Design, characterization and performance evaluation of few-mode EDFA system with propagation up to six modes

摘要

Modal gain equalized system for six mode-erbium doped fiber amplifier (6M-EDFA) is designed and analyzed. For the ease in calculation an appropriate pump combination of linearly polarized LP_(01), LP_(31) and LP_(21) modes required for reduction in differential mode gain (DMG_λ) is mathematically obtained from extended Giles model. An optimized centre depressed erbium ion profile is also obtained separately. The optimum simplified unidirectional pump combination at 980 nm is then used to excite centre depressed erbium ion profile to achieve considerable reduction in DMG_λ. The mathematical analysis is further corroborated with simulation results. The 6M-EDFA system designed provided a minimum DMG-λ of 4.41 dB in six mode group and DMG-λ of 1.37 dB in five mode group. At smaller values of pump power, a mean gain of about 24.364 dB is achieved at 1550 nm and at lower input signal power of -7 dBm per mode. Gain spectrum is observed to be completely flat over C band for all six modes. Further investigation on evolution of amplified spontaneous emission (ASE) is reported for 6M-EDFA for the first time with new performance metric namely ASE ratio to indicate the deviation of noise between six modes. The ASE ratio of about 2.387 is obtained indicating that the designed system is reasonably immune to noise effects. The influence of non ideal high concentration effects is also investigated on the designed system. The DMG_λ values are observed to monotonically increase with length and concentration and decrease with wavelength and input signal powers.