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  • 1.
    Klein, Markus
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Andersson, Magnus
    Umeå University, Faculty of Science and Technology, Physics.
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Physics.
    Fällman, Erik
    Umeå University, Faculty of Science and Technology, Physics.
    Dual-trap technique for reduction of low-frequency noise in force measuring optical tweezers2007In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 46, no 3, p. 405-412Article in journal (Refereed)
    Abstract [en]

    High-resolution long-time force measurements by optical tweezers are often limited by low- frequency (1/f) noise. A dual-trap technique is presented that can reduce such noise in the force signal. It incorporates a second trap (a reference trap) that probes the noise in the system and it is based upon the assumption that the low-frequency parts of the noise from the two traps are correlated. A subtraction of the low-frequency signal from the reference trap from the signal from the force measuring trap will therefore yield a net signal that is significantly less influenced by noise. It is shown that this dual-trap technique can reduce the noise in the force signal up to 60% depending on detection bandwidth.

  • 2.
    Lathdavong, Lemthong
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Shao, Jie
    Institute of Information Optics of Zhejiang Normal University, Jinhua, China, 321004.
    Kluczynski, Pawel
    Siemens AB, IIA SLA R&D, Box 14153, SE-400 20 Göteborg, Sweden.
    Lundqvist, Stefan
    Siemens AB, IIA SLA R&D, Box 14153, SE-400 20 Göteborg, Sweden.
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Methodology for detection of carbon monoxide in hot, humid media by telecommunication distributed feedback laser-based tunable diode laser absorption spectrometry2011In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 50, no 17, p. 2531-2550Article in journal (Refereed)
    Abstract [en]

    Detection of carbon monoxide (CO) in combustion gases by tunable diode laser spectrometry is often hampered by spectral interferences from H2O and CO2. A methodology for assessment of CO in hot humid media using telecommunication DFB lasers is presented. By addressing the R14 line at 6395.4 cm-1, and using a dual-species-fitting technique that incorporates fitting of both a previously measured water background reference spectrum and a 2f-wavelength modulation lineshape function, %-level concentrations of CO can be detected in media with tens percent of water (c(H2O)  ≤ 40 %) at  ≤ 1000 °C with an accuracy of a few percent by the use of a single reference water spectrum for background correction

  • 3.
    Lathdavong, Lemthong
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Westberg, Jonas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Shao, Jie
    Institute of Information Optics of Zhejiang Normal University.
    Dion, Claude M.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Kluczynski, Pawel
    Siemens AB.
    Lundqvist, Stefan
    Siemens AB.
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Faraday modulation spectrometry of nitric oxide addressing its electronic X2Π − A2Σ+ band: I. theory2010In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 49, no 29, p. 5597-5613Article in journal (Refereed)
    Abstract [en]

    We give a simple two-transition model of Faraday modulation spectrometry (FAMOS) addressing the electronic X2Π(ν''=0) − A2Σ+(ν'=0) band in nitric oxide. The model is given in terms of the integrated line strength, S, and first Fourier coefficients for the magnetic-field-modulated dispersive line shape function. Although the two states addressed respond differently to the magnetic field (they adhere to the dissimilar Hund coupling cases), it is shown that the technique shares some properties with FAMOS when rotational-vibrational Q-transitions are targeted: the line shapes have a similar form and the signal strength has an analogous magnetic field and pressure dependence. The differences are that the maximum signal appears for larger magnetic field amplitudes and pressures, ∼1500 G and ∼200 Torr, respectively.

  • 4. Naik, Naren
    et al.
    Beatson, Rick
    Eriksson, Jerry
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Radial-basis-function level-set-based regularized Gauss-Newton-filter reconstruction scheme for dynamic shape tomography2014In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 53, no 29, p. 6872-6884Article in journal (Refereed)
    Abstract [en]

    The dynamic reconstruction problem in tomographic imaging is encountered in several applications, such as species determination, the study of blood flow through arteries/veins, motion compensation in medical imaging, and process tomography. The reconstruction method of choice is the Kalman filter and its variants, which, however, are faced by issues of filter tuning. In addition, since the time-propagation models of physical parameters are typically very complex, most of the time, a random walk model is considered. For geometric deformations, affine models are typically used. In our work, with the objectives of minimizing tuning issues and reconstructing time-varying geometrically deforming features of interest with affine in addition to pointwise-normal scaling motions, a novel level-set-based reconstruction scheme for ray tomography is proposed for shape and electromagnetic parameters using a regularized Gauss-Newton-filter-based scheme. We use an implicit Hermite-interpolation-based radial basis function representation of the zero level set corresponding to the boundary curve. Another important contribution of the paper is an evaluation of the shape-related Frechet derivatives that does not need to evaluate the pointwise Jacobian (the ray-path matrix in our ray-tomography problem). Numerical results validating the formulation are presented for a straight ray-based tomographic reconstruction. To the best of our knowledge, this paper presents the first tomographic reconstruction results in these settings. (C) 2014 Optical Society of America

