Concentration-dependent upconversion in Er/sup 3+/-doped fiber amplifiers: Experiments and modeling

Abstract
Eight different fibers were studied to examine upconversion as a function of erbium concentration below 1000 ppm by weight. These experiments and a computer model of two-particle upconversion made it possible to estimate the upconversion coefficient used in a rate-equation model. A strong correlation was found between amplification reduction and parasitic upconversion in fibers with both Ge- and Ge/Al/P-cores when the erbium-doping concentration is in the range 70-840 ppm by weight. When plotting maximum amplification versus the 980 luminescence on a logarithmic scale, the slope was approximately -2 irrespective of the host material. The validity of the assumed two-particle upconversion model up to 4 mW of pump power is shown.