光纖通訊用面射型光源之研製

Fabrication of Surface-Emitting Light Source for Fiber Optic Communications

李峰旻(Feng-Ming Lee)

指導教授 ： 吳孟奇

國立清華大學/電機資訊學院/電子工程研究所/博士(2008年)

https://doi.org/10.6843/NTHU.2008.00043

摘要

摘要
為因應網際網路蓬勃發展，開發低成本之高速光通訊用光源乃當務之急。相對於一般邊射型雷射而言，面射型雷射具備發光體積小、單一縱模、圓型輸出光束、低製作成本等優勢，因此在中、短距離數據網路，例如:都會網路與區域網路，之應用更具潛力與價值。此外，環顧目前車用視訊傳輸與工業用監控設備市場已急遽增加，具有低成本、易於裝置之塑膠光纖因而受到高度重視，為此開發紅光共振腔發光二極體正可以大大滿足極短距離、低成本之傳輸需求。綜合上述，本論文旨在研究適用於低成本光通訊用之高速短波長、長波長面射型雷射與紅光共振腔發光二極體。
藉由選擇性氧化鄰近於主動區之高含鋁 AlGaAs，氧化侷限型面射型雷射具備優越的光、電侷限效果，因此選擇性氧化技術普遍用來製作高效率面射型雷射。一般為達到大功率光輸出之目的，製作大氧化孔徑面射型雷射乃必要條件，然而大孔徑面射型雷射易有電流擁擠、空間燒洞、多橫向模態數、低動態調變速度等缺點。因此，本論文率先利用環形結構製作大輸出孔徑之 850nm氧化侷限型面射型雷射，此元件透過二次氧化製程可得外部氧化孔徑與內部氧化孔徑分別為15 μm 與 9 μm ，經由靜態特性分析，室溫最大輸出功率為 7.5 mW、於任意電流注入皆可維持穩定之雙模態操作。此外，經由66-m光纖傳輸後之大訊號調變頻寬仍可高達 10 Gb/s，充分顯示出環形面射形雷射滿足高速光纖通訊市場之需求標準。
另一方面，由於矽基光纖於1310 nm 與 1550 nm 有兩個光通訊傳輸窗口，因此經由引入 GaInNAs 充當主動層發光材料，外加運用表面蝕刻技術與反相位鍍膜技術，我們已成功開發出單橫向模態之 1.3 μm 長波長面射型雷射。首先，結合表面蝕刻技術所得之單模態面射型雷射，其表面蝕刻口徑為5 μm，氧化孔徑則可高達13 μm，在此徹底解決以往僅能利用縮小氧化孔徑達到單模態操作的問題，此元件於室溫下最大輸出功率更可高達1 mW、於任意注入電流之旁模抑制比皆可維持 30 dB以上、大訊號調變頻寬為2.5 Gb/s；其次，選用反相位鍍膜技術之面射型雷射，即使氧化孔徑高達 13 μm，當表面鍍膜孔徑縮小至 5 μm，我們可得單模態輸出之特性，此元件於任意電流注入條件下，旁模抑制比皆可高達 35 dB以上。
最後，由於塑膠光纖於650 nm 波段具備一低損耗之通訊傳輸窗口，因此開發紅光通訊光源有高度實用價值，然而紅光波段之面射型雷射高溫操作特性不佳。其次，顧及發光二極體受限於全反射角而導致光萃取受阻。因此於發光二極體引入共振腔結構將有助於大大提升光萃取效率並取代面射形雷射。在此，為進一步降低共振腔發光二極體之電容效應並增加電載子侷限能力，以提升元件操作頻寬與發光效率，於製程技術上，我們採用蝕刻一深達數μm 之平台結構，並利用氧化矽填平技術完成平面型共振腔發光二極體，利用此氧化矽填平技術可提升輸出光功率近乎3倍之多，此外，元件小訊號調變頻寬可高達200 MHz 以上，然而，受限於大面積光檢測器之頻寬，大訊號頻寬量測值為 100 MHz，徹底滿足商用極短距離傳輸通訊需求。

