Optical Field Measurements Research Articles

Optical field measurements for accurate modeling of nonlinear parametric interactions

Using a phase retrieval algorithm on a set of irradiance measurements both the local amplitude and phase of a coherent field beam profile are extracted. As a concrete application, we study a high-gain optical downconversion process in periodically poled lithium niobate (PPLN). For both the pump and signal wave beam profiles, measurements were made along the optical axis. The retrieved phase profiles were used in our nonlinear beam propagation codes to predict output amplitude and phase profile of the pump and signal fields after propagating through a PPLN crystal. The retrieval of the phase profile together with the amplitude is reported here under different conditions in a high-gain system.

Identification from measurements of mechanical fields by finite element model updating strategies

Inverse problem resolution methods are widely used in the determination of material behaviour. The optimisation of the parameters, as inputs into a well-defined system, is obtained from observed outputs such as kinematic field measurements. The aim of this paper is to summarize the research concerning one inverse method, Finite Element Modelling Updating, based on the use of these field measurements. This method is based on a combination of three components, described in the following three sections. First we present the optical field measurements applied to multi-axially loaded objects, together with their performances. Then we outline the use of Finite Element Modelling for achieving a correlation between numerical fields and their experimental counterparts. Finally we describe the identification process, together with cost functions, minimisation procedure and model validation analysis.

Luminance and brightness field distribution of light guiding plate for backlight panel (BLP) by micro molding

AbstractThis research investigates the luminance and the brightness field distribution of the microstructure of a light guiding plate (LGP) by micro injection molding (MIM) and micro injection‐compression molding (MICM). The process of manufacturing a LGP includes photo‐etching, MIM, MICM, and optical field measurement. The results show that the luminance of microstructure of LGPs produced using MICM is better than those made using MIM. The results also indicate that the most important processing parameter is the mold temperature for the luminance distribution of the LGP whether made by MIM or MICM. The maximum luminance of the LGP is 80 Nit (cd/m2) on micro molding. The brightness field distribution of the LGP made using MICM is more uniform than those made using MIM for the same processing parameters. MICM is a more suitable process than MIM for the fabrication of a LGP on a backlight panel (BLP). Copyright © 2008 John Wiley & Sons, Ltd.

Technical progress in designing and controlling of active microsystems

This paper reports on the particularities designing and controlling micro motors fabricated using thin-film techniques and the progress within the iterative development process derived by the results of the characterisations of the active parts. Presented are selected results derived from finite-element method simulations and measurements belonging to the domain of force and optical magnetic field measurements. Furthermore a control concept for micro linear motors is presented. A sliding mode controller is modified with regard to the dynamic which is substantiated by simulations and measurements.

General-Purpose Filter Design for Neural Prosthetic Devices

Brain-driven interfaces depend on estimation procedures to convert neural signals to inputs for prosthetic devices that can assist individuals with severe motor deficits. Previous estimation procedures were developed on an application-specific basis. Here we report a coherent estimation framework that unifies these procedures and motivates new applications of prosthetic devices driven by action potentials, local field potentials (LFPs), electrocorticography (ECoG), electroencephalography (EEG), electromyography (EMG), or optical methods. The brain-driven interface is described as a probabilistic relationship between neural activity and components of a prosthetic device that may take on discrete or continuous values. A new estimation procedure is developed for action potentials, and a corresponding procedure is described for field potentials and optical measurements. We test our framework against dominant approaches in an arm reaching task using simulated traces of ensemble spiking activity from primary motor cortex (MI) and a wheelchair navigation task using simulated traces of EEG-band power. Adaptive filtering is incorporated to demonstrate performance under neuron death and discovery. Finally, we characterize performance under model misspecification using physiologically realistic history dependence in MI spiking. These simulated results predict that the unified framework outperforms previous approaches under various conditions, in the control of position and velocity, based on trajectory and endpoint mean squared errors.

OS9-1-1 Full field optical measurement of large deformation fields in Single Point Incremental Forming (SPIF) Processes

The Single Point Incremental Forming (SPIF) process is an interesting innovative production technique for industries involved in metal forming and metal stamping. SPIF is based on the relative movement of a simple small punch with respect to the blank. The advantages of SPIF are a high flexibility and the fact that there is no need for a die to obtain the final shape of components. Forming strategies in SPIF are strongly dependant on the formability of the used metals. Therefore, the optimization of forming parameters requires an understanding of the strain history during the SPIF process. The present paper reports the strain evolution observed by a Digital Image Correlation (DIC) technique in a partial cone with flanges of 50° during SPIF. The results show a bending/reverse bending mechanism when the tool approaches, passes and moves away from a given material point on the tool path. Also the large strains typical for a SPIF process are measured. The paper discusses how to solve some encountered technical problems and presents procedures to obtain reliable strain results during the DIC-SPIF experiment.

