Study of second generation, high-temperature superconducting coils: Determination of critical current

This paper presents the modeling of second generation (2 G) high-temperature superconducting (HTS) pancake coils using finite element method. The axial symmetric model can be used to calculate current and magnetic field distribution inside the coil. The anisotropic characteristics of 2 G tapes are included in the model by direct interpolation. The model is validated by comparing to experimental results. We use the model to study critical currents of 2 G coils and find that 100μV/m is too high a criterion to determine long-term operating current of the coils, because the innermost turns of a coil will, due to the effect of local magnetic field, reach their critical current much earlier than outer turns. Our modeling shows that an average voltage criterion of 20μV/m over the coil corresponds to the point at which the innermost turns' electric field exceeds 100μV/m. So 20μV/m is suggested to be the critical current criterion of the HTS coil. The influence of background field on the coil critical current is also studied in the paper. © 2012 American Institute of Physics.

Automated sparse 3D point cloud generation of infrastructure using its distinctive visual features

The commercial far-range (>10 m) spatial data collection methods for acquiring infrastructure’s geometric data are not completely automated because of the necessary manual pre- and/or post-processing work. The required amount of human intervention and, in some cases, the high equipment costs associated with these methods impede their adoption by the majority of infrastructure mapping activities. This paper presents an automated stereo vision-based method, as an alternative and inexpensive solution, to producing a sparse Euclidean 3D point cloud of an infrastructure scene utilizing two video streams captured by a set of two calibrated cameras. In this process SURF features are automatically detected and matched between each pair of stereo video frames. 3D coordinates of the matched feature points are then calculated via triangulation. The detected SURF features in two successive video frames are automatically matched and the RANSAC algorithm is used to discard mismatches. The quaternion motion estimation method is then used along with bundle adjustment optimization to register successive point clouds. The method was tested on a database of infrastructure stereo video streams. The validity and statistical significance of the results were evaluated by comparing the spatial distance of randomly selected feature points with their corresponding tape measurements.

Toward automated generation of parametric BIMs based on hybrid video and laser scanning data

Only very few constructed facilities today have a complete record of as-built information. Despite the growing use of Building Information Modelling and the improvement in as-built records, several more years will be required before guidelines that require as-built data modelling will be implemented for the majority of constructed facilities, and this will still not address the stock of existing buildings. A technical solution for scanning buildings and compiling Building Information Models is needed. However, this is a multidisciplinary problem, requiring expertise in scanning, computer vision and videogrammetry, machine learning, and parametric object modelling. This paper outlines the technical approach proposed by a consortium of researchers that has gathered to tackle the ambitious goal of automating as-built modelling as far as possible. The top level framework of the proposed solution is presented, and each process, input and output is explained, along with the steps needed to validate them. Preliminary experiments on the earlier stages (i.e. processes) of the framework proposed are conducted and results are shown; the work toward implementation of the remainder is ongoing.

Graphone model interpolation and Arabic pronunciation generation

This paper extends n-gram graphone model pronunciation generation to use a mixture of such models. This technique is useful when pronunciation data is for a specific variant (or set of variants) of a language, such as for a dialect, and only a small amount of pronunciation dictionary training data for that specific variant is available. The performance of the interpolated n-gram graphone model is evaluated on Arabic phonetic pronunciation generation for words that can't be handled by the Buckwalter Morphological Analyser. The pronunciations produced are also used to train an Arabic broadcast audio speech recognition system. In both cases the interpolated graphone model leads to improved performance. Copyright © 2011 ISCA.

Alternating current loss of second-generation high-temperature superconducting coils with magnetic and non-magnetic substrate

It is widely believed that the second-generation high-temperature superconducting (2G HTS) tapes with magnetic substrates suffer higher transport loss compared to those with non-magnetic substrates. To test this, we prepared two identical coils with magnetic and non-magnetic substrates, respectively. The experimental result was rather surprising that they generated roughly the same amount of transport loss. We used finite element method to understand this result. It is found that, unlike in the single tape where the magnetic field-dependent critical current characteristic can be neglected and the effect of magnetic substrate dominates, the magnetic field-dependent critical current characteristic of 2G tape plays as an equally important role as magnetic substrate in terms of HTS coils. © 2012 American Institute of Physics.

