以非線性極化旋轉機制啟動中心波長可調被動鎖模摻鉺光纖雷射輸出束縛態光固子之穩定性研究

Stability of Tightly Bunched Soliton Pulses in Passively Mode-locked Erbium-doped Fiber Laser Based on Nonlinear Polarization Rotation with Central Wavelength Tunability

林聖峰(Sheng-Fong 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(2014年)

https://doi.org/10.6342/NTU.2014.02980

摘要

被動鎖模摻鉺光纖雷射因其可撓曲共振腔，並使用便利的光纖接頭作為輸出端，使其脈衝輸出位置具有相當大的自由度，因此適合做為超快光源在許多領域（包括材料微型加工、生醫顯像與雷射手術醫療…等）均具有相當大的應用發展潛力。為了滿足架設簡易與耗費低廉的應用需求，過去已有許多的研究團隊投入以非線性極化旋轉機制啟動被動鎖模摻鉺光纖雷射系統的發展與研究。
本研究中，我們利用被動鎖模摻鉺光纖雷射共振腔中極小彎曲的光纖環圈，提供的弱極化相依之功率衰減效應成功啟動非線性極化旋轉機制，若將泵浦功率由100毫瓦提昇至325毫瓦，輸出的脈衝寬度可由5.3皮秒縮短至4.9皮秒，對應的頻譜寬度由0.43奈米增加至0.56奈米。另一方面，在加入極化器於被動鎖模摻鉺光纖雷射腔內提供強極化相依功率衰減之後，當雷射啟動後光固子的脈衝寬度可大幅縮減至390飛秒並使頻譜寬度增加至7.14奈米。值得注意的是在增加泵浦功率後，光固子的脈衝寬與頻譜寬度並無顯著的變化；然而當摻鉺光纖雷射達到一定增益以上後，光固子會分裂成束縛態光固子群。操作在最大泵浦功率325毫瓦下，最多會有18個光固子形成並以短時域間距，束縛在一脈衝波包內並且穩定循環在光纖雷射腔內。基於光固子微擾理論，當雷射腔內的長距離光固子作用力為吸引力時，高增益的被動鎖模摻鉺光纖雷射脈衝就會分裂產生束縛態光固子群。
接著，我們探討利用光纖雙折射與腔內極化線性濾波的合併效應，產生中心波長可調的非線性極化旋轉機制被動鎖模摻鉺光纖雷射。藉由彎曲雷射腔內的光纖形成半徑為3至1公分的極小光纖環圈，迫使光纖中快軸與慢軸的折射率差異增加產生相位差。在泵浦功率等於325毫瓦的情況下，可產生被動鎖模在1570奈米波段其中心波長可調範圍是2.9奈米的光固子脈衝。在1600奈米波段時可調範圍則是擴大至10.2奈米，此相當大的可調範圍差異是受到雷射增益頻譜的曲率影響其增益頻寬縮減效應所誘發。此外，我們也探索了被動鎖模摻鉺光纖雷射在同樣的泵浦功率下，會因為泵方向與形式的不同也產生中心波長偏移的現象。當被動鎖模在1570奈米波段，前向泵浦與後向泵浦兩者產生脈衝的中心波長偏移達5.9奈米；而鎖模在1600奈米波段時，前向泵浦與後向泵浦兩者產生脈衝中心波長偏移僅1.1奈米。兩波段偏移波長的差異起因於摻鉺光纖內激態鉺離子空間分布有所不同，使得增益頻譜與其曲率形式在不同的泵浦分布函數會因應產生變化。因此，除了摻鉺光纖增益飽和引起的折射率改變，光纖雙折射以及增益頻譜的變化也能造成非線性極化旋轉機制被動鎖模摻鉺光纖雷射的中心波長位移。
最後，我們探討了非線性極化旋轉機制被動鎖模摻鉺光纖雷射在不同位置輸出束縛態光固子群的時序顫動與強度波動。當泵浦功率在325毫瓦時，將輸出位置設定在極化器之後，脈衝寬與平均輸出功率分別是400飛秒與3.59毫瓦。若調整腔內極化使得光固子群形成束縛態時則有較穩定的輸出，其強度波動是2.7×10-3以及時序顫動是31皮秒。如果輸出位置設定在摻鉺光纖放大脈衝後，外部量子效率可提昇至原本位置的2.1倍使得平均輸出功率可達7.98毫瓦；此時因為摻鉺光纖的飽和增益引發群速色散與光固子負啁啾現象，光固子脈衝寬由400飛秒進一步縮減為380飛秒，相應強度波動降至1.6×10-3以及時序顫動縮減為12皮秒。基於上述結果得知當高增益非線性極化旋轉機制被動鎖模摻鉺光纖雷射的輸出位置選擇在摻鉺光纖放大脈衝後，其束縛態光固子群會有較高外部量子效率與較低的強度和相位雜訊。

