Gilberto Brambilla

Ultra-low-loss optical fiber nanotapers

Optical fiber tapers with a waist size larger than 1microm are commonplace in telecommunications and sensor applications. However the fabrication of low-loss optical fiber tapers with subwavelength diameters was previously thought to be impractical due to difficulties associated with control of the surface roughness and diameter uniformity. In this paper we show that very-long ultra-low-loss tapers can in fact be produced using a conventional fiber taper rig incorporating a simple burner configuration. For single-mode operation, the optical losses we achieve at 1.55microm are one order of magnitude lower than losses previously reported in the literature for tapers of a similar size. SEM images confirm excellent taper uniformity. We believe that these low-loss structures should pave the way to a whole range of fiber nanodevices.

Optical fibre nanowires and microwires: a review

Optical fibre nanowires and microwires offer a variety of enabling properties, including large evanescent fields, flexibility, configurability, high confinement, robustness and compactness. These distinctive features have been exploited in a wealth of applications ranging from telecommunication devices to sensors, from optical manipulation to high Q resonators. In this paper I will review the fundamentals and applications of nanowires and microwires manufactured from optical fibres.

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.

Optical microfiber coil resonator refractometric sensor.

We present a novel refractometric sensor based on a coated all-coupling optical-fiber-nanowire microcoil resonator which is robust, compact, and comprises an intrinsic fluidic channel. We calculate the device sensitivity and find its dependence on the nanowire diameter and coating thickness. A sensitivity as high as 700 nm/RIU and a refractive index resolution as low as 10(-10) are predicted.

Mid-IR Supercontinuum Generation From Nonsilica Microstructured Optical Fibers

In this paper, the properties of nonsilica glasses and the related technology for microstructured fiber fabrication are reviewed. Numerical simulation results are shown using the properties of nonsilica microstructured fibers for mid-infrared (mid-IR) supercontinuum generation when seeding with near-IR, 200-fs pump pulses. In particular, bismuth glass small-core fibers that have two zero-dispersion wavelengths (ZDWs) are investigated, and efficient mid-IR generation is enabled by phase-matching of a 2.0-mum seed across the upper ZDW into the 3-4.5 mum wavelength range. Fiber lengths considered were 40 mm. Simulation results for a range of nonsilica large-mode fibers are also shown for comparison.

High-sensitivity, evanescent field refractometric sensor based on a tapered, multimode fiber interference

We propose and experimentally demonstrate an enhanced evanescent field fiber refractometer based on a tapered multimode fiber sandwiched between two single-mode fibers. Experiments show that this fiber sensor offers ultrahigh sensitivity [better than 1900 nm/RIU at a refractive index (RI) of 1.44] for RI measurements within the range of 1.33-1.44, in agreement with the theoretical predictions. This is the highest value reported to date (to our knowledge) in the literature.

Embedding optical microfiber coil resonators in Teflon

A method to manufacture optical microfiber coil resonators embedded in Teflon has been demonstrated for what is the first time to the best of our knowledge. The resonator was obtained by wrapping a microfiber on a low refractive index rod and coating it with a Teflon resin. The coating process is investigated and discussed. Resonances in excess of 9 dB, a free spectral range of approximately 0.8 nm, and Q factors greater than 6000 have been observed in the embedded resonator. The device is compact, robust, and portable.

An embedded optical nanowire loop resonator refractometric sensor

A novel refractometric sensor based on an embedded optical nanowire loop resonator is presented. The device sensitivity has been studied in two typical configurations and its dependence on the nanowire diameter and coating thickness determined.

Optical manipulation of microspheres along a subwavelength optical wire

The propulsion of 3 microm polystyrene spheres along a subwavelength optical wire is demonstrated. Velocities in the range of 7-15 microm/s are observed. Simulations are carried out to evaluate the evanescent field at the waveguide-water suspension interface.

The Ultimate Strength of Glass Silica Nanowires

In the past decade nanowires have attracted an increase interest because of their extraordinary mechanical strength. In fact, material properties in the nanoregime are extremely different from those found in macroscopic samples: few crystalline materials have shown a tensile strength in excess of 10 GPa in the form of nanowires. Still the length of defect-free crystalline nanowires is limited to a few millimeters and the strength of long nanowires is compromised by defects. The strength of glass nanowires is less affected by single defects. In this paper we present the ultimate strength of glass silica nanowires manufactured by a top-down fabrication technique; this is the highest value reported for glass materials. The measured ultimate strength is in excess of 10 GPa and increases for decreasing nanowire diameters. Scanning electron micrographs of the broken fragments showed a fragile rupture.