arXiv subjects

N. A. Mortensen

Publications and source records attributed to N. A. Mortensen.

46 records · Page 3Linked to original sources

arXiv2003

Small-core photonic crystal fibers with weakly disordered air-hole claddings

Motivated by recent experimental work by Folkenberg et al. we consider the effect of weak disorder in the air-hole lattice of small-core photonic crystal fibers. We find that the broken symmetry leads to higher-order modes which have generic intensity distributions resembling those found in standard fibers with elliptical cores. This explains why recently reported experimental higher-order mode profiles appear very different from those calculated numerically for ideal photonic crystal fibers with inversion and six-fold rotational symmetry. The splitting of the four higher-order modes into two groups fully correlates with the observation that these modes have different cut-offs.

physics.optics↗

arXiv2003

Mode-Field Radius of Photonic Crystal Fibers Expressed by the V-parameter

We numerically calculate the equivalent mode-field radius of the fundamental mode in a photonic crystal fiber (PCF) and show that this is a function of the V-parameter only and not the relative hole size. This dependency is similar to what is found for graded-index standard fibers and we furthermore show that the relation for the PCF can be excellently approximated with the same general mathematical expression. This is to our knowledge the first semi-analytical description of the mode-field radius of a PCF.

physics.optics↗

arXiv2003

Multi-mode photonic crystal fibers for VCSEL based data transmission

Quasi error-free 10 Gbit/s data transmission is demonstrated over a novel type of 50 micron core diameter photonic crystal fiber with as much as 100 m length. Combined with 850$ nm VCSEL sources, this fiber is an attractive alternative to graded-index multi-mode fibers for datacom applications. A comparison to numerical simulations suggests that the high bit-rate may be partly explained by inter-modal diffusion.

physics.optics↗

arXiv2003

Modal cut-off and the V-parameter in photonic crystal fibers

We address the long-standing unresolved problem concerning the V-parameter in a photonic crystal fiber (PCF). Formulate the parameter appropriate for a core-defect in a periodic structure we argue that the multi-mode cut-off occurs at a wavelength lambda* which satisfies V_PCF(lambda*)=pi. Comparing to numerics and recent cut-off calculations we confirm this result.

physics.optics↗

arXiv2003

Experimental investigation of cut-off phenomena in non-linear photonic crystal fibers

The modal cut-off is investigated experimentally in a series of high quality non-linear photonic crystal fibers. We demonstrate a suitable measurement technique to determine the cut-off wavelength and verify it by inspecting the near field of the modes that may be excited below and above the cut-off. We observe a double peak structure in the cut-off spectra, which is attributed to a splitting of the higher order modes. The cut-off is measured for seven different fiber geometries with different pitches and relative hole size, and a very good agreement with recent theoretical work is found.

physics.optics↗

arXiv2003

Reduced micro-deformation attenuation in large-mode area photonic crystal fibers for visible applications

We consider large-mode area photonic crystal fibers for visible applications where micro-deformation induced attenuation becomes a potential problem when the effective area A_eff is sufficiently large compared to lambda^2. We argue how a slight increase in fiber diameter D can be used in screening the high-frequency components of the micro-deformation spectrum mechanically and we confirm this experimentally for both 15 and 20 micron core fibers. For typical bending-radii (R~16 cm) the operating band-width increases by ~3-400 nm to the low-wavelength side.

physics.optics↗

arXiv2002

Improved large-mode area endlessly single-mode photonic crystal fibers

We numerically study the possibilities for improved large-mode area endlessly single mode photonic crystal fibers for use in high-power delivery applications. By carefully choosing the optimal hole diameter we find that a triangular core formed by three missing neighboring air holes considerably improves the mode area and loss properties compared to the case with a core formed by one missing air hole. In a realized fiber we demonstrate an enhancement of the mode area by ~30 % without a corresponding increase in the attenuation.

physics.optics↗

arXiv2002

Universal spin-polarization fluctuations in 1D wires with magnetic impurities

We study conductance and spin-polarization fluctuations in one-dimensional wires with spin-5/2 magnetic impurities (Mn). Our tight-binding Green function approach goes beyond mean field thus including s-d exchange-induced spin-flip scattering. In a certain parameter range, we find that spin flip suppresses conductance fluctuations while enhancing spin-polarization fluctuations. More importantly, spin-polarization fluctuations attain a universal value 1/3 for large enough spin flip strengths. This intrinsic spin-polarization fluctuation may pose a severe limiting factor to the realization of steady spin-polarized currents in Mn-based 1D wires.

cond-mat.mes-hall↗

arXiv2002

Numerical Aperture of Single-Mode Photonic Crystal Fibers

We consider the problem of radiation into free space from the end-facet of a single-mode photonic crystal fiber (PCF). We calculate the numerical aperture NA=sin theta from the half-divergence angle theta ~ tan^{-1}(lambda/pi w) with pi w^2 being the effective area of the mode in the PCF. For the fiber first presented by Knight et al. we find a numerical aperture NA ~ 0.07 which compares to standard fiber technology. We also study the effect of different hole sizes and demonstrate that the PCF technology provides a large freedom for NA-engineering. Comparing to experiments we find good agreement.

physics.optics↗

arXiv2001

Andreev levels in a single-channel conductor

We calculate the subgap density of states of a disordered single-channel normal metal connected to a superconductor at one end (NS junction) or at both ends (SNS junction). The probability distribution of the energy of a bound state (Andreev level) is broadened by disorder. In the SNS case the two-fold degeneracy of the Andreev levels is removed by disorder leading to a splitting in addition to the broadening. The distribution of the splitting is given precisely by Wigner's surmise from random-matrix theory. For strong disorder the mean density of states is largely unaffected by the proximity to the superconductor, because of localization, except in a narrow energy region near the Fermi level, where the density of states is suppressed with a log-normal tail.

cond-mat.mes-hall↗