137 research outputs found
A curvilinear snake arm robot with gripper-axis fibre-optic image processor feedback
The official version of this article can be found at the link below.A curvilinear robot constructed from a number of modular flexible sections of fixed length and diameter but independently controlled radius and direction of curvature has been equipped with an optical fibre image guide transmitting images from between the gripper jaws to the remote TV camera of Microvision-100, a microcomputer controlled real-time DMA-based vision System that is easily trained to recognise the shape, position and orientation of components. The gripper position and orientation is controlled by feedback from the vision System, the action taken depending on component recognition and inspection for defects. Redundant degrees of freedom enable the curvilinear robot to avoid obstacles and work in confined spaces.The research programme described in this paper is supported by the U.K. Science and Engineering Research Council
Compressive optical interferometry
Compressive sensing (CS) combines data acquisition with compression coding to
reduce the number of measurements required to reconstruct a sparse signal. In
optics, this usually takes the form of projecting the field onto sequences of
random spatial patterns that are selected from an appropriate random ensemble.
We show here that CS can be exploited in `native' optics hardware without
introducing added components. Specifically, we show that random sub-Nyquist
sampling of an interferogram helps reconstruct the field modal structure. The
distribution of reduced sensing matrices corresponding to random measurements
is provably incoherent and isotropic, which helps us carry out CS successfully
Transparent Perfect Mirror
A mirror that reflects light fully and yet is transparent appears
paradoxical. Current so-called transparent or "one-way" mirrors are not
perfectly reflective and thus can be distinguished from a standard mirror.
Constructing a transparent "perfect" mirror has profound implications for
security, privacy, and camouflage. However, such a hypothetical device cannot
be implemented in a passive structure. We demonstrate here a transparent
perfect mirror in a non-Hermitian configuration: an active optical cavity where
a certain prelasing gain extinguishes Poynting's vector at the device entrance.
At this threshold, all vestiges of the cavity's structural resonances are
eliminated and the device presents spectrally flat unity-reflectivity, thus,
becoming indistinguishable from a perfect mirror when probed optically across
the gain bandwidth. Nevertheless, the device is rendered transparent by virtue
of persisting amplified transmission resonances. We confirm these predictions
in two photonic realizations: a compact integrated active waveguide and a
macroscopic all-optical-fiber system.Comment: The paper is highlighted in Nature Photonics:
http://www.nature.com/nphoton/journal/v11/n6/full/nphoton.2017.90.html The
supplementary data is available in:
http://pubs.acs.org/doi/suppl/10.1021/acsphotonics.7b0005
The 2005 - 2007 Bala (Ankara, central Turkey) earthquakes : a case study for strike-slip fault terminations
An intense seismic activity has been observed after the Bala (Ankara, NW central Turkey) earthquakes (30 July 2005: Mw=5.3, 20 December 2007: Mw=5.4, and 26 December 2007: Mw=5.3), continuing up to the present. The epicenters and the focal mechanism solutions of the earthquakes indicate that the right lateral strike-slip AfĆar fault, trending N55-60°W, is responsible for the main shocks. The AfĆar fault is thought to be the NW continuation of the TuzgölĂŒ fault zone, which is one of the main neotectonic elements in central Anatolia. On the other hand, the aftershock distributions of the 2005 event have a NNE trend, and those of the 2007 event show a NW trending. Some focal mechanism solutions of the 2005 Bala earthquake aftershocks indicate normal and oblique normal faulting that corresponds to the NNE-trending Karakeçili fault. It seems that seismic activation of the NNE-trending Karakeçili fault was triggered by the 2005 main shock (Mw=5.3) that occurred on the NW-trending right lateral strike-slip AfĆar fault. The overall neotectonic framework is that the northwestern edge of the TuzgölĂŒ fault zone, represented by the AfĆar fault in Bala, terminates in an extensional system represented by the oblique-slip Karakeçili faul
The 2005 - 2007 Bala (Ankara, central Turkey) earthquakes: a case study for strike-slip fault terminations
An intense seismic activity has been observed after the Bala (Ankara, NW central Turkey) earthquakes (30 July 2005: Mw=5.3, 20 December 2007: Mw=5.4, and 26 December 2007: Mw=5.3), continuing up to the present. The epicenters and the focal mechanism solutions of the earthquakes indicate that the right lateral strike-slip AfĆar fault, trending N55-60°W, is responsible for the main shocks. The AfĆar fault is thought to be the NW continuation of the TuzgölĂŒ fault zone, which is one of the main neotectonic elements in central Anatolia. On the other hand, the aftershock distributions of the 2005 event have a NNE trend, and those of the 2007 event show a NW trending. Some focal mechanism solutions of the 2005 Bala earthquake aftershocks indicate normal and oblique normal faulting that corresponds to the NNE-trending Karakeçili fault. It seems that seismic activation of the NNE-trending Karakeçili fault was triggered by the 2005 main shock (Mw=5.3) that occurred on the NW trending right lateral strike-slip AfĆar fault. The overall neotectonic framework is that the northwestern edge of the TuzgölĂŒ fault zone, represented by the AfĆar fault in Bala, terminates in an extensional system represented by the oblique-slip Karakeçili fault
What is the maximum differential group delay achievable by a space-time wave packet in free space?
