Geometric optics analysis of inverted graded index fibers

We derive a general solution based on geometric optics that describes the light propagation properties in multimode optical fibers with inverted refractive index profiles. Using this general solution, we classify rays according to their propagation properties and calculate the analytical expressions of the ray trajectories inside these fibers under different launching conditions. In addition, we discuss the most suitable propagation conditions that maximize the confinement of light power in the vicinity of the core-cladding interface for sensing purposes.hese results are part of the Grant Nos. PID2021-122505OB- C31, TED2021-129959B-C21 and PDC2022-133053-C21, funded by MCIN/AEI/10.13039/501100011033, by ‘ERDF A way of making Europe’ and by the ‘European Union Next GenerationEU/PRTR’, and also of the Grant Nos. IT1452-22, ELKARTEK KK 2021/00082 and ELKARTEK KK 2021/000 funded by the Gobierno Vasco/Eusko Jaurlaritza

Fabrication Quality Assessment Based on the Coupling of a Dual-Core Microstructured Polymer Optical Fiber

In this paper we report on the theoretical analysis and fabrication of a dual-core microstructured polymer optical fiber (mPOF) and demonstrate how the coupling characteristics of a dual-core mPOF may be a key factor to assess the quality of the fabrication process. The coupling characteristics of this fiber have been tested and, for comparison purposes, simulations regarding the effects of inaccuracies during the manufacturing process were carried out to evaluate the fabrication quality. Results indicate that theoretical, simulation and experimental data are in good agreement, which highlights the uniformity of the microstructure along the fiber and the quality of its fabrication process. In fact, the manufactured mPOF reached a coupling efficiency up to 95.26%, which makes this mPOF appealing for applications in which highly efficient power couplers are required.This work was supported in part by the European Regional Development Fund, in part by the Ministerio de Economía y Competitividad under project RTI2018-094669-B-C31 and RTC2019-007194-4, and in part by the Gobierno Vasco/Eusko Jaurlaritza under projects IT933-16 and ELKARTEK (KK-2021/00092, KK-2021/00082)

Fabrication of Active Polymer Optical Fibers by Solution Doping and Their Characterization

This paper employs the solution-doping technique for the fabrication of active polymer optical fibers (POFs), in which the dopant molecules are directly incorporated into the core of non-doped uncladded fibers. Firstly, we characterize the insertion of a solution of rhodamine B and methanol into the core of the fiber samples at different temperatures, and we show that better optical characteristics, especially in the attenuation coefficient, are achieved at lower temperatures. Moreover, we also analyze the dependence of the emission features of doped fibers on both the propagation distance and the excitation time. Some of these features and the corresponding ones reported in the literature for typical active POFs doped with the same dopant are quantitatively similar among them. This applies to the spectral location of the absorption and the emission bands, the spectral displacement with propagation distance, and the linear attenuation coefficient. The samples prepared in the way described in this work present higher photostability than typical samples reported in the literature, which are prepared in different ways.This research was funded by European Regional Development Fund (ERDF), by Ministerio de Economia y Competitividad (MINECO) (TEC2015-638263-C03-1-R) and by Eusko Jaurlaritza (ELKARTEK KK-2016/0030, ELKARTEK KK-2016/0059, ELKARTEK KK-2017/00033, ELKARTEK KK-2017/00089, IT933-16). The work of Mikel Azkune was supported in part by a research fellowship from the Universidad del Pais Vasco/Euskal Herriko Unibertsitatea (UPV/EHU), Vicerrectorado de Euskera y Formacion Continua, while working on a Ph.D. degree

Use of a novel fiber optical strain sensor for monitoring the vertical deflection of an aircraft flap

The present paper reports the use of a plastic optical fiber-based sensor for elongation measurements in an aircraft flap subjected to different types of flexural loading conditions. The sensor, bonded to the surface of the aircraft structure, relies on measuring the phase shift that occurs between two sinusoidally modulated light signals when the aircraft structure is bent. The light signals are guided through two optical fibers, one of them fixed to the top surface of the flap, and the other one to the bottom surface. The sensor offers good signal stability and repeatability and represents a cost-effective alternative to other more sophisticated health-monitoring systems currently used.Publicad

Review of a Custom-Designed Optical Sensing System for Aero-Engine Applications

Fibre bundle-based reflective optical sensors are good candidates for parameter monitorisation in aero engines. Tip clearance is one of those parameters of great concern that is necessary to monitor. Within this optical technology, the evolution experienced by a custom-designed optical sensor is presented from its first configuration up to the fifth one. The performance of the last configuration is compared with those of other two optical sensors that are also based on a fibre bundle design. The comparison has been carried out in an experimental program in a transonic wind tunnel for aero engines. The proven high resolution and sensitivity of the last configuration of the optical sensor opens up the possibility to detect blade defects, cracks, etc. that could otherwise be hard to track.This work was supported in part by the European Regional Development Fund, in part by the Ministerio de Economía y Competitividad under project RTI2018-094669-B-C31, and in part by the Gobierno Vasco/Eusko Jaurlaritza under projects IT933-16, ELKARTEK (KK-2018/00078 and KK-2019/00051), EKINTZAILE 01560/2019 and HAZITEK (ZL-2020/00380). The work of J. Amorebieta is supported in part by a PhD fellowship from the University of the Basque Country (UPV/EHU), Vicerrectorado de Euskera y Formación Continua

