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.

The temperature on the busbars of electric generators in the end-winding region needs to be monitored continuously. High temperature is associated with curvatures, seams, and poor ventilation. However, this solution has limitations, such as the number of measurement points. This article presents a reconstruction algorithm to estimate high-resolution signals, from readings of a distributed temperature system for monitoring high-power generator bars. The purpose of this work is to measure the temperature in the end-winding region with optical sensors based on Raman distributed temperature sensing (RDTS). The ability of RDTS equipment to perform a measurement is limited by its spatial resolution of 1 m. The proposed algorithm reduces spatial resolution through signal processing without changing the equipment hardware. Through the experimental results, it was possible to perform accurate temperature measurements for lengths up to 15 cm using the sensor element. With this method, the final winding of a generator bar of a 355 MW hydroelectric plant was instrumented in the laboratory. The results show that it is possible to monitor the temperature over this region, which has a length of 45 cm. Therefore, the proposed method has the potential for the use of RDTS systems for temperature acquisition of small, medium, and large electrical machines.

This study presents a theoretical model for polarisation manipulation using electronic polarisation controllers (EPCs) based on fibre squeezing. A method to calculate the EPC configuration in order to transform between two arbitrary states of polarisation (SOP) is presented. After, a technique to deterministically generate four SOPs for use in polarisation-encoded quantum communication systems is proposed. Moreover, the effectiveness of the proposed technique is experimentally assessed through the generation of two pairs of orthogonal SOPs. The experimental implementation used an field programmable gate array (FPGA) board to electrically control the four waveplates of the EPC, reaching a rate of 500 qubit/s. Results show that this polarisation generation process is intrinsically stable, demonstrating its potential for practical implementations of polarisation-encoding quantum key distribution systems using the BB84 and B92 protocols.

Commercial light-emitting diodes are a low-cost and energy-efficient solution for the implementation of optical wireless communication, known as visible light communication (VLC). This technology has a huge growing interest, being recently a research spotlight in the scientific community, especially due to the increasing popularity and rapid development of self-sustainable smart houses and the Internet of Things. As the VLC link is free space, big challenges arise in its implementation. To improve the VLC performance, this work proposes an enhanced system solution integrating an optical amplifier. In this context, organic–inorganic hybrids incorporating a blue-emitting conjugated polymer with high quantum yield (>50%) were synthesised and processed as planar waveguides. The waveguides were tested in a testbed scenario, showing a signal amplitude improvement of 2.5 dB, establishing the proposed approach as a promising cost-effective solution for optical amplification in VLCs.

Visible light communication (VLC) is a promising candidate for future indoor wireless communication. Light emitting diodes (LEDs) are popular choices as transmitters for VLC, since they are energy efficient and have the ability to provide illumination and data transmission simultaneously. VLC is attractive for reasons such as security, the ability to use a licence free spectrum, and broad bandwidth. Previous review articles on VLC mainly focused on VLC network algorithms, implementation challenges, system design, applications, and physical layer technologies. Unlike the existing reviews, this study presents state-of-the-art VLC systems for high-speed data communication in the gigabit range and the techniques to achieve such high data rates. In addition, concepts such as LED modelling and fabrication process, recent commercial advancements of VLC products, and hybrid/heterogeneous networks to achieve high data rate are highlighted in this study. For this purpose, some key technologies of VLC systems, including channel equalisation from both transmitter and receiver sides, higher-order modulation techniques, wavelength division multiplexing, multiple input multiple output technology, LED modelling, and advanced fabrication processes are discussed. This study also covers recent advancements in commercialisation of VLC technology, and recent progress made by various research groups.

The rapid growth of digital services has led to a widespread deployment of opto-electronics that furnish the Internet as an efficient communication backbone. The electroabsorption-modulated laser (EML) is a representative example of a monolithic integrated electro-optic converter that has early become a commodity: it has been widely adopted in telecommunication networks in virtue of its cost- and energy-efficient light generation and modulation. This study reviews the state-of-the-art of EML applications. Despite its simplicity, the EML addresses numerous use cases that require either the transmission or the reception of optical signals, such as equaliser-free high-bandwidth intensity modulation/direct-detection links at low signal drive, analogue signal transmission with high signal integrity, spectral sculpting for dispersion-tolerant transmission and vector modulation. Full-duplex transceiver functionality in lieu of a pair of dedicated half-duplex sub-systems is eventually attained by combining transmission and reception. This strategy of significantly reducing the cost for a bidirectional communication engine will be discussed for coherent digital data and analogue radio-over-fibre transmission and optical ranging. The maturity of EMLs as coherent transceivers will be evidenced by a small penalty for realising full-duplex transmission and the accomplishment of homodyne detection, which obviates digital signal processing for the purpose of signal recovery.

