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Depressed-index-core singlemode bandgap fiberwith very large effective areaR. Jamier, P. Viale, S. Fevrier, and J.-M. Blondy

D)epartment ofPhotonics, Institut de Recherche en Commuanications Optiques et Microondes, UMR CNRS 6615University ofLimoges, 123 avenue Albert TIaomas, 87060 Limoges, France

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S. L. Semjonov, M. E. Likhachev, M. M. Bubnov, E. M. DianovFiber Optics Research Center at GPI, 38 Vavilov Street, Moscow, 119991, Rss'ia

V. F. Khopin, M. Y. Salganskii, A. N. GuryanovInstit ite of Chemistry ofjHigh Putrity Substances, 49 Tropinin Street, Nizhny Nogorod, 603950, Russia

Abstract: We report on an all-silica singlemode, ultra-broad-core bandgap fiber consisting of adepressed-index-core surrounded by cylindrical dielectric mirror. The mode area reaches 530 pm2at 0.866-pm-wavelength with propagation loss equal to 0.27 dB/m.© 2005 Optical Society of AmericaOCIS codes: (060.2280) Fiber design and fabrication' (060.2270) Fiber characterization

1. IntroductionLarge Mode Area Fibers (LMAFs) are currently used to enhance performances of fiber lasers. Holey fibertechnology allows fabrication of efficient fibers [1,2] but is still not as widespread as deposition processes. Moreconventional fibers prepared by deposition processes may also be used. It has recently been demonstrated thatdouble-clad high-order-mode fibers may lead to extreme mode area (2100 pM2 at 1.55 pm) provided a high-ordermode (LP07) is properly excited thanks to a long-period grating [3]. On the other hand, we have recently proposed aLMA Bandgap Fiber (BF) composed of a broad core (diameter D-22X) surrounded by alternating high- and low-index layers, forming a cylindrical dielectric mirror [4]. The fiber was fabricated by Modified Chemical VaporDeposition (MCVD) process. The mode area reached 520 pm2 at 1.55-pm-wavelength. The propagation loss wasmeasured around 1 dB/m. By incorporating a slightly raised-index core and manufacturing highly periodic layers,propagation loss has been decreased down to 10 dB/lkm at 1.064 pm in a 270-pm2-effective area BF [5]. We havealso demonstrated that BFs are less bend-sensitive than step-index fibers (with same mode field diameter) [5].One attractive feature of bandgap fibers is the possibility to guide light in a medium with an index lower than thesmaller cladding index. A depressed-core-index BF is thus a leaky waveguide, which gives large loss discriminationagainst higher-order modes. Consequently a trade-off between singlemodedness and core diameter can be found atthe cost of somewhat high propagation loss. In this communication, a depressed-core-index fiber with 40-pm corediameter is investigated for operation at near infrared wavelengths.

2. Design of the ultra-broad-core baudgap fiber

The refractive index profile (RIP) and associated electric field distribution of the designed fiber are shown in Fig. I a.1.2 0.012 1

i - ~~~~~~~~~~~~~~0.01

v 0.8 0.008 o

: 0.6 - 0.006 2 -.2 .1

5 0.4 0.004 x

/ \ X5 . -00LP11o 0. LP01,,

0 0-0.2 -0.002 0.01

-60 -40 -20 0 20 40 60 0.8 0.05 0.9 0.05Radius, pm Wavelength, pum

(a) (b)Fig. 1: (a) Refractive inldex profile anld electric field distributionL at 0.87 pLm of designLed fiber' (b) Computed conlfinlemenlt lossversus wavelenlgth for the two firs't modes

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The fiber is composed of a 40-prm-broad, fluorine-doped core with index difference relatively to pure silica levelAnl,o,re- 5.10-4 surrounded by a periodic cladding composed of 3 bilayers. The index contrast in the periodiccladding is An = 10.10-3. The width of the high- (resp. low-) index layers is 1.39 pm (resp. 6.45 pm). According toFig. la, the electric field distribution of the fundamental mode is well confined in the core and exhibits a quasi-Gaussian shape. At a wavelength of 0.87 pm, the computed mode area reaches 597 pM2. The confinement loss hasalso been computed versus the wavelength for the two first modes and plotted in Fig. lb. The minimal losswavelength is equal to 0.87 pm. The width of the baudgap at twice the minimal attenuation is 60 nm. Thefundamental mode is the lowest-loss mode. A singlemode behavior is thus expected for a sufficient propagationlength.

3. Experimental results

A fiber has then been prepared by MCVD process. The measured RIP, shown in Fig. 2a, is in good agreement withthe sought one, except a slight index step in the centre of the core attributed to evaporation of F during the collapseof the preform. The fiber outer diameter is 125 pm. The core is 40 pm in diameter. The average width and the indexdifference of the high-index layers are 1 p,m and 10.10-3 respectively.

