| 11. |
- Akram, Muhammad Nadeem, et al.
(författare)
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Influence of base-region thickness on the performance of Pnp transistor-VCSEL
- 2014
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Ingår i: Optics Express. - 1094-4087. ; 22:22, s. 27398-27414
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Tidskriftsartikel (refereegranskat)abstract
- We have recently reported a 980nm GaAs-based three terminal Pnp transistor-vertical-cavity surface-emitting laser (TVCSEL) operating at room temperature with optical power up to 1.8mW. However, the current gain beta = Delta I-c/Delta I-b was near zero just before lasing and became negative after the lasing threshold. The main cause of the negative current gain was found to be a gradual and position-dependent forward-biasing (saturation) of the base-collector junction with increasing bias even before lasing threshold. In this article, detailed multi-physics device simulations are performed to better understand the device physics, and find ways to avoid the premature saturation of the base-collector junction. We have optimized the thickness of the base region as well as its doping concentration and the location of the quantum wells to ensure that the T-VCSEL is in the active mode throughout its range of operation. That is, the emitter-base junction is forward biased and base-collector junction is reversed biased for sweeping the excess charges out of the base region.
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| 12. |
- Alarcon, Alvaro, et al.
(författare)
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Dynamic generation of photonic spatial quantum states with an all-fiber platform
- 2023
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Ingår i: Optics Express. - : Optica Publishing Group. - 1094-4087. ; 31:6, s. 10673-10683
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Tidskriftsartikel (refereegranskat)abstract
- Photonic spatial quantum states are a subject of great interest for applications in quantum communication. One important challenge has been how to dynamically generate these states using only fiber-optical components. Here we propose and experimentally demonstrate an all-fiber system that can dynamically switch between any general transverse spatial qubit state based on linearly polarized modes. Our platform is based on a fast optical switch based on a Sagnac interferometer combined with a photonic lantern and few-mode optical fibers. We show switching times between spatial modes on the order of 5 ns and demonstrate the applicability of our scheme for quantum technologies by demonstrating a measurement-device-independent (MDI) quantum random number generator based on our platform. We run the generator continuously over 15 hours, acquiring over 13.46 Gbits of random numbers, of which we ensure that at least 60.52% are private, following the MDI protocol. Our results show the use of photonic lanterns to dynamically create spatial modes using only fiber components, which due to their robustness and integration capabilities, have important consequences for photonic classical and quantum information processing.(c) 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
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| 13. |
- Alarcon, Alvaro, 1991-, et al.
(författare)
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Polarization-independent single-photon switch based on a fiber-optical Sagnac interferometer for quantum communication networks
- 2020
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Ingår i: Optics Express. - : OPTICAL SOC AMER. - 1094-4087. ; 28:22, s. 33731-33738
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Tidskriftsartikel (refereegranskat)abstract
- An essential component of future quantum networks is an optical switch capable of dynamically routing single photons. Here we implement such a switch, based on a fiber-optical Sagnac interferometer design. The routing is implemented with a pair of fast electro-optical telecom phase modulators placed inside the Sagnac loop, such that each modulator acts on an orthogonal polarization component of the single photons, in order to yield polarization-independent capability that is crucial for several applications. We obtain an average extinction ratio of more than 19 dB between both outputs of the switch. Our experiment is built exclusively with commercial off-the-shelf components, thus allowing direct compatibility with current optical communication systems. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
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| 14. |
- Alcusa-Sáez, E. P., et al.
(författare)
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Acousto-optic interaction in polyimide coated optical fibers with flexural waves
- 2017
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Ingår i: Optics Express. - : Optical Society of America. - 1094-4087. ; 25:15, s. 17167-17173
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Tidskriftsartikel (refereegranskat)abstract
- Acousto-optic coupling in polyimide-coated single-mode optical fibers using flexural elastic waves is demonstrated. The effect of the polyimide coating on the acousto-optic interaction process is analyzed in detailed. Theoretical and experimental results are in good agreement. Although the elastic attenuation is significant, we show that acousto-optic coupling can be produced with a reasonably good efficiency. To our knowledge, it is the first experimental demonstration of acousto-optic coupling in optical fibers with robust protective coating.
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| 15. |
- Alemán Hérnandez, Felipe Ademir, et al.
