8,711 research outputs found
Self-calibrating d-scan: measuring ultrashort laser pulses on-target using an arbitrary pulse compressor
In most applications of ultrashort pulse lasers, temporal compressors are
used to achieve a desired pulse duration in a target or sample, and precise
temporal characterization is important. The dispersion-scan (d-scan) pulse
characterization technique usually involves using glass wedges to impart
variable, well-defined amounts of dispersion to the pulses, while measuring the
spectrum of a nonlinear signal produced by those pulses. This works very well
for broadband few-cycle pulses, but longer, narrower bandwidth pulses are much
more difficult to measure this way. Here we demonstrate the concept of
self-calibrating d-scan, which extends the applicability of the d-scan
technique to pulses of arbitrary duration, enabling their complete measurement
without prior knowledge of the introduced dispersion. In particular, we show
that the pulse compressors already employed in chirped pulse amplification
(CPA) systems can be used to simultaneously compress and measure the temporal
profile of the output pulses on-target in a simple way, without the need of
additional diagnostics or calibrations, while at the same time calibrating the
often-unknown differential dispersion of the compressor itself. We demonstrate
the technique through simulations and experiments under known conditions.
Finally, we apply it to the measurement and compression of 27.5 fs pulses from
a CPA laser.Comment: 11 pages, 5 figures, Scientific Reports, in pres
Domain-adaptive deep network compression
Deep Neural Networks trained on large datasets can be easily transferred to
new domains with far fewer labeled examples by a process called fine-tuning.
This has the advantage that representations learned in the large source domain
can be exploited on smaller target domains. However, networks designed to be
optimal for the source task are often prohibitively large for the target task.
In this work we address the compression of networks after domain transfer.
We focus on compression algorithms based on low-rank matrix decomposition.
Existing methods base compression solely on learned network weights and ignore
the statistics of network activations. We show that domain transfer leads to
large shifts in network activations and that it is desirable to take this into
account when compressing. We demonstrate that considering activation statistics
when compressing weights leads to a rank-constrained regression problem with a
closed-form solution. Because our method takes into account the target domain,
it can more optimally remove the redundancy in the weights. Experiments show
that our Domain Adaptive Low Rank (DALR) method significantly outperforms
existing low-rank compression techniques. With our approach, the fc6 layer of
VGG19 can be compressed more than 4x more than using truncated SVD alone --
with only a minor or no loss in accuracy. When applied to domain-transferred
networks it allows for compression down to only 5-20% of the original number of
parameters with only a minor drop in performance.Comment: Accepted at ICCV 201
Power Laws, Highly Optimized Tolerance, and Generalized Source Coding
We introduce a family of robust design problems for complex systems in uncertain environments which are based on tradeoffs between resource allocations and losses. Optimized solutions yield the “robust, yet fragile” features of highly optimized tolerance and exhibit power law tails in the distributions of events for all but the special case of Shannon coding for data compression. In addition to data compression, we construct specific solutions for world wide web traffic and forest fires, and obtain excellent agreement with measured data
Formation of Compressed Flat Electron Beams with High Transverse-Emittance Ratios
Flat beams -- beams with asymmetric transverse emittances -- have important
applications in novel light-source concepts, advanced-acceleration schemes and
could possibly alleviate the need for damping rings in lepton colliders. Over
the last decade, a flat-beam-generation technique based on the conversion of an
angular-momentum-dominated beam was proposed and experimentally tested. In this
paper we explore the production of compressed flat beams. We especially
investigate and optimize the flat-beam transformation for beams with
substantial fractional energy spread. We use as a simulation example the
photoinjector of the Fermilab's Advanced Superconducting Test Accelerator
(ASTA). The optimizations of the flat beam generation and compression at ASTA
were done via start-to-end numerical simulations for bunch charges of 3.2 nC,
1.0 nC and 20 pC at ~37 MeV. The optimized emittances of flat beams with
different bunch charges were found to be 0.25 {\mu}m (emittance ratio is ~400),
0.13 {\mu}m, 15 nm before compression, and 0.41 {\mu}m, 0.20 {\mu}m, 16 nm
after full compression, respectively with peak currents as high as 5.5 kA for a
3.2-nC flat beam. These parameters are consistent with requirements needed to
excite wakefields in asymmetric dielectric-lined waveguides or produce
significant photon flux using small-gap micro-undulators.Comment: 17
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