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

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

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

U-Shaped and Surface Functionalized Polymer Optical Fiber Probe for Glucose Detection

In this work we show an optical fiber evanescent wave absorption probe for glucose detection in different physiological media. High selectivity is achieved by functionalizing the surface of an only-core poly(methyl methacrylate) (PMMA) polymer optical fiber with phenilboronic groups, and enhanced sensitivity by using a U-shaped geometry. Employing a supercontinuum light source and a high-resolution spectrometer, absorption measurements are performed in the broadband visible light spectrum. Experimental results suggest the feasibility of such a fiber probe as a low-cost and selective glucose detector.This work has been funded in part by the European Regional Development Fund; in part by the Ministerio de Economia y Competitividad under project TEC2015-638263-C03-1-R; and in part by the Gobierno Vasco/Eusko Jaurlaritza under projects IT933-16, IT718-13, and ELKARTEK (KK-2016/0030, KK-2017/00033, KK-2017/00089, KK-2016/0059). Authors are also grateful to the technical and human support provided by SGIKER (UPV/EHU, MICINN, GV/EJ, ERDF, and ESF). 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

Study of the Influence of Various Stress-Based Mechanisms on Polarization of an SM mPOF for the Development of Useful Devices

In contrast to conventional polymer optical fibres (POFs), single-mode microstructured POFs (SM mPOF) exhibit polarization properties that make them potentially interesting for their use in the design and development of polarimetric fibre optic systems. In spite of the theoretical sixfold symmetry of the microstructure that yields zero linear birefringence, a measurement technique reveals us that the SM mPOF behaves as a linear birefringent system with clearly defined optical axes and characteristics. Regarding externally induced birefringence mechanisms acting upon the SM mPOF, either bend- and pressure-induced retardations or twist-induced light rotation follow the behaviour predicted theoretically. More specifically, bend-induced retardation varies linearly with the inverse square of the bending radius of the fibre, and in the case of asymmetrical lateral stress, pressure-induced retardation varies with the applied force. As to twist-induced rotation, the electric field rotates linearly with the angle through which the fibre is twisted. All cases are highly reproducible phenomena that, if used advantageously, may yield useful mPOF-based polarimetric optical devices not exploited yet.This work was supported in part by the European Regional Development Fund, in part by the Ministerio de Economia y Competitividad under Project TEC2015-638263-C03-1-R, and in part by the Eusko Jaurlaritza under Project IT933-16, Project ELKARTEK16/45, and Project ELKARTEK16/86. The work of M. Azkune was supported by a research fellowship from the University of the Basque Country (UPV/EHU), Vicerrectorado de Euskara y Plurilinguismo