Effects of Noise and Detection Error in a Dynamic Adaptive Optics System

It is well known that noise and detection error can affect the performances of an adaptive optics (AO) system. Effects of noise and detection error on the phase compensation effectiveness in a dynamic AO system are investigated by means of a pure numerical simulation in this paper. A theoretical model for numerically simulating effects of noise and detection error in a static AO system and a corresponding computer program were presented in a previous article. A numerical simulation of effects of noise and detection error is combined with our previous numeral simulation of a dynamic AO system in this paper and a corresponding computer program has been compiled. Effects of detection error, readout noise and photon noise are included and investigated by a numerical simulation for finding the preferred working conditions and the best performances in a practical dynamic AO system. An approximate model is presented as well. Under many practical conditions such approximate model is a good alternative to the more accurate one. A simple algorithm which can be used for reducing the effect of noise is presented as well. When signal to noise ratio is very low, such method can be used to improve the performances of a dynamic AO system.

Refractive index and thickness analysis of natural silicon dioxide film growing on silicon with variable-angle spectroscopic ellipsometry

The refractive index and thickness of SiO2 thin films naturally grown on Si substrates were determined simultaneously within the wavelength range of 220-1100 nm with variable-angle spectroscopic ellipsometry. Different angles of incidence and wavelength ranges were chosen to enhance the analysis sensitivity for more accurate results. Several optical models describing the practical SiO2-Si system were investigated, and best results were obtained with the optical model, including an interface layer between SiO2 and Si, which proved the existence of the interface layer in this work as described in other publications.

A Novel Inverse Method For Determining The Refractive Indices Of Medium And Dispersed Particles Simultaneously By Turbidity Measurement

The refractive indices of particles and dispersion medium are important parameters in many colloidal experiments using optical techniques, such as turbidity and light scattering measurements. These data are in general wavelength-dependent and may not be available at some wavelengths fitting to the experimental requirement. in this Study we present a novel approach to inversely determine the refractive indices of particles and dispersion medium by examining the consistency of measured extinction cross sections of particles with their theoretical values using a series of trial values of the refractive indices. The colloidal suspension of polystyrene particles dispersed in water was used as an example to demonstrate how this approach works and the data obtained via such a method are compared with those reported in literature, showing a good agreement between both. Furthermore, the factors that affect the accuracy of measurements are discussed. We also present some data of the refractive indices of polystyrene over a range of wavelengths smaller than 400 nm that have been not reported in the available literature. (C) 2008 Elsevier Inc. All rights reserved.

Evaluation of influence of multiple scattering effect in light-scattering-based applications

The extinction cross sections of a system containing two particles are calculated by the T-matrix method, and the results are compared with those of two single particles with single-scattering approximation. The necessity of the correction of the refractive indices of water and polystyrene for different incident wavelengths is particularly addressed in the calculation. By this means, the volume fractions allowed for certain accuracy requirements of single-scattering approximation in the light scattering experiment can be evaluated. The volume fractions calculated with corrected refractive indices are compared with those obtained with fixed refractive indices which have been rather commonly used, showing that fixed refractive indices may cause significant error in evaluating multiple scattering effect. The results also give a simple criterion for selecting the incident wavelength and particle size to avoid the 'blind zone' in the turbidity measurement, where the turbidity change is insensitive to aggregation of two particles.

Performance analysis of bit error rate for free space optical communication with tip-tilt compensation based on gamma-gamma distribution

In this paper, the gamma-gamma probability distribution is used to model turbulent channels. The bit error rate (BER) performance of free space optical (FSO) communication systems employing on-off keying (OOK) or subcarrier binary phase-shift keying (BPSK) modulation format is derived. A tip-tilt adaptive optics system is also incorporated with a FSO system using the above modulation formats. The tip-tilt compensation can alleviate effects of atmospheric turbulence and thereby improve the BER performance. The improvement is different for different turbulence strengths and modulation formats. In addition, the BER performance of communication systems employing subcarrier BPSK modulation is much better than that of compatible systems employing OOK modulation with or without tip-tilt compensation.

