Photosensitive cladding enables efficient single-frequency fiber laser without hydrogenation

Photosensitive cladding enables efficient single-frequency fiber laser without hydrogenation

Optoelectronics Research Centre (Southampton, England) researchers have demonstrated efficient short-cavity distributed-Bragg-reflector (DBR) and distributed-feedback (DFB) single-frequency fiber lasers based on a photosensitive-cladding erbium/ytterbium-codoped fiber they developed. Boron and germanium doping of the cladding renders it highly photosensitive such that strong gratings (>99%) can be written into the fiber with an ultraviolet laser without hydrogenation of the fiber. When pumped at 980 nm, the DBR device emitted at 1535 nm with a threshold of 4 mW and slope efficiency of about 25% with respect to launched power; it operated with single polarization u¥to 8 mW. The DFB laser--based on a single 5-cm-long grating written in the fiber and centered at 1550.2 nm--exhibited a higher threshold at 14 mW with similar efficiency; single polarization was observed u¥to an output of 6 mW.

The researchers say that separating the photosensitive region of the fiber from the doped core avoids perturbing the composition for efficient energy transfer between Er and Yb ions. The overla¥between the guided optical field and the rare-earth dopants is also not affected. These factors ensure maximum pump-laser absorption and gain per unit length of fiber. The efficiency of both laser types is limited primarily by that of the fiber, and improvements are expected from optimization of the Er/Yb-doped fiber core. Such devices could be a competing technology for semiconductor DFB lasers used in telecommunications.