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A New Metric for Optical Fiber Attenuation

Posted on Jul 27, 2022
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Fiber attenuation is an important physical parameter in the field of fiber optic communication. The primary industry standard for measuring fiber attenuation performance has always been maximum individual cabled fiber attenuation. However, as fiber optic technology has evolved, maximum fiber attenuation and actual fiber loss have become significantly different, requiring a more representative attenuation specification. The new metric for optical fiber attenuation is more practical and can be used for fiber optic cable performance measurement and network design.

Fiber Attenuation Basis and Specifications

Fiber Attenuation is the loss of signal strength or light power as the light signal is transmitted. It's caused by intrinsic factors (absorption, scattering, bending, etc.), or extrinsic components (connectors, splices, splitters, etc). The degree of attenuation determines the maximum distance that the fiber can transmit the signal.

Currently, maximum individual cabled fiber attenuation is mainly used as the specification of fiber attenuation performance. This is because, in the past, optical fiber point discontinuities and attenuation non-uniformities make the attenuation values of some fibers significantly higher than others in the same cable. And average cabled fiber attenuation specifications are not recognized by network designers and end users because averaging doesn't adequately define the fiber attenuation distribution, especially when the cable contains multiple high attenuation fibers. Therefore, manufacturers and industry standards regulate the maximum fiber attenuation, which is the attenuation value of the highest attenuating fiber.

However, over the years, the fiber optic cabling industry has witnessed significant improvements in fiber attenuation and measurement systems of optical fiber and cable performance. As a result, the actual individual fiber attenuation differs significantly from the specified maximum individual fiber attenuation, making the maximum performance of the optical link unattainable. Obviously, new metrics are required that more precisely define the attenuation of fiber-optic cables and the "as built" link.

Link Design Attenuation

The actual cabled fiber attenuation distribution was randomly sampled to obtain average attenuation values for 2 to 20 fibers of varying lengths. As shown below, up to 20 cable links were analyzed to identify the number of spans required for fiber attenuation to approach a steady state. The following figure demonstrates that fiber attenuation gradually stabilizes after only eight concatenated links, generating a technically plausible average attenuation.

Concatenated Cable Attenuation

To obtain a more accurate specification of the fiber attenuation, additional statistical processing is taken. The steady state point presents a limit for the link design attenuation. As shown below, link design attenuation is indicated by the green line, representing a 99.9% confidence threshold against the specification for eight concatenated links. This result reflects the fact that link design attenuation can optimize network design and keep a feasible guard band.

Fiber Attenuation

To be consistent with existing standards, link design attenuation values can be determined using 20 spans. The Monte Carlo simulation shows that eight spans are sufficient to achieve a stable state of the link design attenuation. More spans do not result in a significant increase in capability. Besides, fewer spans contribute to better alignment of the specification with the access network. The following table illustrates that link design attenuation can ensure higher performance for the benefit of customers.

Link Design Attenuation (dB/km) Current Max Cabled Fiber Attenuation in ITU-T G.652.D
1310 nm 1550 nm 1310 nm 1550 nm
0.33 0.19 0.4 0.35

Compared to maximum cabled fiber attenuation, link design attenuation has several distinct advantages. First, longer regeneration distances in long-haul routes can be achieved. Furthermore, FTTH deployment can serve larger areas by increasing radial distances between the optical line terminal (OLT) at the central office and the optical network terminal (ONT) at the house. This allows today's high-performance fibers to achieve the fullest design capabilities in applications.

Conclusion

Nowadays, maximum fiber attenuation is limited to achieving the full performance of fiber optic systems. The new metric, link design attenuation, can effectively determine the loss budget and improve the performance of optical links. Link design attenuation provides a new solution for network design.

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