ABSTRACT We evaluate a hybrid visible light communication (VLC) downlink and beam‐steered infrared (IR) uplink in a 5 × 5 × 3 m indoor room under line‐of‐sight (LOS), non‐line‐of‐sight (NLOS) and user‐mobility scenarios. A MATLAB‐based framework is used to analyse bit error rate (BER), signal‐to‐noise ratio (SNR), spectral efficiency, received power and delay metrics. The evaluation follows classical indoor optical wireless channel models with explicitly parameterised noise, reflection and mobility assumptions to ensure reproducibility. With Turbo coding, the BER falls below once the electrical SNR exceeds 10 dB, following the theoretical BPSK–AWGN waterfall trend whilst exhibiting a clear coding gain relative to the uncoded BPSK benchmark under both LOS and NLOS conditions. VLC achieves high SNR and spectral efficiency near the room centre but degrades towards boundaries due to multipath and shadowing. The IR uplink with adaptive beam steering provides more uniform coverage and robustness in mobility and NLOS conditions. A qualitative comparison with RF systems highlights differences in spectrum availability, interference characteristics and spatial confinement. These findings confirm the feasibility of hybrid VLC–IR systems for smart‐building and indoor IoT scenarios. Future extensions will explore machine‐learning‐based steering and integration with IEEE 802.11bb standards to support large‐scale deployment.
ABSTRACT The continuous increase of data traffic demands necessitates evolution of optical communication systems towards larger capacities enabled by highly performing optical transmitters that generate multilevel and multidimensional ultrahigh data rate signals. Commercially deployed transmitters rely on high‐order electronic digital‐to‐analog converters (eDACs) that drive optical Mach–Zehnder Modulators (MZMs) under particular biasing/driving conditions. However, these conventional transmitters face bandwidth and power consumption limitations arising from the eDACs and the nonlinearity of MZMs transfer function among others. Recently, alternative transmitter architectures have been proposed to perform direct digital to optical conversion, by relying on the utilisation of lower‐order eDACs/drivers in combination with an increased number of MZMs operated with different biasing/driving conditions compared to the conventional transmitters, which are configured in either serial cascade or parallel stacking configurations. These novel transmitter architectures are referred as serial or parallel ‘Optical DACs’ (oDACs) enabling ultrahigh data rate and improved signal quality. In this article, we first discuss the issues of conventional optical transmitters and subsequently analyse selected oDAC architectures that solely rely on simple NRZ drivers or PAM4 eDACs. Simulation studies demonstrate their superior bit rate scalability (up to 3.2 Tbps) and analysis of power consumption reveals savings of about 40% comparing with conventional architectures.
ABSTRACT Free‐space optical (FSO) communication is rapidly advancing as a high‐capacity wireless technology, with growing interest in extending its operation from the traditional near‐infrared to the mid‐infrared (mid‐IR) spectral region. Particularly, the mid‐wave infrared (MWIR) and long‐wave infrared (LWIR) atmospheric transmission windows in the mid‐IR region provide advantages regarding reduced turbulence effects, scattering and absorption, enabling more robust links under adverse conditions. This review presents recent progress in MWIR and LWIR FSO communication systems, highlighting advances in transmitter and receiver technologies, such as quantum cascade lasers (QCLs), lithium niobate modulators and detectors, including quantum cascade detectors (QCDs), mercury cadmium telluride (HgCdTe and MCT) photodiodes and quantum‐well infrared photodetectors (QWIPs). Experimental demonstrations achieving multigigabit to tens‐of‐gigabit data rates with advanced modulation formats, digital equalisation and coherent detection are summarised. The paper discusses integration trends in MWIR and LWIR photonics, coherent communication strategies and hybrid FSO–radio frequency (RF) architectures. Together, these developments outline the pathway for MWIR and LWIR FSO systems to evolve from laboratory demonstrations towards scalable, practical platforms for next‐generation terrestrial and space communication networks.
