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Träfflista för sökning "WFRF:(Gustavsson Johan 1974 ) "

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1.
  • Ma, Y., et al. (författare)
  • Pulse transient hot strip technique adapted for slab sample geometry to study anisotropic thermal transport properties of mu m-thin crystalline films
  • 2014
  • Ingår i: Review of Scientific Instruments. - 0034-6748. ; 85:4
  • Tidskriftsartikel (refereegranskat)abstract
    • A new method based on the adaptation of the Pulse Transient Hot Strip technique to slab sample geometry has been developed for studying thermal conductivity and thermal diffusivity of anisotropic thin film materials (<50 mu m) with thermal conductivity in the 0.01-100 W/mK range, deposited on thin substrates (i.e., wafers). Strength of this technique is that it provides a well-controlled thermal probing depth, making it possible to probe a predetermined depth of the sample layer and thereby avoiding the influence from material(s) deeper down in the sample. To verify the technique a series of measurements were conducted on a y-cut single crystal quartz wafer. A Hot Strip sensor (32-mu m wide, 3.2-mm long) was deposited along two orthogonal crystallographic (x- and z-) directions and two independent pulse transients were recorded. Thereafter, the data was fitted to our theoretical model, and the anisotropic thermal transport properties were determined. Using a thermal probing depth of only 30 mu m, we obtained a thermal conductivity along the perpendicular (parallel) direction to the z-, i.e., optic axis of 6.48 (11.4) W/mK, and a thermal diffusivity of 3.62 (6.52) mm(2)/s. This yields a volumetric specific heat of 1.79 MJ/mK. These values agree well with tabulated data on bulk crystalline quartz supporting the accuracy of the technique, and the obtained standard deviation of less than 2.7% demonstrates the precision of this new measurement technique. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.
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2.
  • Althoff, Karin, 1974-, et al. (författare)
  • Combined segmentation and tracking of neural stem-cells
  • 2005
  • Ingår i: Lecture Notes in Computer Science. - 0302-9743. ; 3540, s. 282-291
  • Tidskriftsartikel (refereegranskat)abstract
    • In this paper we analyze neural stem/progenitor cells in an time-lapse image sequence. By using information about the previous positions of the cells, we are able to make a. better selection of possible cells out of a collection of blob-like objects. As a blob detector we use Laplacian of Gaussian (LoG) filters at multiple scales, and the cell contours of the selected cells are segmented using dynamic programming. After the segmentation process the cells are tracked in the sequence using a. combined nearest-neighbor and correlation matching technique. An evaluation of the system show that 95% of the cells were correctly segmented and tracked between consecutive frames.
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3.
  • Arafin, S., et al. (författare)
  • Comprehensive analysis of electrically-pumped GaSb-based VCSELs
  • 2011
  • Ingår i: Optics Express. - 1094-4087. ; 19:18, s. 17267-17282
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper discusses several performance-related aspects of electrically-pumped GaSb-based buried tunnel junction VCSELs with an emission wavelength of 2.6 mu m based on theoretical and experimental results. These results allow a deeper insight into the internal device physics, such as radial diffusion of carriers, maximum continuous-wave operating temperature, diffraction loss, internal temperature, gain and loss parameters, internal quantum efficiency of the active region etc. These parameters can be taken into account while designing mid-infrared lasers which leads to an improved device performance. A simple thermal model of the devices based on the two-dimensional (2-D) finite element method using the material data from the literature is also presented. In addition, an application-based result utilizing these lasers for the measurement of absolute water vapor concentration by wavelength modulation spectroscopy (WMS) method are also described, hinting that devices are well-suited for the targeted sensing applications.
4.
  • Arafin, S, et al. (författare)
  • Large-Area Single-Mode GaSb-based VCSELs using an Inverted Surface Relief
  • 2010
  • Ingår i: 23rd Annual Meeting of the IEEE Photonics Society, PHOTINICS 2010. - 978-142445368-9 ; s. 61-62
  • Konferensbidrag (refereegranskat)abstract
    • Large-area GaSb-based BTJ VCSELs at ∼2.35 μm were fabricated using an inverted surface relief technique to support the single transverse mode operation. The devices operate in continuous-wave and are (electro-)thermally tunable over 6 nm.
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5.
