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Suppression of cladding-mode coupling loss in fiber Bragg gratings by independent control of refractive index and photosensitive profiles in a single-mode optical fiber

Suppression of cladding-mode coupling loss in fiber Bragg gratings by independent control of refractive index and photosensitive profiles in a single-mode optical fiber
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  1504 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 11, NOVEMBER 2000 Suppression of Cladding-Mode Coupling Loss inFiber Bragg Gratings by Independent Control of Refractive Index and Photosensitive Profiles in aSingle-Mode Optical Fiber J. M. Kim, K. Oh  , Member, IEEE  , T. S. Park, C. S. Kim, and K. Jeong  Abstract— A new method to suppress the cladding-mode cou-pling loss in fiber Bragg gratings, by independent control of thecore refractive index profile and the photosensitive profile, is pro-posed and experimentally demonstrated. Across the core and theinner cladding, a uniform step photosensitive profile was intro-duced by co-doping GeO    and B    O    . The core refractive indexwas selectively raised by further doping Al    O    in the core thathas negligible photosensitivity 244 nm. For strong Bragg gratingsinscribed on the fiber, the cladding-mode coupling loss was sup-pressed below 0.3 dB.  Index Terms— Bragg grating, optical fiber, optical material,waveguide design. I. I NTRODUCTION W ITH wavelength-division-multiplexing(WDM) systemsbeing established as the standard for large-capacityoptical communication systems, demand for device technologyto individually control optical channels has rapidly increased.Fiber Bragg gratings (FBGs) have been intensively studiedfor their inherent narrow band rejection characteristics thatcould be applied in add–drop multiplexers. Bragg gratingswith high reflectivity inscribed on conventional single-modefibers, however, induce significant amount of excess loss in theshorter wavelength side band. The loss is caused by coupling of the fundamental core mode to backward propagating claddingmodes, which incurs insertion loss in neighboring WDMchannels. In order to suppress the cladding-mode couplingloss that severely restricts application of FBGs in WDMsystems, various methods have been reported. Photosensitiveinner-cladding fiber structures which use the mode orthog-onality between the fundamental core mode ( ) and thecladding modes have been experimentally demonstrated [1],[2]. In the photosensitive cladding fibers, it is important tohave the same photosensitivity, both in the core and the innercladding, which requires tedious control of process parameters. Manuscript received April 3, 2000; revised July 6, 2000. This work was sup-ported in part by UFON, the ERC Program in K-JIST, and by the BK21 Projectsponsored by MOE.J. M. Kim and K. Oh are with the Department of Information and Communi-cations, Kwangju Institute of Science and Technology, Oryong-dong, Puk-ku,Kwangju 500-712, Korea (e-mail: S. Park, C. S. Kim, and K. Jeong are with the Optical Communication Re-search Team, Access Network Laboratory, Korea Telecom 62-1, Hwaam-dong,Yusung-gu, Taejeon, 305-348, Korea.Publisher Item Identifier S 1041-1135(00)09576-8.Fig. 1. Schematic of the proposed photosensitive fiber structure. Solid lineand dotted line represent the refractive index profile and photosensitive profile,respectively. Note that the inner cladding and the core, the shaded region,contains the same amount of GeO and B O . Furthermore, germanium contributes both in refractive indexprofile and photosensitive profile such that elaborated wave-guide design to control the modal overlap are strictly limited.These drawbacks of photosensitive inner-clad fibers have led toalternative approaches, such as a high numerical aperture fiber[3] and a depressed inner-clad fiber [4]. In this study, a new photosensitive inner-cladding fiber struc-ture that can suppress the cladding-mode coupling loss by in-dependent control of photosensitive profile and refractive indexprofile is proposed for the first time, to the best knowledge of authors.II. D ATA AND  R ESULTS The schematic of the proposed fiber structure is shown inFig. 1. Solid and dotted lines represent the refractive index pro-file and photosensitive profile, respectively. Both in the coreand in the inner cladding, a certain amount of GeO and B Othat are photosensitive to 244-nm radiation [5] was uniformlydoped to introduce a uniform step photosensitive profile. It iswell known that GeO increases the refractive index of com-posite silicate glass, while B O decreases [6]. As shown in theproposed structure, the index of the layers co-doped with GeOand B O could be matched to that of pristine SiO clad com-pensating the index increment of GeO by B O . The core re-fractive index was raised by doping Al O that has a negligible 1041–1135/00$10.00 © 2000 IEEE  KIM  et al. : SUPPRESSION OF CLADDING-MODE COUPLING LOSS IN FBGs 1505 Fig. 2. Refractive index profile of the proposed fiber before UV irradiation.The index differences 1     for the core and the inner cladding are      2    and 0       2     , respectively. photosensitivity at 244 nm [7]. Note that in the proposed struc-ture, the photosensitive profile is controlled only by GeO andB O while the refractive index profile is controlled by Al O ,independently.The fiber preforms were fabricated by a modified chemicalvapor deposition (MCVD) process