ABSTRACT This paper presents a terahertz metamaterial sensor designed to detect changes in ethanol concentration in water, which is highly relevant for chemical analysis, food safety, and biomedical sensing applications. The sensor comprises three distinct layers: a graphene‐based resonator at the top, an intermediate dielectric spacer layer, and a metallic layer at the bottom, forming a compact and efficient absorber structure. The sensor exhibits an absorption of 99% at a resonant frequency of 6.08 THz for 0% ethanol concentration, with a quality (Q) factor of 31.48. Furthermore, the effects of various graphene material parameters, such as chemical potential and relaxation time, on absorption performance have been systematically investigated through numerical simulations to achieve an optimal response and enhanced sensing performance. As the ethanol concentration increases, the resonant frequency gradually shifts to lower values due to the rise in the effective refractive index near the sensing region, enabling reliable refractive index discrimination. The proposed sensor achieves a high figure of merit (FOM) of 12.4, indicating excellent resolution for refractive index detection, along with a sensitivity of 2369 GHz/RIU and a full width at half maximum (FWHM) of 190 GHz. These results demonstrate that the proposed graphene‐based terahertz sensor offers a promising platform for high‐resolution, noncontact liquid sensing in the terahertz frequency regime.
ABSTRACT Localisation in tunnel environments remains a challenging problem due to the limited availability of GPS signals. To address this issue, this study presents a localisation system that exploits the colour temperature characteristics of indoor LED lighting. Unlike previous LED‐based localisation studies that rely on large colour temperature differences and limited experimental areas, the proposed approach intentionally reduces the colour temperature gap between LEDs and expands the experimental environment to better reflect realistic tunnel conditions. The proposed system analyses both chromaticity and frequency components of LED signals to estimate position. Experiments conducted in a controlled environment simulating tunnel conditions demonstrate that the proposed method achieves positioning accuracy within 10 cm over an expanded two‐dimensional area. Specifically, the X ‐axis coordinate is estimated based on LED colour temperature information, whereas the Y ‐axis coordinate is inferred from frequency‐domain features extracted using bandpass filtering and frequency component ratio (FCR). These results indicate that reliable localisation can be achieved using practical LED configurations even in environments where GPS signals are unavailable.
ABSTRACT This study presents a reinforcement learning‐based power allocation framework for free‐space optical (FSO) nonorthogonal multiple access (NOMA) systems under moderate atmospheric turbulence and intelligent jamming attacks. Modelling the base station (BS)–jammer interaction as a dynamic zero‐sum game, we employ Q‐learning (QL) to enable the BS to adaptively optimise user power distribution. The novelty of this framework lies in its model‐free QL formulation that jointly optimises BS power allocation against an adaptive jamming agent in a stochastic FSO‐NOMA environment, incorporating geometric losses, atmospheric attenuation and log‐normal turbulence fading without requiring explicit channel state information, thereby enabling emergent robust anti‐jamming policies validated through hyperparameter‐tuned simulations. Extensive simulations, comparing the proposed approach against Fixed Equal Power, Random and MaxPower baselines, validate the strategy. Results demonstrate that the BS develops robust anti‐jamming policies, achieving a 15% increase in valid data rate (from 6.4 to 7.4 bits/s/Hz), outperforming nonadaptive strategies that fail under intelligent interference. The efficacy of this resilient, adaptive strategy confirms its potential for securing high‐throughput FSO‐NOMA deployments in adversarial environments.
