Author name used in this publication: Chung-Loong ChoyAuthor name used in this publication: A-Ping ZhangAuthor name used in this publication: Muhtesem Süleyman DemokanAuthor name used in this publication: Xiao-Ming Tao2001-2002 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Chan, HLW

Choy, CL

Demokan, S

Guan, BO

Tam, HY

Tao, X

Zhang, AP

IEEE Photonics Technology Letters

Author name used in this publication: Chung-Loong ChoyAuthor name used in this publication: A-Ping ZhangAuthor name used in this publication: Muhtesem Süleyman DemokanAuthor name used in this publication: Xiao-Ming Tao2001-2002 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishedVoR allowe

PolyU Institutional Repository

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 5, MAY 2002 657Step-Changed Long-Period Fiber GratingsBai-Ou Guan, A-Ping Zhang, Hwa-Yaw Tam, Senior Member, IEEE, Helen L. W. Chan, Chung-Loong Choy,Xiao-Ming Tao, and Muhtesem Süleyman DemokanAbstract—The spectral characteristics of a new type of long-pe-riod grating (LPG) structure, step-changed LPG, are investigatedtheoretically and experimentally. It is shown that the spectral pro-file of such gratings can be tailored by adjusting the step param-eters. A good agreement between simulated results and measuredspectra is obtained. This type of LPG has potential applications ingain flattening of erbium-doped fiber amplifiers.Index Terms—Band-rejection filters, gain flattening, long-periodgratings, mode coupling, optical fiber filters.I. INTRODUCTIONLONG-PERIOD fiber gratings (LPGs) are band-rejectiontransmissive filters which couple light from the core ofa single-mode fiber (SMF) into the cladding, thus producingattenuation bands in the fiber transmission spectrum. LPGshave many applications in both the telecommunications andfiber sensing systems [1]. In some applications, particularlyfor flattening the gain profile of erbium-doped fiber ampli-fiers (EDFA), multiple peaks and special spectral profile arerequired. However, the spectrum of an LPG normally has onlyone attenuation peak around the 1550-nm wavelength range,and so cannot meet this requirement. A reported techniquefor tailoring the LPG spectrum to produce multiple peaks isto introduce phase shift in an LPG [2]. Another technique todesign the LPG spectrum is by cascading two independentLPGs [3]. In this case, the light coupled by the first LPG willbe recoupled into the fiber core by the second LPG, leading tomultipath interference, and introduces large undesirable ripplesin the transmission spectrum. This problem can be eliminatedby either increasing the LPG separation or bending the regionbetween the LPGs. But these result in difficulty of realizingcompact filter structure. Another solution is to utilize LPGswith different period and make the orders of the involvedcladding mode in the two LPGs different with each other[4].This allows the LPGs closely spaced without recouplinginteraction.In this letter, we present a new scheme to tailor the LPGspectra by introducing a step change in the index profile of aconstant pitch LPG. The step-changed LPGs were fabricatedManuscript received October 11, 2001; revised January 21, 2002. This workwas supported by the Center for Smart Materials of the Hong Kong PolytechnicUniversity.B.-O. Guan, H. L. W. Chan, and C.-L. Choy are with the Department of Ap-plied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon,Hong Kong.H.-Y. Tam and M. S. Demokan are with the Photonics Research Center, De-partment of Electrical Engineering, The Hong Kong Polytechnic University,Hung Hom, Kowloon, Hong Kong (e-mail: eehytam@polyu.edu.hk).A.-P. Zhang and X.-M. Tao are with the Institute of Textiles and Clothing,The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong.Publisher Item Identifier S 1041-1135(02)03226-3.using a two-step UV-beam scanning technique with a single uni-form amplitude mask. The spectral characteristics of such LPGswere predicted by numerically solving the coupled mode equa-tions. The predicted spectra are in good agreement with the ex-perimental results.II. THEORETICAL ANALYSISThe coupled mode theory with core and cladding mode de-scriptions of the electromagnetic fields in SMF has been appliedto predict the diffraction efficiency and spectral dependence ofoptical fiber gratings [5], [6]. Since the counterdirectional cou-pling need not be considered in the long-period gratings, thecoupled mode equations then can be expressed as(1)(2)where is the slowly varying amplitudes of the core mode,is that of the cladding modes, and aretheir propagation constants. The coupling coefficients inthe above expressions are(3)where both the core and cladding electric mode are normal-ized. The permittivity perturbation is often modeled as a si-nusoidal function, and the coupled mode equations have beensolved analytically with