| 1. |
- Gao, Qiang, et al.
(författare)
-
Spatiotemporally resolved spectra of gaseous discharge between electrodes triggered by femtosecond laser filamentation
- 2022
-
Ingår i: Applied Physics B: Lasers and Optics. - : Springer Science and Business Media LLC. - 0946-2171. ; 128:10
-
Tidskriftsartikel (refereegranskat)abstract
- Atmospheric pressure discharge plasma is widely utilized in industry and science. However, due to the spatiotemporal uncertainty of the natural discharge, it is difficult to measure the discharge plasma spectra with a high spatiotemporal resolution. This prevents the accurate investigation of discharge plasma evolution and limits further applications. Here, we harnessed a femtosecond laser filament to trigger and guide a high-voltage discharge, i.e., the discharge plasma channel is rigorously controlled by the filament in both space and time. Therefore, the spectra of the plasma channel with a high spatiotemporal resolution could be measured using an imaging spectrometer. The spectra of the whole process of femtosecond laser filament-triggered discharge plasma are thoroughly studied. According to the spectral emission features, the whole process is divided into three stages: femtosecond laser filamentation, streamer propagation, and discharge. The spectral emissions at different stages can be utilized as required according to the spectral emission features. Based on the spatiotemporally resolved spectra of the streamer, the streamer propagation velocity is calculated to be about 3 × 105 m/s. In addition, atomic emissions from a discharge plasma triggered by femtosecond laser filament can be used for one-dimensional component measurements of flow fields.
|
|
| 2. |
- Gluszek, Aleksander, et al.
(författare)
-
Compact mode-locked Er-doped fiber laser for broadband cavity-enhanced spectroscopy
- 2020
-
Ingår i: Applied physics. B, Lasers and optics (Print). - : Springer. - 0946-2171 .- 1432-0649. ; 126:8
-
Tidskriftsartikel (refereegranskat)abstract
- We report the design and characteristics of a simple and compact mode-locked Er-doped fiber laser and its application to broadband cavity-enhanced spectroscopy. The graphene mode-locked polarization maintaining oscillator consumes less than 5 W of power. It is thermally stabilized, enclosed in a 3D printed box, and equipped with three actuators that control the repetition rate: fast and slow fiber stretchers, and metal-coated fiber section. This allows wide tuning of the repetition rate and its stabilization to an external reference source. The applicability of the laser to molecular spectroscopy is demonstrated by detecting CO(2)in air using continuous-filtering Vernier spectroscopy with absorption sensitivity of 5.5 x 10(-8)cm(-1)in 50 ms.
|
|
| 3. |
- Guo, Hao, et al.
(författare)
-
High-speed planar imaging of OH radicals in turbulent flames assisted by deep learning
- 2022
-
Ingår i: Applied Physics B: Lasers and Optics. - : Springer Science and Business Media LLC. - 0946-2171. ; 128:3
-
Tidskriftsartikel (refereegranskat)abstract
- High-speed planar imaging of key combustion species, like hydroxyl radicals (OH), is crucial for understanding the complex chemistry–turbulence interactions in turbulent flames. However, conducting high-speed (kHz) diagnostics is challenging due to the requirements on advanced optical system, including both fast lasers and cameras. In this paper, we report a computational imaging method to artificially achieve higher diagnosing rates based on experimental data at relatively low rates. Sequencies of planar laser-induced fluorescence (PLIF) of OH recorded at 100 kHz in a turbulent flame were first down sampled to 50 kHz, 33.3 kHz and 20 kHz, respectively, and then used as a data source to train several networks. The accuracies of the models were assessed by comparing the predicted images with those from laser measurements. It was found that, among the models tested, convolutional long short-term memory network (CONV-LSTM) can provide the best predictions and is reliable in predicting consecutive images with higher repetition rates. The model can also generate consecutive OH-PLIF images at 200 kHz based on the 100 kHz experimental data. This work sheds light on the hybrid of deep learning-based computational methods with conventional high-speed laser diagnosing techniques, which can potentially increase the temporal resolution of planar optical measurement in turbulent flows, and significantly reduce the computational timing.
|
|
| 4. |
- Mannazhi, Manu, et al.
(författare)
-
Laser-induced fluorescence for studying the influence of potassium and sodium salts on PAH formation in sooting premixed flames
- 2022
-
Ingår i: Applied Physics B: Lasers and Optics. - : Springer Science and Business Media LLC. - 0946-2171. ; 128:4
-
Tidskriftsartikel (refereegranskat)abstract
- Previous studies have shown that alkali salts influence combustion processes and soot formation, although the effects seem to vary across systems. Moreover, fundamental studies on the effect of potassium and sodium salts on formation of polycyclic aromatic hydrocarbons (PAH), which are precursors in soot formation, are scarce. Here, we report a study in which the effects on PAH formation due to the addition of alkali metal salts (KCl, KOH, K2CO3, NaCl and NaOH) to premixed ethylene–air flames were investigated. Different size classes of PAHs were probed using both spectral and 2D measurements of laser-induced fluorescence (PAH-LIF) using the excitation wavelengths 266 nm and 532 nm, while detecting the fluorescence emission at selected wavelength ranges. Elastic light scattering (ELS) measurements were also used to complement the fluorescence data. It was found that potassium and sodium salts do not significantly influence the formation of small PAHs (2–3 rings), while decreasing the concentration of larger PAHs at higher heights above burner (HAB). Another important result was that the anion in the salt (Cl−, OH−, CO32−) negligibly influences the PAH and soot formation processes after dissociation of the salts.
|
|
| 5. |
- Mannazhi, Manu, et al.
