To date, a single-mode large mode area (LMA) PCF with a core diameter up to 100 μm, a large pitch fiber (LPF), modified version of PCF, with core diameter of ~135 μm, and a LPF with 100 W output power and a short length of 40 cm were demonstrated. However, typical LMA-PCFs are made of silica, and the photonic crystal cladding (PCC) contains hollows that limit the further improvement of PCF performance. On one hand, although PCF with heavily doped core contributes to increase the pump absorption efficiency and thus shorten the fiber, the rare-earth-solubility (RES) in silica is at a low level. On the other hand, the hollows in the PCC in a traditional silica and phosphate PCF complicate the splice process of the PCF with the commercially pigtailed optical devices such as the fiber-coupled pump source, thereby weakening the application potential of this kind of fiber in industry.

Researchers at Shanghai Institute of Optics and Fines Mechanics (SIOM/China) reported single-mode ytterbium-doped phosphate all-solid photonic crystal fiber (AS-PCF) with 13.8 W output power and 32% slope efficiency.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, for the first time to knowledge, in a phosphate PCF.[Scientific Reports.5:8490(2015)]