關鍵字

被動鎖模 ； 非線性極化旋轉 ； 摻鉺光纖雷射 ； 束縛態光固子 ； 波長可調

並列摘要

With the flexible fiber patch cord as the output coupler, it is considerably convenient to focus the soliton pulses delivered from the passively mode-locked erbium-doped fiber lasers (EDFLs) on where the users want. Therefore, it is great potential to utilize the passively mode-locked EDFL as the ultrafast light source for many applications. To fulfill the demands of compact and inexpensive ultrafast lasers, passively mode-locked EDFLs based on nonlinear polarization rotation mode-locking (NPRML) mechanism have been developed.
In this thesis, we demonstrated that with the assistance of weak or strong polarization-dependent loss (PDL) in the cavity, NPRML-EDFL shows the transformation on the pulse from single to multiple bunched state with nearly one order of magnitude reduction on pulsewidth. With the bent intracavity fiber providing the weak PDL, the NPRML only shortens the pulsewidth from 5.3 ps to 4.9 ps and correspondingly broadens the spectral linewidth from 0.43 nm to 0.56 nm when enlarging the pump power from 100 mW to 325 mW. With the use of an inserted polarizer providing strong PDL in the EDFL, the fundamental soliton pulsewidth is significantly compressed to 390 fs with the spectral linewidth as wide as 7.14 nm. In particular, the parameters of the soliton pulses is nearly unchanged at different pump powers; however, the soliton pulses splits itself to form the tightly bunched pulses circulating in the EDFL cavity. There are as many as 18 solitons tightly bunched together at the maximum pump power of up to 325 mW. Such a tightly bunched package can be elucidated by the soliton energy quantisation and the long-range soliton interaction according to the perturbation theory in a passively mode-locked EDFL.
Moreover, based on the combined effects of the fiber birefringence and the cavity filtering in the EDFL, the central wavelength tunable NPRML-EDFL is developed. By bending the intracavity fiber to induce the refractive index difference between the fast and slow axes, the wavelength tunable range is 2.9 nm at 1570 nm regime and 10.2 nm at 1600 nm regime. The difference of the tunable range at these two bands is attributed to the fact that the gain narrowing effect is influenced by the shape of the gain profile. On the other hand, the central wavelength shift under the different pumping geometries with the same pump power is also investigated. At 1570 nm regime, the central wavelength offset between the forward pumping and backward pumping condition is 5.9 nm, whereas the forward and backward pumping induced central wavelength offset is only 1.1 nm when mode-locking at 1600 nm regime. Such a difference originates from that the gain spectrum is dependent on the spatial distribution of the excited erbium ions under different pumping schemes. Besides the gain-saturation induced refractive index change, the birefringence variation and the gain spectral variation induced dual-band central wavelength tenability of the NPRML-EDFL is observed and discussed.
Finally, the timing jitter and the intensity stability of the output pulse-train at different intracavity nodes are investigated by operating the NPRML-EDFL under the tightly bunched multi-soliton state. When measuring the output delivered from the node after the polarized mode-locker, the average output power is 3.59 mW and the pulsewidth of the solitons is 400 fs at the pump power of 325 mW. It shows that the multiple soliton pulses at the tightly bunched state are relatively stable, which exhibits the energy fluctuation of 2.7×10-3 and the timing jitter of 31 ps. In contrast, when the output node of the pulse-train is just selected after the EDF amplification, it leads to a larger average output of 7.98 mW at the same pump power. The external quantum efficiency can be enhanced by 2.1 times. At this node, the pulsewidth of the tightly bunched solitons can be compressed from 400 fs to 380 fs due to the gain-induced GVD of the EDF and the negative chirp parameter of the soliton pulse. Moreover, it exhibits even lower energy fluctuation of 1.6×10-3 and smaller timing jitter of 12 ps. These observations signify that the soliton pulses in the tightly bunched state delivered from the output port after the EDF exhibit the better external quantum efficiency and less intensity noise and phase noise.

並列關鍵字

Passive Mode-locking ； Nonlinear Polarization Rotation ； Erbium-doped Fiber Laser ； Tightly Bunched Solitons ； Tunable Wavelength

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