La 2 O 3 ‐Al 2 O 3 ‐SiO 2 Glasses for High‐Power, Yb 3+ ‐Doped, 980‐nm Fiber Lasers

Single‐mode semiconductor pumps have failed to keep pace with the increasing power requirements of Er‐doped fiber amplifiers (EDFAs), so there is a need for high‐powered 980‐nm sources. Yb 3+ ‐doped tapered fiber lasers can provide high‐power output by conversion of a low‐brightness, high‐powered, 920‐nm, multimode broad stripe laser diode to a high‐brightness, 980‐nm, single‐mode output. The tapered fiber laser requires a fiber with high numerical aperture (NA) (>0.4), a rectangular core, and good Yb 3+ spectroscopy for efficient operation. CVD‐based fiber fabrication methods are incapable of delivering fibers with an NA > ∼0.3 or with good efficiency at 980 nm so a new method of high‐NA fiber fabrication was developed. The core glass composition is critical for maintaining a high‐NA fiber with good power extraction while avoiding phase separation, loss, and clustering. The SiO 2 level controlled the NA and interdiffusion between core and clad, while the Al 2 O 3 /La 2 O 3 ratio controlled phase separation. A La 2 O 3 ‐Al 2 O 3 ‐SiO 2 glass was developed that is compatible with a pure SiO 2 cladding glass and has a laser slope efficiency of 70% at 980 nm. The optimized fiber composition yielded 450 mW of 980‐nm power in a single‐mode fiber.