Spectral control of optical gain in a rare earth-doped optical fiber using novel triple layered structures

Novel techniques to control the effective emission cross section of a rare earth-doped optical fiber with a functional cladding structure are reviewed for applications in active fiber devices. We discuss evanescent wave filtering (EWF) technique based on the overlap in the radiative transition cross sections between the emitting ion in the core and the absorbing ion in the inner cladding. Experimental applications of EWF concept are reviewed for a composite gain medium with the erbium ions in the core and samarium ions in the inner cladding for self-gain flattening in C-band. W-type three layered fiber structure is also analyzed in terms of modal guidance and location of the LP₀₁ mode cut-off. Its application in neodymium and thulium-doped silica fiber laser is discussed to facilitate the radiative transition near 940 and 1600 nm region, respectively. The design concepts, fabrication process, and device performances are discussed for each application.