Iet Optoelectronics最新文献

Pixelated two-dimensional (2-D) optical wireless communication systems, with intensity-modulation and direct-detection (IM/DD), are capable to transmit data at high data rates by modulating the data into image intensities at the transmitter. Frequency division modulation (FDM) and space-domain modulation (SDM) are two common modulation schemes in a 2-D pixelated system. Generally, FDM is preferred because FDM benefits from the capability of controlling the spectrum of transmitting signals so that the output signal could be shaped according to the characteristics of a particular optical channel. However, the computational complexity of FDM is relatively high and complex computation is needed at both the transmitter and the receiver. In this work, a fast orthogonal frequency-division multiplexing (FOFDM) modulation which employs 2-D inverse discrete cosine transform (IDCT2) instead of 2-D inverse Fourier transform (IFFT2) is presented. As the image intensities must be real and positive, two schemes are hereby introduced to generate unipolar image intensities. The proposed FOFDM is simulated and discussed in terms of key characterisations including the signal spectrum, spectral efficiency, peak to average power ratio (PAPR), bit error ratio (BER) and computational complexity. Compared with traditional spatial orthogonal frequency-division multiplexing (OFDM) based on IFFT2, the proposed FOFDM has similar BER performance, twice the spatial efficiency for the same constellation size, lower computational complexity and a much simpler structure in the transmitter.

The key challenge in free space optical (FSO) communications is combating turbulence-induced fading. As the channel fading in FSO is quasi-static, the transmission parameters such as the code rates, transmit power and modulation schemes can be modified with respect to the channel state information transmitted via the feedback path. As a result, adaptive channel coding is considered as one of the practical approaches to improve the FSO link performance. In this study, the FSO system with polar codes is investigated and its performance is analysed by determining the optimum code-rate required to achieve a bit error rate of 10⁻⁹ under weak turbulence. It is shown that, using Monte-Carlo simulations for the scintillation indices of 0.12 and 0.2, the successive cancelation list (SCL) decoder offers coding gains of 2.5 and 0.3 dB, respectively, as compared with SC decoder, and for the scintillation index of 0.31, the SC decoder offers a coding gain of 2.5 dB compared to that of the SCL decoder for the code rate.

Optical fibre sensors with modified cladding have been a topic of great interest for decades. Thanks to the new techniques of fabricating nanostructured films, this interest has been renewed. Although there have been many increases in published experimental research, no comprehensive theoretical formulation of the structure is reported in the literature. In this study, a general theory for transmissive modified cladding optical fibre sensors is presented. The model is formulated based on the electromagnetic fields and optical power in the core and cladding of a step-index fibre. The model considers various parameters, especially the radius of the optical fibre core, modified cladding refractive index, and light wavelength. The sensitivity, response, and optical transmission of the sensor are analysed as a function of the parameters. The analyses show that optical transmission reduces dramatically when the modified cladding refractive index approaches the refractive index of the core of the fibre and results in higher sensor response and sensitivity. Increasing the core radius of the fibre will result in lower sensor response with higher sensitivity. Both parameters will increase for a light with a larger wavelength. The results are in good agreement with experimental evidence.

A novel method that converts blue light into the white-light for illumination and provides the high-speed visible light communication link is introduced. Typical phosphors for creating white-light cause low bandwidths and restrictions on changing correlated colour temperatures. The proposed method deals with such issues by utilising combinations of photo-luminescence lights from cost-effective fluorescent acrylic sheets. Characteristics of the light generated from the proposed method show that white-lights with various correlated colour temperatures can be straightforwardly generated whilst providing orders of magnitude higher bandwidth than that from the conventional approaches using the phosphors. A high-speed white-light visible light communication demonstration based on a blue laser-diode and the fluorescent acrylic sheets is presented, showing the data-rate of 2.16 Gb/s with a bit-error-rate less than 3.8 × 10⁻³ by using an optical orthogonal frequency division multiplexing scheme.

The creation of an array of three-dimensional (3D) multifocal spots in the focal region has attracted interest due to potential applications in parallel or simultaneous process areas. Based on the theory of pattern synthesis of antenna array and the electromagnetic time reversal technique, an optimisation-free approach is reported to construct a 3D controllable diffraction-limited spot array in the focal volume of a 4pi focussing system formed by two high-numerical-aperture (NA) objectives. The proposed method can be implemented readily by inversely focussing the field radiated from a virtual spaced-dipole antenna array mounted at the focus of the 4pi configuration. By solving the inverse problem, the required illumination in the pupil plane for producing the 3D spot array can be found. It is demonstrated that the 3D diffraction-limited focal spot array owns the properties of controllable polarisation, scheduled number, tunable location, and adjustable interval. This array may find applications in 3D simultaneous optical manipulation and trapping, 3D parallel fabrication, 3D optical data storage, and so on.

The purpose of this work is to present a method for designing planar waveguide couplers of arbitrary modal electric field distributions. The idea here is to create couplers with unusual electric fields, which confine in one planar waveguide. This is possible if a planar waveguide profile supporting two pre-selected mode fields is designed. Their interference will have to transfer the optical power from one region (bottom layer) to another region (top layer) completely. The inverse transmission-line technique and the desired electric field are applied, based on which the refractive index profile of the couplers exhibiting excellent crosstalk is reconstructed. By choosing this technique, it is feasible to design planar components in sensing applications, such as high power, temperature or pressure sensors and numerous other possibilities, using unusual electric field distributions.

Inclination is one of the important parameters of a target’s surface, which is closely related to the waveform of light detection and ranging. When the target is tilted, the laser spot on the target’s surface changes greatly, and the influence of the target on the echo signal is more complex. Here, the formation process of a laser’s echo signal is explored. On that basis, the spot distribution model in the case of a large incident angle is optimised, and a comparison is made with the conventional method. A simulation system which can produce and display the signal affected by the inclined target is designed. In view of the difficulty in extracting the target's inclination from the echo signal, a method based on waveform analysis is proposed in this article. Then a calculation model is built to retrieve the inclination of the target from the echo signal by data fitting. The maximum measured error is only 0.948°, and the maximum root-mean-squared-error is only 0.45°.