We examine the use of state-of-the-art distributed sensing systems to extract temperature information from the optical fibre infrastructure already of the Electricity Authority of Cyprus power distribution network (~25-year old installation); as a means of optical fibre distributed sensing in the underground cables. The optical fibres are collocated with existing power distribution cables, for the purpose of power line monitoring cable joints that are prone to failure, along with general monitoring for unusual behaviour and potential cable fault conditions. Detection is achieved using DTS: Distributed Temperature Sensors (Silixa Ltd) that use RAMAN-based measurements in combination with BOTDR (Brillouin Optical Time-domain Reflectometry) for high-precision temperature detection. We examine the correlation between the temperature of the power cable with the power consumption provided by the EAC and the weather conditions. Furthermore, our data will give an indication of how important is uniform spacing between power and optical cables. The real-time and continuous monitoring of the temperature of the optical cables through the distributed sensing systems may help identifying abnormal cable behavior (hot spots) and possible future network failures in the power network.
KEYWORDS: Signal to noise ratio, Data acquisition, Tolerancing, Temperature metrology, Computing systems, Tunable filters, Sensing systems, Laser frequency, Data analysis, Statistical analysis
A Brillouin Optical Time-Domain Analysis (BOTDA) Lorentzian data fitting method to estimate the Brillouin Frequency Shift (BFS) is proposed. Data is obtained from an experimental setup used to conduct the temperature and strain measurements. Before Lorentzian fitting the noisy data is averaged and filtered. The proposed method attempts to lower computational complexity in determining the Brillouin frequency. The resulting parameters of a completed BGS curve fitting are used as initial set of parameters for the next location point BGS fitting. Completion of the Lorentzian fitting using the Levenberg-Marquardt nonlinear curve fitting algorithm is achieved in a small number of iterations which improves the performance in obtaining the Brillouin frequency shift.
The Brillouin frequency spectrum profile is characterised by the Lorentzian or the Gaussian function, which is identical to the reflection spectrum of a fibre Bragg grating (FBG), while similarly the Brillouin frequency is linearly dependent on the amount of applied strain or the ambient temperature of the fibre. In this paper, we used the Zero-Crossing fibre Bragg grating demodulation algorithm to optimise the Brillouin frequency shift of a typical single-mode optical fibre, SMF28, in which static and dynamic strain steps are applied. We showed that the FBG demodulation algorithms could be used and improve the response from a Brillouin system even though the strain levels are below the measuring capability of the system.
We report on the femtosecond laser inscription of a fibre Bragg grating array in multimode, gradient-index, CYTOP polymer optical fibre and its demonstration as a quasi-distributed sensor for cantilever health monitoring measurements. We exploit the key advantage of polymer optical fibres, having a significantly lower Young’s modulus compared with silica fibres, for vibration measurements. We also modify the typical multi-mode Bragg grating spectrum through control of the femtosecond laser inscription process, thereby producing gratings having single peak wavelength spectra. The sensor array is used to recover the time-dependent, wavelength response from each Bragg grating sensor and extract the mode shape of the beam. The mode shapes of the beam were used to observe “damage” introduced to the cantilever by adding masses to its surface; adjusting the level of damage by using different weights and placing them at different point across the beam. We show that health monitoring measurements are feasible with polymer based fibre Bragg gratings. The accurate and rapid detection of damage points on structural beams and the damage level is an important parameter for improved maintenance and servicing of beams under load and for the prevention of long-term damage.
The work described in this paper involved two different material fibre Bragg grating (FBG) arrays, investigating their performance as quasi-distributed sensors by capturing the vibrating response of a free-free metal beam close to its resonance frequencies. A six meter length of low-loss, gradient-index, multimode CYTOP fibre and of SMF-28 were used for the inscription of multiple FBG sensors using a femtosecond laser inscription method. The FBG arrays were multiplexed in the wavelength domain using a high-speed commercial demodulator, from which we recovered wavelengthand time-dependent displacement information. We compared the vibration response of the two arrays and using a novel computation algorithm we extract the first mode shape of the free-free metal beam that was exited at its first resonance frequency using a vibrating force.
