Aircraft structures require periodic and scheduled inspection and maintenance operations due to their special operating conditions and the principles of design employed to develop them. Therefore, structural health monitoring has a great potential to reduce the costs related to these operations. Optical fiber sensors applied to the monitoring of aircraft structures provide some advantages over traditional sensors. Several practical applications for structures and engines we have been working on are reported in this article. Fiber Bragg gratings have been analyzed in detail, because they have proved to constitute the most promising technology in this field, and two different alternatives for strain measurements are also described. With regard to engine condition evaluation, we present some results obtained with a reflected intensity-modulated optical fiber sensor for tip clearance and tip timing measurements in a turbine assembled in a wind tunnel.

When it comes to measuring blade-tip clearance or blade-tip timing in turbines, reflective intensity-modulated optical fiber sensors overcome several traditional limitations of capacitive, inductive or discharging probe sensors. This paper presents the signals and results corresponding to the third stage of a multistage turbine rig, obtained from a transonic wind-tunnel test. The probe is based on a trifurcated bundle of optical fibers that is mounted on the turbine casing. To eliminate the influence of light source intensity variations and blade surface reflectivity, the sensing principle is based on the quotient of the voltages obtained from the two receiving bundle legs. A discrepancy lower than 3% with respect to a commercial sensor was observed in tip clearance measurements. Regarding tip timing measurements, the travel wave spectrum was obtained, which provides the average vibration amplitude for all blades at a particular nodal diameter. With this approach, both blade-tip timing and tip clearance measurements can be carried out simultaneously. The results obtained on the test turbine rig demonstrate the suitability and reliability of the type of sensor used, and suggest the possibility of performing these measurements in real turbines under real working conditions.

Polymer optical fibers (POFs) doped with organic dyes can be used to make efficient lasers and amplifiers due to the high gains achievable in short distances. This paper analyzes the peculiarities of light amplification in POFs through some experimental data and a computational model capable of carrying out both power and spectral analyses. We investigate the emission spectral shifts and widths and on the optimum signal wavelength and pump power as functions of the fiber length, the fiber numerical aperture and the radial distribution of the dopant. Analyses for both step-index and graded-index POFs have been done.

We report efficient random lasing in a ground powder of a novel solid-state material based on silica gel containing SiO₂ nanoparticles embedding rhodamine 6G (Rh6G) dye. Basic properties of random lasing such as emission kinetics, emission spectrum, and threshold of stimulated emission are investigated by using real-time spectroscopy. The laser-like emission dynamics can be accurately described by a light diffusive propagation model. The device behavior is close to a conventional ultrafast Q-switched laser, which is an interesting fact aimed to further applications.

The optical properties of trivalent chromium ions in zirconium-barium-lanthanum fluoride glasses [${62\mathrm{Z}\mathrm{r}\mathrm{F}}_4$-${30\mathrm{B}\mathrm{a}\mathrm{F}}_2$-${8\mathrm{L}\mathrm{a}\mathrm{F}}_3$, ${57\mathrm{Z}\mathrm{r}\mathrm{F}}_4$-${34\mathrm{B}\mathrm{a}\mathrm{F}}_2$-${5\mathrm{L}\mathrm{a}\mathrm{F}}_3$-${4\mathrm{A}\mathrm{l}\mathrm{F}}_3$, and 58 ${\mathrm{ZrF}}_4$-${18\mathrm{B}\mathrm{a}\mathrm{F}}_2$-5.${5\mathrm{L}\mathrm{a}\mathrm{F}}_3$-${3\mathrm{A}\mathrm{l}\mathrm{F}}_3$-15NaF] have been investigated by using steady-state and time-resolved luminescence techniques. The thermal quenching of the broadband emission is discussed in terms of the quantum-mechanical single-configurational-coordinate model using the Struck-Fonger method, and good agreement with the experimental results is obtained. The quantum efficiencies for the three glasses are low in accordance with the strong thermal quenching of the luminescence, and the influence of the host material on their values can be well related with the parameters of the quantum-mechanical single-configurational-coordinate model. In order to correlate glass-matrix composition with emission spectral properties, detailed time-resolved measurements have been performed. The results obtained provide evidence for the assumption of two main statistical site distributions for ${\mathrm{Cr}}^{3+}$ in these glasses.