雙波長鎖模之單石量子點雷射

Abstract

本論文研製被動鎖模之單石半導體量子點雷射，並成功地實現單波長鎖模與雙波長鎖模。我們挑選兩片雷射磊晶片樣品：樣品一為均勻堆疊量子點主動層，樣品二為啁啾式堆疊量子點主動層。製程上則是將傳統脊狀波導分隔為雙區段電極，我們分別採用濕式蝕刻與乾式蝕刻進行兩次製程，由於乾式蝕刻的良率不佳，因此委託光電公司進行第三次的製程。我們將此三批元件進行一系列的量測，包括L-I-V光電特性、光譜、RF頻譜與脈衝寬度等；此外，為深入了解操作條件對鎖模雷射特性的影響，我們架設自動量測系統對元件順向電流與逆向偏壓進行陣列掃瞄並作成二維映射圖(2D mapping image)。雙波長鎖模現象除了可透過RF頻譜的拍頻訊號而間接獲得證實，我們也在偵測前加上高品質的長通濾光片或短通濾光片，直接驗證雙波長鎖模並釐清個別波長的效應。

本研究的雙波長鎖模量子點雷射有別於現有的文獻發表，其差異在於量子點堆疊的波長與數量是經過刻意的設計，因此從光譜就能確定雙雷射波長分別來自不同堆疊的基態能階，而非文獻上來自相同堆疊的基態與激態能階，同時我們的雙波長鎖模元件也具有較低操作電流與較高光功率輸出等優點。實驗上所量測到的雙雷射波長約在1270 nm與1233 nm附近，因此高通/低通濾光片可以輕易地選購在1250 nm波段，即可清楚分出兩雷射波長的光功率強度；此外，光脈衝量測的二次對諧波產生對於波長也有很好的解析能力，目前在相同操作條件下所量測的最佳脈衝寬度，長波長約10 ps，而短波長約7 ps。由於鎖模量子點雷射的雙鎖模現象相當新穎而複雜，我們也針對不同吸收與增益比例進行一系列量測，論文中也將討論初步的量測結果。

In this thesis, we have fabricated monolithic passively mod-locked semiconductor quantum-dot (QD) lasers, and successfully achieved both single-wavelength mode-locking and dual-wavelength mode-locking. We selected two epitaxial laser samples; one is with uniformly stacked QD active layers, another is with chirped multilayer QD active region. The device process is based on two-section ridge-waveguide with two separate contacts. We have carried out both wet-etching and dry-etching process; however, the device yield involving dry-etching process is rather low. The third process is therefore outsourced to local optoelectronics company in Taiwan. Three batches of mode-locked lasers are then taken under a series of measurements, including L-I-V characteristics, optical spectrum, RF spectrum, and optical pulse widths. Automated measurement system is also setup to perform two-dimensional mapping of device characteristics on operation conditions of both forward current and reverse bias. The indirect evidence of dual-wavelength mode-locking can be drawn from the beating of two dominant peak frequencies in RF spectrum. To make direct confirmation and resolve contributions of two individual wavelengths, we put high quality long-pass or short-pass optical filters before all the detection and measurement.

The dual-wavelength mode-locked QD lasers in this study are different from those in previous publications, where two lasing wavelengths are originated from ground-state and excited-state emissions. In our chirped multilayer QD sample, the chirped wavelengths and QD stacking numbers are intentionally designed and grown. Therefore two mode-locked wavelengths are easily identified as two ground-state emissions from different QD stacks. Moreover, our dual-wavelength mode-locked devices have advantages of lower operation current and higher optical output. In our experiment, the two lasing wavelengths are around 1270 nm and 1233 nm, which are easily resolved by long-pass or short-pass optical filters with cut-on or cut-off wavelength at 1250 nm. By the way, the second-harmonic generation in optical pulse measurement is also wavelength dependent. Under the certain bias condition, the optimum pulse width is about 10 ps for long wavelength emission, and about 7 ps for short wavelength emission. Since properties of dual-wavelength mode-locked devices are rather novel and complicated, measurements on different absorber-to-gain length ratios are performed and discussed in the thesis.