光通訊系統中的全光訊號處理

Abstract

本論文將介紹及分析數種新穎的全光訊號處理技術，包含了全光的波長轉換、全光的開關移鍵到差分相位移鍵的格式轉換、全光的差分相位移鍵的再生。 為了提升在半導體光放大器中的交叉極化調變之波長轉換頻寬，我們提出了一種稱作差分交叉極化調變的方法，它與傳統的交叉極化調變的不同僅在於一段額外的雙折射延遲線。在差分交叉極化調變的分析部份，包括訊號的數值模擬以及小訊號模型的推導，兩者都能和實驗結果互相對應。其中數值模擬清楚地顯示出波長轉換的效能提昇，而小訊號模型則提供了更多對差分交叉極化調變直觀且深入的理解。 在全光調變格轉換中，我們使用光子晶體光纖中的交叉相位調變，來達成40 Gb/s的開關移鍵到差分相位移鍵的訊號格式轉換。因為我們所使用的光子晶體光纖具有高度的線性雙折射特性，並且其雙折射光軸在整段光纖中是固定的，將受感應光的偏振態控制在相對雙折射光軸45度輸入光纖時，只要受感應光與調變光的波長差超過約6 nm就能明顯降低該全光調變格式轉換效率受調變光偏振態的影響程度，並且偏振態與波長差對於格式轉換的影響有理論的定量分析。由於該格式轉換基於以調變光光強度調變受感應光相位，當我們需要的相位變化不足、或調變光強度雜訊引起相位雜訊，都會影響差分相位移鍵的訊號性能，該問題的造成的損耗也有理論的分析。 關於差分相位移鍵的訊號再生，因為差分相位移鍵訊號是將訊號載在光相位上，其訊號表現會同時受到振幅及相位雜訊影響。因此，除了振幅的再生，差分相位移鍵的再生必須要保留原有相位，甚至要降低相位雜訊。我們提出了一個新的概念：相位雜訊平均器，來降低差分相位移鍵的系統中的相位雜訊。它能夠有效地增加相鄰位元間相位雜訊的相關性，並因此減少了系統的差分相位雜訊。考慮各種不同特性的相位雜訊，我們發現多次的相位雜訊平均皆能使差分相位雜訊收斂，而且無關於系統傳輸長度。最後，對於不同的差分相位移鍵的再生，我們也理論分析了它們的差異。

In this dissertation, new approaches for all-optical wavelength conversion, all-optical on-off keying (OOK) to differential phase-shift keying (DPSK) format conversion and all-optical DPSK regeneration are proposed and analyzed theoretically. In order to improve the wavelength conversion bandwidth of cross polarization modulation (XPoM) in a semiconductor optical amplifier (SOA), a novel conversion scheme named differential cross-polarization conversion (DXPoM) is proposed, and it is realized by simply adding an extra birefringence delay line in the conventional XPoM. In this part, both the large-signal simulation and small-signal model of DXPoM are developed, and they successfully predict results that agree with experimental outcomes. The large-signal simulation of DXPoM evidently shows the performance improvement and match well with experimental results. The small-signal model gives more comprehensible and intuitive physical insight of DXPoM. For format conversion, using the cross-phase modulation (XPM) effect in a nonlinear photonic crystal fiber (PCF), the conversion of OOK-to-DPSK is achieved at 40 Gb/s for the first time. Because the PCF has high linear birefringence, and its birefringent axes remain for entire length, launching the probe at 45 degree relative to the birefringence axes of the PCF and having the pump-probe detuning (PPD) greater than » 6 nm can realize polarization-insensitive XPM-based conversion. The polarization- and PPD-dependent nonlinear phase shift is investigated theoretically. It indicates improper nonlinear strength could generate insuffcient phase shift (< pi) in XPM-based conversion, and the amplitude noise (AN) of an OOK pump will also be converted into the phase noise (PN) of the converted DPSK signal, the penalties induced by these issues are discussed analytically as well. Because the DPSK format carries information on the phase domain, it is influenced by both AN and PN. Therefore, except amplitude regeneration, the regeneration of DPSK signals has to preserve phase information or eliminate PN. A novel all-optical phase noise averager (PNA) is proposed to reduce residual PN in the DPSK transmission system with phase-preserving amplitude regenerators. It can increase the correlation between the PNs of neighboring bits and greatly reduce the differential PN in the transmission system. Considering various PN, the multiple PN averaging effect is investigated, and the effective PN is convergent regardless of the transmission distance. Furthermore, theoretical bit error rates of the DPSK format with various regeneration schemes is presented for comparison.