ABSTRACT In this work, we present a machine learning–augmented simulation study of a MoTe 2 ‐based solar cell incorporating Sb 2 S 3 (antimony sulfide) as the hole transport layer (HTL). MoTe 2 was selected as the absorber due to its strong optical absorption, low toxicity, and compatibility with low‐cost fabrication methods. Using SCAPS‐1D, we optimised the heterojunction structure (Al/FTO/CdS/MoTe 2 /Sb 2 S 3 /Pt) by varying absorber and HTL thickness, doping concentration, defect density, and temperature. The best simulated device (MoTe 2 thickness 0.5 μm, N A ≈ 10 17 cm −3 ) achieves V oc ≈ 1.05 V, J sc ≈ 40.8 mA/cm 2 , FF ≈ 87.6%, and a power conversion efficiency (PCE) of 40.33%. We clarify that this extremely high efficiency represents a theoretical upper bound under idealised assumptions (e.g., negligible nonradiative losses), rather than an experimentally demonstrated result. For comparison, the baseline cell without Sb 2 S 3 yields V oc ≈ 0.95 V, J sc ≈ 38.15 mA/cm 2 , FF ≈ 81.09%, and η ≈ 29.35%. The Sb 2 S 3 layer significantly suppresses back‐surface recombination and improves carrier extraction, thereby enhancing V oc and FF. To streamline design, we generated a dataset of approximately 6735 SCAPS simulations spanning key input variables (thickness, doping density, defect density, temperature) and trained five regression models for performance prediction. Among these, Random Forest regression achieved the highest accuracy ( R 2 ≈ 0.98), effectively capturing nonlinear dependencies. Feature‐importance analysis confirmed that absorber thickness, defect density, and doping are the dominant performance drivers, consistent with the physics‐based trends. This hybrid SCAPS–ML framework provides a fast, data‐driven tool for optimising next‐generation solar cells. Our study advances previous work by explicitly identifying robust parameter ranges, introducing predictive modelling, and clarifying the theoretical bounds of simulated efficiency.
ABSTRACT All‐optical wavelength conversion (AOWC) is essential for overcoming wavelength blocking in wavelength‐division multiplexing systems and improving bandwidth utilization. Integrating two‐dimensional materials with optical microfibers presents a promising route to enhance nonlinear performance. Here, we demonstrate an efficient AOWC device based on black phosphorus (BP)‐deposited highly nonlinear microfiber, which operates through the four‐wave mixing (FWM) process. The converter achieves a conversion efficiency of −33.51 dB at 2.05 μm. Systematic characterisation reveals that the efficiency increases by 4.56 dB as the pump power rises from 1.83 to 2.00 mW, whereas it decreases by 16.76 dB when the signal–pump wavelength spacing broadens from 3.5 to 11 nm, consistent with FWM power dependence and phase‐matching constraints. The device also exhibits excellent stability, with power fluctuation within ± 1.08 dB over 140 min. This work provides valuable insights into the nonlinear coupling between 2D materials and microfibers, supporting the development of practical low‐power high‐speed all‐optical signal processing systems.
ABSTRACT In this paper, we report the design and fabrication of a surface plasmon resonance (SPR) sensor for measuring variations in the refractive index of liquid solutions. The sensor employs a simple Kretschmann configuration in which only two layers of chromium and gold with optimised thicknesses were deposited on a prism and the laser beam reflection is evaluated via image processing instead of photodiode. A computer simulation programme was developed to predict resonance curves by analysing the effect of key parameters. To validate the system, calibration experiments were carried out using sugar solutions with concentrations of 0%, 5%, 15% and 20%. The results show that the implemented SPR sensor achieves a sensitivity of 165.16 deg/RIU, an FWHM of 7 deg, a quality factor of 25.6 RIU −1 and a dynamic range of 1.00–1.37 RIU. These findings demonstrate the reproducibility and robust performance of the proposed setup, highlighting its potential as a low‐cost and reliable platform for refractive index sensing in chemical, biological and industrial applications.
