Uniform Fiber Bragg Grating sensors based on Polarization-Maintaining fibers are designed for simultaneous longitudinal strain and temperature measurement of Carbon Fiber Reinforced Polymer laminates. Panda, Bow tie and Pseudo-rectangle optical fiber shape sections are investigated employing three different embedding methods, for each optical fiber. The simulation accurately takes into account that the fibers are embedded in unidirectional Carbon Fiber Reinforced Polymer and covered between two adhesive polyimide films. An exhaustive, accurate and robust investigation of the mechanisms originating Bragg wavelength shift is developed by considering a complete multiphysical model including: the propagation modes and their interaction, the birefringence, the optomechanical and thermal behaviour of both the optical fiber and the embedding composite material. For the first time, the use of polyimide films is proposed to obtain an increase of temperature sensor sensitivity, reducing the stress-transfer, due to thermal expansion, between the composite laminate and the sensing element. The designed Fiber Bragg Grating sensors are compared and their potential application, for simultaneous strain and temperature measurement of Carbon Fiber Reinforced Polymer is discussed.

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 = ± 𝟑𝟑𝒎 −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 𝝀 𝒔 = 𝟑. 𝟗𝟐 𝝁𝒎, when pumped at 𝝀 𝒑 = 𝟖𝟎𝟖 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-heavilydoped fiber, having same fiber section geometry, same refractive indices and pumped at 𝝀 𝒑 = 𝟖𝟖𝟖 𝒏𝒎. Slope efficiency 𝜼 = 𝟏𝟔. 𝟔𝟕% and input power threshold 𝑷 𝒕𝒉 = 𝟎. 𝟐 𝑾 are obtained for the fiber length 𝑳 𝒇𝒊𝒃𝒆𝒓 = 𝟎. 𝟒 𝒎, dopants concentrations 𝑵 𝑯𝒐 = 𝟖 × 𝟏𝟎 𝟐𝟔 𝒊𝒐𝒏𝒔/𝒎 𝟑 and 𝑵 𝑵𝒅 = 𝟏 × 𝟏𝟎 𝟐𝟔 𝒊𝒐𝒏𝒔/𝒎 𝟑 , and output mirror reflectivity 𝑹 𝒐𝒖𝒕 = 𝟔𝟎%. This result encourages the fabrication of a continuous wave laser based on a Ho:Nd-codoped fluoroindate fiber.