  • Baveja, Prashant, 1985-, et al. (författare)
  • Impact of device parameters on thermal performance of high speed oxide confined 850 nm VCSELs
  • 2012
  • Ingår i: IEEE Journal of Quantum Electronics. - 0018-9197. ; 48:1, s. 17-26
  • Tidskriftsartikel (refereegranskat)abstract
    • We study the impact of device parameters, such as inner-aperture diameter and cavity photon lifetime, on thermal rollover mechanisms in 850-nm, oxide-confined, vertical-cavity surface-emitting lasers (VCSELs) designed for high-speed operation. We perform measurements on four different VCSELs of different designs and use our empirical thermal model for calculating the power dissipated with increasing bias currents through various physical processes such as absorption within the cavity, carrier thermalization, carrier leakage, spontaneous carrier recombination, and Joule heating. When reducing the top mirror reflectivity to reduce internal optical absorption loss we find an increase of power dissipation due to carrier leakage. There is therefore a trade-ff between the powers dissipated owing to optical absorption and carrier leakage in the sense that overcompensating for optical absorption enhances carrier leakage (and vice versa). We further find that carrier leakage places the ultimate limit on the thermal performance for this entire class of devices. Our analysis yields useful design optimization strategies for mitigating the impact of carrier leakage and should thereby prove useful for the performance enhancement of 850-nm, highspeed, oxide-confined VCSELs.
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6.
  • Baveja, P. P., et al. (författare)
  • Assessment of VCSEL thermal rollover mechanisms from measurements and empirical modeling
  • 2011
  • Ingår i: Optics Express. - 1094-4087. ; 19:16, s. 15490-15505
  • Tidskriftsartikel (refereegranskat)abstract
    • We use an empirical model together with experimental measurements for studying mechanisms contributing to thermal rollover in vertical-cavity surface-emitting lasers (VCSELs). The model is based on extraction of the temperature dependence of threshold current, internal quantum efficiency, internal optical loss, series resistance and thermal impedance from measurements of output power, voltage and lasing wavelength as a function of bias current over an ambient temperature range of 15-100 degrees C. We apply the model to an oxide-confined, 850-nm VCSEL, fabricated with a 9-mu m inner-aperture diameter and optimized for highspeed operation, and show for this specific device that power dissipation due to linear power dissipation (sum total of optical absorption, carrier thermalization, carrier leakage and spontaneous carrier recombination) exceeds power dissipation across the series resistance (quadratic power dissipation) at any ambient temperature and bias current. We further show that the dominant contributors to self-heating for this particular VCSEL are quadratic power dissipation, internal optical loss, and carrier leakage. A rapid reduction of the internal quantum efficiency at high bias currents (resulting in high temperatures) is identified as being the major cause of thermal rollover. Our method is applicable to any VCSEL and is useful for identifying the mechanisms limiting the thermal performance of the device and to formulate design strategies to ameliorate them.
7.
  • Baveja, P. P., et al. (författare)
  • Impact of photon lifetime on thermal rollover in 850-nm high-speed VCSELs
  • 2012
  • Ingår i: Proceedings of SPIE - Conference on Vertical-Cavity Surface-Emitting Lasers XVI (VCSELs) / SPIE Photonics West Symposium, San Fransisco, 25-26 January 2012. - 0277-786X. - 9780819489197 ; 8276
  • Konferensbidrag (refereegranskat)abstract
    • We present an empirical thermal model for VCSELs based on extraction of temperature dependence of macroscopic VCSEL parameters from CW measurements. We apply our model to two, oxide-confined, 850-nm VCSELs, fabricated with a 9-mu m inner-aperture diameter and optimized for high-speed operation. We demonstrate that for both these devices, the power dissipation due to linear heat sources dominates the total self-heating. We further show that reducing photon lifetime down to 2 ps drastically reduces absorption heating and improves device static performance by delaying the onset of thermal rollover. The new thermal model can identify the mechanisms limiting the thermal performance and help in formulating the design strategies to ameliorate them.
8.
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9.
  • Davani, H. A., et al. (författare)
  • Polarization investigation of a tunable high-speed short-wavelength bulk-micromachined MEMS-VCSEL
  • 2012
  • Ingår i: Proceedings of SPIE - Conference on Vertical-Cavity Surface-Emitting Lasers XVI (VCSELs) / SPIE Photonics West Symposium, San Fransisco, 25-26 January 2012. - 0277-786X. - 9780819489197 ; 8276
  • Konferensbidrag (refereegranskat)abstract
    • We report the investigation of the state of polarization (SOP) of a tunable vertical-cavity surface-emitting laser (VCSEL) operating near 850 nm with a mode-hop free single-mode tuning range of about 12 nm and an amplitude modulation bandwidth of about 5 GHz. In addition, the effect of a sub-wavelength grating on the device and its influence on the polarization stability and polarization switching has been investigated. The VCSEL with an integrated sub-wavelength grating shows a stable SOP with a polarization mode suppression ratio (PMSR) more than 35 dB during the tuning.
10.
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