Circular gradient-diameter photonic crystal fiber with large mode area and low bending loss.

In this paper, a silica-based sixfold circular gradient-diameter photonic crystal fiber (CGPCF) with five rings is proposed, and the matching conditions of air hole lattices are established considering the gradient and diameter of the air holes. This CGPCF supports endlessly single-mode propagation due to the existence of small air holes near to the fiber core based on the matching rule. Simultaneously, it exhibits an ultralarge effective mode area of 3110  μm in the straight case and 1105  μm in the bending case with a bending radius of 15 cm at the wavelength of 1550 nm. In addition, within an ultrawide communication wavelength range from 1000 to 2000 nm (from 1350 nm to 2000 nm), the effective mode area in the straight (bending) case still maintains more than 3043  μm (981  μm), and the bending loss maintains an ultralow order of magnitude of 10  dB/m at a great degree of bending radius of 15 cm. Moreover, the rotation-symmetric structure of CGPCF can reduce the impact of bending orientation on the optical properties. The proposed CGPCF is highly meaningful for optical fiber communication, high-power laser systems, and optical amplifiers.