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Nonlinearity in Holey Optical Fibres N,(G,l<, Hrotlcrick, P. Uciinctt, U. Ilcwak, 1 . M . Motiro, ,..I. Kiclinrtlson aiitl Y.U. Wcst Oi,tnelrc'tr[iiiic,s i\'cst~clr(:li Cerrtrc,, U/iive,:sity r ~ S o u t l r c i i t y i t o t i , Soutlrciniptoir SO I7 1 II.1

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Abstract: We discuss recent cxpci'iiiiciital results coriccming Llic iisc of niicros(cucturctl oplical litires as iiovcl nonliiicar iilcdia. Continuiiiii gciicratiori, short w;tvclcrigtli stililoii prop;igntion aiitl optical li.cqticiicy comb gciicratioii liavc been olitainctl tliatiks to ii comliinatioii of t i i iusui i l dispersive 1it'qwtics eiitl tlie smal l motlc area avai1aI)lc i n liolcy libres.

[Inley fihers ([IF) provide a iicw pmatligiii for the ti.ansversc guitlaiice of light. tinlike coriveritiorial optical fibers, which use dif'lcrcrit corc a i i d

cladding materials, 111;s ciiii IIC iiiadc lroiri ;I siriglc inaterial. aiitl guitl;iwc is provitlctl by the tlil'fcrcricc i i i effective iiitliccs of the coIc ;ind tlic 'liolcy' clatltling [1,21. 'L'his diflcrciicc arises f'rotii tlic inclusion of ;I central liigli iiidcx tlclcct, wliicli is sui-iwintictl by ii iiiinihci- of air holes. A typical lioley film is shown i n Fig. I . Whcii these liolcs x e iii ~;irigctl pxioclically. sucl i libres c m also posscss a coiiiplctc pliotoiiic liantlgzip 13 I dcperitliiig ot i the relative size of the air holcs ol'tllc period of [lie latticc.

Holey fibers are attl-aclivc lor pliotoiiic dcviccs liccnusc their optical propei-ties c m ~ > c ciigiiiccred over a i i i i icl i broat~er Ixirairictcr range during f , [ 'I i ~ r c ~ i t i o i i : . ' tliari convcntional tibrcs 141. l b r cxainplc, H I 3 caii liavc

anoinalous wavcgtiitlc dispersion at wavclciigllis less than I .3 microns iiiid be single-inotlc,. which is impossihlc i n conventiorinl step-iiitlex fibers 15 1. Sucli filiers of'lix the pntcritial for creating soliton fibel- 1 . . 'iscrs . tloiic at cvcn shorter wavelengths. 111 particular, doping ;I 111' with yttcrbiiim would allow casy fitbricatioii of sub-picosecond high rcpctitioii I J U I X soiirccs at I pin, which woiiltl find iisc in ii variety 01 areas.

As descrihctl diovc, tlie unusual optical propcrtics 01 HFs i.csult 1i.oni the prcseiicc of air holes iii the cladtlitig. It is then natural to ask how tlie holes affect the cllcctivc nonliiicarity of HF, md wlictlier or not it is possible to signiticantly alter i t via the liber desigli. lloley Iihcrs olTei- two clistiiict w;tys to ;icliicvc this. Firstly, hy appropriate clioicc of HP geoiiietry, the iriotle size ciiri be tailored by tis inuch iis three oi.tlers 01 rnagnitutle [cl], ii much larger rntigc tlian that possible i n conventional fiber types. Clianging the iriotle size alters [tic cllkctive nonlinearity of the liber 11y iiicrcnsing/~lccicnsiiig the intensity inside tlic fibel. wliich irtcreases/tlccrcases the rton1iiic;ir pliase cliarigc cxpcrieiicetl by liglit tluiing propngatioii. A m i r e direct way to afl'ect the iiotilincarity would tic Iiy using HI'S in which thc hole sp"cing is less than the waveleiigth of tlic liglit. In this regime, ii Iargc fraction 01 (lie motlc can pt-opigate i i i Llic ail. 171, ancl thus by idling the IiFs with ii suitablc iionliiiear inaterial tlic cflcctive nonliiic;irity could be signilicantly ciilianccd.

Ilolcy libre iiiodes typically Iiiivc ii non-circular sliape tliat reflects tlic way ir i which the air holes i n the cladding arc ari~angctl. This inakcs the iiienstu'criieiit o l the mode size i n tlie fibres tliflicult using colivcntiorial appro~iclies, which typically :issiiiiic a cii cularly syinmctric inotlal profile. To avoid this problerii, we used the iiietliotl ofI3oskovic et cil. 181 to iiicasurc the rioiiliiiearity of our fiber. 'l'his approach involves tlic use of liigli power dual frequency beat signals. ' l l ic fiber iionliiicai-ity creates spectral sidcb;iiids and the intensity mtio between tlic sigiial :iiltl tlie first side haiid gives the nonlinear pliase Cp= 2 ti)

y L 1% whcrc L is ttic efcctive fiber lciigth ant1 1' the signal power.. Using this iiietliotl, tlic eflectivc nonliriearity y ciin be calculated for the fibcr iiiidei. test. Note h i t y is proportional to the inaterial nonlinearity and the effective aIe;i o l the niotle. l'lie HI+ coiisitleretl here iii'c composed solely of silica, ant1 so n2 is kriowii. Hencc this inethod prnvitlcs ii dii-eel way of accurately iiicasuririg the cflectivc riiotlc area i n HFs. This tecliriiquc l w measuring tlie eflectivc iircii is particulal-ly uscltil for Hf's, as it makes iio

ussuiriptions about the inotlc shape. 'The mode of tlic 111: in I'ig. I has 6-loltl (hexagonal) syrnmctry, a~id so cannot be npproxiitiated Iiy a siiiiplc circularly-syriinictlic (iaussiaii. 'l'liis is paiticularly noticcable i r i the wings or at sliortcr wavelengths, whcrc tlic inotle hccoines iiiorc 1iex;tgoiial. More traditional methods, which rely on Craussim optics to cstiiiiate Llic area. woultl htil in these cases.

