Two Dimentional Optical Power Distribution of Side-out Coupled Radaition From TFBGs in MM Fibre

7/29/2019 Two Dimentional Optical Power Distribution of Side-out Coupled Radaition From TFBGs in MM Fibre

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was then characterised in this polarisation over the entire spectralregion. The maximum of the orthogonal polarisation was analogouslyfound and the response of thc dcvice in this polarisation w as measured.All the measurements were recorded after rccoating the FBGs tominimise periodic coupling between the cladding and core modes.The spectral responses of WO FP filters written into the same fibresample are shown in Fig. I. Th e WO FPs are separated by 8 nmspectrally and 2 cm spatially. The length of the individual d evices is3 and 2 cm for 10 0 and 50 GHc, respectively. It can bc seen thatcladding mode coupling degrades the performance of the devices as th espectral bandwidths of the chirped FBGs are larger than the spectralseparation behvccn thc cladding modes induced Inss and the FBGsreflection (-10 nm). This gives an additional loss of about 4 d B overmost of the spectral band except for the last lOnm at th e longwavelength side. To &her improve the largc-band FP performancein terms of insertion loss,a high-photosens itive HiBi optical fibre withcladding mode suppression would be needed. However, to ou r knowl-edge, such a fibre is not availablc on the market.

-5mg -10.-(D.-E -15m-20-

, I .- 1529 1530 1531 1532 1533 1534A. nm

0

5ms -10.-y1.-Es -15I-20 ,.

1549 1550 1551 1552 1553 1554 1555A,nmb

Fig. 2 Detail o rranrmission o FP ,$Item .specha/ m p o n s e meartwedu l o n g , / n ~ t d o t r ed line) and slow polid line) axesso G H ~SRb 100 GH r FSKTable 1: Measurcd FSR, f inesse, peak isolation and polarisation

peak splitting for two FP dev ices

The spectral responses obtained with light polarised along the fastand slow axcs fo r the two FP filters are shown in Fig. 2. The parametersmeasured over the entire spectral band of th e FP filters are summarisedin Table 1. For all the parameters , the range represents the maxim umdcviation from the average value. Fo r the 50 G H r filter, the polarisationisolation is limited by the depth of the FP transmission response, sincethe polarisation peaks are separated by nearly half of the FSR. Also,when the input polarisation is carefully adjusted, there is no ev idence ofpolarisation cross talk that would result from polarisation coupling inthe device (see Fig. 2% slow axis). Therefore, we conclude that thephotoinduced birefringence has no impact on the performance ofthe realised filters, although no special precaution s were taken to align

the fibre birefringence axes with respect to the incidence plane of th eW beam. The polarisation peak separation corresponds to a birefrin-gence of 1.9 x IO-'!Conclusion: We realised high-fin esse all-fibre Fabry-Perm filters in ahigh-birefringence fibre. The finesse of the realised filters was 20 and12 for FSR of 100 and 50 GHz. respectively, This required a totalphotoinduced refractive index change of the order 4 x IO-'. T hefilters showed a spectral separation of 26 GHz between the transmis-sion peaks fo r l ight polarised along the two principal axes. To realisef ilters with higher f inesse, FBGs w ith lower chirp would b e requiredto achicve higher reflectivity with the same refractive index change.The main limitation to the performance of the f ilters war coupling tocladding modes, which resultcd in an insertion loss of 4 dB .Applications of these filters include, for example, polarisation bea msplitters, frequency selective elements fa r multi-wavelength lasers, andDWDM wavelength referencing.Acknowledgment: We acknowledge the support of the Government ofCanada. the Government of Quebcc, TcraXion, and KVH.

0 EE 2003Electronics Letters Online No : 20030428DOI: IO. 049/e1:2003042XR. Slavik, S . Doucet and S. LaRochelle (Centre dh pt iq ue, phutoniqueet loser (COPLj. Department a/ Electr-ical an d ComputerEngineering. Uniwrsit l Luval, Quebec G I K 7P4, Canada)E-mail: [email protected]. Slavik: On leave from IREE A S CR, Prague , CZ.References

