Studies Of Theories And Experiments Of Distributed Fiber Raman Amplifiers

At present, in high capability and long distance transmission system, the technology of fiber transmission is almost the only choice. The capability is expended largely by the technology of dense wavelength division multiplexing (DWDM). The fiber amplifiers, especially Erbium-doped fiber amplifiers are one of the key technologies of DWDM. When the fiber network is developed to be higher speed, broader band and longer distance, the EDFAs can not satisfy the need of development of fiber network because of the wavelength band which is fixed and not broad enough. The Raman amplifiers have the smart and broad (100nm) wavelength band, so they can satisfy the need of development of fiber network. Since the beginning of the 21th century, the Fiber Raman Amplifiers has become a hot study area. For the application in broad band and long distance situation, the theories and experiments of distributed fiber Raman amplifiers are studied. The following is the abstract content.At first, in the chapter of prolegomenon, the evolvement process and the current situation of fiber transmission and fiber optical amplifiers are introduced. The advantages of fiber Raman amplifiers compared with EDFAs are analyzed. The history and current situation of fiber Raman amplifiers are introduced. Especially, the key parts including Raman fibers, pump sources, wavelength combiners, et al. are reviewed.Secondly, the scattering equations of Raman amplifiers are analyzed. The different methods of solving scattering equations are compared. The boundary problems of Raman scattering equations are analyzed and two methods of solving the boundary problem are described detailedly. These efforts ensure that the simulation model of FRAs is correct.Thirdly, the present paper introduces the measurement of basic parameters and the basic experiments of fiber Raman amplifiers including the scheme of experiments and detail process of some experiments. In addition, some devices, such as C-Band, L-Band and C+L-Band broad band sources, were made in experiments. The photonic crystal fiber and femtosecond pulse laser are used to make out a supercontinuum source. The basic problems of gain are studied experimentally, and the result has good agreement with the simulation result. So, the present simulation method can forecast the experiment result correctly. The simulation and experiment results of Raman gain spectra pumped by multi-wavelength are studied. The conclusion can be drawn out that: the method of adding the different Raman gain spectra pumped by single wavelength can be used to optimize the pump wavelength, but can not be used to optimize the pump power.Fourthly, the main advantage of distributed fiber Raman amplifiers is broad band. In the long distance transmission, the accumulation of the gain fluctuation will affect the error rate of signal. So, the flat gain is a very important problem. Genetic algorithm is studied to optimize the flat gain. Different fitness functions are compared and a fitness function verysuitable to FRAs is presented. At last, according to the optimization result, the experiment system is set up and the optimization result agree with the experiment result.Fifthly, when the transmission distance is longer, the influence of the environment is greater, obviously causing random gain fluctuation. In addition, the change of wavelength number will cause strong gain fluctuation. This is the problem of gain equalization. In order to overcome this problem, a real-time automatic control of gain is needed. The paper reviews the technology of automatic control of gain and brings out an original automatic control algorithm using the pseudo-inverse matrix (generalized inverse matrix). The experiments have been done to validate this real-time control technology.Sixthly, in long distance transmission systems, the advantages of distributed fiber Raman amplifiers are obvious. The long distance is the main application situation of FRAs. In experiments, how the FRAs affect the signal power, eyes and signal noise ratio are studied in long distance fiber transmission system without regeneration. The combination of FRAs and dispersion compensation fibers (DCFs) are also studied. Based on the characteristics of DCFs and the distributed amplification, an amplification scheme of mid-span and second order pump is presented. This scheme is analyzed by simulation. The result of simulation shows that this scheme has obviously advantages in improving the noise figure.At last, the conclusion of this paper is presented. The trend of Raman amplifiers is analyzed. We point out that these areas including pump sources (high power semiconductor lasers, tunable multiwavelength fiber lasers), waveguide Raman amplifiers and the application of FRAs in passive optical network et al. need further study.