The lack of stable and coherent natural targets can threat the effectiveness of Interferometric Synthetic Aperture Radar (InSAR) applications. To overcome this issue, active and passive radar reflectors are designed. Thanks to their low cost of construction and maintenance, passive radar reflectors are even more employed as coherent targets to assess potential displacement measurements of land, buildings, and infrastructures. In the present study, different types of passive radar reflectors are investigated by simulating their backscattering characteristics through a 3D electromagnetic software and by calculating their radar cross sections at different azimuth and incidence angles. Simulation results have been examined by considering the characteristics of current SAR satellite missions orbiting on different planes (Sun-Synchronous Orbit/Mid-Inclination Orbit) and considering both passes (Ascending/Descending), as well as by analyzing different orientations of SAR antenna (i.e., Right/Left look sides and incidence angles). Advantages and disadvantages of the investigated passive radar reflectors are highlighted in terms of their visibility on multiple Line Of Sights (LOS). Two carrier frequencies have been selected, that are close to those of operational SAR satellites: 5.405 GHz (C-band) and 9.66 GHz (X-band).
Fiber lasers emitting in the 3-5 μm wavelength range have attracted much interest during the last years, thanks to their wide potential employment in different fields, such as remote sensing, air pollution detection, communication applications, and medical diagnostics. Fluoroindate fibers allow transmission in this range and can be doped with different rare-earth ions, including erbium, holmium, dysprosium, and neodymium. Recent experiments have shown the feasibility of emission at λ = 3.92 μm wavelength employing Ho:Nd co-doped fluoroindate glass, encouraging the investigation on continuous-wave (CW) emission lasers. In this work, a complete model of Ho:Nd co-doped fluoroindate fiber pumped at λp = 808 nm and emitting at λ = 3920 nm is developed, in order to find the unknown energy transfer parameters, thus allowing a correct design. The energy transfer parameter recovering is performed by simulating the fluoroindate fiber via a finite element method (FEM) code, by solving the rate equations with a homemade code and by matching simulations with experimental values reported in literature. The results pave the way for the accurate design of a CW laser emitting at λ = 3920 nm, potentially with better efficiency than lasers based on Ho3+ -heavily-doped fluoroindate fibers. Preliminary fiber laser design has been based on commercially available fluoroindate fibers, including double cladding fibers, in order to choose the best geometry for the fiber laser and investigate its feasibility.
A novel fiber laser based on a fluoroindate glass doped with erbium ions and cladding pumped with red light is designed. In the simulation, the pump beam at 635 nm wavelength is injected in a commercially available double D-shaped, fewmode, optical fiber fabricated by Le Verre Fluoré in order to excite the 4F9/2 energy level. A strong population inversion between 4F9/2 and 4I9/2 energy levels is obtained, thus allowing emission in the 3400-3600 nm band. The electromagnetic analysis of the fiber, performed by the finite element method, shows that up to six signal modes at 3500 nm are supported. An exhaustive mathematical model based on five rate equations for the erbium ion populations, coupled with the power propagation equations for the pump and all the signal modes, is developed. Both cases of forward and bidirectional pumping are considered. All the spectroscopic parameters employed in the model, including the absorption/emission cross sections, the lifetimes and the branching ratios, are taken from the literature. The numerical code allows for evaluating the output signal power, the threshold pump power, and the slope efficiency. The behavior of the laser is studied by varying several parameters, such as the cavity length, the erbium concentration and the output mirror reflectivity. Preliminary simulations show that, with pump powers of a few hundred mW, lasing can be obtained. These results will be improved by using an evolutionary optimization technique, like the particle swarm optimization approach, and promise interesting low-cost applications.
The accurate knowledge of the rare-earth spectroscopic parameters is fundamental for the design of both fiber and integrated active devices. The lifetimes, the branching ratios, the up-conversion, the cross-relaxation, the energy transfer coefficients of the rare-earths must be preliminarily identified before the design. The particle swarm optimization (PSO) is an efficient global search approach; when applied to rare-earth-doped host materials and devices, it permits the rare-earth spectroscopic characterization starting from optical gain measurements. The model for the peculiar case of a SiO2 - SnO2 : Er3+ glass ceramic system is illustrated. Two different, direct and indirect, pumping schemes are considered for the rare-earth spectroscopic characterization. In the direct pumping scheme, a pump at 378 nm wavelength is used to excite the erbium ions. The SnO2 does not take part in the excitation process. On the contrary, in the indirect pumping scheme the SnO2 is involved by exploiting the absorption band around 307 nm wavelength via a proper pump. In this case, the energy transfer between the SnO2 and the Er3+ ions occurs during the amplification process. The fabricated SiO2 - SnO2 : Er3+ glass ceramic slab waveguide is simulated via a finite element method (FEM) code and a homemade code is used to solve the rate equations. In order to identify the value of the SnO2-Er3+ energy transfer coefficient, the ratio between the two simulated optical gains at 1533 nm wavelength, with the direct and indirect pumping schemes, is compared with the ratio between the two emission intensity measurements.
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