1 _ - E - 1.46 _

0.75-1.456

0.5 X C °

1.4520.251

0

0 1.448 -20 0 20-60 -40 -20 0 20 40 60

Radius(pm) XX mm(a) (b)

Fig. 2: (a) Refractive index profile and electric field intensity distribution (solid line: computed using the actual RIP, sign:measured) of fabricated fiber at X 0.866 pm, (b) Observed near field intensity pattern at X = 0.866 pm

The modal behavior of the fabricated fiber was studied using a tuneable Ti:Sapphire cw laser. Example of near fieldintensity pattetn measured at X = 866 nm, shown in Fig. 2b, evidences the singlemode behavior of a 2-m-long pieceof fiber. The measured field pattern is in close agreement with the computed one (See Fig. 2a). Note the small indexstep in the core slightly distorts the field pattern. The calculated mode area is 530 pm2, slightly lower than thetheoretical one due to the distorted field distribution. The far-field pattern has been measured and plotted in Fig. 3.

1

0.75

0.5

0.25-

0-4 -2 0 2 4

Angle,

Fig. 3: Measuied fai field inlten:sity distributinnl

The divergenLce anlgle at 5%c-maximal inltenLsity is (2.1±0.1f)° givinLg a nLumerical aperture as low as 0.036±0.002.The loss spectrumn of the fiber has beenl measured usinlg the cut-back technlique anld reported inl Fig. 4.

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p =20cmp -0

___o o_

0.10.7 0.8 0.9 1 1.1 1.2 1.3 1.4

Wavelength, pm

Fig. 4: Measured loss spectrum of a 3-m-long piece of fiber

Two low-loss spectral regions are observed. First, when the fiber is led straight, the minimal attenuation reaches0.27 dB/m and 2 dBIm at 0.866 pm and 1.16 pm, respectively. The lowest-loss wavelength is in excellent agreementwith the sought one (0.87 pm). The width of the bandgap at twice the minimal attenuation is 108 nm, almost twicethe theoretical one due to slight discrepancy in the high-index layers' thicknesses. Wounding the fiber onto a 20-cm-radius reel adds 0.5 dB/m and I dB/m loss in the two spectral regions.In bandgap fibers, the bandgaps may be up- or down-shifted by scaling the refractive index profile or, equivalently,by scaling the fiber outer diameter during the drawing process. An outer diameter equal to 174 pm should allow forwaveguidance around 1.08 pm in a fiber exhibiting a Gaussian mode area as high as 1020 in2. Additionalexperimental results will be presented at the conference.

4. Conclusion

We have designed, fabricated and experimentally investigated a very large core bandgap fiber exhibiting a depressedindex core. This feature allows for increasing the mode area up to 530 pm2 at 0.866 pm (core diameter D-46X).Moreover, high-order modes experience high loss, making the fiber effectively singlemode for a 2-im-longpropagation length. It is anticipated that scaling the index profile could lead to 1000-pm2-mode-area fibers suitablefor operation at 1.08-p,m wavelength.

References

[I] J.C. Knight, T.A. Birks, R.F. Cregan, P.St.J. Russell, J.-P. de Saodro, "Large mode area photonic crystal fibre", Electronics Letters, 34, 1347-1348 (1998)

[2] W.S. Wong, X. Peng, J.M. McLaughlin, L. Dong, "Robust single-mode propagation in optical fibers with record effective areas", in Proc. ofConference on Lasers and Electro-Optics, Paper CPB 10, Baltimore, Maaryland, USA, 22-27 May 2005

[3] S. Ramachandran, J.W. Nicholson, S. Ghalmi, M.F. Yan, P. Wisk, E. Monberg, F.V. Dimarcello, "Robust, single-moded, broadbandtransmission and pulse compression in a record Aer (2100 pm2) higher-order-mode fiber", in Proc. of 3lh European Conference onOCommunication, Post Deadline paper Th4.4. 1, Glasgow, United Kingdom, 25-29 September 2005

[4] S. Fevrier, P. Viale, F. Gerome, P. Leproux, P. Roy, J.-M. Blondy, B. Dussardier, G. Monnom, "Very large effective area singlemodephotonic bandgap fiber", Electronics Letters, 39, 1240-1242 (2003)

[5] S.Fevrier R. Jamier, J.-M. Blondy, S. L. Semjonov M. E. Likhachev M. M. Bubnov, E. M. Dianov, V. F. Khopin M. Y. SaIganskii A. N.Guryanov "Low Loss Large Mode Area Bragg Fibre" in Proc. of 3 I"' European Conference on Optical Communication, Post Deadline paperTh4.4.3 Glasgow United Kingdom 25-29 September 2005