Neil P. Sessions

Optical fiber nanowires and microwires: fabrication and applications

Microwires and nanowires have been manufactured by using a wide range of bottom-up techniques such as chemical or physical vapor deposition and top-down processes such as fiber drawing. Among these techniques, the manufacture of wires from optical fibers provides the longest, most uniform and robust nanowires. Critically, the small surface roughness and the high-homogeneity associated with optical fiber nanowires (OFNs) provide low optical loss and allow the use of nanowires for a wide range of new applications for communications, sensing, lasers, biology, and chemistry. OFNs offer a number of outstanding optical and mechanical properties, including (1) large evanescent fields, (2) high-nonlinearity, (3) strong confinement, and (4) low-loss interconnection to other optical fibers and fiberized components. OFNs are fabricated by adiabatically stretching optical fibers and thus preserve the original optical fiber dimensions at their input and output, allowing ready splicing to standard fibers. A review of the manufacture of OFNs is presented, with a particular emphasis on their applications. Three different groups of applications have been envisaged: (1) devices based on the strong confinement or nonlinearity, (2) applications exploiting the large evanescent field, and (3) devices involving the taper transition regions. The first group includes supercontinuum generators, a range of nonlinear optical devices, and optical trapping. The second group comprises knot, loop, and coil resonators and their applications, sensing and particle propulsion by optical pressure. Finally, mode filtering and mode conversion represent applications based on the taper transition regions. Among these groups of applications, devices exploiting the OFN-based resonators are possibly the most interesting; because of the large evanescent field, when OFNs are coiled onto themselves the mode propagating in the wire interferes with itself to give a resonator. In contrast with the majority of high-Q resonators manufactured by other means, the OFN microresonator does not have major issues with input-output coupling and presents a completely integrated fiberized solution. OFNs can be used to manufacture loop and coil resonators with Q factors that, although still far from the predicted value of 10. The input-output pigtails play a major role in shaping the resonator response and can be used to maximize the Q factor over a wide range of coupling parameters. Finally, temporal stability and robustness issues are discussed, and a solution to optical degradation issues is presented.

Spectroscopy of Tm3+-doped tellurite glasses for 1470 nm fiber amplifier

Three thulium doped tellurite glass compositions have been investigated. The 1470 nm transition is radiative in these tellurite glasses and the radiative lifetimes are in the range of 350 to 470 µs. The 1470 nm fluorescence is broad with a full width at half maximum of 105 nm. Fibers have been drawn from these glasses with a loss of 0.7 dB/m at 1300 nm. A fiber with an OH fundamental absorption of 200 dB/m at 2.99 µm has an OH first overtone absorption of 0.3 dB/m at 1480 nm. The overlap between the thulium ion 1470 nm emission and the hydroxyl absorption depends on glass composition. Tellurite glasses can accept large concentrations of Tm3+ ions and, as long as the hydroxyl level can be kept low, the effect of concentration quenching can be minimized. Tm3+-doped tellurite glasses represent a viable alternative for the next generation of active components for S-band optical amplifiers. It can be pumped at 795 nm with an absorption of ~38 dB/km/ppm and codoped with Ho3+ to avoid self-termination of the 1470 nm transition. It can also be pumped at 1212 nm as efficiently as at 795 nm, but diodes are not yet available at this wavelength. Using available pump wavelengths of 1064 nm and 1047 nm will require fiber lengths 15 times longer than pumping at 1212 nm.

Fabrication of Submicrometer High Refractive Index Tantalum Pentoxide Waveguides for Optical Propulsion of Microparticles

Design, fabrication, and optimization of tantalum pentoxide (Ta2O5 ) waveguides to obtain low-loss guidance at a wavelength of 1070 nm are reported. The high-refractive index contrast (Deltan ~ 0.65, compared to silicon oxide) of Ta2O5 allows strong confinement of light in waveguides of submicrometer thickness (200 nm), with enhanced intensity in the evanescent field. We have employed the strong evanescent field from the waveguide to propel micro-particles with higher velocity than previously reported. An optical propelling velocity of 50 mum/s was obtained for 8-mum polystyrene particles with guided power of only 20 mW.

