Pietro Bia

Design of mid-infrared amplifiers based on fiber taper coupling to erbium-doped microspherical resonator.

A dedicated 3D numerical model based on coupled mode theory and solving the rate equations has been developed to analyse, design and optimize an optical amplifier obtained by using a tapered fiber and a Er³⁺-doped chalcogenide microsphere. The simulation model takes into account the main transitions among the erbium energy levels, the amplified spontaneous emission and the most important secondary transitions pertaining to the ion-ion interactions. The taper angle of the optical fiber and the fiber-microsphere gap have been designed to efficiently inject into the microsphere both the pump and the signal beams and to improve their spatial overlapping with the rare earth doped region. In order to reduce the computational time, a detailed investigation of the amplifier performance has been carried out by changing the number of sectors in which the doped area is partitioned. The simulation results highlight that this scheme could be useful to develop high efficiency and compact mid-infrared amplifiers.

Fractional Derivative Based FDTD Modeling of Transient Wave Propagation in Havriliak–Negami Media

In this paper, an accurate finite-difference time-domain (FDTD) scheme for modeling time-domain wave propagation in arbitrary dispersive biological media is proposed. The main drawback occurring in the conventional FDTD implementation for such materials is the approximation of the fractional derivatives appearing in the relevent time-domain permittivity model. To overcome this problem, we propose a novel FDTD scheme based on the direct solution of the time-domain Maxwell equations by using the Riemann-Liouville operator for fractional differentiation. The feasibility of the proposed method is demonstrated by simulating the transient wave propagation in general bulk and slab dispersive materials with dielectric spectrum described by Cole-Cole, Cole-Davidson, and Havriliak-Negami formulas. In particular, the comparison between the numerical results and those evaluated by using an analytical method based on the Fourier transformation and the matrix formulation for lossy layered media demonstrates the accuracy of the proposed FDTD scheme in a broadband frequency range.

A novel FDTD formulation based on fractional derivatives for dispersive Havriliak-Negami media

A novel finite-difference time-domain (FDTD) scheme modeling the electromagnetic pulse propagation in Havriliak-Negami dispersive media is proposed. In traditional FDTD methods, the main drawback occurring in the evaluation of the electromagnetic propagation is the approximation of the fractional derivatives appearing in the Havriliak-Negami model equation. In order to overcome this problem, we have developed a novel FDTD scheme based on the direct solution of the time-domain Maxwell equations by using the Riemann-Liouville operator for fractional differentiation. The scheme can be easily applied to other dispersive material models such as Debye, Cole-Cole and Cole-Davidson. Different examples relevant to plane wave propagation in a variety of dispersive media are analyzed. The numerical results obtained by means of the proposed FDTD scheme are found to be in good accordance with those obtained implementing analytical method based on Fourier transformation over a wide frequency range. Moreover, the feasibility of the proposed method is demonstrated by simulating the transient wave propagation in slabs of dispersive materials. HighlightsFDTD modeling for electromagnetic pulse propagation in complex media.Evaluation of the transmittance and reflectance in slab of dispersive materials.Approximations of fractional derivatives using finite differences.

Design of fiber coupled Er3+:chalcogenide microsphere amplifier via particle swarm optimization algorithm

Abstract. A mid-IR amplifier consisting of a tapered chalcogenide fiber coupled to an Er3+-doped chalcogenide microsphere has been optimized via a particle swarm optimization (PSO) approach. More precisely, a dedicated three-dimensional numerical model, based on the coupled mode theory and solving the rate equations, has been integrated with the PSO procedure. The rate equations have included the main transitions among the erbium energy levels, the amplified spontaneous emission, and the most important secondary transitions pertaining to the ion-ion interactions. The PSO has allowed the optimal choice of the microsphere and fiber radius, taper angle, and fiber-microsphere gap in order to maximize the amplifier gain. The taper angle and the fiber-microsphere gap have been optimized to efficiently inject into the microsphere both the pump and the signal beams and to improve their spatial overlapping with the rare-earth-doped region. The employment of the PSO approach shows different attractive features, especially when many parameters have to be optimized. The numerical results demonstrate the effectiveness of the proposed approach for the design of amplifying systems. The PSO-based optimization approach has allowed the design of a microsphere-based amplifying system more efficient than a similar device designed by using a deterministic optimization method. In fact, the amplifier designed via the PSO exhibits a simulated gain G=33.7  dB, which is higher than the gain G=6.9  dB of the amplifier designed via the deterministic method.

Fractional-Calculus-Based FDTD Algorithm for Ultrawideband Electromagnetic Characterization of Arbitrary Dispersive Dielectric Materials

A novel finite-difference time-domain algorithm for modeling ultrawideband electromagnetic pulse propagation in arbitrary multirelaxed dispersive media is presented. The proposed scheme is based on a general, yet computationally efficient, series representation of the fractional derivative operators associated with the permittivity functions describing the frequency dispersion properties of a given dielectric material. Dedicated uniaxial perfectly matched layer boundary conditions are derived and implemented in combination with the basic time-marching scheme. Moreover, a total field/scattered field formulation is adopted in order to analyze the material response under plane-wave excitation. Compared with alternative numerical methodologies available in the scientific literature, the proposed technique features a significantly enhanced accuracy in the solution of complex electromagnetic propagation problems involving higher order dispersive dielectrics, such as the ones typically encountered in geoscience and bioengineering applications.