28 Gb/s unequalized PAM3 modulation of an 850 nm VCSEL for next-generation datacommunication links

An 850 nm vertical-cavity surface-emitting laser is modulated at 28 Gb/s using pulseamplitude modulation with three levels. Unequalized transmission over 100 m of OM3 MMF is demonstrated, with advantages over NRZ and PAM4 modulation. © 2012 OSA.

Design methodology of HTS bulk machine for direct-driven wind generation

This paper introduces the design methodology of HTS bulk generator for direct-driven wind turbine. The trap field capability of HTS bulks offer the potential of maintaining similar or even higher magnetic loading level without the iron circuit in the generator. This so-called air-cored design can reduce the weight and increase the power outing per volume of the machine. The detailed design method of the air-cored HTS bulk machine is presented; 3D modeling is applied to consider the total trapped field of bulk arrays; a case study is performed to demonstrate the advantages of air-cored HTS bulk machine over conventional permanent magnet machine. Our results show that the air-cored HTS bulk machine has the potential to maintain the same magnetic loading level as that of the conventional permanent magnet machine. More importantly, it can reduce the total machine weight by 30%. © 2002-2011 IEEE.

Ultrashort pulse generation in diode laser devices

The generation of ultrashort optical pulses by semiconductor lasers has been extensively studied for many years. A number of methods, including gain-/Q-switching and different types of mode locking, have been exploited for the generation of picosecond and sub-picosecond pulses [1]. However, the shortest pulses produced by diode lasers are still much longer and weaker than those that are generated by advanced mode-locked solid-state laser systems [2]. On the other hand, an interesting class of devices based on superradiant emission from multiple contact diode laser structures has also been recently reported [3]. Superradiance (SR) is a transient quantum optics phenomenon based on the cooperative radiative recombination of a large number of oscillators, including atoms, molecules, e-h pairs, etc. SR in semiconductors can be used for the study of fundamental properties of e-h ensembles such as photon-mediated pairing, non-equilibrium e-h condensation, BSC-like coherent states and related phenomena. Due to the intrinsic parameters of semiconductor media, SR emission typically results in the generation of a high-power optical pulse or pulse train, where the pulse duration can be much less than 1 ps, under optimised bias conditions. Advantages of this technique over mode locking in semiconductor laser structures include potentially shorter pulsewidths and much larger peak powers. Moreover, the pulse repetition rate of mode-locked pulses is fixed by the cavity round trip time, whereas the repetition rate of SR pulses is controlled by the current bias and can be varied over a wide range. © 2012 IEEE.

Thermal modelling of a tubular linear machine for marine renewable generation

A lumped parameter thermal model has been constructed for a tubular linear machine that has been designed for use in a marine environment. It shows good correlation to both steady state and transient experimental tests on the machine. The model has been developed for a stationary machine in a laboratory environment - the modelling techniques used and enhancements to enable the application of the model directly to marine scenarios are discussed.

Light generation in silicon photonic crystal cavities

Far-field optimized photonic crystal nanocavities are used to strongly increase light generation from crystalline silicon. Low-power continuous-wave harmonic generation as well as efficient room temperature light-emission from optically-active defects are demonstrated in these devices. © 2011 IEEE.

1.5 GHz picosecond pulse generation from a monolithic waveguide laser with a graphene-film saturable output coupler

We fabricate a saturable absorber mirror by coating a graphenefilm on an output coupler mirror. This is then used to obtain Q-switched mode-locking from a diode-pumped linear cavity channel waveguide laser inscribed in Ytterbium-doped Bismuthate Glass. The laser produces 1.06 ps pulses at ∼1039 nm, with a 1.5 GHz repetition rate, 48% slope efficiency and 202 mW average output power. This performance is due to the combination of the graphene saturable absorber and the high quality optical waveguides in the laser glass. © 2013 Optical Society of America.