Showing papers in "Journal of the Optical Society of America in 2010"

Intrinsic limitations of Shack-Hartmann wavefront sensing on an extended laser guide source

Abstract: In this paper we investigate the behavior of various centroiding methods (weighted center of gravity, matched filtering, and correlation) classically used in Shack–Hartmann wavefront sensing when dealing with an elongated asymmetric spot. We study the impact of model errors on these centroiding methods at high signal-to-noise ratios, and, using a one-dimensional formalism, we show that the associated estimates all suffer from a bias uncorrelated with the actual spot displacement if its shape is not known precisely. Additionally, we show that the correlation method provides an estimate with a unitary gain whatever the parameters used, while the other two methods introduce a non-unitary gain in the estimation process. Finally, we show that the sampling of the spot structures after filtering by some convolution kernels is crucial to get an unbiased estimate of the spot displacement.

Tomographic wavefront error using multi-LGS constellation sensed with Shack-Hartmann wavefront sensors

Abstract: Noise effects induced by laser guide star (LGS) elongation have to be considered globally in a multi-LGS tomographic reconstruction analysis. This allows a fine estimation of performance and the comparison of different launching options. We present a modal analysis of the wavefront error with Shack–Hartmann wavefront sensors based on quasi-analytical matrix formalism. Including spot elongation and the Rayleigh fratricide effect, edge launching produces similar performance to central launching and avoids the risk of possible underestimation of fratricide scatter. Performance improves slightly with an optimized centroid estimator and is not affected by a slight field-of-view truncation of the subapertures. Finally we discuss detector characteristics for a LGS Shack–Hartmann wavefront sensor.

Designing microstructured polymer optical fibers for cascaded quadratic soliton compression of femtosecond pulses: erratum

Abstract: The dispersion of index-guiding microstructured polymer optical fibers is calculated for second-harmonic generation. The quadratic nonlinearity is assumed to come from poling of the polymer, which is chosen to be the cyclic olefin copolymer Topas. We found a very large phase mismatch between the pump and the second-harmonic waves. Therefore the potential for cascaded quadratic second-harmonic generation is investigated in particular or soliton compression of femtosecond pulses. We found that excitation of temporal solitons from cascaded quadratic nonlinearities requires an effective quadratic nonlinearity of 5 pm/V or more. This might be reduced if a polymer with a lower Kerr nonlinear refractive index is used. We also found that the group-velocity mismatch could be minimized if the design parameters of the microstructured fiber are chosen so the relative hole size is large and the hole pitch is of the order of the pump wavelength. Almost all design-parameter combinations resulted in cascaded effects in the stationary regime, where efficient and clean soliton compression can be found. We therefore did not see any benefit from choosing a fiber design where the group-velocity mismatch was minimized. Instead numerical simulations showed excellent compression of λ=800 nm120 fs pulses with nanojoule pulse energy to few-cycle duration using a standard endlessly single-mode design with a relative hole size of 0.4.

Transmission enhancement of slow light by a subwavelength plasmon-dielectric system

Abstract: We present a theoretical and numerical analysis of a subwavelength plasmon-dielectric system that incorporates a periodic metal grating deposited on a dielectric waveguide and supports transmission enhancement of slow light at infrared wavelength for the s polarization. We find that a Fano resonance mechanism to produce this novel phenomenon is based on the interaction of the discrete waveguide-plasmon hybridization modes with the incident photon continuum, which is different from the popular cases with surface plasmonic modes excited by p polarized incident light. The further analysis of the Fano effect indicates that group velocity of slow light and transparent efficiency can be controlled in a large range by the coupling strength, and a more than 20-fold transmission enhancement corresponding to the group velocity of 0.005c is obtained as compared to the case without the dielectric waveguide substrate.