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Reflective side-polished optical fiber submersion sensor using an optical fiber mirror for remote sensing

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Reflective side-polished optical fiber submersion sensor using an optical fiber mirror for remote sensing
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  IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 8, APRIL 15, 2007 583 Reflective Side-Polished Optical Fiber SubmersionSensor Using an Optical Fiber Mirror forRemote Sensing Cherl-Hee Lee, Jang-Hee Kim, Jae-Hee Park, and Jae-Won Song  Abstract— A reflection-type side-polished optical fiber submer-sion sensor (RSS) with an optical fiber mirror for remote sensing isproposed. When compared with a conventional transmission-typeside-polished optical fiber submersion sensor (TSS), the RSS withan optical fiber mirror provides an improved response, and allowsefficient remote submersion sensing. In experiments, the RSS withan optical fiber mirror detected submersion with 1 dB increasedthroughput power gain when compared with a conventional TSS,and provided efficient remote sensing at a distance of 1 km basedon the resonance wavelength shift and transmission power gain.  Index Terms— Optical fiber mirror, reflective, remote sensing,side-polished, submersion sensor. I. I NTRODUCTION A side-polished optical fiber coupler exhibits band-rejectionfiltering characteristics at a coupling wavelength, and thecoupling wavelength is sensitively shifted by the refractiveindex of the external material on the upper plane waveguide(PWG). This property has already been applied to the sensingof various physical parameters, such as the refractive index of liquids, temperature, humidity, and water, etc. In the case of optical fiber networks, the submersion of splicing points underwater has a detrimental effect on the service quality of theoptical fiber networks and lifetime of the cable. And multiplecables of optical fiber, telephone line, cable TV, and electricalpower line are together dispersed through an undergroundpathway of conduits linked by utility-pipe conduit. In this case,moisture entering the cable or splice cases quickly leads topermanent physical damage and potential multiple service out-rages. Therefore, these submerged locations need to be detectedand restored to normal conditions as quickly as possible. Thus,to detect the occurrence of submersion and enable a speedyrecovery of the optical fiber networks to normal conditions,various submersion sensors have been developed using opticalfiber bending [1], [2] and side-polished optical fibers [3], [4], where side-polished optical fiber submersion sensors havedemonstrated the advantages of an immediate response, highrecurrence, low insertion loss, mechanical stability, and simplefabrication process. However, since conventional side-polishedoptical fiber submersion sensors are transmission-type devices,their use is restricted as regards measuring long distances. Manuscript received October 2, 2006; revised November 27, 2006.C.-H. Lee, J.-H. Kim, and J.-W. Song are with the School of Electronic andElectrical Engineering and Computer Science, Kyungpook National Unversity,Daegu 702-701, Korea (e-mail: chlee@ee.knu.ac.kr).J.-H. Park is with the School of Electronic Engineering, Keimyung Univer-sity, Daegu 704-701, Korea.Digital Object Identifier 10.1109/LPT.2007.894280Fig. 1. Structure of RSS. Furthermore, since the optical source and detector are requiredto be at opposite ends of the sensor system in a transmis-sion-type device, the efficiency and monitoring costs are majorbarriers to the realization of a practical fiber sensor system.Thus, a submersion measurement system using a reflectiveside-polished optical fiber submersion sensor (RSS) with anoptical fiber mirror serving as a reflector is proposed as moresuitable for remote sensing. The mirror as a reflector is formedat the end surface of single-mode optical fiber (SMF) by thefusion splicing technique [5], [6]. By connecting the optical fiber mirror to one end of the RSS, this allows both the op-tical source and the detector to be located at the other end,thereby, improving the monitoring efficiency and reducing themaintenance costs. The proposed RSS with an optical fibermirror also has an increased throughput power gain comparedto a conventional transmission-type side-polished optical fibersubmersion sensor (TSS), as the transmitted light and lightreflected from the mirror means that coupling occurs twicewithin the same resonance wavelength. The proposed RSSwith