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In this paper, a global multi-trace method for the scattering of time-harmonic waves by a structure that can contain, in addition to dielectric and perfectly conducting regions also perfectly conducting thin sheets, is presented. The method is direct in the sense that the unknowns are traces and jumps of the fields in the structure. The flexibility of the method and the correctness of the solution will be demonstrated by realistic examples.

Practical realization of gain-switched Dy3+-doped ZBLAN fiber laser operating at 2.94 µm is reported. The laser is pumped by a 1.1 µm Q-switched ytterbium (III) fiber laser, which was constructed in-house. The ZBLAN fiber laser generates a stable pulse train with repetition rates spanning the range from 25 to 100 kHz. At the repetition rate of 50 kHz, the pulse width is 183 ns while energy and peak power are 0.72 µJ and 4 W, respectively.

In this contribution a discussion of design and practical realization of dysprosium ion doped, and erbium ion doped, fluoride glass fiber lasers is given. The specific operating wavelength range that is covered corresponds is near 3000 nm. The discussion is centered around the pulsed Q-switched operation of the fiber laser.

Mid-infrared (MIR) spectroscopy and imaging underpin many applications, ranging from industrial process monitoring to environmental analysis, and offer a new era in healthcare and clinical diagnostics. The spectral region from 1.5 to $12\ \mu \mathrm{m}$ is of interest since it contains strong characteristic absorption peaks associated with the vibrational transitions of many important molecules. A principal requirement for rapid measurement is bright, fibre-based, sources that, if possible, should also be robust and low cost. This paper describes our work to use sources based on the photoluminescence from Pr3+ and Dy3+ doped GeAsGaSe fibres to provide affordable carbon dioxide (CO 2 ) sensing in heritage buildings. Limitations to the Beer Lambert law, previously reported by others, were observed in the sensing system, and the sensors were consequently calibrated empirically. Simulation results show that the use of commercial filters provides a promising approach to realising more compact, real-time, sensing.

This paper investigates the impact of lightning segmented diverter strips on antenna performance. A fully coupled electromagnetic model is considered, where the antenna is enclosed by a realistic dielectric radome profile on which segmented diverter strips are placed. The geometric model of a radome with lightning diverter strips is generated by using a computer graphics method for seamlessly morphing two surfaces together. The antenna performance is characterised by both the S11 parameter and the far field profile.

We investigate the effects of switching the state of a PIN diode from insulating to conducting as a component within a Solid-State Receiver Protection (SSRP) system. The investigation follows a component adding process, ultimately arriving at a full configuration of a metallic post insulated from the waveguide by two dielectric blocks in the Off-state, which is then connected at one end in the On-state. The effects of the diameter of the post are also studied. The S-parameters of the system are calculated from the voltage and current transmission line observations sampled directly with a single time-domain numerical method based upon an unstructured mesh.

In this contribution it is shown that broadband emission stretching from 2000 nm up to 6000 nm can be achieved when using selenide-chalcogenide glass. It is also shown experimentally that by appropriate selection of pumping wavelengths the shape of the output spectrum can be varied to suit a particular application.

This paper presents two methods for the segmentation of medical images based on evolutionary algorithms. Such image segmentation is a useful first step in the efficient and speedy analysis of medical images and provides a basis for the next stages of the image analysis process. In particular, the processing of images obtained from the inside of the human body allows for more effective diagnostics of lesions of human tissues. The first approach developed is based on evolutionary algorithms which divide the image into a given number of areas based on the intensity of RGB colours. The second segmentation method is based on the detection of edges in an image. An evolutionary algorithm was also used for this segmentation, based on the search for threshold values in the grey level in the image. We also compare the above segmentation methods with the numerical k-means segmentation method.