Aerosol variability in the Adriatic Sea from automated optical field measurements and Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS)

The aerosol optical properties in the Adriatic Sea are presented using a 9‐year time series (1996–2005) of automated measurements collected on the Acqua Alta Oceanographic Tower (AAOT) in the northern part of the basin and a coincident satellite record obtained from an atmospheric correction scheme adapted for European seas and applied to the Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS). At AAOT, the overall averages of aerosol optical thickness τa at 500 nm and Ångström exponent α are 0.29 ± 0.21 and 1.51 ± 0.34, respectively. The average single scattering albedo varies from 0.957 at 440 nm to 0.910 at 1020 nm. The aerosol size distribution derived by optical data inversion exhibits an increase of the radius of the accumulation mode with τa. From 402 coincident data records, the agreement between satellite and field τa is remarkable, with mean absolute percentage differences of 17–20% in the 412–870 nm spectral range. On the other hand, the satellite record tends to filter out occurrences of high τa. The satellite‐derived products are used to analyze the seasonal cycle over the Adriatic basin, showing minima in winter and maxima in summer (τa(500) from 0.06 to 0.23). Then, the results of radiative transfer simulations are combined with the satellite‐derived seasonal cycles of τa and cloud fraction to determine the clear‐sky aerosol direct radiative effect for the Adriatic Sea. At the surface, the aerosol load results in a monthly average cooling effect ranging from −1 W m−2 in winter up to −9.6 W m−2 in August and a corresponding atmospheric warming from 0.5 to 4.6 W m−2.

Evaluation of remote sensing based terrestrial productivity from MODIS using regional tower eddy flux network observations

The Moderate Resolution Spectroradiometer (MODIS) sensor has provided near real-time estimates of gross primary production (GPP) since March 2000. We compare four years (2000 to 2003) of satellite-based calculations of GPP with tower eddy CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> flux-based estimates across diverse land cover types and climate regimes. We examine the potential error contributions from meteorology, leaf area index (LAI)/fPAR, and land cover. The error between annual GPP computed from NASA's Data Assimilation Office's (DAO) and tower-based meteorology is 28%, indicating that NASA's DAO global meteorology plays an important role in the accuracy of the GPP algorithm. Approximately 62% of MOD15-based estimates of LAI were within the estimates based on field optical measurements, although remaining values overestimated site values. Land cover presented the fewest errors, with most errors within the forest classes, reducing potential error. Tower-based and MODIS estimates of annual GPP compare favorably for most biomes, although MODIS GPP overestimates tower-based calculations by 20%-30%. Seasonally, summer estimates of MODIS GPP are closest to tower data, and spring estimates are the worst, most likely the result of the relatively rapid onset of leaf-out. The results of this study indicate, however, that the current MODIS GPP algorithm shows reasonable spatial patterns and temporal variability across a diverse range of biomes and climate regimes. So, while continued efforts are needed to isolate particular problems in specific biomes, we are optimistic about the general quality of these data, and continuation of the MOD17 GPP product will likely provide a key component of global terrestrial ecosystem analysis, providing continuous weekly measurements of global vegetation production

Investigations on possibilities of inline inspection of high aspect ratio microstructures

LIGA is the basic idea of promising developments for the manufacturing of microelectromechanical system parts containing high aspect ratio microstructures. Aim of the work is a brief discussion of the starting-points for inline process inspection within a direct LIGA technology using deep X-ray lithography for the production of micromechanical gear wheels with critical dimensions of ∼35 μm width at ∼1 mm height as well as to show methodic and technical measuring possibilities. Firstly, results of the determination of residual solvent content distribution within ultra-thick SU-8 films are shown obtained from refracted near field optical measurements. Furthermore, the capability of X-ray computer tomographic imaging is discussed and measurements for the determination of the three-dimensional shape of high aspect ratio microstructures are practically demonstrated with microscopic and interferometric optical methods. Finally, first results demonstrate the potential of the optical coherence tomography for several further important measurement tasks, among others, e.g. for the imaging of the distribution of mechanical stress at the resist–substrate interface. The results show that much information which is essential in the LIGA process can be achieved with recently available measurement methods. However, further development of non-destructive measurement techniques would be desirable for an effective inline process control of mass production of micromechanical parts.

The spectroscopic fingerprint of TNT between 395 and 495 nm determined from transmission near field optical microscopy measurements

Deposits of trinitrotoluene (TNT) prepared with an aerosol jet are examined with surface reflection Fourier transform infrared spectroscopy, scanning electron microscopy, atomic force microscopy and near field optical microscopy. Significant structure is observed in the vibrational spectrum of TNT particles in the N–O stretching region around 1530 cm −1 and in the spectroscopic finger print determined from transmission near field scanning optical microscopy (NSOM) measurements between 395 and 405 nm as well as in the absorption spectra of bulk quantities of TNT. About 7 fg of TNT are detected in the transmission NSOM measurements.