In this study, the authors present a single-band tunable metamaterial absorber in the 3.00–6.00 THz region. This absorption peak (90.00% amplitude) is excited by the cavity resonance at the resonance frequency of 4.55 THz. The cavity resonance mode is achieved in the spacer cavity formed by the metal/strontium titanate (STO)/SU-8 photoresist (SU-8)/STO/metal layers. The effect of structural parameters on this absorption peak is measured in two groups of experiments. As the diameter of holes array D increases, the cavity resonance peak is enhanced to 94.00% and shifted to 4.79 THz. However, for the increase in the thickness of the SU-8 layer T3, this peak is increased to 98.00% and moved to 3.88 THz. To verify the dual detection functions, two sets of experiments are carried out. In the third set of experiments, this absorption peak is increased to 99.80% and shifted to 5.60 THz with the increasing environmental temperature. In the fourth set of experiments, samples are covered by three liquids, and both the absorption amplitude and the resonance frequency are reduced. This tunable metamaterial absorber shows possibilities for applications in environmental temperature sensing and liquid sensing.

This study aims at optimising the graphene coverage in a microresonator for the design of an efficient modulator by using a combination of graphene induced losses and phase shift. The graphene integration in the ring can provide tunability by means of altering the loss and coupling coefficient by changing the graphene coverage or its chemical potential. The fundamental parameters of graphene integrated all-pass ring resonator are extracted based on the transmission characteristics using finite-difference time-domain method. As compared to conventional graphene-oxide-semiconductor straight waveguide, graphene integrated on an all-pass ring resonator shows enhanced optical absorption and phase shift due to field enhancement factor. A modulator based on graphene tunable loss and phase shift is investigated taking into account the field enhancement factor. For 1.55 μm radiation and ring radius of 3.75 μm with 22% graphene coverage, a maximum modulation depth of 12.6 dB is demonstrated based on electrostatic control of graphene chemical potential. The proposed modulator can feature operational speed in the range of 10 GHz that is useful in a variety of electro-optic applications.

An accurate power output prediction of the photovoltaic system is pivotal to eliminate the extra cost and the negative impact in the utility grid integrated with photovoltaic power sources. The power output of a photovoltaic system is predicted by introducing a long short-term memory method. Moreover, the influence of noise data on prediction results is eliminated with the empirical mode decomposition. To further improve the accuracy and stability of the prediction method, the parameters of long short-term memory neural networks are determined with a sine cosine algorithm. The performances of the long short-term memory method in terms of root mean square error, mean absolute error, and coefficient of determination in January and August are analysed, respectively. Compared with other prediction schemes, the long short-term memory method provides superior accuracy for photovoltaic power output prediction.

In this study, the authors have theoretically investigated the effects of metallic nanoparticles (NPs) on the performance of nanowire-based plasmonic photodetectors. For this goal, they propose a novel structure for plasmonic photodetectors in which the metallic NPs are deposited on a semiconductor nanowire sandwiched between two ohmic contacts. To calculate the eigenstates of the system, they have solved the Schrodinger equation for the nanowire in a nanostructure system with metallic NPs. Obtaining the system's eigenstates, the nanowires absorption coefficient, dielectric constant, and refractive index have been calculated. Also, the dependency of these parameters on the input wavelength has discussed. Besides, to find out the impact of metallic NPs, they have calculated the current of the system. They also have made a comparison of the system's responsivity in the simple condition (where there are no NPs effects) and the hybrid condition, in which the effects of NPs are also be considered. In fact, the effects of the NPs in the enhancement of the electrical field, and their role in the scattering of the field are the main fields of studies in this work.

The liquid-infiltrated photonic crystal fibre (LI-PCF) is proposed for guiding terahertz radiation. Geometrical asymmetry is achieved by introducing a large ellipse in the core. By filling the ellipse with liquid cocaine, the optical properties of the photonic crystal fibre (PCF) are theoretically examined using finite element method-based COMSOL multiphysics software. At an operating frequency of 1 THz, the proposed LI-PCF demonstrates a sensitivity of 87.02% and confinement loss in the order of 10⁻⁴ cm⁻¹. The PCF also demonstrates extremely low effective material loss <0.01 cm⁻¹, a birefringence of 0.018, large effective mode area of 1.11 × 10⁵ µm², a high numerical aperture of 0.45 and near-zero ultra-flattened chromatic dispersion of 1.4351 ± 0.5883 ps/THz/cm. The design simplicity and high sensitivity, strong confinement factor, low material losses and high birefringence of the fibre suggest that the proposed fibre may be convenient for PCF-based cocaine sensing, for application in the security and defence industries.