Due to lattice disorder, glassy semiconductors have energy levels in their bandgaps (gap states) that can be populated by sub-gap illumination or due to recombination of photo-excited carriers. The non-linear optical (NLO) response induced by high-intensity femtosecond (fs) laser pulses develops together with the gap states excitation. These effects in non-crystalline semiconductors have not yet been well studied. In this work, we use two realizations of the pump-probe method [1,2] to study the NLO response of chalcogenide glassy semiconductors of the systems As 40 S x Se 1-x and As 40 Se x Te 1-x with fs resolution in time upon illumination by fs laser pulses with the peak wavelengths λ p of 0.79 and 1.57 μm. By the partial replacement of S by Se in the As 40 S x Se 1-x system and of Se by Te in the As 40 Se x Te 1-x system, the bandgap energy Eg was varied so that the ratio R= hv/E g of the photon energy hv to the bandgap energy Eg was tailored in the range between 0.3 and 0.9. Magnitudes of the NLO coefficients of refraction n 2 and absorption β 2 have been obtained from measurements at each λ p with thin glass samples (thickness ∼1 mm) of each composition. Comparison of the data with the dispersion function G 2 (R) derived in the theory of NLO response of direct-gap crystalline semiconductors [3] (Fig.1a) allowed us to reveal that in the range 0.5 2 for a glass composition decreases with R in a similar way but it is always positive-valued. By solution of kinetic equations for densities of the photo-excited charge carriers, the dielectric constant e was evaluated by using the Drude-Lorentz model. We found that dynamics of the NLO response develop differently with or without a single-photon excitation of gap states [4,5]. In the range 0.3 2 increases with R (Fig.la). Here in the samples with R > 0.38, the two-photon excitation of gap states is possible; this is confirmed by Fig. 1b where the time-resolved NLO response of the samples is compared with the typical trace (inset) obtained in [2] for two-beam coupling signal from fused silica at the wavelength of 1.22 μm (R

Lightning strike protection is critical for aircraft made from carbon fiber composites that are less conductive than metals. In areas prone to initial lightning attachment, such as the wingtips and the nose radome, additional protection in the form of solid metallic or segmented diverter strips is typically required. However, generating a realistic model geometry of a wingtip or a radome nose with full lightning protection is a significant task in its own right as the diverter strips and the simulation mesh need to be aligned onto a non-developable surface. The paper explores the use of geometric techniques for the spatial manipulation of three-dimensional objects in order to generate realistic aircraft nose radome models incorporating lightning protection features. These models are then used to explore the effectiveness of a lightning protection scheme when high voltage is directly injected into the radome nose.

In this paper the Unstructured Transmission Line Modelling (UTLM) method is used to model wave propagation along single wires, T -junctions and crossed-junction wiring structures. The accuracy of the direct meshing of wires is achieved by introducing mesh refinement around the wire. The paper shows the computationally efficient capability of the UTLM method for modelling electrically thin wiring structures and junctions.

The paper investigates the use of a space deformation technique for the 3D manipulation of textile antenna deformations. The results obtained using Green Coordinates for space deformations are compared against published results for antennas bent over a cylindrical surface. The method is also applied to model crumpling and twisting of textile antennas.

The paper studies the spectral properties of Parity-Time (PT) symmetric photonic structures based on circular microresonators. A semi-analytical model based on solving the Boundary Integral Equation (BIE) is used to model coupled resonators satisfying the PT-symmetric condition. Unique properties such as asymmetric transmission and a localised defect state are observed. A potential benefit of these properties is the generation of a single frequency laser source for spectroscopy or telecommunication applications.

The RWG basis widely used in the discretisation of the EFIE is traditionally defined over interior edges of the structure's mesh when modelling charge-neutral scattering problems. In this paper, a careful extension to the RWG function space that allows for flow of current in and out of a structure via its boundary edges while maintaining current conservation is constructed. This space is appropriate for the computation of capacitance, where (i) a constant voltage difference is maintained between two components making up the device, and (ii) the total current is conserved. Using this basis implicitly enforces a constant potential difference between the metal plates (cf. natural boundary conditions), and explicitly places a restriction on the current (cf. essential boundary conditions). It improves upon the classic delta gap excitation of capacitive structures in that it is asymptotically independent of the triangulation used to describe the device's geometry.

The established time domain Transmission-Line Modelling (TLM) numerical method is extended to include models of (i) saturable dispersive gain and (ii) non-linear dielectric material. The method is used to simulate a photonic memory device based on a nonlinear Parity-Time (PT) structure.