ABSTRACT The incorporation of sixth‐generation (6G) technologies into underwater optical wireless communication (UWOC) holds the potential for Gbps‐class maritime connectivity; however, the field is presently affected by significant overestimations in the systematic link budget found in existing literature. Certain modelling discrepancies found in the literature—such as using absorption instead of total beam attenuation, assumptions of coherent detection in IM/DD systems, neglecting the Hermitian symmetry factor in DCO–OFDM throughput and the unexamined application of laboratory‐measured AI gains to analytical budgets—result in range forecasts that surpass physically consistent predictions by two to three times. This study introduces a revised intensity‐modulated direct‐detection (IM/DD) framework for DC‐biased optical OFDM (DCO–OFDM) that corrects for each of these issues. The net throughput is rigorously calculated to be 776 Mbps for QPSK, explicitly considering the 511 usable subcarriers dictated by Hermitian symmetry in a 1024‐point transform. Symbol‐level Monte Carlo simulations across 40.9 million bits per distance point confirm the analytical BER predictions within 0.3 orders of magnitude at error rates relevant to operational use, whereas a controlled experiment with an MLP equaliser shows that, in a known, stationary and linear LTI channel with perfect channel state information (CSI) provided to the minimum mean squared error (MMSE), the gains of +4 to −6 dB that are often associated with deep learning equalisers diminish to zero. This highlights the necessity of channel uncertainty, transmitter nonlinearity and rapid fading as essential conditions for achieving the benefits of AI‐assisted equalisation. The revised framework offers a self‐validated range of 44.8 m (only with MRC diversity) and a projected range of 46 m (assuming literature‐based CSI prediction) at BER in clear ocean conditions. System‐level evaluations of proton‐exchange membrane fuel cells indicate that AI‐driven load smoothing yields minimal efficiency improvements (< 0.2%) at average AUV power levels, confirming that the 1.8× endurance benefit over lithium‐ion batteries is solely due to energy density rather than intelligent power management. By providing fully disclosed parameters and empirical validation for every asserted advantage, this research establishes a scientifically credible benchmark for future calibration of 6G subsea architectures.
ABSTRACT Based on Young's double‐slit experiment, the diffraction of light has been recognised as an exclusive property of the wave nature of light. However, if a mass (e.g., steel bead) is incident with light, three distinct characteristic diffraction patterns (i.e., concentric circles, dots, and sawteeth) are observed in the slits as well as in the mass. This study examined each case of distinct direct/indirect collisions between light and the steel bead, and all cases were geometrically interpreted for intuitive understanding.
Metal oxides have been widely studied in recent years due to their promising applications in catalysis and optoelectronic devices. Among these metal oxides, Indium Oxide (In 2 O 3 ), attracts a special attention exhibiting excellent electrical, optical and catalytic properties. Therefore, the investigation of the chemical composition, electronic structure, and optical properties of this compound is very important to define the defects that can affect its practical application in peculiar areas. Consequently, the properties of the compound can be developed to obtain optimal results. In this context, here we carry out a complex study of physicochemical surface properties of In 2 O 3 using a new approximation for exchange correlation potential denoted nKTB‐mBJ and a tandem consisting of five very powerful experimental techniques. The spectroscopic measurements used in present work involve such ultrahigh vacuum (UHV) precise methods as X‐ray photoelectron spectroscopy (XPES), Auger electron spectroscopy (AES), reflective electron energy loss spectroscopy (REELS), Ultraviolet photoelectron spectroscopy (UPS) and temperature dependent/independent photoluminescence spectroscopy (PLS) possessing high sensitivity while characterising the material surface and interfaces properties. These techniques were used to investigate the surface stoichiometry and valence band structure of In 2 O 3 when being subjected to ion cleaning in UHV conditions. A robust combination between experiments via UPS and theory employing density functional theory DFT lead us to corroborate three well distinguish sub‐peaks located at (4.4, 11.0 and 13.8 eV) and attribute each one to the appropriate orbitals of In and O. Furthermore, photoluminescence spectroscopy (PLS) was also used to carry out room temperature (RT) measurements, as well as those at variable temperatures and high vacuum conditions achieved by using a cryostat coupled with a multistage axial‐flow turbo molecular EdwardnEXT240D pump, to investigate the impact of temperature on the bandgap states of In 2 O 3 . The results present good stability of In 2 O 3 under the ion sputtering process. The PLS spectra reveal the thermal sensitivity of the intrinsic point defect energy levels in the optical band gap of bulk In 2 O 3 and a high emission around 580 nm at RT conditions, indicating that the compound under study is very useful for optoelectronic applications. The chromaticity CIE 1931 diagram shows the temperature effect at the structural defect states and hence the emission of bulk In 2 O 3 . Furthermore, we have employed a number of theoretical approaches to study in detail the structural and electronic configuration of In 2 O 3 . In particular, our data indicate that the calculated energy band gap of 3.30 eV, the intense visible emission at 2.1 eV assigned to oxygen vacancies and the electron effective mass of 0.31 m 0 collectively confirm the n ‐type character of In 2 O 3 that are of high importance for the practical application of this compound.
