Deyerl, Hans Jürgen

El-Bez, Jonathan

Jensen, Jesper Bo Damm

Kristensen, Martin

Peucheret, Christophe

Plougmann, Nikolai

Sørensen, Henrik Rokkjær

We present two fibre Bragg gratings with reduced in-band dispersion for DWDM applications. The gratings were designed by the inverse scattering method and fabricated using the novel polarization control method for UV-writing of advanced gratings

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Downloaded from orbit.dtu.dk on: Dec 17, 2017Low-Dispersion Fibre Bragg Gratings Written Using the Polarization Control MethodDeyerl, Hans Jürgen; Plougmann, Nikolai; Jensen, Jesper Bo ; El-Bez, Jonathan; Sørensen, HenrikRokkjær; Peucheret, Christophe; Kristensen, MartinPublished in:ECOCPublication date:2002Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Deyerl, H. J., Plougmann, N., Jensen, J. B. D., El-Bez, J., Sørensen, H. R., Peucheret, C., & Kristensen, M.(2002). Low-Dispersion Fibre Bragg Gratings Written Using the Polarization Control Method. In ECOC (Vol. 3,pp. 1-2). IEEE.Graftings And Filters 7.2.7LOW-DISPERSION FIBRE BRAGG GRATINGS WRITTEN USING THEPOLARIZATION CONTROL METHODHans-JUrgen Deyerl, Nikolai Plougmann, Jasper Bo Damm Jensen, Jonathan El-Bez, Henrik Rokkijr Sorensen,Christophe Peucheret, Martin KristensenCOM, Technical University of Denmark, Buikding 345v, DK-2800 Kgs. Lyngby, Denmarke-mail: hjdOcom.dtu.dkAbstract We present two fibre Bragg gratings with reduced in-band dispersion for DWDM applications. Thegratings were designed by the inverse scattenng method and fabricated using the novel polarization controlmethod for UV-writng ofadvanced gratings.IntroductionDue to the growth of the internet andtelecommunication over the last decade, there hasbeen an increasing demand for bandwidth. The densewavelength division rnultiplexing (DWDM) technologyplays an important role for increasing thetransmission capacity in optical communicationnetworks. The intemational standard for the channelspacing has recently been lowered from 100GHz to50GHz and may become 26GHz in the future at themoderate bit-rate of 1 OGbitWs. To meet thesedemands, the ideal optical filter should have a squarespectral response and keep a linear phase throughoutthe filter pass-band leading to zero cross-talk and lowdispersion,Fibre Bragg gratings (FBGs) are widety used foradd/drop rnultiplexing (ADM). When apodizedappropriately, they exhibit a near ideal optical filterfunction, due to their narrow optical spectrum [1].However, FBGs suffer from non-linear phaseresponse and thus dispersion both outside and insidethe stop-band, which could limit their application inhigh bit-rate optical communication systemns (2].Recently two alternative ways for reducing the in-band dispersion properties of FBGs have beendemonstrated. One is based on an asymmetric sinc-like design [3], while the other one uses a symmetricFBG design applying an optimised cosine progressionto the apodization profile [4].In this paper we present the design and fabrication oflow-dispersion asymmetric FBGs. The presentedFBGs are wrinen with the novel polarization controlmethod providing an altemative, more robust way ofgrating fabrication, when compared to the ditheringrnethod [33.FBG designThe inverse Fourier transform of a square filterfunction gives a sinc-function for the refractive indexmodulation. However, applying Fourier theory for thedesign of FBGs is only justified for weak gratings.When dealing with the response of strong FBGs, e.g.when the light in the stop band no longer penetratesthe full length of the modulation profile without anysignifcant attenuation, Fourier theory cannot beapplied for the full analysis of the spectral response ofthe FBG. Deviations from the Fourier analysis appearwhen higher strength FBG with certain properties,e.g. low dspersion, are designed. Severalapproaches, which are based on the inversescattering technique, have been developed toaddress this issue (5].The target designs of the linear-phase (dispersionfree) Bragg grating presented here include zerodispersion throughout the spectral response. Twogratings with a grating length of L=23 mm have beendesigned, one (a) with a transrmission-loss of 60dBand a bandwidth of 100GHz and a second one (b)with a transmission-loss of 20dB and a bandwidth of50GHz. The mnodulation profiles were generated bysolving the inverse scattering problem by the layer-peeling algorithm implemented in the commercialprogram IFO_Gratings [6]. Figure 1 shows thenormalized refractive index profile for both gratingsalong wih the simnulated spectral response and thepredicted group delay (launching the light from left toright).a) b)Fig. 1. Normalized refractve index profile for twolinear phase FBGs (upper frames) along with thecorresponding simulatedspectral response and groupdelay (dashed line) (lower frames).The refractive index profile is asymmetric andincludes multiple phase-shfts. This asymmetry willAuthorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on March 10,2010 at 08:19:50 EST from IEEE Xplore.  