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Québec, Canada

Law P.-C.,University of Illinois at Urbana - Champaign | Liu Y.-S.,University of Illinois at Urbana - Champaign | Croteau A.,INO | Dragic P.D.,University of Illinois at Urbana - Champaign
Optical Materials Express

We present measurements and modeling of the effect of P2O5 doping on the acoustic damping and temperature sensitivity coefficients of silica fibers. In particular, the Brillouin gain spectrum of a highly P-doped fiber is measured and investigated at different temperatures. It is found that the acoustic damping coefficient (proportional to the Brillouin spectral width) of phosphorus pentoxide (1.41 × 105 m-1 for bulk P2O5 at 11 GHz) is similar to, but larger than, that of germanium dioxide. Additionally, the acoustic velocity (and thereby the Stokes' shift) is found to be much less dependent on temperature in P2O5 (+ 0.12 m/s/°C) than in SiO2 (+ 0.56 m/s/°C). Using these coefficients (the thermo-acoustic coefficients), the modeled and unique slopes of the Stoke'-shift-versus-temperature curves for the four observed acoustic modes each lie within 3% of the measured values. Finally, utilizing both the thermo-optic and thermo-acoustic coefficients, a design example is presented where a composition is determined for which the dependence of the Brillouin frequency shift on temperature is minimized. In this example, the calculated temperature sensitivity is less than 5 kHz/°C over the temperature range -100 °C < T < 100 °C for the molar composition 0.54P2O5:0.46SiO2. © 2011 Optical Society of America. Source

Law P.-C.,University of Illinois at Urbana - Champaign | Croteau A.,INO | Dragic P.D.,University of Illinois at Urbana - Champaign
Optical Materials Express

We present measurements and modeling of the effect of P2O5 doping on the strain sensitivity coefficients of silica fibers. In particular, the Brillouin gain spectrum of a heavily P2O5-doped fiber is measured and investigated at different strains. We provide measurements of the strainoptic coefficient (SOC) and the strain-acoustic coefficient (SAC), obtained to be + 0.139 and + 9854m/sec/ε, respectively, both of which are less than the pure silica values. The Pockels' coefficients p11 and p12 for bulk P2O5 are also estimated via Brillouin gain measurements. Using the strain coefficients, the modeled and unique slopes of the Stokes'-shift-versusstrain curves for the four observed acoustic modes in the fiber each lie within 2% of the measured values. © 2012 Optical Society of America. Source

Carlson C.G.,Urbana University | Carlson C.G.,United States Air Force Academy | Keister K.E.,Urbana University | Dragic P.D.,Urbana University | And 2 more authors.
Journal of the Optical Society of America B: Optical Physics

Emission spectra in the ̃240-1100 nm wavelength region as well as the temporally resolved decay of Yb3+ and point defect spontaneous emission have been recorded when aluminosilicate optical fibers doped with Yb are irradiated with ̃160 fs laser pulses having a central wavelength of ̃250 nm (hω=5 eV). Photoexcitation of the fibers in this region of the deep ultraviolet (UV) provides access simultaneously to the Type II Si oxygen deficiency center (ODC), the non-bridging oxygen hole center (NBOHC: an oxygen-excess defect), and the Ge ODC. Emission from all of these defects in the ultraviolet and/or visible is observed, as is intense fluorescence at 976 nm from Yb3++. Absorption measurements conducted in the ̃230-265 nm region with a sequence of UV light-emitting diodes reveal a continuum peaking at ̃248 nm and having a spectral width of ̃18 nm (FWHM), confirming that the 250 nm laser pump is photoexciting predominantly the ODC. The temporal histories of the optically active defect and rare earth ion emission waveforms, in combination with time-integrated spectra, suggest that the Si ODC(II) triplet state directly excites Yb3++ as well as at least one other intrinsic defect in the silica network. Prolonged exposure of the Yb-doped fibers to 250 nm radiation yields increased Yb3++, NBOHC, and Si ODC(II) singlet emission which is accompanied by a decline in Si ODC(II) triplet fluorescence, thus reinforcing the conclusion-drawn on the basis of luminescence decay constants-that the triplet state of Si ODC(II) is the immediate precursor to the NBOHC and is partially responsible for Yb ion emission at 976 nm. This conclusion is consistent with the observation that exposure of fiber to 5 eV radiation slightly suppresses ODC absorption in the ̃240-255 nm region while simultaneously introducing an absorption continuum extending from 260 nm to below 235 nm (hω≈5.28 eV). These results suggest that ODC →E' center conversion assumes a role in excitation transfer to Yb3++. © 2010 Optical Society of America. Source

