JEOS:RP - Journal of the European Optical Society Rapid publications
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Probing canonical geometrical objects by digital spiral imaging
Get PDFWe recently proposed a novel concept to remotely acquire information of objects, based on the discrete orbital angular momentum of ligh. Here we use two different experimental schemes for implementing the technique. We use a canonical phase jump as a target to test the methods and to compare the results
Terahertz imaging: a new non-destructive technique for the quality control of plastic weld joints
Get PDFWe present the first investigation of plastic weld joints using terahertz waves. Terahertz time-domain spectroscopy clearly reveals contaminations like metal or sand within the weld joint of two high-density polyethylene sheets. Furthermore, areas can be identified where the welding process has failed and the parts to be joined are separated by a small air gap. We show that a three layer structure of polyethylene-air-polyethylene has a characteristic, frequency-dependent transmission behaviour. This allows for a distinction between welded and non-welded material as well as for the calculation of the air layer thickness from the relative transmission spectrum. Consequently, terahertz time-domain spectroscopy provides a promising new non-destructive and even contactless technique, which is desired by the plastics industry for detecting a variety of deviations from the ideal welding process
Experimental validation of the reverse polar decomposition of depolarizing Mueller matrices
Get PDFWe experimentally assess the validity of the reverse polar decomposition (R. Ossikovski et al., Opt. Lett. 32, 689 (2007)), which describes any Mueller matrix as a product of a depolarizer, a diattenuator and a retarder with the diattenuator placed after the depolarizer and not before, as in the well-known Lu and Chipman’s forward decomposition. The raw data are Mueller images of a depolarizer (dilute milk at variable concentrations), followed by two tilted glass plates as a diattenuator and a mica retardation plate. While the reverse decomposition accurately reconstructs the component matrices in all cases, the usual forward decomposition provides reasonable values only for the trace of the depolarizer matrix, the other quantities being affected by gross errors. The potential interest of this decomposition for biological samples is briefly discussed
Excitation back transfer in a statistical model for upconversion in Er-doped fibres
Get PDFWe report a new analytical approach to evaluate the accuracy of a statistical model of the migration assisted upconversion in Er-doped fibres. Unlike the mean-field approach to the excitation back transfer which was used in previous statistical model, we use a new approximation accounting for the variance of population of the first excited level. Implementing these results, we find that the accuracy of upconversion rate calculations is within 13 % if the concentration of erbium ions is smaller than the critical one
A Model to optimize a microwave PBG accelerator based on generic unit cell
Get PDFIn this paper a numerical method, based on the well known Floquet-Bloch theory, useful to analyze the physical properties of a PBG based accelerator, is presented. The proposed model has been developed to analyze a 2D lattice characterized by a generic inclination angle between the two primitive translation vectors, thus resulting very useful when a periodic structure without an equilateral triangular or square cell has to be investigated. The numerical method has been optimized in order to account several number of space harmonics with a low CPU time and memory consumption. Comparisons with more complex numerical methods demonstrate the accuracy of our model. Several simulations have been performed to find all the geometrical parameters including the inclination angle of the unit cell, filling factor and index contrast. The proposed method, through an optimization procedure of the photonic band structure, allows to obtain a large spectral purity, high order mode suppression and high Q-values
Improved perturbation method for absorption map reconstruction in diffusion optical tomography
Get PDFThe perturbation method is an effective approach for optical tomography reconstruction, however its success depends to a great extent on how close initial estimates are to the actual solutions. In addition, the linear perturbation method can only be applied to the reconstruction of differences between two similar states. To overcome these limitations, we present a pre-scaling technique applied to the qualitative reconstruction of the absorption map. In this method, the initial estimate of the absorption coefficient is scaled to an appropriate value before it is employed in iteration. The scaled estimate leads to the forward solution on the boundary close to the measured data. In
the simulated experiments, reconstructions were performed with and without the pre-scaling technique. The results demonstrate that the proposed technique extends the selection of the initial estimate of optical properties, and makes it feasible that the absolute value of optical properties can also be used in the linear perturbation approach
Non-Bragg band gaps in 1D metamaterial aperiodic multilayers
Get PDFWe investigate the existence of non-Bragg band gaps in 1D aperiodic photonic structures, namely the Fibonacci and Thue-Morse lattices combining ordinary positive index materials and dispersive metamaterials. Both structures present new band gaps which, in contrast with the usual Bragg gaps, are not based on interference mechanisms. It is known that one of these non-Bragg gaps corresponds to zero (volume) averaged refractive index. In this paper we demonstrate the existence of two new non-Bragg gaps in aperiodic photonic crystals: the zero permeability, and the zero permittivity gaps. Some distinctive aspects of these polarization-selective gaps are outlined and the impact on the photonic spectra produced by the level of the generation of the aperiodic structure is analyzed
Strehl ratio and optimum focus of high-numerical-aperture beams
Get PDFWe analytically calculate the focus setting for a beam with a high numerical aperture (NA) that optimizes its Strehl ratio in the case of small aberrations up to the ’just’ diffraction-limited value (Strehl ratio ≥0.80). The optimum focus setting deviates from the one that follows from a minimization of the wavefront aberration with the aid of the Zernike aberration coefficients. This deviation stems largely from the fact that the
common quadratic approximation of the focus term becomes inadequate in the high-NA case. Fundamental high-NA amplitude nonuniformity in the exit pupil of an optical system in the case of a linearly polarized incident beam also nfluences the optimum focus setting. Results for spherical aberration and astigmatism are presented for an NA-value
of 0.95
Mode suppression in a microcavity solid-state dye laser
Get PDFA solid-state dye laser with a microcavity whose size is comparable to the lasing wavelength, is modeled by means of the finite element method. The position of the pumping source affects the lasing mode spectrum. In comparison with a single point source at the edge of the cavity, a random distribution of excitation sources in the central gain part of the microcavity leads to suppression of odd longitudinal modes and to higher output efficiency of the laser radiation in even modes. The modeling and simulation results are explained by simple physical considerations
Technology and performances of silicon oxynitride waveguides for optomechanical sensors fabricated by plasma-enhanced chemical vapour deposition
Get PDFThe technology and performances of a micromachined channel waveguides, based on PECVD deposition of silicon oxynitride (SiOxNy) thin films, is presented. The deposition parameters of the PECVD process are studied in connection with their optical, mechanical and chemical properties. Waveguide deign is optimized allowing single mode, low loss propagation and high efficiency of coupling with single mode optical fiber. The proposed technology is applied to fabricate the pigtailed Mach-Zehnder interferometers, where the coupling from optical fiber to waveguide is based on the etch of U-grooves, supporting fibers in the same substrate as the waveguide substrate