Radio-over-fibre (RoF) distributed antenna system (DAS) technology has been investigated for the distribution of ultra-high frequency (UHF) radio frequency identification (RFID) signals. RoF DAS allows for reduced number of readers and centralised placement of readers which facilitates easy system maintenance, but it is important to find a low-cost solution that can achieve comparable performance to a conventional RFID system. In this work, a low-cost vertical-cavity surface-emitting laser (VCSEL)-based multimode fibre (MMF) link has been developed and demonstrated for passive UHF RFID applications. The reported results show almost the same performance when compared with a conventional RFID system. In addition, simple spatial antenna diversity schemes are tested, with improved performance reported in comparison with an RFID-RoF system without diversity. Also, an investigation of RFID over fibre with RoF non-linearity is carried out showing that phase-reversal amplitude shift keying (PR-ASK) RFID modulation allows for higher levels of RF carrier and modulated signal power than the ASK RFID, for low levels of non-linearity.

This study analysed GaN nanophosphors based white light-emitting diodes (WLEDs) with ultraviolet (UV) excitation. Graphene quantum dots (GQDs) used as a charge transfer medium to enhance the performance in terms of luminous efficacy and colour quality. The improvement in colour rendering and colour temperature has been observed with the increase in injection current. The luminous efficacy of radiation also gets improved with injection current and maximises up to 255 lm/W at 260 mA along with 90 colour rendering index and 7100 K correlated colour temperature. Mapping of higher surface temperature for GQD-based devices shows a better thermal stability, indicating the good heat dissipation capability of GQDs because of excellent thermal conductivity. Therefore, proposed WLEDs were found more competent with improved thermal quenching of phosphors for rare-earth-free solid-state lighting as compared to the blue chip-excited yellow phosphors.

The performance of multiple-input and multiple-output (MIMO) flip-orthogonal frequency division multiplexing (flip-OFDM) for indoor optical wireless communications (OWCs) is studied here. To improve the system performance in terms of the bit error rate (BER), the singular value decomposition (SVD)-based MIMO technique is applied together with the bit loading technique. The authors, especially, focus on optimising the BER performance of SVD-based MIMO-OFDM OWC systems under the impact of the OFDM symbol design, the configuration of MIMO transmitters, the required data rates, the available bandwidth, and the angular diversity based on tilting receiver photo detectors (PDs). The system BER expressions are analytically derived and verified to be accurate through comparison with simulation results. The authors find that the 9 × 4-MIMO system is the optimal configuration in terms of a large coverage of low BER regions and low computational complexity for the 5 m × 5 m room. In addition, polar angle tilting of PDs can effectively improve the BER performance. Finally, in case of light dimming control with a low total transmit optical power, the system can benefit from a high multiplexing gain with a large tilting polar angle, as indicated by improved condition numbers of MIMO channel matrices.

A double-image asymmetric cryptosystem is proposed by using cylindrical diffraction and spectrum fusion and compression in the zigzag-scanned domain of discrete cosine transform (DCT). First, each plaintext of double image is converted into spectrum coefficients (SCs) by DCT, and the SCs are zigzag scanned. Second, the first 1/4 of SCs is resized to two low-frequency images, the second 1/4 of SCs is resized to two high-frequency images, while the last 1/2 of SCs is discarded. Third, the two low-frequency and two high-frequency images are fused and merged into three rounds of encryption process by using the complex combination, random phase encoding, and phase reserving–truncation in the cylindrical diffraction transform domain. Finally, the plaintexts of the double image are successfully encrypted into a ciphertext and three private keys with each only 1/8 and totally 1/2 of the size of the plaintext, which gains a high compression efficiency. In the proposed cryptosystem, the quality of the reconstructed image is improved due to the compression in the zigzag-scanned DCT domain, and the security is improved owing to the asymmetric cylindrical diffraction and spectrum-fusion algorithm. The effectiveness and flexibility of the proposed cryptosystem have been validated by the numerical simulation results.

A multilayer graphene-based optical modulator is proposed in this study. The structure is made of several layers of graphene on hexagonal boron nitride (hBN), working at 193 to 200 THz (1.5 to 1.55 µm) band. In this modulator, the electric field passes through graphene layers instead of passing through the silicon waveguide, causing very low insertion losses about 4 × 10⁻³ dB for 40 µm long device. It is shown that the transverse magnetic mode has a more efficient propagation in the modulator. Due to the passage of light from graphene layers and the graphene's optical characteristics, light transmission can be controlled with less variation in gate voltage, and it has the better switching performance concerning monolayer, bilayer and diagonal graphene optical modulators.

