半導體量子點寬頻光源

Abstract

光學斷層掃瞄系統近年來在生醫領域蓬勃發展，由於系統需求之寬頻光 源波段恰與光通訊波段ㄧ致，也因此帶動光電領域之通訊光源的研發，其中 最受注目的首推低價砷化鎵基板上以砷化銦量子點為增益介質的長波長半導 體光源。半導體量子點磊晶的自我成長機制雖使得量子點本身的形狀、大小 及成份組成呈現寬廣的分布，然透過適當設計的多層堆疊結構可以克服飽和 增益不足的問題，同時又可發揮非均勻光譜展寬的本質特性，因此是半導體 寬頻光源的最佳選擇。 NSC96 的計劃中，我們以數位啁啾式堆疊量子點結構研製出具新奇發光 特性的半導體雷射，下半年將繼續分析探討此特殊量子點堆疊結構以最佳化 雷射元件的光電特性。在接下來的兩年計畫提案，NSC97 將以最佳化量子點 堆疊結構來研製具有實質寬頻的半導體寬頻光源，特別著重於降低光源垂直 發散角，以減少未來光纖耦合封裝所可能造成的嚴重損失，我們將研製兩項 候選的元件，亦即量子點超高亮度二極體及量子點寬頻雷射，然後以NSC96 架設之可調光延遲干涉儀來量測光源的同調長度以決定光源的解析度。 NSC98 將在NSC96 與NSC97 的基礎下研製具有等效寬頻之掃頻式波長可調 光源，我們將先進行首次量子點雷射與光子晶體的光電元件整合，探討光子 晶體對於雷射波長的調控，然後以結合光子晶體之內腔式架構以及結合半導 體光放大器與繞射光柵之外腔式架構來進行評估以實現快速而穩定的波長可 調，我們最終希望將此元件交付生醫研發單位進行影像掃描的測試。

Optical coherence tomography (OCT) systems have attracted great attentions in bio-medical imaging in recent year. Light emitters for optical communication are therefore spotlighted because the optical communication wavelength band is coincident with the required emission wavelength band of the OCT. Of the special interest is the long-wavelength semiconductor emitters based on gain medium of InAs quantum dots (QDs) grown on low-cost GaAs substrates. The self-assembled growth of QDs is subjected to inhomogeneity in shapes, sizes and compositions, which results in lower saturated gain. With multilayer QDs structure, however, this disadvantage can be somewhat alleviated. Nonetheless, the non-homogeneous spectral broadening characteristics are the key to the semiconductor broadband light sources. In NSC96, we have fabricated novel semiconductor lasers based on our digitally chirpy-stacked QD structure and will optimize their device performance in the remaining months. Regarding to the following two-year proposal, we will implement the semiconductor light emitters with broad emission spectrum in NSC97. Special focus will be put on lowering the vertical beam divergence so that fiber coupling loss can be reduced. The candidate devices for broadband light emitters are QD superluminescent diodes (SLDs) and QD broadband lasers. We will perform the coherence length measurement of these two semiconductor light emitters based on the interferometer settled in NSC96. With efforts from NSC96 and NSC97, we will be equipped to develop the tunable and swept-wavelength light sources in NSC98. We will carry out the first integration of photonic crystal with QD lasers and investigate its wavelength tuning range in the beginning. Then, both intra-cavity and external-cavity configurations are evaluated in tuning the emission wavelength. Multi-section emitter with photonic crystal section is integrated in the intra-cavity configuration, while semiconductor optical amplifier combined with diffraction grating is adapted in the external-cavity configuration. Our goal is to hand out the high-speed and stable swept-wavelength devices to bio-medical research unit for direct OCT imaging test.