225 research outputs found
Molecular relaxation effects in hydrogen chloride photoacoustic detection
A photoacoustic (PA) sensor has been developed to monitor hydrogen chloride at sub-ppm level in the 1740-nm region. The system was designed to control the process in the novel low-water-peak optical fiber manufacturing process. Relaxation effects in hydrogen chloride PA detection in oxygen-helium and nitrogen-helium gas mixtures are presented, showing that the generation of the PA signal is strongly affected by the ratio of these substances. In addition, the role of water vapor in the PA signal is investigate
Near-infrared laser photoacoustic detection of methane: the impact of molecular relaxation
A photoacoustic sensor has been developed for trace-gas monitoring using a near-infrared semiconductor laser emitting in the 2Μ3 band of methane at 1.65Όm. The apparatus was designed for on-line process control in the manufacturing of the novel low-water-peak fibres developed for optical telecommunications. The importance of collisional relaxation processes in the generation of the photoacoustic signal is reported in the particular case of CH4 detection in dry O2 and O2-N2 mixtures. The negative influence of these effects results in a strongly reduced and phase-shifted photoacoustic signal, induced by a fast resonant coupling between the vibrational states of methane and oxygen, associated with the slow relaxation of the excited oxygen molecules. An unusual parabolic response of the sensor with respect to the methane concentration has been observed and is discussed. Finally, the beneficial effect of several species, including water vapour and helium, acting as a catalyst to hasten the relaxation of the CH4-O2 system, is demonstrate
Ammonia trace measurements at ppb level based on near-IR photoacoustic spectroscopy
A photoacoustic sensor using a laser diode emitting near 1532nm in combination with an erbium-doped fibre amplifier has been developed for ammonia trace gas analysis at atmospheric pressure. NH3 concentration measurements down to 6ppb and a noise-equivalent detection limit below 3ppb in dry air are demonstrated. Two wavelength-modulation schemes with 1f and 2f detection using a lock-in amplifier were investigated and compared to maximise the signal-to-noise ratio. A quantitative analysis of CO2 and H2O interference with NH3 is presented. Typical concentrations present in ambient air of 400ppm CO2 and 1.15% H2O (50% relative humidity at 20°C) result in a NH3 equivalent concentration of 36ppb and 100ppb, respectivel
An Optimized Spline-Based Registration of a 3D CT to a Set of C-Arm Images
We have developed an algorithm for the rigid-body registration of
a CT volume to a set of C-arm images.
The algorithm uses a gradient-based iterative minimization of a least-squares measure
of dissimilarity between the C-arm images and projections of the
CT volume. To compute projections, we use a novel method for fast
integration of the volume along rays. To improve robustness and
speed, we take advantage of a coarse-to-fine processing of the
volume/image pyramids. To compute the projections of the volume,
the gradient of the dissimilarity measure, and the multiresolution
data pyramids, we use a continuous image/volume model based on
cubic B-splines, which ensures a high interpolation accuracy and a
gradient of the dissimilarity measure that is well defined
everywhere. We show the performance of our algorithm on a human
spine phantom, where the true alignment is determined using a set
of fiducial markers
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On Automorphisms and Focal Subgroups of Blocks
Given a p-block B of a finite group with defect group P and fusion system on P, we show that the rank of the group is invariant under stable equivalences of Morita type. The main ingredients are the construction, due to BrouĂ© and Puig, a theorem of Weiss on linear source modules, arguments of Hertweck and Kimmerle applying Weissâ theorem to blocks, and connections with integrable derivations in the Hochschild cohomology of block algebras
Wavelet entropy and fractional Brownian motion time series
We study the functional link between the Hurst parameter and the Normalized
Total Wavelet Entropy when analyzing fractional Brownian motion (fBm) time
series--these series are synthetically generated. Both quantifiers are mainly
used to identify fractional Brownian motion processes (Fractals 12 (2004) 223).
The aim of this work is understand the differences in the information obtained
from them, if any.Comment: 10 pages, 2 figures, submitted to Physica A for considering its
publicatio
Descent of Equivalences and Character Bijections
Categorical equivalences between block algebras of finite groupsâsuch as Morita and derived equivalencesâare well known to induce character bijections which commute with the Galois groups of field extensions. This is the motivation for attempting to realise known Morita and derived equivalences over non-splitting fields. This article presents various results on the theme of descent to appropriate subfields and subrings. We start with the observation that perfect isometries induced by a virtual Morita equivalence induce isomorphisms of centres in non-split situations and explain connections with Navarroâs generalisation of the AlperinâMcKay conjecture. We show that Rouquierâs splendid Rickard complex for blocks with cyclic defect groups descends to the non-split case. We also prove a descent theorem for Morita equivalences with endopermutation source
Characterization of laser propagation through turbulent media by quantifiers based on the wavelet transform: dynamic study
We analyze, within the wavelet theory framework, the wandering over a screen
of the centroid of a laser beam after it has propagated through a time-changing
laboratory-generated turbulence. Following a previous work (Fractals 12 (2004)
223) two quantifiers are used, the Hurst parameter, , and the Normalized
Total Wavelet Entropy, . The temporal evolution of both
quantifiers, obtained from the laser spot data stream is studied and compared.
This allows us to extract information of the stochastic process associated to
the turbulence dynamics.Comment: 11 pages, 3 figures, accepted to be published in Physica
Generation of Arbitrary Frequency Chirps with a Fiber-Based Phase Modulator and Self-Injection-Locked Diode Laser
We present a novel technique for producing pulses of laser light whose
frequency is arbitrarily chirped. The output from a diode laser is sent through
a fiber-optical delay line containing a fiber-based electro-optical phase
modulator. Upon emerging from the fiber, the phase-modulated pulse is used to
injection-lock the laser and the process is repeated. Large phase modulations
are realized by multiple passes through the loop while the high optical power
is maintained by self-injection-locking after each pass. Arbitrary chirps are
produced by driving the modulator with an arbitrary waveform generator
Multi-hydrogenated compounds monitoring in optical fibre manufacturing process by photoacoustic spectroscopy
Sub-ppm hydrogen chloride (HCl) and water vapour (H2O) monitoring using photoacoustic spectroscopy in optical fibre manufacturing is reported. The development and performance of a sensor based on an acoustic resonant configuration is described, and on-site measurements are presented. Two DFB lasers emitting in the 1370nm and 1740nm range were used for the detection of H2O and HCl, respectively. A detection limit (defined for a SNR=3) of 60ppb for HCl and 40ppb for H2O was achieved. Contamination sources of the carrier gas used for the fibre preform manufacturing are identified and discusse
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