In this paper, an ormocomp polymer nanowire with possible use in integrated-optics sensing applications is presented. We discuss the structure design, the fabrication process and present results of the simulation and characterization of the optical field profile. Since the nanowires are designed and intended to be used as integrated optics devices, they are attached to tapered and feed waveguides at their ends. The fabrication process in this work is based mainly on the nanoimprint technique. The method assumes a silicon nanowire as an original pattern, and polydimethylsiloxane (PDMS) as thesoft mold. The PDMS mold is directly imprinted on the ormocomp layer and then cured by UV light to form the polymer based nanowire. The ormocomp nanowires are fabricated to have various dimensions of width and length at a fixed 500nm thickness. The length of the nanowires is varied from 250 µm to 2 mm, whereas the width of the structures is varied between 500nm and 1µm. The possible optical mode field profile that occurs in the proposed polymer nanowire design is studied using the H-field finite element method (FEM). In the characterization part, the optical field profile and the intensity at the device output are the main focus of this paper. The various lengths of the nanowires show different characteristics in term of output intensity. An image processing is used to process the image to obtain the intensity of the output signal. A comparison of the optical field and output intensity for each polymer nanowire is also discussed.
Today there is an increasing surge in Surface Plasmon based research and recent studies have shown that a wide range of plasmon-based optical elements and techniques have led to the development of a variety of active switches, passive waveguides, biosensors, lithography masks, to name just a few. The Terahertz (THz) frequency region of the electromagnetic spectrum is located between the traditional microwave spectrum and the optical frequencies, and offers a significant scientific and technological potential in many fields, such as in sensing, in imaging and in spectroscopy. Waveguiding in this intermediate spectral region is a major challenge. Amongst the various THz waveguides suggested, the metal-clad waveguides supporting surface plasmon modes waves and specifically hollow core structures, coated with insulating material are showing the greatest promise as low-loss waveguides for their use in active components and as well as passive waveguides. The H-field finite element method (FEM) based full-vector formulation is used to study the vectorial modal field properties and the complex propagation characteristics of Surface Plasmon modes of a hollow-core dielectric coated rectangular waveguide structure. Additionally, the finite difference time domain (FDTD) method is used to estimate the dispersion parameters and the propagation loss of the rectangular waveguide.
We present femtosecond laser inscribed phase masks for the inscription of Bragg gratings in optical fibres. The principal
advantage is the flexibility afforded by the femtosecond laser inscription, where sub-surface structures define the phase
mask period and mask properties. The masks are used to produce fibre Bragg gratings having different orders according
to the phase mask period. The work demonstrates the incredible flexibility of femtosecond lasers for the rapid
prototyping of complex and reproducible mask structures. We also consider three-beam interference effects, a
consequence of the zeroth-order component present in addition to higher-order diffraction components.
We present the results of femtosecond laser microstructuring of optical fibres by direct access of the fibre end face, both
at the surface and several hundred microns into the fibre, to realise one- and two-dimensional grating structures and
optical fibre splitters, respectively. We show the versatility of this simple but effective inscription method, where we
demonstrate classic multiple slit diffraction patterns and show the potential for coarse wavelength division multiplexing
for sensor signals. A key advantage for the fibre splitter is that the inscription method avoids the use of oil immersion
that compensate for the fibre curvature in the standard side writing method.
Microfluidics are important micro-scale devices that can be used to manipulate very small volumes of fluids on the order
of nano- to femto-litres. The control and sorting of nano-particles is a primary goal using this technology. There is
particular interest in the use of microstructure optical fibres for the transfer of fluids, whereby the guided light interacts
with a fluid in the region of the air-hole structure. We study the fluid transport capabilities of microstructure fibres with
cross sections containing circular or elliptical holes, considering the effects of flow rates, fluid viscosity, and the channel
diameter. The role of heat flux is considered in relation to the fluid characteristics. We solve the time-dependent Navier-
Stokes equations and the convection-diffusion equation.
Photonic crystal fibres (PCF) and more commonly microstructure fibres, remain interesting and novel fibre types
and when suitably designed can prove to be "ideal" for sensing applications, as the different geometrical
arrangement of the air holes alters their optical wave-guiding properties, whilst also providing tailored dispersion
characteristics. This impacts the performance of grating structures, which offer wavelength encoded sensing
information. We undertake a study on different air hole geometries and proceed with characterization of fibre Bragg
and long period gratings, FBG and LPG, respectively that have been inscribed (using either a femtosecond or
ultraviolet laser system) within different designs of microstructured fibre that are of interest for sensing applications.
The terahertz (THz) frequency region of the electromagnetic spectrum is located between the traditional microwave
spectrum and the optical frequencies, and offers a significant scientific and technological potential in many fields, such
as in sensing, in imaging and in spectroscopy. Waveguiding in this intermediate spectral region is a major challenge.
