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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 19 — Sep. 13, 2010
  • pp: 19456–19461
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Ultralow bending loss fibers with higher-order mode strippers

Ngoc Hai Vu, Jin-Tae Kim, Eun-Sun Kim, Chang-Hyun Jung, Kyung-Goo Lee, and In-Kag Hwang  »View Author Affiliations


Optics Express, Vol. 18, Issue 19, pp. 19456-19461 (2010)
http://dx.doi.org/10.1364/OE.18.019456


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Abstract

We propose and demonstrate bend-insensitive fibers equipped with higher-order mode strippers. The mode stripper is realized by filling a section of air holes with epoxy to attenuate any higher-order modes that are excited at fiber junctions and are confined by the air holes surrounding the core. We found that the higher-order modes are well suppressed with 5 cm-long epoxy columns. An ultralow bending loss of 0.025 dB/turn at a bend diameter of 10 mm, together with single-modeness, is experimentally demonstrated in a bend-insensitive fiber with six air holes 16 μm in diameter.

© 2010 OSA

1. Introduction

Fiber-to-the-home (FTTH) applications are growing as the number of high speed internet subscribers increases worldwide. When standard single-mode fibers (SMFs) are used for FTTH, the fiber cable must be installed very carefully to avoid small bends in the fiber path that can cause significant signal loss [1

1. I. Sakabe, H. Ishikawa, H. Tanji, Y. Terasawa, T. Ueda, and M. Itou, “Bend-Insensitive SM Fiber and Its Applications to Access Network Systems,” IEICE Trans. Electron. E88-C(5), 896–903 (2005). [CrossRef]

]. For conventional single mode fibers, the smallest bending diameter allowed is around 30 mm, and this restriction causes high cost and large space for the fiber installation. Several designs of bend insensitive fibers (BIFs) have been proposed to reduce the bending loss of single-mode fibers. These fibers are characterized by depressed cladding, low index trench [2

2. P. R. Watekar, S. Ju, and W. T. Han, “Single-mode optical fiber design with wide-band ultra low bending-loss for FTTH application,” Opt. Express 16(2), 1180–1185 (2008). [CrossRef] [PubMed]

4

4. L.-A. de Montmorillon, F. Gooijer, N. Montaigne, S. Geerings, D. Boivin, L. Provost, P. Sillard, “All-Solid G.652.D Fiber with Ultra Low Bend Losses down to 5 mm Bend Radius,” OFC’09, San Diego, CA, paper OTuL3 (2009).

], hole-assisted cladding [5

5. D Nishioka, T Hasegawa, T Saito, E Sasaoka, and T Hosoya, “Development of Holey Fiber Supporting Extra Small Diameter Bending,” SEI Tech. Rev. 58, 42–47 (2004).

7

7. T. W. Wu, L. Dong, and H. Winful, “Bend performance of leakage channel fibers,” Opt. Express 16(6), 4278–4285 (2008). [CrossRef] [PubMed]

], or a nano-engineered ring in the cladding [8

8. M.-J. Li, P. Tandon, D. C. Bookbinder, S. R. Bickham, M. A. McDermott, R. B. Desorcie, D. A. Nolan, J. J. Johnson, K. A. Lewis, and J. J. Englebert, “Ultra-Low Bending Loss Single-Mode Fiber for FTTH,” J. Lightwave Technol. 27(3), 376–382 (2009). [CrossRef]

].

2. Fabrication of the higher-order mode stripper

For our experiments, we selected a hole-assisted fiber with six relatively large air holes fabricated by Optomagic, Inc.(Ansan-si, Korea), as shown in Fig. 1. The Ge-doped core has a diameter of d = 9.0 μm. The air holes have a diameter of D = 16 μm, and they are located at the triangular lattice with a lattice constant of Λ = 23 μm. The cutoff wavelength was ~1240 nm, and the mode field diameter was 10.2 μm at 1550 nm. The attenuation was 0.38, 0.342, 0.300, 0.203, and 0.208 dB/km at 1260, 1310, 1385, 1550, and 1620 nm, respectively. This fiber exhibits very low bending loss due to the large hole diameter, but there exist several higher-order modes or inner-cladding modes that are strongly guided by the six air holes.

