Libin Fu

Progress in advanced specialty fibers is the foundation to further breakthroughs in fiber lasers. Recently, we have been working to advance several areas of developments in specialty fibers and would like to review these efforts here. The first topic is in the further development of all-glass large core leakage channel fibers (LCF) for robust and practical solutions for power scaling. The second area is the development of wide band air-core fibers with an innovative square lattice cladding and the demonstration of a factor of two improvements in bandgap over conventional hexagonal lattice. These air-core fibers are critical for fiber delivery solution of both CW and pulsed fiber lasers in the future. The last topic is a new development in design and simulation of SBS gains in optical fibers by incorporating leaky acoustic modes. These leaky acoustic modes have been mostly overlooked so far. It is essential that they are considered in SBS simulations in fibers, because they are normal solutions to the acoustic waveguide equations and have similar loss to guided acoustic modes where the acoustic mode loss is dominated by material loss. This leads to much improved resolution of SBS gain spectrum in fibers and to new design insights to the limit of SBS suppression based on anti-guide acoustic waveguide designs.

We report a strictly single-mode optical fiber with a record core diameter of 84µm and an effective mode area of ~3600µm² at 1µm. We also demonstrate fundamental mode operation in an optical fiber with a record core diameter of 252µm and a measured mode field diameter (MFD) of 149µm at 1.03µm, i.e. an effective mode area (A_eff) of ~17,400µm² at 1.03µm, an A_eff of 31,600µm² at 1.5µm. All these fibers have near parabolic index profiles with a peak refractive index difference ΔN≈~6×10^-5, i.e. a record low numerical aperture (NA) of ~0.013 in an optical fiber. This low refractive index difference was achieved by frozen-in thermal stress as a result of two different types of glass in the fibers. When the fundamental mode was excited in the 252µm core fiber using a 1.03µm ASE source, the output beam was measured to have M²_x=1.04 and M²_y=1.18.