When installing Fibre Optic cable care must be taken to ensure that cable is not bent, stretched or deformed.The best case is that the fibre core will break and be faulty, the worst case is that the fibre optic core will be deformed or damaged and cause signal distortion that results in intermittent faults.

Two Types of Fibre Optic Cable

In data networking, two type of cable are in common use – single-mode (SMF) and multimode (MMF). The core is embedding in a layer of cladding that helps to protect and strengthen the cable. The cladding also contains a reflective layer that is critical to laser propagation.

Fibre Breakage

The glass core in a fibre optic cable is fragile. It is slightly thicker than a human hair but made of glass (more rarely a plastic material may be used for multi-mode). Manufacturers have been able to design and manufacture the core material to be somewhat elastic and resilient to bending. Single mode uses special type of glass that is extruded into a solid medium to protect it. MMF is made from glass but being thicker (at 62.5µm compared to 9µm) is more robust. Because of this SMF is more sensitive to breakage than MMF.

When the core is stretched or bent beyond a certain point the core will physically break into two parts. The cladding and buffer around the cable core helps to prevent damage. The glass core has some movement to protect against movement, thermal expansion/shrinking, and for installation.

When the physical integrity is compromised two outcomes are possible. The best case is that two pieces of core are not physically aligned and no laser light will propagate. Something that is broke can be located and fixed. 

A less common case is that core materials will be partially aligned after the break and pass a partial signal. The network may or may not work due to the the drop of laser power. Laser receivers require

Intermittent operation may happen as the cable expands/shrinks with temperature, vibration or movement and core loses alignment or the gap expands to reduce laser power to a non-functional level.

A fibre optic cable relies on complete internal reflection and this scenario still supports that but a substantial amount of signal power can be lost at this interface plus reflections at the break/air interface will have secondary effects.

Cracking of the Fibre Core

It is also possible that the fibre core might be damaged instead of a break. For SMF the glass might only crack and cause imperfection in the medium that would reduce signal propagation and causes reflections. For MMF the is more likely to be damaged by the flexing and cause power loss. In the case graded fibre, this is quite damaging to the signal as it will be badly distorted.

Best and Worse Cases

When working with fibre optic patch leads, it is common for people to trap them in door or stretch them by pulling on them. While patch leads are designed to be more flexible compared the cabling used in risers, it is still susceptible to breakage in the best case. “Best case” means that the cable doesn’t work. Worst case is when the fibre core is partially damaged and likely to cause intermittent operation.

Intermittent is much worse than broken. Another reason for replacing cables is that the fibre connectors are dirty, scratched or faulty and new cables will improve the power level received. Cleaning the cables and SFP connectors can also resolve intermittent problems.

In my opinion, thats why replacing the cable often fixes intermittent problems in network. It is very hard to test or confirm a faulty fibre optic patch lead. A faulty patch lead can cause waveform deformation through propagation distortion or power loss through a cracked or misaligned core. And according to the temperature fluctuation, ambient vibration by fans and coolers, or just being moved the network might have problems.

EtherealMind’s First Law of Networking is “Always Check the Physical”. This is why.

Case Study

The image below shows a fibre optic patch where the rack door has been pressing on the patch where the strain relief ends and rather obviously bend radius has been severely compromised. While patch cables are fairly robust this cable is faulty when the door is closed but working when the door is open.

Thats worse than just broken.