Simulation and Measurement of Medium-Frequency Signals Coupling From a Line to a Loop Antenna.

The underground-mining environment can affect radio-signal propagation in various ways. Understanding these effects is especially critical in evaluating communications systems used during normal mining operations and during mine emergencies. One of these types of communications systems relies on medium-frequency (MF) radio frequencies. This paper presents the simulation and measurement results of recent National Institute for Occupational Safety and Health (NIOSH) research aimed at investigating MF coupling between a transmission line (TL) and a loop antenna in an underground coal mine. Two different types of measurements were completed: 1) line-current distribution and 2) line-to-antenna coupling. Measurements were taken underground in an experimental coal mine and on a specially designed surface test area. The results of these tests are characterized by current along a TL and voltage induced in the loop from a line. This paper concludes with a discussion of issues for MF TLs. These include electromagnetic fields at the ends of the TL, connection of the ends of the TL, the effect of other conductors underground, and the proximity of coal or earth. These results could help operators by providing examples of these challenges that may be experienced underground and a method by which to measure voltage induced by a line.

Transformation-optics-inspired anti-reflective coating design for gradient index lenses.

Recent developments in transformation optics have led to burgeoning research on gradient index lenses for novel optical systems. Such lenses hold great potential for the advancement of complex optics for a wide range of applications. Despite the plethora of literature on gradient index lenses, previous works have not yet considered the application of anti-reflective coatings to these systems. Reducing system reflections is crucial to the development of this technology for highly sensitive optical applications. Here, we present effective anti-reflective-coating designs for gradient index lens systems. Conventional anti-reflective-design methodologies are leveraged in conjunction with transformation optics to develop coatings that significantly reduce reflections of a flat gradient index lens. Finally, the resulting gradient-index anti-reflective coatings are compared and contrasted with conventional homogeneous anti-reflective coatings.

Conformal mappings to achieve simple material parameters for transformation optics devices.

The transformation optics technique for designing novel electromagnetic and optical devices offers great control over wave behavior, but is difficult to implement primarily due to limitations in current metamaterial design and fabrication techniques. This paper demonstrates that restricting the spatial transformation to a conformal mapping can lead to much simpler material parameters for more practical implementation. As an example, a flat cylindrical-to-plane-wave conversion lens is presented and its performance validated through numerical simulations. It is shown that the lens dimensions and embedded source location can be adjusted to produce one, two, or four highly directive planar beams. Two metamaterial designs for this lens that implement the required effective medium parameters are proposed and their behavior analyzed.

Optical planar chiral metamaterial designs for strong circular dichroism and polarization rotation.

Planar chiral metamaterials comprising double-layer dielectricmetal- dielectric resonant structures in the shape of a gammadion are presented in the near-infrared regime. The unit cell of the doubly-periodic metamaterial design is optimized using the genetic algorithm for maximum circular dichroism and for maximum optical activity. A circular dichroism value in excess of 50% is predicted for the optimized design. Maximum polarization rotatory powers in terms of the minimum allowed transmittances are also obtained and presented.

Hybrid vanadate waveguiding configurations for extreme optical confinement and efficient polarization management in the near-infrared.

Vanadate materials such as CaVO3 and SrVO3 were recently proposed as promising alternatives to their conventional transparent conducting oxide counterparts owing to the superior capability for simultaneous realization of high optical transparency and high electrical conductivity originating from strong electron-electron interactions. Here we show that, in addition to their remarkable optoelectronic properties as conducting materials, their incorporation into planar waveguiding configurations could enable outstanding optical performance that is otherwise difficult to achieve with conventional material building blocks, especially metals. Starting from the guided wave at a single CaVO3/dielectric interface, the unique dispersion relationship and propagation property of the fundamental mode are revealed and compared to the conventional surface plasmon polariton associated with a silver/dielectric planar configuration. The superior confinement capability and the unique modal attenuation of the CaVO3-based waveguiding platform are further demonstrated via investigating silicon-based hybrid guiding schemes integrated with a CaVO3 nanostructure. By leveraging the pronounced polarization dependent loss in the hybrid configuration, an ultra-compact TE-pass polarizer is numerically demonstrated at telecommunication wavelengths. This transformative design features a reduced footprint and enhanced optical performance when benchmarked against the current state-of-the-art in hybrid silicon polarizers. The combination of these vanadate materials with traditional waveguiding platforms thereby opens new avenues towards miniaturized functional integrated photonic devices, and potentially enables a variety of intriguing applications at the sub-diffraction-limited scale.

Efficient Cross-talk Reduction of Nanophotonic Circuits Enabled by Fabrication Friendly Periodic Silicon Strip Arrays.

Reduction of the crosstalk between adjacent photonic components has been regarded as one of the most effective, yet most challenging approaches for increasing the packing density of photonic integrated circuits. Recently, extensive efforts have been devoted to this field, leading to a number of elaborate designs, such as waveguide supperlattice and nanophotonic cloaking, among others. Here we develop a simple and efficient crosstalk reduction approach for silicon-based nanophotonic circuits by introducing a periodic array of silicon strips between adjacent waveguides. Studies indicate that the coupling lengths can be extended by more than two orders of magnitude for a waveguide pair with an edge-to-edge distance of ~λ/3 at the telecommunication wavelength. Further investigations reveal that our method is effective for both strongly and weakly confined silicon photonic modes, and works well over a broad band of operational wavelengths. In addition, the crosstalk reduction technique is shown to be capable of improving the coupling lengths of other elements as well, such as vertical silicon slot waveguides. Our approach offers a promising platform for creating ultra-compact functional components that is fabrication friendly, thereby providing a feasible route toward the realization of photonic integrated circuits with ultra-high packing densities.

A Hybrid Approach for Efficient Modeling of Medium-Frequency Propagation in Coal Mines.

An efficient procedure for modeling medium frequency (MF) communications in coal mines is introduced. In particular, a hybrid approach is formulated and demonstrated utilizing ideal transmission line equations to model MF propagation in combination with full-wave sections used for accurate simulation of local antenna-line coupling and other near-field effects. This work confirms that the hybrid method accurately models signal propagation from a source to a load for various system geometries and material compositions, while significantly reducing computation time. With such dramatic improvement to solution times, it becomes feasible to perform large-scale optimizations with the primary motivation of improving communications in coal mines both for daily operations and emergency response. Furthermore, it is demonstrated that the hybrid approach is suitable for modeling and optimizing large communication networks in coal mines that may otherwise be intractable to simulate using traditional full-wave techniques such as moment methods or finite-element analysis.