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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 11 — May. 23, 2011
  • pp: 10595–10603
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Investigation on multi-core fibers with large Aeff and low micro bending loss

Katsunori Imamura, Yukihiro Tsuchida, Kazunori Mukasa, Ryuichi Sugizaki, Kunimasa Saitoh, and Masanori Koshiba  »View Author Affiliations


Optics Express, Vol. 19, Issue 11, pp. 10595-10603 (2011)
http://dx.doi.org/10.1364/OE.19.010595


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Abstract

To realize large effective area (Aeff) multi-core fibers (MCFs), the design to suppress the cross-talk and the influence of the cladding diameter on the micro bending loss were investigated. As a result, the MCFs with large Aeff over 100 μm2 and low micro bending loss were successfully fabricated. The results indicate the importance of fiber design to realize large Aeff MCFs including fiber diameters, which largely affect the micro bending loss property. Additionally, MCF with large Aeff, low attenuation loss and suppressed cross-talk was successfully realized by optimizing the fiber design. The cross-talk properties could be estimated by the simulation based on the coupling power theory taking the influences of the longitudinal fluctuation of core diameter into account.

© 2011 OSA

1. Introduction

For the design of the large Aeff MCFs, the interference between cores and the micro bending loss are the most significant problems, which needs to be addressed. In this paper, we studied the design to suppress the increase of the interference between cores in case of the large Aeff fibers and found out that the Aeff can be enlarged without increasing the cross-talk between cores by keeping the macro bending loss constantly. The influence of the cladding diameter on the micro bending loss was investigated and the solid MCFs with large Aeff over 100 μm2 and low micro bending loss were actually fabricated by means of the adequate design with the sufficient cladding thickness. The low cross-talk properties were successfully realized by the design optimization and could be estimated by the simulation based on the coupled power theory taking the longitudinal fluctuation of core diameter into account.

2. Micro bending losses of multi-core fibers with different cladding diameters

2.1 Design of multi-core fibers with large Aeff and low cross-talk

At first, we investigated the design strategy required for enlargement of Aeff of MCFs. As the Aeff is increased, we need to pay much attention to the increase of interference between cores. We thoroughly investigated the relation between Aeff at 1550 nm, macro bending loss at 20 mm diameter at 1550 nm and core pitch required for the 100 km transmission. The target cross-talk was set to −30 dB at 1550 nm after 100 km transmission. As the result, we found out that the core pitch can be maintained even when Aeff was enlarged, if we set the macro bending loss conditions to be the same (Fig. 1
Fig. 1 Relationship between Aeff, macro bending loss and core pitch required to realize 100 km transmission with low cross-talk.
). By using this concept, we can design the large Aeff solid MCFs by shifting the cut-off wavelength (λc) and keeping the macro bending loss constant without increase of cross-talk between cores.

On the other hand, a high core density packaging technique is proposed by Koshiba et al. [6

6. M. Koshiba, K. Saitoh, and Y. Kokubun, “Heterogeneous multi-core fibers: proposal and design principle,” IEICE Electron. Express 6(2), 98–103 (2009). [CrossRef]

]. It was reported that the power-conversion efficiency can be decreased drastically by combining some cores with slightly different refractive index in one fiber. However, the reported Aeff was same as or smaller than conventional SMF. This method can be applied for the large Aeff MCFs in this study, so we investigated the effect of this technique on our large Aeff design and obtained the relation between core diameter and maximum cross-talk as shown in Fig. 2
Fig. 2 Cross-talk between neighboring cores (@ 1550 nm).
. The effective refractive index was changed just by changing the core diameter a little keeping the core Δ ( = 0.31%) for a simplification of fabrication process. The core pitch was set to 42 μm, which corresponds to the pitch of 75 μm in the case of identical cores. As a result, by changing the core diameter just by 0.2 μm, the cross-talk between neighboring cores (different cores) can be decreased under −60 dB. We optimized the core diameter to keep the cable cut-off wavelength (λcc) below 1530 nm and macro bending loss below 5 dB/m even when the core diameters are changed to +/− 0.2 μm (Table 1

Table 1. Design Properties of Each Core

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). From these calculations, it was confirmed that the solid MCFs with Aeff as 100 μm2 can be realized with high core density.