關鍵字

面射型光源 ； 面射型雷射 ； 微共振腔發光二極體 ； 單一橫向模態

並列摘要

Abstract
Surface emitting light source now is regarding as a very important light source for many optoelectronic applications, such as high-speed LANs, computer links, optical interconnects, laser printing, display, optical mouse etc. In addition, the surface emitting light source can roughly be divided into two categories: surface emitting lasers and surface emitting LEDs. Among them, the mainstream is to develop vertical- cavity surface emitting lasers (VCSELs) and resonant-cavity light emitting diodes (RCLEDs) due to urgent need for optical communication. With respect to oxide-confined VCSELs, since the oxidized AlGaAs layer provides both excellent current and optical confinements, it gives rise to a reduced threshold current, a high efficiency, an enhanced modulation bandwidth, and a higher light output power as compared to other VCSEL structures. However, the oxide-confined VCSELs with a larger aperture (> 10 μm) would launch many higher-order transverse modes and exhibit the strong mode competition due to large refractive-index step, spatial hole burning, current crowding, as well as thermal effects. These effects would further degrade the performance of the devices with a large aperture such as the limits to the maximum light output power and the high frequency response. In order to overcome drawbacks mentioned above, we fabricate ring-shape VCSELs. These ring-shape VCSELs at room temperature exhibit a threshold current of 3.65 mA, a maximum light output power of 7.5 mW at 25 mA, and a differential resistance of 65 Ω. In addition, these devices exhibit a stable high-order-mode behavior over the entire operation current range resulting from the uniform carrier distribution and the weak mode competition. On the other hand, this TO-packaged 850-nm VCSEL for small-signal analyses shows a maximum modulation frequency of about 8 GHz corresponding to a modulation current efficiency factor (MCEF) of 2.47 GHz/mA1/2 and a clear and symmetric eye-opening feature at 10.34 Gb/s at 18 mA for both back-to-back and 66-m transmission test. These results ensure that the TO-packaged VCSELs can fulfill the OC-192 SONET mask-test.
On the other hand, due to the continuously increasing demand for higher network capabilities of extending transmission distance at high data rates, it gives rise to the VCSELs toward launching longer wavelengths. A 1.3 μm VCSEL based on GaInAsN active layer would be the dominant candidate because it can be pseudomorphically grown on GaAs substrates by utilizing the well-established AlGaAs/GaAs distributed Bragg reflectors (DBRs). Besides, the relatively large conduction-band offset of GaInAsN/GaAs also exhibits a better high-temperature performance than that of InP-based material systems. Nonetheless, the large oxide-aperture VCSELs will tend to lase in high-order Laguerre-Gaussian modes at elevated current levels and leads to a problem in fiber coupling and result in the mode partition noise, which will further deteriorate the optical signal during data transmission. Attempts to control the mode dynamics that can be easily carried out by the following methods: surface relief (i.e. shallow etching technique) and anti-phase coating. Based on these two techniques, we have succeed in the 1.3 μm single-mode planar-type GaInAsN VCSELs with two Ga0.65In0.35As0.99N0.01 SMQWs as the active region grown by MOVPE. A circular surface relief and a thick silicon oxide was utilized to support the single fundamental mode and to planarize the VCSELs, respectively. The VCSELs with a 12-μm-diameter oxide-confined aperture and a 5-μm-diameter surface-relief aperture at room temperature exhibit a threshold current of 3 mA, a slope efficiency of 0.14 mW/mA, and a single-mode behavior. These VCSELs show a maximum light output power of 1 mW for the single fundamental mode with a transverse-mode suppression of more than 30 dB at the current level of 15 mA. Furthermore, the maximum operation temperature of the VCSELs is 90℃. Finally, the VCSELs also show a clear eye-opening feature and are well operated at 2.488 Gb/s under a bias current of 12.6 mA. These results confirm the 1.3 μm single-mode planar-type GaInAsN VCSELs have the potential capacity for fiber optic applications. On the other hand, the anti-phase coating VCSELs with four different diameters in Ge-coated apertures was formed in the center of the device to improve the characteristics of transverse mode. In addition, a thick silicon oxide film was used to planarize the VCSELs. The VCSELs with a 13-μm-diameter oxide-confined aperture and a 7-μm-diameter Ge-coated aperture at room temperature exhibit a stable single-mode behavior and a transverse-mode suppression of more than 35 dB over the entire operational range.
In addition, nowadays, LED have been widely used in short-distance low-cost local area networks (LANs) over polymethyl methacrylate (PMMA) plastic optical fiber (POF), which exhibits the minimum attenuation rate in the 650 nm wavelength. Nonetheless, LEDs have typically an inherent low light extraction efficiency due to the existence of high difference in refractive index at the semiconductor-air interface so that only a few of the light is available to escape from the surface. One of particular interesting researches is focused on RCLED mainly because of the feasibility of increased extraction efficiency with micro-cavity structure. Based upon the demand of low-cost mass productions, we will propose an alternative method to realize the planar-type 650-nm RCLEDs by using silicon oxide. The device with a SiOx planarized layer exhibits a low operating voltage of 2.3 V at 20 mA, a maximum light output power of 304 μW at 15 mA, and the best external quantum efficiency of 3 % at 1.2 mA. In addition, the SiOx-planarized device shows emitting peak wavelength at 647 nm at 20 mA and exhibits less temperature sensitivity than that of their counterpart. The RCLED with a 30-μm diameter has the maximum 3-dB frequency bandwidth of 275 MHz at a driving current of 40 mA. Finally, the SiOx-planarized device also shows a clear eye-opening feature as operating at 100 Mbit/s at 20 mA.

並列關鍵字

surface-emitting light source ； VCSELs ； RCLEDs ； single transverse mode

參考文獻

[1] B. Mukherjee, Optical Communication Networks, McGraw Hill, New York (1997).

[3] Paul Green, “Progress in optical networking,” IEEE Communications Magazine, vol. 39, pp.54-61 (2001).

[4] H. Soda, K. Iga, C. Kitahara, and Y. Suematsu, “GaInAsP/InP surface emitting injection lasers,” Jpn. J. Appl. Phys., vol. 18, no. 12, pp. 2329–2330 (1979).

[5] F. Koyama, S. Kinoshita, and K. Iga, “Room-temperature cw operation of GaAs vertical cavity surface-emitting laser,” IEICE Trans., vol. E47, no. 11, pp. 1089–1090 (1988).

[6] F. Koyama, S. Kinoshita, and K. Iga, “Room-temperature continuous wave lasing characteristics of a GaAs vertical cavity surface-emitting laser,” Appl. Phys. Lett., vol. 55, no. 3, pp. 221–222 (1989).