The mode stripper was realized by filling the air holes with an epoxy for a certain length as shown in Fig. 2
Fig. 2 Schematic diagram of cladding mode stripper.
. Since the refractive index of epoxy is higher than that of silica, the inner-cladding modes cannot be confined by total internal reflection. Instead, it is only weakly guided by Fresnel reflections at the silica/epoxy boundary [10

10. J. Hsieh, P. Mach, F. Cattaneo, S. Yang, T. Krupenkine, K. Baldwin, and J. A. Rogers, “Tunable microfluidic optical-fiber devices based on electrowetting pumps and plastic microchannels,” IEEE Photon. Technol. Lett. 15(1), 81–83 (2003). [CrossRef]

]. If the epoxy length in the ends of fiber is long enough, the inner-cladding modes will be completely diffused into the outer cladding to be scattered or absorbed, and only the core mode can propagate in the BIF. The fundamental core mode also can be slightly affected by high index column in terms of optical loss or dispersion. But such effects will be negligible if the epoxy column is only a-few-cm long. In the mode stripping region, the fiber does not have low-index barriers and bend loss occurs as in conventional single mode fibers. Therefore, we assumed that the stripper section is protected from any sharp bends.

The epoxy was injected into the air channels using a dipping method. The BIF was dipped in a long tube filled with epoxy as shown in Fig. 3(a)
Fig. 3 Fabrication method of the mode stripper. (a) Dipping of a bend-insensitive fiber in a tube filled with epoxy, (b) side view of epoxy-filled fiber, (c) SEM image of cross-section of the mode stripper, and (d) the epoxy infiltration length as a function of epoxy tube length.
. Then, the epoxy was injected into air channels automatically by epoxy column pressure and capillary force. This method allows simultaneous treatment of multiple fibers. In our experiment, UV Epoxy (NOA 60, Norland, Inc.) with a refractive index of 1.56 and a low viscosity (300 CPS) was chosen for fast injection and curing. After about 12 hr of dipping, the epoxy was cured under UV radiation, and the filling length (l) was measured from optical microscope images (Fig. 3(b)). In Fig. 3(d), the epoxy length is plotted as a function of the dipping depth, showing linear dependence. Figure 3(c) is a scanning electron microscope image of the fiber cross section in which the six air holes are filled with epoxy.

3. Suppression of modal interference by the mode stripper

Higher-order mode suppression by the mode stripper was also demonstrated in far-field patterns of the fiber output. The SLD was replaced with a 1550-nm narrow-linewidth laser in the setup shown in Fig. 4(a). The SMF at the back end of the BIF was removed, and the far field of the BIF output was observed using an infrared vidicon camera. Figure 5
Fig. 5 (a)-(f) Far field patterns at the end of the fiber for epoxy lengths of l = 0, 10, 20, 30, 40, and 50 mm, respectively.
shows the far fields for the epoxy length changing from 0 to 50 mm. Many speckles and asymmetric pattern appeared in non-treated BIFs of Fig. 5(a) due to the interference of multiple spatial modes. The field pattern gradually changed to the symmetric Gaussian shape of the fundamental mode as the epoxy length increased. The mode pattern in Fig. 5(f) was found to be very stable under any strain or stress applied to the BIF, whereas such a perturbation produced blinking and reshaping of the pattern in other cases. The results are in good agreement with those of the experiment shown in Fig. 4.