Transparent perovskite solar cells (T‐PSCs) offer a promising solution for integrating photovoltaic technology into windows and other transparent surfaces without compromising aesthetics. In this study, a periodic oxide–dielectric–oxide (ITO/ZnS/ITO) structure is designed and numerically optimised to enhance optical absorption whilst maintaining high transparency. Using CST Microwave Studio, a unit cell is simulated to analyse absorption, transmission, and reflection spectra and evaluate the optical performance of the proposed structure. The optimised design exhibited an average absorption of approximately 93% with a peak of 98% and an average visible transmission of 92%. Owing to its symmetrical geometry, the proposed structure showed insensitivity to both oblique incidence and polarisation angles. The absorption behaviour has been further verified using multiple reflection theory, which showed good agreement with the simulation results. Additionally, antireflection coatings are incorporated to achieve maximum transmission and thermal stability. Overall, the proposed ITO‐based structure demonstrates effective light trapping and represents a strong candidate for integration into transparent perovskite solar cell applications.
The design of optical elements often requires precise optimisation where metaheuristic algorithms have emerged as a powerful approach. This study aims to optimise a guided-mode resonance reflectance filter chosen due to its extreme parameter sensitivity, which makes it an exemplary platform for evaluating metaheuristic algorithms in photonics. We rigorously compare six established metaheuristic techniques (particle swarm optimisation (PSO), genetic algorithm (GA), shuffled frog leaping algorithm (SFLA), artificial bee colony (ABC), imperialist competitive algorithm (ICA), and differential evolution (DE)) using critical performance metrics. This systematic analysis not only benchmarks algorithm effectiveness for guided-mode resonance filters but also establishes guidelines for optimising parameter-sensitive photonic devices.
In this paper, we put forward an accurate multiple scattering channel model for ultraviolet (UV) signals propagating through the atmosphere. The model incorporates both the altitude-dependent variation of the refractive index structure parameter and the relative height between the UV transmitter and receiver, factors which have often been overlooked in previous studies. To validate the proposed model, we compare it with the established turbulence model through simulations and experiments at transmission distances ranging from 600 to 1000 m. The results demonstrate that the proposed model achieves higher accuracy. Furthermore, we investigate the effects of wind speed, the nominal value of the refractive-index structure parameter at ground level, and the number of photon scattering events on the channel path loss. This work establishes a solid theoretical foundation for the development of UV communication technology.
This paper proposes a hybrid optical modulation scheme that integrates spectrally efficient quadrature amplitude modulation (QAM) and power-efficient multipulse pulse position modulation (MPPM) for underwater wireless optical communication (UWOC) to achieve high spectral and energy efficiencies and combines it with LDPC codes to mitigate the adverse effects imposed by the underwater turbulence channel. Considering the effect of both direct-current (DC) bias level and modulation index on signal-to-noise ratio (SNR), for the first time, an approximate closed-form bit error rate (BER) expression for the hybrid L -QAM-MPPM UWOC system over turbulence channels is derived and verified using Monte Carlo (MC) simulations. Subsequently, comparisons among the hybrid L- QAM-MPPM, conventional MPPM and QAM systems with comparable spectral efficiency for exponentiated Weibull (EW)-modelled turbulence channel indicate that hybrid modulation offers resilience to turbulence, although the BER in the additive white Gaussian noise (AWGN)-only channel is higher. Finally, the effectiveness of LDPC codes in improving the performance of hybrid modulations is discussed, with coded 128-QAM-(12, 2) MPPM offering ∼6.5 and 5.8 dB code gains at a BER of 10 − 4 ${10}^{-4}$ in strong and moderate turbulence, respectively. These results demonstrate the promise of hybrid modulation with LDPC as a robust and efficient modulation scheme for UWOC systems.