2.2 Examination of the influence of the cladding diameter on micro bending loss properties

Based on the simulation results described in section 2.1, the solid MCFs, which have high productivities, were actually fabricated by means of stack & draw method. Because the outer cores tend to be close to the cladding edge, the micro bending losses can be considered as one issue of the MCFs. So, we investigated the influence of the cladding diameter on micro bending loss properties. Two fibers with different cladding diameters (Fiber #1: 141 μm, Fiber #2: 215 μm) were fabricated to compare their micro bending loss properties. The cross sections of designed and fabricated fibers are shown in Figs. 3
Fig. 3 Cross section of designed fiber with air holes as markers.
and 4
Fig. 4 Cross sections of fabricated fibers (a) Fiber #1: 141 μm, (b) Fiber #2: 215 μm.
, respectively. The core pitch was 40 μm, which is almost same as the design, and the coating thickness were set to be equal for each fiber to keep the same influence of coating. To recognize the core arrangement, 3 air holes were arranged as markers at the region where they would not influence the optical properties.

Optical properties of two fibers are shown in Table 2

Table 2. Optical Properties of Fiber #1 and Fiber #2 (@ 1550 nm)

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. Because there are 3 kinds of cores, center core (core A) and other two neighboring cores (B, C) were measured. As shown in the attenuation loss spectra (Fig. 5
Fig. 5 Attenuation spectra of fabricated fibers (a) Fiber #1: 141 μm, (b) Fiber #2: 215 μm.
), loss increase in the longer wavelength region were observed in case of fiber with small cladding diameter (Fiber #1), especially in outer cores (B, C). On the other hand, in case of Fiber #2 with large cladding diameter, loss increases at longer wavelength were successfully suppressed in each core. As the result, the effect of large cladding diameter for the reduction of micro bending losses was clearly confirmed.

3. Improvement of cross-talk properties by optimization of core pitch

3.1 Design optimization of core pitch taking the core density into account

However the attenuation loss was drastically improved by means of sufficiently thick cladding as reported in the section 2, it still needs to be investigated because there was a problem, which is a higher cross-talk. As shown in Table 3

Table 3. Cross-Talk after 2 km Transmission

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, the cross-talks between core A and core B, C after 2 km transmission were relatively large (about −20 dB) compared to the simulation results (< −60 dB after 100 km). Additionally, the loss increase observed at the longer wavelength region in case of core A of Fiber #1, which has sufficiently thick cladding, can also be considered due to the higher cross-talk. It was assumed that this high cross-talk was caused by the smaller core diameter difference and smaller core pitch than the designed values. So, we optimized the design taking these factors into account, to improve the optical properties of the large Aeff MCFs.

About the core diameter difference, we tried to solve the problem by setting some margins at the design stage. On the other hand, optimum core pitch was reexamined from the point of views of the cross-talk and a core density. The core density was evaluated by normalized core numbers, which are defined as this expression,
normalizedcorenumbers=NMCFSSMFSMCF
(1)
which means the effective core numbers of MCFs within an unit area, namely, the area of conventional SMF with 250 μm coating diameter (Fig. 6
Fig. 6 Definition of the normalized core numbers of MCFs.
). The coating thickness of the MCFs was set to be the same as the conventional SMF. In Fig. 7
Fig. 7 Core pitch dependence of cross-talk after 100 km transmission and normalized core numbers.
, the core pitch dependences of the cross-talk and normalized core numbers are shown. Here, the cross-talk between the identical cores was calculated, which is the most dominant factor of the cross-talk. As a result, it was found that the cross-talk could be suppressed drastically by increasing the core pitch, while the influences to the normalized core numbers were relatively small. So, an enlargement of the core pitch could be regarded as an effective way to improve the cross-talk properties. This time, we set the core pitch as 46 μm because the cross-talk could be suppressed by 30 dB from that of Fiber #2, which had the core pitch of 40 μm.

3.2 Confirmation of the optimized design by fiber fabrication

3.3 Estimation of the cross-talk properties by means of coupled power theory

Though modeling of the cross-talk properties were based on the coupled mode theory in this work, new method for the simulation of cross-talk by applying the coupled power theory has recently been suggested [10

10. K. Takenaga, S. Tanigawa, N. Guan, S. Matsuo, K. Saitoh, and M. Koshiba, “Reduction of Crosstalk by Quasi-Homogeneous Solid Multi-Core Fiber,” in Proceedings of Optical Fiber Communications Conference (2010), paper OWK7. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2010-OWK7

]. It was described that more practical cross-talk characteristics can be explained by using the coupled power theory instead of the coupled mode theory, which calculate the interference between cores in case of the ideal situation with the constant core diameter in the fiber length. On the other hand, it is known that the longitudinal characteristics of fibers can be estimated by means of the bi-directional optical time-domain reflectometer (OTDR) method [11

11. K. Nakajima, M. Ohashi, and M. Tateda, “Chromatic dispersion distribution measurement along a single-mode optical fiber,” J. Lightwave Technol. 15(7), 1095–1101 (1997). [CrossRef]

