Single-Mode Fiber with Ultra-Low-Loss and Large-Effective-Area

Lei Zhang, Hongyan Zhou,Jun Wu, Shengya Long, Ruichun Wang, R. Matai

Key Laboratory of Optical Fiber and Cable Manufacture Technology, Wuhan 430073, China

Yangtze Optical Fiber and Cable Joint Stock Limited Company, Wuhan 430073, China

+86-027-6788-7602 · zhanglei@yofc.com

Abstract

A new communication fiber with ultra-low loss (typical value: 0.165dB/km) & large effective area (typical value: 110μm²) at 1550nm wavelength is proposed. Compared to conventional single-mode fiber, it can contribute more benefits for high speed long reach communication system, especially for long-haul terrestrial network. Its optical characteristics are discussed, including splicing, macro-bending, micro-bending and cable temperature cycling test and attenuation change during cabling process.

Keywords: ultra low loss, large effective area, G.654, long-haul, terrestrial cable.

1.  Introduction

In recent years, long distance and large capacity are urgent demands for transmission system. Especially for China, the first batch long-haul networks (most of them are standard G.652.D) are installed in 1990s and have to retired due to the 25-year’s lifetime. So how to choose the next generation fiber for long-haul, terrestrial cable is urgent problem for both network operators and optical companies.

First of all, let us define the crucial characteristics of next generation fiber. To increase capacity in long haul transmission system, the transmission speed must be speed up, so 400G or beyond 400G is the visible trend for the next generation communication. Since coherent receiver and digital signal process (DSP) technique have been widely used in 100G and 100G beyond communication, dispersion and polarization mode dispersion (PMD) can be compensated in electric field in a certain range and will not be the limitation for next generation fiber and cable. But transmission loss and nonlinear effect are still the most important limitation factors for long reach transmission. It is known that lower loss can increase the output optical signal-to-noise (OSNR) and Q-factor of transmission system for the same distance, or extend the transmission reach. So ultra low loss is designed for long distance application^[1] and larger effective area (A_eff. for short) can support higher optimum launch power into the fiber which is good for improving OSNR. To achieve better system performance, we combine the ultra low loss and large A_eff. into one fiber and it will be a more perfect fiber for the next generation fiber.

It’s well known that ultra low loss and large effective area fiber, G.654.B and G.654.D fiber are widely used in transoceanic cable system, but the terrestrial cable installation, application and service environments are quite different from submarine cable. So it requires more performance optimization and design for terrestrial ultra low loss & large effective area fiber. A new category G.654.E has been strongly proposed in 2013, which is used in terrestrial large capacity transmission system. The new ITU-T proposal of G.654.E for terrestrial cable will be released before the end of 2016.

In this paper, we introduce the design and characteristics of ultra low loss & large A_eff fiber for long-haul terrestrial cable. And the key fiber and cable performance for terrestrial application are also discussed.

2.  How to quantitative calculate the contribution of A_eff. & loss to the system

As we discussed above, lower attenuation and larger effective area can improve next generation system’s performance. Main fiber manufacturers use Fiber-Of-Merit (FOM) to quantitative evaluate the contribution of loss and A_eff.[2-4]. The FOM is usually easily predicted as a function of fiber parameters.

（1）

Here, n₂, L, L_eff mean nonlinear refractive index, span length, and effective length, respectively.

According to the FOM calculation, we obtain the FOM value of different fiber in Table 1 and the table also shows the great advantages of fiber which combine ultra low loss and large effective area together than only ultra low loss fiber with 80μm² A_eff. or larger A_eff. with common attenuation.

Table 1.  FOM values of fibers with different attenuation & A_eff.

3.  Fiber design and manufacture

Figure 1. Schematic of fiber RI profile structure

As shown in Figure 1, compared with conventional F doped outer cladding structure’s ultra low loss & large effective area fiber, pure SiO2 is used as our fiber’s outer cladding, which is possible to make our fiber products more cost competitive and easier to control the drawing process.

Table 2 shows the possible G.654.E specification and our ultra low loss and large effective fiber characteristics. We can see our fiber can meet or even exceed current most strict specification proposal.

Table 2. G.654.E proposals and our G.654 fiber characteristics

*TBD: still to be discussed in the ITU-T Q5 group.