Entanglement purification with higher error threshold for imperfection of local operations and bell state measurements

We analyse further the entanglement purification protocol proposed by Feng et al. (Phys. Lett. A 271 (2000) 44) in the case of imperfect local operations and measurements. It is found that this protocol allows of higher error threshold. Compared with the standard entanglement purification proposed by Bennett et al. [Phys. Rev. Lett. 76 (1996) 722], it turns out that this protocol is remarkably robust against the influences of imperfect local operations and measurements.

Negative refractive index in a four-level atomic system

We propose a scheme for realizing negative refractive index in a four-level atomic system. It is shown that such a system can simultaneously exhibit negative permittivity and negative permeability in an optical frequency range. Furthermore, by analysing the dispersion property of the left-handed material, we find that the probe beam can be controlled from superluminal to subluminal or vice versa via choosing appropriate parameters.

Electromagnetically induced negative refractive index in a V-type four-level atomic system

We propose a scheme for realizing negative refractive index in a V-type four-level atomic system. It is shown that the negative refractive index can be achieved in a wide frequency band based on the effect of quantum coherence. It is also found that the frequency band of negative refractive index and the absorption property of left-handed material are manipulated by the pump and control fields. Furthermore, left-handed material with reduced absorption is possible by choosing appropriate parameters. (c) 2006 Elsevier B.V. All rights reserved.

Improvement of sensitivity and refractive-index changes in LiNbO3 : Ce : Cu crystals with UV light

A nonvolatile recording scheme is proposed using LiNbO3:Ce:Cu crystals and modulated UV light to record gratings simultaneously in two centres and using red light to bleach the grating in the shallow centre to realize persistent photorefractive holographic storage. Compared with the normal UV-sensitized nonvolatile holographic system, the amplitude of refractive-index changes is greatly increased and the recording sensitivity is significantly enhanced by recording with UV light in the LiNbO3:Ce:Cu crystals. Based on jointly solving the two-centre material equations and the coupled-wave equations, temporal evolutions of the photorefractive grating and the diffraction effciency are effectively described and numerically analysed. Roles of doping levels and recording-beam intensity are discussed in detail. Theoretical results confirm and predict experimental results.

Improvement in refractive-index change in LiNbO3 : Ce : Cu by applying an external electric field

By jointly solving two-centre material equations with a nonzero external electric field and coupled-wave equations, we have numerically studied the dependence of the non-volatile holographic recording in LiNbO3:Ce:Cu crystals on the external electric field. The dominative photovoltaic effect of the non-volatile holographic recording in doubly doped LiNbO3 crystals is directly verified. And an external electric field that is applied in the positive direction along the c-axis (or a large one in the negative direction of the c-axis) in the recording phase and another one that is applied in the negative direction of the c-axis in the fixing phase are both proved to benefit strong photorefractive performances. Experimental verifications are given with a small electric field applied externally.

Refractive-index change and sensitivity improvement in holographic recording in LiNbO3:Ce:Cu crystals with green light

Nonvolatile holographic recording is performed with green light in LiNbO3:Ce:Cu crystals. The refractive-index change and the recording sensitivity are times better than those obtained by recording with red light, and higher optical fixing efficiency is obtained. Correspondingly, theoretical investigations are given.

Corrections to the paraxial approximation solutions in transversely nonuniform refractive-index media

The coupled differential recurrence equations for the corrections to the paraxial approximation solutions in transversely nonuniform refractive-index media are established in terms of the perturbation method. All the corrections (including the longitudinal field corrections) to the paraxial approximation solutions are presented in the weak-guidance approximation. As a concrete application, the first-order longitudinal field correction and the second-order transverse field correction to the paraxial approximation of a Gaussian beam propagating in a transversely quadratic refractive index medium are analytically investigated. (C) 1999 Optical Society of America [S0740-3232(99)00310-5].