Restrictions apply. Gratings And Filters 7.2.7not affect the arnplitude spectral response of theFBG, but will affect the phase response whenilluminated from the opposite direction.Fabrication and group delay measurementThe grating is written using the recently developedpolarization controlled method for UV-writing of fibreBragg gratings (FBGs) (7P. This method relies onchanging the polarization of the UV beam duringexposure and it requires only a single scan atconstant speed with constant UV-fluence. Briefly, apolarization beam splitter is scanning along the phasemask together with the polarized UV-beam. Twospatially separated Bragg gratings are inscripted inthe core of the fibre by the diverging s- and p- fractionof the UV-light. A high-power polarizer mounted in arotation stage controls the polarization of the UVbeam and therefore the relative intensities of the UV-light in s- or p-polarization. If the distance betweenphase-mask and fibre is chosen so that the phase-shift of the two inscripted gratings is equal to X, weare able to write FBGs with advanced apodizationprofiles including multiple discrete phase-shifts [7].A deuterium loaded (100bar) highly non linear(HNLF) fibre (from OFS FTtel Denmark) with NA-0.33is used and a pulsed Excimer laser operating at248nm and 40 Hz is emnployed as the UV source.The group delay has been determined by the phase-shift technique using an external cavity laser as thesource and a modulation frequency of 1GHz [8].Results and DiscussionFig. 2(a) shows the measured reflection andtransmission spectrum along with the group-delaymeasurements (top framne) for the 100GHz bandwidthlinear-phase grating when probed as indicated in Fig.1(a). The spectral response is confirmed to be nearsquare with a side lobe suppression of 15 dB and anearly constant transmission loss of 60 dB.Furthermore Fig. 2(a) shows, that the group delay isnearly constant wih a small ripple of 30ps and ingood agreement with simulations. In Fig. 2 (middleframe) the measured group delay spectrum is alsopresented when launching the light from the reversedirection. The measured group delay is in excellentagreement with theoretical prediction.Rg. 2(b) shows the results for the 50GHz bandwidthgrating. Also for this design we achieve a near squarefilter function with a side lobe suppression of 20 dB atan almost constant transmission loss of 20dB. Themeasured group delay (top frame) is in excellentagreement with the theoretical prediction and thevariation is within 20 ps in the reflective bandwidth.Both gratings exhbit much better dispersioncharacteristics when compared to 'standard'Gaussian or sinc apodized FBGs 13, 41.Due to the beam size (beam-waist of 0.7 mmn) of theused UV-light, the design of the modulation profilewas limited to 6 phase-shifts at a grating length of 23mm. Using a CW-laser with a better mode quality anda smnaller beam size could improve the quality of thedesign and fabrication of FBGs.a) b)Fig 2: Measured refiecton, group delay andtransmission spectra of the two deigned linearphaseFBGs. Top frames: launching the light fom te left(designed); middle frames: launching the light fromthe right (opposite) direction; lower frames:transmion spectra. The dashed Jines reprosentsimnulatad data.ConclusionsDispersion-reduced optical filters using 23mmn longFBGs for 100GHz and 50GHz channel spacing inDWOM are presented. Both designs were written withthe novel polarization control method and prove thatthe new writing method is capable of writingadvanced, designed FBGs and is only limited by thebeam profile and beam size of the UV-source and thenon-linear photosensitvity of the fibre.References1 T. A. Strasser et al Proc. to OFC'96 (1996), PD82 G. Nykolak et al IEEE Photon. Tech. Left. 10(1998) 13193 M. Ibsen et al Proc. to OFC'01 (2001), MCI4 T. Shibata et al Proc. to OFC'01 (2001), WDD845 L. Poladian Opt. Lett. 25 (2000), 7876 IFO-Gratings, Version4.0 by Optiwave Corporation(http:l/www.optiwave.com)7 J. B. D. Jensen et al Opt. Lett., (2002), in press8 C. Caspar et al Electron. Lett., 33 (1997), 1624.W* 1404 WAS 461t 1091.Wova"04"Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on March 10,2010 at 08:19:50 EST from IEEE Xplore.  Restrictions apply. 

Low-Dispersion Fibre Bragg Gratings Written Using the Polarization Control Method

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Low-Dispersion Fibre Bragg Gratings Written Using the Polarization Control Method

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