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.
Air-core photonic bandgap fibers offer many unique properties and are critical to many emerging applications. A notable
property is the high nonlinear threshold which is the key for applications at high peak powers. The strong interaction of
light and air is also essential for a number of emerging applications, especially those based on nonlinear interactions and
spectroscopy. For many of those applications, much wider transmission bandwidths are desired to accommodate a wider
tuning range or a large number of optical wavelengths involved. All demonstrated air-core photonic bandgap fibers so far
have a cladding of hexagonal lattice. The densely packed geometry of hexagonal stacking does not allow large nodes in
the cladding, which are essential for a further increase of photonic bandgaps. On the other hand, a photonic cladding with
a square lattice can potentially provide much larger nodes and consequently wider bandgap. In this work, the potentials
of much wider bandgap with square lattice cladding are theoretically studied.