Two time-resolved experimental methods have been used for characterization of the non-linear optical response of chalcogenide glasses of the system As-S-Se-Te in IR and THz ranges upon excitation by femtosecond laser pulses at 800 nm wavelength. Photoinduced conductivity and refractivity were studied by using a rate equation model.

A ray dynamical approach is developed for the study of large-core asymmetric step index fibres (SIF), especially those made from chalcogenide glasses (ChGs) which can exhibit very high refractive index, large numerical aperture, and which are transparent at mid-infrared wavelengths. The model allows for deformations of the SIF away from concentric circular structures, and for the light rays captured by the fibre to behave chaotically within the asymmetric boundaries of the fibre. Chaotic and periodic rays can be classified by the Poincare surface of sections (SOSs). In the model, the ray dynamics in the SIF are approximated by dividing the SOSs into pixels; the construction of a transfer matrix stores all the mapping probabilities. The light intensity distribution in the SOSs is efficiently propagated using the constructed transfer matrix, providing a viable alternative to propagating all the rays in the SIF by brute force ray tracing. The model enables the rapid calculation of the power accumulated in the fibre core following an arbitrary excitation.

Broadband terahertz time domain spectroscopy (THz-TDS) and time resolved terahertz spectroscopy (TRTS) were performed on a 54 μm thick chalcogenide glass (As 30 Se 30 Te 40 ) sample using a two-color laser induced air plasma THz system in transmission and reflection configurations, respectively. Two absorption modes were observed at 2–3 THz and 5–8 THz. The photo-induced conductivity can be well described by the Drude-Smith conductivity model, which shows significant carrier localization effects. A fast refractive index change was observed 100 fs before the conductivity reached its maximum with two orders of magnitude larger amplitude than expected for the optically induced THz Kerr effect, indicating that the free carriers are responsible for the transient index change.

In this contribution a multiwavelength erbium ring laser is presented. The multiwavelength operation isachievedby using a novel in-line Mach-Zehnder (MZ) fibre interferometer based on multicore microstructured fibre (MC-MOF).The MZ interferometer acts as a periodic spectral filter. The modal properties of the MC-MOF are analysed using the finite difference method. Dual wavelength operation is demonstrated with an optical signal-to-noise ratio (OSNR) of 35 dB and a peak separation of 2 nm.

The mid-infrared (MIR) spans the 3 – 25 µm wavelength range. Rare-earth-ion doped selenide-chalcogenide glasses are being developed for direct-emission MIR fibre lasers. The true Pr3+ absorption cross-section in the 3.5 – 6 µm wavelength region of a Pr3+-doped (500 ppmw of Pr3+ i.e. 9.47×1019 Pr3+ ions cm−3) GeAsGaSe host-glass is presented, after numerically removing the underlying, extrinsic vibrational absorption due to [H-Se-] contamination of the host-glass.

We discuss the numerical modelling of lanthanide-ion doped chalcogenide glass fibre lasers for operation in the mid-infrared wavelength region. We extract the modelling parameters from emission and absorption measurements using Judd-Ofelt and McCumber theory. Numerical algorithms are developed based on the experimentally extracted fibre parameters. The simulation results predict lasing with slope efficiency of at least 20 % provided, that the fibre loss can be kept at the level of 1 dB/m or less.

Much effort has been devoted to the study of glasses that contain the chalcogen elements (sulfur, selenium and tellurium) for photonics' applications out to MIR wavelengths. In this paper we describe some techniques for determining the refractive index dispersion characteristics of these glasses. Knowledge of material dispersion is critical in delivering step-index fibres including with high numerical aperture for mid-infrared supercontinuum generation.

An interferometric pump-probe method with the femtosecond resolution in time is used to study the third-order non-linear optical response and photosensitivity of a variety of chalcogenide glasses of the system As-S-Se.

This paper analyses the eigenfrequencies of an infinitely long chain of microresonators with PT symmetry. The results show that the depth of modulation of the real part of the refractive index can be used to control the threshold point of the PT structure.