an optical fiber mirror can monitor submersion based onboth the throughput power gain and the resonance wavelengthshift. Experimental results confirm a significant remote sensingof 1 km was achieved when using an optical fiber mirror as areflector.II. P RINCIPLE OF  O PERATION The RSS consists of a side-polished SMF and a multimodePWG as shown in Fig. 1. The operation principle is based onthe evanescent field coupling between a side-polished SMF andan PWG. Strong power transfer from the fiber to the PWG oc-curs only if the effective index of two waveguide is matched. The th coupling wavelength is approx-imately given by the following equation from the transverse 1041-1135/$25.00 © 2007 IEEE  584 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 8, APRIL 15, 2007 Fig. 2. Dispersion curves of PWG. resonance conditions of an PWG [3], [7]:If the refractive index of the external material of the PWG ischanged, the phase-matching condition is also changed. Fig. 2shows the dispersion curves calculated from the transverse res-onance condition of the PWG with thickness of 170 m. Thecross points of the two lines, which represent the same effectiveindex between the PWG and the SMF ), repre-sent the coupling wavelength. We can expect the phase shift of 1.6 nm for distilledwater (the refractiveindex, 1.33)and 3.8 nmfor petroleum (the refractive index, 1.435).III. E XPERIMENT The RSS used in the test was composed of a side-polished fi ber and PWG. A standard SMF with a clad diameter of 125 m and core diameter of 9 m was used as the side-pol-ished  fi ber and a microscope cover glass was used as the PWG,shown in Fig. 1. The microscope cover glass with a thickness( ) of 170 m was separated by spacers with a thickness ( )of 65 m. The  fi ber was polished to allow evanescent  fi eldcoupling of the  fi ber to the multimode PWG. The thickness of the residual cladding was approximately 1 m based on theliquid drop method [8]. The refractive index of the  fi ber core,clad, and PWG was 1.450, 1.444, and 1.523, respectively. Therefractive index of the NOA 65(adhesive, Norland) used to fi x the PWG to the side-polished  fi ber block was 1.52. TheRSS has polarization dependence, and strongly depends onthe dispersion of the PWG. So the effect of the phase shiftcannot be neglected. The coupling wavelength of TMpolarization is shifted to shorter wavelength than that of TEpolarization. The phase shift between TE and TM of 1.0 nm forair (the refractive index, 1.0), 0.8 nm for water (the refractiveindex, 1.33), and 0.08 nm for petroleum (the refractive index, Fig. 3. Experimental setup.Fig. 4. Optical  fi ber mirror. 1.435). The polarization-dependence loss of the RSS was0.7 dB for air, 0.58 dB for distilled water, 0.34 dB for petro-leum, as measured by a PDL_Multimeter (PS3650,Uniphase).The experimental setup is shown in Fig. 3. A wideband erbiumampli fi ed spontaneous emission source was used as the lightsource and an optical spectrum analyzer was used to detectthe resonance wavelength shift and throughput power gain of the RSS. An optical circulator with an insertion loss of 0.5 dBand isolation greater than 40 dB for all circulating ports wasplaced to feed back the source light and used as an output portto measure the optical power and spectra.An optical  fi ber mirror is generally fabricated by evaporationof metal or multilayer dielectric materials to obtain high re fl ec-tivity. As aluminum easily evaporates and sticks well to everymaterial including plastic, it is widely used as a metal  fi lm. Theoptical  fi ber mirror was made on a piece of cleaved  fi ber withtheendsurfacecoatingwherethesurfacecoatingwasfabricatedusing aluminum deposited on the end of an SMF at a thicknessof 2 m by thermal evaporation with a re fl ectance of 72%, asshown in Fig. 4. When an aluminum sticks to the end of optical fi ber, its re fl ectivity of the optical  fi ber mirror is [9]Here, is the effective index of an optical  fi ber and is therealpartoftherefractiveindexofaluminum; istheimaginarypartoftherefractiveindexofaluminum.Theoptical fi bermirrorwas spliced to the one end of the RSS that was then placed in avessel  fi lled with distilled water.As shown in Fig. 5, when the RSS was submerged in dis-tilled water, it produced a signi fi cantly different spectral re-sponse when compared with the TSS. The solid line in Fig. 5representsthetransmissionoftheRSS inairbeforesubmersion,where the minimal transmission dip due to coupling occurred at1540 nm and the minimal transmission was 15.06 dB. Mean-while, the dotted line in Fig. 5 represents the transmission of the