Polarization optimized focusing of light and coupling to sub-wavelength antennae

Gerd Leuchs, Susanne Quabis

This chapter describes the polarization optimized focusing of light and coupling to sub-wavelength antennae. In a set-up for focusing of light, the lens system transforms the transverse field distribution at the input into the transverse field distribution at the focal plane. The parameters of the input beam are the transverse intensity and phase distribution as well as its state of polarization. The various optical rays directed by the lens to the focal spot typically carry different polarizations even if the input beam has a homogeneous polarization distribution across the beam. Locally the polarization is linear everywhere but the direction of the electric field vector depends on the transverse coordinates within the cross section of the beam in such a way that the electric field vectors oscillate in the radial direction. In the limit of high numerical aperture, focusing the field distribution at and near the focus is calculated using the Debye approximation. The effects of the vector properties of light on the structure of the focus are best observed if two special input beams are compared with identical intensity distribution but different polarization properties

Degree of polarization for partially coherent general beams in turbulent atmosphere

H. T. Eyyuboglu, Y. Baykal, Y. Cai

The degree of polarization is found for optical excitations of cosh-Gaussian, cos-Gaussian and annular-Gaussian beams in a turbulent atmosphere. The related formulation is based on the beam coherence polarization matrix. The self and mutual coherence functions appearing in the beam coherence polarization matrix are evaluated, when the above mentioned excitations exhibit partial source coherence for self and cross fields. Plots showing the variation of the degree of polarization are provided versus the propagation length when the source size, displacement parameter, structure constant and the degree of source coherence for self and cross fields change.

Quasi-phase-matched high harmonic generation in hollow core photonic crystal fibers

H. Ren, A. Nazarkin, J. Nold, P. St. J. Russell

The potential of hollow core photonic crystal fiber as a nonlinear gas cell for efficient high harmonic generation is discussed. The feasibility of phase-matching this process by modulating the phase of ionization electrons using a counter-propagating laser field is shown. In this way, harmonics with energies of several hundreds of eV can be produced using fs-laser pump pulses of mu J energy. (C) 2008 Optical Society of America

Squeezing based on nondegenerate frequency doubling internal to a realistic laser

UL Andersen, P Tidemand-Lichtenberg, P Buchhave

We investigate theoretically the quantum fluctuations of the fundamental field in the output of a nondegenerate second-harmonic generation process occurring inside a laser cavity. Due to the nondegenerate character of the nonlinear medium, a field orthogonal to the laser field is for some operating conditions independent of the fluctuations produced by the laser medium. We show that this fact may lead to perfect squeezing for a certain polarization mode of the fundamental field. The experimental feasibility of the system is also discussed.

Optimal unambiguous state discrimination of two density matrices: Lower bound and class of exact solutions

P Raynal, N Lutkenhaus

Recently the problem of unambiguous state discrimination of mixed quantum states has attracted much attention. So far, bounds on the optimum success probability have been derived [T. Rudolph, R. W. Spekkens, and P. S. Turner, Phys. Rev. A 68, 010301(R) (2003)]. For two mixed states they are given in terms of the fidelity. Here we give tighter bounds as well as necessary and sufficient conditions for two mixed states to reach these bounds. Moreover we construct the corresponding optimal measurement strategies. With this result, we provide analytical solutions for unambiguous discrimination of a class of generic mixed states. This goes beyond known results which are all reducible to some pure state case. Additionally, we show that examples exist where the bounds cannot be reached.

Active laser radar systems with stochastic electromagnetic beams in turbulent atmosphere

Yangjian Cai, Olga Korotkova, Halil T. Eyyuboglu, Yahya Baykal

Propagation of stochastic electromagnetic beams through paraxial ABCD optical systems operating through turbulent atmosphere is investigated with the help of the ABCD matrices and the generalized Huygens-Fresnel integral. In particular, the analytic formula is derived for the cross-spectral density matrix of an electromagnetic Gaussian Schell-model (EGSM) beam. We applied our analysis for the ABCD system with a single lens located on the propagation path, representing, in a particular case, the unfolded double-pass propagation scenario of active laser radar. Through a number of numerical examples we investigated the effect of local turbulence strength and lens' parameters on spectral, coherence and polarization properties of the EGSM beam. (C) 2008 Optical Society of America

Phase-preserving 2R regeneration of a WDM RZ-DPSK signal using a nonlinear amplifying loop mirror

K. Cvecek, K. Sponsel, C. Stephan, G. Onishchukov, R. Ludwig, C. Schubert, B. Schmauss, G. Leuchs

A modified NALM setup for phase-preserving amplitude regeneration of WDM-DPSK signals is proposed. Demultiplexing-multiplexing within the NALM fiber loop is used to avoid WDM channel crosstalk. A negative power penalty of 1.2 dB was obtained. (C) 2007 Optical Society of America.

Propagation properties of anomalous hollow beams in a turbulent atmosphere

Yangjian Cai, Halil T. Eyyuboglu, Yahya Baykal

Propagation of circular and elliptical anomalous hollow beams in a turbulent atmosphere is investigated in detail. Based on the extended Huygens-Fresnel integral, analytical formulae for the average irradiance of circular and elliptical anomalous hollow beams propagating in a turbulent atmosphere are derived. The irradiance and spreading properties of circular and elliptical anomalous hollow beams in a turbulent atmosphere and in free space are studied numerically. It is found that circular and elliptical anomalous hollow beams at short propagation distance in turbulent atmosphere have similar propagation properties to those of free space, while at long propagation distance, circular and elliptical anomalous hollow beams eventually become circular Gaussian beams in a turbulent atmosphere, which is much different from their propagation properties in free space. The conversion from an anomalous hollow beam to a circular Gaussian beam becomes quicker and the beam spot spreads more rapidly for a larger structure constant, a shorter wavelength and a smaller waist size of the initial beam. (C) 2008 Elsevier B.V. All rights reserved.

Efficiency of coherent-state quantum cryptography in the presence of loss: Influence of realistic error correction

Matthias Heid, Norbert Luetkenhaus

We investigate the performance of a continuous-variable quantum key distribution scheme in a practical setting. More specifically, we take a nonideal error reconciliation procedure into account. The quantum channel connecting the two honest parties is assumed to be lossy but noiseless. Secret key rates are given for the case that the measurement outcomes are postselected or a reverse reconciliation scheme is applied. The reverse reconciliation scheme loses its initial advantage in the practical setting. If one combines postselection with reverse reconciliation, however, much of this advantage can be recovered.

Forward Brillouin Scattering in Tapered Optical Fibers

M. S. Kang, A. Brenn, G. S. Wiederhecker, P. St. J. Russell

We experimentally study forward Brillouin scattering in tapered fibers. Circularly symmetric acoustic phonons resonant in fibers are observed by using a novel photoacoustic measurement technique. Strong acousto-optic interaction can take place in highly tapered fibers. (C) 2008 Optical Society of America