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Träfflista för sökning "L773:1094 4087 ;pers:(Andersson Engels Stefan)"

Sökning: L773:1094 4087 > Andersson Engels Stefan

  • Resultat 1-10 av 12
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1.
  • Abrahamsson, Christoffer, et al. (författare)
  • Time and wavelength resolved spectroscopy of turbid media using light continuum generated in a crystal fiber
  • 2004
  • Ingår i: Optics Express. - 1094-4087. ; 12:17, s. 4103-4112
  • Tidskriftsartikel (refereegranskat)abstract
    • We report a novel system for time-resolved diffuse remission spectral measurements, based on short light continuum pulses generated in an index-guided crystal fiber, and a spectrometer-equipped streak camera. The system enables spectral recordings of absorption and reduced scattering coefficients of turbid media in the wavelength range 500 - 1200 nm with a spectral resolution of 5 nm and a temporal resolution of 30 ps. The optical properties are calculated by fitting the solution of the diffusion equation to the time-dispersion curve at each wavelength. Example measurements are presented from an apple, a finger and a pharmaceutical tablet. (C) 2004 Optical Society of America.
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2.
  • Alerstam, Erik, et al. (författare)
  • Improved accuracy in time-resolved diffuse reflectance spectroscopy.
  • 2008
  • Ingår i: Optics Express. - 1094-4087. ; 16:14, s. 10440-10454
  • Tidskriftsartikel (refereegranskat)abstract
    • Significant improvements in the accuracy of time-resolved diffuse reflectance spectroscopy are reached by using a Monte Carlo scheme for evaluation of measured photon time-of-flight distributions. The use of time-resolved diffusion theory of photon migration, being the current standard scheme for data evaluation, is shown defective. In particular, the familiar problem sometimes referred to as absorption-to-scattering coupling or crosstalk, is identified as an error related to the breakdown of the diffusion approximation. These systematic errors are investigated numerically using Monte Carlo simulations, and their influence on data evaluation of experimental recordings are accurately predicted. The proposed Monte Carlo-based data evaluation avoids these errors, and can be used for routine data evaluation. The accuracy and reproducibility of both MC and diffusion modeling are investigated experimentally using the MEDPHOT set of solid tissue-simulating phantoms, and provides convincing arguments that Monte Carlo-based evaluation is crucial in important ranges of optical properties. In contrast to diffusion-based evaluation, the Monte Carlo scheme results in optical properties consistent with phantom design. Since the MEDPHOT phantoms are used for international comparisons and performance assessment, the performed characterization is carefully reported.
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3.
  • Axelsson, Johan, et al. (författare)
  • Spatially varying regularization based on spectrally resolved fluorescence emission in fluorescence molecular tomography
  • 2007
  • Ingår i: Optics Express. - 1094-4087. ; 15:21, s. 13574-13584
  • Tidskriftsartikel (refereegranskat)abstract
    • Fluorescence molecular tomography suffers from being mathematically ill-conditioned resulting in non-unique solutions to the reconstruction problem. In an attempt to reduce the number of possible solutions in the underdetermined system of equations in the reconstruction, we present a method to retrieve a spatially varying regularization map outlining the feasible inclusion position. This approach can be made very simple by including a few multispectral recordings from only one source position. The results retrieved through tissue phantom experiments imply that initial reconstructions with spatially varying priors reduces artifacts and show slightly more accurate reconstruction results compared to reconstructions using no priors.
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4.
  • Holm, Jesper, et al. (författare)
  • High power 404 nm source based on second harmonic generation in PPKTP of a tapered external feedback diode laser
  • 2008
  • Ingår i: Optics Express. - 1094-4087. ; 16:4, s. 2486-2493
  • Tidskriftsartikel (refereegranskat)abstract
    • We present results on a 404 nm laser system based on second harmonic generation in a new compact external cavity configuration. We obtain a stable 318 mW cw diffraction limited output from the system with a mode-matched pump power of 630 mW. We observe up to 620 mW SHG, when the cavity is operating in scanning mode. The pump source is an external cavity grating feedback tapered diode laser operating at 808 nm. We find that thermal effects in the nonlinear crystal severely limit the efficiency of the setup with high input powers.
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5.
  • Kamran, Faisal, et al. (författare)
  • Computationally effective solution of the inverse problem in time-of-flight spectroscopy
  • 2015
  • Ingår i: Optics Express. - 1094-4087. ; 23:5, s. 6937-6945
  • Tidskriftsartikel (refereegranskat)abstract
    • Photon time-of-flight (PTOF) spectroscopy enables the estimation of absorption and reduced scattering coefficients of turbid media by measuring the propagation time of short light pulses through turbid medium. The present investigation provides a comparison of the assessed absorption and reduced scattering coefficients from PTOF measurements of intralipid 20% and India ink-based optical phantoms covering a wide range of optical properties relevant for biological tissues and dairy products. Three different models are used to obtain the optical properties by fitting to measured temporal profiles: the Liemert-Kienle model (LKM), the diffusion model (DM) and a white Monte-Carlo (WMC) simulation-based algorithm. For the infinite space geometry, a very good agreement is found between the LKM and WMC, while the results obtained by the DM differ, indicating that the LKM can provide accurate estimation of the optical parameters beyond the limits of the diffusion approximation in a computational effective and accurate manner. This result increases the potential range of applications for PTOF spectroscopy within industrial and biomedical applications. (C) 2015 Optical Society of America
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6.