  • 5.
    Schedin, Staffan
    et al.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Hallberg, Per
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Behndig, Anders
    Umeå University, Faculty of Medicine, Department of Clinical Sciences, Ophthalmology.
    Analysis of long-term visual quality with numerical 3D ray tracing after corneal crosslinking treatment2017In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 56, no 35, p. 9787-9792Article in journal (Refereed)
    Abstract [en]

    A numerical 3D ray tracing model was used to evaluate the long-term visual effects of two regimens of corneal crosslinking (CXL) treatment of 48 patients with the corneal degeneration keratoconus. The 3D ray tracing analyses were based on corneal elevation data measured by Scheimpflug photography. Twenty-two patients were treated with standard CXL applied uniformly across the corneal surface, whereas 26 patients underwent a customized, refined treatment only at local zones on the cornea (photorefractive intrastromal CXL; PiXL). Spot diagrams, spot root-mean-square (RMS) values, and Strehl ratios were evaluated for the patients prior to and 1, 3, 6, and 12 months after treatment. It was found that the group of patients treated with PiXL, on average, tended to attain a long-term improvement of the corneal optical performance, whereas only minor changes of the optical parameters were found for group treated with standard CXL. Our results confirmed that standard CXL treatment stabilizes the corneal optical quality over time, and thus halts the progression of the corneal degeneration. In addition to stabilization, the results showed that a significantly higher proportion of subjects treated with PiXL improved in RMS, 3, 6, and 12 months after treatment, compared to with CXL (p<0.05). This finding indicates that the PiXL treatment might improve optical quality over time.

  • 6.
    Schedin, Staffan
    et al.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Hallberg, Per
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Behndig, Anders
    Umeå University, Faculty of Medicine, Department of Clinical Sciences, Ophthalmology.
    Three-dimensional ray-tracing model for the study of advanced refractive errors in keratoconus2016In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 55, no 3, p. 507-514Article in journal (Refereed)
    Abstract [en]

    We propose a numerical three-dimensional (3D) ray-tracing model for the analysis of advanced corneal refractive errors. The 3D modeling was based on measured corneal elevation data by means of Scheimpflug photography. A mathematical description of the measured corneal surfaces from a keratoconus (KC) patient was used for the 3D ray tracing, based on Snell's law of refraction. A model of a commercial intraocular lens (IOL) was included in the analysis. By modifying the posterior IOL surface, it was shown that the imaging quality could be significantly improved. The RMS values were reduced by approximately 50% close to the retina, both for on-and off-axis geometries. The 3D ray-tracing model can constitute a basis for simulation of customized IOLs that are able to correct the advanced, irregular refractive errors in KC.

  • 7.
    Shao, Jie
    et al.
    Institute of Information Optics of Zhejiang Normal University.
    Lathdavong, Lemthong
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Westberg, Jonas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Kluczynski, Pawel
    Siemens AB.
    Lundqvist, Stefan
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Faraday modulation spectrometry of nitric oxide addressing its electronic X2Π - A2Σ+ band: II. experiment2010In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 49, no 29, p. 5614-5625Article in journal (Refereed)
    Abstract [en]

    A first demonstration of Faraday modulation spectrometry (FAMOS) of nitric oxide (NO) addressing its strong electronic X2Π(ν″=0)−A2Σ+=0) band is presented. The instrumentation was constructed around a fully diode-laser-based laser system producing mW powers of ultraviolet light targeting the overlapping Q22(21/2) and R12Q(21/2) transitions at ∼226.6nm. The work verifies a new two-transition model of FAMOS addressing the electronic transitions in NO given in an accompanying work. Although the experimental instrumentation could address neither the parameter space of the theory nor the optimum conditions, the line shapes and the pressure dependence could be verified under low-field conditions. NO could be detected down to a partial pressure of 13µTorr, roughly corresponding to 10ppb·m for an atmospheric pressure sample, which demonstrates the feasibility of FAMOS for sensitive detection of NO addressing its strong electronic band. A first demonstration of Faraday modulation spectrometry (FAMOS) of nitric oxide (NO) addressing its srtong electronic X2Π - A2Σ+ band is presented. The instrumentation was constructed around a fully diode-laser-based laser system producing mW powers of ultraviolet light targeting the overlapping Q22(21) and QR12(21) transitions at around 226.6 nm. The work verifies a new two-transition model of FAMOS addressing the electronic transitions in NO given in an accompanying work. Although the experimental instrumentation could address neither the parameter space of the theory nor the optimum conditions, the line shapes and the pressure dependence could be verified under low-field conditions. NO could be detected down to a partial pressure of 13 μTorr, roughly corresponding to 10 ppb · mfor an atmospheric pressure sample, which demonstrates the feasibility of FAMOS for sensitive detection of NO addressing its strong electronic band.

