714 research outputs found
A Broadband Scalar Vortex Coronagraph
Broadband coronagraphy with deep nulling and small inner working angle has
the potential of delivering images and spectra of exoplanets and other faint
objects. In recent years, many coronagraphic schemes have been proposed, the
most promising being the optical vortex phase mask coronagraphs. In this paper,
a new scheme of broadband optical scalar vortex coronagraph is proposed and
characterized experimentally in the laboratory. Our setup employs a pair of
computer generated phase gratings (one of them containing a singularity) to
control the chromatic dispersion of phase plates and achieves a constant
peak-to-peak attenuation below over a bandwidth of 120 nm
centered at 700 nm. An inner working angle of ~\lambda/D is demonstrated along
with a raw contrast of 11.5 magnitudes at 2\lambda/D.Comment: Accepted, 6 pages, 6 image
Common pulse retrieval algorithm: a fast and universal method to retrieve ultrashort pulses
We present a common pulse retrieval algorithm (COPRA) that can be used for a
broad category of ultrashort laser pulse measurement schemes including
frequency-resolved optical gating (FROG), interferometric FROG, dispersion
scan, time domain ptychography, and pulse shaper assisted techniques such as
multiphoton intrapulse interference phase scan (MIIPS). We demonstrate its
properties in comprehensive numerical tests and show that it is fast, reliable
and accurate in the presence of Gaussian noise. For FROG it outperforms
retrieval algorithms based on generalized projections and ptychography.
Furthermore, we discuss the pulse retrieval problem as a nonlinear
least-squares problem and demonstrate the importance of obtaining a
least-squares solution for noisy data. These results improve and extend the
possibilities of numerical pulse retrieval. COPRA is faster and provides more
accurate results in comparison to existing retrieval algorithms. Furthermore,
it enables full pulse retrieval from measurements for which no retrieval
algorithm was known before, e.g., MIIPS measurements
Impedance generalization for plasmonic waveguides beyond the lumped circuit model
We analytically derive a rigorous expression for the relative impedance ratio
between two photonic structures based on their electromagnetic interaction. Our
approach generalizes the physical meaning of the impedance to a measure for the
reciprocity-based overlap of eigenmodes. The consistence with known cases in
the radiofrequency and optical domain is shown. The analysis reveals where the
applicability of simple circuit parameters ends and how the impedance can be
interpreted beyond this point. We illustrate our approach by successfully
describing a Bragg reflector that terminates an insulator-metal-insulator
plasmonic waveguide in the near-infrared by our mpedance concept
- …