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.
Traditional CARS microscopy using picosecond (ps) lasers has been applied to a wide variety of applications;
however, the lasers required are expensive and require an environmentally stable lab. In our work, we demonstrate
CARS microscopy using a single femtosecond (fs) laser combined with a photonic crystal fiber (PCF) and optimal
chirping to achieve similar performance to the ps case with important added advantages: fs-CARS utilizes
versatile source that allows CARS to be combined with other multiphoton techniques (e.g. SHG, TPF) for
multimodal imaging without changing laser sources. This provides an attractive entry point for many researchers
to the field. Furthermore, optimal chirping in fs-CARS also opens the door to the combination and extension
of other techniques used in ps CARS microscopy such as multiplex and FM imaging. The key advantage with
chirped fs pulses is that time delay corresponds to spectral scanning and allows for rapid modulation of the
resonant CARS signal. The combination of a fs oscillator with a PCF leads to a natural extension of the
technology towards an all-fiber source for multimodal multiphoton microscopy. An all-fiber system should be
more robust against environmental fluctuations while being more compact than free-space systems. We have
constructed and demonstrated a proof of concept all-fiber based source that can be used for simultaneous CARS,
TPF and SHG imaging. This system is capable of imaging tissue samples and live cell cultures with 4 μs/pixel
dwell time at low average powers.
Leakage channel fibers have demonstrated their ability to significantly extend the effective mode area of a fundamental
mode while maintaining robust single mode operation. These fibers are designed to have strong built-in mode filtering
which effectively suppresses the propagation of all higher order modes while keeping fundamental mode loss to a
minimum, and, therefore, effectively extending the regime of single mode operation. Recently all-glass leakage channel
fibers have been demonstrated as a significant improvement over designs with air holes. These all glass leakage channel
fibers not only can be manufactured with much improved consistence and uniformity. They can also be handled and used
as conventional fibers. More importantly, mode distortions from collapse of air holes in photonic crystal fibers during
splicing and other end face treatments are largely eliminated. We will review some of the recent progress in this area.
The nonlinearity in optical fibres can be enhanced significantly by reducing the effective mode area or by using materials
with higher nonlinear-index coefficient (n2). In this paper we combine these two concepts and experimentally
demonstrate enhanced Kerr nonlinear effects in tapered highly nonlinear As2Se3 chalcogenide fibre. We taper the fibre to sub-wavelength waist diameter of 1.2 μm and observe enhanced nonlinearity of 63,600 W-1km-1. This is 40,000 times larger than in silica single-mode fibre, owing to the 400 times larger n2 and almost 100 times smaller effective mode area. We also discuss the role of group velocity dispersion in these highly nonlinear fibre tapers.
Chalcogenide glass based optical waveguides offer many attractive properties in all-optical signal processing because of the large Kerr nonlinearity (up to 420 × silica glass), the associated intrinsic response time of less than 100 fs and low two-photon absorption. These properties together with the convenience of a fiber format allow us to achieve all-optical signal processing at low peak power and in a very compact form. In this talk, a number of non-linear processing tasks will be demonstrated including all-optical regeneration, wavelength conversion and femtosecond pedestal-free pulse compression. In all-optical regeneration, we generate a near step-like power transfer function using only 2.8 m of fiber. Wavelength conversion is demonstrated over a range of 10 nm using 1 m of fiber with 7 ps pulses, peak power of 2.1 W, and 1.4 dB additional penalty. Finally, we will show efficient compression of low-power 6 ps pulses to 420 fs around 1550 nm in a compact all-fiber scheme.
These applications show chalcogenide glass fibers are very promising candidate materials for nonlinear all-optic signal processing.
We demonstrate a high power erbium-ytterbium co-doped large-core fiber laser with narrow linewidth, an M2 value of 1.7 and a broad tuning range. The fiber was cladding-pumped by a diode stack emitting at 975 nm. The laser had a linewidth around 0.16 nm and was tuned from 1533 nm to 1566 nm by compression-tuning a fiber Bragg grating. Output powers in excess of 30 W were obtained over the entire laser tuning range which was limited by the low gain at wavelengths shorter than 1533 nm and by the grating fabrication wavelength at 1566 nm. The laser slope efficiency was ~30% and the threshold ~3.3 W. Our results underline the capability for efficient, broad-band, high-power operation of large-core Er-Yb doped fibers and demonstrate compatibility with telecom components like standard single-mode fibers and fiber Bragg gratings.