This paper presents a coupled electromagnetic-thermal model for modelling arc discharge typical of lightning strike phenomena. The model is implemented in the Transmission Line Modelling (TLM) method and tested on a canonical case where a plasma conducting channel is established between two metal electrodes.

In this paper, the unstructured transmission line modelling (UTLM) method is applied to solve the canonical wire coupling problem by explicitly meshing wires. The nearend crosstalk of two single-core copper wires above a ground plane is evaluated using both UTLM method and experiment. The results obtained show very good agreement, which validates the viability of direct-wire modelling in the UTLM method.

We consider the modelling and design of high power ZBLAN glass based erbium (III) doped fibre lasers operating in the Q-switched regime. The detailed description of all models used is given. The results are calculated for a standard set of modelling parameters used in the literature, which correspond to a commercially available erbium doped ZBLAN fibre. We apply a time domain method which uses a steady state model to obtain the approximation of the initial conditions and study the dependence of the pulse shape on laser operating parameters.

We present a study of mid-infrared photoluminescence emitted from Tb3+:GeAsGaSe chalcogenide bulk glass and fibre. Absorption and emission cross-sections were calculated based on measured results. The decay characteristic related to the potential laser transition 7F 5 → 7F 6 has been measured and investigated. The observed high values of emission and absorption cross-sections and long lifetime from the level 7F 5 → 7F 6 indicate that this glass is a good candidate for future development of a mid-infrared fibre laser.

Electromagnetics education constitutes core knowledge in electrical and/or electronics (EE) engineering courses across the globe. Although there exists a global general consensus pertaining to the content of elementary electromagnetics, the methods of delivery differ from institution to institution. The current approaches to teaching elementary electromagnetics at the undergraduate level range from assessing students' performance purely from examinations, such as midterm tests and a final exam, to evaluating students' understanding by incorporating assignment questions and coursework. In this paper, the module H63FWA Fields Waves and Antennas, which is being taught at The University of Nottingham, is taken as an example where coursework that contains a research element is introduced into a previously 100% exam-based module.

In this paper, a novel compact 2D photonic crystal bandpass resonant filter is proposed and designed on the basis of coupling matrix theory between the waveguide and point defect resonators with perturbing dielectric posts. The coupling matrix concept offers an accurate method for a direct synthesis of narrow-band filters and provides initial estimate of structure parameters for optimization design of PhC bandpass filters. By fine adjusting the structure parameters such as the radius of the coupling rod, dielectric rod, and perturbing rod, the filter characteristics are studied by using the finite difference time domain method.

Advances in small scale fabrication processes have led to the advent of very thin flexible devices such as flexible RFID tags and smart clothing. In a geometrical sense, these present themselves as curved 2D open surfaces embedded in a 3D domain. When simulating EM behavior on these surfaces at low frequencies, a full 3D field model can become computationally expensive in terms of memory and run times. The objective of this paper is to present a method for applying a 2D unstructured Transmission Line Method (TLM) simulation to open, curved surfaces embedded in a 3D domain, by providing a one-to-one mapping of the geometry to a 2D flat plane The simulation results are then mapped back to the original 3D geometry, negating the need for a full 3D simulation. Further, we demonstrate that if the surface material parameters are altered in the vicinity of high curvature, the proposed method is still effective

In this paper the dispersion characteristics of the two-dimensional TLM method, based upon an unstructured triangular mesh, are investigated and compared against those of the TLM method based on structured rectangular mesh. This is done numerically on an example of plane wave propagation in an homogeneous free space. Phase errors of plane wave propagation are extracted for a variety of triangular meshes and compared to an equivalent conventional TLM mesh. The computational resources needed for TLM method based on triangular and rectangular grid to obtain the same numerical dispersion are also compared.

We discuss the modelling and design of mid-infrared fibre lasers for realisation in a chalcogenide glass host doped with lanthanide ions. Three lanthanide dopant ions are considered: terbium, dysprosium and praseodymium. The laser configuration exploits the advantages of a cascade lasing scheme, which allows for a reduction in both the side effects of the long lifetime of the lower lying state and heat generation within the laser structure.

