Reduced Coupling Loss Research Articles

A novel power divider with arbitrary power division and high coupling efficiency with a single mode fibre

To improve the coupling efficiency of a conventional Ti: LiNbO3 channel waveguide with a single mode fibre, a promising Ti-Mg double diffused method has been employed to fabricate a single mode power divider. Calculated numerical results have revealed the feasibility of reducing coupling loss as well as radiation loss, and a set of efficient fabrication parameters for this single mode device can be established. The dependence of asymmetric branching angles and output dividing-power-ratios has been studied by adjusting the branching angle.

Reduced coupling loss using a tapered-rib adiabatic-following fiber coupler

We present the design and experimental demonstration of a tapered-rib adiabatic-following fiber coupler (TRAFFIC). This device is an adaptation of a Shani-Henry mode converter fabricated in (Al)GaAs and designed to increase the coupling efficiency of conventional optical fibers to tightly-confined semiconductor rib waveguide devices. This approach offers the potential of significantly reducing fiber butt coupling losses from the typical values of 7 to 10 dB to values of <1 dB. This long-standing packaging problem is one of the major impediments to the widespread acceptance of semiconductor-based optoelectronics. Moreover, the design can be implemented with minimal increase in fabrication complexity since it uses only a straightforward modification to epitaxial growth, and one additional lithography and etching step.

Experimental results on instability caused by non-uniform current distribution in the 30 kA NbTi Demo Poloidal Coil (DPC-U) conductor

Two 30 kA, NbTi Demo Poloidal Coils, DPC-U1 and DPC-U2, were fabricated and tested in the Demo Poloidal Coil project at the Japan Atomic Energy Research Institute. DPC-U1 and -U2 have a large current, forced flow cooling, cable-in-conduit conductor, which is composed of 486 strands. The strand surfaces are insulated by formvar to reduce coupling losses between the strands. DPC-U1 and -U2 reached their design current, but exhibited instability during charge, in many cases resulting in a coil quench. Such a quench occurred even at a current one-tenth of the conductor critical current. To clarify the cause of the instability, a detailed investigation on the quench current and normal voltage behaviour was carried out by charging the coil in several ways to the coil quench, and by measuring the stability of the coil at a current of 16–21.5 kA. These experimental results revealed the existence of non-uniformity of current distribution among the strands in the conductor, even under slow charging. This non-uniformity of current distribution caused the instability of the coil. The time constant of current redistribution is very large due to the insulation between the strands. However, if part of the conductor can be forced to go normal without coil quench occurring, a redistribution of current takes place and the current distribution becomes more uniform. It was then demonstrated that the current distribution could become uniform by applying heat to the conductor to generate intentional normalcy. Consequently, the possibility of stable operation of the DPC-U was suggested.

Cooling characteristics of a large‐current capacity AC superconducting cable

AbstractDiameter of the superconducting ac composite strand is small, typically 0.1 mm, because the strand must be twisted in a very short pitch to reduce coupling losses. Therefore, current capacity of the single strand is small and in the range of ∼10 A. In large‐scale electric power apparatus, the conductors must be able to carry large currents and hundreds of these composite strands should be bundled. The composite strand is highly unstable and susceptible to a very small disturbance due to a frictional wire motion because the main matrix of the wire is highly resistive CuNi.For stable operation of an ac superconducting winding, every strand in the bundle cable should be fixed firmly. An effective technique to fix strands is to impregnate the winding by epoxy. In this case, the ac losses in the winding are to be cooled by heat conduction through the epoxy. Therefore, it is important to estimate the temperature rise of the winding to discuss the fact that the epoxy impregnation technique is applicable to the ac superconducting electric power apparatus.In this paper, the mechanism of thermal conduction of the epoxy‐impregnated winding at the 4.2 K region is discussed based on experimental data and the temperature rise of a large‐scale cable bundles by 7 × 7 × 7 strands calculated considering thermal resistivity at the interface between the epoxy and the strand. The calculated value agrees well with the measured value.

Multifilamentary superconductor with alternating twist in a transverse pulsed field

A multifilamentary superconductor with alternating twist, proposed earlier in order to reduce coupling losses in longitudinal fields, is investigated in a transverse pulsed field. The induced currents are calculated and a formula for the coupling loss is derived. Magnetization measurements on samples with ordinary and alternating twist have been performed and good agreement with the theory is obtained. The frequency-dependent coupling losses are enhanced by a factor of 2, independent of the alternation rate of twist.

Leaky coaxial cable for communication in high speed railway transportation

The practical performance of leaky coaxial cables is affected by the surrounding materials. Coupling loss and transmission loss vary according to the distance between a concrete wall and the cable. Some experiments were conducted using concrete walls on an experimental railway, and several kinds of installation methods were tested. It was confirmed that the effect of snow was small. It is also shown that the newly developed antenna, the main beam of which had the same directivity pattern as that of the leaky coaxial cable, was very effective in reducing coupling loss and fluctuation of received power.