This letter demonstrates an approach for tuning fiber Bragg grating sensors with optical energy carried in the same optical fiber. Optical energy carried in the optical fiber was used to heat in-fiber Bragg gratings in order to alter the grating's optical response to surrounding media. The functional enhancement of optically heated Bragg gratings as sensor devices is demonstrated by a dual-function Bragg grating temperature and level sensing array for liquid at room and cryogenic temperatures. © 2005 American Institute of Physics

Buric, M

Chen, KP

Jewart, C

McMillen, B

Xu, W

Kevin P. Chen

Ben McMillen

Michael Buric

Chuck Jewart

Wei Xu

Crossref

English

Self-heated fiber Bragg grating sensors

D-Scholarship@Pitt

Self-heated fiber Bragg grating sensorsKevin P. Chen, Ben McMillen, Michael Buric, Chuck Jewart, and Wei Xu  Citation: Applied Physics Letters 86, 143502 (2005); doi: 10.1063/1.1895485 View online: http://dx.doi.org/10.1063/1.1895485 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/86/14?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in Pressure effects on the temperature sensitivity of fiber Bragg gratings AIP Conf. Proc. 1511, 1570 (2013); 10.1063/1.4789229  Development of optical fiber Bragg grating force-reflection sensor system of medical application for safeminimally invasive robotic surgery Rev. Sci. Instrum. 82, 074301 (2011); 10.1063/1.3606502  A multiplexed fiber Bragg grating sensor for simultaneous salinity and temperature measurement J. Appl. Phys. 103, 053107 (2008); 10.1063/1.2890156  Fiber Bragg grating microphone system Appl. Phys. Lett. 89, 161109 (2006); 10.1063/1.2363977  Highly sensitive fiber Bragg grating refractive index sensors Appl. Phys. Lett. 86, 151122 (2005); 10.1063/1.1904716    This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:  130.49.166.8On: Wed, 17 Sep 2014 16:36:24Self-heated fiber Bragg grating sensorsKevin P. Chen,a! Ben McMillen, Michael Buric, and Chuck JewartDepartment of Electrical and Computer Engineering, University of Pittsburgh,Pittsburgh, Pennsylvania 15261Wei XuStockerYale Inc., Salem, New Hampshire 03079sReceived 14 December 2004; accepted 10 February 2005; published online 28 March 2005dThis letter demonstrates an approach for tuning fiber Bragg grating sensors with optical energycarried in the same optical fiber. Optical energy carried in the optical fiber was used to heat in-fiberBragg gratings in order to alter the grating’s optical response to surrounding media. The functionalenhancement of optically heated Bragg gratings as sensor devices is demonstrated by adual-function Bragg grating temperature and level sensing array for liquid at room and cryogenictemperatures. © 2005 American Institute of Physics. fDOI: 10.1063/1.1895485gFiber Bragg gratings sFBGsd are key components foroptical sensing and communication.1 In-fiber sensing compo-nents offer several important advantages over other opticaland electronic devices, including immunity to electromag-netic fields, long lifetimes, high sensitivity, and the capabilityof working in harsh environments. Despite the advantages ofpurely optical fiber components, their performance and ap-plications have been limited by their total passivity. Passivesensor elements do not support active adjustment of sensingparameters to adapt to a changing environment. In order toenhance the functionality of fiber-optic components, inten-sive research has been carried out in the past few years di-rected toward the development of tunable FBGs. Tuning byvarious methods including on-fiber electrical heating,2–4electrowetting,5 piezo-electric actuation,6 and mechanicalactuation7 has been demonstrated. Despite advances in tun-ability, a common drawback for all active fiber componentsstill remains. In order to tune a fiber device, an energysource, an energy delivery mechanism, and on-fiber actuatorsare required. To date, electricity has been the only form ofenergy used to power in-fiber devices. An electrical cablemust run along with the optical fiber to supply this energy.Additional cabling, however, leads to many problems includ-ing an increase in manufacturing cost due to the additionalon-fiber electrical contacts, the need for delicate packaging,and the elimination of electromagnetic immunity. Due tothese issues, such fiber components would no longer be suit-able for use in hostile environments. Improvements in theperformance and functionality of an electrically driven activefiber component are achieved at the expense of many of theintrinsic advantages possessed by a purely optical compo-nent.In this letter we demonstrate a concept that active FBGsensors can be directly powered