Showing papers in "Journal of the European Optical Society: Rapid Publications in 2007"

Terahertz imaging: a new non-destructive technique for the quality control of plastic weld joints

Abstract: We 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.

Numerical analysis of a slit-groove diffraction problem

Abstract: We present a comparison among several fully-vectorial methods applied to a basic scattering problem governed by the physics of the electromagnetic interaction between subwavelength apertures in a metal film. The modelled structure represents a slit-groove scattering problem in a silver film deposited on a glass substrate. The benchmarked methods, all of which use in-house developed software, include a broad range of fully-vectorial approaches from finite-element methods, volume-integral methods, and finite-difference time domain methods, to various types of modal methods based on different expansion techniques.

Dyakonov-Tamm wave at the planar interface of a chiral sculptured thin film and an isotropic dielectric material

Abstract: Surface waves, named here as Dyakonov--Tamm waves, can exist at the planar interface of an isotropic dielectric material and a chiral sculptured thin film (STF). Due to the periodic nonhomogeneity of a chiral STF, the range of the refractive index of the isotropic material is smaller but the range of the propagation direction in the interface plane is much larger, in comparison to those for the existence of Dyakonov waves at the planar interface of an isotropic dielectric material and a columnar thin film.

Probing canonical geometrical objects by digital spiral imaging

Abstract: We 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.

Slow light by means of coherent population oscillations: laser linewidth effects

Abstract: Several groups have recently reported the observation of slow light propagation speeds based on the process of coherent population oscillations (CPO) under conditions such that the laser line width exceeds the width of the CPO transparency window. We explain this apparently paradoxical result by noting that the CPO effect is sensitive only to intensity variations of the incident laser field that occur on time scales comparable to or shorter than the population relaxation time of the material medium. Laser linewidths are associated primarily with phase fluctuations or with intensity fluctuations on much shorter time scales.

Experimental validation of the reverse polar decomposition of depolarizing Mueller matrices

Abstract: We 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.

Computing Zernike polynomials of arbitrary degree using the discrete Fourier transform

Abstract: The conventional representation of Zernike polynomials R_n^m(rho) gives unacceptable results for large values of the degree n. We present an algorithm for the computation of Zernike polynomials of arbitrary degree n. The algorithm has the form of a discrete cosine transform which comes with advantages over other methods in terms of computation time, accuracy and transparancy. As an application we consider the effect of NA-scaling on the lower-order aberrations of an optical system in the presence of a very high order aberration.

High-efficiency wide-band metal-dielectric resonant grating for 20 fs pulse compression

Abstract: More than 95% average efficiency TE-polarization diffraction over a 200 nm wavelength range centered at 800 nm is obtained by a metal-dielectric grating structure with non-corrugated mirror. 98% maximum –1st order diffraction efficiency and a wide band top-hat spectrum are demonstrated experimentally opening the way to the high-efficiency Chirped Pulse Amplification of femtosecond pulses as short as 20 fs.

Non-Bragg band gaps in 1D metamaterial aperiodic multilayers

Abstract: We 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.

High sensitivity photonic crystal pressure sensor

Abstract: A two-dimensional photonic crystal microcavity coupled with a waveguide is proposed to realise a high sensitive force sensor, designed on a GaAs membrane A theoretical model is developed to evaluate the change of the refractive index induced by the application of the force onto a sensing surface A linear calibration curve is obtained relating the resonant drop position to the applied force

Tolerancing of single point diamond turned diffractive optical elements and optical surfaces

Abstract: Single point diamond turning has an increasing importance with the production of the surfaces for different optical systems such as infrared systems, prototype production of mobile phone cameras or head mounted displays with plastic lenses or master manufacturing for the injection moulding of plastic lenses for mass products. Tolerances which occur during single point diamond turning of aspheric surfaces and diffractive elements or during polar coordinate laser plotting of Computer-generated holograms will be treated. In both cases we expect similar tolerances, because the work piece is rotated in both diamond turning and laser plotting. The purpose is to understand the typical tolerances and to simulate their influence to the aberrations in the optical system. © The Author. All rights reserved [DOI: 10.2971/jeos.2007.07028]

Visualization of paper structure by optical coherence tomography: Monte Carlo simulations and experimental study