the assumptions that it is a two-modeinteraction process.We model the grating index modulation as a rectangular func-tion instead of the sinusoidal approximation commonly adoptedin previous works because the LPGs are fabricated by usingan amplitude mask whose period is many orders larger thanthe wavelength of UV light (248 nm). The index perturbationsof step-changed LPGs is defined as [refer to Fig. 1](4)where is a conventional rectangular index modulationof the normal long-period gratings, is the step ratiowhich describes the amplitude of the step change, is thegrating length, and is a normalized parameterwhich describes the position of the step-change along thegrating. Compared with conventional long-period gratings,two adjustable parameters and are introduced in thestep-changed long-period gratings. The following sectionexplains how the spectral profile of the LPGs can be tailoredby adjusting these two parameters.1041-1135/02$17.00 © 2002 IEEEAuthorized licensed use limited to: IEEE Xplore. Downloaded on October 22, 2008 at 03:36 from IEEE Xplore.  Restrictions apply.658 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 5, MAY 2002Fig. 1. Schematic diagram of the refractive index modulation of thestep-changed LPG.In general, the spectral characteristic of LPGs can be ana-lyzed using transfer matrix based on the analytical fundamentalmatrix solution of (1)–(2), by considering the coupling betweenthe core mode and the corresponding -thcladding mode. This two-mode interaction assumption is satis-fied in conventional LPGs, because its spectral separations be-tween resonant peaks are much wider than the bandwidth of in-dividual peaks. This method has also been extended to calculatethe spectra of cascaded or phase-shifted LPGs [7], where an ad-ditional matrix representing the optical path difference or phaseshift between two adjacent gratings has been inserted betweentwo fundamental matrixes.The step-changed LPGs can be regarded as two concatenatedLPGs with zero separation, but different index modulation am-plitude. For weakly step-changed LPGs , the transfermatrix method can be directly used to analyze their spectra bymultiplying two fundamental matrixes with respective gratingparameters. But for strongly step-changed LPGs , theaforementioned two-mode coupling treatment is not a good as-sumption. The resonant wavelength of the LPGs is very sensi-tive to the average index modulation, and the wavelength shiftmore than 80 nm during the LPG inscription was reported [8].Therefore, for step-changed LPGs with larger , loss peaks andtheir sidelobes corresponding to different cladding modes mayoverlap and multimode interaction may occur. To consider theseproblems, the transmission spectra of this new type of LPGswere analyzed by directly employing the numerical integrationsof coupled mode equations. Two cladding modes are selected to-gether with the core mode in the numerical simulations. This ap-proach by treating the LPGs as rectangular profile and selectingmore than one cladding mode is more rigorous, and more exactresults are expected than the conventional two-mode interactiontreatments.III. RESULTS AND DISCUSSIONSThe LPGs were written using a 248-nm KrF excimer laserand an amplitude mask. The fibers used in the experiments wereCorning SMF-28 fibers that were previously photosensitized bysoaking in a hydrogen atmosphere. The period of the amplitudemask was 450 m. The step-changed LPG was written with atwo-step UV-beam scanning technique. Initially, a 2-cm normalLPG was written by exposing the hydrogen-sensitized fiber tothe scanned UV-beam through the amplitude mask. Then theLPG was re-exposed by scanning the UV-beam from the begin-ning of the grating to a certain position ,while both the amplitude mask and the fiber remained fixed.The UV-beam scanning speed during the first and second expo-sures and the length of the second exposure determine the stepFig. 2. Experimental (solid curve) and theoretical (dashed curve) spectra ofthe step-changed LPGs with L = 2:0 cm,  = 1:6, (a)  = 0, (b)  = 0:25,(c)  = 0:4, (d)  = 0:525, (e)  = 0:625.ratio and step location , respectively. During the LPG in-scription, both UV-beam scanning and the laser operation werecontrolled by a computer. The LPGs were annealed at 110 C for6 h after they were written, then their transmission spectra weremeasured with a broadband light source and an optical spectrumanalyzer (OSA).Fig. 2 shows the spectra of a group of step-changed LPGswritten with the same UV-beam scanning speed (same ), butdifferent exposure lengths (different ) during the second expo-sure. The LPG without the step change has only one attenuationpeak, corresponds to cladding mode, in the wavelengthrange 1500–1650 nm. The introduction of the step-change re-sults in the appearance of a second attenuation peak at the longerwavelength side of the original peak. The new attenuation peakincreases with the length of the second exposure (i.e., ). Atthe same time, the original peak decreases, and the separationbetween the peaks decreases as well. When the length of thesecond exposure approaches (i.e., ), two peaks willAuthorized licensed use limited to: IEEE Xplore. Downloaded on October 22, 2008 at 03:36 from IEEE Xplore.  