A Fibre-Optic Platform for Sensing Nitrate Using Conducting Polymers.

Monitoring nitrate ions is essential in agriculture, food industry, health sector and aquatic ecosystem. We show that a conducting polymer, poly(3,4-ethylenedioxythiophene) (PEDOT), can be used for nitrate sensing through a process in which nitrate ion uptake leads to oxidation of PEDOT and change of its optical properties. In this study, a new platform is developed in which a single-mode fibre coated at the tip with PEDOT is used for nitrate sensing. A crucial step towards this goal is introduction of carbonate exposure to chemically reduced PEDOT to a baseline value. The proposed platform exhibits the change in optical behaviour of the PEDOT layer at the tip of the fibre as it undergoes chemical oxidation and reduction (redox). The change in optical properties due to redox switching varies with the intensity of light back reflected by the fibre coated with PEDOT. The proposed platform during oxidation demonstrates linear response for the uptake of nitrate ions in concentrations ranging between 0.2 and 40 parts per million (ppm), with a regression coefficient R2=0.97 and a detection limit of 6.7 ppm. The procedure for redox switching is repeatable as the back reflection light intensity reaches ±1.5% of the initial value after reduction.

Design and verification of an external radiobiological beam port on a 16.5 MeV GE PETtrace proton cyclotron.

PURPOSE: Protons and heavy ions are considered to be ideal particles for use in external beam radiotherapy due to the superior properties of the dose distribution. While a photon (x-ray) beam delivers considerable dose to healthy tissues around the tumor, a proton beam that is delivered with sufficient energies has: a low entrance dose (the dose in front of the tumor); a high-dose region within the tumor, known as the Bragg peak; and, no exit dose beyond the tumor. Proton therapy is the next major step in advancing radiotherapy treatment. The purpose of this project was to adapt an existing radioisotope production cyclotron, a General Electric (GE) PETtrace, to enable radiobiological studies using proton beams. During routine use the PETtrace delivers 16.5 MeV protons to target with beam currents in the range of 10-100 µA resulting in dose rates in the order of kGy/s. To achieve the aim of the project the dose rate had to be reduced to the Gy/min range, without attenuating the proton energy below 5 MeV. This paper covers the design, construction and validation of the beam port. METHODS: Monte Carlo simulations were performed, using GEANT4, SRIM and PACE4 to design the beam port and optimize its components. Once the beam port was fabricated, validation experiments were performed using EBT3 and HD-V2 Gafchromic™ films, and a Keithley 6485 picoampere meter. RESULTS AND CONCLUSION: The external beam port was successfully modeled, designed and fabricated. By using a 0.25 mm thick gold foil and a brass pin-hole collimator the beam was spread from a narrow full beam diameter of 10 mm to a wide beam with a 5% flatness area in the center of the beam that had a diameter of ~20 mm. In using this system the dose rate was reduced from kGy/s to ~30 Gy/min.

Unified theory of whispering gallery multilayer microspheres with single dipole or active layer sources.

The development of a fast and reliable whispering gallery mode (WGM) simulator capable of generating spectra that are comparable with experiment is an important step forward for designing microresonators. We present a new model for generating WGM spectra for multilayer microspheres, which allows for an arbitrary number of concentric dielectric layers, and any number of embedded dipole sources or uniform distributions of dipole sources to be modeled. The mode excitation methods model embedded nanoparticles, or fluorescent dye coatings, from which normalized power spectra with accurate representation of the mode coupling efficiencies can be derived. In each case, the emitted power is expressed conveniently as a function of wavelength, with minimal computational load. The model makes use of the transfer-matrix approach, incorporating improvements to its stability, resulting in a reliable, general set of formulae for calculating whispering gallery mode spectra. In the specific cases of the dielectric microsphere and the single-layer coated microsphere, our model simplifies to confirmed formulae in the literature.

Optimal light collection from diffuse sources: application to optical fibre-coupled luminescence dosimetry.

A model is developed to evaluate the light collection of a diffuse light source located at the tip of an optical fibre. The model is confirmed experimentally and used to evaluate and compare the light collection efficiency of different fibre-coupled luminescence dosimeter probe designs. The model includes contributions from both meridional and skew rays, and considers the light collection from an optically attenuating scintillator. Hence the model enables the optimisation of different, but useful and new probe materials such as BeO ceramic. Four different dosimeter architectures are considered, including previously investigated probe designs; the butt-coupled and reflective wall, along with two novel designs. The novel designs utilise a combination of the scintillating material and transparent media to increase the light collection. Simulations indicate that the novel probes are more efficient in light collection for applications in which it is necessary to minimise the volume of the scintillating material.

Efficient excitation of surface plasmons in metal nanorods using large longitudinal component of high index nano fibers.

We report theoretical calculations of the mode fields of high index lead silicate and silicon nano fibers, and show that their strong longitudinal component enables efficient excitation of surface plasmons within a silver nanorod placed at the fiber tip. An excitation efficiency 1600 times higher than that of the standard single mode fibers has been achieved using a 350nm diameter silicon fiber at 1.1µm wavelength, while a factor of 640 times higher efficiency is achieved for a 400nm diameter lead silicate F2 glass fiber. The strong localized field emerging from the end of the rod serves as a nano-scale source with adjustable beam width, and such sources offer a new approach to high-resolution microscopy, particle manipulation and sensing.

