Andrea Annunziato

For the first time, micro-tapering of fluoride glass optical fibers is exploited as a feasible technique to obtain long period gratings. The spectral properties of the fabricated long period gratings are accurately predicted by means of finite element method modal analysis and coupled mode theory. An efficient optical grating inscription is performed by using a commercial glass processing system, based on graphite filament heating. The periodic tapering, obtained via heating and stretching technique, of a section of zirconium fluoride optical fiber, provides a measured transmission attenuation with a dip larger than 15 dB at the designed resonant wavelength. A good agreement between the simulation and the characterization is obtained. These results pave the way for a number of applications, among which notch filtering in fiber amplifiers and sensing in the Mid-Infrared spectral range.

This manuscript illustrates the design and characterization of the first fiber-based photonic lantern, tailored for an efficient operation in the Mid-Infrared spectral range. The design and fabrication of the proposed device involve the use of three distinct indium fluoride optical fibers into a surrounding low refractive index glass capillary. An adiabatic transition is achieved through a controlled manufacturing process based on heating and drawing. The modal behavior of the photonic lantern is investigated along the transition via electromagnetic modal analysis, at the wavelength ${\bm{\lambda}} \ = \ \text{3.34}\ {\bm{\mu} \mathbf{m}}$ . Mode-group selectivity is obtained through the use of optical fibers with different characteristics. This implies that the light launched into a particular optical fiber evolves into specific mode groups at the photonic lantern multi-mode end. Experimental results demonstrate the possibility to excite the LP ₀₁ mode and two, odd and even, LP ₁₁ propagation modes, with losses below 1.3 dB and 1.7 dB, respectively. These results emphasize the feasibility of the proposed photonic lantern based on indium fluoride glass, with potentials for diverse applications, e.g., in communications, astrophotonics, remote sensing and spectroscopy.

A novel approach to monitor the degree of bending via flexible photonics devices, using ratiometric power variation in a few-mode optical waveguide, is proposed. To demonstrate its feasibility, a sensor exploiting a Bragg grating, approximately aligned to the neutral axis, is designed, fabricated and characterized. The reduced thickness of the proposed planar photonic sensor is uniquely limited by optical confinement requirements, enabling for an ultra-thin and highly flexible planar device, based on all-glass platform. Finite Element Method, Beam Propagation Method and Coupled Mode Theory are employed in the design to model the electromagnetic and mechanical phenomena occurring during the three-point bending test. The experiment demonstrates that the planar device withstands tight curvature without mechanical failure. The device shows that by increasing the bending the reflected power from the fundamental mode decreases and the reflected power from the higher order mode increases. The measured ratiometric power sensitivity versus displacement is KP_R = −0.78 dB /mm with negligible variation over a 40 °C thermal range. Therefore, exploiting both the Bragg wavelength shift and the mode optical power change, the proposed sensor can be employed for multiparameter sensing purposes, e.g. simultaneous temperature and curvature monitoring.

In this work, a novel continuous wave fiber laser, pumped at ${\lambda }_p = \ 1550\ nm$ and emitting at ${\lambda }_s = \ 4\ \mu m$ , has been designed and optimized. It is based on a step-index, double-cladding, praseodymium-doped fluoroindate glass fiber, available on market, having dopant concentration ${N}_{Pr} = \ 8000\ ppm$ . For a realistic design, measured spectroscopical parameters have been taken into account, writing a five-level rate equation model. The design is carried out by employing a homemade code solver. The best predicted slope efficiency of about $\eta \ = \ 33{\bm{\ \% }}$ and pump power threshold ${P}_{th} = {\bm{\ }}0.007\ W$ have been obtained for a fiber length ${L}_{fiber} = \ 0.4\ m$ and output mirror reflectivity ${R}_{out} = \ 30{\bm{\ \% }}$ . These values are very interesting with reference to the state of the art and promise the fabrication of high beam quality optical sources in the middle infrared range, by employing conventional erbium-doped fiber pumping lasers, with a potentially easy all-in-fiber integration.