Optimization of the Design of High Power

In this paper, the optimization of the design of rare earth-doped cladding-pumped fiber amplifiers is investigated to improve their performance with respect to the constraints associated with space missions. This work is carried out by means of a computer code based on particle swarm optimization (PSO) and rate equation model. We consider a fiber that is radiation tolerant at the space dose levels, and we characterize the radiation response of the amplifier based on it. By simulations, we study how the design of the radiation-tolerant double-cladding Er3+/Yb3+-codoped fiber amplifiers (EYDFAs) can improve the global system response in space. The rate equations model includes the first and secondary energy transfer between Yb3+ and Er3+, the amplified spontaneous emission and the most relevant upconversion and cross relaxation mechanism among the Er3+ ions. The obtained results highlight that the developed PSO algorithm is an efficient and reliable tool to perform the recovering of the most relevant spectroscopic parameters and the optimum design of this kind of devices. These results demonstrated that the performance of high power optical amplifiers can be optimized through such a coupled approach, opening the way for the design of radiation-hardened devices for the most challenging future space missions.

\hbox{Er}^{3+}/\hbox{Yb}^{3+}

The interaction of electromagnetic fields and biological tissues has become a topic of increasing interest for new research activities in bioelectrics, a new interdisciplinary field combining knowledge of electromagnetic theory, modeling, and simulations, physics, material science, cell biology, and medicine. In particular, the feasibility of pulsed electromagnetic fields in RF and mm-wave frequency range has been investigated with the objective to discover new noninvasive techniques in healthcare. The aim of this contribution is to illustrate a novel Finite-Difference Time-Domain (FDTD) scheme for simulating electromagnetic pulse propagation in arbitrary dispersive biological media. The proposed method is based on the fractional calculus theory and a general series expansion of the permittivity function. The spatial dispersion effects are taken into account, too. The resulting formulation is explicit, it has a second-order accuracy, and the need for additional storage variables is minimal. The comparison between simulation results and those evaluated by using an analytical method based on the Fourier transformation demonstrates the accuracy and effectiveness of the developed FDTD model. Five numerical examples showing the plane wave propagation in a variety of dispersive media are examined.

-Codoped Fiber Amplifiers for Space Missions by Means of Particle Swarm Approach

A Body Area Nano-NETwork represents a system of biomedical nano-devices that, equipped with sensing, computing, and communication capabilities, can be implanted, ingested, or worn by humans for collecting diagnostic information and tuning medical treatments. The communication among these nano-devices can be enabled by graphene-based nano-antennas, which generate electromagnetic waves in the Terahertz band. However, from a perspective of the electromagnetic field propagation, human tissues generally introduce high losses that significantly impair the communication process, thus limiting communication ranges. In this context, the aim of this contribution is to study the communication capabilities of a Body Area Nano-NETwork, by carefully taking into account the inhomogeneous and disordered structure offered by biological tissues. To this end, the propagation of Pulsed Electric Fields in a stratified media stack made up by stratum corneum, epidermis, dermis, and fat has been carefully modeled. First, electric and magnetic fields, as well as the Poynting vector, have been calculated through an accurate Finite-Difference Time-Domain dispersive modeling based on the fractional derivative operator. Second, path loss and molecular absorption noise temperature have been evaluated. Finally, channel capacity and the related transmission ranges have been estimated by using some baseline physical interfaces. Moreover, the comparison with respect to reference values already available in the literature is presented too. Obtained results clearly highlight that new research efforts are needed to ensure the considered communications due to the severe impairment suffered by electromagnetic waves.

Fractional Calculus-Based Modeling of Electromagnetic Field Propagation in Arbitrary Biological Tissue

The electromagnetic analysis of a special class of 3D dielectric lens antennas is described in detail. This new class of lens antennas has a geometrical shape defined by the three-dimensional extension of Gielis’ formula. The analytical description of the lens shape allows the development of a dedicated semianalytical hybrid modeling approach based on geometrical tube tracing and physical optic. In order to increase the accuracy of the model, the multiple reflections occurring within the lens are also taken into account.

Terahertz electromagnetic field propagation in human tissues: A study on communication capabilities

In this paper, an accurate finite-difference time-domain (FDTD) scheme for modeling the electromagnetic pulse propagation in arbitrary dispersive media is presented. The main mathematical drawbacks encountered while solving this class of problems by means of the FDTD technique is the approximation of the fractional derivatives appearing in the time-domain permittivity response pertaining such materials. In order to overcome this issue, the proposed scheme solves the Maxwells equations directly in the time-domain by using the Riemann-Liouville fractional derivative operator. The feasibility of the proposed method is demonstrated by simulating the ultra-wideband wave propagation in general stratified Raicu dispersive media displaying multiple relaxation times response.