Gamma and proton radiation effects in erbium-doped fiber amplifiers: Active and passive measurements

Commercially available Er-doped fibers were irradiated with 5.6- and 28-MeV protons and Co-60 gamma rays, up to levels of 50 krad. White-light transmission spectra under passive conditions (no pump or signal) were measured at several radiation levels for the six types of fibers that were tested. The spectra were used to evaluate the relative radiation sensitivity of the fibers and compare gamma versus proton-induced damage for two fiber types. The amount of radiation damage for the fibers was observed to scale inversely with the Ge concentration. Samples from three of the fiber types were configured as optical amplifiers using 980-nm and 1550-nm pump and input signals. In situ measurements of the gain, noise figure, and amplified spontaneous emission (ASE) were made as a function of pump power at several levels of radiation. A computer code, based on a conventional Er-doped fiber amplifier (EDFA) model, was written to simulate performance, using input data provided by the fiber vendor and anchored to measurements made prior to radiation. A comparison between the simulations and experimental data shows that, in certain fibers where the damage is significant, the radiation-induced loss determined from amplifier measurements can be substantially less than that determined from passive transmission spectra.