by in-fiber light as illus-trated in Fig. 1. In contrast to a passive sensor, optical poweris delivered together with a sensing signal through the samefiber that contains FBGs. Optical taps are fabricated in de-sired sections of the optical fiber to release high-power laserlight as an energy source. An optical conversion membranecoated on the outside of the optical fiber will serve as atransducer converting high-power laser energy to other en-ergy forms. The optical characteristics of FBGs can then beadjusted using this optical energy. By eliminating the electriccabling, on-fiber electric contacts, and delicate packaging,the functional enhancement of FBG sensors can be achievedwithout compromising any intrinsic advantages of in-fiberpassive sensors.To demonstrate this concept, we present a dual-functionactive FBG sensor array for liquid temperature and levelsensing at both room and cryogenic temperatures. Currently,the temperature and level of liquid hydrogen in cryogenicfuel tanks for space missions is detected with an array ofpoint sensors using silicon diodes. During operation, the sili-con diodes are heated by current pulses. The voltage dropacross a diode- sensor is used to gauge the temperature riseof the heated diode. When the diode is submerged in cryo-genic fuel, the temperature rise of the heated diode is limitedby the boiling point of the liquid hydrogen. The temperaturedifference between the submerged diodes and diodes outsidethe liquid is utilized to determine the liquid level. Since liq-uid fuel does not settle to the bottom of the tank in a micro-gravity environment, a large number of diode point sensorsare needed to precisely gauge the distribution and the totalmass of the liquid fuel inside fuel tanks. Each point sensorrequires two metal leads to measure the voltage and supplycurrent to the sensor. This incurs a large number of electricalfeedthrough lines for one cryogenic fuel tank. The largenumber of metal feedthrough lines increases heat leakageand the potential for mechanical and electrical failure in ahigh-g environment. The large number of electronic sensorsadAuthor to whom correspondence should be addressed; electronic mail:kchen@engr.pitt.eduFIG. 1. Schematic illustration of active FBG which is heated by in-fiberhigh-power laser light.APPLIED PHYSICS LETTERS 86, 143502 s2005d0003-6951/2005/86~14!/143502/3/$22.50 © 2005 American Institute of Physics86, 143502-1 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:  130.49.166.8On: Wed, 17 Sep 2014 16:36:24and metal leads are also easily susceptible to electromagneticinterference, because any ac current imposed upon them isrectified and appears as an erroneous temperature offset.This practical problem can be readily resolved by theactive fiber sensing technology proposed in this letter. Thedual-function level/temperature sensor demonstrated in thisletter consists of a FBG array powered by in-fiber light.When the power light is turned off, the FBG can be used asa temperature sensor to precisely gauge the temperature dis-tribution in the fuel tank. When the power light is turned on,optical energy carried by the fiber is used to heat fiber Bragggrating sensor arrays to distinguish between the liquid andgas phases. The spectral thermal response of grating sensorsis detected and transmitted back through the same opticalfiber. This work promises a true one-fiber solution for cryo-genic level/temperature sensing without a single electric wireand with only one fiber feedthrough.The self-heated FBG level sensor demonstrated hereininvolves two sections of fiber as shown in Fig. 2.8 A FBGwas first written in a single-mode standard fiber. The reflec-tion spectrum of the FBG was monitored with an opticalspectrum analyzer and a broadband source via a circulator.The single-mode fiber was fusion spliced with a 100-mm-diam core multimode fiber. Ten watts of 910 nm laser lightfrom a high-power diode laser array was coupled into thefree end of the multimode fiber using a pair of 203 micro-scope objectives. The 910 nm laser light propagating in themultimode fiber leaked into the cladding of the single-modefiber through the spliced junction. The light leakage profilewas measured using a cut-back technique. By measuring 910nm laser light coupling out of the fiber as a function ofdistance from the splice junction, it was found that the lightenergy leaking out from the fiber cladding falls off exponen-tially with distance. Ninety percent of the light was lostwithin 10 cm of the splice junction. A uniform silver filmwas plated on the FBG section of the fiber to absorb light.The coated section began approximately 5 mm from thesplice junction and