RSS after submersion, where the minimal transmission was15.07dBat1541.6nm.Assuch,thecouplingwavelengthshiftwas approximately 1.6 nm, which was almost the same as theTSS; however, the throughput power increased about 2.04 dBcompared to the 1540 nm in air and was increased 1 dB morethan the TSS at the 1540 nm. Since the resonant wavelengthshift is only dependent on the refractive index of the distilledwater, regardless of the sensor type, the fact that the RSS had  LEE  et al. : RSS USING AN OPTICAL FIBER MIRROR FOR REMOTE SENSING 585 Fig. 5. Wavelength response.Fig. 6. In the sample materials (a) wavelength response; (b) throughput powergain. the same coupling wavelength response as the TSS sensor elim-inates any doubt as to false behavior by the optical  fi ber mirror.Plus, an increased throughput power gain was obtained due tothe twofold coupling of the transmitted light and light re fl ectedby the mirror within the same coupling wavelength, making itmore useful for monitoring submersion than a TSS.When the RSS was dipped in the different sample materials(distilled water, river water, sea water, foul water, muddy water,petroleum, and edible oil), the resonance wavelength andthroughput power gain were changed by the overlay samplematerials, as shown in Fig. 6. The coupling wavelength isalmost the same in the water samples, as shown in Fig. 6(a), but TABLE IP ARAMETERS OF  S AMPLE  M ATERIALS very different in the oils. As the refractive index of petroleumis different from that of water, the coupling wavelength of thepetroleum is shifted about 4 nm, which was double that of the water samples. Thus, petroleum and water can easily bediscriminated. Fig. 6(b) shows the throughput power gain from1540 nm with the sample materials. The throughput power gainis almostthe same in the water samples, but the power gain withthe oils is double that with the water samples. The RSS clearlyexhibited a different response to the water samples comparedto the oil samples, con fi rming its suitability for application insubmergence monitoring system. For measurement of remotesensing, 1-km-length  fi ber bundle was inserted between theRSS and circulator, and remote sensing of various lengths of  fi bers can be easily measured using the RSS system. Table Ishows the parameters of sample materials used in the test.IV. C ONCLUSION This letter presented a new type of RSS using an optical  fi bermirror. The resulting throughput power gain is increased by al-most1dBcomparedtoaconventionalTSS,plusremotesensingcan be accomplished at a distance of 1 km. When the RSS issubmerged under sample materials (distilled water, river water,sea water, foul water, muddy water, petroleum, and edible oil),it can discriminate water and oil. Thus, remote sensing using anRSS sensor with an optical  fi ber mirror was successfully real-ized.R EFERENCES[1] S. Tomita, H. Tachino, and N. Kasahara,  “ Water sensor with optical fi ber, ”  J. Lightw. Technol. , vol. 8, no. 12, pp. 1832 – 1892, Dec. 1990.[2] W. C. Michie, B. Culshaw, I. McKenzie, M. Konstantakis, N. B.Graham, C. Moran, F. Santos, E. Bergqvist, and B. Carlstrom,  “ Dis-tributed sensor for water and pH measurements using  fi ber optics andswellable polymeric systems, ”  Opt. Lett. , vol. 20, pp. 103 – 105, 1995.[3] K.-R. Sohn, K.-T. Kim, and J.-W. Song,  “ Optic  fi ber sensor forwater detection using a side-polished  fi ber coupler with a planarglass-overlay-waveguide, ”  Sens. Actuators A , vol. 101, pp. 137 – 142,2002.[4] C.-H. Lee, D.-H. Lee, K.-H. Kwon, and J.-W. Song,  “ Submersionsensor with side-polished  fi ber and planar waveguide in a manhole, ”  Jpn. J. Appl. Phys. , vol. 45, no. 10A, pp. 7771 – 7772, 2006.[5] J. T. Krause, W. A. Reed, and K. L. Walker,  “ Splice loss of single-mode  fi ber as related to fusion time, temperature, and index pro fi lealteration, ”  J. Lightw. Technol. , vol. LT-4, no. 7, p. 837, Jul. 1986.[6] J. Park and H. Taylor,  “ Fabrication of low re fl ectance optical  fi bermirror, ”  Electron. Lett. , vol. 32, pp. 2342 – 2343, 1996.[7] D. G. Moodie and W. Johnstone,  “ Wavelength tunability of compo-nents based on the evanescent coupling from a side-polished  fi ber toa high-index-overlay waveguide, ”  Opt. Lett. , vol. 18, pp. 1025 – 1027,1993.[8] C. A. Millar, M. C. Brierley, and S. R. Mallinson,  “ Exposed-coresingle-mode- fi ber channel-dropping  fi lter using a high-index overlaywaveguide, ”  Opt. Lett. , vol. 12, no. 4, pp. 284 – 286, 1987.[9] M. Ohring  , The Material Science of Thin Films . New York: Aca-demic, 1991.
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