31 January 2003

SLAVlK. K., OllCET, S. and LAROCHELLE. 5.: 'High-performance all-fiberFabry-Perot filters with superim posed chirped Bragg gratings',J Lighmave Techno/., 2003 (i o be published)DOUCET. s., SLAviK, R.. and LAROCHBLLE. s.: 'High-finesse large bandFLbbry-Pemt fibre filter with supe rimp osed chirp ed Bragg gratings'.Electrun. LIII., 2002, 38, pp. 402403ERDOGAN, T , and MILRAHI. v : 'Characterization of UV-inducedbirefringence in photosensitive Ge-duped silica optical fibers', J OpI.Soc. A m . B, Opt. Phyf., 1994, 11, pp.2100-2105MARTIN. J., and OUELLET~E.F.: 'Novel writing technique of long andhighly reflective in-fibre gratings', Electrwn. Lerl., 1994.30, pp. 91 1-812

Two-dimensional optical power dis tributionof side-out-coupled radiation from t il tedFBGs in multimode fibreK. Zhou, G . Simpson, X. Chen, L. Zhang an d 1. Bennion

Two-dimensional optical power distribution of th e out-coupledradiatian fram UV-imctibcd tilted Ebrc Brugg gratings (TFBGs)in multimode fibre (MMF) has been side-detected with highspatial/npcchal rcs~ lufion. howing a near-identical radiation m odeprofile io that measured from the fibrc-cnd dcfcction method. Acomparative investigation of the radiation charactetistics of TFBGIfabricated in singlemode fibre (SMF) and MMF clcarly indicates thatthe radiation owcoupling is stronger and spatially innre confined inMMF. The unique spatial-to~ spectrvl encoding properry of thecoupling mechanism offers potential application in low-cost WDMdevices.

Introduction: Tilted fibre Bragg gratings (TFBGs) have been de mon-strated fo r applications in WDM channel monitoring [ I ] , EDFA gainflattening [2], polarisation discrimination [ 3 ] and optical sensinginterrogation [4 ] . The index distr ibution in a TFBG structure isgiven by:

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where 6 n is the amplitude of the index change, A is the normal periodof the grating, I nd x arc the co-ordinate axes defining the fibreorientation and 0 is the in temal h n g e t il t angle, as sho un in F ig. I h .The tilted srmcture enables light coupling between the m re m ade andthe radiation modes, generating desirable spectral characteristics forsome applications. In the fabrication, the tilted structure is introducedeither by rotating the mask in the phase-mask fabrication :systemor thefibre in the hvo-beam-holographic fabrication setup. To date, mostdemonstrated tilted grating devices have been fabricated in singlemodefibre (SMF). Owing to its Small COIC diametcr (-8 pm), the tiltedstruchire pem rbatio n over the cross-section of thc core i! of the ordcro f a few grating periods. This le ads to relatively weal: wavelengthselective radiation aut-coupling. In contrast, multimode fibre (MMF)ha s a much larger core diameter (50or 62 .5 p ) , cover ing t en s o f thetilted fringes, resulting in significantly stronger wavelength selectiveradiation out-coupling.

detector I 4 . . . . L

cylindricallens ..I:II

index

40 ,

- * < . , . , , , . , . , . , . ,-120 -110 -100 -90 -80 -70 4 0position, nm

Fig. 2 Side-our-coupkd radiaredpowerdisnihution of TFBCjibricared inMMFWwolength differenceof 5 n m between neighbour curves

p l a Wa b

Fig. 1 Schrmoric diogruni < r u r r u n g e m e n r und geomen-ita: definition o jS OUCIQlt la Schematic diagram of nidc-delcclian arrangementb Ccomcrrical definition of tilted grating ~tmctu re

Most published work to date has evaluated the spectral responses ofTFBGs by measuring the transmission, using the fibre-md detectionmethod and there has been no detailed investigation of the side-out-coupled radiation profilcs. Side-detection of radiation inodcs offersmany application-specific advantages. For example. in conjunction witha CCD array detector the function of side-tapping ligiit with highspectral resolution may be utilised to implement a spectrometer or toform the basis for low-cost WDM devices. In this Letter, we report, forth e first time to our ho wlc dge , the measurement of thc two-dimen-sional radiation power distribution of thc TFBG in MMF with highsparial/spcctral resolution employing the side-detection technique.