  • Khoptyar, Dmitry, et al. (författare)
  • Broadband photon time-of-flight spectroscopy of pharmaceuticals and highly scattering plastics in the VIS and close NIR spectral ranges
  • 2013
  • Ingår i: Optics Express. - 1094-4087. ; 21:18, s. 20941-20953
  • Tidskriftsartikel (refereegranskat)abstract
    • We present extended spectroscopic analysis of pharmaceutical tablets in the close near infrared spectral range performed using broadband photon time-of-flight (PTOF) absorption and scattering spectra measurements. We show that the absorption spectra can be used to perform evaluation of the chemical composition of pharmaceutical tablets without need for chemo-metric calibration. The spectroscopic analysis was performed using an advanced PTOF spectrometer operating in the 650 to 1400 nm spectral range. By employing temporal stabilization of the system we achieve the high precision of 0.5% required to evaluate the concentration of tablet ingredients. In order to further illustrate the performance of the system, we present the first ever reported broadband evaluation of absorption and scattering spectra from pure and doped Spectralon (R). (C) 2013 Optical Society of America
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7.
  • Levitz, D, et al. (författare)
  • Determination of optical scattering properties of highly-scattering media in optical coherence tomography images
  • 2004
  • Ingår i: Optics Express. - 1094-4087. ; 12:2, s. 249-259
  • Tidskriftsartikel (refereegranskat)abstract
    • We developed a new algorithm that fits optical coherence tomography (OCT) signals as a function of depth to a general theoretical OCT model which takes into account multiple scattering effects. With use of this algorithm, it was possible to extract both the scattering coefficient and anisotropy factor from a particular region of interest in an OCT image. The extraction algorithm was evaluated against measurements from an integrating sphere on a set of tissue phantoms and yielded valid results. Finally, a preliminary ex vivo OCT investigation on human aortic specimen indicated that the algorithm may contribute importantly to differentiation between normal and atherosclerotic arteries. We conclude that this algorithm may facilitate tissue characterization by OCT. (C) 2004 Optical Society of America.
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8.
  • Liu, Haichun, et al. (författare)
  • Potential of multi-photon upconversion emissions for fluorescence diffuse optical imaging
  • 2014
  • Ingår i: Optics Express. - 1094-4087. ; 22:15, s. 17782-17790
  • Tidskriftsartikel (refereegranskat)abstract
    • The spatial resolution of fluorescence molecular imaging is a critical issue for the success of the technique in biomedical applications. One important method for increasing the imaging resolution is to utilize multi-photon emissions. In this study, we thoroughly investigate the potential of the multi-photon upconversion emissions from rare-earth-doped upconverting nanoparticles for the improvement in spatial resolution of diffuse optical imaging. It is found that the imaging resolution is increased by a factor of 1.45 through employing two-photon upconversion emission compared with using the linear emission, and can be further elevated by a factor of 1.23 by using three-photon upconversion emission. In addition, we demonstrate that the pulsed excitation approach holds the promise of overcoming the low quantum yield associated with the high-order upconversion emissions. (C) 2014 Optical Society of America
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9.
  • Samsoe, E, et al. (författare)
  • Improvement of spatial and temporal coherence of a broad area laser diode using an external-cavity design with double grating feedback
  • 2004
  • Ingår i: Optics Express. - 1094-4087. ; 12:4, s. 609-616
  • Tidskriftsartikel (refereegranskat)abstract
    • We demonstrate a novel technique for narrow bandwidth and highly improved lateral mode operation of a high-power broad area laser diode. The system uses simultaneous feedback from the first diffracted order and the zeroth reflected order of a diffraction grating. The two feedback paths lead to simultaneously improvement of the spectral and spatial properties of the laser diode. The laser system operates in the well-known asymmetric double-lobed far field pattern with the larger lobe being extracted as the output. The bandwidth of the output beam is measured to 0.07 nm, which corresponds to an improvement of a factor of 17 compared to the bandwidth of the freely running laser. The output from the system contains 54% of the energy reaching the grating, or 75% of the power reflected into the zeroth order. The improvements in both the spatial and temporal coherence opens the possibility of using this laser system in applications such as frequency doubling and pumping of optical parametric oscillators. (C) 2004 Optical Society of America.
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10.
  • Sekar, Sanathana Konugolu Venkata, et al. (författare)
  • Multi-variable compensated quantum yield measurements of upconverting nanoparticles with high dynamic range : a systematic approach
  • 2022
  • Ingår i: Optics Express. - 1094-4087. ; 30:10, s. 16572-16584
  • Tidskriftsartikel (refereegranskat)abstract
    • Non-linear materials such as upconverting nanoparticles (UCNPs) are emerging technology with fast-growing applications in various fields. The power density dependence of the emission quantum yield (QY) of these non-linear materials makes them challenging to characterize using currently available commercial QY systems. We propose a multimodal system to measure QY over a wide dynamic range (1:104), which takes into account and compensates for various distorting parameters (scattering, beam profile, inner filter effect and bandwidth of emission lines). For this, a beam shaping approach enabling speckle free beam profiles of two different sizes (530 µm or 106 µm) was employed. This provides low noise high-resolution QY curves. In particular, at low power densities, a signal-to-noise ratio of >50 was found. A Tm-based core-shell UCNP with excitation at 976 nm and emission at 804 nm was investigated with the system.
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  • Resultat 1-10 av 12

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