In this study, a vertical coupled single ring resonator add/drop filter is simulated and the effect of different design parameters on the crosstalk suppression bandwidth is numerically investigated. Crosstalk suppression is defined as the difference between drop port and through port spectral responses. In a vertically coupled ring resonator, coupling efficiency between the evanescent tails of modes on the bus and bent waveguides is controlled by vertical separation and lateral deviation between waveguides. Designing a small size (single ring) add/drop filter that provides an increased crosstalk suppression bandwidth (to drop 10 Gbps non-return to zero signals) is proposed. As the analytical calculations cannot produce an accurate prediction of the design parameters, a simulation-driven design optimisation procedure is used to determine the values of the design parameters that produce a 21 GHz crosstalk suppression bandwidth.

In this study, a novel design of porous octagonal photonic crystal fibre with a metal-coated central hole is investigated and analysed in the terahertz (THz) range. The suggested structure is analysed near a frequency of 1.0 THz by using the full vectorial finite difference method. The effects of the structure geometrical parameters and bending radius on the mode effective index, effective mode area and attenuation of the proposed design are investigated. The numerical results reveal that the effective mode area with a metal-coated central hole is smaller than that of porous core without a metal-coated central hole by 80%. In addition, very low attenuation can be obtained because of the porosity inside the core region.

The authors theoretically investigated the cross-gain modulation and the related cross-phase modulation in a travelling wave quantum dot-in-a-well (QDWELL) semiconductor optical amplifier (SOA). They show that the strong enough pumping wave with an optical power in the range of 1 mW up to several milliwatts synchronises the carrier dynamics in the QDWELL SOA, increasing its operation rate to the frequency of 240 Gb/s and decreasing the patterning effect and chirp.

Bulk InGaAs layer under slight tensile strain, embedded in InGaAsP barriers and grown on InP, was used as semiconductor optical amplifier active layer for polarisation insensitive amplification. The material bandstructure was obtained by solving the Lüttinger–Kohn Hamiltonian, including tetragonal strain contribution. Study of the InGaAs material gain was performed by taking into account the effect of k-dependent bandgap shrinkage. The semiconductor optical amplifiers device amplified spontaneous emission and noise figure have been investigated as a function of temperature, carrier density and barrier height. Good agreement was obtained with the trends observed in the experimental characteristics. The authors show that a 100 nm bandwidth can be obtained with a difference between transverse electric and transverse magnetic emission kept constant, as required for polarisation independent amplifiers.

The authors report 1.3-μm InAs/GaAs quantum-dot (QD) lasers monolithically grown on a Si substrate by optimising the dislocation filter layers (DFLs). InAlAs/GaAs strained layer superlattices (SLSs) have been presented as DFLs in this study. A distinct improvement in the InAs/GaAs QDs was observed when using InAlAs/GaAs SLSs because of the effective filtering of threading dislocations. Consequently, a laser with a threshold current density of 194 A/cm² at room temperature and an operating temperature as high as 85°C is successfully demonstrated. These results show the potential for integrating III–V QD materials on a Si platform via InAlAs/GaAs SLSs as DFL.

The effect of post-growth annealing on InAs/GaAs quantum dot solar cells (QDSCs) is studied. A significant improvement in photoemission, photocurrent density and spectral response was observed with post-growth annealing. The optimal anneal temperature was found to be 700°C, which lead to an 18% improvement in current density from 4.9 mA cm⁻² for as-grown sample to 5.8 mA cm⁻². We assign this enhanced performance to the reduced density of inherent point defects that was formed at the quantum dot (QD) and GaAs barrier. Post-growth thermal annealing of QDSCs is demonstrated as a simple route for achieving improved device performance.

The study presents a theoretical investigation of the impact of individual electron and hole dynamics on the photovoltaic characteristics of InAs/GaAs quantum dot solar cells. The analysis is carried out by exploiting a model which includes a detailed description of quantum dots (QD) kinetics within a drift-diffusion formalism. Steady-state and transient simulations show that hole thermal spreading across the closely spaced QD valence band states allows to extract the maximum achievable photocurrent from the QDs; on the other hand, slow hole dynamics turns QDs into efficient traps, impairing the short circuit current despite the extended light harvesting provided by the QDs.

Quantum well laser diodes with low far-field divergence remain a requirement for many applications such as optical interconnects and data networks, pump sources and next generation holographic red–green–blue displays requiring compact, high power, visible light sources with high spatial and spectral coherence. Many designs exist, but the structure must be easy to grow reproducibly, which has commercial advantages. The authors' low far-field divergence design widens the vertical mode in such a way as to decrease the far-field divergence without significantly reducing the confinement factor, thus keeping threshold current lower. In this study, the authors calculate the sensitivity of their design, which has high refractive index mode expansion layers inserted in the cladding, to unintentional variations in layer thickness and composition during growth. They obtain consistency in measured far-fields for three wafers grown over an interval of a year, with a full-width-half-maximum vertical far-field divergence of 17° for a narrow design (Design A) and just under 13° for a very narrow design (Design B). They have demonstrated a useful, reproducible design, adding to the range of versatile semiconductor lasers available for every application.

