Large-mode-area Bragg fiber with microstructured core for suppression of high-order modes

Photonic bandgap Bragg fibers are promising for designing large-mode-area structures owing to their high bend immunity. However, at a large core diameter, filtering of high-order modes (mainly, the LP 11 mode) becomes difficult, because the propagation constant of such modes is close to that of the fundamental LP 01 mode. In this paper, we demonstrate the possibility to suppress high-order modes in Bragg fibers by introducing low-index inclusions into the Bragg fiber core. Numerical analysis shows that an appropriate choice of the position and types of such inclusions allows one to increase the LP 11 mode radiation loss without increasing the optical loss of the fundamental LP 01 mode. The Bragg fiber with two B-doped and two F-doped rods in the core was fabricated and studied. The fundamental LP01 mode at 1064 nm had a mode-field area of about 340μm 2 and an optical loss below 0.2 dB/m at a bending radius of 15 cm. The LP 11 mode was not observed in both bent and straight fibers at this wavelength. Only the LP 21 mode was detected in a straight fiber; however, it was completely suppressed after propagating a length of 60 cm in a fiber bent with a radius <50 cm.