Soliton transmission in cascaded distributed erbium-doped fiber amplifiers

Soliton is considered to be the carrier for high bit–rate and long distance fiber communication system. Because of fiber loss, soliton is amplified periodically to maintain its power. Recently, lumped erbium-doped fiber amplifier (LEDFA) is successfully demonstrated to amplify the soliton.1 However, soliton experiences large power perturbation with LEDFA and it is stable only when the soliton period is much larger than the pump period.2 Thus the soliton pulsewidth can not be too short for long distance transmission. To reduce power perturbation, distributed erbium-doped fiber amplifier (DEDFA) is suggested.3 We numerically study soliton propagating in the cascaded DEDFAs of different doping densities. The results show that, for the soliton period less than the pump period, the soliton pulseshape can maintain well with the cascaded DEDFAs of proper doping density, but optical bandpass filter between DEDFAs is required to reduce neighboring interaction and maintain soliton separation. For example, with lpsec/km/nm dispersion, 30km pump period and 650GHz bandwidth filter, the pulseshape and separation of a 5psec soliton pair (9.9km soliton period) with 10 pulsewidth separation can maintain well over 10Mm. In this case, the maximum power variation is 3.2%; while, with LEDFA, it is 84%. Furthermore, the required pump power is greatly reduced to compare with the Raman amplifiers.