Acoustic Emission Monitoring of Defects in Buckling CFRP Composite Panels

The presence of impact damage in a carbon fibre composite can reduce its capacity to support an in-plane load, which can lead to an unexpected or premature failure. This paper reports on an investigation into two slender carbon/fibre epoxy panels, one un-damaged and one with an artificial delamination introduced using an embedded section of PTFE. The reported tests form part of a larger series of investigations using differing sizes of artificial delamination and real impact damage. An investigation of wave velocity propagation at varying angles to the composite lay up was completed to assist in source location. The specimens were loaded under, uniaxial in-plane loading and monitored using four resonant acoustic emission sensors. A full field optical measurement system was used to measure the global displacement of the specimens. Analysis of AE waveforms and AE hit rate were used to assess the buckling of the panel. The results compared favourably with the optical measurement results.

A "Reef-Up" approach to classifying coral habitats from IKONOS imagery

Monitoring coral reef benthos with satellites has typically followed a sensor-down approach, with the classification algorithm driven by statistics derived from the imagery. I adopt a method, drawing on statistics derived from hyperspectral optical field measurements of substrate reflectance to train image classification. In order to calibrate the satellite data with direct physical measurements of reflectivity, it is necessary to process both the imagery and in situ data to common units of albedo. Building upon a proof-of-concept study conducted by the author in the Red Sea, the link is made by correcting the remote sensing data for the effect of varying bathymetry using in situ measurement of water column optical properties and a digital elevation model constructed from a vessel-based acoustic survey, thereby yielding units of substrate reflectance. Extensive ground verification of the predictive benthic habitat map resulting from image classification showed that eight substrate classes were resolved with an overall accuracy of 69% down to a depth of 6 m, including live and dead coral framework. As compared to conventional from-image classification techniques, the reef-up method offers the potential for higher thematic accuracy while maintaining a greater degree of flexibility for repeat survey using platforms of higher spectral and spatial resolution, expected to come online in the near future. The fact that image acquisition and optical ground-truthing did not occur concurrently, is of particular relevance in confirming that in situ measurements can be made independent of image acquisition and retrospectively linked to appropriate substrate classes. Considering the wealth of hyperspectral data already acquired for shallow reef facies, the work highlights the potential of the reef-up approach for quantifying substrate distribution in coral environments using both air- and spaceborne platforms.

Experimental and Numerical Study of Intense Shear Banding for Al-Alloy under Uniaxial Tension

The occurrence of intense shear band is a prelude to failure in many Al-sheet materials. In the present study, a full field optical system measurement technique (digital image correlation) and the finite element method are used to characterize the sequence of deformation in uniaxial tension before and after the intense shear band formation in AA6111-T4. The results indicate good agreement between the measurement and the predictions in terms of shear band width, strain distribution along the gauge length and the failure mode.

Fourier fringe processing using a regressive Fourier-transform technique

Since the introduction of a fourier fringe algorithm by Takeda, it has been possible to determine the phase of a particular light source impinging on an object from one sole image. This has led to applications in many whole field optical measurement techniques such as ESPI, holography, profilometry and so on. However, the basic processing technique, in case of the 2D-Fourier transform, is subject to a major drawback. Because this technique supposes periodicity in a fringe image, the so-called leakage effects occur. This gives rise to non-negligible errors, which can be resolved by using a regressive Fourier transformation technique. In the method introduced in this article, the fringe signal is represented by a model using sines and cosines where the frequency is not fixed (which is the case for classical FFT-techniques). The coefficients of those sines and cosines together with the frequency components are then estimated locally by means of a frequency domain system identification technique. This allows the fringe pattern to be unwrapped without any distortion. This method will be applied in particular to Fourier-transform profilometry (determines object geometry using shifts of projected fringes) although it can be used in any of the techniques mentioned above. Moreover, it will be shown that the proposed method can deal with other distortions that occur in practice such as over-modulation and varying fringe visibility. The proposed technique will be validated on both simulations and on a profile measurement of a pipe section.

