| 摘要: | 在現今光纖通訊（OI）市場中，主要是以850奈米的垂直共振腔面射型雷射（VCSEL）來當作短距離（<300米）主要的訊號發射端，因為VCSEL的製作成本低、元件面積小且低耗能，最主要的是直接調變即可實現高速的傳輸數據。而在數據中心裡使用到大量的光連結，成本和能源效率就成為關鍵的議題。因此很多研究團隊試圖想改善這個問題，如採用短波長分波多工（SWDM）技術來傳輸，它是由四個不同波長的VCSEL當作訊號發射端，接收這四個的波長訊號的同時再由一條多模光纖（MMF）來傳輸數據，如此一來，光纖的數目可減少為原來的四分之一。除了光纖成本外，每條通道的通道速度也是很重要的議題，現在VCSEL的開關鍵控調變（OOK）以56 Gbit/s為目標來滿足下一代光纖通訊的要求。
現在幾乎所有的高速VCSEL都是使用水氧化製程來侷限電流，但是其產生的薄氧化層會造成明顯的寄生電容，這是限制頻寬的原因之一，因此我們利用氧化層掏離技術來解決這問題，此方法是利用化學性濕蝕刻將氧化鋁給去除，以空氣取而代之，因空氣的介電常數低於氧化鋁，所以寄生電容降低，進而提升速度。另外氧化鋁與周圍的砷化鋁鎵（AlGaAs）晶格常數不匹配，可能會在高電流操作時導致一些缺陷，氧化層掏離技術正好可以解決問題，提高元件的可靠度。除了速度之外，傳輸距離也是一個很重要的因素，這是因為隨著物聯網和雲端的蓬勃發展，數據需求量越來越大，使得數據中心越蓋越大，距離也將成為一個問題。在長距離傳輸下，所面臨到模態色散的問題，使得數據在傳輸的過程中失真，所以我們利用鋅擴散技術使上層布拉格反射鏡（DBR）的原子排序紊亂並降低擴散區域的反射率，因此可以抑制其它非主要的模態，使得模態數目減少，從而實現無失真的長距離傳輸。
在本論文中，我們結合鋅擴散和氧化層掏離技術來製造VCSEL。首先，我們研發出旁模抑制比（SMSR）大於30dB的單模850奈米VCSEL，並且能以26 Gbit/s數據速率在OOK光收發模組調變下成功地實現以OM4 MMF傳輸的高比特率距離乘積（14 Gbit/s × 2.0 km）。而且也利用單模850奈米VCSEL做成陣列元件，呈現出近圓形對稱的遠場圖形、發散角非常窄（~4°）且輸出功率高達187.4毫瓦。另外，在高速VCSEL研發上，分別有頻寬高達29 GHz的850奈米VCSEL，使用前向錯誤更正（FEC）和決策回饋等化器（DFE）處理實現以一公里OM4 MMF的54 Gbit/s無錯誤數據傳輸。另一種是頻寬高達31 GHz的940奈米VCSEL，並不需要FEC和DFE處理就實現以50公尺OM5 MMF的50 Gbit/s無錯誤數據傳輸，這些高速VCSEL都已經應用於SWDM系統上。現在我們利用鋅擴散與氧化層掏離技術運用到新穎的主動層設計VCSEL結構，從而實現從室溫到高溫(85 °C)下擁有不變的速度表現，而且在環境溫度高達150 °C下，仍能擁有25Gbit/s無錯誤數據傳輸特性。
;In today′s optical communications (OI) market, vertical-cavity surface-emitting lasers (VCSELs) with central wavelengths at 850 nm have mainly been used as the primary signal emitters for short distances (<300 m). This is because VCSELs have a low-cost of fabrication, small device area, low power consumption, and most importantly have the capability of high-speed direct modulation. Data centers require an enormous number of links, which makes cost and energy efficiency critical issues. Therefore, many research teams are trying to solve this problem, some by using shortwave wavelength division multiplexing (SWDM) technology which utilizes multiple VCSELs at different wavelengths coupled into one multimode fiber (MMF). Consequently, the number of MMFs can be reduced. In addition to the cost of fiber, the data rate of each channel is also a very important issue. The targeted the on-off keying (OOK) modulation speed of a VCSEL is 56 Gbit/s to meet the requirements of the next generation of OI channels.
Up till now, almost all high-speed VCSELs utilize the wet oxidation process for current confinement, but this thin oxide layer causes significant parasitic capacitance, which is one of the factors limiting bandwidth. In order to overcome this problem, we demonstrate an oxide-relief technique which uses selective wet chemical etching to remove AlOx and replace it with air, whose dielectric constant is lower than that of AlOx. As the parasitic capacitance is reduced, the speed will increase. In addition, the lattice constant of AlOx does not match that of the surrounding AlGaAs layers, which may cause some defects during high-current operation. The oxide-relief technique can solve this problem and increase the reliability of the devices. Therefore, the oxide-relief technique not only can enhance the bandwidth but also can increase the reliability. In addition to the problem of speed, transmission distance is also very important. With the rapid development of the Internet of Things and the Cloud, the amount of data that needs to be handled has increased enormously and data centers are becoming bigger and bigger, making long-distance transmission an issue. Under long-distance transmission, problems of mode and chromatic dispersion arise, which distorts the data to be transmitted. Therefore, we utilize the Zn-diffusion technique, which disorders the top Distributed-Bragg-Reflector (DBR) mirrors and reduces the reflectivity in the diffused area, to suppress the higher-order modes. Since the number of modes becomes less, the influence of modal dispersion will thus decrease, which will help to achieve long-distance transmission without distortion.
In this dissertation, we incorporate the Zn-diffusion and oxide-relief technique to fabricate a VCSEL. First, we demonstrated single-mode 850 nm VCSELs with a side-mode suppression ratio (SMSR) of more than 30 dB, and obtained a maximum data rate up to 26 Gbit/s. Under OOK modulation formats we successfully demonstrated a high bit rate-distance product (14 Gbit/s × 2.0 km) for OM4 MMF transmission. In addition, we have demonstrated a single-mode 850 nm VCSEL array structure with excellent lasing performance. A stable (invariable) near circular far-field pattern with a narrow full-width half maximum (FWHM) divergence angle (~4°) under the full range of bias current and a high maximum single-lobe output power (187.4 mW) under continuous wave (CW) operation can be achieved. This has enabled the development of high-speed VCSELs, one of which is a 850 nm VCSEL whose electrical-to-optical (E-O) bandwidth achieves 29 GHz. In addition by using forward error correction (FEC) and decision feedback equalization (DFE) processing, at room-temperature (RT) we were able to obtain error free data transmission for 54 Gbit/s back-to-back (BTB) through a 1 km OM4 fiber. With another 940 nm VCSEL, an E-O bandwidth of 31 GHz was achieved with and 50 Gbit/s BTB data transmission under RT. Error-free transmission over a 50 meter OM5 fiber can be successfully achieved without using pre-emphasis or equalization techniques. These high-speed VCSELs have been applied to SWDM systems. A Zn-diffusion/oxide-relief VCSEL structure with a novel active layer design, which can achieve invariant high-speed performance from RT to high temperature (85 °C), has been studied. When the ambient temperature increases to 150 °C, it can achieve 25 Gbit/s error-free data transmission. |
