REDUCTION OF AMPLIFIED SPONTANEOUS EMISSION FROM A TRANSMITTED SOLITON SIGNAL USING A NONLINEAR AMPLIFYING LOOP MIRROR AND A NONLINEAR-OPTICAL LOOP MIRROR

The amplified spontaneous emission (ASE) from concatenated erbium-doped fiber amplifiers (EDFA's) in a 10 Gb/s 500 km soliton transmission system has been successfully reduced through the use of a nonlinear amplifying-loop mirror (NALM). By using the NALM the ratio of the signal peak to ASE was improved from 17.4:1 to 24.9:1. A similar result was obtained for a nonlinear optical-loop mirror (NOLM) with a 6:4 directional coupler. It is noted that pulse transmission through the NALM or the NOLM gives rise to frequency chirp, which means that these fiber devices are not appropriate for use at the midpoint of a long-distance soliton transmission. The chirp characteristics caused by the NALM were also described and the optimum condition for nonlinear switching was obtained.