This proceeding at : 23 rd International Conference en Optical Fiber Sensors (OFS23). Took place in 2014, June, 02-06, in Santander (Spain).The design and development of a plastic optical fiber (POF) macrobend temperature sensor is presented. The sensor has a linear response versus temperature at a fixed bend radius, with a sensitivity of 8.2 ∙ 10−4(ºC)−1  〖8.2.10〗^(-4) 〖(°C) 〗^(-1)  and a 8% non-linearity full scale error. The sensor system uses the power variation between two discrete wavelengths for auto reference purposes. An analysis for selecting operation wavelengths has been carried out in order to optimize the response of the sensor. The proposed sensor can be used in harsh environment and has a low-costThis work has been sponsored by the Spanish Ministry of Economía y Competitividad under grant TEC2012-37983-C03-02.Publicad

Pinzón Castillo, Plinio Jesús

Pérez Garcilópez, Antonia Isabel

Tapetado Moraleda, Alberto

Vázquez García, María Carmen

Zubía, Joseba

e-Archivo (Univ. Carlos III de Madrid e-Archivo)

This is a postprint version of the following published document: A. Tapetado, P. J.  Pinzón, J. Zubia, I. Pérez, C. Vázquez."Self-referenced temperature sensor based on a polymer optical fiber macro-bend". OFS-23 Proc. SPIE 9157, 23rd International Conference on Optical Fibre Sensors, 915716(June 2, 2014). Availble in http://www.dx.doi.org/10.1117/12.2059297© 2014 Society of Photo Optical Instrumentation Engineers. One print or electronic copy may be made for personal use only. Systematic electronic or print reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.     Self-referenced Temperature Sensor Based on a Polymer Optical Fiber Macro-Bend  A. Tapetado*a, P. J. Pinzóna, J. Zubiab, I. Péreza, C. Vázqueza a Dpto. Tecnología Electrónica, Universidad Carlos III de Madrid, Avda. De la Universidad 30, CP 28911 Leganés (Madrid), Spain; b Dpto. de Electrónica y Telecomunicaciones, Universidad del País Vasco, Euskal-Herriko Unibertsitatea, Alameda de Urquijo S/N, Bilbao 48013, Spain. ABSTRACT The design and development of a plastic optical fiber (POF) macrobend temperature sensor is presented. The sensor has a linear response versus temperature at a fixed bend radius, with a sensitivity of 8.2 ∙ 10−4(ºC)−1 and a 8% non-linearity full scale error. The sensor system uses the power variation between two discrete wavelengths for auto reference purposes. An analysis for selecting operation wavelengths has been carried out in order to optimize the response of the sensor. The proposed sensor can be used in harsh environment and has a low-cost. Keywords: POF Sensor, temperature, intensity, bend loss, loop, self-referenced. 1. INTRODUCTION  The use of temperature sensors in today’s society is continually growing. There are a large number of applications in which it is necessary to measure temperature, for instance: automotive industry, air-conditioning control, medical applications, among others. Some decades ago, world sensor market was made up of traditional temperature sensors such as thermocouples, thermistors and resistance temperature detectors. These sensors are not well suited to be used in the presence of electromagnetic disturbances or inflammable atmospheres. These problems together with the advantage in fabrication of optical fiber and in low-cost components for optical fiber communications promoted the development of innovative temperature sensors based on optical fibers.  Many optical fiber sensors have been developed based on amplitude or phase techniques [1]. Different interferometric configurations have been proposed as temperature sensors such as Mach-Zehnder [2] and Fabry-Perot [3]. These sensors are characterized by their large sensitivities. But their complexity of implementation and the higher price of the equipment needed, make them unsuitable for the applications described above. Temperature amplitude modulation sensors are based on optical power variations. Some proposed sensors are based on frustration of total internal reflection [4], light generation [5] and Fiber Bragg Grating wavelengths shifts [6]. The amplitude temperature sensors usually are cheaper than the phase modulation sensors but they need to use a reference technique to avoid false readings caused by fluctuations of the light source or other undesired losses [7]. In this paper, the authors propose a low-cost intensity macro-bend temperature sensor based on polymer optical fiber (POF). Other techniques to measure the temperature with a macro-bend sensor have been developed using silica optical fiber [8]. The main advantage of using POF fiber instead glass fiber is the large diameter of the fiber, making them less fragile and easier to handle, reducing development and maintenance cost. Although polymer based sensors have a smaller temperature range. The novelty of this work is to develop and implement a self-reference technique based on a single macro-bend loop. The sensor system used a relation between two different wavelengths to avoid possible errors related to optical power fluctuations of the light. Previous POF macrobend temperature sensor as the one reported in [4] used a dummy fiber-optic sensor, making them less reproducible. 