Fibre optics and fibre waveguides
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- Optical propagation, dispersion and attenuation in fibres | Fibre optic sensors; fibre gyros | Optical fibre testing and measurement of fibre parameters | Other fibre optical devices and techniques | Optical fibre fabrication, cladding, splicing, joining | Gradient-index (GRIN) fibre devices and techniques | Fibre couplers and connectors | Other fibre optical properties
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This study presents the development and validation of a fibre Bragg gratings (FBGs)-based sensor system for the assessment of strain in the midpalatal suture in subjects using rapid palatal expanders (RPEs). The ex-vivo experiments were made by means of positioning two RPEs in a porcine palatal region. The RPEs used were the Hyrax, a tooth-borne expander and MARPE (microimplant-assisted rapid palatal expansion), a bone-borne expander. In order to define the regions in the palatal region for the sensors positioning, a finite-element analysis was performed in a porcine head subjected to the loadings caused by an RPE. In addition, a strain transfer model was used to obtain a correction coefficient that approximates the strain estimated by the FBG to the actual strain in the structure under shear and normal stress. Results show high linearity in the sensors characterisation tests with the advantages of compactness, intrinsic safe operation and multiplexing capabilities of FBGs. In the RPE analysis, a higher strain was estimated in the anterior region, which is in accordance with the simulation and previously reported results, where MARPE showed a higher strain (with an exponential pattern) than Hyrax as the number of activations increase.
A novel liquid-core photonic crystal fibre (LC-PCF) based on the Zeonex substrate has been proposed for sensing the varying concentration of kerosene in petrol and diesel fuels in terahertz frequencies. Fuel sample infiltration is achieved by designing a single hexagon in the porous core region. The LC-PCF has been numerically studied using a finite element method. By filling the core hexagon with different petrol–kerosene and diesel–kerosene concentrations, sensitivity and transmission profiles of the proposed LC-PCF have been examined. The impact of different design parameters of the fibre on its performance has been analysed, allowing the optimum design parameter set to be selected for further analysis. The proposed fibre shows relative sensitivities above 96% and effective material losses in the order of 10−3. With negligible confinement losses, the proposed fibre demonstrates near-zero ultra-flattened group velocity dispersion profiles within 1–1.6 THz. It is envisaged that the proposed fibre may be fabricated easily due to its simple design, and thus, serving as a significant step towards understanding THz pulse interaction with liquid petrochemical products.
In optical fibre communication system, improved modulation schemes have been proposed to increase capability and high bit rate transmission. One such technique is optical duobinary modulation, which can able to broadcast high-speed optical signals over bandwidth-limited channels. The conventional modulation schemes employ the non-return-to-zero modulation and return-to-zero modulation technique. These schemes decrease spectrum width and increase the dispersion tolerance. However, the effect of inter-symbol inference presents between the pulses. These techniques give a reduced performance for high bit rate transmissions and transmission over long distances. Hence in this work, the implementation of duobinary modulation improves the performance of optical communication along with the provision of enhanced security through bio-inspired chaos masking scheme. The bio-inspired chaos-masking scheme generates the security key using the spider security pattern generator to provide improved security. The parameters of the proposed technique are analysed for different transmission rates and the performance improvement is validated.
The authors experimentally demonstrate an ultra-low repetition rate passive mode-locked fibre ring laser with a semiconductor optical amplifier works like as a gain medium and non-linear polarisation rotation. A unidirectional erbium-doped fibre amplifier provides additional gain to increase cavity power. By exploiting non-linear polarisation rotation combined with dispersion management stable 5.18 ps optical pulses with 67.2 kHz repetition rate and energy of 1.75 nJ were generated with an extinction ratio >35 dB. The central wavelength is 1557.89 nm and the optical 3-dB bandwidth 1.82 nm very appropriate for optical applications that often require high-peak-power, low repetition rate and picosecond pulses.
In this work, the authors review different design setups of polarisation beam splitters (PBSs) developed in Si3N4 platforms. They analyse different approaches based on directional couplers and interferometers. Then the authors present the design and simulation results of a PBS based on two cascaded multi-mode interferometers that operate in the whole C-band. Finally, the authors compare the devices in terms of insertion loss, extinction ratio, bandwidth, footprint and fabrication tolerances.
In this study, the authors present theoretical and experimental results of wideband beamforming networks steered by a single linear chirped fibre Bragg grating (CFBG). The standard single-sideband modulation technique is followed to validate the wideband (at 18 GHz) operation of the proposed system. CFBG has been fabricated by phase mask technology for the desired specification to be compatible with the antenna array. To the authors knowledge, the effect of dispersion slope feature of fabricated FBG on the performance of beam-steering capability of the antenna is reported for the first time in this study. Theoretically preceded by experimental testing, it was found that the scanning angle increased with the rise in the number of antenna elements and the frequency of modulating microwave signal.