(författare)
-
Two-dimensional laser-induced incandescence for soot volume fraction measurements : issues in quantification due to laser beam focusing
- 2020
-
Ingår i: Applied Physics B: Lasers and Optics. - : Springer Science and Business Media LLC. - 0946-2171. ; 126:12
-
Tidskriftsartikel (refereegranskat)abstract
- Two-dimensional laser-induced incandescence (LII) measurements usually involve the use of a cylindrical lens to illuminate the planar region of interest. This creates a varying laser fluence and sheet width in the imaged flame region which could lead to large uncertainties in the quantification of the 2D LII signals into soot volume fraction distributions. To investigate these effects, 2D LII measurements using a wide range of laser pulse energies were performed on a premixed flat ethylene–air flame while employing a cylindrical lens to focus the laser sheet. Using shorter focal length of the focusing lens resulted in larger variation of the LII signal profiles across the flame. A heat – and – mass – transfer - based LII model was also used to simulate the measurements and good agreement was found. The ratio between focal length (FL) and image length (IL) was introduced as a useful parameter for estimating the bias in estimated soot volume fractions across the flame. The general recommendation is to maximize this FL/IL ratio in an experiment, which in practice means the use of a long focal length lens. Furthermore, the best choices of laser fluence and detection gate width are discussed based on results from these simulations.
|
|
| 6. |
- Sepman, Alexey, et al.
(författare)
-
Measuring NO and temperature in plasma preheated air using UV absorption spectroscopy
- 2020
-
Ingår i: Applied physics. B, Lasers and optics (Print). - : Springer. - 0946-2171 .- 1432-0649. ; 126:6
-
Tidskriftsartikel (refereegranskat)abstract
- A new fast sensor for simultaneous high temperature diagnostics (above 800 K) of nitrogen oxide (NO) concentration and gas temperature (T) was developed based on the spectral fitting of low-resolution NO UV absorption near 226 nm. The sensor was intended for process control in future low-carbon footprint heavy process industries using renewable powered electro fuels (e.g. H2, NH3) or plasma torches as heat source. Due to excitation of molecular vibration, the shape of the selected NO feature, including (0, 0), (1, 1), and (2, 2) vibrational transitions of the A2Σ+ − X2Π2 electronic system had a strong temperature sensitivity at temperatures above 800 K. The fitting was made using the well-known NO molecular constants of the A2Σ+ − X2Π2 electronic system. To reduce the computational time, a library of the molecular spectra calculated at different temperatures was created. The fitting of an experimental spectrum representing the convolution of the instrument line function of the spectrometer with the molecular spectra was performed using the pre-calculated library spectra. Based on comparison with conventional measurement methods, the accuracy of the developed sensor was within 15% for NO and about 40 K for T, clearly showing the potential for fast in situ diagnostics in hot process gases. © 2020, The Author(s).
|
|
| 7. |
- Török, Sandra, et al.
(författare)
-
Laser-induced incandescence (2λ and 2C) for estimating absorption efficiency of differently matured soot
- 2021
-
Ingår i: Applied Physics B: Lasers and Optics. - : Springer Science and Business Media LLC. - 0946-2171. ; 127:7
-
Tidskriftsartikel (refereegranskat)abstract
- Two-wavelength and two-color laser-induced incandescence (2λ–2C-LII) was used to study the absorption properties of three types of cold soot of different maturity from a mini-CAST soot generator. LII fluence curve analysis allowed for estimating absorption wavelength dependence in terms of dispersion coefficients ξ by the use of two excitation wavelengths (532 and 1064 nm). The estimated ξ (based on E(m, λ) ∝ λ1−ξ) spanned from ~ 1.2 for the mature soot, up to 2.3 for the young soot. The results for the mature soot showed good agreement with previous measurement using multi-wavelength extinction. For the young soot, however, some discrepancy was observed suggesting a weaker wavelength dependence (lower ξ) from the LII fluence analysis. Furthermore, an estimation of the E(m, λ) for the different types of soot was done from the experimental fluence curves with temperature analysis in the low-fluence regime and simulations using an LII model. Additionally, uncertainties and limitations were discussed. Finally, it should be pointed out that caution has to be taken when interpreting 2λ-LII results to obtain quantitative absorption properties of less mature soot, which may be influenced by thermal annealing during the laser pulse and by absorption from non-refractory species externally/internally mixed with the soot.
|
|
| 8. |
- Wang, Jinlei, et al.
