Exploring the Polarization Dependent Mode-locking in Erbium Doped Fiber Laser with Graphene and MoS2 Saturable Absorbers

陳婷卉(Ting-Hui Chen)

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.2015.10774

摘要

被動鎖模摻鉺光纖雷射已廣泛地應用在許多科學領域，包括基礎科學研究、生醫顯像、材料微型加工、光通訊科技等。近期許多團隊的研究指出，二維材料可作為良好飽和吸收體以飛秒級脈衝雷射。以二維材料作為飽和吸收體的被動鎖模光纖雷射系統內，腔內極化對雷射特性的影響鮮少被詳細地討論。因此，我們探索與偏振相關脈動以石墨稀與二硫化鉬作為飽和吸收體摻鉺光纖雷射之研究。
首先，我們研究以非線性極化旋轉與飽和吸收體雙重機制產生混成鎖模摻鉺光纖雷射之極化相關表現，並藉由修改後之被動鎖模主方程式進行混成鎖模光纖雷射動態行為之模擬分析。光纖雷射共振腔內之極化敏感元件(單模光纖與摻鉺光纖)可誘發一微弱之非線性極化旋轉效應，由非線性極化旋轉效應產生之最大自振幅調變係數為5.36×10-4。透過極化控制器的調整可以產生一不穩定之被動鎖模摻鉺光纖雷射，其雷射脈衝寬為395 飛秒，但極化控制器可調控範圍僅有34度。於共振腔內加入石墨烯飽和吸收體後可以提升自振幅調變係數至1.456×10-3，並產生一穩定之鎖模光纖雷射，其脈衝寬度可縮短至352飛秒，極化控制器可調控範圍增大至149度。非線性極化旋轉與飽和吸收體雙重機制的混成鎖模機制在自啟動產生脈動的初始階段扮演重要腳色，可以幫助穩定摻鉺光纖雷射，並進一步優化雷射脈衝特性。我們的研究中討論了非線性極化旋轉與鎖模結果的相依性原因，並解釋需要調整腔內之極化控制器來產生非線性極化旋轉隨著正弦函數變化的自振幅調變係數以優化鎖模脈衝特性。
接著，我們研究由非線性極化旋轉與二硫化鉬飽和吸收體雙重機制產生混成鎖模摻鉺光纖雷射，並詳細探討極化控制器如何優化鎖模脈衝特性。在弱的非線性極化旋轉效應下，控制極化控制器內兩片四分之一波片旋轉角度，藉由調控半玻片可使脈衝寬度由382飛秒壓縮至377飛秒。但半玻片之調控角度範圍只有21度可啟動鎖模雷射。在加入二硫化鉬飽和吸收體後，隨著極化控制器的調整，弱非線性極化旋轉效應與強飽和吸收效應的混成效應可使自振幅調變參數由4.02×10-4變化至1×10-3，藉此產生一穩定鎖模光纖雷射並優化脈衝寬度至305飛秒，極化控制器半玻片之調控角度範圍增加至76度。此時自相位調變係數仍然與尚未加入二硫化鉬飽和吸收體時相同。二硫化鉬飽和吸收體增加自振幅調變係數，並增加可鎖模的極化角度範圍但不改變自相位調變。
最後，我們比較各類二維飽和吸收體產生被動鎖模摻鉺光纖雷射之輸出特性，二維材料分別為石墨稀、二硫化鉬、N型與P型碲化鉍。首先分析二維飽和吸收體的非線性吸收，石墨稀擁有最大的自振幅調變係數9.2x10-4 (二硫化鉬：4x10-4、N型碲化鉍： 7.2x10-4、P型碲化鉍：7.9x10-4)。當系統操作在低功率下時，自振幅調變係數主導鎖模光纖雷射的形成。於實驗中，各類二維飽和吸收體皆可產生低於700飛秒之脈衝寬度，尤其石墨稀因為擁有最大的自振幅調變效應，引此可產生最窄之鎖模雷射脈衝寬度615飛秒，雖然二硫化鉬與碲化鉍在產生脈衝的能力沒有較石墨稀好，仍然可以成功的穩定鎖模摻鉺光纖雷射，而二硫化鉬的鎖模能力又弱於碲化鉍。隨著增加激發雷射功率產生光固子鎖模效應，此時群延遲色散效應與自相位調變效應相互補償，因此進一步壓縮雷射脈衝寬度，壓縮比例約為0.55。石墨稀飽和吸收體能得到最窄的脈衝寬度343飛秒，與最寬的光譜寬度7.86奈米。當操作在鎖模門檻的功率下，飽和吸收體自振幅調變能力主導摻鉺光纖雷射脈動，說明飽和吸收體對於被動鎖模的重要性，二維飽和吸收體透過好的鎖模能力在自振幅調變的階段產生低於皮秒等級之被動鎖模摻鉺光纖雷射，所有二維材料在相似的條件壓縮下皆能產生低於400飛秒的脈衝寬度。