The authors analyse a method, which leads to an efficient and precise algorithm for synthesising coaxial, optical fibre couplers of arbitrary modal electric-field distributions. Their purpose is to design a fibre optic index profile supporting two pre-selected mode fields such that their interference transfers all the optical power spatially from the core region to a cladding part of the optical fibre completely. To deal with this problem, they apply inverse transmission-line techniques. Using the desired electric field, they work inversely and synthesise the appropriate refractive index profile of the couplers with excellent crosstalk. They envisage one can apply this technique to design fibre optic coaxial components acting as filters in sensing and numerous other applications.
A coherent interference-free tunable microwave photonic notch filter with a pair of negative and positive coefficients generated by employing polarisation multiplexing and cross gain modulation in a semiconductor optical amplifier is proposed and demonstrated through simulation results. The negative and positive coefficients of the notch filter are realised through two orthogonal polarisation states of a single continuous wave laser diode. The free spectral range of microwave photonic filter is tuned by varying the differential group delay of a birefringent single-mode fibre. The proposed filter shows good tunability, easy implementation and is cost-effective.
The authors present a new method for determining the optical properties of arbitrary refractive index elliptical core fibres. The method applies the elliptic cylindrical coordinate system to Maxwell's equations and appropriately, using Fourier Transforms, operated on an elliptical core optical fibre waveguide, derives a flexible algorithm, which can be used to determine the optical properties of elliptical fibre waveguides. By dividing the fibre into a series of uniform consecutive thin dielectric layers transversely across the elliptical cross section with suitable transforming functions representing voltage and current themselves based on the electromagnetic field components, leads to transverse transmission line equations. The resonance frequencies of the cascaded matrices determine the mode propagation constants. The analysis leads to a numerical algorithm for calculating the exact modes of propagation constants. The method is analytically exact and computationally accurate, leading to algorithmic implementation, without making use of Mathieu functions which are solutions for step index waveguide. The advantage of this work is also the fact that it can be applied to arbitrary refractive index profiles. They present results of modal diagrams for various fibre ellipticities and also apply this method to determine the birefringence of graded index profile fibres and mode cut-off frequencies.
The optical time domain reflectometry (OTDR) is the only investigation tool for the optical fibre continuity measurement and is capable of verifying inline splices and locating fibre faults. As per the literature review, most of the distributed fibre sensors are designed using OTDR principle. Dynamic range plays a major role in such instruments. Dynamic range enhancement of OTDR is proposed using the lifting wavelet transform (LWT)-modified particle swarm optimisation (MPSO) scheme. This scheme enables us to design customised lifting wavelet filters to improve the signal-to-noise ratio which in turn improvises the dynamic range. This study proposes and demonstrates the application of LWT along with the MPSO evolutionary algorithm to obtain optimum threshold, in order to mitigate the noisy lifting wavelet coefficients effectively. The proposed scheme is employed for OTDR measurement up to 50 km (silica optical fibre), using 10 dBm of laser input power. As compared with the conventional wavelet regularised deconvolution schemes, the authors’ proposed scheme offers a 3.42 dB enhancement in the dynamic range under a lower computational complexity requirement. The proposed study is carried out using computer simulation using MATLAB 15.0 software. The results were experimentally validated.
This study presents a Brillouin distributed sensor (BDS) with high spatial resolution for the monitoring of temperature and strain simultaneously. Fourier deconvolution (FD), Fourier regularised deconvolution (FRD), and Fourier regularised denoising deconvolution (FRDD) algorithms have been explored to measure the Brillouin backscattered power using the optical time-domain reflectometry technique. The improvement of temperature and strain resolutions along with the spatial resolution using the FRDD algorithm is observed for 50 km sensing range. The numerical simulation process of the proposed sensing system has been simultaneously incorporated to extract temperature and strain information. The simulation results show that the temperature and strain resolutions are observed as 5°K and 138 μɛ, respectively, using FRDD compared with the FD algorithm which shows temperature and strain resolutions as 54°K and 1588 μɛ, respectively, at a 50-km distance. A high spatial resolution of 9 m is observed of the proposed BDS over the sensing range of 50 km using the FRDD algorithm. In addition, the denoising capability of the proposed FRDD algorithm is validated using the experimentally obtained Rayleigh backscattered signal.
Two-dimensional (2D) metasurface integrated with strip waveguide in silicon-on-insulator (SOI) has been designed to achieve high coupling efficiency for 3.8 μm wavelength. The optimisation in period and radius has been achieved using 3D finite-difference time-domain (FDTD). The calculated coupling efficiency in the in-plane waveguide for the out-of-plane surface illumination is over 90% with a bandwidth of 1 μm. The design is consistent with the available lithography using 400 nm thick SOI for mid-IR applications. Finally, monolithic integration can be achieved using standard multi-project wafer run.
Fibre Bragg Grating (FBG) sensor is a new sensor with numerous advantages. How to apply in the safety monitoring of landslide engineering is still in experiment and research stage. Here, FBG is used to monitor the landslide process of the gravel soil pile. Before monitoring experiment, the article first tests FBG packaging and burying effect by two tests in lab, one is sand pile landslide test which proves vertical buried is feasible and the other is loading test on the steel bar sensor which proves the wavelength change of FBG on steel bar sensor has a good linear relationship with the loading change. Then, buried the steel bar sensor in the gravel soil slope after simple encapsulation. Stress and strain monitoring results of grating sensors are obtained in landslide which is formed through the simulation of excavation construction. The experimental data show that through correct embedding FBG sensors can effectively monitor the stress–strain change of the slope. FBG provides a new way for the landslide early warning.