2. PRINCIPLE OF OPERATION A curved step index optical fiber schematic is shown in Figure 1. *atapetad@ing.uc3m.es; phone +34 91624 6040 23rd International Conference on Optical Fibre Sensors, edited by José Miguel López-Higuera, Julian Jones, Manuel López-Amo, José Luis Santos, Proc. of SPIE Vol. 9157, 915716 © 2014 SPIE · CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2059297Proc. of SPIE Vol. 9157  915716-1    α YXO, X´||XO,XRρcnCorenCladding2ρCore Rayρϕ cYZ RaynCore Figure 1. Schematic of the frontal and top view of a bend section. The bending radius is R and the core radius is ρCore. The refractive indices of the core and cladding are nCore and nCladding, respectively. Optical power is launched at the beginning of the rectilinear section of the fiber. The guidance of the rays in the core is achieved by ensuring that the propagation angle, α, satisfies the condition: 0 ≤ α ≤ αc, where the critical angle (αc) is given by αc = sin−1(ncladding ncore⁄ ). The expression of the numerical aperture (NA) for the rectilinear section is given by:   1 22 2sinCore Core CladdingNA n n n      (1) However, in the bend section, the guidance of the rays in the core can follow two ways. Only the rays entering the bent part of the fiber in the meridional plane remain with the same angle of incidence along a given ray path. On the other hand, the skew rays entering this plane, after the successive reflections within the core, do not follow a simple repeatable pattern because of the asymmetry introduced by bending the fiber. So the local numerical aperture changes at a given location of the bent optical fiber; this dependence is given by [4]:   12 222, , 1cosCladding CoreCoreCoren RNA R nn R             (2) where ϕ is the ray angle at the beginning of the bend, which varies from 0º to 180º and ρ is the radial position in the core satisfying the relation 0 ≤ 𝜌 ≤ 𝜌𝐶𝑜𝑟𝑒. In this intensity sensor, the losses induced by the bending effect depend on changes in the numerical aperture. Numerical aperture changes on the fiber are related to variations of the refractive index of the cladding and core with temperature. The numerical aperture in the bent section of the fiber versus temperature can be expresses as:       12 222, , , 1cosCladding CoreCoreCoren T RNA T R n Tn T R             (3) 3. EXPERIMENTAL RESULTS AND CALIBRATION CURVES In this section, the sensor manufacturing process, the experimental set-up and the measurement of the sensor calibration curve are reported. A commercial step index POF fiber with a good tensile strength and flexing is used. These characteristics provide good mechanical properties at the time of manufacturing the sensor. From the middle section of the fiber length, the buffer coating is stripped. The length of this stripped section is about 30mm. The fiber sensor is formed by creating a single 180º loop (½ turn) with a bend radius of 2mm. The buffer coating in the junction of the two branches Proc. of SPIE Vol. 9157  915716-2    is fixed by cyanoacrylate adhesive. With this simple method, a stable macro-bend fiber temperature sensor can be manufactured. The schematic of the experimental set-up is shown in Figure 2. The light source is a 360 to 2500nm tungsten halogen bulb (Avantes® AvaLigth-Hal). The fiber-optic sensor is fixed close to the rectangular highly conductive metal base plate of a heating unit (Linkam® LTS350). The temperature of the hot plate is controlled with a controller unit (Linkam® TP94). Temperature measurements are taken at a 1mm distance from the hot plate surface. Setting a distance between the sensor and the hot plate prevents possible changes in the refractive index of the cladding due to the contact with other materials as it is discussed in a previous work [4]. For measuring the real temperature of the optical fiber sensor at 1mm distance, an independent electronic temperature sensor (Texas Instruments® LM35) is used in the measurements. Light Source   Avantes® AvaLight-HalFiber-optic SensorHot plateIndependent Temperature SensorLensCOL-UV/VISSpectrometer Avantes® AvaSpect-128Multimode Silica FiberϕCore=100µmSMA ConnectorsMultimode POF FiberϕCore=980µmϕCladding=1mmST ConnectorsLensCOL-UV/VISMultimode Silica FiberϕCore=100µmSMA ConnectorsMultimode POF FiberϕCore=980µmϕCladding=1mmST Connectors Figure 2. Schematic of the experimental set-up for characterizing the POF temperature sensor. In order to avoid false readings caused by fluctuations of the light source or other undesired losses, a relation between transmittances at two different wavelengths (𝜆1, 𝜆2) is measured to implement a self-referenced technique. The transmittances at two wavelengths (𝜏𝜆1 , 𝜏𝜆2) can be expressed as:     11 1 11 0RP TF TP T       (4)     22 2 22 0RP TF TP T       (5) where Pλ1and Pλ2  are the output optical powers for two wavelengths at a given temperature (T). PR λ1and PR λ2  are the output power for both wavelengths at a reference temperature (T0 = 23ºC), which are considered as the 100% transmission. βλ1  and βλ2 are factors including the attenuation of the fiber leads, collimation losses and connector losses at each wavelength. Finally, 𝐹𝜆1 and 𝐹𝜆2 are the optical power modulation function versus temperature at each wavelength.  