Amongst the various THz waveguides suggested, metal-clad plasmonic waveguides and specifically hollow core
structures, coated with insulating material are the most promising low-loss waveguides used in both active and passive
devices. Optical power splitters are important components in the design of optoelectronic systems and optical
communication networks such as Mach-Zehnder Interferometric switches, polarization splitter and polarization
scramblers. Several designs for the implementation of the 3dB power splitters have been proposed in the past, such as the
directional coupler-based approach, the Y-junction-based devices and the MMI-based approach. In the present paper a
novel MMI-based 3dB THz wave splitter is implemented using Gold/polystyrene (PS) coated hollow glass rectangular
waveguides. The H-field FEM based full-vector formulation is used here to calculate the complex propagation
characteristics of the waveguide structure and the finite element beam propagation method (FE-BPM) and finite
difference time domain (FDTD) approach to demonstrate the performance of the proposed 3dB splitter.
We demonstrate the development of femtosecond laser inscribed superstructure fiber gratings (fsSFG) in silica optical
fibre. We utilise a single step process, to inscribe low loss and polarisation independent, sampled gratings in optical
fibres using the point by point femtosecond laser inscription method. Our approach results in a controlled modulated
index change with complete suppression of any overlapping LPG structure leading to highly symmetric superstructure
spectra, with the grating reflection well within the Fourier design limit. We also solve Maxwell's equations and calculate
the back reflection spectrum using the bidirectional beam propagation method (BiBPM). Experimental results validate
our numerical analysis and the estimation of inscription parameters such as ac index modulation, wavelength and the
relative peak strength. We also explore how changes in the grating's period influence the reflection spectrum.
We report on the first inscription of fibre Bragg gratings in microstructure optical fibre using a NIR femtosecond laser
system, inscribed using the point-by-point method, and without the use of any oil immersion techniques to remove the
effects of fibre curvature or the role of the holey microstructure. The Bragg gratings are second order and recorded in a
microstructure optical fibre that is highly birefringent, single-mode at 1550nm, photosensitive and compatible with FBG
inscription technology due to the minimized number of air holes surrounding the fibre core. Both axial strain and
temperature sensitivity are measured.
The use of high intensity femtosecond laser sources for inscribing fibre gratings has attained significant interest. The
principal advantage of high-energy pulses is their ability for grating inscription in any material type without
preprocessing or special core doping. In the field of fibre optical sensing LPGs written in photonic crystal fibre have a
distinct advantage of low temperature sensitivity over gratings written in conventional fibre and thus minimal
temperature cross-sensitivity. Previous studies have indicated that LPGs written by a point-by-point inscription scheme
using a low repetition femtosecond laser exhibit post-fabrication evolution leading to temporal instabilities at room
temperatures with respect to spectral location, strength and birefringence of the attenuation bands. These spectral
instabilities of LPGs are studied in photonic crystal fibres (endlessly single mode microstructure fibre) to moderately
high temperatures 100°C to 200°C and their performance compared to fusion-arc fabricated LPG. Initial results suggest
that the fusion-arc fabricated LPG demonstrate less spectral instability for a given constant and moderate temperature,
and are similar to the results obtained when inscribed in a standard single mode fibre.
We report on the temperature response of FBGs recorded in pure PMMA and TOPAS holey fibres. The gratings are fabricated for operational use at near IR wavelengths, using a phase mask and a CW He-Cd laser operating at 325nm. The room temperature grating response is non-linear and characterized by quadratic behaviour for temperatures from room temperature to the glass transition temperature, and this permanent change is affected by the thermal history of the gratings. We also report the first FBG inscription in microstructured polymer optical fibres fabricated from TOPAS. This material is fully polymerized and has very low moisture absorption, leading to very good fibre drawing properties. Furthermore, although TOPAS is chemically inert and bio-molecules do not readily bind to its surface, treatment with Antraquinon and subsequent UV activation allows sensing molecules to be deposited in well defined spatial locations. When combined with grating technology this provides considerable potential for label-free bio-sensing.