Finally, the bending loss of the BIF was measured using narrow line-width lasers and an optical power meter. Two or three turns of fiber loops were made for a bend diameter in the range of 8 to 14 mm. Each measurement was repeated three times and the average values were recorded. Figure 6
Fig. 6 Bending loss of the BIF at various wavelengths.
shows the results obtained for the various wavelengths. The bending loss increased with the wavelength as mode confinement becomes weaker. At 1550 nm, the loss was about 0.025 dB/turn for a bend diameter of 10 mm, which is comparable to the lowest value ever reported. We believe that the loss can made even lower simply by increasing the air hole sizes. It is an important advantage of our scheme that there is no specific requirement for the holes sizes or layouts to maintain single modeness of the fiber.

The suggested BIF design with epoxy infiltrated at both ends has the additional benefit of hole sealing. The epoxy will prevent the air holes from being polluted by external gases or liquids. It is known that the bending loss of hole-assisted fibers dramatically increases at low temperatures due to water condensation on the hole surfaces. This problem can be solved effectively if the air holes are filled with dry nitrogen before applying epoxy. Although the required mode stripper length of 5 cm seems to be acceptable when applied to fiber connectors, it may be further reduced by optimization. Proper selection of the coating material surrounding the fiber cladding, as well as optimization of the refractive index and absorption coefficient of the epoxy, are being considered to enhance the attenuation of cladding modes.

4. Conclusion

Acknowledgement

This work was supported by the Regional Research Center for Photonic Materials and Devices, Chonnam National University, and the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2008-331-C00115).

References and links

1.

I. Sakabe, H. Ishikawa, H. Tanji, Y. Terasawa, T. Ueda, and M. Itou, “Bend-Insensitive SM Fiber and Its Applications to Access Network Systems,” IEICE Trans. Electron. E88-C(5), 896–903 (2005). [CrossRef]

2.

P. R. Watekar, S. Ju, and W. T. Han, “Single-mode optical fiber design with wide-band ultra low bending-loss for FTTH application,” Opt. Express 16(2), 1180–1185 (2008). [CrossRef] [PubMed]

3.

J. M. Fini, P. I. Borel, M. F. Yan, S. Ramachandran, A. D. Yablon, P. W. Wisk, D. Trevor, D. J. DiGiovanni, J. Bjerregaard, P. Kristensen, K. Carlson, P. A. Weimann, C. J. Martin, A. McCurdy, “Solid Low-Bend Loss Transmission Fibers using Resonant Suppression of High-Order Modes,” ECOC’08, Brussels, paper Mo.4.B.4 (2008).

4.

L.-A. de Montmorillon, F. Gooijer, N. Montaigne, S. Geerings, D. Boivin, L. Provost, P. Sillard, “All-Solid G.652.D Fiber with Ultra Low Bend Losses down to 5 mm Bend Radius,” OFC’09, San Diego, CA, paper OTuL3 (2009).

5.

D Nishioka, T Hasegawa, T Saito, E Sasaoka, and T Hosoya, “Development of Holey Fiber Supporting Extra Small Diameter Bending,” SEI Tech. Rev. 58, 42–47 (2004).

6.

Y. Bing, K. Ohsono, Y. Kurosawa, T. Kumagai, and M. Tachikura, “Low-loss Holey Fiber,” Hitachi Cable Rev. 24, 1–5 (2005).

7.

T. W. Wu, L. Dong, and H. Winful, “Bend performance of leakage channel fibers,” Opt. Express 16(6), 4278–4285 (2008). [CrossRef] [PubMed]

8.

M.-J. Li, P. Tandon, D. C. Bookbinder, S. R. Bickham, M. A. McDermott, R. B. Desorcie, D. A. Nolan, J. J. Johnson, K. A. Lewis, and J. J. Englebert, “Ultra-Low Bending Loss Single-Mode Fiber for FTTH,” J. Lightwave Technol. 27(3), 376–382 (2009). [CrossRef]

9.

D. Boivin, L.-A. de Montmorillon, L. Provost, and P. Sillard, “Coherent Multipath Interference in Bend-Insensitive Fibers,” IEEE Photon. Technol. Lett. 21(24), 1891–1893 (2009). [CrossRef]

10.

