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A modified sigmoid sliding mode control (MS-SMC) approach is proposed for stabilizing and tracking a quadrotor system with a nonlinear sliding surface, where the dynamics model is underactuated, highly coupled, and nonlinear. The constructed nonlinear sliding surface is based on the traditional sliding mode surface with a modified sigmoid function, allowing the initial value to quickly reach equilibrium. A new type of nonlinear SMC is applied for performance improvement of the quadrotor using the proposed modified sigmoid sliding surface. To control the quadrotor effectively, a double-loop control method is used to design the control rate, in which the position subsystem is the outer loop, and the attitude subsystem is the inner loop.With the Lyapunov function, the stability of the overall closed-loop system is ensured by stabilizing each subsystem step by step. Moreover, from a practical point of view, the system performance under the model uncertainties and external disturbances are also considered. The simulation results show that the proposed MS-SMC performs better than the conventional sliding mode control (CSMC) and the back-stepping sliding mode control (BS-SMC) in terms of stabilization and tracking against external disturbances.

The spectroscopic characteristics of a series of Dy3+/Tm3+-codoped LaF3 single crystals have been studied for mid-IR emissions through Dy3+:6H13/2 → 6H15/2 transition. Optimal mid-IR emission was achieved in 3 at% Tm3+ and 2 at% Dy3+ co-doped LaF3 sample under the excitation of 790 nm. Its emission cross section was calculated to be 2.56 × 10–21 cm2 at the peak wavelength (2840 nm) with a FWHM of 270 nm, and the fluorescence lifetime was 3.694 ms. The fact of intense 2.84 µm emission was achieved by 790 nm pumping proves the effective energy transfer from Tm3+ to Dy3+ ions. The possible energy transfer routes were analyzed in detail, and the microscopic transfer constants were also estimated based on the phonon-assisted energy transfer theory. As a result, Dy3+/Tm3+-codoped LaF3 single crystal can be a potential medium for mid-IR lasers.

In this paper, a series of Dy3+/Gd3+ codoped LaF3 (LDG) single crystals were grown by the Bridgman–Stockbarger method. The influence of non-optically-active Gd3+ ions on the luminescence of Dy3+ ions in LaF3 crystals has been explored. Dy3+/Gd3+ ions substitute for the La lattice in LaF3, and the local site symmetry of each subset of Dy3+ will be distorted from C2 to lower symmetry C1. Based on the measured low temperature excitation and emission spectra, it can be proved that Dy3+ ions occupy the same kind of spectroscopic site in Dy3+/Gd3+ codoped LaF3 crystals, and codoped Gd3+ ions do not change the spectroscopic site symmetry of Dy3+. When Gd3+ ions were codoped into the Dy3+:LaF3 crystal, Dy–Gd clusters were formed around single Dy3+ ions to isolate Dy3+ ions from each other and weakened the concentration quenching effect of Dy3+ ions. When 2 at% Dy3+ and 1 at% Gd3+ ions were doped into the LaF3 crystal, the strongest yellow luminescence was obtained under excitation at 349 nm and 453.75 nm, respectively. While doped with 2 at% Dy3+ and 2 at% Gd3+ ions, the LDG crystal exhibited the strongest mid-infrared fluorescence emission intensity. This concludes that the LDG crystals are suitable for achieving a yellow laser (4F9/2 → 6H13/2 transition) pumped using a InGaN blue laser diode (LD) and a mid-infrared laser (6H13/2 → 6H15/2 transition) pumped using a 1280 nm LD.

Enhanced 2.86 µm emission corresponding to Ho3+: 5I6→5I7 was achieved in Nd3+/Ho3+ codoped CaLaGa3O7 crystal for the first time. The detailed spectroscopic properties and energy transfer mechanism of the as-grown crystal were investigated. The results show that the Nd3+ ion is not only a very good sensitizer in a Ho3+ doped CaLaGa3O7 crystal, but also an appropriate deactivated ion with efficient depopulation of the Ho3+: 5I7 level for enhancing the 2.86 µm fluorescence emission. And the energy transfer efficiency of Ho3+: 5I7→Nd3+: 4I13/2 is estimated to be 93%, which indicates that the self-termination effect for the 2.86 µm holmium laser is suppressed successfully in Nd3+/Ho3+: CaLaGa3O7 crystal. Besides, the thermal properties of Nd3+: CaLaGa3O7 crystal were also studied. In conclusion, the introduction of Nd3+ is favorable for achieving an enhanced 2.86 µm emission in Nd3+/Ho3+: CaLaGa3O7 crystal, which can act as a new promising candidate for mid-infrared lasers.