In this work, a self-oscillating parametric optical frequency comb generator is proposed and experimentally demonstrated. It consists of an electroabsorption modulated laser (EML) based dual-loop optoelectronic oscillation loop and a parametric frequency mixer. In the experiments, a 20 GHz radio-frequency signal is produced to drive the EML. The output pulses from the EML are amplified to work as pump seeds for the frequency mixer. In the mixer, a length of high non-linear fibre (HNLF) is firstly used to introduce chirps to the pump pulses, a section of single mode fibre is then employed for pulse compression, and finally another HNLF is applied for parametric frequency mixing. When the pump power is 28 dBm, a nine-line comb with a frequency spacing of 20 GHz and an optical signal-to-noise ratio >15 dB is generated. More comb lines can be obtained when another frequency mixer is cascaded.

This study investigates the performance of a decode-and-forward-relay-based hybrid free space optical (FSO)/radio frequency (RF) system with selection combining. The proposed hybrid system considers an FSO sub-system as a default transmission option and RF sub-system is utilised when FSO is in outage. For the proposed system, the outage threshold signal-to-noise ratio (SNR) is an important parameter and the optimum value for the same has been calculated to achieve the target average symbol error rate (SER) and high capacity. The presented model considers the effect of transmit beam waist, detector size, and jitter variance explicitly for the FSO link. Transmit beam waist and detector size are optimised to minimise the average SER considering various weather conditions and link distances. A unified performance analysis is addressed and exact expressions for outage probability and average SER are obtained. Simple asymptotic expressions are also derived for high SNR to obtain the diversity order. Large coding gains are observed for optimum beam waist compared to any arbitrary beam waist for same average SER. Significant performance improvement is also observed for the proposed system compared to the individual FSO and RF systems. The theoretical and asymptotic analyses are verified by Monte-Carlo simulations.

Amplify-and-forward cooperative underwater optical wireless communication employing optical improved quadrature spatial modulation (OIQSM) is proposed in this study. It uses multiple signal constellations, thereby improving the system performance. It uses multiple lasers but it activates only a few of them depending on the message bits and can transmit double modulated symbols in a single symbol interval. The OIQSM system performance is compared in terms of bit error rate (BER) with other existing schemes in the literature. The analysis is conducted for weak oceanic turbulence conditions, which is generally prevalent in deep oceans. The cost and power consumption analysis of OIQSM is performed to highlight its benefits over other schemes. It is observed that OIQSM yields a signal-to-noise ratio gain of at least 5.5 dB over the conventional optical spatial modulation scheme for any BER value between and .

In recent times, several applications requiring highly accurate indoor positioning systems have been developed. Since the global positioning system is unavailable/less accurate in the indoor environment, alternative techniques such as visible light positioning (VLP) are considered. The VLP system benefits from the wide availability of illumination infrastructure, energy efficiency and the absence of electromagnetic interference. However, there is a limited number of studies on three-dimensional (3D) VLP and the effect of multipath propagation on the accuracy of the 3D VLP. This study proposes a supervised artificial neural network to provide accurate 3D VLP whilst considering multipath propagation using receiver diversity. The results show that the proposed system can accurately estimate the 3D position with an average root mean square (RMS) error of 0.0198 and 0.021 m for line-of-sight (LOS) and non-LOS link, respectively. For 2D localisation, the average RMS errors are 0.0103 and 0.0133 m, respectively.

A novel C + L band hybrid erbium-doped fibre amplifier (EDFA) using a two-stage configuration was proposed and demonstrated experimentally. The amplifier is composed of a 0.5-m long hafnium bismuth erbium co-doped fibre (EDF) to provide gain within the C-band and a 4-m long zirconia-based EDF to provide gain within the L-band. The proposed amplifier was examined based on the multi-wavelength input source. A parallel pumping distribution technique was used to mitigate the amplifier complexity. The C + L band amplifier achieved a gain flattening of over 55 nm bandwidth for the three levels of the input powers. A gain-flattening of roughly 10.9, 15.5, and 19.2 dB were obtained, respectively, for the input signal powers of −5, −10, and −15 dBm. An average noise figure of 6.4, 5.4, and 4.7 dB was achieved, respectively, for the input signal powers of −5, −10, and −15 dBm.