Performance of Combined TDM and HOD Raman Pumping Scheme in a Dispersion-Compensated Link

隨著對高容量的分波多工的要求，使得目前的光纖通訊技術不斷的往高容量發展，因此對光放大器的需求日益增加。而現今的放大器要注意雜訊對信號的影響之外，還須注意光纖中色散、放大器的雜訊、光纖裡的非線性現象所引起損害這三者之間的權衡取捨(tradeoff)。 在這本論文中，我們將研究在色散補償系統使用新激發機制的拉曼放大器的光纖色散、放大器雜訊、光纖非線性現象三者之間的表現，並且試著找出最佳的權衡取捨。 有好幾種光放大器，而光纖拉曼放大器具有可調整的放大頻寬和很平坦的增益曲線。分佈式的光纖拉曼放大器直接使用傳輸光纖為增益介質在一組光纖跨距裡。拉曼放大器會隨著不一樣的激發機制而有不一樣的雜訊表現方式,因此研究激發機制就是一個好的研究方向。首先我們會討論各種的激發機制的優缺點，在我們的研究中主要是討論結合分時多工和高階激發機制的拉曼放大器的雜訊表現。這樣的激發機制可以降低泵浦和泵浦間的交互作用。早先的光纖拉曼放大器，只使用一種光纖對現在的光纖通訊系統來說是相當不合時宜的因為除了光纖的損耗問題之外還有其他的效應也是考慮。例如光纖色散的問題，光纖的色散來說可以使用色散補償光纖(DCF)來補償色散的現象，但相關的研究確是著墨的比較少，所以在這本論文中我們將會呈現色散補償光纖中加入結合分時多工和高階激發機制的拉曼放大器的實驗結果。 在實驗的架構中我們把實驗架構分成兩個部分。第一個部分是高階拉曼激發機制的激發方向的討論，第二個部分是色散補償的先後順序的討論,這兩個部分四種實驗結構都有個別的雜訊表現方式，我們會分別從雜訊指數(Noise Figure)和品質因數(Q-factor)實驗的結果來分析每一種實驗架構的優缺點。最後從結果看到預先色散補償的八十公里的光纖通訊系統中並且使用正向高階拉曼激發機制在雜訊指數和品質因數之間有最好的權衡取捨。The ever-increasing demand for ultra high-capacity wavelength-division-multiplexed (WDM) fiber transmission systems pushes the existing technologies to their limits. New concepts are required which allow for a better tradeoff between the three limiting factors: accumulation of amplifier noise, chromatic dispersion, and impairments due to nonlinear effects. In this thesis, the performance of Raman amplification with a newly proposed pumping scheme in a dispersion-compensated link will be studied. The study considers the three limiting factors and tries to achieve the best tradeoff among them. The motivation of our study is as follows. There are many different types of optical amplifiers and among them, fiber Raman amplifiers (FRAs) have the advantage of flexible amplification wavelength band and flat gain profile. They are often used in the distributed form to improve the OSNR of a long fiber span. The pumping schemes of FRAs greatly affect their performance and attract many research interests in recently years. The pumping scheme that we are interested is a combined time-division-multiplexed (TDM) and high-order (HOD) pumping scheme. This scheme can reduce the undesired pump-pump interactions while maintaining low nonlinear degradation. Previous studies of Raman pumping schemes mostly focused on a link with only one kind of fiber. However, this may not be true in current systems. For example, dispersion compensation is necessary in many systems, especially in long-haul systems, and the use of dispersion-compensating fibers (DCFs) is the most popular way to achieve dispersion compensation. The characteristics of DCFs are quite different from those of transmission fibers such as standard single-mode fibers (SSMFs). Therefore, the Raman amplification in a dispersion-compensated link consisting of SSMFs and DCFs will be very different from that in a link with only one kind of fiber. In this thesis, FRAs’ characteristics with focus on their noise sources will be introduced. Different Raman pumping schemes and their advantages and disadvantages will also be discussed. For the combined pumping scheme under study, there are two possible ways to launch the HOD pump. For a dispersion-compensated link, there are a couple of possible ways to arrange the fibers. Among them, two of them will be considered – post-compensation and pre-compensation. The four configurations (two pumping directions × two fiber arrangements) will be demonstrated and analyzed. The two metrics used are equivalent noise figure (eNF) and Q‐factor. The experimental results indicate that for a link consisting of an 70 km standard single‐mode fiber (SSMF) and an 11 km dispersion‐compensating fiber (DCF), the configuration of pre‐compensation and forward second‐order pumping achieves the best tradeoff between noise figure degradation and nonlinear effects.Chapter 1 Introduction ............................................................................................ 1 1.1. Raman Amplifiers in Optical Communication Systems ................................ 1 1.2. Thesis Outline ................................................................................................ 2 Chapter 2 Fiber Raman Amplifiers ......................................................................... 4 2.1. Overview of Fiber Raman Amplifiers............................................................ 5 2.1.1. Stimulated Raman Scattering............................................................................................6 2.1.2. Spontaneous Raman Scattering ........................................................................................9 2.1.3. Raman Gain Spectrum....................................................................................................10 2.1.4. Noise in Fiber Raman Amplifiers ...................................................................................12 2.2. Noise in Fiber Raman Amplifiers ................................................................ 14 2.2.1. Amplified Spontaneous Emission Noise..........................................................................14 2.2.2. Double Rayleigh Scattering (DRS).................................................................................16 2.2.3. Pump-Pump Four-Wave Mixing (FWM).........................................................................18 2.2.4. Pump-Signal Four-Wave Mixing (FWM)........................................................................20 Chapter 3 Combined TDM and HOD Raman Pumping Scheme in a Dispersion-Compensated Link .................................................................................... 22 3.1. Pumping Schemes of Raman Amplifiers ..................................................... 24 3.1.1. Pumping Directions ........................................................................................................24 3.1.2. Time-Division Multiplexing Pumping Scheme................................................................27 3.1.3. Higher-Order Pumping Scheme......................................................................................28 3.1.4. Combined TDM and HOD Pumping Scheme .................................................................29 3.2. Combined TDM and HOD Raman Pumping Scheme in a Dispersion-Compensated Link ............................................................................... 31 3.2.1. Dispersion-Compensated Links ......................................................................................32 3.2.2. Arrangement of Fibers....................................................................................................34 3.2.3. Experimental Setup.........................................................................................................35 3.2.4. Results and Discussions..................................................................................................39 3.3. Noise Figure Measurement and Analysis .................................................... 40 3.3.1. Experimental Setups .......................................................................................................40 3.3.2. Results and Discussions..................................................................................................41 Chapter 4 Q-Factor Measurement and Analysis ................................................... 43 4.1. Motivation.................................................................................................... 43 4.2. Q-Factor Measurement and Analysis .......................................................... 46 4.2.1. Experimental Setup.........................................................................................................46 4.2.2. Results and Discussions..................................................................................................47 Chapter 5 Conclusion............................................................................................ 5