Lanthanide ion doped glass materials find many applications in photonics. In recent years a particular attention has been drawn to lanthanide ion doped low phonon energy glasses and their potential application as amplifiers and lasers delivering mid-infrared coherent light. We present a comprehensive study of the photoluminescence measured from chalcogenide glass samples doped with praseodymium ions. The study includes Judd-Ofelt analysis and simulations performed consistently on the basis of the rate equations approach. The results allow for estimating the actual efficiency of the particular radiative transitions and also for extracting the parameters characterising the non-radiative processes.

In this contribution we present the sensing capabilities of a novel N-path Mach-Zehnder interferometer (MZI) that relies on a multicore microstructured optical fibre (MC-MOF) connected between two sections of the standard single mode fibre. The modal properties of the MC-MOF structure are analyzed experimentally by measuring near field profiles. The dependence of the N-path MC-MOF MZI sensitivity on temperature, tensile strain and bending is investigated. The results suggest that such interferometer is a good candidate for a tensile strain or bending sensor.

Chalcogenide glasses are promising materials for mid-infrared (MIR) fibre optic applications i.e. 3-25 μm wavelength. Their low phonon energy is attractive not only for passive mid-IR transmission fibre-conduits but also for active fibre. For the latter, rare earth (RE-)-ion doped chalcogenide glasses have been investigated, as direct emission mid-IR fibre lasers. The quality of the chalcogenide glass fibre is key to this application. RE-ion addition to the chalcogenide glasses can lead to devitrification. Gallium has been identified as a useful additive to help successfully solubilise the RE-ions and it is thought that gallium complexes the RE-ions in the glass matrix. We investigate here replacing gallium with indium. Indium shares Group 13 of the New Periodic Table with gallium and so exhibits similar chemical behaviour. Indium is heavier than gallium, potentially promoting lower phonon energy locally at the RE-ion. We have found enhanced fluorescence behaviour of the RE-ion in bulk glasses formulated with indium rather than gallium. Here, drawing of step index fibre (SIF) based on ~ 500 ppmw (parts per million by weight) Pr 3+ -doped Ge-As-(In or Ga)-Se chalcogenide glasses is successfully carried out. MIR fibre loss of the Pr 3+ -doped SIFs is measured as well as fibre photoluminescence in the range 3-6 μm.

Scattering of a normally incident plane wave by a dielectric slab with air-filled periodic inclusions of circular cylinder form is investigated. Comparison with the related homogeneous slab problem shows the presence of modes associated with the periodic inclusions. The corresponding resonances are sharp. Lasing threshold and the real-valued lasing frequency are then sought by introducing a negative imaginary part into the slab refractive index to develop a laser eigenvalue problem (LEP). The dependencies of lasing threshold and frequency on geometrical parameters are found, and indicate that the periodic modes have high lasing efficiency.

In this paper a perturbation method for relaxing the minimum link length in the Transmission Line Method (TLM) mesh is presented. The method is assessed in terms of accuracy and computational time on two canonical examples of PEC cylindrical cavity.

An infinite grating of identical dielectric or metal cylinders or strips illuminated by a plane wave has high-quality optical resonances near the Rayleigh anomalies [1-16]. These resonances are present for any periodic 2-D structures and have striking properties as their Q-factors increase if the grating period becomes larger. Besides, in such a resonance the domain of high-intensity scattered field occupies a very wide domain along the grating. In this paper we investigate a configuration where we have a dielectric slab and an infinite grating of air holes inside it. This structure is closer to practical applications in nano and micro-optics and our study shows the presence of the grating resonances having complicated behaviour.

The results of the numerical modeling of the transmittance of an all-solid photonic band gap fiber are presented. For the modeling, a vectorial “wide-angle” Fourier transform based beam propagation method is applied. The actual fiber structure, having morphological deformations of a hexagonal lattice, is considered. The coexistence of high-order total internal reflection modes of high-index rods and the fundamental photonic band gap mode is demonstrated.