Erperimenl and re.szr1t.s:Fig. Io shows the side detection system formeasuring two-dimensional radiation power out-coupled from aTFBG. The grating is immersed in index-matching gel to maximisethe radiation mode coupling efficiency. A cylindrical l ens is used toefficiently collect the radiated light an to thc detector. A tunable laser(HP8168D) is used as a light source to e xamine the spatial-to-spectralrelationship of the out-coupled radiation. The detcctor is scanned inth c x an d y directions using two motorised translation stages. Themovement of the stages, the operation of the tunable laser and thepower reading of the detector are synchronised using a computer.

If radiation mod e out-coupling is spectrally distribut,:d along thegrating position within the fibre, an optical spectrum analyscr functioncan bc implemented. This function is of enormous utility since it ca neasily facilitate lov-cost WDM devices for applications in telecommu-nications and optical sensing. To this end we have systematicallyinvestigated the radiation power distribution of TFBGs in both SMFand M MF using the side-detection mcthod. With the system shown inFig. 1, the out-coupled radiation at different wavelengths ca n beeffectively measured along the TFBG. Fig. 2 shows a lypical opticalpower distnbution of a TFBG in MMF measured in one dimension(along the x-axis) over the radiation mode coupling range from 1480 to1560nm. When the launching wavelength changes the correspondingfocus point on the .r-axis moves, giving a clear spatial-to-spectralencoding relationship with a high conversion efficiency of-0.6 mm/nm.

bFig. 3 Contour imager of c o l l eoed radiated p o w r for tilted gratingsU MMPb SM F

We further examined the out-coupled radiation in two dimensions byremoving the cylindrical lens in the side-detection system and scanningthe detector in the x-y plane as shown in Fig. Ib . Fig. 3 shows theimages of the contour plot of the radiation power collected over onescanning process for two TFB Gs in MMF and SMF, respectively. Thesetwo TFBGs were fabricated and measured under the same conditions.The Figure shows clearly that the spatial distribution of the radiatedpower is more confined and stronger in the TFBG fabricated in M M Fthan in SMF, Fig. 4plots the maximum azimuthal angle (as defined inFig. I h ) of the radiation power distribution obtained from the twoTFBCs measured over the wavelength range from 1480 to 1540nm. tis cvidem from Fig. 4that the entire azimuthal angle disiribution rangeis much smaller for the TFBG in M M F (12-18") than for that in SM F(22-28"). This further confirms the better spatial confinement of th eout-coupled power from the TFBG in M M f . It is also noted from Fig. 4that the angular distribution of the MMF device output exhibits aparabolic characteristic giving a minimum angle at 1515nm .

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h mFig. 4 Maximirm uzimuthol angle o radiarion di.wibution sf TFBGs inMMF on d SMF

from f ibre endfmmfibre Side

2, nmFig. 5 Compunsun of rmn.mion profle o TFBG n ,MMFKadiation profile is invenedly ploned for purpose of companion

Fig. 5 comparatively plots the radiated spectral profiles measuredfrom the fibre-side and fibre-end, respectively, showing the anticipatedC X C e l l e n t agreement.

Conclusion: We have investigatcd the two-dimensional spatial distri-bution of the radiated power out-coupled from TFB Gs in both SMFand MMF using the side-detection method. The outstanding spatial-to-spectral cncading property of the side out-coupled radiationstrongly suggests that, in conjunction with a CC D array detector,low-cost WDM devices could be implemented using the TFBG. Ou rinvestigation reveals clearly that the out-coupled radiation is strongerand much more confined in the TFBGs fabricated in MME The two-dimensional measurement of out-coupled power shows excellentagreement with the spectrum obtained from the fibre cnd.

A c k n o d e d g w s : This work was carried ou t under the UK DTI-EPSRC LINK OSDA schemc and we acknowledge ou r EMPIREproject partners of BAE SYSTEMS, Indigo Photonics Ltd andDeutsch Ltd for their technical support an d useful discussions.