The group velocity of 'space-time' wave packets propagation-invariant
pulsed beams endowed with tight spatio-temporal spectral correlations can
take on arbitrary values in free space. Here we investigate theoretically and
experimentally the maximum achievable group delay that realistic finite-energy
space-time wave packets can achieve with respect to a reference pulse traveling
at the speed of light. We find that this delay is determined solely by the
spectral uncertainty in the association between the spatial frequencies and
wavelengths underlying the wave packet spatio-temporal spectrum and not by
the beam size, bandwidth, or pulse width. We show experimentally that the
propagation of space-time wave packets is delimited by a
spectral-uncertainty-induced `pilot envelope' that travels at a group velocity
equal to the speed of light in vacuum. Temporal walk-off between the space-time
wave packet and the pilot envelope limits the maximum achievable differential
group delay to the width of the pilot envelope. Within this pilot envelope, the
space-time wave packet can locally travel at an arbitrary group velocity and
yet not violate relativistic causality because the leading or trailing edge of
superluminal and subluminal space-time wave packets, respectively, are
suppressed once they reach the envelope edge. Using pulses of width 4ps
and a spectral uncertainty of 20 pm, we measure maximum differential
group delays of approximately 150 ps, which exceed previously reported
measurements by at least three orders of magnitude
Coral record of Younger Dryas Chronozone warmth on the Great Barrier Reef
Author Posting. © American Geophysical Union, 2020. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography and Paleoclimatology 35(12), (2020): e2020PA003962, doi:10.1029/2020PA003962.The Great Barrier Reef (GBR) is an internationally recognized and widely studied ecosystem, yet little is known about its sea surface temperature (SST) evolution since the Last Glacial Maximum (LGM) (~20 kyr BP). Here, we present the first paleoâapplication of Isopora coralâderived SST calibrations to a suite of 25 previously published fossil Isopora from the central GBR spanning ~25â11 kyr BP. The resultant multicoral Sr/Caâ and ÎŽ18Oâderived SST anomaly (SSTA) histories are placed within the context of published relative sea level, reef sequence, and coralgal reef assemblage evolution. Our new calculations indicate SSTs were cooler on average by ~5â5.5°C at Noggin Pass (~17°S) and ~7â8°C at Hydrographer's Passage (~20°S) (Sr/Caâderived) during the LGM, in line with previous estimates (Felis et al., 2014, https://doi.org/10.1038/ncomms5102). We focus on contextualizing the Younger Dryas Chronozone (YDC, ~12.9â11.7 kyr BP), whose Southern Hemisphere expression, in particular in Australia, is elusive and poorly constrained. Our record does not indicate cooling during the YDC with nearâmodern temperatures reached during this interval on the GBR, supporting an asymmetric hemispheric presentation of this climate event. Building on a previous study (Felis et al., 2014, https://doi.org10.1038/ncomms5102), these fossil Isopora SSTA data from the GBR provide new insights into the deglacial reef response, with nearâmodern warming during the YDC, since the LGM.This work was funded by National Science Foundation (NSF) award OCE 13â56948 to B. K. L, with NSF GRFP support DGEâ11â44155 to L. D. B., and the Australian Research Council (grant no. DP1094001) and ANZIC IODP. Partial support for B. K. L's work on this project also came from the Vetlesen Foundation via a gift to the LamontâDoherty Earth Observatory. T. F. received funding from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)âProject number 180346848, through Priority Program 527 âIODP.â A. T. received support from the UK Natural Environment Research Council (NE/H014136/1 and NE/H014268/1). M. T. thanks Ministry of Earth Sciences for support (NCPOR contribution no. Jâ84/2020â21). L. D. B. would also like to thank Kassandra Costa for her input regarding error analysis.2021-06-1
Response of the Great Barrier Reef to sea level and environmental changes over the past 30,000 years
Previous drilling through submerged fossil coral reefs has greatly improved our understanding of the general pattern of sea-level change since the Last Glacial Maximum, however, how reefs responded to these changes remains uncertain. Here we document the evolution of the Great Barrier Reef (GBR), the world\u27s largest reef system, to major, abrupt environmental changes over the past 30 thousand years based on comprehensive sedimentological, biological and geochronological records from fossil reef cores. We show that reefs migrated seaward as sea level fell to its lowest level during the most recent glaciation (~20.5-20.7 thousand years ago (ka)), then landward as the shelf flooded and ocean temperatures increased during the subsequent deglacial period (~20-10âka). Growth was interrupted by five reef-death events caused by subaerial exposure or sea-level rise outpacing reef growth. Around 10âka, the reef drowned as the sea level continued to rise, flooding more of the shelf and causing a higher sediment flux. The GBR\u27s capacity for rapid lateral migration at rates of 0.2-1.5âmâyrâ1 (and the ability to recruit locally) suggest that, as an ecosystem, the GBR has been more resilient to past sea-level and temperature fluctuations than previously thought, but it has been highly sensitive to increased sediment input over centennial-millennial timescales
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