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| 2. |
- Alerstam, Erik, et al.
(författare)
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Improved accuracy in time-resolved diffuse reflectance spectroscopy.
- 2008
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Ingår i: Optics Express. - 1094-4087. ; 16:14, s. 10440-10454
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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. |
- Andersson, Mats, et al.
(författare)
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Spectroscopic studies of wood-drying processes
- 2006
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Ingår i: Optics Express. - 1094-4087. ; 14:8, s. 3641-3653
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Tidskriftsartikel (refereegranskat)abstract
- By the use of wavelength-modulation diode laser spectroscopy, water vapor and oxygen are detected in scattering media nonintrusively, at 980 nm and 760 nm, respectively. The technique demonstrated is based on the fact that free gases have extremely sharp absorption structures in comparison with the broad features of bulk material. Water vapor and oxygen measurements have been performed during the drying process of wood. The results suggest that the demonstrated technique can give information about the drying process of wood to complement that of commercially available moisture meters. In particular, the time when all the free water has evaporated from the wood can be readily identified by a strong falloff in the water vapor signal accompanied by the reaching of a high-level plateau in the molecular oxygen signal. Furthermore, the same point is identified in the differential optical absorption signal for liquid water, with a sharp increase by an order of magnitude in the ratio of the signal intensities at 980 nm and 760 nm.
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| 4. |
- Andrekson, Peter, 1960, et al.
(författare)
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Ultrafast, atto-Joule switch using fiber-optic parametric amplifier operated in saturation
- 2008
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Ingår i: Optics Express. - 1094-4087 .- 1094-4087. ; 16:15, s. 10956-10961
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Tidskriftsartikel (refereegranskat)abstract
- ll-optical manipulation of signals carried by lightwaves is attractive because controlling the light directly can be more efficient, allows a multitude of signal formats, and can also prove most cost effective. We implemented a novel scheme for ultrafast optical switching using very small control energy that relies on the use of a saturated fiber-optic parametric amplifier. Approximately 19 aJ (150 photons) of control pulse energy was needed for 50% extinction of the signal which is three to four orders of magnitude smaller than in other all-optical switching demonstrations. This allows the consideration of novel practical approaches to implement all-optical switching devices and all-optical subsystems for telecommunications and other applications.
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| 5. |
- Atlasov, K.A., et al.
(författare)
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Wavelength and loss splitting in directly coupled photonic-crystal defect microcavities
- 2008
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Ingår i: Optics Express. - 1094-4087. ; 16:20, s. 16255-16264
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Tidskriftsartikel (refereegranskat)abstract
- Coupling between photonic-crystal defect microcavities is observed to result in a splitting not only of the mode wavelength but also of the modal loss. It is discussed that the characteristics of the loss splitting may have an important impact on the optical energy transfer between the coupled resonators. The loss splitting - given by the imaginary part of the coupling strength - is found to arise from the difference in diffractive outof-plane radiation losses of the symmetric and the antisymmetric modes of the coupled system. An approach to control the splitting via coupling barrier engineering is presented. © 2008 Optical Society of America.
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| 6. |
- Atlasow, Kirill A., et al.
(författare)
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Photonic-crystal microcavity laser with site-controlled quantum-wire active medium
- 2009
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Ingår i: Optics Express. - 1094-4087. ; 17:20, s. 18178-18183
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Tidskriftsartikel (refereegranskat)abstract
- Site-controlled quantum-wire photonic-crystal microcavity laser is experimentally demonstrated using optical pumping. The single-mode lasing and threshold are established based on the transient laser response, linewidth narrowing, and the details of the non-linear power input-output charateristics. Average-power threshold as low as ~240 nW (absorbed power) and spontaneous emission coupling coefficient β~0.3 are derived.
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| 7. |
- Axelsson, Johan, et al.
(författare)
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Spatially varying regularization based on spectrally resolved fluorescence emission in fluorescence molecular tomography
- 2007
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Ingår i: Optics Express. - 1094-4087. ; 15:21, s. 13574-13584
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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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| 8. |
- Azzouz, H, et al.
(författare)
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Levitated droplet dye laser
- 2006
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Ingår i: Optics Express. - 1094-4087. ; 14:10, s. 4374-4379
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Tidskriftsartikel (refereegranskat)abstract
- We present the first observation, to our knowledge, of lasing from a levitated, dye droplet. The levitated droplets are created by computer controlled pico-liter dispensing into one of the nodes of a standing ultrasonic wave (100 kHz), where the droplet is trapped. The free hanging droplet forms a high quality optical resonator. Our 750 nL lasing droplets consist of Rhodamine 6G dissolved in ethylene glycol, at a concentration of 0.02 M. The droplets are optically pumped at 532 nm light from a pulsed, frequency doubled Nd:YAG laser, and the dye laser emission is analyzed by a fixed grating spectrometer. With this setup we have achieved reproducible lasing spectra in the visible wavelength range from 610 nm to 650 nm. The levitated droplet technique has previously successfully been applied for a variety of bio-analytical applications at single cell level. In combination with the lasing droplets, the capability of this high precision setup has potential applications within highly sensitive intra-cavity absorbance detection. (c) 2006 Optical Society of America.
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| 9. |
- Barrios, Carlos Angulo, et al.
(författare)
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Demonstration of slot-waveguide structures on silicon nitride / silicon oxide platform
- 2007
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Ingår i: Optics Express. - 1094-4087. ; 15:11, s. 6846-6856
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Tidskriftsartikel (refereegranskat)abstract
- We report on the first demonstration of guiding light in vertical slot-waveguides on silicon nitride/silicon oxide material system. Integrated ring resonators and Fabry-Perot cavities have been fabricated and characterized in order to determine optical features of the slot-waveguides. Group index behavior evidences guiding and confinement in the low-index slot region at O-band (1260-1370nm) telecommunication wavelengths. Propagation losses of < 20 dB/cm have been measured for the transverse-electric mode of the slot-waveguides.
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| 10. |
- Beckers, Jacques M., et al.
(författare)
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Very high-resolution spectroscopy for extremely large telescopes using pupil slicing and adaptive optics
- 2007
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Ingår i: Optics Express. - 1094-4087. ; 15:5, s. 1983-1994
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Tidskriftsartikel (refereegranskat)abstract
- Under seeing limited conditions very high resolution spectroscopy becomes very difficult for extremely large telescopes (ELTs). Using adaptive optics (AO) the stellar image size decreases proportional with the telescope diameter. This makes the spectrograph optics and hence its resolution independent of the telescope diameter. However AO for use with ELTs at visible wavelengths require deformable mirrors with many elements. Those are not likely to be available for quite some time. We propose to use the pupil slicing technique to create a number of sub-pupils each of which having its own deformable mirror. The images from all subpupils are combined incoherently with a diameter corresponding to the diffraction limit of the sub-pupil. The technique is referred to as "Pupil Slicing Adaptive Optics" or PSAO. (c) 2007 Optical Society of America.
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