Proposal of a new method for narrowing and moving the stimulated region of cochlear implants: animal experiment and numerical analysis

We have proposed the tripolar electrode stimulation method (TESM) for narrowing the stimulation region and continuously moving the stimulation site for cochlear implants. The TESM stimulates the auditory nerve array using three adjacent electrodes which are selected among the electrodes of an electrode array within the lymphatic fluid. Current is emitted from each of the two lateral electrodes and received by the central electrode. The current received by the central electrode is made equal to the sum of the currents emitted from the lateral electrodes. In this paper, we evaluate whether or not TESM works according to a theory which is based on numerical analysis using an electrical equivalent circuit model of the auditory nerve fibers. In this simulation, the sums of the excited model fibers are compared to the compound action potentials (CAP's) which we obtained through animal experiments. To identify the main parameter while maintaining the amplitude of the CAP (the sum of the fired fibers), we assumed the presence of some parameters from the radial current density profile. In the case of the width value among the parameters being kept constant, the amplitude of the CAP was almost constant; thus, the number of the fired fibers was also almost constant. The width value equals the distance between the points at which the profile of the radial current density of the electrode array and the line of the radial threshold current density of the electrode array intersect. It is possible to determine the measure of the stimulation region or site by controlling the width value and the ratios of the currents emitted from the lateral electrodes. As a result, we succeeded in narrowing the stimulation region by controlling the sum of the currents emitted from the two lateral electrodes. Also we succeeded in continuously moving the stimulation site by modifying the currents emitted from the two lateral electrodes.     

The theory of endfire arrays

The general King-Sandler array theory has been examined in detail for the case of endfire arrays. Since it is not necessary to assume identical current distributions, a distinction is made between specification of the base voltages and currents. The driving-point impedances for specified base voltages and currents are presented for arrays of up to 25 elements. The effect of interaction between the element currents in distorting the radiation pattern is shown for the 15-element endfire array. The results indicate that the unequal current distributions have a pronounced effect in determining the driving-point impedances, sidelobe levels and back-to-front ratios of endfire arrays.     

A study of steering capability of a microstrip phased array using inter-injection locked oscillators

An innovative, phase coherent, power combining method, known as inter-injection locking applied to linear limited scan phased arrays is investigated. In this novel method, each antenna element in the array is supplied by power from a separate oscillator. However, the oscillator phases are allowed to be synthesized coherently by means of a coupling network and injection currents, being the input controls to the system of coupled phases oscillators. The inter-injection-locked phased arrays studied in this paper provide more design freedom than conventional phased arrays. They also result in systems more adapted to solid state monolithic integration technology.     

Ultra sharp crystalline silicon tip array used as field emitter

This article describes the fabrication of single crystal silicon field emission tip arrays. Each array consists of 2500 tips. We used 4 in. (100) oriented n type silicon wafers 0.008 – 0.020 Ωcm, Sb doped. The tips were formed using a RIE process. We achieved crystalline emitter tip radiuses of 1.5 – 2 nm. The extraction grid is a self aligned, sputter deposited Ti_0.1W_0.9 film. The radiuses of the extraction grid apertures range from 300 to 150 nm and have a tip to tip spacing from 10 to 5 μm. The testing was done in vacuum with a distance of 500 μm between extraction grid and anode. We have seen maximum stable array currents up to 2 μA. An anode current of 10 nA was initially detected at a minimal gate bias of about 14 V.     

The theory of broadside arrays

The general King-Sandler array theory has been examined in detail for the case of broadside arrays. Since it is not necessary to assume identical current distributions on every element in the array, a distinction is made between specified base currents and voltages. The driving-point impedances for specified base currents and voltages are presented for arrays of up to 25 elements. The effect of interaction between the element currents in the base impedances and radiation patterns is clearly shown for the broadside array. The results indicate that the major effect of unequal current distributions in the broadside array is to cause important variations in the driving-point impedances and little effect in the radiation patterns.     

