開發短腔長窄線寬之1550奈米分佈式回饋雷射

Abstract

在自動駕駛應用的驅動下，能同時提供距離與速度資訊、並具高抗擾性的FMCW LiDAR技術備受矚目，而其性能實現的關鍵為具備窄線寬之雷射光源。本論文以短腔長、窄線寬之1550奈米分佈式回饋(Distributed Feedback, DFB)雷射為主題，探討腔長對雷射性能之影響。我們比較600與900微米腔長的DFB雷射，兩顆雷射皆具備相同磊晶結構與製程條件。透過分析兩種腔長之DFB雷射在直流方面的特性，驗證此短腔長設計是否具備性能上的優勢。 為了確保線寬量測準確性，我們架設了延遲自外差干涉儀(Delayed self-heterodyne interferometer, DSHI)線寬量測平台，同時開發Python擬合工具，分析經由長延遲光纖與短延遲光纖的功率譜密度，從而精確地提取雷射的本質線寬。實驗結果明確指出，腔體長度為600微米的雷射各項性能均優於900微米的雷射。在25°C的環境溫度下，短腔長雷射展現出更低的閾值電流（8.7 mA）、更高的斜率效率（0.39 W/A）與更大的最大輸出光功率（61.5 mW），最關鍵的是實現了更窄的本質線寬，其最小值為18.04 kHz，顯著優於長腔長雷射的25.8 kHz。此外，研究也發現雷射的最小線寬會出現在其PCE達到峰值的偏壓電流點。 綜合上述，本論文成功證明，透過精心的磊晶與結構設計，短腔長DFB雷射達成更佳的輸出功率，更大的調頻範圍，更高的斜率效率，更小的閾值電流，與更窄的本質線寬。

Driven by autonomous driving applications, FMCW LiDAR technology has garnered significant attention for its ability to provide both distance and velocity information with high immunity to interference. The key to its performance is a laser source with a narrow linewidth. The thesis focuses on a compact size, narrow-linewidth 1550 nm DFB laser, investigating the impact of cavity length on its performance. We conducted a comparative analysis of DFB lasers with cavity lengths of 600 µm and 900 µm, both fabricated using the same epitaxial structure and process, by examining their DC characteristics to evaluate whether the compact size design offers any performance advantages. To ensure accurate linewidth measurement, we established a DSHI platform. We also developed a Python fitting tool to precisely analyze the power spectral density from both long and short delayed fibers, enabling the accurate extraction of the DFB laser's intrinsic linewidth. The experimental results clearly show that the 600 µm laser outperforms the 900 µm design in key aspects. Specifically, at 25°C, the 600 µm laser exhibits a lower threshold current of 8.7 mA, a higher slope efficiency of 0.39 W/A, and a greater maximum output power of 61.5 mW. Most critically, the 600 µm laser achieved a narrower intrinsic linewidth, with a minimum value of 18.04 kHz, which is significantly better than the 25.8 kHz from the 900 µm laser. Furthermore, the study found that the minimum linewidth occurs at the bias current where the PCE peaks. In summary, this thesis successfully demonstrates that with careful epitaxial and structural design, a short-cavity DFB laser can achieve higher output power, wider frequency tuning range, greater slope efficiency, a lower threshold current, and a narrower intrinsic linewidth.