Author name used in this publication: Peng, Gang-DingAuthor name used in this publication: Wang, Yi-Ping2006-2007 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Jin, W

Peng, GD

Rao, YJ

Wang, DN

Wang, Y

Applied Physics Letters

Author name used in this publication: Peng, Gang-DingAuthor name used in this publication: Wang, Yi-Ping2006-2007 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishedVoR allowe

PolyU Institutional Repository

Asymmetric long period fiber gratings fabricated by use of CO2 laserto carve periodic grooves on the optical fiberYi-Ping WangaDepartment of Electrical Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong,People’s Republic of China and State Key Laboratory of Advanced Optical Communication Systemsand Networks, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of ChinaD. N. Wangb and Wei JincDepartment of Electrical Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong,People’s Republic of ChinaYun-Jiang RaoSchool of Communication and Information Engineering, University of Electronic Scienceand Technology of China, Chengdu 610054, People’s Republic of ChinaGang-Ding PengSchool of Electrical Engineering and Telecommunications, University of New South Wales, Sydney 2052,AustraliaReceived 10 June 2006; accepted 20 August 2006; published online 9 October 2006An asymmetric long period fiber grating LPFG with a large attenuation of −47.39 dB and a lowinsertion loss of 0.34 dB is fabricated by use of focused CO2 laser beam to carve periodic grooveson one side of the optical fiber. Such periodic grooves and the stretch-induced periodic microbendscan effectively enhance the refractive index modulation and increase the average strain sensitivityof the resonant wavelength of the LPFG to −102.89 nm/m. The resonant wavelength and the peakattenuation of the LPFG can be tuned by 12 nm and 20 dB, respectively, by the application ofa stretching force. © 2006 American Institute of Physics. DOI: 10.1063/1.2360253Long period fiber grating LPFG is one of the widelyused passive optical fiber devices. Various LPFG fabricationtechniques have been demonstrated, including ultraviolet la-ser irradiation,1 CO2 laser heat,2,3 hydrofluoric acid etchingcorrugation,4 and application of periodic microbend.5 Thestrain sensitivity obtained for the CO2-laser-induced LPFGswithout physical deformation is usually very low, only−0.45 nm/m.2,3 In this letter, a technique of fabricatingasymmetric LPFG by use of focused CO2 laser beam tocarve periodic grooves on one side of the optical fiber ispresented. The LPFGs obtained exhibit a large peak trans-mission attenuation of −47.39 dB and a low insertion loss of0.34 dB. Moreover, the average strain sensitivity of resonantwavelength of the LPFG is increased to −102.89 nm/m.Our experimental setup is shown in Fig. 1. A CO2 laserSYNRAD 48-1 with a maximum output power of 10 W, alight-emitting diode light source, and an optical spectrumanalyzer HP 70004A were used. The optical fiber CorningSMF-28 was situated in the focal plane of the CO2 laserbeam. One of the fiber ends was fixed and a small weight of5 g was used at the free end of the fiber to avoid theweight-induced macrobend and to provide a tensile strain inthe fiber. The focused CO2 laser beam scanned repeatedly forM times along the X direction at a location, corresponding tothe first grating period, of the fiber via a two-dimensionaloptical scanner under the computer control. Then the laserbeam was shifted by a grating period along the Y directionand scanned repeatedly for M times to generate the nextgrating period. This scanning and shifting process was car-ried out for N times N is the number of grating periods untilthe final grating period was created. The above mentionedprocess was repeated for K cycles until a high quality LPFGwas produced. The repeated scanning of the focused CO2laser beam created a local high temperature in the fiber,which led to the gasification of SiO2 on the surface of thefiber. As a result, periodic grooves were carved on the fiberas shown in Fig. 2. Such grooves induce periodic refractiveindex modulation along the fiber axis due to the photoelasticeffect, thus creating a LPFG. The typical depth and width ofthe grooves obtained in our LPFGs were 15 and 50 m,respectively. The depth of the grooves depends on a numberof parameters, such as the diameter of the focused laser lightspot, the scanning speed, the average laser output power de-termined by the repetition rate and the width of the laserpulses, and the values of M and K. In our experiments, thediameter of the focused CO2 laser beam spot, the line speedof the scanning, the pulse repetition rate, the pulse width, andthe average output power of the CO2 laser were 35 m,2.326 mm/s, 10 kHz, 4.1 s, and 0.5 W, respectively.aElectronic mail: ypwang@china.combElectronic mail: eednwang@polyu.edu.hkcElectronic mail: eewjin@polyu.edu.hk FIG. 1. Experimental setup for LPFG fabrication.APPLIED PHYSICS LETTERS 89, 151105 20060003-6951/2006/8915/151105/3/$23.00 © 2006 American Institute of Physics89, 151105-1Downloaded 31 Mar 2011 to 158.132.161.52. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsFigure 3 illustrates the transmission spectra of theLPFGs with grating periods grooves of 20, 40, 60, 70, and80, respectively. It can be seen from this figure that, with theincrease of the number of grating periods, the resonant wave-length of the LPFG shifts toward the shorter wavelength, thetransmission attenuation is increased, and the 3 dB band-width of the attenuation is decreased. The transmission spec-trum evolution of a LPFG with the increased number ofscanning cycles is illustrated in Fig. 4. One high qualityLPFG with a large peak attenuation of −47.39 dB at the reso-nant wavelength of 1520.10 nm and a low insertion loss of0.34 dB can be obtained after nine scanning cycles.In order to observe the strain characteristic of the LPFGproduced, an external stretching force was applied to aCO2-laser-carved LPFG with a resonant wavelength of1569.53 nm and a peak transmission attenuation of−21.062 dB. As shown in Fig. 5, with the tensile strain in-creasing, the resonant wavelength of the LPFG shifts to1557.39 nm and the peak attenuation first increases rapidlyto −41.193 dB and then decreases. However, the insertionloss of the LPFG hardly changes during this process.When an optical fiber with asymmetric structure, e.g.,periodic grooves on one side of the fiber, is stretched longi-tudinally, small lateral bends, i.e., periodic microbends, willbe induced in the grooved section of the fiber.6 So periodicmicrobends will be observed when a CO2-laser-carved LPFGwith asymmetric grooves is stretched, as shown in Fig. 6.Thus, refractive index modulation in the stretchedCO2-laser-carved LPFG can be expressed asn = nresidual + ngroove + nstretch, 1where nresidual is the initial refractive index perturbation in-duced by the residual stress relaxation resulting from thehigh local temperature, which is similar to the case of theCO2-laser-induced LPFGs without periodic grooves;2,3ngroove is the initial refractive index perturbation induced bythe periodic grooves on the fiber, which is similar toncorrugated in the corrugated LPFGs fabricated by hydrofluo-ric acid etching;4 nstretch is the refractive index perturbationinduced by the stretching force and can be expressed asnstretch = nstrain + nmicrobend, 2where nstrain is the refractive index perturbation induced bythe difference between the stretch-induced tensile strains inthe grooved and the ungrooved regions due to the photoelas-tic effect;4,6 nmicrobend is the refractive index perturbationinduced by the stretch-induced microbends.5,6 Such stretch-induced microbends effectively enhance refractive indexmodulation in the CO2-laser-carved LPFGs, which is similarto the case of the microbend-induced LPFG.5 Therefore, therefractive index modulation efficiency in theCO2-laser-carved LPFG is higher than that of theCO2-laser-induced LPFG without periodic grooves. As a re-sult, a large peak attenuation of −47.39 dB and a low inser-tion loss of 0.34 dB can be obtained in the CO2-laser-carvedLPFG, as shown in Fig. 4. By contrast, the peak attenuationof the LPFGs without periodic grooves is typically −25 dB.1–3Both the difference between the stretch-induced tensilestrains in the grooved and the ungrooved regions and theamplitude of the stretch-induced microbends increase withthe increase of the stretching force, which leads to the in-crease of nstrain and nmicrobend. As shown in Fig. 5, theresonant wavelength shifts toward the shorter wavelength by−12 nm and the peak attenuation increases by 20 dBwhen a stretching force is applied. It can also be seen fromFig. 5 that the transmission attenuation changes to the oppo-site direction when the tensile strain is increased beyond acritical value of 100 . This is due to the overcouplingbetween the fundamental core mode and the cladding modeFIG. 2. Photograph, obtained from a charge-coupled device camera, of theCO2-laser-carved LPFG with a grating pitch of 400 m.FIG. 3. Color online Transmission spectrum evolution of the LPFG with agrating pitch of 400 m while N varies from 20 to 80, where M =20 andK=1.FIG. 4. Color online Transmission spectrum evolution of the LPFG with agrating pitch of 400 m while K varies from 1 to 9, where N=20 and M=8.FIG. 5. Color online a Variations of the resonant wavelength and thepeak attenuation, b transmission spectrum evolution of theCO2-laser-carved LPFG with the tensile strain.151105-2 Wang et al. Appl. Phys. Lett. 89, 151105 2006Downloaded 31 Mar 2011 to 158.132.161.52. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionswhen a large refractive index modulation in the grating leadsto the occurrence of the maximum coupling efficiency. Withthe increase of the tensile strain before the overcoupling, theresonant wavelength of the CO2-laser-carved LPFG shiftsfrom 1569.93 to 1559.58 nm with an extremely high averagesensitivity of −102.89 nm/m, compared with the value ob-tained with the CO2-laser-induced LPFGs without periodicgrooves, which is only −0.45 nm/m.3 It becomes clear thatthe strain sensitivity of the resonant wavelength of theLPFGs written by the CO2 laser in the same type of opticalfiber Corning SMF-28 can be increased by 229 times bycarving periodic grooves on one side of the fiber. As reportedby Lin et al.,4 the resonant wavelengths of the corrugatedLPFG with symmetric grooves are insensitive to the tensilestrain. Thus, during the LPFG fabrication, asymmetric peri-odic grooves, instead of symmetric periodic grooves, shouldbe created to increase the strain sensitivity of resonant wave-length. The CO2-laser-carved LPFG can be used both as atunable filter with a large wavelength tuning range and as ahighly sensitive strain sensor based on wavelength shiftand/or intensity modulation. Compared with the strain sen-sors based on piezoelectric effect7 or magnetostrictioneffect,8 our LPFG sensor exhibits the advantages of electro-magnetic immunity, easy fabrication, simple configuration,and no backreflection.Single side incidence of CO2 laser beam induces anasymmetric index profile within the cross section of theCO2-laser-carved LPFG, which is similar to the case of theCO2-laser-induced LPFG without physical deformation.3,9–11Consequently, a clear polarization dependence exists in theCO2-laser-carved LPFG as shown in Fig. 7, where the aver-age loss and the polarization dependent loss PDL are mea-sured by an Agilent 81910A photonic all-parameter analyzer.The maximum PDL of 1.35 dB in the CO2-laser-carvedLPFG is close to the value, i.e., 1.2 dB, of that in theCO2-laser-induced LPFGs without periodic grooves.9,10 Thisindicates that the periodic grooves will not enhance the po-larization dependence in the LPFG. The reason is that, al-though periodic grooves increase the refractive index modu-lation, they are confined within the outer cladding region anddo not strongly influence the asymmetry of index profilewithin the cross section of the core and inner cladding of theLPFG.During our LPFG fabrication process, the focused CO2laser beam with a high energy density, resulting from a smallfocused light spot with a diameter of only 35 m, scansrepeatedly and heats continuously at each grating groove lo-cation. As a result, periodic grooves are carved on the surfaceof the fiber. By contrast, there is no physical deformation inthe LPFGs fabricated by Davis et al.2 and Rao et al.3 usingthe CO2 laser. Such periodic grooves in our LPFGs haveessentially no contribution to the insertion loss, similar to thecase of the corrugated LPFG fabricated by hydrofluoric acidetching.4 This is due to the fact that these grooves are totallyconfined within the outer cladding and have no influence onthe light transmission in the fiber core. Experimental resultsshow that, as long as the grooves do not touch the centralregion near the fiber core, no obvious insertion loss is ob-served. The insertion loss of the LPFG is mainly due to thenonperiodicity and the disorder of the refractive indexchange.In conclusion, asymmetric LPFGs with large peak at-tenuation of −47.39 dB and low insertion loss of 0.34 dBhave been fabricated by use of focused CO2 laser beam tocarve periodic grooves on one side of the optical fiber. SuchLPFGs combine the features of the CO2-laser-induced LPFGwithout physical deformation, the corrugated LPFG createdby hydrofluoric acid etching, and the microbend-inducedLPFG. The resonant wavelength of the LPFG obtained canbe tuned by 12 nm with an extremely high sensitivity of−102.89 nm/m and the peak attenuation can be changed by20 dB.This work was supported by the research grants No.G-YX51 of the Hong Kong Polytechnic University in aPostdoctoral Research Fellowship scheme and the NationalScience Foundation of China No. 60507013.1A. M. Vengsarkar, P. J. Lemaire, J. B. Judkins, V. Bhatia, T. Erdogan, andJ. E. Sipe, J. Lightwave Technol. 14, 58 1996.2D. D. Davis, T. K. Gaylord, E. N. Glytsis, S. G. Kosinski, S. C. Mettler,and A. M. Vengsarkar, Electron. Lett. 34, 302 1998.3Y. J. Rao, Y. P. Wang, Z. L. Ran, and T. Zhu, J. Lightwave Technol. 21,1320 2003.4C. Y. Lin, L. A. Wang, and G. W. Chern, J. Lightwave Technol. 19, 11592001.5C. H. Lin, Q. Li, A. A. Au, Y. Jiang, E. B. Wu, and H. P. Lee, J. LightwaveTechnol. 22, 1818 2004.6M. Vaziri and C. L. Chen, Appl. Opt. 32, 6399 1993.7Y. R. Wu and J. Singh, Appl. Phys. Lett. 85, 1223 2004.8H. J. Mamin, B. A. Gurney, D. R. Wilhoit, and V. S. Speriosu, Appl. Phys.Lett. 72, 3220 1998.9H. S. Ryu, Y. Park, S. T. Oh, Y. J. Chung, and D. Y. Kim, Opt. Lett. 28,155 2003.10B. L. Bachim and T. K. Gaylord, Appl. Opt. 42, 6816 2003.11Y. P. Wang and Y. J. Rao, IEEE Sens. J. 5, 839 2005.FIG. 6. Schematic diagram of the CO2-laser-carved LPFG a before and bafter a stretching force F is applied, where , D, and W are grating pitch, thedepth, and the width of the grooves, respectively.FIG. 7. Color online Average loss and PDL of the CO2-laser-carved LPFGin the free state.151105-3 Wang et al. Appl. Phys. Lett. 89, 151105 2006Downloaded 31 Mar 2011 to 158.132.161.52. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

Asymmetric long period fiber gratings fabricated by use of CO₂laser to carve periodic grooves on the optical fiber

Abstract

Similar works

Full text

Available Versions

PolyU Institutional Repository