A simulation study evaluating the performance of high-density electrode arrays on myocardial tissue

Multielectrode arrays used to detect cellular activation have become so dense (electrodes per square millimeter) as to jeopardize the basic assumptions of activation mapping; namely, that electrodes are points adequately separated as to not interfere with the tissue or each other. This paper directly tests these assumptions for high-density electrode arrays. Using a finite element model with modified Fitzhugh-Nagumo kinetics, we represent electrodes as isopotential surfaces of varying widths and spacing ratio (SR) (center-to-center spacing divided by electrode width). We examine the signal strength and ability of a single electrode to detect activation due to a passing wavefront. We find that high-density arrays do not cause significant wavefront curvature or alter activation timing in the underlying tissue. Relationships between signal strength, cross talk, and array design are explained by the interaction of the propagating wavefront and induced sources on the isopotential electrodes. Sensitivity analysis shows that these results may be generalized to a wide range of physiologically relevant designs and applications. We conclude that electrode array designs in which electrode spacing greatly exceeds electrode diameter are overly conservative and that arrays with a SR of less than 2.0 may perform successfully in electrophysiological studies.     

Selective activation of distant nerve by surface electrode array

Neural prostheses for restoring lost functions can benefit from selective activation of nerves with limited number and density of electrodes. Here, we show by simulations and animal experiments that multipoint simultaneous stimulation with a surface electrode array can selectively activate nerves in a bundle at a desired location in between the array or at a desired depth, which are referred to as lateral or depth-wise gating stimulation, respectively. The stimulation broadly generates action potentials with cathodic source electrodes, and simultaneously blocks unnecessary propagation with downstream anodic gate electrodes. In general, stimulation with a small diameter electrode can affect a nearly hemispherical region, while a large electrode is effective at a more vertically compressed region, i.e., a surface of nerve bundle. The gating stimulation takes advantage of the size effects by utilizing an asymmetrical electrode array. The array of the lateral gating stimulation is designed to have four electrodes; a pair of large source electrodes and a pair of small gate electrodes. The depth-wise gating stimulation array consists of two electrodes; a large gate and small source electrodes. The simulation first demonstrated that appropriate combination of currents at the source and gate electrodes can change recruitment patterns of nerves with lateral or depth-wise selectivity as desired. We then applied the lateral gating stimulation on the rat spinal cords and obtained a preliminary support for the feasibility.     

Analysis of Current–Voltage Measurements on Long-Wavelength HgCdTe Photodiodes Fabricated on Si Composite Substrates

We have performed a detailed study of dark current versus voltage to understand existing limitations in dark current and address the nonuniformity of dark current in devices fabricated on HgCdTe grown on silicon substrates. One interesting observation is that trap-assisted tunneling, g-r currents, are not found close to zero bias in certain devices. Devices from the low end of the R ₀ A distribution show heavy shunting paths close to zero bias. We believe that these shunting paths may be the limiting cause of tail distributions in fabricated focal plane array tail distributions. Possible causes for these shunting paths are surface charges associated with dislocation cores and impurity gettering at dislocation cores. The measured non-anti-reflection (AR)-coated quantum efficiency (QE) was 0.576 at 78 K and displays the classical response versus wavelength. The measured QE on isolated single devices is consistent with the 256 × 256 focal-plane array mean QE. Obtained average dark currents are on the order of mid 10⁻⁵ A cm^–2, which is one order of magnitude higher than dark currents obtained from arrays on lattice-matched substrates. On average, arrays on lattice-mismatched substrates show performance characteristics inferior to those of arrays fabricated on lattice-matched substrates. This inferior performance is due to array pixel operability, as can be seen from the tail of the distribution and the average dark currents, which are one order of magnitude higher than those obtained on lattice-matched substrates.     

Superconducting joints for silver-clad BSCCO tapes

The advent of high-T_c superconducting tapes for a variety of applications has resulted in the need for superconducting joints. Although long-length tapes can be custom-made for some applications, interconnections between subcoils of a magnet or shorter-length conductors for coil winding and current leads, make the development of such joints imperative. Additionally, high-quality short-or medium-length conductors, which are easier to make, can be joined for improved performance. Employing a novel chemical etching technique, we have fabricated lap joints between short lengths of silver-clad BSCCO tapes. Each joint was formed by etching the silver away and bringing together the exposed superconductor cores of two tapes together. The joined tapes were then subjected to a series of thermomechanical treatments. Detailed microstructural and electrical characterization within and across the joint was performed. Critical currents of up to 37 A within the joint region and 10 A through the joint region(at 77K) have been achieved.     