4.  Fiber and cable performance
4.1  Fiber attenuation

Figure 2.  Attenuation spectrum comparison of standard G.652.D fiber with ultra low loss & large A_eff. fiber

Lower attenuation can reduce the amount of repeaters and lower the network maintenance cost for long-haul network. So continuous reducing the fiber attenuation is the eternal target for fiber manufacturers. Figure 2 shows the attenuation spectrum comparison of standard G.652.D with our ultra low loss & large A_eff.. fiber. The standard G.652 fiber loss is 0.2dB/km at 1550nm wavelength while our ultra low loss fiber can reach 0.165 dB/km@1550nm.

As shown in formula 1, the fiber loss composition

（2）

The contribution of Rayleigh scattering loss to the overall spectral-loss is given as:

（3）

Where A is the Rayleigh contributor and B represent the combined effect of random and periodic micro-bending, waveguide imperfection, and other scattering losses.

It’s well known that Rayleigh contributor A have direction relation with concentration factor and density factor. In the theory, we can low by reducing fiber dopant and low by reduce fiber fictive temperature.

Impurity is mainly OH groups. And the OH absorption has influence on the fiber loss only around the water peak center.

Infrared (IR) absorption is very small at short wavelength region.

Macro-bending loss is dependent on the fiber index profile, the radius of curvature of the fiber axis, and the ratio of the operating wavelength to the cut-off wavelength.

Loss spectrum model decomposition of ultra low loss & large Aeff. fiber is shown in Figure 3. The Rayleigh scattering loss of ultra low loss & large Aeff. fiber is significantly lower than standard G.652.D as shown in Table 3.

Figure 3. Loss spectrum model decomposition of ultra low loss & large A_eff. Fiber

Table 3. Loss spectrum composition of standard G.652.D and ultra low loss fiber

In the loss spectrum, we can evacuate the contribution value of different parts and the information can help us further optimize the fiber loss.

4.2  Splicing performance

Currently, most of terrestrial cable use G.652.D fiber. So if we choose ultra low loss and large A_eff. fiber as next generation long-haul fiber, the compatible problem is a key point. The direct reflection is the splicing performance of ultra-low loss & large A_eff fiber splicing with standard G.652.D fiber.

There are several factors can affect the splicing loss, but the mode field mismatch is the key of splicing loss. As shown in Figure. 4, the typical splicing loss for A_eff 110μm² with standard fiber is lower than A_eff 130μm² with standard fiber, which is the main reason for we choose 110μm² as the more appropriate effective area in next generation fiber.

Figure 4.Splicing performance for standard fiber with different A_eff fiber (110μm2 and 130μm2)

Figure 5.Self splicing performance

And the self splicing performance of ultra low loss fiber with A_eff -110μm² and standard G.652.D are also tested.  Because relative larger mode field help G.654 fiber reduce the self splicing loss, ultra low loss and A_eff  -110μm² fiber has lower splicing loss than standard G652.D fiber, and the results are shown in Figure.5. Considering most of splicing in the long-haul network are self splicing with one type fiber, so it can obviously reduce the link loss if we take ultra low loss & large effective area fiber as next generation fiber.

4.3  Macro-bending loss

The terrestrial cable installation and application environment are more complicated than submarine cable. Terrestrial cable has to pass some corners or have some extra-length in cable cabinet, so we must make sure the fiber has better macro-bending performance than submarine fiber.

The trench assisted structure is a mature bending insensitive design method in G.657 fiber [5]. In our fiber design, we optimize a reasonable trench volume to achieve a better bending resistance performance.  As shown in Figure.6, our ultra low loss & large effective area fiber have excellent macro-bending than standard single mode G.652.D fiber, even exceed the G.657.A1 specification.

Figure6.  Macro-bending comparison

At the same time, the excellent macro-bending performance can remain much attenuation/power margin which will ensure stable overtime while materials are aging and guarantee the fiber performance stability under 25-year lifetime.

4.4  Micro-bending loss

Figure7. Micro-bending loss comparison

Micro-bending performance is very important factor for cabling design and cabling processing. Excellent micro-bending performance can reduce the difficulty of cable design and cabling process and also can improve the cable performance’s stability in different application conditions under extreme environment. Figure 7 depicts the micro-bending comparison of our ultra low loss & A_eff  -110μm² fiber with standard single mode

G.652.D fibre, we can find our fiber have a excellent micro-bending performance, the typical micro-bending induced loss under all wavelength is lower than 0.5 dB/km.

