Improvement of 1.57 ∼ 1.61 μ band amplification efficiency by recycling wasted backward ASE through the unpumped EDF section

Broad and flat gain bandwidth of erbium doped fiber amplifiers is one of essential elements to wavelength division multiplexed (WDM) transmission systems and their related optical networks. The requirement of wide bandwidth amplifiers ensuing transmission capacity increase, has evoked the latest interest in new erbium gain band from 1570nm to 1610nm, which is the second gain window of erbium ion. As a direct solution to meet those requirements, a new material, such as tellurite based fiber has attracted much attention [1]. However, the relatively non-uniform gain spectrum and immaturity of the related researches still make immediate, practical applications of this technology somewhat difficult. Apart from solutions involving new material composition, silica based erbium-doped amplifiers with various structures have been used to obtain wide gain bandwidth beyond a conventional band (called C-band, 1530 nm ∼ 1560nm) [2]. For these approaches, relatively longer lengths of erbium doped fibers are commonly required with high pump power sources, to adjust the population inversion of the EDF at an approximately 30 ∼ 40% level, and correspondingly to get the amplification in a long wavelength range (1570nm ∼ 1610nm, called L-band). By combining this L-band amplifier with a conventional EDFA in parallel configuration, it was demonstrated that a gain bandwidth of more than 80nm can be achieved for a silica based EDFA as well [3]. However, the relatively short history of the development of the L-band amplifier still retains several issues for further optimization, including long EDF length and high pump power from low conversion efficiency [4].