Optimal Design of Flattened Gain Spectrum of Raman Amplifiers Based on Photonic Crystal Fibers

An efficient optimal algorithm, named SAA-PA in brief, based on the simulated annealing algorithm (SAA) and the Powell algorithm (PA) for the PCF-based Raman amplifiers (PCF-RA) design has been applied for the first time, in order to achieve the flat Raman gain characteristics. The pump spectra (wavelengths and powers) have been optimized to achieve low gain ripple (GR) over C+L band. The PCF considered to study the RA has the following parameters: Lambda = 4 mum, d/Lambda = 0.625, Sixty signals (1540-1600 nm) spaced at 1 nm are launched to the PCF of 10-km length with an input power of -10 dBm/ch. Various pump wavelengths and power distributions have been considered with the aim to reduce the gain ripple as much as possible. Simulation results show that the lowest GR of 0.72 dB with a peak gain of 8.03 dB in an 10 km ULL-PCF can be attained over C+L band. Also, it has been easily found that, when the gain and bandwidth are approximately the same, the lowest GR decreases with the addition of pump number, i.e., the GR are 1.27, 1.11, 0.85, and 0.72 dB for 3, 4, 5, and 6 backward pumps, respectively. All this shows that PCF is an appropriate candidate for high gain Raman amplifier.