Broadband and gain-flattened optical fiber amplifiers

Broadband and gain-flattened erbium-doped fiber amplifiers are being widely developed to meet the requirements posed by wavelength-division-multiplexed transmission systems/networks pursuing the ultimate capacity. One approach is the improvement of erbium-doped fiber itself for broader gain bandwidth or better gain flatness. In addition to the well-known aluminum-codoped silica fibers and the fluoride glass fibers, rather new materials, such as tellurite glass, or hybrid configurations which utilize two or more erbium- doped fibers with different glass compositions are being examined. Another approach is the use of gain-equalizing optical filters. One of the most promising gain equalizing techniques is the long-period fiber grating. In order to maintain the gain flatness even under variable operational conditions, control schemes of the optical fiber amplifiers are also a critical issue.