Raman Fiber Amplifier Gain Study

Raman fiber amplifier(RFA)is a kind of optical amplifier that can amplify signal light directly through nonlinear interaction of high intensity pump and signal light in fiber. Whereas the spectrum of Raman gain coefficient of optical fiber is not flat, the gain of signal light with different wavelength induced by a single pump is rather different, when wavelength division multiplexing (WDM) signal light is amplified by a single pump, the gain spectrum of the WDM signal light is not flat. The flatness of the WDM signal light has important impact on the Optical signal-to-noise ratio (OSNR). On the other hand the crosstalk of the WDM signal at fan-out of demultiplexer will increase with the power difference of the each channel. Therefore, the gain of optical amplifier to WDM signal must have small ripple. One of the solutions is using two or more pumps with different wavelength and power according to the range of signal light to be amplified and Raman gain coefficient of the fiber. Moreover, the saturation characteristic of Raman fiber amplifier is also one of the important parameters and the total power of signal light input into the fiber is limited by it. In this paper, first, mechanism of Raman fiber amplifier is analyzed using classical electromagnetism. Secondly, basing on the theory of on-off gain of small signal ,the Raman gain coefficient for frequency shift between 0.5 and 20 THz of standard sigle mode optical fiber is measured by pump-probe method through a super luminescent diode (SLD)as a broadband small signal probe source. Thirdly, numerical simulation analysis of gain characteristic of Raman fiber amplifier for C band WDM signal light is made according to the Raman gain coefficient of the fiber measured before through target and four rank Runge-Kutta method. At the invariability of wavelength and maximum power of each of two pumps, schematic of powers of two pumps for best flatness on C band WDM optical gain was found out, at the same time, the factors of resulting in gain saturation is analyzed, too. Finally, the experimentations are performed to verify the conclusion drawn form the numerical simulation analysis, it find that the numerical simulation results agree basically with the experiment results. It indicates that the numerical simulations are correct. Both of the numeric simulations and the experimentations indicate that the C band Raman fiber amplifier can be satisfied with some gain flatness when numbers and wavelengths of pump are installed while powers of pump are arranged properly. The method of measuring fiber Raman gain coefficient is adopted to other typefiber as well as and the pump power collocated method is useful for the research of other band Raman fiber amplifier...