Outdoor Cabled Optical Fiber Lifetime Reliability

Outdoor Cabled Optical Fiber Lifetime Reliability
AEN 92, Revision: 1

This Applications Engineering Note (AE Note) addresses the reliability of optical fibers in outside
plant cables comprising a black, medium density polyethylene (MDPE) jacket.  It addresses
both fiber reliability and cable component (materials) reliability.  Many factors also affect cable
reliability and this AE Note addresses these factors as well.

Fiber Reliability

Mechanical reliability models predict structural failure rates for optical fibers under given stress
conditions.  Corning supports the Two Region Power Law Model which describes crack growth
for low stress applied over a long time (i.e. deployed fiber in a cable) and crack growth for fast
events such as proof testing. These results, coupled with manufacturing test data are used to
determine safe stress guidelines.  When adhering to Corning fiber and cable manufacturer’s
recommended practices, modeling indicates reliable fiber operation for 30 years or more.
Furthermore, Corning Cable systems is aware of deployed systems continuing to operate
reliability for over 30 years.

Cable Component Reliability

The materials most impacted by aging are the cable jacket and the optical fiber itself.  Outside
plant cable jacket materials used today are not exceptionally different from those used in early
installations and were chosen for their proven long-term performance in copper cable designs.
Material and cable manufacturers continue to use industry approved aging tests to support
material selection.
Fiber designs and manufacturing techniques have improved significantly over the years and
fibers that were once cutting edge may not be capable of meeting the demands of today’s
higher performance systems.  Performance parameters such as polarization mode dispersion,
now considered critical for high data rate systems, were not even specified or measured for
fibers manufactured before or around 1994.  Other optical parameters previously specified have
also improved.  Therefore, care must be taken to not mistakenly label a cable’s performance as
degraded when it could just simply be that the legacy fiber was never designed for the specific
application or current specified performance.

Installed Cable Fiber Performance Factors

The performance of installed cabled fiber depends on a number of factors listed below including
the cable environment, the type of installation, cable design, and the cable material.  Factors
which may affect certain installations are: structural cable damage caused during or after initial
installation, improper cable design, the splicing quality and splice closure work, exposure to
periods of severe environmental loading or certain chemicals, or exposure to temperature and
humidity conditions outside of specified performance levels.

Direct Buried Installations
- Type of soil (e.g., rocky, sandy, etc.)
- Depth of cable
- Underground hazards (e.g., rodents, pressure points)
- Soil settling or erosion
- Chemicals if present (contamination from hydrocarbon or other chemical spills)
- type of chemical(s)
- concentration
- duration of exposure
- Cable structural damage due to improper installation, dig-ups, lightning strikes, rodents,
   or other means
- Cable design (e.g., armored or all-dielectric, and type of armoring if present)
- Residual cable stress
- Improper hardware solution or improper use of correct hardware
- Violation of tension or minimum bend radius

Duct Installations

- Chemicals (e.g., use of incompatible pulling lubricants, or presence of other chemicals)
- Residual cable stress
- Number, type and size of other cables present in the duct
- Damage from subsequent cable pulls into the same duct
- Damage from installation:
- Improper duct construction
- Exceeding the tensile rating and fiber strain rating
- Violation of tension or minimum bend radius
- Routing in manholes
- Frozen water in duct (from not using duct plugs) resulting in compression failures
- Improper hardware solution or improper use of correct hardware

Aerial Installations

- Environmental loading conditions (e.g., ice/wind)
- Periods of severe environmental loading (e.g., hurricane, ice storm, etc.)
- Damage from rodents, debris (e.g., falling tree limbs), or shotguns
- Improper hardware solution or improper use of correct hardware

Conclusion
There are many considerations when addressing cabled fiber lifetime performance.  Assuming
the appropriate cable is selected for the application and the cable is installed correctly, modeling
and real world experience indicates reliable operation for approximately 30 years for outside
plant cables with black, MDPE jackets.
The best gauge of the health of a system is typically the optical properties of the cabled fiber
when compared to industry standards at time of purchase.  Measurements such as multiwavelength attenuation, Polarization Modal Dispersion, and Chromatic dispersion, as well as bidirectional OTDR and Optical Spectral Analyses can be taken in order to determine the ability of
a link to support higher data rates.  While this testing can not guarantee against future failure
due to cable aging, it can confirm the performance and potential upgradeability of the link or
integration of an older cable into a new telecommunications system.