had a resistance of 2.2 V/cm. Uniformgratings with lengths of 10 mm were used in the experi-ments.The liquid sensing experiments were carried out in bothwater and liquid nitrogen. Figure 3 shows a Bragg gratingspectral response in both water and air. When the 910 nmpower laser was turned off, the grating served as a tempera-ture sensor. To determine whether the grating is submersedinside the liquid slevel sensingd, the 910 nm laser was turnedon to inject 115 mW to heat the grating. When the grating issubmersed under water, the resonance peak ssolid traced ofthe heated grating is shifted about 60 pm from the unheatedpeak sdotted traced. When the grating was pulled above thewater level, the grating reflection peak rapidly shifted over1.4 nm from 1541 to 1542.4 nm. The reflection spectrum ofthe FBG is also changed, probably due to the nonuniformsilver coating on the fiber. However, the dramatic thermalresponse for the heated grating provided unambiguous detec-tion of the liquid level. The distinct thermal responses ofheated gratings have their origins in the disparate thermalproperties of air and water. The thermal responses of heatedgratings in air, water, and liquid nitrogen as a function ofinput laser power are further characterized by Fig. 4. As ex-pected, the heated grating exposed to air produced the largestresonance wavelength shift. The peak shift of the heatedgrating in ambient room temperature air follows a linearvariation with input laser power with a slope of 15 pm permW. Thus, a 10 mW laser input will produce a 150 pmreflection peak shift. This is in contrast to a 20 pm shift inwater and a 5 pm shift in liquid nitrogen. The grating wave-length shifts in water and liquid N2 were re-plotted using areduced vertical scale as shown in the inset of the same fig-ure. Compared with air, most liquids have much larger spe-cific heat capacities and thermal convection rates, therefore,it is much harder to raise the temperature of the grating inliquid than in an air/gas environment. Furthermore, when thegrating temperature rises up to the boiling point of the sur-rounding liquid s77 K for liquid N2d, the temperature of thegrating will be held at the boiling point until the surroundingliquid is totally evaporated. This is clearly illustrated in theFIG. 2. Sketch of a grating level sensor array, light from a high-power laserdiode was coupled into a multimode sMMd fiber that is fusion spliced to theSM fiber. High-power 910 nm laser light in the multimode fiber leaks intothe cladding of the single-mode fiber through the spliced junction and heatsthe silver coating of FBGs. The actual fusion spliced SM to MM junction isshown in the figure.FIG. 3. Bragg grating response in and out of water when it was heated by115 mW 910 nm diode laser power.FIG. 4. Resonance wavelength shifts of heated gratings in air, water, andliquid N2 as a function of input laser power. The grating wavelength shifts inwater and liquid N2 re-plotted using a reduced vertical scale is shown in theinset of the same figure.143502-2 Chen et al. Appl. Phys. Lett. 86, 143502 ~2005! This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:  130.49.166.8On: Wed, 17 Sep 2014 16:36:24inset of Fig. 4. The resonance wavelength of FBG increasedonly by 20 pm and subsequently remained constant overvarying injection power up to 680 mW. In contrast, the re-flection peak of the FBG submerged in water increasedmonotonically to 170 pm with the same injected laser power.Based on this experimental setup, a four-point sensorarray was also demonstrated to test the liquid/gas discrimi-nation by raising the sensor array from the liquid. To dem-onstrate a sensor array, the 100/400 mm multimode fiberwas replaced by a 62.5 mm core/125 µm cladding gradedindex multimode fiber to increase the power laser lightpropagation distance from the splice junction. This fiber pro-vided better mode matching than the previous multimodefiber. The acrylic coating of the single-mode fiber was care-fully stripped to ensure a clean silica fiber surface.An array of four 5 mm gratings was written in a single-mode fiber sCorning SMF-28d. The grating sensors were 3cm apart with resonance wavelengths at 1535.6 nm ssensor1d, 1537 nm ssensor 2d, 1538.8 nm ssensor 3d, and 1540.6 nmssensor 4d, respectively. Sensor 4 was approximately 2 cmfrom the splice junction. The reflection spectrum of the un-heated grating array is shown as the dot trace in Fig. 5.Sensor 1 swith the shortest resonance wavelengthd was thetopmost grating, and Sensor 4 swith the longest resonancewavelengthd was the lowest grating. Figure 5sad shows thereflection spectrum of the optically heated grating arraywhen Sensor 