along with a solu-tion-soaking technique [8]. The flow rate of vapor precursorsSiCl ,GeCl ,andBCl ,werekeptconstantbothinthecladdingand the core deposition. The inner-cladding layers were consol-idated as deposited using conventional MCVD process. Thenthe porous core layers were deposited at a lower temperatureand Al O was doped in the core region by solution dopingfollowed by consolidation. The ratio of inner-clad diameter tocore diameter was three, which is wide enough to utilize modeorthogonality. The refractive index profile of the fiber drawnfrom the preform was measured using S14 Refractive IndexProfiler 1 and the result is shown in Fig. 2. The index differencesfor the core and the inner cladding referenced to outercladding are and , respectively. In orderto confirm the uniform-step photosensitivity profile, a segmentof the bare fiber was directly exposed to the KrF excimer laserbeam without any focusing optics. After UV irradiation, theindex profile of the fiber was measured as shown in Fig. 3.The index differences for the core and the inner claddingreferenced to outer cladding are and ,respectively. Nearly the same amount of average index increaseof was observed both in the core and in the innercladding within experimental errors, which confirmed theuniform photosensitive profile across the core and the cladding.Hydrogen was, then, loaded to the fiber at room temperaturein a high pressure. The output from a KrF excimer laser of 240mJ/cm at 5 Hz was irradiated to the prepared fiber over aphase mask to imprint a Bragg grating with a proper focusingoptics. The transmission spectrum of the imprinted Bragggrating is shown in Fig. 4. The cladding-mode coupling losswas suppressed below 0.3 dB for the band rejection efficiencyover 25 dB at the Bragg wavelength. The cladding-modecoupling loss was found to lower than that of depressed innercladding [4]. Due to the nearly matched cladding nature, theproposed fiber was also found less sensitive to the phase mask  1 From York Technology Ltd., York House, School LN, Chanders Ford,Hampshire, U.K.Fig. 3. Refractive index profile of the proposed fiber after UV irradiation. Theindex differences, 1     , for the core and the inner cladding are      2     and      2    , respectively.Fig. 4. The transmission spectrum of a strong Bragg grating inscribed on theproposed fiber. Note that the resolution of the optical spectrum analyzer was0.05 nm. tilt angle than the depressed cladding. In the proposed fiberstructure, various combinations of previously reported fibertypes could be achieved. For example high numerical apertureover 0.2 could be obtained doping Al O alone by a vaporphase delivery [9] such that combination of high numericalaperture structure [3] and photosensitive clad structure [1], [2] could be achieved to further reduce the cladding-mode loss inBragg gratings. The proposed fiber structures could be appliedto various fiber devices that require the core mode control, aswell as low cladding-mode coupling loss, such as chirped fibergratings for dispersion compensators and low-loss reflectors inRaman fiber laser cavities.III. C ONCLUSION Independent control of refractive index and photosensitivitywith a three-layered optical fiber structure was demonstrated byselective doping of GeO , B O , and Al O in silica. Uniformphotosensitive profile across the core and inner cladding wasexperimentally confirmed. Cladding-mode coupling loss below0.3 dB for the Bragg reflectivity over 25 dB was obtained. Fur-therelaboratedwaveguidedesignbyAl O dopingisbeingpur-suedforoptimizationofcladding-modecouplinglossreduction.  1506 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 11, NOVEMBER 2000 R EFERENCES[1] E. Delevaque, S. Boj, J. F. Bayon, H. Poignant, J. Lemellot, and M.Monerie, “Optical fiber design for strong grating photoimprintingwith radiation mode suppression,” in  Conf. Optical Fiber Communica-tion—Tech. Dig. Series , vol. 8, Feb. 26–Mar. 3 1995, pp. 343–346.[2] K. Oh, J. M. Kim, H. S. Seo, U. C. Paek, M. S. Kim, and B. H.Choi, “Suppression of cladding mode coupling in Bragg grating usingGeO –B O codoped photosensitive cladding optical fiber,”  Electron. Lett. , vol. 35, no. 5, pp. 423–424, 1999.[3] T. Komukai and M. Nakazawa, “Efficient fiber grating formed on highN.A dispersion shifted fibers,”  Jpn. J. Appl. Phys. , vol. 34, no. 31, pp.1286–1287, 1995.[4] L. Dong, L. Reekie, J. L. Cruz, J. E. Caplen, J. P. De Sandro, and D.N. Payne, “Optical fibers with depressed claddings for suppression of coupling into cladding modes in fiber Bragg gratings,”  IEEE Photon.Technol. Lett. , vol. 9, pp. 64–66, Jan. 1997.[5] D. L. Williams, B. J. Ainslie, J. R. Armitage, R. Kashyap, and R.Campbel, “Enhanced UV photosensitivity in boron codoped ger-manosilicate fibers,”  Electron. Lett. , vol. 29, pp. 45–47, 1993.[6] S.E.MillerandA.G.Chynoweth,Eds., Opt.FiberTelecommun. . NewYork: Academic, 1979, pp. 188–191.[7] L. Dong, J. Pinkstone, P. S. J. Russell, and D. N. Payne, “Ultraviolet ab-sorption in modified chemical vapor deposition preforms,”  J. Opt. Soc. Amer. B , vol. 11, no. 10, pp. 2106–2111, 1994.[8] S.B.Poole,D.N.Payne,andL.M.E.Fermann,“Fabricationoflow-lossoptical fibres containing rare-earth ions,”  Electron. Lett. , vol. 21, pp.737–738, 1985.[9] J. R. Simpson and J. B. Macchesney, “Optical fibers with an Al2O3-doped silicate core composition,”  Electron. Lett. , vol. 19, pp. 261–262,1983.
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