43 research outputs found
FEM investigation of leaky modes in hollow core photonic crystal fibers
Hollow-core holey fibers are promising candidates for low-loss guidance of
light in various applications, e.g., for the use in laser guide star adaptive
optics systems in optical astronomy. We present an accurate and fast method for
the computation of light modes in arbitrarily shaped waveguides. Maxwell's
equations are discretized using vectorial finite elements (FEM). We discuss how
we utilize concepts like adaptive grid refinement, higher-order finite
elements, and transparent boundary conditions for the computation of leaky
modes in photonic crystal fibers. Further, we investigate the convergence
behavior of our methods. We employ our FEM solver to design hollow-core
photonic crystal fibers (HCPCF) whose cores are formed from 19 omitted cladding
unit cells. We optimize the fiber geometry for minimal attenuation using
multidimensional optimization taking into account radiation loss (leaky modes).Comment: 8 page
Impact of Sodium Layer variations on the performance of the E-ELT MCAO module
Multi-Conjugate Adaptive Optics systems based on sodium Laser Guide Stars may
exploit Natural Guide Stars to solve intrinsic limitations of artificial
beacons (tip-tilt indetermination and anisoplanatism) and to mitigate the
impact of the sodium layer structure and variability. The sodium layer may also
have transverse structures leading to differential effects among Laser Guide
Stars. Starting from the analysis of the input perturbations related to the
Sodium Layer variability, modeled directly on measured sodium layer profiles,
we analyze, through a simplified end-to-end simulation code, the impact of the
low/medium orders induced on global performance of the European Extremely Large
Telescope Multi-Conjugate Adaptive Optics module MAORY.Comment: 7 pages, 5 figures, SPIE conference Proceedin
Focal-plane wavefront sensing for active optics in the VST based on an analytical optical aberration model
We study a novel focal plane wavefront sensing and active optics control scheme at the VST on Cerro Paranal, an f/5.5 survey telescope with a 1x1 degree field of view and a 2.6m primary mirror. This scheme analyzes the elongation pattern of stellar PSFs across the full science image (256 Mpixels) and compares their second moments with an analytical model based on 5th-order geometrical optics. We consider 11 scalar degrees of freedom in mirror misalignments and deformations (M2 piston, tip/tilt and lateral displacement, detector tip/tilt, plus M1 figure astigmatism and trefoil). Using a numerical optimization method, we extract up to 4000 stars and complete the fitting process in under one minute. We demonstrate successful closed-loop active optics control based on maximum likelihood filtering
Optimization of cw sodium laser guide star efficiency
Context: Sodium laser guide stars (LGS) are about to enter a new range of
laser powers. Previous theoretical and numerical methods are inadequate for
accurate computations of the return flux and hence for the design of the
next-generation LGS systems.
Aims: We numerically optimize the cw (continuous wave) laser format, in
particular the light polarization and spectrum.
Methods: Using Bloch equations, we simulate the mesospheric sodium atoms,
including Doppler broadening, saturation, collisional relaxation, Larmor
precession, and recoil, taking into account all 24 sodium hyperfine states and
on the order of 100 velocity groups.
Results: LGS return flux is limited by "three evils": Larmor precession due
to the geomagnetic field, atomic recoil due to radiation pressure, and
transition saturation. We study their impacts and show that the return flux can
be boosted by repumping (simultaneous excitation of the sodium D2a and D2b
lines with 10-20% of the laser power in the latter).
Conclusions: We strongly recommend the use of circularly polarized lasers and
repumping. As a rule of thumb, the bandwidth of laser radiation in MHz (at each
line) should approximately equal the launched laser power in Watts divided by
six, assuming a diffraction-limited spot size.Comment: 15 pages, 12 figures, to be published in Astronomy & Astrophysics,
AA/2009/1310
Dynamic phase separation of fluid membranes with rigid inclusions
Membrane shape fluctuations induce attractive interactions between rigid
inclusions. Previous analytical studies showed that the fluctuation-induced
pair interactions are rather small compared to thermal energies, but also that
multi-body interactions cannot be neglected. In this article, it is shown
numerically that shape fluctuations indeed lead to the dynamic separation of
the membrane into phases with different inclusion concentrations. The tendency
of lateral phase separation strongly increases with the inclusion size. Large
inclusions aggregate at very small inclusion concentrations and for relatively
small values of the inclusions' elastic modulus.Comment: 6 pages, 6 figure