Giovanni Breglio

Electro-optical switch and continuously tunable filter based on a Bragg grating in a planar waveguide with a liquid crystal overlayer

The possibility of using smectic liquid crystals in active wave- guide devices is explored through the analysis of both an integrated electro-optic switch and a continuously tunable filter. The two devices are based on a Bragg grating in planar waveguide with a liquid crystal over- layer, which enables changing the spectral behavior of the device. The fast and bistable switching of smectic C* in the surface-stabilized liquid crystal structure is used to investigate the possibility of realizing an inte- grated electro-optical switch. The principal advantage of this device is its spectral signature, which enables us to overcome the problems of inten- sity dependent devices. The soft-mode of smectic A* liquid crystals, enabling a continuous modulation of extraordinary refractive index, is used to design an integrated wavelength filter in the wavelength range of interest for optical communications. The principal advantages of such device include fast tuning speed, wide tuning range, low power dissipa- tion, and low cost.

An optoelectronic sensor for cure monitoring in thermoset-based composites

In this paper, we present the design and the experimental results of an optoelectronic fiber optic sensor able to monitor the polymerization reactions of thermoset polymer-matrix composite processing. These kinds of materials are now widely used in large industrial areas: aeronautical, aerospace, automotive and civil due to their low cost/low weight features. Unfortunately, their final properties are strongly dependent on the processing stage. In order to ensure the setting of correct processing parameters (i.e. temperature and pressure), it is crucial to monitor, during the reaction, the process variables such as degree of cure (DOC) and viscosity. In this work, we point out our attention upon the use of a fiber optic sensor, which can be embedded into the composite component, to monitor the cross-linking formation, occurring during the curing process. Based on the Fresnel refraction principles, the proposed sensor scheme is sensitive to refractive index variation. Experimental results are shown on sensor capability to monitor the cure kinetics, and a novel calibration method is proposed in order to link the optoelectronic response directly to the DOC of the reacting resin.

All-silicon optical temperature sensor based on Multi-Mode Interference.

In this paper we present a novel approach to temperature sensing with optoelectronic devices which relies on the usage of bare silicon as the transducing material. The device is composed by a single mode input waveguide, a MMI region where a number of higher order modes is also allowed to propagate and two output waveguides. The refractive index variation in the MMI section due to temperature shifts induces different phase velocities of the various propagating modes. The position of the input and output waveguides together with the length and width of the MMI section are chosen in order to maximize the sensitivity of the device. Analytical calculations are presented together with BPM simulations aimed to the maximization of the sensitivity of the sensor as a function of its geometries.

Experimental Detection and Numerical Validation of Different Failure Mechanisms in IGBTs During Unclamped Inductive Switching

The physics of the different failure modes that limit the maximum avalanche capability during unclamped inductive switching (UIS) in punchthrough (PT) and not PT (NPT) insulated-gate bipolar transistor (IGBT) structures is analyzed in this paper. Both 3-D electrothermal numerical simulations and experimental evaluations support the theoretical analysis. Experimental results for UIS test show that, at low time duration (or inductance value) of the test, the UIS limit moves from energy limitation to current limitation. While the energy limitation is well known, the current-limited failures are less studied. In this paper, the current limit for UIS test is analyzed in detail, and the cause is attributed to a filamentary current conduction due to the presence of a negative differential resistance (NDR) region in the IC- VCE curve in breakdown. The filamentary current conduction locally increases the current density causing early device latch-up and possible device failure at a current much lower than the one dictated by energy limitations. The physical parameters that affect both the onset of NDR region and the failure current are discussed for both an NPT trench IGBT structure with a local lifetime control and a PT trench IGBT structure with a field-stop layer.

Analytical model for thermal instability of low voltage power MOS and SOA in pulse operation

Thermal instability presented by some high current power MOS has been shown to limit significantly the SOA capability. In this paper, we present a new analytical model to explain this type of instability in transient operation, based on an analytical formulation for both the positive temperature coefficient of the drain current and for the thermal resistance. The model is capable of predicting the onset of thermal instability for a given device structure and layout, and can be used both to define the allowed SOA of the device and as a design guide to design more rugged devices.

Thermal instabilities in high current power MOS devices: experimental evidence, electro-thermal simulations and analytical modeling

The phenomenon of the thermal instability presented by some high current power MOS has been intensively investigated, both by experimental means and by numerical simulations. An analytical expression for the positive temperature coefficient of the Drain current has been developed and a model for the thermal instability in transient operation has been proposed. The results explain the main causes of the thermal instability and give some rules to evaluate the possible failure occurrence for a given device.

Fast silicon-on-silicon optoelectronic router based on a BMFET device

In this paper, a new configuration for an optoelectronic router is presented. The device is composed by a single-mode input waveguide, a two-mode active section where the light steering by means of mode mixing between the fundamental and the first higher order mode is achieved. A terminal Y-branch is placed to separate the output channels. Active modulation is achieved by means of a three-terminal electronic device, which injects and controls the free carrier plasma inside the active region. The usage of a three-terminal device allows extremely fast dynamic response. Switching times of few nanoseconds are obtained together with an average crosstalk of -11 dB.

Electro-thermal instability in multi-cellular Trench-IGBTs in avalanche condition: Experiments and simulations

This paper reports on the results of a study on electro-thermal instability induced in multi-cellular Trench-IGBTs in avalanche condition. Experimental measurements, made on T-IGBTs, show possible inhomogeneous current distribution under Unclamped Inductive Switching (UIS) confirmed by transient infrared thermography measurements. Together with this, an analytical modeling of avalanche behavior has been included in a compact electro-thermal simulator to study the interaction between a large numbers of elementary cells of T-IGBTs forced in avalanche condition. Electro-thermal simulations qualitatively replicate the possible inhomogeneous operation observed experimentally. Finally a possible theoretical interpretation of the instability in avalanche condition for T-IGBT is given.

Reliability enhancement with the aid of transient infrared thermal analysis of smart Power MOSFETs during short circuit operation

Abstract The usage of novel measurement techniques enhances the capabilities of researchers and power device manufacturers to understand and address reliability problems in novel Smart Power Devices. Along this line of argument, this work describes a method to improve the reliability of the smart Power MOSFET devices by design. The design optimization process involves Silicon layout, interconnections, packaging and protection strategy as well. Accurate thermal transient analyses, made possible by the unique features of a custom infrared radiometric microscope experimental setup which allows dynamic temperature detection with a bandwidth of 1 MHz over the chip area, indicated the way to minimize peak temperature and to verify the effect of the optimization.

Analysis of feasibility on the use of fiber Bragg grating sensors as ultrasound detectors

The spectral response changing of a Fiber Bragg grating due to the interactions with an ultrasound wave have been numerically analyzed by a transfer matrix approach has been used, taking into account the geometrical and elasto-optic effects. Numerical analysis show that its possible to characterize the ultrasound wave, only if the ratio between the ultrasonic wavelength and the length of the used grating exceeds an established value, which depends on both the characteristics of the Bragg grating and the ultrasound amplitude itself.