利用聲波調製應用於可見光通訊系統

Abstract

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，可見光通訊系統能結合一般照明與通訊兩種功能，使其成為了未來相當具有潛力的通訊方式。隨著近年來無線通訊需求大增，提升元件的操作速率一直都是重要的議題。藍光發光二極體已逐漸取代傳統燈泡成為現今主要之照明光源，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旨。 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，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。 以表面聲波進行聲光調製之元件已廣泛被使用。本論文嘗試利用指狀電極產生低頻之波面聲波提升垂直式發光二極體特性，當二極體受到108 MHz表面聲波的影響之下，會產生聲電流，進而提升二極體約2.5% 的發光強度；同時，亦會形成壓電材料的共振現象，而此結果在本研究中被認定與表面聲波所給定的操作頻率並無直接相關。不同共振模態下的共振頻率也會互相影響以達到頻率上的上轉換，最終產生接近1 GHz的光震盪。而本研究也顯示可以將接近1 GHz 之光震盪以20 MHz之載波進行頻率調變，並應用於可見光通訊系統當中。

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. In recent years, the devices with acousto-optic modulation have been widely used. This thesis attempts to enhance the optical properties of VLED by using surface acoustic waves (SAW) generated by interdigital transducers. Since surface acoustic waves at 108 MHz were fed into the VLED, the acoustoelectric current were generated, and the optical output power was also improved about 2.5%. Meanwhile, the piezoelectric vibration was occurred, and the resonance frequencies of different vibration modes are independent with the frequency of SAW. Resonance frequencies comes from different vibration modes would affect each other and achieve frequency up-conversion. Finally, ~GHz optical vibration was achieved and observed. Thus, the 20 MHz RF signal was given and thought of as the carrier which can carry the SAW induced GHz optical oscillation signals. It is an important result to demonstrate a novel A/O modulation for visible light communication applications.