先進光調變與多工技術在直接偵測光傳輸系統上的性能評估與分析

Abstract

在本論文中我們將簡介數種已提之先進光調變格式以及光多工技術，其工作原理以及目前所遇到的議題將一併於論文中予以討論以及部分解決。其中光多工技術包含光正交頻分多工 (OFDM) 以及光碼分多工 (OCDMA) 兩種技術，而光調變格式主要是高頻譜效能(>1 bit/symbol) 之格式包含多階相位或多階振幅調變格式 。 針對直接偵測OFDM下我們提出射頻輔助正交分頻多工(RF-tone assisted OFDM)以及廣義虛擬單邊帶正交頻分多工(Generalized Virtual SSB-OFDM)等兩種技術。此技術置放一射頻次載波於信號頻帶邊緣作為遠端直接偵測參考信號之用。使用我們的技術後，和同性質的直接升頻(direct up-conversion) 調變器相比，可節省一半數位轉類比器 (DAC) 的取樣速度，而其DAC有限的取樣速度在高容量 (≥ 10 Gbps) 的光通訊傳輸下目前乃是主要瓶頸。此外，我們成功地傳輸10 Gbps OFDM訊號經過1600 km的標準單模光纖後僅3 dB的損傷，驗證此技術在未來光通訊的應用潛力。 於光調變格式中，我們實驗以及理論驗證其使用簡易、低廉的單顆光調變器產生數種複雜傳輸訊號的可能性。這在未來講求低價格，高效能的光通訊網路上是一大躍進。除此之外我們更進而分析數種不同調變格式的性能理論上限: 量子限制 (quantum limit)。 此一限制給出最佳性能指標以簡化工程應用上所能達成的最佳性能。 最後我們研究光碼分多工技術在局部網路(LAN)上的應用。我們使用費比布洛自注入雷射以及光纖光柵串列提出廉價光碼分多工光源。藉著針對每一個光柵施以溫度或應力的控制我們可以改變所要傳送地點的用戶碼。接著提出交叉頻率分組方式以降低在多用戶光碼分多工系統下所遭遇到的多重存取干擾。最後，我們首次實驗驗證多載波碼分多工系統於光接取網路上的應用可能。

In this dissertation, we review and propose several new approaches for orthogonal frequency division multiplexing (OFDM), advanced modulation formats, and code division multiplexing (OCDM) techniques in optical transmission systems. For OFDM, we propose an RF-tone assisted OFDM and virtual single-sideband OFDM (VSSB-OFDM) systems for direct detected optical transmission. With an RF tone inserted at the edge of the signal band, the sensitivity and CD tolerance are both improved compared to the previous power modulated SSB-OFDM. Besides, our approach also relaxes the bandwidth requirement for digital to analog converter (DAC) which would be very critical in a high speed transmission with a data rate of ≥ 10 Gbps. In addition, we firstly propose a 2x2 matrix equalization technique to jointly compensate the distortions resulted from both the transmission and the imbalances of the optical modulator. Moreover, we successfully transmit a 4-QAM, 10-Gbps signal through 1600 km uncompensated standard single mode fiber (SSMF) with only ~ 3 dB penalty, which is smaller than any other proposed direct-detected transmission systems. For advanced modulation formats, we numerically evaluate and experimentally demonstrate the generation of complex modulation formats by using one dual-drive Mach-Zehnder modulator (DD-MZM). We also analyze the performance bound, i.e. quantum limit, of 4ASK format. We further propose a phase modulated 4ASK (PM4ASK) format which shows a better CD, PMD and filtering tolerances compared to conventional 4ASK format. For CDMA, we propose a simple and cost-effective fast frequency hopping CDMA (FFH-CDMA) light source which uses a self-injected Fabry Perot laser and one string of fiber Bragg grating array (FBGA). By tuning the stress of each individual grating, we can encode the transmitted data by the destination codeword. Aimed to eliminate the inherent multiple access interference (MAI) in optical FFH-CDMA, we propose a frequency-interleaved multi-group approach which fully uses the frequency gap between the gratings to reduce the spectrum overlapping among different users, and design the new codeword searched by computers which is suitable for this proposed technique. Finally we versify this technique by some simulation results.