Fabrication and characterization of active polymer optical fibers with a ring-doped structure

This paper employs the solution-doping technique for the fabrication of polymer optical fibers (POFs) doped with two perylene derivatives, Lumogen Yellow 083 and Lumogen Red 305, in different combinations. With the solution-doping technique is very easy to control the amount of dopant penetration into the core of non-doped uncladded fibers, allowing the fabrication of active POFs with a novel ring-doped structure. In addition to manufacturing the fibers, these have also been optically characterized. Specifically, the influence of the combination of dopants, pumping power and wavelength, as well as the light propagation distance, have been measured and analyzed. Furthermore, time-resolved emission characteristics have also been measured to determine the fluorescence lifetimes and to extract information about the energy transfer between the dopants. Finally, the aim of this work has been to investigate the performance of the aforementioned POFs for fluorescent lighting applications, with a special focus on tunable light sources, and also for sensing applications.These results are funded in part by the Ministerio de Ciencia e Innovación -under projects RTC2019-007194-4, PID2021-122505OB-C31 and TED2021-129959B-C21,- and in part by Gobierno Vasco/Eusko Jaurlaritza under projects IT1452-22 and ELKARTEK (KK‑2021/00082, KK‑2021/00092)

Optical Fiber Sensors for Aircraft Structural Health Monitoring

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 tunnelThe authors would like to thank Professor A. Guemes for the permission to use the FBG responses shown in Figure 2. This work has been sponsored by the Ministerio de Economia y Competitividad (Spain) and FEDER funds under project TEC2012-37983-C03-01, the Gobierno Vasco/Eusko Jaurlaritza under projects IT664-13, ETORTEK14/13 and by the University of the Basque Country (UPV/EHU) through programs UFI11/16, US13/09 and EUSKAMPUS

Optical Tip Clearance Measurements as a Tool for Rotating Disk Characterization

An experimental investigation on the vibrational behavior of a rotating disk by means of three optical fiber sensors is presented. The disk, which is a scale model of the real disk of an aircraft engine, was assembled in a wind tunnel in order to simulate real operation conditions. The pressure difference between the upstream and downstream sides of the disk causes an airflow that might force the disk to vibrate. To characterize this vibration, a set of parameters was determined by measuring the tip clearance of the disk: the amplitude, the frequency and the number of nodal diameters in the disk. All this information allowed the design of an upgraded prototype of the disk, whose performance was also characterized by the same method. An optical system was employed for the measurements, in combination with a strain gauge mounted on the disk surface, which served to confirm the results obtained. The data of the strain gauge coincided closely with those provided by the optical fiber sensors, thus demonstrating the suitability of this innovative technique to evaluate the vibrational behavior of rotating disks.This work has been funded in part by the Fondo Europeo de Desarrollo Regional (FEDER); by the Ministerio de Economia y Competitividad under project TEC2015-638263-C03-1-R; by the Gobierno Vasco/ Eusko Jaurlaritza under projects IT933-16 and ELKARTEK (KK-2016/0030 and KK-2016/0059) and by the University of the Basque Country UPV/EHU under program UFI11/16

Influence of Humidity on Fiber Bragg Grating Sensors

We demonstrate the influence of the relative humidity (RH) on the wavelength of fiber Bragg grating sensors (FBGS), performing tests with five FBGS at different humidity and temperature conditions. These tests were performed in a climate chamber whose RH changes according to a scheduled profile from 30% to 90%, in steps of 10%. These profiles were repeated for a wide range of temperatures from to , in steps of . Two different types of instrumentation methods have been tested, spot welding and epoxy bonding, in two different materials, steel and carbon fiber reinforced polymer (CFRP). We discuss the results for each type of sensor and instrumentation method by analyzing the linearity of the Bragg wavelength with RH and temperature.Sponsored by the Ministerio de Economia y Competitividad under Project TEC2012-37983-C03-01 and Eusko Jaurlaritza/Gobierno Vasco under Projects IE11-302, SPE11CA001, S-PE12CA001, and IT664-13 and by the University of the Basque Country (UPV/EHU) through Program UFI11/16

Sensing Applications in Aircrafts Using Polymer Optical Fibres

We report on recent advances in the use of inexpensive polymer optical fibres (POFs) for sensing applications in avionics. The sensors analysed in this manuscript take advantage of the unique properties of polymers, such as high flexibility, elasticity, and sensitivity, and they range from strain, elongation, and vibration interrogators to level and temperature meters, leading to cost-effective techniques for structural health monitoring in aircraft structures. We also highlight recent power-supply methods using Power-over-POF in order to feed sensors remotely, and we discuss the constraints imposed by connectors on the performance of POF networks in aircrafts