Iet Optoelectronics最新文献

This article presents a novel design for guiding terahertz (THz) waves, which utilises a photonic crystal fibre with a porous core (PC-PCF). This approach is substantiated by a Topas-based octagonal-shaped core with rectangular air slots. By utilising Finite Element Method and Perfectly Matched Layer boundary condition, the guiding properties of a structure consisting of octagonal air holes integrated with circular air slots in the cladding region have been investigated. The proposed structure exhibits a birefringence value of 0.05 and a low effective material loss of 0.013 cm⁻¹. Moreover, it exhibits flattened dispersion measuring $��0.6�\pm �0.16��p�s�/�T�H�z�/�c�m�$ at a wide-ranging frequency of 0.4 to 1.6 THz. In addition to that, the proposed structure offers 60% high core power fraction, 10⁻¹⁸ cm⁻¹ confinement loss at 65% porosity of 290 μm core diameter and 1.6 THz frequency. Other significant waveguide properties: effective refractive index, effective area, spot size, and beam divergence of the proposed fibre are numerically analysed and discussed rigorously along with the fabrication viability of the fibre with existing techniques.

The technical feasibility of developing an Internet of Things multi-user communication system is evaluated based on a central digital multiband carrierless amplitude and phase transmitter, which broadcasts data on multiple channels for a number of low-cost/low-power devices. It addresses the issue of carrier synchronisation, which is critical in real-world implementations because of imperfections of devices and the delays of the system. A simulation model for the traditional Costas Loop is presented, along with performance results, which demonstrate the system's ability to synchronise with pull-in and lock ranges of ±800 and ±900 Hz, respectively. The loop requires 1.194 ms to be in the locked state, allowing the system to lock within 6 symbols period. In addition, the authors measured the performance of the system in the presence of noise and interference from other modulated bands. The results showed that noise and interference did not degrade the system's performance. Although the system was unable to lock when energy was present in adjacent bands, alternative options such as a high order phase-locked loop and hybrid frequency-division multiple access and time-division multiple access, can improve system performance without significantly increasing the cost and complexity of the devices.

The authors propose to arrange the light emitting diodes (LEDs) on the ceiling for an indoor visible light communication system in order to minimise the secrecy outage probability (SOP) at the location of the legitimate user (LU). More precisely, by deriving the secrecy capacity for a multiple-input single-output-VLC system, we evaluate the SOP at the LU's location which is used as the appropriate metric to determine the position of the LEDs on the ceiling. To mitigate the effect of the eavesdropper, we derive the optimal beamforming coefficients which are used at the transmitter LED array with the aim of maximising the signal-to-noise ratio at the LU. Simulation results show that for a SOP of 10⁻⁴, the proposed SOP-based LED arrangement with optimal beamforming offers up to 3 dB gain in transmit power compared to the classical uniform LED arrangement with sub-optimal zero-forcing beamforming. Moreover, they show that by using the proposed method, the SOP is kept below a desired threshold, over a wider area inside the indoor environment compared to classical methods.

The improvement of light trapping inside the active layer of perovskite solar cells (PSCs) was numerically investigated. The light absorption probability was improved by incorporating periodic arrays of mesoscopic electron-transporting layer into the absorber layer (CH₃NH₃PbI₃) of the PSCs. Accordingly, chalcopyrite (CuInSe₂) and bismuth selenide (Bi₂Se₃) were introduced in the form of hexagonal pillars with an optimum radius of 35 nm and a height of 292 nm. It was found that the proposed PSCs can significantly extend the broadband light absorption from the visible spectrum to the near-infrared (NIR) region compared to planar PSCs that have an identical active layer thickness. After optimization, the PSCs based on MP-CuInSe₂ and MP-Bi₂Se₃ showed the best performance with an enhancement of respectively 32% and 54 % in the photocurrent density, (with values of 29.94 and 34.93 mA.cm⁻²), as compared to the planar PSC (with a photocurrent density of 22.69 mA.cm⁻²). This enhancement resulted from a more effective carrier transport due to the mesoscopic structures.

Downlink data transmission of multi-user indoor optical wireless communications and its corresponding fairness analysis is studied by the authors. It is assumed that access points (APs) may use infrared beam (IR-beam) laser or light emitting diodes (LEDs) for data transmissions. For the case of APs with IR-beams, since the bandwidth of the transmitted optical signal can be very large than that of the LED’s, a typical time-division multiple access (TDMA) scheme is utilised to serve multiple users and the respective achievable rates are derived. Then, for a general fairness-scheduling utility function with the capability of adjusting the degree of fairness known as α-proportional fairness, optimal time-sharing coefficients are analytically derived in closed-forms. To compare the performance of the IR-based communications for the LED-based APs, two non-cooperative and cooperative data transmission schemes are also studied and the corresponding achievable rates are presented based on the classical superposition coding. Finally, to compare the performance of the two types of APs, per user data rate of the proposed systems are compared through numerical results for different scenarios to highlight advantages and drawbacks of the two types of APs. Also, by considering the Jain's index as a fairness metric, the impact of the system parameters on the fairness is discussed through numerical results.