Multiphoton absorption and cooperative upconversion excitation in Er3+-doped fibers.

Multiphoton and cooperative upconversion excitation of Er3+ ions has been studied in two Er3+/Ge-doped optical fibers with different Er3+ concentrations by using 120-fs pulses at 1.51 μm. In the case of high Er3+ concentration it is demonstrated that the excitation of the excited levels is dominated by cooperative upconversion, whereas multiphoton absorption is mainly responsible for the excitation of the excited levels in the case of low Er3+ concentration. Lifetimes of the excited levels are measured to lie between 1 and 10 μs.