Optimization on bandwidth in the third-order pumped distributed fiber Raman amplifier

The effect of higher-order amplified spontaneous scattering in Raman amplification is investigated. With the effect, a structure of thirdorder distributed fiber Raman amplifier (DFRA) is introduced and its bandwidth is studied and optimized. A method of multiorder pumps with multiwavelengths is adopted, which is different from that of multiwavelength pumps for the conventional DFRA. Simulation results indicate that pumps in different orders have different functions: the first-order pump can improve the gain flatness and extend the bandwidth, the secondorder pump can increase the gain value, whereas the third-order pump can provide powers for pumps in other orders and improve the noise figure (NF) together with them; a flat spectrum with the bandwidth of 80 nm, gain of 24 dB, ripple of less than 1 dB, and a low NF over a span of 120 km is achieved. All these results are beneficial to design a broadband higherorder Raman pumping DFRA in future experimental work. A modified genetic algorithm is also proposed in optimizing the bandwidth.