Rare-earth (RE) doped double-clad fibers have been investigated for many years; one of the primary objectives is to enlarge their core diameter and shorten the length, which is especially important in single-frequency fiber laser and pulsed laser. Many solutions have been suggested to break the limitation of the core diameter in a traditional silica step-index fiber, including chirally-coupled-core (CCC) fibers, leaky channel fibers, gain-guiding fibers, photonic bandgap fiber, multi-trench fiber, distributed modal filtering rod fiber, large pitch fiber, photonic crystal fibers (PCF).

By developing the PAC and the propagation loss of the fiber, researchers at Shanghai Institute of Optics and Fines Mechanics (SIOM/China) achieved a shortened fiber with higher slope efficiency and output power. By altering the diameter of the rods around the doped core and thus breaking the symmetry of the fiber, a polarization-maintaining AS-PCF with degree of polarization of >85% （with polarization extinction ratio of 11 dB）was also achieved, in a phosphate PCF.They demonstrated an Yb³⁺doped phosphate single mode all-solid photonic crystal fiber with output power of 13.8 W and 32% slope efficiency. 

This study was published on Scientific Reports on Feb.16, 2015.[ http://www.nature.com/srep/2015/150216/srep08490/full/srep08490.html?WT.ec_id=SREP-639-20150217]

This research was supported by the Chinese National Natural Science Foundation.