Restrictions apply.GUAN et al.: STEP-CHANGED LONG-PERIOD FIBER GRATINGS 659Fig. 3. Experimental (solid curve) and theoretical (dashed curve) spectra ofthe step-changed LPGs with L = 2:0 cm,  = 0:25, (a)  = 1:6, (b)  = 2:1,(c)  = 2:26.merge into one because the LPG becomes a normal LPG witha higher index modulation. The introduction of the step-changealso leads to a sidelobe at the shorter wavelength side of theoriginal peak. The results obtained from theoretical simulationare also shown in Fig. 2 (in dashed line). It can be seen that thetheoretical spectra are in very good agreement with the experi-mental data.Fig. 3 shows the spectra of another group of step-changedLPGs that were written with the same exposure length (same), but different UV-beam scanning speed (different ) duringthe second exposure. With the increase of the amplitude of thestep-change, a third peak appears at the larger wavelength sideof the original peak. The original peak decreases with the in-crease of the new peaks, and the separations between the peaksstay constant. The LPGs in Figs. 2(b) and 3(a) have differentindex modulation amplitude although their and values arethe same, thus they exhibit different transmission spectra.From both theoretical simulations and experimental results,it can be seen that the spectral profile of the LPGs can be tai-lored and multiple peaks can be produced by introducing a stepchange in the grating index profile. Since the bandwidth of theLPGs attenuation peak is inversely proportional to the gratinglength, the spectral shape of the step-changed LPGs can be moreeffectively controlled by adjusting the step parameters andin combination with selecting the desired grating length . ThisFig. 4. Original (darker curve) and flattened (lighter curve) ASE spectrum ofan EDFA.characteristic of the step-changed LPGs has potential applica-tions in EDFA gain flattening. This is illustrated in Fig. 4. Theamplified spontaneous emission (ASE) spectrum of a 20-m-longEr-doped fiber pumped by a 980-nm laser diode is flattened towithin 2 dB over 30 nm using a step-changed LPG. The param-eters of the step-changed LPG used here are as follows: gratingperiod 455 m, cm, , and .IV. CONCLUSIONA special LPG structure, step-changed LPG, has been in-vestigated theoretically and experimentally. The step-changedLPGs were fabricated using a two-step UV-beam scanning tech-nique with a single uniform amplitude mask. The spectral char-acteristics of this type of LPG were simulated by numericallysolving the coupled-mode equations and the results obtainedfrom the simulation shown good agreement with the measure-ment spectra. It has been shown that the spectral profile of suchLPG can be tailored by adjusting the step parameters and thisproperty has potential applications in EDFA gain-flattening.REFERENCES[1] M. Vengsarkar, P. J. Lemaire, J. B. Judkins, V. Bhatia, T. Erdogan, andJ. E. Sipe, “Long-period fiber gratings as band-rejection filter,” J. Light-wave Technol., vol. 14, pp. 58–65, Jan. 1996.[2] J. R. Qian and H. F. Chen, “Gain flattening fiber filters usingphase-shifted long period fiber gratings,” Electron. Lett., vol. 34, pp.1132–1133, May 1998.[3] A. M. Vengsarkar, J. R. Pedrazzanni, J. B. Judkins, P. J. Lemaire, N.S. Bergano, and C. R. Davidson, “Long-period fiber-grating-based gainequalizers,” Opt. Lett., vol. 21, pp. 336–338, Mar. 1996.[4] M. Harumolo, M. Shigehara, M. Kakui, H. Kananori, and M. Nishimura,“Compact long-period grating module with multi-attenuation peaks,”Electron. Lett., vol. 36, pp. 512–513, Mar. 1998.[5] T. Erdogan, “Cladding-mode resonance in short- and long-period gratingfilters,” J. Opt. Soc. Amer. A—Optics Image Science and Vision, vol. 14,pp. 1760–1773, Aug. 1997.[6] , “Fiber grating spectra,” J. Lightwave Technol., vol. 15, pp.1277–1294, Aug. 1997.[7] B. H. Lee, Y.-J. Kim, Y. Chung, W.-T. Han, and U.-C. Paek, “Analyticsolution for cascaded long period fiber gratings,” IEICE Trans. Commu-nications, vol. E84-B, pp. 1247–1254, May 2001.[8] B. O. Guan, H. Y. Tam, H. L. W. Chan, C. L. Choy, and M. S. Demokan,“Growth characteristics of long-period gratings in hydrogen-loaded fiberduring and after 193 nm UV inscription,” Meas. Sci. Technol., vol. 12,pp. 818–823, July 2001.Authorized licensed use limited to: IEEE Xplore. Downloaded on October 22, 2008 at 03:36 from IEEE Xplore.  Restrictions apply.

Step-changed long-period fiber gratings

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