The authors report on the design and the preliminary characterisation of two active non-linear sources in the terahertz and near-infrared range. The former is associated to difference-frequency generation between whispering gallery modes of an AlGaAs microring resonator, whereas the latter is granted by parametric generation in a waveguide via modal phase matching. Both devices rely on embedded quantum-dot lasers, which allow for low-threshold currents and unconventional geometries. They also include specific degrees of freedom that open a practical route towards phase matching, either during the lithographic process or during operation.

The performance of optical inter-satellite communications using microwave photonics technologies employing intensity modulation with direct detection (IMDD) and phase modulation (PM) with interferometric detection are investigated and demonstrated with a numerical example. The theoretical model considering the saturated gain and amplifier spontaneous emission noise of optical booster amplifier and optical preamplifier is constructed for both architectures. Furthermore, the authors compare the performance of PM with interferometric detection to optimum biased IMDD in terms of radio frequency (RF) gain, RF noise figure and compression dynamic range (CDR). The numerical results show that the PM with interferometric detection can achieve better RF gain, RF noise figure and CDR than the optimum biased IMDD link as the laser output power is lower than 14.34, 12.45 and 4.2 dBm, respectively.

A Q-switched Brillouin fibre laser (BFL) is successfully demonstrated using multi-walled carbon nanotubes (MWCNTs) embedded in polyvinyl alcohol (PVA) film as a passive saturable absorber (SA) for the first time. The SA is obtained by sandwiching the developed MWCNTs–PVA film between two FC/PC fibre connectors after depositing index-matching gel onto the fibre ends. The proposed Q-switched BFL incorporates a 5 km long dispersion shifted fibre in a ring cavity structure to generate Stokes shifted by 0.08 nm from the Brillouin pump wavelength. The BFL starts to generate a Q-switching pulse train at threshold pump power of 5 dBm. As the BP power is varied from 5.0 to 6.0 dBm, the repetition rate of the Q-switched BFL exhibits an increasing trend from 27.75 to 30.21 kHz, whereas the pulse width exhibits a decreasing trend from 3.25 μs to 1.11 μs. The maximum pulse energy of 0.13 nJ is obtained at maximum BP power of 6.0 dBm.

The behaviour of distributed temperature sensors based on spontaneous Raman scattering using deconvolution algorithm is studied for improvement of temperature and spatial resolution in Raman distributed temperature sensor (RDTS). The deconvolution algorithms such as Fourier deconvolution (FourD), Fourier regularised deconvolution and Fourier wavelet regularised deconvolution (FourWaRD) are investigated in this study. Numerical simulation shows that the temperature resolution is enhanced by six times using the proposed FourWaRD algorithm compared with conventional FourD algorithm. In this study, for a light source with 60 ns pulse width, a spatial resolution of 3 m is achieved using the FourWaRD algorithm. The proposed sensing system exhibits a temperature resolution of 0.45 K because of the reduction of noise. In addition, the signal to noise ratio (SNR) of proposed RDTS is realised using Indium Gallium Arsenide avalanche photo detector over a 30 km sensing range. The authors have achieved about 4 dB improvement of SNR using FourWaRD algorithm compared with FourD algorithm at a distance of 25 km for an input power of 10 dBm. The numerical simulation results indicate that the proposed deconvolution algorithms are beneficial complements for RDTS to realise accurate detection and temperature monitoring of sharp temperature variations.

A methodology to control the eye opening distribution of mapping multiplexing technique (MMT) system is proposed and presented. The authors modelled and analysed a method to enhance the receiver sensitivity of MMT system by using optimised dual-drive Mach–Zehnder modulator. The proposed method offers larger eye opening for distorted Level-3 eyes. Based on the numerical calculation, the optimisation leads toward minimal eye closure. The performance of proposed system is shown to be improved by 6.1 dB in terms of receiver sensitivity compared with the conventional MMT system. Throughout the paper, the authors also discuss a numerical comparative analysis between the proposed MMT model, conventional MMT model, 33% return-to-zero on–off-keying (33%RZ-OOK), 50% RZ-OOK, 67% carrier-suppressed RZ-OOK and 4-ary amplitude shift keying in terms of positive chromatic dispersion tolerance and spectral efficiency.

In this study, the authors investigate the performance of selection diversity-aided subcarrier intensity modulation (SIM)/spatial modulation (SM) for free-space optical communications. Selection diversity is a receiver diversity technique which can enhance reception robustness in atmospheric turbulence-induced fading. Moreover, it is most attractive in terms of the complexity when compared with the other diversity schemes. Through computer simulations, the performance of the proposed selection diversity-aided SIM/SM is compared in terms of bit error rate with that of the conventional SIM in an additive white Gaussian noise and in an outdoor environment with log-normal atmospheric channel. Furthermore, the error performance are mathematically analysed in the atmospheric turbulence channel at the spectral efficiency of 2 bits/s/Hz. Numerical results show that the performance of the proposed SIM/SM with two receivers is better than that of a conventional SIM with four receivers at the spectral efficiency of 4 bits/s/Hz. It means that the SIM/SM can release the complexity with less diversity photodetectors over the atmospheric turbulence channel. Additionally, it is shown that the performance of the proposed SIM/SM outperforms that of a conventional SIM regardless of the irradiance variance value. Therefore, the proposed selection diversity-aided SIM/SM can be a good solution to achieve reliable communication with low complexity under high spectral efficiency, even under the impaired conditions of a turbulence fading channel.