Crawled News Article
Site: http://www.nature.com/nature/current_issue/

Time is running out for Indian scientists to build a facility that would let them compete in one of the hottest races in physics. The India-based Neutrino Observatory (INO) — an effort to learn about the masses and other properties of mysterious particles called neutrinos — is under threat as a result of baseless rumours about its aims and environmental impact. Despite a government go-ahead in January 2015 to build a massive detector under a mountain in the southern state of Tamil Nadu, opposition from environmentalists and state politicians means that not a single grain of earth has been shifted. Neutrinos are abundant subatomic particles that are extremely hard to detect. Billions pass through each square centimetre of Earth every second, but barely any leave a trace. The INO would study neutrinos produced when cosmic rays strike the atmosphere, and would seek to reveal the relative masses of the three known types of neutrino. The measurements could lead to Nobel-prize-worthy insights into the relationship between nature’s four fundamental forces, as well as the imbalance between matter and antimatter in the Universe. But if the INO is not built soon, other projects — including one that broke ground in China a year ago — may get there first, says D. Indumathi, a theorist at the Institute of Mathematical Sciences in Chennai who is part of the INO collaboration, and coordinates outreach for it. “Longer than a year of delay and I think it will be difficult to have viable physics goals, at least of the current type,” she says. Conceived in 2001 and originally slated for completion in 2012, the INO has faced a rocky path to construction. To shield the enormous detector from the confounding zoo of subatomic particles that pummels Earth’s surface, the facility needs to be built more than a kilo­metre underground. The first earmarked site was ruled out in 2009 after a lengthy battle with conservationists over its proximity to an elephant and tiger reserve. The current site, in the Tamil Nadu district of Theni, faced opposition as soon as it was put forward in 2010. Local villagers worried that the facility would deplete or contaminate their restricted water supply, and cut off access to land for grazing livestock, says Indumathi. But, she says, villagers consented after scientists assured them that the facility would not interfere with their resources. Since then, however, local environmental organizations and regional politicians have taken up the issue, and the list of objections has swelled to include fears that the lab will emit radiation and store nuclear weapons, and that the excavation will threaten a nearby dam. The rumours are untrue, says Naba Mondal, a physicist at the Tata Institute of Fundamental Research in Mumbai who leads the INO collaboration. INO scientists have visited schools and held community meetings to counter misconceptions. But many villagers have turned against the project. “They don’t know what the truth is, and I can understand that,” says Mondal. At the root of the rumours is mistrust of the state and the scientific establishment, says Govind Krishnan, an Indian journalist who has closely followed the project. He believes that the fears that have been raised lie “in the realm of fantasy”, but are understandable given the poor environmental record of past state-sponsored construction projects. Govind disagrees with activists who say that INO scientists have ignored the project’s impact on the poor, but he says that scientists’ efforts have been hampered by class and linguistic barriers. India’s government allocated 15 billion rupees (US$225 million) to construction when it gave the INO the green light last year, but the Madras High Court in Chennai brought the project to a standstill in March following a petition from local activists and politicians. The court said that the Tamil Nadu Pollution Control Board must give consent before construction can start. This is normally a routine, 45-day step, but the process has so far taken 9 months, says Mondal. The politically contentious nature of the project means that the local board may well delay until after state elections in May. “I am confident that it will eventually be approved, but the question is when,” says Mondal. The delay is damaging the morale of students and researchers on the project, he adds. Meanwhile, China expects to complete the Jiangmen Underground Neutrino Observatory in 2019. To remain competitive, the INO must start construction in the next few months, says Mondal. “Science is something you have to do in time. If you are not in time, your results may not be that important.” But neutrino physicists say that even if the INO loses the race, its findings would help to corroborate discoveries at other detectors. The INO takes a unique approach — using 50,000 tonnes of magnetized iron to separate atmospheric neutrino observations from their antineutrino counterparts. That will make its results interesting whenever they come out, says Mark Messier, a physicist at Indiana University Bloomington and co-spokesperson for the NOvA Neutrino Experiment at Fermilab in Batavia, Illinois, which also has a chance of solving the neutrino-mass mystery. Researchers point to other benefits, too. Putting a physics laboratory deep underground gives India the opportunity to host research into areas such as dark matter, they say — and it is empowering for Indian scientists to bring a major physics facility to fruition. “Already I’ve seen the tremendous difference it’s made to students having an experiment on which they call the shots,” says Indumathi. “So I really don’t care whether we get a Nobel prize or not.”

Andersson M.,Swedish Defence Research Agency | Gustafsson F.,Linkoping University | Prevost D.,INO | St-Laurent L.,INO
IEEE Journal on Selected Topics in Signal Processing

We investigate the unsupervised K -means clustering and the semi-supervised hidden Markov model (HMM) to automatically detect anomalous motion patterns in groups of people (crowds). Anomalous motion patterns are typically people merging into a dense group, followed by disturbances or threatening situations within the group. The application of K-means clustering and HMM are illustrated with datasets from four surveillance scenarios. The results indicate that by investigating the group of people in a systematic way with different K values, analyze cluster density, cluster quality and changes in cluster shape we can automatically detect anomalous motion patterns. The results correspond well with the events in the datasets. The results also indicate that very accurate detections of the people in the dense group would not be necessary. The clustering and HMM results will be very much the same also with some increased uncertainty in the detections. © 2007-2012 IEEE. Source

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