Depressed-clad large mode area amplifier fiber with selective doping yielding near diffraction-limited beam quality

Large mode area (LMA) optical fibers are finding widespread use nowadays in high power fiber lasers and amplifiers. The lower numerical apertures allow for larger core diameters and therefore reduced intensity of guided lightwaves whilst preserving the near single-mode guidance. As the core diameter is made larger though, conventional LMA fibers support a growing number of modes and beam propagation factor - M² - gets worse unless provision is made to avoid the latter (even if bending-induced losses as a result of coiling the fiber to a prescribed diameter is assumed). Diverse strategies have been reported in the literature in the last decade or so to address the aforementioned issue. Selective doping [1-2] and multi-layer claddings with depressed index inner layer [3] are two such schemes. The former favors the amplification of the fundamental mode through confinement of rare-earth dopants to the central portion of the core whereas the latter results in increased differential bending losses as a result of the lower effective numerical aperture seen by higher-order modes (HOMs). Both of these methods are shown herein to be quite effective at suppressing HOMs for fibers with large core diameters when implemented all together.