Measuring thermal and mechanical stresses on optical fiber in a DC module using fiber Bragg gratings

Fiber Bragg gratings (FBGs) can be used as sensors to monitor stress and test temperature during the processing and handling of optical fiber. As the FBG experiences a combination of mechanical and thermal loading, the return Bragg wavelength will shift proportionately to the magnitude of the load. This paper discusses the use of these sensors in quantifying induced stress on fiber during the packaging of a dispersion-compensating module (DCM) and the ensuing environmental exposure. There are two potential fiber-failure modes for fiber wound in DCMs, namely microbend-induced attenuation and fiber failure from fatigue. The ability to quantify fiber stress provides a useful feedback tool in the design phase of these modules that can aid in reducing the risk of mechanical and optical failure modes. A practical characterization process was developed to decouple thermal and stress effects on FBGs based on results from current literature and from this study. Uncoated Bragg sensors were found to respond linearly between -40 to 80/spl deg/C. Gratings with a protective polymer recoat departed from the linear behavior of the uncoated gratings below -5/spl deg/C. It was determined that the recoat material places less than 25 MPa (3.6 klbf/in/sup 2/) of axial compression on the fiber at -40/spl deg/C. Four gratings with different Bragg wavelengths were spliced into 10 km of fiber and wound into a DCM. The wind-induced stress on all four gratings quickly relaxed. The module was then thermal cycled between -40 and +75/spl deg/C. The overall stress on each grating was acceptably low for reliability purposes. The maximum stress of 17 MPa (2.5 klbf/in/sup 2/) was observed at the lowest temperature.     

A high-precision contrast injector for small animal x-ray digital subtraction angiography

The availability of genetically altered animal models of human disease for basic research has generated great interest in new imaging methodologies. Digital subtraction angiography (DSA) offers an appealing approach to functional imaging in small animals because of the high spatial and temporal resolution, and the ability to visualize and measure blood flow. The micro-injector described here meets crucial performance parameters to ensure optimal vessel enhancement without significantly increasing the total blood volume or producing overlap of enhanced structures. The micro-injector can inject small, reproducible volumes of contrast agent at high flow rates with computer-controlled timing synchronized to cardiopulmonary activity. Iterative bench-top and live animal experiments with both rat and mouse have been conducted to evaluate the performance of this computer-controlled micro-injector, a first demonstration of a new device designed explicitly for the unique requirements of DSA in small animals. Injection protocols were optimized and screened for potential physiological impact. For the optimized protocols, we found that changes in the time-density curves for representative regions of interest in the thorax were due primarily to physiological changes, independent of micro-injector parameters.     

The change in initial lung volume during exercise

The lung volumes from which inhalation and exhalation proceed change during exercise. There have been previous attempts to predict these lung volumes but none have been completely successful. Prediction of initial lung volume is important in the accurate calculation of respiratory power during exercise, especially when the subject is encumbered with respiratory protective apparatus and exercising above the anaerobic threshold. Although there is no direct work advantage to varying lung volumes at the beginning of inspiration or expiration, such changes appear to be dictated by the asymmetry of lung recoil pressure about the lung relaxation volume. Equalizing inspiratory and expiratory elastic pressures has been found to match available data on lung initial volume     

On the interaction of adaptive filtering, identification, andcontrol

The aim of the article is to add to the evidence of the need for interaction between the two field of (i) signal processing and (ii) control and estimation. The focus in on the the interaction specialities from (i), i.e., adaptive filtering, and (ii)i.e., adaptive identification and control. For evidence, three basic communication systems applications of adaptive filtering are used to evoke system identification (and adaptive control) style problems amenable to analytical tools. The applications raise a number of unresolved issues that challenge existing theory. The connections drawn to adaptive identification and control problems (and relevant analytical tools) suggest approaches to these challenges     

Application of coupled electro-thermal and physics-of-failure-based analysis to the design of accelerated life tests for power modules

In the reliability theme a central activity is to investigate, characterize and understand the contributory wear-out and overstress mechanisms to meet through-life reliability targets. For power modules, it is critical to understand the response of typical wear-out mechanisms, for example wire-bond lifting and solder degradation, to in-service environmental and load-induced thermal cycling. This paper presents the use of a reduced-order thermal model coupled with physics-of-failure-based life models to quantify the wear-out rates and life consumption for the dominant failure mechanisms under prospective in-service and qualification test conditions. When applied in the design of accelerated life and qualification tests it can be used to design tests that separate the failure mechanisms (e.g. wire-bond and substrate-solder) and provide predictions of conditions that yield a minimum elapsed test time. The combined approach provides a useful tool for reliability assessment and estimation of remaining useful life which can be used at the design stage or in-service. An example case study shows that it is possible to determine the actual power cycling frequency for which failure occurs in the shortest elapsed time. The results demonstrate that bond-wire degradation is the dominant failure mechanism for all power cycling conditions whereas substrate-solder failure dominates for externally applied (ambient or passive) thermal cycling.     

