高速垂直共振腔面射型雷射及垂直共振腔電晶體雷射之開發及特性研究

Fabrication and Characterization of High-Speed Vertical-Cavity Surface-Emitting Laser and Vertical Cavity Transistor Laser

吳承翰(Cheng-Han Wu)

指導教授 ： 吳肇欣

國立臺灣大學/電機資訊學院/電子工程學研究所/博士(2019年)

https://doi.org/10.6342/NTU201903598

摘要

隨著網際網路的快速發展，光纖傳輸已廣泛使用在長距離的骨幹網路、中距離的都會網路及短距離的區域網路。光通訊的資料傳輸速率需求也越來越高，目前10 Gb/s及25 Gb/s的高速雷射光源已經發展成熟並應用在40/100 Gb/s乙太網路系統，而400 Gb/及Tb/s級系統架構也在全球最大光通訊研討會(Optical Fiber Conference, OFC)被探討。在2015 OFC，預估 2020 年400 Gb/s光收發模組開始實際應用於數據中心。為了達成400 Gb/s系統架構，光發射器為重要一環，為達到更高的輸出頻寬及更低的功耗，單通道50 Gb/s垂直共振腔面射型雷射(Vertical-Cavity Surface-Emitting Laser, VCSEL)為必要發展的項目。
本論文旨在研發下一代光通訊網路之高速雷射，在論文第二章，我們開發了單通道零誤碼率(Error-free) 50 Gb/s 850nm垂直共振腔面射型雷射，目標展示單顆面射型雷射高速50 Gb/s傳輸及零誤碼率操作，我們使用半波長(0.5-λ)共振腔並讓氧化孔徑縮小到3.3 μm，其閾值電流(Ith)為0.8 mA，最大頻寬27.6 GHz，我們使用不歸零(Non-Return-to-Zero, NRZ)開關移鍵(On-Off Keying, OOK)格式在室溫下通過50 Gb/s零誤碼率數據傳輸。
第三章探討VCSEL在高溫下的特性。在高溫下，由於增益-共振腔補償設計VCSEL的增益頻譜(gain spectrum)會逐漸紅移並在65˚C時有最低閾值電流(0.62 mA)。從室溫到85˚C，頻寬由27.6 GHz降到25 GHz。我們透過微波量測及建模擬合萃取出快速的電子電洞復合(τrec = 0.053增加至0.063 ns)及較低的光子生命時間(τp = 3.2增加至4.3 ps)並歸納雷射之頻寬限制來自於熱效應導致的光子密度飽和。
然而，二極體雷射由於主動區載子動力學的劣勢，電子電洞注入量子井後會堆積並等待復合，終究會限制其頻寬，而且其過高的共振頻率峰值會導致眼圖過衝(overshooting)進而扭曲數據傳輸的波形而增加誤碼率。因此，我們將VCSEL與電晶體結合，開發出垂直共振腔電晶體雷射(Vertical Cavity Transistor Laser, VCTL)，其主動區少數載子分布變成傾斜，主動區不再像是水庫累積載子，復合慢的載子將會被基-集極的逆偏電流掃至集極，進而減少復合發光時間，因此可以降低其共振峰值而得到較“平”光頻率響應及不失真的傳輸波形。第四章，我們使用選擇性氧化技術(selective oxidation)增加VCTL的載子侷限並增加復合放光效率進而降低閾值電流至2.4 mA，其旁模抑制比(Side Mode Suppression Ratio, SMSR)為31.76 dB為單模操作。在80 K時，最大光頻寬為11.1 GHz。其頻率響應相較於二極體雷射更“平”，將來應用於數據傳輸可得到不失真的波形及更低的誤碼率。

關鍵字

光通訊 ； 半導體雷射 ； 垂直共振腔面射型雷射 ； 電晶體雷射 ； 光頻率響應

並列摘要

Due to the rapid growth of data traffic demand, 850 nm vertical-cavity surface-emitting lasers (VCSELs) have been used in high-speed short reach optical interconnects in the data center. Current 10 Gb/s and 25 Gb/s products are maturely applied to 40G/100G Ethernet transceiver and 50 Gb/s per channel is predicted to be utilized in next-generation applications. Therefore, to achieve high data-rate transmission, the bandwidth of the VCSEL must be enhanced. In this dissertation, we develop the high-speed 850 nm oxide-confined VCSEL with 0.5-λ cavity and 3.3 μm aperture. We demonstrate 27.6 GHz optical modulation bandwidth with low threshold current of 0.8 mA. The VCSEL also passes the direct modulated 50 Gb/s error-free back-to-back data-rate transmission in non-return-to-zero (NRZ) configuration at room temperature without any error-correction technique.
High temperature operation of the VCSEL is an important topic especially for data center application. Hence, the temperature-dependent property from room temperature up to 85 ˚C of the VCSEL is performed. Due to the gain-cavity offset design, the lowest Ith occurs at 65 ˚C. The optical bandwidth of the 3.3 μm aperture VCSEL decreases from 27.6 to 25 GHz when temperature increases from 25 to 85 ˚C. A microwave determination method of these physical characteristics is utilized to extract the fundamental parameters based on experimental data and small-signal equivalent circuit model. With the microwave determination, the e-h recombination lifetime increases from 0.053 (25 ˚C) to 0.063 ns (85 ˚C) and the cavity photon lifetime increases from 3.2 (25 ˚C) to 4.3 ps (85 ˚C). The bandwidth limitation mainly arises from the photon density saturation which is induced by self-heating effect.
Fundamentally, the bandwidth of the VCSEL is limited by the carrier dynamics in the active region. The carriers tend to accumulate and wait for recombining which cause the “carrier choking” in the quantum-well. Thus, a large resonance peak and the slower recombination lifetime on average are expected which result in the overshooting in eye diagram and degrade the signal integrity in data-rate transmission. Consequently, the new carrier dynamics in active region with combining a VCSEL and a transistor is developed. A vertical kind of transistor laser with small volume, high energy efficiency, and high Q cavity is presented, namely, vertical cavity transistor laser (VCTL). The “tilted” minority carrier profile in the active region allows a faster radiative recombination process with sweeping the slow recombined carrier out to the third collector terminal. As a result, a resonance-free optical response of a 4.7 × 5.4 μm2 aperture VCTL with 11.1 GHz bandwidth at 80 K is exhibited. The single-mode laser operation with side mode suppression ratio (SMSR) of 31.76 dB is shown. The “flat” optical responses of these high Q active region transistor laser devices are projected to provide higher signal integrity for improving eye-opening for better error-free data transmission. These results also yield the capability of signal mixing and processing employing advantageously the enhanced voltage-modulation capability as well as the usual current-modulation.

並列關鍵字

optical interconnect ； semiconductor laser ； vertical-cavity surface-emitting laser ； transistor laser ； optical response

參考文獻

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