The use of high intensity femtosecond laser sources for inscribing fibre gratings has attained significant interest. The
principal advantage of high-energy pulses is their ability for grating inscription in any material type without pre-processing
or special core doping - the inscription process is controlled multi-photon absorption, void generation and
subsequent local refractive index changes. The formation of grating structures in photonics crystal fibre has proven
difficult, as the presence of holes within the fibre that allow wave-guidance impair and scatter the femtosecond
inscription beam. Here we report on the consistent manufacture of long period gratings in endlessly single mode
microstructure fibre and on their characterisation to external perturbations. Long period gratings are currently the subject
of considerable research interest due to their potential applications as filters and as sensing devices, responsive to strain,
temperature, bending and refractive index. Compared to the more mature fibre Bragg grating sensors, LPGs have more
complex spectra, usually with broader spectral features. On the other hand they are intrinsically sensitive to bending and
refractive index. Perhaps more importantly, the fibre design and choice of grating period can have a considerable
influence over the sensitivity to the various parameters, for example allowing the creation of a bend sensor with minimal
temperature cross-sensitivity. This control is not possible with FBG sensors. Here we compare the effects of symmetric
and asymmetric femtosecond laser inscription.
We report on the temperature response of FBGs recorded in pure PMMA and TOPAS holey fibers. The gratings are
fabricated for operational use at near IR wavelengths, using a phase mask and a cw He-Cd laser operating at 325nm. The
room temperature grating response is non-linear and characterized by quadratic behavior for temperatures from room
temperature to the glass transition temperature, and this permanent change is affected by the thermal history of the
gratings. We also report the first FBG inscription in microstructured polymer optical fibers fabricated from TOPAS. This
material is fully polymerized and has very low moisture absorption, leading to very good fiber drawing properties.
Furthermore, although TOPAS is chemically inert and bio-molecules do not readily bind to its surface, treatment with
Antraquinon and subsequent UV activation allows sensing molecules to be deposited in well defined spatial locations.
When combined with grating technology this provides considerable potential for label-free bio-sensing.
In this paper we report on investigations of some of the factors that have a bearing on the reliability and repeatability of polymer fibre Bragg gratings. The main issues discussed are the fibre preform composition, the fibre drawing conditions and the thermal history of the fibre grating.
The annealing properties of Type IA Bragg gratings are investigated and compared with Type I and Type IIA Bragg gratings. The transmission properties (mean and modulated wavelength components) of gratings held at predetermined temperatures are recorded from which decay characteristics are inferred. Our data show critical results concerning the high temperature stability of Type IA gratings, as they undergo a drastic initial decay at 100°C, with a consequent mean index change that is severely reduced at this temperature However, the modulated index change of IA gratings remains stable at lower annealing temperatures of 80°C, and the mean index change decays at a comparable rate to Type I gratings at 80°C. Extending this work to include the thermal decay of Type IA gratings inscribed under strain shows that the application of strain quite dramatically transforms the temperature characteristics of the Type IA grating, modifying the temperature coefficient and annealing curves, with the grating showing a remarkable improvement in high temperature stability, leading to a robust grating that can survive temperatures exceeding 180°C. Under conditions of inscription under strain it is found that the temperature coefficient increases, but is maintained at a value considerably different to the Type I grating. Therefore, the combination of Type I and IA (strained) gratings make it possible to decouple temperature and strain over larger temperature excursions.
We describe recent research into devices based on fibre Bragg gratings in polymer optical fibre. Firstly, we report on the inscription of gratings in a variety of microstructured polymer optical fibre: single mode, few moded and multimoded, as well as fibre doped with trans-4-stilbenmethanol. Secondly, we describe research into an electrically tuneable filter using a metallic coating on a polymer fibre Bragg grating. Finally we present initial results from attempts to produce more complex grating structures in polymer fibre: a Fabry-Perot cavity and a phase-shifted grating.
The characterization and optimization of optical microring resonator-based optical filters on deeply etched GaInAsP-Inp
waveguides, using the finite element-based beam propagation approach is presented here. Design issues for directional
coupler- and multimode interference coupler-based devices, such as field evolution, optical power, phase, fabrication
tolerance and wavelength dependence have been investigated.
The characterization and optimization of optical microring resonator-based optical filters on deeply etched GaInAsP-Inp waveguides, using the finite element-based beam propagation approach is presented here. Design issues, such as coupling , field evolution, power and phase considerations have also been investigated.
Design issues such as optical transmission, interference mechanisms and splitting ratio of a compact parabolically tapered Multimode Interference-based 3dB power splitter on an InP-based deeply etched ridge waveguide, using the finite element-based beam propagation method approach, are presented.
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