Transmission analysis on the resonant-cavity combiner of mobileradio transmitters

A general approach to the transmission loss analysis of resonant-cavity combiners of the type used in cellular and in other mobile radio systems is described. A simple closed-form solution is derived for a particular type of combiner in which the cavities are connected in a star configuration via transmission lines that are odd multiples of quarter-wavelength long at the center frequency. This is an exact solution at the center of the combiner band in which one cavity is tuned to this frequency and other cavities are tuned symmetrically in pairs about this frequency. For narrow-band combiners, such as those used in cellular systems, the transmission loss obtained from this solution should be accurate for the entire band. The analysis is valid over wide ranges of combiner parameters: loaded and unloaded Q's of cavities, frequency spacings, and the number of ways it is combined. The comparison of the analytic results with laboratory measurements, although limited in scope, shows that they are in good agreement     

MBE growth and properties of ZnO on sapphire and SiC substrates

Molecular beam epitaxy (MBE) of ZnO on both sapphire and SiC substrates has been demonstrated. ZnO was used as a buffer layer for the epitaxial growth of GaN. ZnO is a würtzite crystal with a close lattice match (<2% mismatch) to GaN, an energy gap of 3.3 eV at room temperature, a low predicted conduction band offset to both GaN and SiC, and high electron conductivity. ZnO is relatively soft compared to the nitride semiconductors and is expected to act as a compliant buffer layer. Inductively coupled radio frequency plasma sources were used to generate active beams of nitrogen and oxygen for MBE growth. Characterization of the oxygen plasma by optical emission spectroscopy clearly indicated significant dissociation of O₂ into atomic oxygen. Reflected high energy electron diffraction (RHEED) of the ZnO growth surface showed a two-dimensional growth. ZnO layers had n-type carrier concentration of 9 × 10¹⁸ cm⁻³ with an electron mobility of 260 cm²/V-s. Initial I-V measurements displayed ohmic behavior across the SiC/ZnO and the ZnO/GaN heterointerfaces. RHEED of GaN growth by MBE on the ZnO buffer layers also exhibited a two-dimensional growth. We have demonstrated the viability of using ZnO as a buffer layer for the MBE growth of GaN.     

Signal processing hardware in a radio LAN demonstrator

In order to demonstrate the practicality of high speed radio LANs, such as HIPERLAN, a hardware demonstrator has been built. To overcome the inter-symbol interference caused by the dispersive nature of the indoor radio channel, a decision feedback equaliser has been included in the system. No central synchronisation is provided in a network and a node must be able to derive all synchronisation from a received packet. This paper describes all of the signal processing hardware built for the demonstrator. The demonstrator is not standards compliant but the physical layer parameters are very similar and the signal processing required in HIPERLAN equipment will be very similar to that in the demonstrator.     

Grain nucleation and texture analysis of electroless copper deposition on a palladium seed layer

Electroless copper grains were deposited on a Pd seed layer under varying bath conditions. The seed layer was determined to have a (111) texture using grazing incident x-ray (GIX) diffraction. Multiple nucleation sites in the grain boundaries were imaged using a scanning tunneling microscope. Continual copper growth produced row-like structures. The texture of the electrolessly deposited copper (ED-Cu) grains were determined to be (111). No radial grain orientation for the Pd seed layer or the ED-Cu thin film was detected using GIX diffraction. Atomic force microscope images indicated continual Cu nucleation throughout the deposition process. PdH was formed as a by-product of the electroless deposition process, and detected by x-ray diffraction.     

Molecular-beam epitaxial growth of HgCdTe infrared focal-plane arrays on silicon substrates for midwave infrared applications

Molecular beam epitaxy has been employed to deposit HgCdTe infrared detector structures on Si(112) substrates with performance at 125K that is equivalent to detectors grown on conventional CdZnTe substrates. The detector structures are grown on Si via CdTe(112)B buffer layers, whose structural properties include x-ray rocking curve full width at half maximum of 63 arc-sec and near-surface etch pit density of 3–5 × 10⁵ cm⁻² for 9 μm thick CdTe films. HgCdTe p⁺-on-n device structures were grown by molecular beam epitaxy (MBE) on both bulk CdZnTe and Si with 125K cutoff wavelengths ranging from 3.5 to 5 μm. External quantum efficiencies of 70%, limited only by reflection loss at the uncoated Si-vacuum interface, were achieved for detectors on Si. The current-voltage (I-V) characteristics of MBE-grown detectors on CdZnTe and Si were found to be equivalent, with reverse breakdown voltages well in excess of 700 mV. The temperature dependences of the I-V characteristics of MBE-grown diodes on CdZnTe and Si were found to be essentially identical and in agreement with a diffusion-limited current model for temperatures down to 110K. The performance of MBE-grown diodes on Si is also equivalent to that of typical liquid phase epitaxy-grown devices on CdZnTe with R₀A products in the 10⁶–10⁷ Θ-cm² range for 3.6 μm cutoff at 125K and R₀A products in the 10⁴–10⁵ Θ-cm² range for 4.7 μm cutoff at 125K.