For the first time, a 2×2 optical fiber coupler based on indium fluoride optical fibers is designed via coupled mode theory, fabricated via fused biconical tapering technique, and characterized in the mid-infrared spectral range. A conventional glass processing system, Vytran GPX-2400, is employed for the 2×2 optical fiber coupler fabrication, overcoming the main limitation related to the narrow temperature range for indium fluoride optical fiber processing. Moreover, the fabricated device exhibits negligible surface crystallizations. The experimental results are in optimum agreement with the simulations, demonstrating that reproducible manufacturing of low-loss fused optical fiber components for the mid-infrared spectral range is possible. In particular, the fabricated 2×2 optical fiber coupler is characterized by a measured coupling ratio CR= 72.2:27.8 at the wavelengthλ = 3.34 μm with low excess loss E_L = 0.88 dB

In this paper, for the first time, an erbium-doped fluoroindate fiber laser emitting up to 3.91 μm is designed and optimized by means of a numerical investigation performed via a home-made computer code. It is cladding pumped with red light at 635 nm. The employed fiber is commercially available from Le Verre Fluoré and exhibits a double D-shaped geometry. Continuous-wave laser emission is obtained thanks to the population inversion between the ⁴ F _9/2 and ⁴ I _9/2 energy levels. The model takes into account measured spectroscopic parameters for the absorption, stimulated emission and spontaneous decay processes. The device performance is investigated by varying several parameters, such as the input pump power, the fiber length, the dopant concentration, the output mirror reflectivity and the signal wavelength. The proposed device is very versatile and is optimized for different scenarios, including: the shortest fiber, the highest output power and the lowest threshold. Simulation results show that the best performance in terms of emission bandwidth is obtained for the laser with the lowest threshold, i.e., only 25 mW, predicting a broadband coherent emission in the 3.25--3.91 μm wavelength range and paving the way to the fabrication of a low-cost and easy-to-pump middle infrared fiber laser.

A high curvature flexible Bragg grating, in planar technology, is designed, fabricated and experimentally characterized by employing silica glass. The curvature sensing mechanism for slightly off-axis core is theoretically explained and experimentally demonstrated. The electromagnetic behavior of the proposed sensor is investigated through conformal mapping, modal analysis, and coupled-mode theory. The experimental measurement reveals different sensitivities to positive and negative curvatures. The reflection spectrum of the planar Bragg grating sensor and the relationship between its resonant wavelengths and the curvature is measured for a large curvature range up to 1/ R = ± 33 m ^-1 . The interest toward off-axis core on planar flexible platform for curvature monitoring is due to the lateral stiffness and low Young's modulus of the substrate.

In this work, a novel mid-infrared continuous wave laser, based on a fluoroindate fiber co-doped with holmium and neodymium, is designed to emit at ${{\bm{\lambda }}}_{\bm{s}} = {\bm{\ }}3.92{\bm{\ \mu m}}$ , when pumped at ${{\bm{\lambda }}}_{\bm{p}} = {\bm{\ }}808{\bm{\ nm}}$ . The laser is modeled considering a nine-level system, by taking into account experimental spectroscopical parameters. Since the energy transfer coefficients are unknown, they have been evaluated starting from the measured emission spectra of the bulk glass, reported in literature, and comparing their ratio with respect to the ratio between the simulated signal gain coefficients. The designed laser promises higher slope efficiency and power threshold lower than those obtainable with a holmium-heavily-doped fiber, having same fiber section geometry, same refractive indices and pumped at ${{\bm{\lambda }}}_{\bm{p}} = {\bm{\ }}888{\bm{\ nm}}$ . Slope efficiency ${\bm{\eta \ }} = {\bm{\ }}16.67{\bm{\% }}$ and input power threshold ${{\bm{P}}}_{{\bm{th}}} = {\bm{\ }}0.2{\bm{\ W}}$ are obtained for the fiber length ${{\bm{L}}}_{{\bm{fiber}}} = {\bm{\ }}0.4{\bm{\ m}}$ , dopants concentrations ${{\bm{N}}}_{{\bm{Ho}}} = {\bm{\ }}8 \times {10}^{26}{\bm{\ ions}}/{{\bm{m}}}^3$ and ${{\bm{N}}}_{{\bm{Nd}}} = {\bm{\ }}1 \times {10}^{26}{\bm{\ ions}}/{{\bm{m}}}^3$ , and output mirror reflectivity ${{\bm{R}}}_{{\bm{out}}} = {\bm{\ }}60{\bm{\% }}$ . This result encourages the fabrication of a continuous wave laser based on a Ho:Nd-codoped fluoroindate fiber.

An innovative microstructured flat optical fiber is designed to obtain a multiaxial strain sensor for composite material monitoring. The sensing regions are constituted by the two eyelets where Bragg gratings are written. To achieve multiaxial sensing, a suitable microstructure is designed close to only one of the eyelets. The effect of the strain field, evaluated via a 3D finite element method approach, is considered to obtain the change of the refractive index distribution. The electromagnetic modal analysis and the coupled mode theory are exploited to evaluate the Bragg wavelength shift for the slow and fast axis fundamental modes, guided in the two eyelets and affecting the sensor response. The designed microstructured flat optical fiber is technologically feasible and promises sensing performance higher than that obtainable with the conventional optical fibers. In addition, flat optical fiber can be embedded in composite materials reducing the drawbacks related to both orientation and excess resin.