This study accessed the performance of the four-level Manchester coding (4-MC) and four-level modified Manchester (4-MM) technique at the bit rates of 20 and 40 Gb/s for the first-order and second-order optical equalisers. The performances were evaluated with respect to bit error rate (BER), power penalty, eye diagrams, and dispersion tolerance. Based on the results by utilising the second-order optical equaliser, at 20 Gb/s, the transmission distance of 4-MC and 4-MM was extended by 25 and 27 km of standard single-mode fibre (SSMF), respectively, while at the bit rate of 40 Gb/s, the 4-MC and 4-MM transmission distance was extended by 6.5 and 7.5 km of SSMF, respectively. Hence, the second-order optical equaliser can adequately compensate for the fibre dispersion and consequently the overall BER performance. Therefore, the proposed system could serve as an alternative to the next-generation high-speed optical transmission links.

Multicore fibre (MCF) has attracted considerable attention in mobile fronthaul networks as a promising solution to face the ever-increasing mobile traffic demand and to support network capacity scaling in a cost-effective manner. However, MCF-based systems suffer from the inter-core crosstalk (IC-XT) noise, which could incur severe performance degradation. In this study, we investigate for the first time, to our knowledge, the performance of an analogue radio-over-fibre mobile fronthaul link employing MCF under the joint impact of fibre dispersion and frequency chirp. Herein, the authors consider the radio frequency (RF) power fading induced by fibre dispersion and frequency chirp. In addition, the frequency-dependent IC-XT power fluctuations induced by dispersion and frequency chirp have been discussed. Closed-form equations for the carrier-to-noise ratio penalty due to power fading and IC-XT are derived. Moreover, new expressions for the average and variance of the crosstalk transfer function are obtained. Also, the analysis accounts for inter-core skew, laser relative intensity noise, shot noise and thermal noise. It has been shown that both the desired signal and IC-XT signals show the same behaviour in the presence of fibre dispersion and frequency chirp. Moreover, results show that RF power fading is the key issue.

A tandem solar cell including of CuIn_1−y Ga _y Se₂/Si_1 − x Ge _x was exhibited by integrating a CuIn_1−y Ga _y Se₂ solar cell on a Si_1 − x Ge _x solar cell to achieve a conversion efficiency of about 26% by controlled current matching. Electrical analyses were used to determine the performance parameters of the bottom and top sub-cells. In this work, an experimental Si_0.73Ge_0.27 solar cell has been modelled. The photovoltaic characteristics of the Si_0.73Ge_0.27 solar cell are in good agreement with its experimental counterpart. Afterwards, a copper–gallium–diselenide (CGS)/Si_0.73Ge_0.27 tandem solar cell with 24.1% efficiency has been designed. Additionally, the bandgap combination of CuIn_1−y Ga _y Se/Si_1 − x Ge _x structure has been studied. The findings indicate that CuIn_1−y Ga _y Se/Si_1 − x Ge _x tandem cell with 0.7 < y< 1 and 0 < x< 0.7 can achieve acceptable efficiency and the optimised CGS/Si device with 26.1% efficiency is proposed. In CGS/Si_0.73Ge_0.27 tandem cell, the current matching is obtained when the CGS thickness of the top cell is 1 µm and the Si_0.73Ge_0.27 absorber thickness of the bottom cell is 1.9 µm. The current matching condition for this device degrades the fill factor, although increases the current, so the device does not achieve maximum output power.

A novel InP-based metal-coated sidewall, hybrid plasmonic multimode interference (MCS-HP-MMI) power splitter has been proposed. Based on two methods of multimode interference self-imaging and finite difference time domain simulations, a 1 × 2 MCS-HP-MMI power splitter was numerically analysed and compared with an HP-MMI power splitter with the same specifications showing 70% reduction in length due to the negative Goos–Hänchen shift in the metal-coated sidewalls of the MCS-HP-MMI power splitter. The width and layer stack specifications of the device were optimised using response surface methodology to minimise the multimode interference length and maximise the optical power transmission. The total optical power transmission of 98% was achieved for an MCS-HP-MMI width and length of 662 and 190 nm, respectively, with a wavelength of 1550 nm.

The stability of the central wavelength of the erbium-doped fibre source (EDFS) is one of the most important parameters to ensure the accurate operation of navigation systems based on high-accuracy fibre optic gyroscopes (FOGs). In this study, the method of stabilisation of the EDFS as a part of the FOG is presented. The main advantage of this method is the possibility of quick and simple matching of temperature-compensating spectral filters by changing the pump ratio without modifying the optic scheme. The EDFS optic scheme based on the double-pass bidirectional pumping configuration. It makes it possible to obtain the world-class stability of the central wavelength of the EDFS as a component of different devices with the spectral selectivity and set the constant value of the optic power of the output light (OL). Also, this is the first study of the dependence of the temperature coefficient of the central wavelength on the pump ratio. The proposed method allows compensating the spectral characteristics of the FOG, achieving the OL central wavelength stability around 0.7 ppm/°C and the FOG scale factor stability around 0.73 ppm/°C.