Abstract

It has been reported previously that the co-operative upconversion process that occurs in high concentration erbium doped fibers result in gain degradation when these fibers are used as amplifiers, even for pump wavelength free from excited state absorption (ESA).^1,2 This paper reports the results of an experimental study of codirectionally and contradirectionally pumped high concentration erbium doped fiber amplifiers (EDFA's) and shows through numerical simulations of the same that upconversion losses also leads to differences between the gains under co and contradirectional pumping. The fiber used for the study had 1200 ppm-wt of erbium doped in Al/P silica matrix, a numerical aperture of 0.13 and core-cladding diameters of 6.8–80 μm. The pump wavelength used was the 514.5 nm line from an Argon-ion laser. Upconversion losses were taken into account in the numerical simulation using the model developed by Masuda et al.¹ According to Morkel et al.,³ for short fiber lengths, co and contradirectional gains are nearly equal when pump wavelengths at which ESA exists are used for pumping EDFA's. They have also reported that for fiber lengths longer than the optimum length, contradirectional gain is higher than codirectional gain. Our experimental studies however show a different trend. For the short fiber length of 1.5 m studied by us, contradirectional gain was found to be lower than codirectional gain. But the longer fiber length of 5 m studied by us gave higher gain for contradirectional pumping. Our numerical simulations show that for the 1.5 m EDF used, the co and contradirectional gain expected without taking upconversion losses into account is 15.35 dB, when pumped by 0.15 W at 514.5 nm. But for the same fiber length and same pump power, the numerical study showed that the codirectional gain was less by 2.62 dB compared to the gain obtained without taking upconversion into account. Similarly the contradirectional gain was less by 6.97 db when upconversion losses were included. On the other hand, for the longer fiber length of 5 m and for the same pump power of 0.15 W at 514.5 nm, our numerical study shows that contradirectional gain remains higher than codirectional gain even after including the effect of upconversion losses. Thus, for fiber lengths shorter than the optimum fiber length, we find both experimentally and theoretically, that contradirectional gain is lower than codirectional gain. In other words, for a fixed fiber length, if pump power exceeds the optimum pump power needed for that length, contradirectional gain tends to decrease more than the codirectional gain, due to upconversion losses. For such cases, numerical simulations show that the difference between the co and contradirectional gains increases and tends to a nearly constant value with further increase in pump power.

© 1995 IEEE