This paper proposes a hybrid optical modulation scheme that integrates spectrally efficient quadrature amplitude modulation (QAM) and power‐efficient multipulse pulse position modulation (MPPM) for underwater wireless optical communication (UWOC) to achieve high spectral and energy efficiencies and combines it with LDPC codes to mitigate the adverse effects imposed by the underwater turbulence channel. Considering the effect of both direct‐current (DC) bias level and modulation index on signal‐to‐noise ratio (SNR), for the first time, an approximate closed‐form bit error rate (BER) expression for the hybrid L ‐QAM‐MPPM UWOC system over turbulence channels is derived and verified using Monte Carlo (MC) simulations. Subsequently, comparisons among the hybrid L‐ QAM‐MPPM, conventional MPPM and QAM systems with comparable spectral efficiency for exponentiated Weibull (EW)‐modelled turbulence channel indicate that hybrid modulation offers resilience to turbulence, although the BER in the additive white Gaussian noise (AWGN)‐only channel is higher. Finally, the effectiveness of LDPC codes in improving the performance of hybrid modulations is discussed, with coded 128‐QAM‐(12, 2) MPPM offering ∼6.5 and 5.8 dB code gains at a BER of in strong and moderate turbulence, respectively. These results demonstrate the promise of hybrid modulation with LDPC as a robust and efficient modulation scheme for UWOC systems.
Despite perovskite solar cells (PSCs) having seen significant improvements in efficiency, their path to industrialisation and commercialisation is hindered by considerable challenges. A major concern is the toxicity associated with lead‐based perovskites, which also suffer from rapid degradation. To mitigate these issues, researchers are investigating lead‐free alternatives. In this study, we developed a device structure for a lead‐free PSC utilising Cs 2 TiBr 6 and La 2 NiMnO 6 as absorbers. By modifying the thickness of these layers with distinct bandgap values, we achieved current matching in the design, which was carried out utilising the solar cell capacitance simulator (SCAPS‐1D) with the AM 1.5 G 1 sun light spectrum. The proposed optimised device structure includes layers of FTO, SnO 2 , Cs 2 TiBr 6 , La 2 NiMnO 6 , GO and Au. SCAPS‐1D is also utilised to assess the impact of defect density, different hole transport layers (HTLs) and operating temperature on the performance of the proposed double‐absorber solar cell (DASC). Additionally, it is also examined how the back‐reflective coating and varying solar spectra affect the proposed structure. At 300 K, the optimised design achieved a power conversion efficiency (PCE) of 33.38%, an open circuit voltage ( V oc ) of 0.9158 V, a short‐circuit current density ( J sc ) of 43.47 mA/cm 2 and a fill factor (FF) of 83.85%.
This paper presents the design and analysis of a tunable metamaterial absorber based on the combination pattern of two‐dimensional molybdenum disulfide (MoS 2 ) and vanadium dioxide (VO 2 ) resonator. The design of the absorber structure is simple and includes an array of silicon on which the pattern of MoS 2 and vanadium is placed. By using the ability to adjust the MoS 2 carrier concentration and VO 2 phase transition properties, the absorber has dual control over its optical properties through temperature and voltage settings. This design provides the possibility of dynamic control of the resonance frequencies of the absorber. The absorber structure shows 99.3% absorption at 1375.8 nm and 94.3% at 1550.9 nm. By adjusting the parameters of the absorber structure, the absorption peak at wavelengths 1545.6 and 1743 nm reached 99.4% and 95.4%, respectively. The tunability of MoS 2 and vanadium allows precise tuning of absorption peak locations and makes the structure suitable for applications such as sensors, detectors, optical filters and telecommunication devices that require precise wavelength placement.
