被動鎖模摻鉺光纖雷射光固子與混沌脈衝操作態之研究

Abstract

在本論文中，我們藉由偏振疊加波鎖模機制來鎖模光纖雷射以產生光脈衝序列，雷射共振腔是由4m的摻鉺光纖以及6m的單模光纖所組成，輸出的脈衝重複頻率為20MHz。我們藉由調變共振腔中的偏振控制器與980 nm雷射二極體的幫浦功率，可以讓雷射操作在兩個截然不同的操作態-光固子態與混沌脈衝態。在這兩個操作態中，混沌脈衝操作態的輸出光譜較為平滑且頻寬寬度為12nm，而光固子態的輸出光譜則有著對稱的Kelly sidebands，其頻寬寬度為10nm。兩者的輸出脈衝寬度皆為300飛秒左右，但在示波器上可以觀察到混沌操作態下的脈衝序列高度是不規律的。混沌操作態下的脈衝經單模光纖傳輸後，在強度自相關量測儀的量測中可以看到一個狹窄的中心尖峰，並且這個狹小尖峰的寬度幾乎不受色散影響，這正是混沌脈衝操作態的特色之一。在光固子操作態下﹐我們比較了Kelly sidebands 對光固子傳輸實驗的影響性，利用了帶通濾波器來濾掉Kelly sidebands，結果發現有Kelly sidebands的光固子脈衝經色散位移光纖傳輸後，有較清楚的四波混和效應。相較而言，在混沌操作態中我們只用了24mW的平均功率在色散位移光纖中傳輸就能產生頻寬約120nm的超連續平坦光譜。最後，我們比較了兩操作態下的脈衝壓縮實驗，光固子操作態最短可以壓縮至223飛秒，而混沌操作態則無法有好的脈衝壓縮。

In the thesis, we use the polarization additive pulse mode-locking (P-APM) technique to mode-lock the laser. The laser cavity constitutes 4m Er-doped fiber and 6m SMF-28, with the pulse repetition rate of 20MHz. By adjusting the polarization controllers and the 980 nm LD pump power inside optical cavity, we can let laser operate under two very different operation states - soliton and chaotic pulse states. The optical spectrum under the chaotic state is smooth and the 3dB bandwidth is 12nm. In contrast, the optical spectrum under the soliton state has the symmetric Kelly sidebands and the 3dB bandwidth is 10nm. The output pulse-width is 300fs for both cases, but we can observe chaotic-like pulse amplitude variation under the chaotic state by using an oscilloscope. When the chaotic pulse train propagates through a section of SMF-28, we can observe a narrow center coherent spike in the auto-correlation trace, which width is almost not affected by the dispersion. We also compare the difference of propagation for the soliton case without and with Kelly sidebands. A band-pass filter can be used to filter out the Kelly sidebands. The result is that the soliton pulse without Kelly sidebands has shown obvious four-wave mixing effects after propagating through the dispersion-shifted fiber. In contrast, for the chaotic operation state, we can just use 24mW of the average power to produce a 120 nm relatively flat optical spectrum after propagating through the dispersion-shifted fiber. Finally we compare the pulse compression possibility of the two operation states. The shortest compressed pulse we can achieve is 223 fs for the soliton case. In comparison, the chaotic pulse cannot be compressed well.