0 EE 2003 28 JMW 2003Electronics Letters Online ho: 20030426Dol: lO.l04Y/el:20030426K. Zhou, G. Simpson, X. Chen, L. Zhang and 1. Bennion (PhotonicsResearch Group, Elecrronic Engineering, Aston University,Birmingham, 8 4 7E7: Unired Kingdom)ReferencesI WESTBROOK. R.S., FEUEK. K . S . , RLYES. El., STEINYURZEL. P.,

LGCLETON. R.J . . ERNST, R.G., REITII. L . A . , and GILL. D M : Application offiber Biagg grating filtrrjtap module to a wavelen gth-locked low-chirpdirectly-modulated10G b j s Rz transmitter. OFC 2002, A naheim, CA,USA, Paper ThGG 49, pp. 680-682ELECTRONKS LE77ERS 17thApril 2003 Vol. 39

2 KASHYAP, R. , WYATT, R. , and C A M P B E L L , R . J : Wideband gain flattenederbium fibre amplifier using a photosensitive fibre blared grating,Electrun. Lei?.,1993, 29 , (2). pp . 154-156LABIDI. II.,DERARROS. C., LETIERON, R., and RIANT, I.: Slanted Bmgggrating with ultm-low polarization dependan t loss. OF C 2002. Anaheim,CA , USA, Paper TuQ5, . I13LIU. Y., ZHANG, L. , and BENNION, I.: Fabricating fibre ed ge fillws witharbitmy spectral response based on tilted chirped grating SINCIU~~S,Mea.?.ci. Technol.. 1999, 10 , pp. LI-L3

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Efficient ID-based authenticated keyagreement protocol based onWeil pairingKyungah Shim

The we8knerier of Q reccntly proposed Smarts ID-based authenticatedhuo~puss cy agreement p r O i O E O l ar e discussed. An rficient IU-busedauthmlicated key agrccment pmtoco l with the optimal number o fevaluations o f We d pairing is proposed.

Inrrodrtcrion: Several new cryptosyrtemr based on bilinear pairingshave been proposed recently. The existence of Weil pairing (o r Tatepairing) was thought 10 be a bad thing in cryptography. It w as shownthat thc discrete logarithm problem in supcrsingular curvcs w asreducible to that in an extension of the underlying field via Weilpairing [ I ] . This led to supcrsingular curvos being avoided forcryptographic use. This h a t i o n changed with the work of Joun[2], who gave a simple one-round tripartite Diffie-Hellman protocolbased on th e Weil pairing an supersingular cuwes. Thcreaficr, Bonchand Franklin [3] developed an ID-based encryption scheme using theWeil pairing. The scheme allows the replacement of a PKI with asystem where ones identity becomes the public key and a ke ygeneration centre helps generate users private keys. Smart [4] morerecently proposed an ID-based authenticated key agrcemcnt prolocolwhich also makes use of the Weil pairing and combines thc idcas ofBoneh and Franklin [3] with the tripartite Diffie-Hellman protocol ofJoux [2]. In this Letter, we show that Sm ar ts protocol doe s notprovide full forward secrecy. In addition, we propose an efficient ID-based two-pass authenticated key agreement protocol which has anoptimal number of evaluations of Weil pairing.Modrfied W e i l p o i r i n g : Let p be a pr ime such that p=2(mod 3 ) an dp = 6 q - 1 for some prime q > 3 . Le t E be a supersingular curvedefined by p=x+ I over Let H be a collision resistant hashfunction H ( 0 , I}*+n. Ou r ID-based scheme will make use of asimple algorithm for converting an arbitrary string (D E o , 1 ) * to Bp oint Q , D E E / F ~We refer to this algorithm as MapToPoinl,, [3 ]which works as follows:1. Compute y o=H ( /D ) and x0=(,$- l)(*p-)3(modp).2. Let Q = (xu. yo)t / E F Set Qm = 6Q. Then Q, has ordcr y asrequired.

Le t p q be the subgroup of E contains all elements of order 4. Th eWeil pairing an the curve E/F,,, is a mapping e: G,, x G,+ ,, Wedefine the modified Weil pairing2: Gqx G, + pq . 2(P. Q)= e ( P ,Q(Q))

where 4(.r, y )= i x , y), I# i is a solution of x ~ I =0 (mod p )and G, is a group of points with orde rq. The modified Wcil pairing thensatisfies the following prope rties:1. Bilinear: G(a.i? b .Q )=Q(~Q)h , . f o ra l / P .QEG , . . d , h E E .2. Nan-degenerate: There exists a point P t G, such that 2(.p P)# 13. Polynomial-time computable: i . (P. Q) is computable in polynomialtime.

Smartk ID-based prorocoi ond ir r wrokness: The ID-based keyexchange scheme is specified by three mndomised algorithms:Setup, Extract, KeyAgrecmcnt.No. 8 653

Two Dimentional Optical Power Distribution of Side-out Coupled Radaition From TFBGs in MM Fibre

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