(författare)
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Frequency comb enhanced Brillouin microscopy
- 2020
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Ingår i: Optics Express. - : Optica Publishing Group. - 1094-4087. ; 28:20, s. 29540-29552
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Tidskriftsartikel (refereegranskat)abstract
- Brillouin light scattering (BLS) microscopy is a well established and powerful technique to study acoustic and magnetic excitations in the frequency domain with sub-micron spatial resolution. Many other spectroscopic techniques have benefited from the introduction of femtosecond laser sources to optically pump and stimulate the sample under investigation. In BLS microscopy, the use of femtosecond lasers as the excitation source introduces several challenges, primarily since the measured frequency shift is small and the signal levels are weak due to the low duty cycle of typical femtosecond lasers. Here we present a method to evade these challenges. A strong enhancement of the weak scattering amplitude on selected modes is observed by pumping the sample with a high repetition rate frequency comb laser source. The laser beam can be focused to the diffraction limit, providing a micron pumping area. We can thus preserve the innate high frequency and spatial resolution of BLS microscopy. Furthermore, we are able to induce a point-like source of mode-selected elementary excitations which propagate away from the pumping spot. We conclude that we have demonstrated frequency comb pumped BLS microscopy as an attractive tool for studies of ultrafast induced laser dynamics directly in the frequency domain. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
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| 16. |
- 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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| 17. |
- Amer, Eynas, et al.
(författare)
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Stimulated Raman scattering detection for chemically specific time-resolved imaging of gases
- 2016
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Ingår i: Optics Express. - 1094-4087. ; 24:9, s. 9984-9993
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Tidskriftsartikel (refereegranskat)abstract
- stimulated Raman scattering (SRS) imaging technique based on spatial modulation of the pump beam has been used to study gases. The SRS gain signal was separated from the Stokes beam background in the spatial frequency domain. The SRS signal shows linear behaviour with the gas pressure at a range from 1.0 to 8.0 bars. The signal is linearly proportional to the pump beam intensity while it is enhanced with increasing the Stokes beam intensity to a certain limit than it saturates. Further, the chemical specificity of the technique has been investigated. Two sharp peaks with line width at half maximum of about 0.30 nm have been obtained at Stokes beam wavelengths of 629.93 nm and 634.05 nm corresponding to the methane and ethylene gases, respectively. The results show that SRS imaging is a promising technique to provide chemical specificity as well as spatial and temporal information of gaseous species
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| 18. |
- Amer, Eynas, et al.
(författare)
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UV laser interaction with a fluorescent dye solution studied using pulsed digital holography
- 2013
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Ingår i: Optics Express. - 1094-4087. ; 21:21, s. 25316-25323
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Tidskriftsartikel (refereegranskat)abstract
- A frequency tripled Q-switched Nd-YAG laser (wavelength 355 nm, pulse duration 12 ns) has been used to pump Coumarin 153 dye solved in ethanol. Simultaneously, a frequency doubled pulse (532 nm) from the same laser is used to probe the solvent perpendicularly resulting in a gain through stimulated laser induced fluorescence (LIF) emission. The resulting gain of the probe beam is recorded using digital holography by blending it with a reference beam on the detector. Two digital holograms without and with the pump beam were recorded. Intensity maps were calculated from the recorded digital holograms and used to calculate the gain of the probe beam due to the stimulated LIF. In addition numerical data of the local temperature rise was calculated from the corresponding phase maps using Radon inversion. It was concluded that about 15% of the pump beam energy is transferred to the dye solution as heat while the rest is consumed in the radiative process. The results show that pulsed digital holography is a promising technique for quantitative study of fluorescent species
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| 19. |
- Amotchkina, Tatiana, et al.
(författare)
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Stress compensation with antireflection coatings for ultrafast laser applications : from theory to practice
- 2014
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Ingår i: Optics Express. - : Optical Society of America. - 1094-4087. ; 22:24, s. 30387-30393
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Tidskriftsartikel (refereegranskat)abstract
- Each complicated coating, in particular, a dispersive mirror consists of dozens of layers. Thin films layers have mechanical stresses. After summing up stresses from all layers, the resulting stress is high enough to bend even a relatively thick substrate. To avoid this effect we suggest depositing an antireflection coating (AR) at the back-side of the substrate which together with suppression of unwanted reflections from the back side will also compensate this stress. We demonstrate unique, extremely thick and sophisticated AR coating consisting of 71 layers with the total physical thickness of 7.5 µm. This AR coating completely compensates stress from the dispersive mirror coated on the front side and minimizes unwanted reflections.
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| 20. |
- 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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