Low-frequency loop antenna arrays: Radiation field of systems of horizontally oriented loops

Previously we calculated the impedance and antenna currents of loop arrays resulting from ground reaction and mutual interaction [1]. Here we make use of those results and develop a general theory of the radiation field for horizontally oriented loop arrays. Radiation patterns of a linear array of 100 horizontally oriented loops are computed numerically in their dependence on ground conductivity, distance, and current phase difference between adjacent loops.     

Frequency stabilization of power-combining grid oscillator arrays

In this paper, we report the results of phase locking of grid oscillator arrays. First, a voltage-controlled grid oscillator array with a center frequency of 4.7 GHz and with a 300-MHz electric tuning range was locked to a frequency synthesizer through a phase-locked loop. Second, a 4 × 4 and a 6 × 6 grid oscillator arrays were locked by way of the injection locking. In both methods, a simple loop antenna mounted on the reflection mirror was used for taking/injecting signals from/to the array. Results show that the phase noise performance is improved significantly in the locked oscillator arrays     

Zig–Zag-Array Superconducting Resonators for Relatively High-Power Applications

Since power handling of superconducting resonators is severely limited by current density saturation, it is proposed to form resonators having their incident power and resulting currents divided within interior arrays of "basic resonators" (A properly designed array of basic resonators acts as a single resonator.) Though other forms of basic resonators may also be attractive, in this paper, in all examples, the basic resonator used is a "zig-zag resonator" having a fundamental resonance at f₀. This type of resonator is attractive because it is relatively compact and tends to keep the energy confined to a region close to the surface of the substrate. This latter is important so that even if a large number of basic resonators are used, energy will not be radiating out to the normal-metal resonator housing, which would greatly lower the unloaded Q. The use of parallel and cascade connections of basic resonators are analyzed and are found to both yield an increase in power handling proportional to the number of basic resonators used. However, these two types of connections have different characteristics with regard to introducing spurious modes, and it is found that it will usually be desirable to use both types of connections within an array. A sizable number of computed explorative examples are presented containing as many as 64 zig-zags. Calculation of the current densities within the zig-zags shows them to be remarkably uniform throughout the arrays. Measured high-temperature superconductor resonator results are presented, which confirm the principles involved. Unloaded Q's measured at 77 K were consistently well above 100 000. This array technique should also be useful for some relatively high-power planar normal-metal filters.     

Multiplexed Readout of Superconducting Bolometers

Studies of emission in the far-infrared and submillimeter from astrophysical sources require large arrays of detectors containing hundreds to thousands of elements. A multiplexed readout is necessary for practical implementation of such arrays, and can be developed using SQUIDS, such that, e.g., a 32 × 32 array of bolometers can be read out using ≈100 wires rather than the >2000 needed with a brute force expansion of existing arrays. These bolometer arrays are made by micromachining techniques, using superconducting transition edge sensors as the thermistors. We describe the development of this multiplexed superconducting bolometer array architecture as a step toward bringing about the first astronomically useful arrays of this design. This technology will be used in the SAFIRE instrument on SOFIA, and is a candidate for a wide variety of other spectroscopic and photometric instruments.     

Simple approaches of wavelength registration for monolithically integrated DWDM laser arrays

A simple approach is proposed to allocate the wavelengths of monolithically integrated sampled grating distributed Bragg reflector (SGDBR) laser arrays for applications in dense wavelength-division-multiplexing (DWDM) systems. The lasers in an array are thermally controlled altogether and each laser is operated with two electrodes. Arrays of accurate wavelengths can be realized by fine adjusting the bias current and phase current of each laser to output a uniform wavelength comb, and then varying the heat sink temperature to align all channels to the desired wavelength grid. Such procedures can provide arrays of high sidemode suppression ratio (SMSR) and no mode hopping over a wide range of operation condition. We demonstrated 50-GHz spaced arrays of accurate and stable wavelengths.     

