Study On Group-Velocity Dispersion In Raman Fiber Amplifier

Fiber Raman amplifier (FRA) is full communication windows amplifier based on the fiber, and it has excellent features of high gain, high output power, quick response speed, small non-linear influence and high signal noise ratio. Now it is more and more important with the widely used of DWDM technology as an online amplifier. However, with the development of high-speed communication systems, dispersion in FRA becomes an important factor limiting system performance, so effect of dispersion to performance of FRA and dispersion compensation has gradually become research hot.The paper researches group-velocity dispersion effect in FRA and its dispersion compensation utilizing theory analysis and numerical simulation. First, transmission of signal in FRA pumped by continuous wave (CW) and pulse with limited width is simulated based on the numerically solution of Nonlinear Schrodinger Equations by split-step Fourier method, and effect of dispersion-induced walkoff effects and the broadening of the pulse to maximal action length of FRA and the signal power is also analyzed, furthermore, effect of pulse width of pump to FRA is discussed. Results show that, for the FRA pumped by CW, the maximal action length and the signal power of FRA are decided mainly by the pulse broadening level, which is in turn determined by the dispersion parameter of the signal pulse, but for the case of limited width pulse, although they are affected greatly both by the pulse walkoff and the pulse broadening, walkoff has larger influence than pulse broadening when walkoff parameter is quite large, furthermore, the effect of walkoff is more important when pulse width is smaller.Chirped fiber grating is used for the purpose of compensating the dispersion in FRA. First, reflective spectrum, time delay curve and dispersion curve of chirped fiber gratings are simulated based on the numerically solution of coupled-mode equations, and influence of main parameters such as grating length, chirp coefficient and coupling coefficient to them is analyzed. Study shows that shape of reflective spectrum, time delay curve and dispersion curve of chirped fiber gratings are decided by grating length and chirp coefficient, and has little relationship with coupling coefficient, but the peak of reflectivity is related to the three. Secondly, time delay curve with better linear identity and reflective spectrum with flatter top are got by using Gauss function. Finally, use the linearly chirped fiber grating with the Gauss function to compensate thedispersion in FRA. Although the top of compensated pulse is got a little narrow, the broaden pulse can restored 90% to the original.