Abstract: This paper analyzes the capability of an optical coherence tomography (OCT) setup to visualize paper structure by comparing experimental and Monte Carlo (MC) simulated OCT images In the experiments, we used an OCT setup at 910 nm with the coherence length of an SLD source of 75 mum Several different refractive index matching agents were applied to paper samples to enhance the visualization of their inner structure The obtained OCT images show improved contrast in inner structure and enhanced visualization depth A set of corresponding simulations were then performed using a previously developed MC code A multilayer paper sheet model with non-planar fiber-air interfaces was developed to characterize the paper samples MC simulations were carried out to study the effect of the setup parameters on the obtained OCT images A comparison with the experimental data shows that the developed model provides a satisfactory correspondence between simulated and experimental images

Design and fabrication of an all-dielectric grating with top-hat high diffraction efficiency over a broad spectral range

Abstract: Between 98% and 100% flat top efficiency –1st order TE diffraction over a 40 nm wavelength range centered at 800 nm can be obtained by an all-dielectric grating structure where the corrugation is etched in a high index layer unlike in the state of the art. 98% maximum efficiency and a wide-band top-hat diffraction efficiency spectrum are demonstrated experimentally opening to high efficiency femtosecond pulse compression for high average power laser machining.

Enhanced transmission through arrays of subwavelength holes in gold films coated by a finite dielectric layer

Abstract: Enhanced transmissions through a gold film with arrays of subwavelength holes are theoretically studied, employing the rigid full vectorial three dimensional finite difference time domain method. Influence of air-holes shape to the transmission is firstly studied, which confirms two different resonances attributing to the enhanced transmission: the localized waveguide resonance and periodic surface plasmon resonances. For the film coated with dielectric layers, calculated results show that in the wavelength region of interest the localized waveguide resonant mode attributes to sensing rather than the periodic gold-glass surface plasmon mode. Although the detected peak is fairly broad and the shift is not too pronounced, we emphasize the contribution for sensing from the localized waveguide resonant mode, which may opens up new ways to design surface plasmon based sensors.

All-optical switch based on liquid-crystal infiltrated photonic bandgap fiber in transverse configuration

Abstract: We demonstrate optically controlled switching in a photonic bandgap fiber filled with liquid crystal using transverse coupling geometry. Fiber samples made from silica and lead silicate are studied. For the latter one, a continuous and fairly flat operating range from 600 to 1700 nm was achieved. The insertion loss was 3 dB and the extinction ratio better than 20 dB.

Energy and momentum flux in a high-numerical-aperture beam using the extended Nijboer-Zernike diffraction formalism

Abstract: We describe the energy and momentum flux in the case of an aberrated optical imaging system with a high numerical aperture (NA). The approach is based on the extended Nijboer-Zernike diffraction theory, that, in its high-NA version, yields an accurate analytic representation of the electromagnetic field vectors in the focal region of imaging systems that suffer from aberrations and/or transmission defects. In an earlier publication, we have derived the electromagnetic energy density from the field vectors. In this paper, we expand our analysis to the energy flow (Poynting vector) and to the quantities related to the linear and angular momentum of the radiation. Several examples of the energy and momentum flow are presented. In particular, we show how the linear and angular momentum distribution in the focal region depend on the initial polarization state and on the parameters describing the wavefront shape of the converging beam. For the angular momentum flow, we show how the separation between spin and orbital momentum is modified when going from the paraxial case to a high-NA focused beam.

Technology and performances of silicon oxynitride waveguides for optomechanical sensors fabricated by plasma-enhanced chemical vapour deposition

Abstract: The 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.

Effects of partial coherence on frequency combs

Abstract: We give an analytical model to study the deviations from an ideal frequency comb when the effects of random variations, from pulse to pulse, in the amplitude, phase, shape, chirp, and timing jitter are simultaneously taken into account. This model is consistent with the previous experimental research and extends the theoretical work on spectral line widths of mode-locked lasers.

Mode suppression in a microcavity solid-state dye laser

Abstract: A 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 ...

Concealment by uniform motion

Abstract: The perceived lateral position of a transmitted beam, upon propagating through a slab made of homogeneous, isotropic, dielectric material at an oblique angle, can be controlled through varying the velocity of the slab. In particular, by judiciously selecting the slab velocity, the transmitted beam can emerge from the slab with no lateral shift in position. Thereby, a degree of concealment can be achieved. This concealment is explored in numerical calculations based on a 2D Gaussian beam.

Microlenses with annular amplitude and phase masks

Abstract: We present theoretical and experimental investigations of microlenses with both amplitude and phase masks. The light field in the focal region has been measured with a high resolution Mach-Zehnder interferometer with z-scan. The experimental results show good agreement with simulation. We show that we can obtain effects as diverse as squeezing or expansion in longitudinal and lateral directions, focal point splitting and focal shift. These effects may be of interest for applications such as Shack-Hartmann wavefront sensing, integral imaging, optical tweezers and confocal microscopy.