This paper presents a new discretisation of Cartesian meshes based on spatial anti-aliased filtering of geometry boundaries in TLM. The method is compared against some other techniques presently used to alleviate stair casing error when representing circular boundaries. The antialiasing method provides the best convergence to actual resonances for E-waves amongst the techniques studied. For H-waves, the new method produces uniform convergence, but an anisotropic sub-pixel smoothing produces smaller error for finer meshes.

In this paper, the temporal dynamics of the non-linear response of chalcogenide glass samples of the system As-S-Se have been studied near the bandgap frequencies of the samples by using the interferometric pump-probe method. The pumping laser frequency was located in the spectral range of the Urbach tail or weak-absorption tail at the edge of the fundamental absorption band (so-called Tauc edge), depending on the glass composition. Peculiarities of the non-linear response of each glass composition depend on the relation of cross-sections of the two- and single- photon absorption at the 790 nm wavelength of the pumping laser.

Dielectric resonators are used in a variety of application, including filters, oscillators, frequencies meters and tuned amplifiers. To date, a range of approximate semi-analytical methods have been used to model microwave dielectric resonators, namely magnetic wall method [1], variational method [2], and various integral equation methods [3,4]. Of special interest are the contour IE methods that use the Muller IEs and the Method of Analytical Regularisation (MAR) [5,6] to convert the original equations set to the matrix equations with more favourable features, namely to the Fredholm type equations of the second kind.conditions imposed on the disk median section [7]. Our main objective is to extend the Muller IEs to the full 3D case without making any approximating assumptions. In this paper we present a valuable intermediate step, namely method based on the combination of Muller IE and the Body of Revolution (BOR) approach [4]. The BOR method is IE based method that is applicable to bodies that possess axial (rotational) symmetry and can thus be obtained by rotating a so-called generic arc around the axis of symmetry.

In this paper, we discuss challenges in modelling circular geometries in Cartesian time domain numerical methods. The main focus of this paper is to reduce the apparent shift in the deduced resonance frequencies. We show that this shift is completely dependent on the discretisation technique and hence propose a resolution to minimise it. In addition, we discuss the limitations of extracting resonances from the time domain signal obtained from such a numerical model using a FFT and compare complex frequency extraction techniques. We show that, despite its high computational complexity, a modified difference Prony method provides best accuracy. We use this method to show that our proposed method of discretising to maintain resonator area is uniformly convergent with mesh refinement.

To study the electromagnetic wave scattering by a thin dielectric disk covered by graphene it is necessary to couple the Maxwell boundary value problem for a thin disk with a phenomenological model of graphene conductivity. One of the main challenges of such an approach is to involve the near-zero thickness of the graphene cover into the model. Another difficulty is to find a numerically effective and stable method to solve this boundary problem. We propose to couple the equivalent resistive boundary conditions as a model of each graphene cover with two-side generalized boundary conditions (GBS) as a model of a thin dielectric disk and then obtain effective GBS for a sandwich-like disk. To reduce the problem to a matrix equation we follow the method of analytical regularization developed for a thin dielectric disk scattering problem.

In this article a novel highly-birefringent erbium-doped microstructured optical fiber is presented. The modal properties of the structure are analyzed numerically using a fully vectorial finite difference method. The emission properties of the structure have also been investigated. The emission at 1550 nm has been measured. The realised fiber can find potential applications in metrology or telecommunication optical systems.

The electromagnetic properties of a silver nanowire grating embedded into a dielectric slab are investigated, using a Maxwellian formalism, to determine and distinguish effects caused by the structure periodicity and by the presence of silver wires which entail plasmon resonances. The interaction of these two effects gives an opportunity to control the qualitative characteristics of the plasmon resonance while the variation of the slab refractive index allows managing its wavelength.

The excitation of surface-plasmon resonances on a graphene disk is studied numerically as a 3D electromagnetic wave-scattering problem. Mathematical model for the scattering of a horizontal magnetic dipole field by a graphene disk is based on the Maxwell equations with the electrically-resistive boundary conditions on the disk surface, where graphene electron conductivity is included as a parameter and determined from the Kubo formalism. It is shown that plasmon resonance frequencies in the terahertz range shift with variation of the chemical magnetic potential of the graphene. Far field radiation patterns are plotted in the most intensive resonances.