]. This time, we investigated the longitudinal distribution of core diameter of the fabricated MCFs by using bi-directional OTDR method, and applied the obtained results to the estimation of cross-talk properties based on the coupled power theory. The results of the longitudinal distribution of mode field diameter (MFD) in case of Fiber #3 are shown in Fig. 11
Fig. 11 Longitudinal fluctuation of MFD @ 1550 nm (Fiber #3).
. The noises in the first and last part of figures are caused by the influence of the fresnel reflection at the each side of the fiber. We estimated the longitudinal fluctuations of the core diameters from these MFD distributions and applied them to the calculation of the cross-talk properties based on the coupled power theory in order to reproduce more practical situation than the former case [10

10. K. Takenaga, S. Tanigawa, N. Guan, S. Matsuo, K. Saitoh, and M. Koshiba, “Reduction of Crosstalk by Quasi-Homogeneous Solid Multi-Core Fiber,” in Proceedings of Optical Fiber Communications Conference (2010), paper OWK7. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2010-OWK7

] which supposed that the core diameters were changed constantly. The obtained results are shown in Fig. 12
Fig. 12 Fiber length dependence of cross-talk based on the coupled power theory taking the longitudinal fluctuation of core diameters into account (a) Fiber #1, (b) Fiber #2, (c) Fiber #3.
. The bending diameter of fiber was set to 280 mm and the twist rotated in every 20 m was considered. As a result, it was confirmed that cross-talk properties are successfully calculated by the combination of the bi-directional OTDR method and coupled power theory.

4. Conclusion

To realize large Aeff multi-core fibers, the design strategy to suppress the cross-talk between cores and the micro bending loss were investigated. The MCFs with the large Aeff over 100 μm2 and low micro bending loss were successfully fabricated and the obtained results indicate the importance of fiber design to realize large Aeff MCFs including fiber diameters, which largely affect the micro bending loss property. The low cross-talk after long distance transmission was realized by the design optimization and those properties were successfully explained by using the coupled power theory taking the longitudinal fluctuation of core diameter into account.

Acknowledgment

This research is supported by the National Institute of Information and Communications Technology (NICT), Japan under “Research on Innovative Optical Fiber Technology”.

References and links

1.

S. Inao, T. Sato, S. Sentsui, T. Kuroha, and Y. Nishimura, “Multicore optical fiber,” in Optical Fiber Communication, 1979 OSA Technical Digest Series (Optical Society of America, 1979), paper WB1. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-1979-WB1

2.

M. J. Gander, W. N. MacPherson, R. McBride, J. D. C. Jones, L. Zhang, I. Bennion, P. M. Blanchard, J. G. Burnett, and A. H. Greenaway, “Bend measurement using Bragg gratings in multicore fibre,” Electron. Lett. 36(2), 120–121 (2000). [CrossRef]

3.

Y. Huo, P. K. Cheo, and G. G. King, “Fundamental mode operation of a 19-core phase-locked Yb-doped fiber amplifier,” Opt. Express 12(25), 6230–6239 (2004), http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-12-25-6230. [CrossRef] [PubMed]

4.

N. N. Elkin, A. P. Napartovich, V. N. Troshchieva, and D. V. Vysotsky, “Diffraction modeling of the multicore fiber amplifier,” J. Lightwave Technol. 25(10), 3072–3077 (2007). [CrossRef]

5.

T. Morioka, “New generation optical infrastructure technologies: ‘EXAT Initiative’ towards 2020 and beyond,” in Proceedings of OptoElectronics and Communications Conference (2009), paper FT4.

6.

M. Koshiba, K. Saitoh, and Y. Kokubun, “Heterogeneous multi-core fibers: proposal and design principle,” IEICE Electron. Express 6(2), 98–103 (2009). [CrossRef]

7.

K. Imamura, K. Mukasa, and T. Yagi, “Multi-core holey fibers for the long-distance (>100 km) ultra large capacity transmission,” in Proceedings of Optical Fiber Communications Conference (2009), paper OTuC3. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2009-OTuC3

8.

C. Sethumadhavan, X. Liu, B. Zhu, and D. W. Peckham, “Transmission of a 1.2-Tb/s 24-carrier no-guard-interval coherent OFDM superchannel over 7200-km of ultra-large-area fiber,” in Proceedings of European Conference and Exhibition on Optical Communications (2009), paper PD2.6.

9.

