同步至非同步雷射鎖模態轉換之實驗研究

Abstract

非同步諧波鎖模光纖雷射和其他鎖模雷射相比有許多優點，包括如較高的超模噪音抑制比、較短的脈衝寬度、以及利用低頻電子即可來作腔長控制等，未來亦有可能會廣泛的應用在光通訊以及生醫光電的領域。在本論文的研究中，我們透過改變調變頻率來控制相位調變混合鎖模光纖雷射由同步鎖模狀態過渡到非同步鎖模狀態，同時觀察各項雷射特性在鎖模態轉換過程中的變化並作理論預測和實驗結果的比較。由於理論預測所使用的參數和實際實驗狀況無法完全相同，因此我們注重的是變化趨勢的相符，而非實際數值上的等同，而所比較的雷射特性則包含雷射輸出功率的改變、非同步狀態的存在範圍、雷射中心波長、半高寬、脈衝寬度以及峰值功率的改變等。透過這些比較，我們驗證了隨著調變頻率的改變，雷射可以操作在三種不同的鎖模狀態：同步短脈衝、同步長脈衝以及非同步短脈衝狀態，這三種不同鎖模狀態的穩態鎖模特性就決定了當調變頻率改變時各項雷射參數的轉變趨勢。

In comparison to others mode-locked lasers, the asynchronous harmonic mode-locked fiber lasers have exhibited several advantages including higher super-mode suppression ratio, shorter pulse width, and the possibility of achieving cavity length control with low frequency electronics. These lasers may find applications in the area of optical communication and biophotonics. In this thesis work, we vary the detuning frequency of a phase-modulation hybrid harmonic mode-locked Er-doped fiber laser to make it transit from synchronous to asynchronous mode-locking. The lasing characteristics during transition are measured and are compared with the theoretical predictions. Because the parameters used in theoretical calculation are not exactly corresponding to the experimental conditions, we will emphasize the tendency comparison when the detuning frequency is scanned. Compared lasing characteristics include the variation of laser output power, the existence range of the asynchronous state, and the variation of the lasing central wavelength, FWHM bandwidth, FWHM pulse width and pulse peak power. After the comparison, we verify that there are three distinct mode-locked states during the transition: the synchronous short-pulse, synchronous long-pulse and asynchronous short-pulse state. Their steady state characteristics determine the transition behavior of the lasing parameters.