J. Hsieh, P. Mach, F. Cattaneo, S. Yang, T. Krupenkine, K. Baldwin, and J. A. Rogers, “Tunable microfluidic optical-fiber devices based on electrowetting pumps and plastic microchannels,” IEEE Photon. Technol. Lett. 15(1), 81–83 (2003). [CrossRef]

OCIS Codes
(060.2400) Fiber optics and optical communications : Fiber properties
(060.4005) Fiber optics and optical communications : Microstructured fibers

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: April 20, 2010
Revised Manuscript: June 17, 2010
Manuscript Accepted: June 22, 2010
Published: August 30, 2010

Citation
Ngoc Hai Vu, Jin-Tae Kim, Eun-Sun Kim, Chang-Hyun Jung, Kyung-Goo Lee, and In-Kag Hwang, "Ultralow bending loss fibers with higher-order mode strippers," Opt. Express 18, 19456-19461 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-19-19456


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References

  1. I. Sakabe, H. Ishikawa, H. Tanji, Y. Terasawa, T. Ueda, and M. Itou, “Bend-Insensitive SM Fiber and Its Applications to Access Network Systems,” IEICE Trans. Electron. E88-C(5), 896–903 (2005). [CrossRef]
  2. P. R. Watekar, S. Ju, and W. T. Han, “Single-mode optical fiber design with wide-band ultra low bending-loss for FTTH application,” Opt. Express 16(2), 1180–1185 (2008). [CrossRef] [PubMed]
  3. J. M. Fini, P. I. Borel, M. F. Yan, S. Ramachandran, A. D. Yablon, P. W. Wisk, D. Trevor, D. J. DiGiovanni, J. Bjerregaard, P. Kristensen, K. Carlson, P. A. Weimann, C. J. Martin, A. McCurdy, “Solid Low-Bend Loss Transmission Fibers using Resonant Suppression of High-Order Modes,” ECOC’08, Brussels, paper Mo.4.B.4 (2008).
  4. L.-A. de Montmorillon, F. Gooijer, N. Montaigne, S. Geerings, D. Boivin, L. Provost, P. Sillard, “All-Solid G.652.D Fiber with Ultra Low Bend Losses down to 5 mm Bend Radius,” OFC’09, San Diego, CA, paper OTuL3 (2009).
  5. D Nishioka, T Hasegawa, T Saito, E Sasaoka, and T Hosoya, “Development of Holey Fiber Supporting Extra Small Diameter Bending,” SEI Tech. Rev. 58, 42–47 (2004).
  6. Y. Bing, K. Ohsono, Y. Kurosawa, T. Kumagai, and M. Tachikura, “Low-loss Holey Fiber,” Hitachi Cable Rev. 24, 1–5 (2005).
  7. T. W. Wu, L. Dong, and H. Winful, “Bend performance of leakage channel fibers,” Opt. Express 16(6), 4278–4285 (2008). [CrossRef] [PubMed]
  8. M.-J. Li, P. Tandon, D. C. Bookbinder, S. R. Bickham, M. A. McDermott, R. B. Desorcie, D. A. Nolan, J. J. Johnson, K. A. Lewis, and J. J. Englebert, “Ultra-Low Bending Loss Single-Mode Fiber for FTTH,” J. Lightwave Technol. 27(3), 376–382 (2009). [CrossRef]
  9. D. Boivin, L.-A. de Montmorillon, L. Provost, and P. Sillard, “Coherent Multipath Interference in Bend-Insensitive Fibers,” IEEE Photon. Technol. Lett. 21(24), 1891–1893 (2009). [CrossRef]
  10. J. Hsieh, P. Mach, F. Cattaneo, S. Yang, T. Krupenkine, K. Baldwin, and J. A. Rogers, “Tunable microfluidic optical-fiber devices based on electrowetting pumps and plastic microchannels,” IEEE Photon. Technol. Lett. 15(1), 81–83 (2003). [CrossRef]

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