  • 8. Shao, Jie
    et al.
    Xiang, Jindong
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ying, Chaofu
    Wavelength-modulated tunable diode-laser absorption spectrometry for real-time monitoring of microbial growth2016In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 55, no 9, p. 2339-2345Article in journal (Refereed)
    Abstract [en]

    It is important to monitor and assess the growth of micro-organisms under various conditions. Yet, thus far there has been no technique to do this with the required speed and accuracy. This work demonstrates swift and accurate assessment of the concentration of carbon dioxide that is produced by use of a wavelength-modulated tunable diode-laser based absorption spectroscopy (WM-TDLAS). It is shown by experiments on two types of bacteria, Staphylococcus aureus and Candida albicans, that the technique can produce high signal-to-noise-ratio data from bacteria grown in confined spaces and exposed to limited amounts of nutrients that can be used for extraction of growth parameters by fitting of the Gompertz model. By applying the technique to S. aureus bacteria at various temperatures (in the 25 degrees C to 42 degrees C range), it is specifically shown that both the maximum growth rate and the so-called lag time have a strong temperature dependence (under the specific conditions with a maximum of the former at 37 degrees C) that matches conventional models well for bacterial growth. Hence, it is demonstrated that WM-TDLAS monitoring CO2 is a user-friendly, non-intrusive, and label-free technique that swiftly, and with high signal-to-noise-ratio, can be used for rapid (on the Hz scale) and accurate assessment of bacterial growth. 

  • 9.
    Stangner, Tim
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hanqing, Zhang
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Tobias, Dahlberg
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Krister, Wiklund
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Andersson, Magnus
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Step-by-step guide to reduce spatial coherence of laser light using a rotating ground glass diffuser2017In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 56, no 19, p. 5427-5435Article in journal (Refereed)
    Abstract [en]

    Wide field-of-view imaging of fast processes in a microscope requires high light intensities motivating the use of lasers as light sources. However, due to their long spatial coherence length, lasers are inappropriate for such applications, as they produce coherent noise and parasitic reflections, such as speckle, degrading image quality. Therefore, we provide a step-by-step guide for constructing a speckle-free and high-contrast laser illumination setup using a rotating ground glass diffuser driven by a stepper motor. The setup is easy to build, cheap, and allows a significant light throughput of 48%, which is 40% higher in comparison to a single lens collector commonly used in reported setups. This is achieved by using only one objective to collect the scattered light from the ground glass diffuser. We validate our setup in terms of image quality, speckle contrast, motor-induced vibrations, and light throughput. To highlight the latter, we record Brownian motion of micro-particles using a 100x oil immersion objective and a high-speed camera operating at 2000 Hz with a laser output power of only 22 mW. Moreover, by reducing the objective magnification to 50x, sampling rates up to 10,000 Hz are realized. To help readers with basic or advanced optics knowledge realize this setup, we provide a full component list, 3D-printing CAD files, setup protocol, and the code for running the stepper motor.

  • 10.
    Wadbro, Eddie
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Hallberg, Per
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Schedin, Staffan
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Optimization of an intraocular lens for correction of advanced corneal refractive errors2016In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 55, no 16, p. 4378-4382Article in journal (Refereed)
    Abstract [en]

    Based on numerical 3D ray tracing, we propose a new procedure to optimize personalized intra-ocular lenses (IOLs). The 3D ray tracing was based on measured corneal elevation data from patients who suffered from advanced keratoconus. A mathematical shape description of the posterior IOL surface, by means of a tensor product cubic Hermite spline, was implemented. The optimized lenses provide significantly reduced aberrations. Our results include a trade-off study that suggests that it is possible to considerably reduce the aberrations with only minor perturbations of an ideal spherical lens. The proposed procedure can be applied for correction of aberrations of any optical system by modifying a single surface.

  • 11.
    Zakrisson, Johan
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Schedin, Staffan
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Andersson, Magnus
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Cell shape identification using digital holographic microscopy2015In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 54, no 24, p. 7442-7448Article in journal (Refereed)
    Abstract [en]

    We present a cost-effective, simple and fast digital holographic microscopy method based upon Rayleigh-Sommerfeld back propagation for identification of the geometrical shape of a cell. The method was tested using synthetic hologram images generated by ray-tracing software and from experimental images of semi-transparent spherical beads and living red blood cells. Our results show that by only using the real part of the back-reconstructed amplitude the proposed method can provide information of the geometrical shape of the object and at the same time accurately determine the axial position of the object under study. The proposed method can be used in flow chamber assays for pathophysiological studies where fast morphological changes of cells are studied in high numbers and at different heights.

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