b I.

l i ' i ~ r e 1: A ' y ~ i c ~ ~ l llolcy fi"

. dt visible wavclciigtlls. Solitons h;ivc heen generated using 850nni liglit in it H t [cl], and this could he

0-7803-5947-W00/$10.0~2000 IEEE 59 1

'1'0 pelform tltc nonlinear iiieiisui"xitts we tised i i tliorlc- scedetl crbitiirt doped airiplilier chain. The input wits tlci.ivctl I'rom two tuneable l)17B lascrs couplctl togcthcr i i i i d thc resultant beat signal cxtcrnally motlulatctl to 11rodiicc 511s square ptilses at a iqictition rate of 200 k l b . After ainpliiicatioli the pcak powcI in the pulses was -lOOniW, ancl wc couplctl 50% 01' this power into tlic HP (length I . 175111). Wc also tcstctl (1.9 in ) of clispcrsioii-sttilled iibcr (DSP) to test thc procetlui,c. We rccortletl tlic output spectra at ii I~tngc of ~ O W C I X alltl ~ ~ t c a ~ ~ r c t l t k amount OT self 1)liase iriotlul;itioit f m r i the tlegrcc of spectral cnrichinciit, which gives LIS the nonliiie;ir phasc. l'tic results obtniiictl io(. both the USF aittl MI: aIc ~IIOWII i i t Fig. 2 aloiig with tltc icsiilts of least squares linear tits. As cxpecietl the nonlinear phasc inciciiscs lincarly with IIcak powcI antl the slope ol' tltc fit gives thc rioiilincacity, FOI. the holey libcr wc obtained y=i.56 10~"' W-'. This gives ail cfl'cctivc aica ol' !3,9ptc? i is coinpared to our tlteorctical estimatc o l 14Uni2. flericc

i d

i i i this HI:, tlic riiotle ai-ca is app~oxiniatcly four tiiiics sinaller than in thc DSP.

I'he fibre showii in Fig.1 has a zero-tlispcrsiori wavclcrtgtli at approxiinately I . I microns r9j. ancl i i s s~iowti atiovc. it also has ii sinall effective core arca. Iti addition, this lilire is single-inodc over iiii cxlreincly broad wavelength mrigc. As first deinonstratctl in Kef [ 101, this combinntion of propertics c m he used to geiieratc i i broadband contiriuiirri

g 5 % -1Ddll

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~. spcctruin. A range of efficient nonlinear pIoccsses occ~ir when this fib1.e is pumped ;it 1.1 inicroris, ne:ii' its zero dispersion wavelength. 'lhis produces a bluadband continuum spcctrunr I'roin at least 400.- 1850nin, :is S I I O W I I in Figure 3.

F'igiire 3: Continuum gciici'ation using the f W shown i n Figure 1.

III conclusion, we have siicccssfiilly measured the Kem nonlinearity for ii typical liolcy fiber, and we tirid that it is considerably cnh;inced by the small effective iireii of this HV. Note that inode areas iis small as Ipni' are possihle for diffemit liole iirrartgenicnts. The technique described here is ii fast easy way to ineasure the effective area of HFs. In the fiiturc, we hope to construct holey fibres out o f othcr more highly nonlinear materials as well using tlicsc fihres to construct soliton f i l m lasers.

I) J.C. Knight, b. Managan, T.A. Birks, P X J . Russoll, and J.P. de Sandro, Opt. Lett. 21, 1547 (1096). 2) T.A. Birks, J.C. Knight, and P.3.J. Kussell, Opt. Lett. 22, 961 (1997). 3) J.C. Knight, J. Uroeng. T.A. Rirks, antl P.St.J. Kussell, Sciciice 2x2, 1476 (1998). 4) T.M. Moriro, U.J. Richardson, N.G.R. Broderick, and P.J. Hennett, J. Light. 'l'ech. 17, (1999). 5 ) J.M. Scitior, (\em Optical Fiber Cominunications] (Freetitice Hall International Lttl, Maylaittls Aveirue,

Hempstead, UK, 1992). 6) W.J. Wadsworth, J.C. Knight, A. Ortigosa-Blanch, J . AlTiaga, H. Silvestrc antl P3t.J . Russell, Elect.

I.ctt. 36, 53, (2000). 7 ) 'Tanya M. Moiiro, U. J. Kichardson and P. J . Bennett, Elect. Lett. 35, 1188 (1999). 8) Boskovic, S.V. Chernikov, J.K. Taylor, I,. (ironer-Nielscn, and O.A. Levring, Opt. Lett. 21, 1966

(1996). 9) 1'. J. Bennclt, T. M. Monro and D. J. Richardson, Opt. Lett. 24, 1203 (1999). 10) J.K. Ranka, R.S. Wiiitlelcr, and A.J. Stentz, Conference on Lasers ancl Electro-Optics, OSA

Postdeadline Technical Digest (Optical Society of America, Washington DC, 1999), CPD8.

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