超大反常色散摻鉺鎖模光纖雷射的脈衝特性研究

Abstract

本論文研究一個由400m長單模光纖組成共振腔的被動摻鉺鎖模光纖雷射,我們藉由偏振疊加波鎖模的機制來鎖模產生鎖模雷射脈衝,並使用30/70的光耦合器把70%的能量輸出腔外,剩下的30%傳回腔內做來維持共振,輸出的脈衝重複頻率為500kHz。此雷射藉由調整共振腔內的偏振控制器可達到不同的穩定鎖模狀態,像是長方波和短脈衝輸出等。在方波操作條件下,單一個雷射脈衝能量最高可以達到142J，並且方波寬度隨著幫浦能量增加而呈現線性遞增的趨勢,在此過程中方波不會因幫浦能量加大而遭破壞,此正是方波操作狀態下的一大特色，其優點是可以獲得大脈衝能量而持續穩定。 此雷射的雙波長現象也是個有趣的狀態,其輸出包含方波和短脈衝兩種狀態的疊合,不論是在時域或是頻域上皆可觀測到兩脈衝疊加的跡象,我們藉由腔外濾波器去選取此狀態的特定頻段,可以定性上去驗證確實是兩個狀態疊加。 另外我們感到興趣的是分別在短脈衝和方波操作狀態下,輸出脈衝的頻譜相位分布特性。我們藉由實驗上光濾波後的脈衝寬度量測結果來估算頻域二階啁啾與三階啁啾的大小,也同時對濾波後的短脈衝來進行壓縮實驗,藉由色散補償光纖設法將其壓縮至160皮秒以下,然後使用光自相干涉儀來量測脈衝寬度。

This thesis work mainly studies Er-doped passive mode-locked fiber laser configuration. The cavity length mainly consists of 400 meters-long single mode fiber and 1.65 meters long Er-doped gain fiber. We use the polarization additive pulse mode-locking (P-APM) technique to mode-lock the laser. An optical output coupler with the ratio of 30/70 (30% power back into the cavity) is used to extract the optical pulses and the pulse repetition rate is 500 kHz. By adjusting the polarization controller inside the cavity, we can produce different stable mode-locked states including the long square pulse state and the short pulse state. In the square pulse operation regime, a single pulse can reach the pulse energy of 142 nJ by increasing the pump power. The pulse-width increases proportionally without the pulse breaking. Dual-wavelength phenomenon is another interesting mode-locked state of the laser. It is a combination of two different pulse states: one is the short pulse state and the other is the square pulse state. We filter the optical spectrum to observe the changes of the pulse shape in the time domain to confirm their properties. We are also interested in the analysis of the spectral chirp characteristics in both the square pulse and short pulse regimes by sliding-filtering the pulses. We filter the spectra of the optical pulses in the two different mode-locked regimes and record the resulting pulses in the time domain. The spectral 2nd and 3rd order chirps are estimated based on the measurement results. In the meantime, we try to compress the filtered optical pulses by connecting a section of dispersion compensation fiber (DCF) and different lengths of single mode fiber (SMF). The optical pulse-width was measured by an autocorrelator and the minimum pulse-width can be below 160ps with proper lengths of the DCF and SMF fibers.