In this review paper some recent advances on optical sensors based on photonic crystal fibres are reported. The different strategies successfully applied in order to obtain feasible and reliable monitoring systems in several application fields, including medicine, biology, environment sustainability, communications systems are highlighted. Emphasis is given to the exploitation of integrated systems and/or single elements based on photonic crystal fibers employing Bragg gratings (FBGs), long period gratings (LPGs), interferometers, plasmon propagation, off-set spliced fibers, evanescent field and hollow core geometries. Examples of recent optical fiber sensors for the measurement of strain, temperature, displacement, air flow, pressure, liquid-level, magnetic field, and hydrocarbon detection are briefly described.

A review on the recent progress in modeling and fabrication of medium infrared (Mid-IR) fiber lasers is reported. The main objective is to illustrate some recent examples of continuous wave optical sources at wavelengths longer than those commonly employed in telecom applications and allowing high beam quality. A small number of Mid-IR lasers, among the large variety of schemes, glasses, dopants and pumping schemes reported in literature, is selected on the basis of their slope efficiency and threshold pump power. In particular, tellurite, fluoride and chalcogenide fiber lasers are considered. More details are given with reference to the novel pumping schemes.

A time-dependent numerical model of a dysprosiumdoped ZBLAN glass fiber is developed in order to design a pulsed laser emitting at about 3μm wavelength, by employing an in-band pumping scheme. A number of design parameters are changed to optimize the laser performance. Gain-switching regime with an output signal peak power close to 59W and a full width at half maximum (FWHM) pulse duration shorter than 184 ns is simulated for a fiber with dopant concentration of 2000 ppm, by employing a pulsed input pump with a peak power of 5W and a repetition rate of 100 kHz at the wavelength of 2.8 μm. These characteristics are very promising and theoretically predict the feasibility of a laser which can find application in many areas such as chemical, biological and environmental monitoring.

A gain-switched pulsed laser based on a commercial, heavily holmium-doped fluoroindate glass (FIG) fiber, is designed to emit in the middle-infrared range, at the wavelength =3.92 μm. The laser, pumped at =888 nm, is modeled by a six-level system, by taking into account experimental spectroscopic parameters, to identify a feasible laser configuration. An output signal peak power of about P_s^peak=14.62 W with a full width at half maximum (FWHM) pulse duration less than _s=73 ns and pulse energy E_s=1.214 J is predicted, by considering an input peak power of P_p^peak=10 W, and pump repetition rate of f=100 kHz, by employing a 8 cm-long fluoroindate fiber with holmium concentration N_Ho=100 000 ppm. The obtained result encourages the construction of a pulsed laser based on commercially available optical fiber, for applications in different fields as sensing and biomedicine.

A comprehensive model for designing robust all-in-fiber microresonator-based optical sensing setups is illustrated. The investigated all-in-fiber setups allow light to selectively excite high-Q whispering gallery modes (WGMs) into optical microresonators, thanks to a pair of identical long period gratings (LPGs) written in the same optical fiber. Microspheres and microbubbles are used as microresonators and evanescently side-coupled to a thick fiber taper, with a waist diameter of about 18 µm, in between the two LPGs. The model is validated by comparing the simulated results with the experimental data. A good agreement between the simulated and experimental results is obtained. The model is general and by exploiting the refractive index and/or absorption characteristics at suitable wavelengths, the sensing of several substances or pollutants can be predicted.

We report a comprehensive characterization of tunable continuous-wave (CW) and passive Q-switching laser performance of Dy-doped zirconium fluoride fiber emitting around 3 µm. The in-band pumped CW laser operation is investigated for pump wavelengths varying from 2.7 µm to 2.825 µm, for fiber lengths ranging from 0.4 m to 2 m, and for output coupling efficiency from 10% to 50%, leading to a maximum laser slope efficiency of 44% and a tuning range larger than 300 nm. With Findlay-Clay analysis and Rigrod analyses, optimal cavity parameters are retrieved, paving the way for further optimizations in performance. The passively Q-switched laser operation of Dy-doped fluoride fiber is achieved employing a semiconductor saturable absorber mirror for the first time, demonstrating a stable operation with a minimum pulse duration of 580 ns, a highest repetition frequency of 103 kHz and a pulse energy up to 300 nJ.