關鍵字

被動鎖模 ； 非線性極化旋轉 ； 摻鉺光纖雷射

並列摘要

Ultrafast mode-locked fiber lasers have been comprehensively utilized in several applications, including fundamental scientific researches, biophotonic imaging, micromachining, and optical communications, etc.. Recently, two-dimensional (2D) material has shown its potential to be a nice saturable absorber for passively mode-locked lasers. However, the intracavity polarization dependent performances of passively mode-locked laser system with 2D saturable absorber have yet to be investigated. In this thesis, the polarization dependent mode-locking in erbium doped fiber laser with graphene and MoS2 saturable absorbers are explored.
Firstly, the hybrid mode-locking of erbium-doped fiber laser (EDFL) induced by nonlinear polarization rotation (NPR) and saturable absorption (SA) is explored. For the first time, a hybrid mode-locking related Haus master equation is established to characterize the competition between NPR and SA, thus assisting to stabilize the hybridly mode-locked EDFL after parametric optimization. A weak polarization-dependent loss (PDL) caused by the polarization-sensitive devices in the EDFL cavity can induce NPR effect with a self-amplitude modulation (SAM) value (NPR) of -5.36×10-4 by detuning the orientation of polarization controller (PC), which generates an unstable passively mode-locked EDFL with a pulsewidth of 395 fs. Inserting graphene can enhance the SAM ability with an enhanced SAM ranging from 3.84×10-4 to 1.456×10-3 by controlling the PC orientation, which can shorten the EDFL pulsewidth to 352 fs. The polarization angle dependent SAM coefficient plays an important role on self-starting the pulsation at initial stage. Our investigation declares the reason why most passively mode-locked EDFL with graphene saturable absorber would exhibit a polarization dependent mode-locking performance, and explain the requirement on the intracavity PC to detune the sinusoidal SAM from the residual NPR so as to optimize the SAM based mode-locking process.
Secondly, the hybrid mode-locking of an EDFL with incorporated MoS2 and the NPR based saturable absorbers is investigated. The weak NPR effect can be controlled by a PC to change the EDFL pulsewidth from 382 to 377 fs, however, a small tolerant range of half-wave plate HWP orientation of 21˚ limits the EDFL pulsation. Adding MoS2 can enhance the SAM and shorten the EDFL pulsewidth to 305 fs with allowable HWP increasing up to 76o, implying the enhanced stabilization of passive mode-locking in EDFL. Inducing the weak NPR with strong SA effect significantly increases the SAM coefficient from 4.02×10-4 to 1×10-3, whereas the SPM coefficient remains unaltered as that without adding the MoS2. The MoS2 saturable absorber offsets SAM beyond its threshold to enlarge the polarization tolerance without altering the SPM. Adjusting the PC with unintentional polarization dependence impacts the NPR induced PDL so as to strengthen the SAM for optimizing the mode-locking of EDFL.
Eventually, 2D materials, including graphene, molybdenum disulfide (MoS2), N-type and P-type bismuth telluride (Bi2Te3) are utilized to passively mode-lock the EDFLs. The SAM of these 2D saturable absorbers are investigated to compare their mode-locking force on the EDFL pulsation. The nonlinear absorption analyzed that the graphene possesses the largest SAM coefficient of 9.2x10-4 which is better than the other 2D saturable absorbers (MoS2：4x10-4, N-type Bi2Te3：7.2x10-4, P-type Bi2Te3：7.9x10-4). When operating the EDFLs at mode-locking threshold, those 2D saturable absorbers can generate the EDFL pulses with sub-700 fs pulsewidth in the low-gain condition. Especially, the graphene can generate the shortest EDFL pulse with 615-fs pulsewidth due to its largest SAM ability. Even if both of MoS2 and Bi2Te3 show worse pulsation results than graphene, they can still achieve the successful and stable mode-locking in EDFL. Moreover, the MoS2 reveals a weaker mode-locking force than that of the Bi2Te3. The importance of SAM coefficient on passive mode-locking is demonstrated, where the SAM of saturable absorber dominates the EDFL pulsation. By inducing the soliton mode-locking with increasing the pumping powers, the passively mode-locked EDFL with graphene saturable absorber also produced the narrowest pulsewidth of 343 fs, which is ~1.12 times smaller than that with MoS2 saturable absorber. Because the soliton compression is dominated by the compensation of group delay dispersion (GDD) and SPM, therefore, all the EDFL pulses with four saturable absorbers were further narrowed with a compression ratio of ~0.55. The 2D saturable absorbers possess excellent mode-locking force, so as to generate the sub-ps passively mode-locked EDFLs in the SAM region. By inducing the soliton mode-locking to further compress the EDFL pulse, all the 2D saturable absorbers can produce the passively mode-locked EDFL pulses with sub-400 fs pulsewidth.

並列關鍵字

Passive Mode-locking ； Nonlinear Polarization Rotation ； Erbium-doped Fiber Laser

參考文獻

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