研究光激發式半導體雷射之時域狀態

Abstract

本論文是利用光激發式半導體雷射為平台，研究自鎖模現象下的各種時域狀態。首先，我們系統地找出如何在光激發式半導體雷射中達成自鎖模之條件，實驗結果顯示自鎖模必須同時伴隨著第一階高階橫模態的產生。若沒有高階橫模態，雷射在時間上的表現不會是穩定的鎖模脈衝序列。利用在腔內放置一可操控半徑的孔洞，我們定量地控制雷射共振腔中可輸出的高階橫模態數量，實驗結果發現過多的高階橫模態會使得雷射輸出趨向於混亂的時域狀態。此外，透過精準量測橫模的頻率值，我們發現一非線性透鏡效應存在於半導體增益介質中，並且理論計算中必須考慮此非線性透鏡效應，方可與實驗結果吻合。我們推測非線性透鏡的存在是造成自鎖模現像的關鍵物理機制。除了典型的脈衝序列，我們也發現利用微小的反射回饋，可以幫助縱模態之間的鎖相，進而產生亮暗脈衝對。更進一步地，藉由腔外的部分反射回饋，我們也觀察到間歇性的自鎖模暗脈衝序列。我們相信本文所研究的光激發式半導體雷射之時域狀態無論是在學術發展或是產業應用皆能夠提供相當大的幫助與價值。

This thesis presents several optical experiments to investigate the phenomenon of self mode locking (SML) in optically pumped semiconductor lasers (OPSLs). First of all, we systematically explore the criterion for achieving the SML operation in an optically pumped semiconductor laser with a linear cavity. Experimental results reveal that the occurrence of SML can be assisted by the existence of the first high-order transverse mode, the continuous SML cannot be observed in the absence of high-order transverse modes. Numerical analysis is performed to confirm that the critical pump power for obtaining the SML operation agree very well with the pump threshold for exciting the TEM1,0 mode. Moreover, we quantitatively investigate the influence of multiple high-order transverse modes on the temporal state in an OPSL with a nearly hemispherical cavity. A physical aperture is inserted into the cavity to manipulate the excitation of high-order transverse modes. While more high-order transverse modes are excited, it is experimentally found that the pulse train is modulated by more beating frequencies of transverse modes. The temporal state becomes the random dynamics when too many high-order transverse modes are excited. Nonlinear refractive index is strongly associated with SML operation. Therefore, we propose another method to measure the nonlinear lensing in the OPSL gain chip. We precisely measure the values of transverse frequency spacing and longitudinal frequency spacing, and find that a focus lens must be considered for the numerical fitting based on ABCD law. According to comparison with experimental results and theoretical calculations, the focal length is found to be independent with pump power, pump mode size, cavity length, and radius of curvature of the output coupler. In addition to typical mode-locked pulses, we originally observe an intriguing phenomenon of SML in an OPSL related to the formation of bright-dark pulse pairs. We experimentally demonstrated that under the influence of the tiny reflection feedback, the phase locking between lasing longitudinal modes can be assisted to form bright-dark pulse pairs in the scale of round-trip time. A theoretical model based on the multiple reflections in a phase-locked multi-longitudinal-mode laser is developed to confirm the formation of bright-dark pulse pairs. Finally, we present a simple and cost-efficient OPSL design supporting stable high power operation. This design is based on the combination of a concave output coupled with an output reflectivity of 88% and a set of coupled resonators. With a coupled mirror reflectivity of 50% and an incident pump power of 24 W, we can achieve 6.3 W of average output power and the slope efficiency was 41%. Moreover, we have observed dark pulse emission with strong external reflection. The great agreement between experimental results and the numerical fitting validates that the strong external reflection is necessary for the generation of self-mode-locked dark pulse emission in an OPSL.