Expect the Best, Prepare for the Worst —
I’ve been fortunate to participate in multiple emergency and planned fiber optic restorations. In this article I address the 2 main categories of restorations, and how they can impact optical attenuation and dispersion values with legacy installations.
Restorations are either proactively planned or reactive emergency situations. In either case, it’s important to have a restoration plan that has been designed to address the many variations of your cable plant as well as one that will provide benefits in time, expense, and safety. I recommend the development and continued updating of your restoration plans. In particular, the restoration plan should identify the possible locations where a restoration may occur, and address aerial, underground, and ducted locations.
Several years ago, when developing a custom emergency restoration training program for Matanuska Telephone Association (MTA) in Alaska, I spent time with their staff to review their network and the challenges caused by the terrain and environment. In doing so we looked at worse case scenarios that impacted their customers and the telecom networks that serve them.
Our benchmark was the 1964 Alaskan earthquake, also known as the Good Friday earthquake. It measured 9.2 on the Richter scale and caused ground fissures, collapsing structures, and tsunamis, resulting from the earthquake. Wikipedia states that it is the second largest documented earthquake in history and caused 131 deaths.
Using this as a benchmark, we created a plan to remedy a similar situation. We had to address the terrain and physical routing MTA serves as we created our restoration plan for an earthquake of this magnitude. So, we worked on the assumption that ring topologies would be limited and critical infrastructure such as bridges and roads may not be usable. Given the scope of damages that could occur with a future earthquake, the need for emergency communications would be critical — whether POTs lines, cellular, or dedicated lines.
Therefore, our plan included 4 fundamental elements:
Element 1: Aerial, underground, and ducted, repair options
Element 2: Options for retrievable and non-retrievable slack restorations
Element 3: Staff and training requirements
Element 4: Review of OSP restoration products
Thankfully, MTA created this type of proactive plan — because in November 2018, a 7.1 earthquake hit the region. Despite that, the MTA network survived with minimal damage due to planning that addressed the worst-case scenario.
Retrievable and Non-Retrievable Slack
During emergency restoration planning, it’s important to address both retrievable and non-retrievable slack scenarios. Retrievable slack scenarios are forgiving as the manufacturer and type of fiber will be the same and the largest variation is the intrinsic tolerances of the fibers.
These tolerances will vary based on the age of the fiber and the date of the installation. As you can see from Figure 1, the tolerances of optical fibers have improved over the years. In fact, .1 dB splices may not be obtainable when splicing older fibers. In the case of emergency restorations where speed is the priority, splices with higher splice losses may be allowed. In the future these will be respliced to optimize attenuation.
With non-retrievable slack scenarios, the largest impact occurs where a new section of optical cable must be installed between the 2 damaged cables. Just as we would use dual wavelength bi-directional testing to confirm the fiber break locations, we are also looking for any stresses that would increase the fiber’s attenuation beyond the actual break locations.
This can be some distance from the actual break point should the cable have been directly buried. In this case there would be 3 sections to identify: B would be the actual break point, while the A and C locations would be identified by the extent of the damage or stresses in either direction.
Beware of optical attenuation in a non-retrievable slack scenario that is caused by optical mismatches and fiber tolerances. In the case of optical mismatches, the replacement section may not use the same optical fiber, type, and manufacturer. These are both possible problems of increased attenuation and even chromatic dispersion (CD). Of the 3 major fiber manufacturers in North America, each uses their own technique for manufacturing preforms used to manufacture the optical fiber. For example, Corning fibers are manufactured using the outside vapor deposition (OVD) process, while OFS uses inside vapor deposition (IVD) and Prysmian uses either advanced plasma and vapor deposition (APVD) or plasma activated chemical vapor deposition (PCVD). Each of these fiber types have a different index of refraction (I.R.) but also melt at different temperatures when splicing. It is important that the splicers know which fiber types are being spliced so they can select the best settings for their fusion splicer.
A second concern would be if the replacement section was a different fiber type. For example, if the original fiber was a standard ITU-T G.652 and the new section was a G.655 (or other), this would affect the optical dispersion values as well as the optical attenuation. In a few examples the cutoff wavelength may be higher than the legacy fiber being installed. Most fibers have a cut off wavelength of 1260 nm but several are as high as 1450 nm. If transmitting a 1310 nm signal over a fiber with a cutoff wavelength of 1450 nm, the fiber would now propagate multiple modes causing modal dispersion. It is important that the technicians make sure the correct fiber type is installed to prevent optical mismatches in non-retrievable restorations.
Help From the OTDR
The key instrument for locating faults is the Optical Time Domain Reflectometer (OTDR). Not only does it measure optical attenuation of the fiber and splices, but also provides distance measurements making it ideal for acceptance testing, troubleshooting and restorations in the outside plant.
It is the key instrument to locate faults as well as the extent of any damage preceding or following the actual break point. These minor stresses are normally due to macrobending or microbending stresses. For this reason, OTDR tests should occur at dual wavelength 1310 nm/1550 nm wavelengths. If the attenuation of a splice or span increases at 1550 nm nanometers this confirms that the internal fibers are being stressed. Locating the location and removing the cause of the stress should rectify the problem in most cases.
If the cable structure has been damaged and continues to stress the internal fibers then a decision must be made whether to cut the cable back to remove the stressed location(s) or to accept the higher attenuation values. Hopefully during the initial testing of the faulty span, the technician can access the original test reports and OTDR graphs to use as comparisons at both wavelengths.
After the restoration, the OTDR is used to document total length, splice performance, and cable distances. These measurements are usually performed at the standard single-mode wavelengths of 1310 and 1550 nanometers, but in some cases such as with DWDM networks, measurements are made at 1550 and 1625 nanometers. After OTDR testing is complete and all parts of the network meet specifications, the end-to-end attenuation measurements are performed using an optical loss test set. At this time the system documentation is completed and the as-built diagrams are updated.
Another consideration is where the new splice location will be placed for safe access. Locating it some distance from where the actual damage occurred provides an opportunity to address future needs such as cable slack storage, aerial to underground transitions and obstructions. It also could be the best splice closure for future access and fiber protection.
Identifying the Extent of the Damage
KEY POINT: Performing bi-directional dual wavelength testing identifies the extent of the damage before and after the break point.
If mid entries are being used, this is an excellent time to inspect and confirm that excess slack is properly stored and routed in trays (or baskets) along with the correct routing of transition tubes and individual fibers to the appropriate splice tray. Care must be taken to dress, route, and secure, the transition tubes as the manufacturer recommends without stressing the internal fibers.
Poorly or improperly sealed closures can allow the ingress of water. “Flash” testing of the splice closure will confirm whether the closure is properly sealed to prevent water ingress which can induce future freezing that places tremendous stresses on the fibers.
Practice Makes Perfect
Periodic review of your restoration plans is recommended along with periodic testing of dark fibers. Performing these tests in winter and summer can help identify worse case fluctuations caused by temperature variations. Using the OTDR’s overlay capabilities can help easily identify changes in the attenuation levels.
While these are suggestions, the key is to have the right restoration plan in place for YOUR network and customers. Then, periodically review the choices you have made with your engineering, construction, and installation/repair staff.