Measurement of Shrinkage Stresses in PMMA Bone Cement

Polymethyl methacrylate (PMMA) bone cement comprises liquid methyl methacrylate monomer and PMMA beads, the former encapsulating the beads and polymerising within 10 minutes of mixing. Up to 7% volumetric shrinkage accompanies the exothermic polymerisation reaction and subsequent cooling induces residual thermal stresses when the cement is restrained around implant components. The authors have measured shrinkages, calculated residual stresses by closed form solutions and measured stresses by a range of methods. Full field optical displacement measurement has been used to derive strains and stresses in rings of cement cooling by 50oC under representative restraints and the hole drilling technique has been applied to cured PMMA cement. A strain gauged transducer developed to measure shrinkage forces in cement rings for derivation of contact stresses. The theoretical results predict stresses in the range 10-25MPa for a range of curing temperatures. These results are supported by the experimental methods and also by subsequent finite element models. The acquisition of these results required experimental characterisation of proprietary PMMA cements, particularly in terms of elastic modulus (up to 2.65GPa) and Poisson’s Ratio (0.455). It is concluded that the thermally induced stresses are sufficient to cause cracking around hip replacement femoral stems at the stem/cement interface, in the immediate post-curing phase and prior to functional loading of the implant. Cracks of this type have been reported from clinical studies and the authors have reproduced such cracks in laboratory models of hip replacements. It is proposed that the cracks are likely to propagate by the mechanism of fatigue under cyclic loading. It is concluded that thermal stresses are important in failure of hip replacements by aseptic loosening, the most common reported indicator for implant revision. There are few references in literature that address this issue directly, however the results of this work are supportive of the measurements and observations reported and provide an explanation and understanding of the initiation of failures.

Optical Field Measurement of Nano-Apertures with a ScanningNear-Field Optical Microscope

We investigate optical near-field distributions of theunconventional C-apertures and the conventional square apertures inpreliminary experiment with an aperture scanning near-field opticalmicroscope. These nano-apertures are fabricated in Au film on a glasssubstrate with focused ion beam technology. The experimental resultsindicate the uptrend of output light intensity that a C-aperture enablesthe intensity maximum to increase at least 10 times more than a square aperturewith same unit length. The measured near-field light spot sizes ofC-aperture and square aperture with 200-nm unit length are 439 nm× 500 nm and 245 nm × 216 nm, respectively.

Binomial states and the phase distribution measurement of weak optical fields

We show that the eight-port interferometer used by Noh, Foug\`{e}res, and Mandel [Phys. Rev. Lett. {\bf 71}, 2579 (1993)] to measure their operational phase distribution of light can also be used to measure the canonical phase distribution of weak optical fields, where canonical phase is defined as the complement of photon number. A binomial reference state is required for this purpose, which we show can be obtained to an excellent degree of approximation from a suitably squeezed state. The proposed method requires only photodetectors that can distinguish among zero, one and more than one photons and is not particularly sensitive to photodetector imperfections.

New Pockels field sensor with ferroelectric liquid-crystal phase modulator

A new optical field sensor for measuring the electric field close to an electrical discharge has been developed. The new system that uses both P- and S-polarized light-division and azimuth angle modulation methods has been devised and investigated. By using a ferroelectric liquid-crystal phase modulator instead of an azimuth angle modulator, it has been possible to eliminate noise from the optical field measurement system, specifically, the birefringence effects in a long optical fiber cable. The optical cable is used for connecting a Pockels field sensor located close to the discharge with the main system located at the ground. Furthermore, it has been possible to compensate for the sensitivity difference between two optical detectors for converting P- and S-polarized light waves to an electrical signal. From this, it is found that by using a light reflection technique, the measurable minimum field strength is reduced from 100 kV/m to 1.0 kV/m in the discharge region. If the smallest phase change in the system is set to be less than 10−3 [rad], the measurable minimum field strength is reduced to 0.1 kV/m. The possible use of the probe is also considered for checking for damage caused by cosmic rays to the insulating skin of a spacecraft.

OS01W0308 Evaluation of notch tip strains using whole field optical measurement techniques

Structural components subjected to notched geometry often show localized yielding upon heavily imposed external loading. Accurate inelastic notch tip strains, required for reliable fatigue design and to prevent catastrophic failure, are frequently calculated by Neuber and Glinka approximations. It is well known that they are extremely efficient, in terms of reduced computational demands, but give poor accuracy because the approaches rely on simple implicit deformation plasticity. In this study, whole field optical measurement techniques, such as reflective moire interferometry and electronic speckle pattern interferometry (ESPI), are applied and compared with the notch-tip strain predictions proposed by Neuber and Glinka. Included are also results obtained by FEM analysis.

Fatigue testing and buckling characteristics of impacted composite specimens

Fatigue testing has been performed in constant amplitude tension–compression loading on impact damaged carbon fibre/epoxy composite laminates. Two layups were tested. The shape and the amplitude of the buckles were measured using an optical whole field measurement technique. The experimental conditions were chosen to be representative of damages in aircraft structures. The R-value was found to have a small influence on the fatigue life indicating that the compressive part of the load cycle has more importance than the tensile part, this because the compressive load caused local buckling around the damage zone. The buckling was inward on the impact side and outward on the backside with larger buckles on the backside. Also the backside buckles showed a larger growth. Fatigue lives were compared with an analytical model that was found to give conservative predictions.