Coherent Large Mode Area (LMA) and Polarization Maintaining LMA (PLMA) Er/Yb co-doped fiber features a unique low NA (0.09) core design, achieved without sacrificing high pump conversion efficiency and developed for applications where robustly single-mode output beam quality is critical. The high NA (0.46) cladding waveguide (250 and 300µm) allows for efficient coupling of high pump powers, while the large core diameter (25 and 30µm) maintains a large-mode field diameter and short device length thereby minimizing deleterious nonlinear effects such as SBS and SRS. The design of these LMA products has been finely tuned to achieve ultra-high efficiencies while suppressing parasitic effects at 1µm, offering unmatched stability when operating at high powers. Utilizing the NuCOATFA coating technology, these LMA fibers provide excellent preservation of beam quality and extended operating life at the high power levels demanded by today’s industrial fiber laser applications.