High-Efficiency Erbium-Doped Fiber Amplifier using Mode Field Diameter Adjusting Technique,

For efficient operation of an Er-doped fiber amplifier EDFA at 1.55 um band it is essential to maximize the pump intensity in the Er-doped region because of the three level system. A high NA structure of EDF is effective to reduce the mode field diameter MFD of the pump and to achieve strong pump intensity 21, and has resulted in excellent gain performance 3,4. In practical use, however, such small MFD fibers have a connectionsplicing problem due to the mode field diameter mismatching. For example, approximately 1 dB splice loss is expected between an EDF with a 5 um MFD and a conventional dispersion shifted fiber DSF with a 8 um MFD. The smaller of EDF, the higher splice loss. The increase of splicing loss causes noise figure degradation in pre-amplifiers and decreasing of conversion efficiency in booster amplifiers. In this paper, we report the splicing loss less than 0.05 dB between an EDF with a small MFD and a conventional DSF. The MFDs have been adjusted by the thermal diffusion of core dopant after the splicing. We also describe the gain and noise characteristics of the EDFA with this adjusting technique.