Optical quality ZnSe films and low loss waveguides on Si substrates for mid-infrared applications

Zinc selenide (ZnSe) is a promising mid-infrared waveguide material with a high refractive index and wide transparency. Optical quality ZnSe thin films were deposited on silicon substrates by RF sputtering and thermal evaporation, and characterized and compared for material and optical properties. Evaporated films were found to be denser and smoother than sputtered films. Rib waveguides were fabricated from these films and evaporated films exhibited losses as low as 0.6 dB/cm at wavelengths between 2.5 µm and 3.7 µm. The films were also used as isolation/lower cladding layers on Si with GeTe4 as the waveguide core and propagation losses were determined in this wavelength range.

Spectroscopy of ytterbium-doped tantalum pentoxide rib waveguides on silicon

The design, fabrication and spectroscopic characterization of ytterbium-doped Ta2O5 rib waveguide are described. The waveguides are fabricated on silicon substrates and operate in a single mode at wavelengths above 970 nm. The peak absorption cross-section was measured to be 2.75 × 10−20 cm2 at 975 nm. The emission spectrum was found to have a broad fluorescence spanning from 990 nm to 1090 nm with the fluorescence emission peak occurring at a wavelength of 976 nm. The excited-state life time was measured to be approximately 260 µs.

Integrated optical waveguides and inertial focussing microfluidics in silica for microflow cytometry applications

A key challenge in the development of a microflow cytometry platform is the integration of the optical components with the fluidics as this requires compatible micro-optical and microfluidic technologies. In this work a microflow cytometry platform is presented comprising monolithically integrated waveguides and deep microfluidics in a rugged silica chip. Integrated waveguides are used to deliver excitation light to an etched microfluidic channel and also collect transmitted light. The fluidics are designed to employ inertial focussing, a particle positioning technique, to reduce signal variation by bringing the flowing particles onto the same plane as the excitation light beam. A fabrication process is described which exploits microelectronics mass production techniques including: sputtering, ICP etching and PECVD. Example devices were fabricated and the effectiveness of inertial focussing of 5.6 µm fluorescent beads was studied showing lateral and vertical confinement of flowing beads within the microfluidic channel. The fluorescence signals from flowing calibration beads were quantified demonstrating a CV of 26%. Finally the potential of this type of device for measuring the variation in optical transmission from input to output waveguide as beads flowed through the beam was evaluated.

Mid-infrared GeTe4 waveguides on silicon with a ZnSe isolation layer

GeTe4 waveguides were designed and fabricated on silicon substrates with a ZnSe isolation layer. GeTe4 has a refractive index of 3.25 at a wavelength of 9 μm and a lower refractive index isolation layer is needed to realise waveguides on silicon. Numerical modelling was carried out to calculate the thickness of the isolation layer (ZnSe, refractive index ~2.4) required to achieve low loss waveguides. For a loss between 0.1 and 1.0 dB/cm it was found that a ~ 4 μm thick ZnSe film is required at a wavelength of 9 μm. ZnSe thin films were deposited on silicon, GeTe4 waveguides were fabricated by lift-off technique and were characterised for mid-infrared waveguiding.

Spectroscopy of high index contrast Yb:Ta2O5 waveguides for lasing applications

Ytterbium-doped waveguides are required for compact integrated lasers and Yb-doped Ta2O5 is a promising candidate material. The design, fabrication and spectroscopic characterisation of Yb:Ta2O5 rib waveguides are described. The peak absorption cross-section was measured to be 2.75 x 10-20 cm2 at 975 nm. The emission spectrum was found to have a fluorescence emission peak at a wavelength of 976 nm with a peak cross-section of 2.9 x 10-20 cm2 and a second broad fluorescence band spanning from 990 nm to 1090 nm. The excited-state life time was measured to be 260 µs.

Ytterbium-Doped Tantalum Pentoxide Waveguides: Spectroscopy for Compact Waveguide Lasers

Ytterbium-doped materials are common gain media in high-performance laser systems. In this work, the first spectroscopic investigation of ytterbium-doped tantalum pentoxide (Yb:Ta2O5) for compact waveguide laser applications is presented.

Sub-wavelength focusing of high intensities in microfibre tips

Sub-wavelength efficient intensity confinement has been demonstrated in nanostructured optical microfibre tips. Focus Ion Beam (FIB) milling was used to nanostructure gold-coated optical microfibre tips and form apertures at the apex. Simulations were carried out to optimize the device design. Enhanced transmission efficiency (higher than 10-2) was achieved in spot sizes of ~λ /10. Nanostructured microfibre tips have the potential for a number of applications including optical recording, photolithography and scanning near-field optical microscopy (SNOM).