Dy3+ doped and Dy3+/Nd3+ codoped LaF3 single crystals were successfully grown by the Bridgman-Stockbarger method in a self-made medium frequency furnace. The crystal structures of LaF3 were investigated, and their geometry optimizations were carried out. The electronic structures including band structure and density of state were calculated by using first-principles calculations based on density functional theory. The polarized absorption and emission spectra including near-infrared and mid-infrared fluorescence spectra were measured at room temperature. The energy transfer mechanism from Nd3+ ion to Dy3+ ion was presented, and the microscopic transfer parameters were calculated based on the phonon-assisted energy transfer theory. In summary, two promising routes of Dy3+/LaF3 crystal pumped by 1280 nm LD and Dy3+/Nd3+/LaF3 crystal pumped by 790 nm LD were proposed for a mid-infrared laser.

In this exploration, the 2 at% Dy3+ and 3 at% Y3+ codoped LaF3 single crystal was grown by the temperature gradient technique. The influence of non-optically-active Y3+ ions on the luminescence of Dy3+ ions in LaF3 crystals has been explored. Geometry optimization demonstrates that the crystallographic symmetry for Dy3+ ions can be distorted from C2 to lower symmetry C1. Low temperature excitation and emission spectra prove that Dy3+ ions occupy the same kind of spectroscopic site in Dy3+/Y3+ codoped LaF3 crystals. The Judd-Ofelt (J-O) theory analysis and room temperature fluorescence emission spectra have testified that the fluorescence emission intensity of Dy3+ with Y3+ is several times than that without Y3+ in LaF3 at yellow and mid-infrared region. Codoping with non-optically-active Y3+ ions could weaken the concentration quenching effect of Dy3+ and strengthen the luminescence intensity of Dy3+ ions.

High-quality single crystals of different Er3+ concentration doped SrGdGa3O7(SGGO) were grown successfully by the Czochralski method. The absorption, up-conversion, near-infrared and mid-infrared fluorescence spectra, as well as the fluorescence decay curves of Er:4I13/2 and 4I11/2 levels in Er:SGGO crystal were measured at room temperature. As the concentration of Er3+ ions increase, the spectra of Er:SGGO crystals show weaker near-infrared emission and stronger mid-infrared emission. The lifetime of the 4I13/2 state decreases by nearly 7times while the lifetime of the 4I11/2 state decreases by only about 1.3 times in the Er:SGGO crystals. These results indicate that the heavily Er3+ doped SGGO crystals is beneficial to improve the 2.7 μm laser.

Most simulation-based noise maps are important for official noise assessment but lack local noise characteristics. The main reasons for this lack of information are that official noise simulations only provide information about expected noise levels, which is limited by the use of large-scale monitoring of noise sources, and are updated infrequently. With the emergence of smart cities and ubiquitous sensing, the possible improvements enabled by sensing technologies provide the possibility to resolve this problem. This study proposed an integrated methodology to propel participatory sensing from its current random and distributed sampling origins to professional noise simulation. The aims of this study were to effectively organize the participatory noise data, to dynamically refine the granularity of the noise features on road segments (e.g., different portions of a road segment), and then to provide a reasonable spatio-temporal data foundation to support noise simulations, which can be of help to researchers in understanding how participatory sensing can play a role in smart cities. This study first discusses the potential limitations of the current participatory sensing and simulation-based official noise maps. Next, we explain how participatory noise data can contribute to a simulation-based noise map by providing (1) spatial matching of the participatory noise data to the virtual partitions at a more microscopic level of road networks, (2) multi-temporal scale noise estimations at the spatial level of virtual partitions, and (3) dynamic aggregation of virtual partitions by comparing the noise values at the relevant temporal scale to form a dynamic segmentation of each road segment to support multiple spatio-temporal noise simulations. In this case study, we demonstrate how this method could play a significant role in a simulation-based noise map. Together, these results demonstrate the potential benefits of participatory noise data as dynamic input sources for noise simulations on multiple spatio-temporal scales.