利用新穎面射型雷射於高速長距離光互聯系統

Abstract

迄今，網際網路已發展超過20年，近來更出現了許多嶄新的熱門應用，如物聯網以及海量資料。而數據中心提供大量資料儲存與管理的能力，可說是上述應用的關鍵基礎之一。使用垂直共振腔面射型雷射(VCSEL)與多模光纖組成的光互聯系統除了具有成本上的優勢，同時光互聯系統也能夠支持高速以及長距離的傳輸。然而，系統可達的最大頻寬受限於多模光纖中的模態色散，且隨著傳輸距離增加而更嚴重。因此，使用少模態的垂直共振腔面射型雷射可以減少多模光纖中被激發的模態，進而增加傳輸距離。但是隨著浮現的問題是，少模態的垂直共振腔面射型雷射會有比較嚴重的空間稍孔現象，而此一現象會嚴重的限制雷射的可調製頻寬。因此，此篇論文提出，同時透過鋅擴散與氧化掏離技術來達到少模態輸出，並且最佳化垂直共振腔面射型雷射的結構減少空間稍孔現象的影響。在實驗中，我們可以達到單模輸出的垂直共振腔面射型雷射，然而，其可調製頻寬仍然是受到限制。因此，我們在傳輸實驗中使用高頻普效益的分離複頻調變技術(DMT)以及位元承載演算法(bit loading)，在有限的頻寬下達到最大的傳輸速度。在使用單模輸出的垂直共振腔面射型雷射與分離複頻調變技術，本實驗成功達到目前世界上最高的傳輸速度距離乘積(BRDP)。本實驗所提出之光互聯系統架構具有低成本、高速、長距離的特性，因此非常適合使用在現代以及未來大型數據中心。

An obvious trend of cloud applications, e.g. the internet of things and big data, has changed every aspect of our lives. Capable to manage and store such huge volume of data, data centers have been a key basis to realizing these cloud applications. Cost-effective, high-speed, and long-reach optical interconnects based on vertical-cavity surface-emitting lasers (VCSELs) and multimode fibers (MMFs) will be needed in modern/future mega data centers. However, the modal dispersion of MMFs leads to a severe limitation in available system bandwidth with increasing the transmission distance. VCSELs with fewer transverse modes can excite fewer modes and facilitate elongating the transmission distance. Nevertheless, the spatial hole burning effect usually limits the modulation speed of the VCSEL with less transverse modes. Therefore, it is essential to optimize the VCSEL structure to achieve few-mode and high-speed characteristics. In this work, we design Zn-diffusion and oxide-relief structures to control the number of transverse modes of VCSELs and mitigate the spatial hole burning effect simultaneously. According to our results, the modulation bandwidth of the few-mode VCSEL is slightly sacrificed. In order to boost the data throughput, we use high spectral-efficiency discrete multitone (DMT) modulation with bit-loading algorithm, which also facilitate the system bandwidth requirement. By using the 850 nm single-mode VCSEL with optimized structures and DMT modulation with bit-loading algorithm, we have successfully achieve a record-high bit-rate distance product (BRDP). Therefore, the proposed system may be another alternative for low-cost, high-speed, and long-reach optical interconnection in modern/future data centers.