152 research outputs found
Extraction of carrier lifetime in Ge waveguides using pump probe spectroscopy
Carrier lifetimes in Ge-on-Si waveguides are deduced using time-resolved infrared transmission pump-probe spectroscopy. Dynamics of pump-induced excess carriers generated in waveguides with varying Ge thickness and width is probed using a CW laser. The lifetimes of these excess carriers strongly depend on the thickness and width of the waveguide due to defect assisted surface recombination. Interface recombination velocities of 0.975 x 10(4) cm/s and 1.45 x 10(4) cm/s were extracted for the Ge/Si and the Ge/SiO2 interfaces, respectively. Published by AIP Publishing
A closed-loop digitally controlled MEMS gyroscope with unconstrained Sigma-Delta force-feedback
In this paper, we describe the system architecture and prototype measurements of a MEMS gyroscope system with a resolution of 0.025 degrees/s/root Hz. The architecture makes extensive use of control loops, which are mostly in the digital domain. For the primary mode both the amplitude and the resonance frequency are tracked and controlled. The secondary mode readout is based on unconstrained Sigma Delta force-feedback, which does not require a compensation filter in the loop and thus allows more beneficial quantization noise shaping than prior designs of the same order. Due to the force-feedback, the gyroscope has ample dynamic range to correct the quadrature error in the digital domain. The largely digital setup also gives a lot of flexibility in characterization and testing, where system identification techniques have been used to characterize the sensors. This way, a parasitic direct electrical coupling between actuation and readout of the mass-spring systems was estimated and corrected in the digital domain. Special care is also given to the capacitive readout circuit, which operates in continuous time
Regression modeling for digital test of ΣΔ modulators
The cost of Analogue and Mixed-Signal circuit
testing is an important bottleneck in the industry, due to timeconsuming
verification of specifications that require state-ofthe-
art Automatic Test Equipment. In this paper, we apply
the concept of Alternate Test to achieve digital testing of
converters. By training an ensemble of regression models that
maps simple digital defect-oriented signatures onto Signal to
Noise and Distortion Ratio (SNDR), an average error of 1:7%
is achieved. Beyond the inference of functional metrics, we show
that the approach can provide interesting diagnosis information.Ministerio de Educación y Ciencia TEC2007-68072/MICJunta de Andalucía TIC 5386, CT 30
Direction-dependent Optical Modes in Nanoscale Silicon Waveguides
On-chip photonic networks have the potential to transmit and route
information more efficiently than electronic circuits. Recently, a number of
silicon-based optical devices including modulators, buffers, and wavelength
converts have been reported. However, a number of technical challenges need to
be overcome before these devices can be combined into network-level
architectures. In particular, due to the high refractive index contrast between
the core and cladding of semiconductor waveguides, nanoscale defects along the
waveguide often scatter light into the backward-propagating mode. These
reflections could result in unwanted feedback to optical sources or crosstalk
in bidirectional interconnects such as those employed in fiber-optic networks.
It is often assumed that these reflected waves spatially overlap the
forward-propagating waves making it difficult to implement optical circulators
or isolators which separate or attenuate light based on its propagation
direction. Here, we individually identify and map the near-field mode profiles
of forward-propagating and reflected light in a single-mode silicon waveguide
using Transmission-based near-field scanning optical microscopy (TraNSOM). We
show that unlike fiber-optic waveguides, the high-index-contrast and nanoscale
dimensions of semiconductor waveguides create counter propagating waves with
distinct spatial near-field profiles. These near-field differences are a
previously-unobserved consequence of nanoscale light confinement and could
provide a basis for novel elements to filter forward-propagating from reflected
light
Advanced code-division multiplexers for superconducting detector arrays
Multiplexers based on the modulation of superconducting quantum interference
devices are now regularly used in multi-kilopixel arrays of superconducting
detectors for astrophysics, cosmology, and materials analysis. Over the next
decade, much larger arrays will be needed. These larger arrays require new
modulation techniques and compact multiplexer elements that fit within each
pixel. We present a new in-focal-plane code-division multiplexer that provides
multiplexing elements with the required scalability. This code-division
multiplexer uses compact lithographic modulation elements that simultaneously
multiplex both signal outputs and superconducting transition-edge sensor (TES)
detector bias voltages. It eliminates the shunt resistor used to voltage bias
TES detectors, greatly reduces power dissipation, allows different dc bias
voltages for each TES, and makes all elements sufficiently compact to fit
inside the detector pixel area. These in-focal-plane code-division multiplexers
can be combined with multi-gigahertz readout based on superconducting
microresonators to scale to even larger arrays.Comment: 8 pages, 3 figures, presented at the 14th International Workshop on
Low Temperature Detectors, Heidelberg University, August 1-5, 2011,
proceedings to be published in the Journal of Low Temperature Physic
- …