發光二極體與高頻驅動電路設計應用於光通訊介面之研究

Design of Light-emitting Diodes and High-frequency Driver Circuits for Optical Communication

游子軒(Zi-Xuan You)

指導教授 ： 黃建璋

國立臺灣大學/電機資訊學院/光電工程學研究所/碩士(2018年)

https://doi.org/10.6342/NTU201803965

摘要

光通訊系統已廣泛運用於我們現今的生活中，在無線通訊領域，可見光通訊能結合一般照明與通訊兩種功能，使其成為了未來頗具潛力的通訊方式。隨著近年來通訊需求大增，提高元件的操作速率一直都是重要的課題。藍光發光二極體已逐漸取代傳統燈泡成為現今主要照明光源，然而其調變頻寬常受限於較長的自發性輻射複合生命週期，因此在以照明功能為基礎的前提下，提升元件操作速率是本論文研究的主題。
一般發光二極體的元件尺寸越小，可以達到越高的頻寬，但同時也犧牲了光強。本論文將一般藍光發光二極體結合光子晶體的結構，以期能提升元件頻寬，且同時保有適當光強。在使用開關鍵控調變的方式下，我們的元件以178 MHz的頻寬，達到400 Mbit/s 的資料傳輸速率。為了在有限的頻寬裡進一步提升資料傳輸量，我們也使用了正交分頻多工的調變方式進行訊號傳輸，並成功以光子晶體發光二極體，達到2 Gbit/s 的資料傳輸速率。我們的研究顯示，以傳統照明用藍寶石基板磊晶而成的發光二極體，再加入光子晶體結構設計後，能有效提升元件頻寬，並能達到可觀的資料傳輸速率，在未來可見光通訊系統中的應用具有極大的潛力。
在有線的光通訊系統中，垂直共振腔面射型雷射結合單多模光纖的傳輸方式已廣泛應用於短中距離的系統上。有別於傳統以金屬導線作為傳輸媒介的方式，光纖通訊有較低的能量耗損與串擾，更能保持訊號的完整性。隨著資料傳輸量的需求增加，除了雷射的頻寬需要提升，控制雷射的驅動電路也需要進一步提升操作速率。因此本論文嘗試以砷化鎵為基板所製成的高電子遷移率電晶體，設計雷射驅動電路並量測其電性，在輸入端振幅300 mVpp的開關鍵控調變下，驅動電路可以達到 32 Gbit/s 的操作速率且誤碼率小於10-12，並有700 mVpp的輸出電壓擺幅；而使用四階脈波振幅調變，輸入25 Gbit/s 的資料速率與振幅350 mVpp 的訊號可以得到930 mVpp 的總輸出電壓擺幅，且誤碼率小於10-11，能通過KP4-FEC 之要求。此結果顯示，以砷化鎵高電子遷移率電晶體設計製作的驅動電路，能擁有足夠的輸出電壓擺幅，將能有效高速調變雷射，並實際應用於高頻光通訊系統中。

關鍵字

發光二極體 ； 可見光通訊 ； 光子晶體 ； 頻率響應 ； 垂直共振腔面射型雷射驅動電路

並列摘要

Optical communication system has been widely used in our daily life. For wireless communications, visible light communication (VLC) has shown its potential as an alternative choice of radio source attribute to the ability of integrating with the exiting lighting infrastructure. With the increasing demand for data content in recent years, improving the component bandwidth has become an important topic. The blue-light light emitting diode luminaries have gradually replaced the traditional light bulbs for solid-state lighting. However, the modulation bandwidth of LEDs is usually limited by the spontaneous carrier lifetime. Therefore, improving the bandwidth of conventional LEDs is the object of this thesis.
Generally, there is a trade-off of high-speed performance of the LEDs. By shrinking the mesa size, LEDs can achieve higher bandwidth but sacrifice the light intensity. In this thesis, we used the photonic crystal (PhC) structure embedded in LEDs (PhCLEDs) to improve the performance of the devices and maintain the appropriate light output power. The eye pattern up to 400 Mb/s using on-off keying (OOK) modulation is achieved with a bandwidth of 178 MHz. To further increase the bit rate beyond the bandwidth limit, we also used 16-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) modulation to transmission data and a data rate up to 2 Gbit/s is obtained with PhCLEDs. The experiments reveal the bandwidth of the LEDs can be improved with photonic crystal structure, and PhCLEDs shown a great potential in visible light communication system.
For wireline optical communication system, data transmission using vertical-cavity surface-emitting lasers (VCSELs) and single or multimode fibers has been widely used. Unlike the traditional way of using metal conductor as transmission medium, optical fiber communication possess smaller power loss and crosstalk as compared to copper wirelines, which can maintain signal integrity. As the demand for data transmission increases, the bandwidth of the laser needs to be increased. Besides, the laser driver circuits also need to further improve the operating speed. To this end, this thesis attempts to use GaAs-based high electron mobility transistors (HEMTs) to design the laser driver circuits and demonstrate the electrical characteristics. When using OOK modulation with an input amplitude of 300 mVpp, the output voltage can reach about 700 mVpp in amplitude with a BER < 10-12 at a data rate of 32 Gbit/s. Furthermore, 4-level pulse-amplitude modulation (PAM-4) scheme was carried out to transmit a 25 Gbit/s data rate with BER = 10-11, and the jitter tolerance is larger enough to pass KP4-FEC requirement. A 930 mVpp output voltage swing was obtained under an input amplitude of 350 mVpp. The results indicate that the VCSEL driver designed in GaAs HEMT technology can support enough output voltage swing, and is capable for modulating a VCSEL in high speed optical communication systems.

並列關鍵字

light-emitting diodes ； visible light communication ； photonic crystals ； frequency response ； VCSEL driver

參考文獻

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