4.5  Cable TCT performance

As we discussed above, the land temperature is not as stable as ocean, so the terrestrial cable have to experience more severe temperature change and still keep the link loss stable. Cable temperature cycling Test (TCT) is tested for checking the attenuation change with temperature. 12 core ultra low loss & A_eff  -110μm² fibers are put in one GYTA cable tube and Figure 7 depicts the schematic of  our cable structure

Figure 8. The schematic of cable construction

Due to our fiber’s excellent micro-bending and macro-bending performance, during the temperature changes from -40 to +70 centigrade, our ultra-low loss & A_eff  -110μm² cable’s attenuation changes are less than 0.01dB/km, which is far better than IEC and ITU-T specification.

Figure 9. The fiber attenuation change with temperature: 12 colors represent the loss changes of 12 core fibers

4.6  able attenuation change during the cabling process

Figure 10. The fiber attenuation change during the cabling process

For conventional G.654 fiber, relative larger mode will bring more micro and macro-bending loss sensitivity and the fiber attenuation will fluctuate upward during the cabling process. In the worst case, the attenuation after cabling will be 0.01dB/km higher than fiber original attenuation before cabling.

In Figure 10, the different color bars are one fiber’s attenuation change during the cabling process, the left blue bar is the original fiber attenuation on the reel before cabling and orange bar is the attenuation after the cable. The attenuation after cabling at 1550nm keeps the same level or even lower than the original fiber attenuation before cabling due to the excellent micro-bending performance. So our fiber is more easier for the cabling process control.

5.  Conclusions

In this paper, we introduced a single-mode fiber with ultra-low loss (0.165dB/km) and large effective area (110μm²) at 1550nm wavelength. Considering the complicated cable installation and application environment on the land, the fiber compatibility, micro & macro-bending, TCT and attenuation change during the cabling process are tested, The new fiber show excellent performance in all tests and can ensures the good performance for the cabling link in actual system use. The fiber is the visible choice for the next 400G and 400G higher communication.

6.  References

[1]     Chengliang Zhang, Yufei Chen, Runhan Wang.et al. “Demonstration of Real-time 1.2 Tb/s Transmission over 4 Types of Fiber with Nyquist WDM Prototype System”. 2014OFC, W2A.21,( 2014)

[2]     N. Bergano, OFC 2009, SubOptic 2010(2010)

[3]     G. Charlet, ECOC 2010, paper We.8.F.1(2010)

[4]     V. Curri, A. Carena, and G. Bosco,et. “Fiber Figure of Merit Based on Maximum Reach,” OFC 2013, OTh3G.2 (2013)

[5]     Q. Han et al., “Large-Scale Production Technology for G.657 Fiber,” Proc. 59th IWCS, pp. 114-116 (2010)

[6]     Runhan Wang, Shengya Long, Hongyan Zhou et al.  “Single Mode Optical Fiber with Large Effective Area for Long Haul Applications”. International Wire & Cable Symposium，Proceedings of the 61th IWCS, (2012)

7.  Authors

Lei Zhang is of chief engineer of R&D center of YOFC. He received his M.S degree in functional materials from Dalian University of Technology, China. He joined Yangtze Optical Fiber and Cable Co. Ltd. (YOFC) in 2010. His current research interests are bending insensitive, low loss and ultra low loss G.654 fiber. His mailing address: zhanglei@yofc.com

Hongyan Zhou is an engineer of R&D center of YOFC. She received her M.S degree in optical fiber communication from Beijing University of Posts and Telecommunications, China. She joined YOFC in 2012. Now she works in the new fiber product R&D and fiber test and transmission system test field. Her mailing address: zhouhongyan@yofc.com

Jun Wu is an engineer of R&D center of YOFC. He received his M.S degree in Material Science from Wuhan Institute of Technology, China. He joined YOFC in 2013. His current research interest is ultra-low loss G.652 fiber. His mailing address: wujun_02737@yofc.com

Shengya Long is the manager of R&D department in R&D center of YOFC. He received his M.S degree in Material Science from Wuhan University of Technology, China. He joined YOFC in 2005. He has been related to fiber product R&D and process modification and improvement. His mailing address: longshengya@yofc.com

Ruichun Wang is the general manager of R&D center of YOFC. He specializes in preform making and drawing process, and has led many R&D projects for improvement of optical fiber products. He received his M.S. degree in Material Science from Zhejiang University, China. His mailing address: wangruichun@yofc.com

Raadjkoemar Matai is the vice general manager of manufacture center and optical fiber technology director of YOFC. He received his BS degree in Chemical Engineering at the technology university in Den Haag (Holland) in 1984. Since 1992 he is working in YOFC. He was involved in numerous product and process development projects related to single and multiple mode optical fibers. His mailing address:R.Matai@yofc.com