Implementation of new superconducting neural circuits using coupledSQUIDs

A novel superconducting neuron circuit and two types of variable synapses, which are based on superconducting quantum interferometer devices (SQUIDs), are presented. A neuron circuit with good input-output isolation and steep threshold characteristics is accomplished using a combination of a single-junction SQUID coupled to a double-junction SQUID. The quantum state of the single-junction SQUID represents the neuron state, and the output voltage of the double-junction SQUID, which is operated in a nonlatching mode with shunt resistors, is a sigmoid-shaped function of the input. Both variable synapse circuits are composed of multiple shunted double-junction SQUIDs. The first type changes its conductance value by using both superconducting and voltage states. The second variable synapse circuit changes its output current digitally by switching its bias currents. Besides numerical simulations of the circuit characteristics, we have fabricated superconducting neural chips in a Nb/AlO_x/Nb Josephson junction technology. The fundamental operation of each element and a 2-bit neural-based A/D converter have been successfully tested. A learning system with a variable synapse is also discussed     

Use of 2-dimensional arrays to determine the uniformity ofJosephson junctions

The linewidths of phase-locked, oscillating arrays of high temperature superconducting Josephson junctions have been used to estimate such statistical information for several junction processes using a fitting process to simulate results. Statistical data from arrays consisting of several hundred to many thousand junctions operating, and at least partially phase locked, at 77K are being used to characterize and improve junction processes. Spreads on critical currents for three different processes; step edge, edge SNS and electron-beam defined nanobridges, have ranged from ±3% to 15% (lσ) and on normal state resistances from ±2% to 11%     

Transparent metal oxide electrode CID imager

The use of metal oxide transparent conductive electrodes in a charge-injection device (CID) imaging array has resulted in a quantum efficiency of approximately 70 percent, uniform throughout the visible, with useful array response out to 3500 Å. The advantages of using highly transparent metal oxide electrodes for the fabrication of frontside illuminated arrays is described. A new high density CID cell is described, which does not require any contact windows or diffused regions. This cell structure is simply composed of two crossed rows and column electrodes and is easily fabricated. Because no contacts are required, it can be reduced in size for large high density arrays. The results for a 32 × 32 self-scanned array are presented.     

取样光栅分布反馈激光器阵列器件研究

采用了一种基于取样光栅原理制作多通道增益-折射率耦合型光栅的方法,成功制作了8波长分布反馈(DFB)激光器阵列,阵列中各激光器的阈值电流为30～40 mA,注入电流为100 mA时的平均输出光功率为10mW,阵列器件实现了波长的可选择性激射,相邻激光器间的频率间距为200 GHz,验证了用取样光栅方法制作DFB激光器阵列的可行性。     

Lumped 50-Ω Arrays of SNS Josephson Junctions

For the first time, lumped series arrays of Josephson junctions have been fabricated with a transmission line-matched 50-Omega resistance. These arrays also have the thousands of junctions necessary to produce a metrologically significant voltage. This approach is expected to increase the output voltage per array and to optimize their performance for Josephson voltage standards. Traditional Josephson arrays for voltage standards have used distributed microwave structures, where array lengths are several multiples of the driving wavelength. The lumped arrays in this work have physical lengths shorter than a quarter of the microwave drive wavelength and total normal-state resistances nearly equal to the transmission line impedance. Fabrication of these arrays was made possible by use of a newly developed Nb-(MoSi₂-Nb)_n stacked junction technology. We present measurements of the microwave response of lumped arrays with total normal resistances up to 54 Omega and with various termination resistances. A simple numerical model is presented that accounts for the spatial distribution of the microwave current and for the nonuniformity of the junction critical currents. The resulting simulations agree well with experimental results     

0.1-μm gate-length superconducting FET

A superconducting field-effect transistors (FET) with a 0.1-μm-length gate electrode was fabricated and tested at liquid-helium temperature. Two superconducting electrodes (source and drain) were formed on the same Si substrate surface with an oxide-insulated gate electrode by a self-aligned fabrication process. Superconducting current flowing through the semiconductor (Si) between the two superconducting electrodes (Nb) was controlled by a gate-bias voltage