G. Charlet, M. Salsi, P. Tran, M. Bertolini, H. Mardoyan, J. Renaudier, O. Bertran-Pardo, and S. Bigo, “72x100Gb/s Transmission over Transoceanic Distance, Using Large Effective Area Fiber, Hybrid Raman-Erbium Amplification and Coherent Detection,” in Proceedings of Optical Fiber Communications Conference (2009), paper PDPB6. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2009-PDPB6

10.

K. Takenaga, S. Tanigawa, N. Guan, S. Matsuo, K. Saitoh, and M. Koshiba, “Reduction of Crosstalk by Quasi-Homogeneous Solid Multi-Core Fiber,” in Proceedings of Optical Fiber Communications Conference (2010), paper OWK7. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2010-OWK7

11.

K. Nakajima, M. Ohashi, and M. Tateda, “Chromatic dispersion distribution measurement along a single-mode optical fiber,” J. Lightwave Technol. 15(7), 1095–1101 (1997). [CrossRef]

OCIS Codes
(060.2280) Fiber optics and optical communications : Fiber design and fabrication
(060.4005) Fiber optics and optical communications : Microstructured fibers

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: April 1, 2011
Revised Manuscript: May 8, 2011
Manuscript Accepted: May 9, 2011
Published: May 13, 2011

Citation
Katsunori Imamura, Yukihiro Tsuchida, Kazunori Mukasa, Ryuichi Sugizaki, Kunimasa Saitoh, and Masanori Koshiba, "Investigation on multi-core fibers with large Aeff and low micro bending loss," Opt. Express 19, 10595-10603 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-11-10595


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References

  1. S. Inao, T. Sato, S. Sentsui, T. Kuroha, and Y. Nishimura, “Multicore optical fiber,” in Optical Fiber Communication, 1979 OSA Technical Digest Series (Optical Society of America, 1979), paper WB1. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-1979-WB1
  2. M. J. Gander, W. N. MacPherson, R. McBride, J. D. C. Jones, L. Zhang, I. Bennion, P. M. Blanchard, J. G. Burnett, and A. H. Greenaway, “Bend measurement using Bragg gratings in multicore fibre,” Electron. Lett. 36(2), 120–121 (2000). [CrossRef]
  3. Y. Huo, P. K. Cheo, and G. G. King, “Fundamental mode operation of a 19-core phase-locked Yb-doped fiber amplifier,” Opt. Express 12(25), 6230–6239 (2004), http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-12-25-6230 . [CrossRef] [PubMed]
  4. N. N. Elkin, A. P. Napartovich, V. N. Troshchieva, and D. V. Vysotsky, “Diffraction modeling of the multicore fiber amplifier,” J. Lightwave Technol. 25(10), 3072–3077 (2007). [CrossRef]
  5. T. Morioka, “New generation optical infrastructure technologies: ‘EXAT Initiative’ towards 2020 and beyond,” in Proceedings of OptoElectronics and Communications Conference (2009), paper FT4.
  6. M. Koshiba, K. Saitoh, and Y. Kokubun, “Heterogeneous multi-core fibers: proposal and design principle,” IEICE Electron. Express 6(2), 98–103 (2009). [CrossRef]
  7. K. Imamura, K. Mukasa, and T. Yagi, “Multi-core holey fibers for the long-distance (>100 km) ultra large capacity transmission,” in Proceedings of Optical Fiber Communications Conference (2009), paper OTuC3. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2009-OTuC3
  8. C. Sethumadhavan, X. Liu, B. Zhu, and D. W. Peckham, “Transmission of a 1.2-Tb/s 24-carrier no-guard-interval coherent OFDM superchannel over 7200-km of ultra-large-area fiber,” in Proceedings of European Conference and Exhibition on Optical Communications (2009), paper PD2.6.
  9. G. Charlet, M. Salsi, P. Tran, M. Bertolini, H. Mardoyan, J. Renaudier, O. Bertran-Pardo, and S. Bigo, “72x100Gb/s Transmission over Transoceanic Distance, Using Large Effective Area Fiber, Hybrid Raman-Erbium Amplification and Coherent Detection,” in Proceedings of Optical Fiber Communications Conference (2009), paper PDPB6. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2009-PDPB6
  10. K. Takenaga, S. Tanigawa, N. Guan, S. Matsuo, K. Saitoh, and M. Koshiba, “Reduction of Crosstalk by Quasi-Homogeneous Solid Multi-Core Fiber,” in Proceedings of Optical Fiber Communications Conference (2010), paper OWK7. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2010-OWK7
  11. K. Nakajima, M. Ohashi, and M. Tateda, “Chromatic dispersion distribution measurement along a single-mode optical fiber,” J. Lightwave Technol. 15(7), 1095–1101 (1997). [CrossRef]

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