Mercury ion (Hg2+) is one of the common pollutants which exists in the natural environment far and wide. The detection of Hg2+ usually requires large laboratory equipment, which limits their field applications. Here, a miniaturised optical fibre sensor based on quantum dots (QDs) for detection of Hg2+ is developed. The sensor is based on the principle of fluorescence quenching caused by Hg2+ and evanescent wave sensing. It is an all-fibre sensor which composed of an optical fibre probe module, an optical module and a signal acquisition module. Through miniaturisation design of optical system and the structure of the sensor, the sensor is reduced to the volume of 120 × 120 × 100 mm. The experimental results show that in the miniaturised sensor both has high sensitivity and fast response speed. Meanwhile, the ion anti-interference experiment shows that the sensor has good selectivity for Hg2+ detection. In general, the proposed sensor has significant potential in the field of biochemistry, environmental monitoring, and food safety.
Compared with the traditional electrical resistance strain gauge, the fibre Bragg grating (FBG) is a new material that can be used to produce the strain sensor, which has many advantages such as convenient installation, low temperature drifting, less signal interference and higher transmitted quality. In this study, FBG sensor system configuration, design principle, numerical simulation and loading tests for the FBG strain gauge are expatiated extensively. At the last, one-axial and three-axial strain gauges are produced and some conclusions have been put forward. So the FBG strain gauge can meet the precision of the structure monitoring and replace the traditional sensors.
In this study, the authors deal with the problem of simultaneous measurement of temperature, pressure, and strain using novel technologies of optical fibre (OF) sensors. Indeed, they address the problem of early surveillance systems to detect disasters in real-time. Through these sensors, they try to predict prospective and possible disasters (landslides, fire, earthquake etc.). Temperature, pressure, and strain should be simultaneously monitored to prevent betimes any damages. They propose a novel design of disaster system, which is able to measure the variation of temperature, pressure, and strain using optical sensors based on Brillouin scattering, two types of these sensors are discussed Brillouin optical time division reflectometers and Brillouin optical time division analysis (BOTDA). Thus, they indicate the performance of BOTDA sensors on disaster detection systems. However, these sensors cannot measure pressure variation, so they introduce the application of hybrid BOTDA/fibre Bragg grating (FBG) system in order to predict disasters caused by pressure.
Optical fibers have demonstrated, against all early predictions, that their geometry is extremely well-suited for handling high optical powers. However, over the years, many refinements have been done to the basic structure of these waveguides to allow guiding and generating high-powers. As a result of this development the fibers employed in high-power laser systems differ significantly from those typically used in other fields such as telecommunications and/or sensors. Therefore, this chapter is devoted to describing and explaining the main intricacies of those optical fiber designs presently used for high-power operation. In this context, the main goal of this chapter is that the reader gets an overview of the different fiber designs and building blocks of high-power optical fibers. The chapter is divided into three main parts: a brief historical overview of the evolution of this technology, a description of the main constituents of fibers for high-power operation (including the core, the pump cladding and materials), and, finally, an outlook in which novel trends in the design of these waveguides are briefly described.
A passively Q-switched erbium-doped fibre laser was demonstrated by using tris-(8-hydroxyquinoline)aluminium (Alq3) organic material as a passive saturable absorber (SA). The SA was fabricated by a drop casting technique at room temperature and incorporated into a laser cavity by inserting it between two optical fibre ferrules. The Q-switched fibre laser operates at a centre wavelength of 1559 nm with a full-width half maximum of 1 nm. The generated pulse train is stable and has a pulse width decreased from 6.65 to 1.2 µs and the pulse repetition rate increased from 31.65 to 144.5 kHz as the pump power increased from 20 to 122 mW. The maximum pulse energy obtained is 63.89 nJ. This work indicates that the Alq3 as SA has a great potential to be utilised for pulsed laser generation. To the best of the authors' knowledge, this is the first time an organic material is utilised as a SA device for generating a stable Q-switched laser pulse.
The thermal response of packaged biconic taper filter with a free spectral range of 1.7 nm is investigated over a temperature range of 0–70°C. The filter length dependence with temperature is estimated from the spectral offset. This device presented thermal stability of 3 × 10−3 nm/°C, measured at λ 0 = 1550 nm.
High-voltage transformers are essential for power systems, particularly in the transmission and distribution sectors. The reliability of this equipment typically depends on their insulation condition. Thus, considerable attention must be accorded to the development of diagnostic and monitoring systems for predicting the condition of transformer oil. At present, several electrical, physical and chemical diagnostic techniques have been applied to transformer condition monitoring. This study presents a review of the application of optical sensors to transformer condition monitoring. Optical-sensor-based monitoring of various parameters of transformer oil as part of transformer condition monitoring is discussed. The classification and sensing principles of optical sensors are also reviewed.