1 stopmostd is pulled out from the water with600 mW input laser power. The resonance peak for Sensor 1shifted 350 pm from 1535.7 to 1536.05 nm. Due to the muchlarger specific heat and thermal convection rates of waterthan those of air, reflection peaks for heated gratings remain-ing in the water shifted less than 10 pm. In practice, a smallerwavelength shift would be enough to ensure unambiguousdetection of the liquid level. Similar behavior was observedwhen Sensors 2, 3 and 4 were pulled out from water in serial.Figures 5scd and 5sdd also show nonuniform peak shifts forthe different gratings while the input laser power is reducedto 550 mW and then to 400 mW. This is due to the nonuni-form leakage profile of the laser light in a single-mode fiber.The power was reduced to avoid the spectral deformation ofSensor 4 due to overheating.In conclusion, this work illustrates an innovative solu-tion to power in-fiber components and to enhance the func-tionality of passive fiber sensor networks. We use fibers notonly for optical signal delivery, but also as multi-functioncables that can deliver optical power for on-fiber self-heating. The concept of active fiber sensors directly drivenby in-fiber light provides a dimension to design agile andmultifunctional fiber sensors without sacrificing any intrinsicadvantages of passive fiber sensors. In this letter, an opticallyheated fiber Bragg grating sensor was used to unambigu-ously determine the liquid level at room temperature as wellas at cryogenic temperatures. It promises an ideal solutionfor cryogenic fuel gauging in a microgravity environment.The optical energy utilized herein can also be converted intoenergy of other forms for on-fiber actuation. We believe thatthe ability of tuning in-fiber devices directly by in-fiber op-tical energy will enable a wide array of agile fiber devicesbased on FBGs, fiber interferometers, and microstructure fi-bers. On the other hand, the optical power supply configura-tion presented in this letter still requires two opticalfeedthrough lines to inject sensing light and power light fromthe opposite side of fiber. This drawback can be overcome byusing a double-clad fiber for both optical power and sensingsignal delivery. Using double-clad specialty fibers, the opti-cal power source can be launched together with the 1.55 mmprobing light source from the same side to power a largegrating array. Optical taps created by ion implantation canrelease the optical power only at sections of the fiber whereFBGs are located. The entire fiber network will only use onefeedthrough to connect the detection/power unit to the fibernetwork. The demonstration of light-powered tunable gratingarrays using double-clad fiber will be presented elsewhere.1K. H. Hill, Y. Fujii, D. C. Johnson, and B. S. Kawasaki, Appl. Phys. Lett.32, 647 s1978d.2J. A. Rogers, B. J. Eggleton, J. Pedrazzani, and T. Strasser, Appl. Phys.Lett. 74, 3131 s1999d.3H. G. Limberger, N. H. Ky, D. M. Costantini, R. P. Salathe, C. A. P.Muller, and G. R. Fox, IEEE Photonics Technol. Lett. 10, 361 s1998d.4P. Mach, M. Dolinski, K. W. Baldwin, J. A. Rogers, C. Kerbage, R. S.Windeler, and B. J. Eggleton, Appl. Phys. Lett. 80, 4294 s2002d.5P. Mach, T. Krupenkin, S. Yang, and J. A. Rogers, Appl. Phys. Lett. 81,202 s2002d.6G. R. Fox, C. A. P. Muller, N. Setter, D. M. Costantini, N. H. Ky, and H.G. Limberger, J. Vac. Sci. Technol. A 15, 1791 s1994d.7C. Goh, M. Mokhtar, S. Bulter, S. Set, K. Kikuchi, and M. Ibsen, IEEEPhotonics Technol. Lett. 15, 557 s2003d.8K. P. Chen, L. J. Cashdollars, and W. Xu, IEEE Photonics Technol. Lett.16, 1897 s2003d.FIG. 5. Spectra responses of a four-grating sensor array when the fiber waspulled out in serial from water. Sensor 1 was the topmost grating and Sensor4 was the lowest sensor. The dot traces are the spectral responses for theunheated sensor.143502-3 Chen et al. Appl. Phys. Lett. 86, 143502 ~2005! This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:  130.49.166.8On: Wed, 17 Sep 2014 16:36:24

This letter demonstrates an approach for tuning fiber Bragg grating sensors with optical energy carried in the same optical fiber. Optical energy carried in the optical fiber was used to heat in-fiber Bragg gratings in order to alter the grating’s optical response to surrounding media. The functional enhancement of optically heated Bragg gratings as sensor devices is demonstrated by a dual-function Bragg grating temperature and level sensing array for liquid at room and cryogenic temperatures

Self-heated fiber Bragg grating sensors

Abstract

Similar works

Full text

Available Versions

Crossref

D-Scholarship@Pitt

Name not available

Name not available