13,142 research outputs found
Dominance of backward stimulated Raman scattering in gas-filled hollow-core photonic crystal fibers
Backward stimulated Raman scattering in gases provides a promising route to
compression and amplification of a Stokes seed-pulse by counter-propagating
against a pump-pulse, as has been already demonstrated in various platforms,
mainly in free-space. However, the dynamics governing this process when seeded
by noise has not yet been investigated in a fully controllable collinear
environment. Here we report the first unambiguous observation of efficient
noise-seeded backward stimulated Raman scattering in a hydrogen-filled
hollow-core photonic crystal fiber. At high gas pressures, when the backward
Raman gain is comparable with, but lower than, the forward gain, we report
quantum conversion efficiencies exceeding 40% to the backward Stokes at 683 nm
from a narrowband 532-nm-pump. The efficiency increases to 65% when the
backward process is seeded by a small amount of back-reflected
forward-generated Stokes light. At high pump powers the backward Stokes signal,
emitted in a clean fundamental mode and spectrally pure, is unexpectedly always
stronger than its forward-propagating counterpart. We attribute this striking
observation to the unique temporal dynamics of the interacting fields, which
cause the Raman coherence (which takes the form of a moving fine-period Bragg
grating) to grow in strength towards the input end of the fiber. A good
understanding of this process, together with the rapid development of novel
anti-resonant-guiding hollow-core fibers, may lead to improved designs of
efficient gas-based Raman lasers and amplifiers operating at wavelengths from
the ultraviolet to the mid-infrared.Comment: 6 pages and 8 figures in the main section. 4 pages and 5 figures in
the supplementary sectio
Introducing the Spatial Conflict Dynamics indicator of political violence
Modern armed conflicts have a tendency to cluster together and spread
geographically. However, the geography of most conflicts remains under-studied.
To fill this gap, this article presents a new indicator that measures two key
geographical properties of subnational political violence: the conflict
intensity within a region on the one hand, and the spatial distribution of
conflict within a region on the other. We demonstrate the indicator in North
and West Africa between 1997 to 2019 to show that it can clarify how conflicts
can spread from place to place and how the geography of conflict changes over
time
Are Recent Peculiar Velocity Surveys Consistent?
We compare the bulk flow of the SMAC sample to the predictions of popular
cosmological models and to other recent large-scale peculiar velocity surveys.
Both analyses account for aliasing of small-scale power due to the sparse and
non-uniform sampling of the surveys. We conclude that the SMAC bulk flow is in
marginal conflict with flat COBE-normalized Lambda-CDM models which fit the
cluster abundance constraint. However, power spectra which are steeper
shortward of the peak are consistent with all of the above constraints. When
recent large-scale peculiar velocity surveys are compared, we conclude that all
measured bulk flows (with the possible exception of that of Lauer & Postman)
are consistent with each other given the errors, provided the latter allow for
`cosmic covariance'. A rough estimate of the mean bulk flow of all surveys
(except Lauer & Postman) is ~400 km/s towards l=270, b=0.Comment: 8 pages, 3 figures. To appear in Proceedings of the Cosmic Flows
Workshop, Victoria, B. C., Canada, July 1999, eds. S. Courteau, M. Strauss,
and J. Willic
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