Limin Xiao

High finesse microfiber knot resonators made from double-ended tapered fibers

We fabricated optical microfiber knot resonators from thin tapered fibers (diameter down to 1 μm) linked to untapered fiber at both ends. We demonstrated a finesse of about 100, over twice as high as previously reported for microfiber resonators. Low-loss encapsulation of microfiber knot resonators in hydrophobic silica aerogel was also investigated.

Tapered fibers embedded in silica aerogel

We embedded thin (down to 2 µm diameter) tapered fibres in silica aerogel with low loss. The aerogel is rigid but behaves refractively like air, protecting the tapered fibre without disturbing light propagation along it.

Stable low-loss optical nanofibres embedded in hydrophobic aerogel

Nanofibres, optical fibres narrower than the wavelength of light, degrade in hours on exposure to air. We show that encapsulation in hydrophobic silica aerogel (refractive index 1.05) provides protection and stability (over 2 months) without sacrificing low attenuation, strong confinement and accessible evanescent field. The measured attenuation was <0.03 dB/mm, over 10 × lower than reported with other encapsulants. This enables many nanofibre applications based on their extreme small size and strong external evanescent field, such as optical sensors, nonlinear optics, nanofibre circuits and high-Q resonators. The aerogel is more than a waterproof box, it is a completely-compatible gas-permeable material in intimate contact with the nanofibre and hydrophobic on both the macroscopic and molecular scales. Its benefits are illustrated by experiments on gas sensing (exploiting the aerogels porosity) and supercontinuum generation (exploiting its ultra-low index).

More than threefold expansion of highly nonlinear photonic crystal fiber cores for low-loss fusion splicing

We have formed low-loss fusion splices from highly nonlinear (HNL) photonic crystal fibers (PCFs) with small cores and high air-filling fractions to fibers with much larger mode field diameters (MFDs). The PCF core was locally enlarged by the controlled collapse of holes around the core while keeping other holes open. The fiber was then cleaved at the enlarged core and spliced to the large MFD fiber with a conventional electric arc fusion splicer. Splice losses as low as 0.36 dB were achieved between a PCF and a standard single-mode fiber (SMF) with MFDs of 1.8 microm and 5.9 microm, respectively.

Optofluidic microchannels in aerogel

We report optofluidic waveguides made by filling microchannels in aerogel with water. The aerogel cladding is a nanoporous material with an extremely low refractive index of ~1.05, giving a large index step from the water core. Channels were formed by removing embedded optical fibers, which could be nonuniform or multiple. The porosity of the aerogel allowed air to be displaced from the channel, preventing the trapping of bubbles. The attenuation of red light in the highly multimode water core waveguide was no greater than 1.5 dB/cm.

Silica aerogel in optical fibre devices

Silica aerogel is a very light and highly porous form of silica glass, with densities and refractive indices much lower than those of ordinary solids. We describe optical interactions with aerogel surrounding tapered fibres or inside the holes of photonic crystal fibres. This enables a new class of fibre devices exploiting the properties of the aerogel as a rigid yet porous replacement for air, as a nonlinear medium and as a host for dopants. Examples include a stable package for fused couplers, a gas sensor and a nonlinear light source.

Hydrophobic photonic crystal fibers

We propose and demonstrate hydrophobic photonic crystal fibers (PCFs). A chemical surface treatment for making PCFs hydrophobic is introduced. This repels water from the holes of PCFs, so that their optical properties remain unchanged even when they are immersed in water. The combination of a hollow core and a water-repellent inner surface of the hydrophobic PCF provides an ultracompact dissolved-gas sensor element, which is demonstrated for the sensing of dissolved ammonia gas.

Aerogel-enhanced tapers and fibres

We replace the air around tapered silica fibres or within hollow-core photonic crystal fibres with silica aerogel. This retains the important device characteristics, including gas permeability, but adds protection and enables new devices.

Tapered fibres embedded in silica aerogel

We embedded thin (down to 2 µm diameter) tapered fibres in silica aerogel with low loss. The aerogel is rigid but behaves refractively like air, protecting the tapered fibre without disturbing light propagation along it.

Modifying photonic crystal fibres

We use controlled hole collapse in photonic crystal fibres to reduce the splice loss between dissimilar fibres. We also describe a hollow-core PCF as a support for a waveguide core made from silica aerogel.