(författare)
-
Mosquito counting system based on optical sensing
- 2020
-
Ingår i: Applied Physics B: Lasers and Optics. - : Springer Science and Business Media LLC. - 0946-2171. ; 126:2
-
Tidskriftsartikel (refereegranskat)abstract
- Mosquitos, sometimes carrying deadly diseases such as malaria, zika, and dengue fever, cause much concern. To control mosquitos, it is important to effectively monitor their presence and behavioral trends. We have constructed two optical sensing systems for insects based on light attenuation and light backscattering, respectively. The systems, which were tested with the potentially dangerous Aedes albopictus and Culex pipiens, were able to extract the wing-beat frequency, when they passed impinging light, derived from light-emitting diodes. We could achieve distinction between the sexes of A. albopictus and C. pipiens based on the wing-beat frequency. Finally, we propose a statistical method suitable for the system to improve the accuracy of counting.
|
|
| 9. |
- Zhang, Xiang, et al.
(författare)
-
Backward lasing of singly ionized nitrogen ions pumped by femtosecond laser pulses
- 2020
-
Ingår i: Applied Physics B: Lasers and Optics. - : Springer Science and Business Media LLC. - 0946-2171. ; 126:3
-
Tidskriftsartikel (refereegranskat)abstract
- We report on the observation of backward lasing at 391.4 nm of nitrogen ions pumped by linearly polarized intense femtosecond pulses at 800 nm. The strongly enhanced spectral intensity at 391.4 nm, as well as the amplification of an externally injected backward seeding pulse, confirm that the backward 391.4 nm signal is due to optical amplification in the nitrogen gas plasma. Compared to the forward emission at 391.4 nm, the optimal backward emission is achieved at a lower gas pressure around 10 mbar, which is due to asymmetry of the backward and forward directions rooted in the traveling excitation geometry. Comparison of the signals in pure nitrogen and air revealed a strong quenching effect of the oxygen molecules, preventing backward lasing action in ambient air.
|
|
| 10. |
- Du, Zaifa, et al.
(författare)
-
Efficiency enhancement of micro-light-emitting diode with shrinking size by localized surface plasmons coupling
- 2024
-
Ingår i: Applied Physics B: Lasers and Optics. - 0946-2171. ; 130:3
-
Tidskriftsartikel (refereegranskat)abstract
- The external quantum efficiency (EQE) enhancement of different sized GaN micro-light-emitting diodes (μLEDs) by using localized surface plasmons (LSPs) have been studied. Silver nanoparticles (Ag NPs) are attached to the sidewalls of μLEDs by spin-coating so as to be effectively coupled with the multiple quantum well (MQW) of μLEDs and generate the LSPs. In the μLEDs with 20 × 20 μm2 large mesas, the LSPs can effectively inhibit the efficiency droop. Compared to the μLED samples without the LSPs coupling, the EQE has been enhanced by about 8% at a high current density of 20,000 A/cm2. This work confirms the effectiveness of the LSPs technology in improving the μLED performances, which is originally practiced only on the basal faces of conventional LEDs.
|
|
| 11. |
- Ramgren, Birgitta, et al.
(författare)
-
Endovascular therapy in basilar artery occlusion in Sweden 2016–2019—a nationwide, prospective registry study
- 2022
-
Ingår i: Neuroradiology. - : Springer Science and Business Media LLC. - 0028-3940 .- 1432-1920. ; 64:5, s. 959-968
-
Tidskriftsartikel (refereegranskat)abstract
- Purpose: We present the first nationwide study on endovascular therapy for basilar artery occlusion (BAO) from early hospital management to 3-month outcome. Methods: Data were collected on all acute ischaemic stroke patients registered 2016–2019 in the two national quality registers for stroke care and endovascular therapy (EVT), receiving EVT for BAO and subclassified into proximal, middle and distal. Results: In all, 251 patients were included: 69 proximal, 73 middle and 109 distal BAO. Patients with proximal BAO were younger (66, middle 71, distal 76, p < 0.0001), less often female (27.5%, middle 47.9%, distal 47.7%, p = 0.015), more often smokers (28.6%, middle 20.3%, distal 11.5%, p < 0.0001), and fewer had atrial fibrillation (13.2%, middle 24.7%, distal 48.6%, p < 0.0001). Level of consciousness and NIHSS score did not differ by BAO subtype and 52.2% were alert on admission. Time from groin puncture to revascularization was significantly longer in patients with proximal BAO (71, middle 46, distal 42 min, p < 0.0001), and angioplasty and/or stenting was more often performed in patients with proximal (43.4%) and middle (27.4%) than distal (6.4%) BAO (p < 0.0001). Cumulative 90-day mortality was 38.6% (proximal 50.7%, middle 32.9%, distal 34.9%, p = 0.02). Older and pre-stroke dependent patients had higher mortality, as did patients in whom angioplasty/stenting was performed. Conclusion: We confirm a serious outcome in BAO despite endovascular therapies, and demonstrate important differences relating to occlusion location in baseline characteristics, procedural time, therapeutic measures and outcome. Further in-depth analyses of factors affecting outcome in BAO are warranted.
|
|