From Equation (4) and (5), the self-referenced output transmittance ratio (γSR) is defined as:     1 1 1 12 2 2 2SRF TF TF T              (6) Therefore the ratio of the output transmittances depends on the power sensor variations with temperature and on a constant ratio between the insertion losses at the different wavelengths. In order to find the output transmittance ratio that offer the highest linearity and sensitivity, the transmittance at different wavelengths has been measured from 360 to 886nm with 4nm resolution using a spectrometer (Avant® AvaSpect-128), in a temperature range from 29 to 70ºC at 2ºC intervals. A searching algorithm based on Mathworks® Matlab® code has been developed to evaluate the ratio of transmittances for all possible pairs of wavelengths. As a result it is obtained that λ1=529nm and λ2=835nm are the best option. Figure 3 shows the measured calibration curves at the two selected wavelengths. At a 1mm distance, the temperature of the independent electronic temperature sensor varies from 27.2 to 50.2ºC. Proc. of SPIE Vol. 9157  915716-3     Figure 3. Self-referenced transmittance ratio versus temperature from measured calibration curves at 529 and 835nm. The measurements show a good linear regression coefficient and sensitivity for the proposed wavelengths.  The linear regression coefficient and sensitivity are 98% and 8.21 ∙ 10−4 (ºC)−1. A statistical analysis of the calibration curve shows that the sensor has a linear response with a non-linearity full scale error of 8.7%. The designed system allows resolutions of 2ºC. CONCLUSION In conclusion, macrobending loss, caused by the different thermo-optic coefficients of the cladding and core of a step-index Polymer Optical Fibers (POFs) has shown to be an usable technique for temperature sensing. The sensor is able to measure a temperature range from +27.2 to +50.2ºC. The sensor sensitivity for a bend radius of 2mm is 8.21 ∙ 10−4 (ºC)−1. A self-referenced technique using a single macro-bend loop in order to reduce the repeatability error associated with the manufacturing process is developed. Self-referenced sensor system shows a resolution below 2ºC and 8.7% non-linearity full scale error. The sensors can be used in a large number of applications, for instance: instrumentation process, automotive industry, air-conditioning control, among others. ACKNOWLEDGMENTS This work has been sponsored by the Spanish Ministry of Economía y Competitividad under grant TEC2012-37983-C03-02. REFERENCES [1] V. Busurin, A. S. Semenov, and N. Udalov, "Optical and fiber-optic sensors (review)," Soviet Journal of Quantum Electronics, vol. 15, p. 595, 2007. [2] Y. Wang, Y. Li, C. Liao, D. Wang, M. Yang, and P. Lu, "High-temperature sensing using miniaturized fiber in-line Mach–Zehnder interferometer," Photonics Technology Letters, IEEE, vol. 22, pp. 39-41, 2010. [3] H. Y. Choi, K. S. Park, S. J. Park, U.-C. Paek, B. H. Lee, and E. S. Choi, "Miniature fiber-optic high temperature sensor based on a hybrid structured Fabry–Perot interferometer," Optics letters, vol. 33, pp. 2455-2457, 2008. [4] A. Moraleda, C. García, J. Zaballa, and J. Arrue, "A Temperature Sensor Based on a Polymer Optical Fiber Macro-Bend," Sensors, vol. 13, pp. 13076-13089, 2013. [5] D. Hernandez, G. Olalde, A. Beck, and E. Milcent, "Bicolor pyroreflectometer using an optical fiber probe," Review of Scientific Instruments, vol. 66, pp. 5548-5551, 1995. [6] Y.-j. Rao, D. J. Webb, D. A. Jackson, L. Zhang, and I. Bennion, "In-fiber Bragg-grating temperature sensor system for medical applications," Lightwave Technology, Journal of, vol. 15, pp. 779-785, 1997. [7] D. S. Montero, C. Vázquez, I. Möllers, J. Arrúe, and D. Jäger, "A self-referencing intensity based polymer optical fiber sensor for liquid detection," Sensors, vol. 9, pp. 6446-6455, 2009. [8] G. Rajan, Y. Semenova, and G. Farrell, "All-fibre temperature sensor based on macro-bend singlemode fibre loop," Electronics Letters, vol. 44, pp. 1123-1124, 2008. γSR = 8.21∙10-04 + 9.77∙10-01R² = 98.1%98,799,4100,1100,8101,5102,2102,9103,6104,30,9960,9991,0021,0051,0081,0111,0141,0171,02027 29 31 33 35 37 39 41 43 45 47 49 51Transmittance, τ[%]Self-referencedtransmittance ratio, γSRTemperature [ºC]Self-referenced transmitance ratio Calibration curve at 529nmCalibration curve at 835nm Linear fit (Self-referenced ratio)Proc. of SPIE Vol. 9157  915716-4

Self-referenced temperature sensor based on a polymer optical fiber macro-bend

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