High output power erbium-doped fiber grating ring laser

positive at M Of 15%. For M above 15%, the M' is directly proportional to M. To explain the above observations, it is necessary to consider an enhanced absorption, in which the amount of absorption increases as a result of the increase in the number of incident photons. When photons with 788 nm are irradiated onto a system that has energy-level structures of ErZ+, electrons in the ground level 41,512 are excited to the 41,,, level. The excited electrons relax to the metastable level 41,3/2 and are excited to the ZH,, ,2 level as a resuh o l the upconversion p~ocess .~ Most of the electrons at the 'HI , i z level relaxnonradiatively to the 'S3, , level and then to the 4H,5,2 and 41,,,2 levels. The transition from the 4S,/2 to the level may lead to stimulated emission in addition to spontaneous emission. The enhanced absorption can be intensified if this stimulated emission process occurs because the light generated by stimulated emission will induce the transition of electrons from the " S 3 / 2 to the 41,3,, level, thereby contributing to the absorption of incident photons.