Nonlinear polarization bistability in optical nanowires.

Using the full vectorial nonlinear Schrödinger equations that describe nonlinear processes in isotropic optical nanowires, we show that there exist structural anisotropic nonlinearities that lead to unstable polarization states that exhibit periodic bistable behavior. We analyze and solve the nonlinear equations for continuous waves by means of a Lagrangian formulation and show that the system has bistable states and also kink solitons that are limiting forms of the bistable states.

Light confinement within nanoholes in nanostructured optical fibers.

We report fabrication of the lead silicate microstructured fibers (MOFs) with core holes as small as 20 nm, the smallest holes fabricated within the core of an optical fiber to date. We show that light confinement and average mode intensity within such holes are strongly dependent on the hole size. Light confinement within 80 nm and 250 nm core hole within the fabricated MOFs has been experimentally characterized using Scanning Near-field Optical Microscopy (SNOM).

Fluorescence-based sensing with optical nanowires: a generalized model and experimental validation.

A model for the fluorescence sensing properties of small-core high-refractive-index fibers (optical nanowires) is developed and compared quantitatively with experiment. For the first time, higher-order modes and loss factors relevant to optical nanowires are included, which allows the model to be compared effectively with experiment via the use of fluorophore filled suspended optical nanowires. Numerical results show that high-index materials are beneficial for fluorescence-based sensing. However, both numerical and experimental results show that the fluorescence signal is relatively insensitive to core size, except for low concentration sensing where nanoscale fiber cores are advantageous due to the increased evanescent field power.

Enhancement of fluorescence-based sensing using microstructured optical fibres.

We develop a generic model of excitation and fluorescence recapturing within filled microstructured optical fibres (MOFs) with arbitrary structure and demonstrate that the light-matter overlap alone does not determine the optimal fibre choice. Fibre designs with sub-wavelength features and high-index glasses exhibit localised regions of high intensity, and we show that these regions can lead to approximately two orders of magnitude enhancement of fluorescence recapturing. Here we show how this regime can be exploited for sensing and demonstrate experimentally in-fibre excitation and fluorescence recapturing within a filled, solid-core MOF.

Enhancement of stimulated Brillouin scattering of higher-order acoustic modes in single-mode optical fiber.

Solving the elastic wave equation exactly for a GeO2-doped silica fiber with a steplike distribution of the longitudinal and shear velocities and density, we have obtained the dispersion, attenuation, and fields of the leaky acoustic modes supported by the fiber. We have developed a model for stimulated Brillouin scattering of these modes in a pump-probe configuration and provided their Brillouin gains and frequencies for an extended range of core sizes and GeO2 doping. Parameter ranges close to cutoff of the acoustic modes and pump depletion enhance the ratio of higher-order peaks to the main peak in the Brillouin spectrum and are suitable for simultaneous strain-temperature sensing.

Subpeaks in the Brillouin loss spectra of distributed fiber-optic sensors.

Subpeaks in the Brillouin loss spectra of distributed fiber-optic sensors were observed for what is believed to be the first time and studied. We discovered that the Fourier spectrum of the pulsed signal and the off-resonance oscillation both contributed to subpeaks. The off-resonance oscillation at frequency /v - vB/ is the oscillation in the Brillouin time domain when beat frequency v of the two counterpropagating laser beams does not match local Brillouin frequency vB. This study is important in differentiating the subpeaks from actual strain-temperature peaks.

Brillouin spectral deconvolution method for centimeter spatial resolution and high-accuracy strain measurement in Brillouin sensors.

Combining a dc and a short pulse (approximately 1 ns) as the probe beam in the pump-probe configuration of Brillouin-based distributed sensors allows us to represent the Brillouin spectrum as a top Lorentzian-like portion and a bottom Gaussian-like portion. Because of the interaction of these two parts, the Lorentzian-like portion carries spatial information that can be extracted within centimeter spatial resolution. Using this information, we develop a spectrum deconvolution method, which considers the location correlation of the strain distribution, to find the number of Brillouin peaks and their frequencies in the top Lorentzian-like portion and hence achieve accurate strain information. An optimum level of dc to pulse power for the best signal and position detection capability is discussed.

Distributed brillouin scattering sensor for discrimination of wall-thinning defects in steel pipe under internal pressure.

A distributed Brillouin scattering sensor has been employed to identify several inner wall cutouts in an end-capped steel pipe by measuring the axial and hoop strain distributions along the outer surface of the pipe. The locations of structural indentations that constitute 50-60% of the inner pipe wall are found and distinguished by use of their corresponding strain-pressure data. These results are quantified in terms of the fiber orientation, defect size and depth, and behavior relative to those of unperturbed pipe sections.

Effect of the finite extinction ratio of an electro-optic modulator on the performance of distributed probe-pump Brillouin sensor systems.

The effect of the finite extinction ratio of an electro-optic modulator (EOM) on the Brillouin frequency measurement of a distributed Brillouin-based fiber optic sensor is studied. An EOM with a finite extinction ratio limits the application of Brillouin optical time domain analysis in a distributed Brillouin-based fiber optic sensor. This results in confusion in specifying the location of the strained region and in error in detecting the Brillouin frequency and hence in strain and temperature measurement.