True LMA fiber featuring a unique low NA (<0.1) high concentration Tm-doped core design. It is fully optimized for high slope efficiency (composition has demonstrated > 130% quantum efficiency) when pumped at 793 nm. This extraordinary efficiency is due to composition enabled cross relaxation of Thulium ions in the core. The high Tm concentration allows short device lengths and high pump conversion efficiency while the low NA (few moded) core design is ideal for applications where robust single-mode beam quality is critical. The high NA (0.46) 400 µm pump cladding wavelength allows for efficient coupling of high pump powers. The large core diameter (25 µm) maintains a large mode field diameter and short device length, thereby minimizing non-linear effects such as SBS and SRS.