Excitation back transfer in a statistical model for upconversion in Er-doped fibres

Abstract: We 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.

Quantitative analysis of the thermal distortions in a 100W CW Nd:YAG ceramic slab laser.

Abstract: Making use of an interferometric diagnostic system, we analyze the thermo-mechanical distortions taking place in the slab shaped ceramic Nd:YAG active medium of a 100 W class laser. These distortions are collected in different pumping regimes, both in static situations and during transient warm-up, and compared to the results of computer simulations. This procedure enables us to determine the relevance of different stress causes and thus to increase the specific power extraction of the active slab module.

Polarization and coherence for vectorial electromagnetic waves and the ray picture of light propagation

Abstract: We develop a complete geometrical picture of paraxial light propagation including coherence phenomena. This approach applies both for scalar and vectorial waves via the introduction of a suitable Wigner function and can be formulated in terms of an inverted Huygens principle. Coherence is included by allowing the geometrical rays to transport generalized Stokes parameters. The degree of coherence for scalar and vectorial light can be expressed as simple functions of the corresponding Wigner function.

A Model to optimize a microwave PBG accelerator based on generic unit cell

Abstract: In 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.

Active microcavity and coupled cavities in one-dimensional photonic crystal

Abstract: The propagation of light in one-dimensional SiO2-TiO2 coupled cavity photonic crystal is investigated. In particular the potential application in light amplification is proposed considering the small group velocity that characterizes the propagation at the edge of the resonance band due to the defects. Then, by means of a transfer-matrix method and a mode matching method code, an estimation of the photon lifetime and of the field intensity in a three-coupled cavity-photonic crystal is reported comparing it with those pertaining to a microcavity photonic crystal. This calculation allows us to underline the role of the light–matter interaction time with respect to that of the number of the active medium layers in the optical amplification.

Third order Bragg grating filters in small SOI waveguides

Abstract: Third order grating filters fabricated in small Silicon-on-Insulator rib waveguides are demonstrated Variations in grating etch depth and duty cycle are considered, and a maximum experimental reflection of 42% is demonstrated for gratings of 1500 µm in length, with a grating period of approximately 689nm and an etch depth of 200nm Agreement with modeling is shown to be good

A Stokes-based spectro-polarimetric analysis of the amplified spontaneous emission in a semiconductor optical amplifier

Abstract: Semiconductor Optical Amplifiers (SOAs), key devices for future all-optical communication systems, are inherently polarisation-dependent, which is a major drawback for most networks applications. In spite of numerous studies carried out in order to design polarisation-insensitive structures, no complete spectro-polarimetric characterization of a SOA has been published so far. In particular, the spectral and polarimetric behaviour of the Amplified Spontaneous Emission (ASE), acting as a partly polarized broadband source, is of interest, since ASE draws from the same carrier reservoir as the amplified signal. In this paper, we present a full spectro-polarimetric characterization of ASE emitted from a commercial, strained-bulk SOA within the frame of the Stokes formalism. This formalism not only allows a determination of the degree of polarisation (DOP) of ASE directly from its Stokes vector, but also gives access to a full, spectrally resolved characterization of its polarized fraction with respect to the bias current applied to the SOA. The way the state of polarisation of that fraction is governed by the dependence of the material gain upon polarisation is spectrally resolved, quantified, and discussed. The same study is performed when a polarized signal is injected into the SOA.

Axial spectral scans of polarization dependent third harmonic generation in a multimode photonic crystal fiber

Abstract: We demonstrate a nondestructive axial scanning technique for the spectrally resolved analysis of femtosecond nonlinear-optical transformation in photonic crystal fibers. This technique is applied to map the generation of a polarization-switched third harmonic of femtosecond Cr:forsterite laser pulses in a multimode silica photonic crystal fiber. Obtained results confirmed the intermodal phase-matching to be responsible for the observed polarization dependent multipeak third-harmonic generation. The axial scans revealed, that it is necessary to distinguish between the low and high energy excitation regime of the fiber sample. The proposed technique allows to measure the spectra of nonlinear signals generated in a photonic crystal fiber as a function of the propagation distance without cutting the fiber.

Improved perturbation method for absorption map reconstruction in diffusion optical tomography

Abstract: The 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.