Understanding Optical Loss in Fiber Networks And How To Tackle It

Optical fiber is a fantastic medium for propagating light signals, and it rarely needs amplification in contrast to copper cables. High-quality single mode fiber will often exhibit attenuation (loss of power) as low as 0.1dB per kilometer.

Power or strength of the signal, will always be higher at the head end or central office of the network connection than at the customer end. Because there will be no light attenuation during the entire time period of the network connection. If the light attenuation is to great then performance suffers. So understanding and measuring these losses is really important in the network installation and testing.

For network planners, the bulk of the loss budget is spent between the final node and the customer’s network terminal. Splitters add significant loss to this part of the network - far greater than fiber connectors and other passive components.

Guidelines On What Loss To Expect When Testing Fiber Optic Cables

To be able to judge whether a fiber optic cable plant is good, one does a insertion loss test with a light source and power meter and compares that to an estimate of what is a reasonable loss for that cable plant. For network planners, the bulk of the loss budget is spent between the final node and the customer’s network terminal. Splitters add significant loss to this part of the network - far greater than fiber connectors and other passive components. When measuring the attenuation effects of these components, we use the terms insertion loss (IL) and return loss (RL).

Measure the attenuation effect

Insertion loss

Put simply, insertion loss is the measurement of light that is lost between two fixed points in the fiber. It can occur when optical fibers are spliced together, connected, or sent through additional passive network components. IL is often attributed to misalignment, contamination, or poorly manufactured connectors (ferrules) and has long been used to advocate fusion splicing. 

Another reason for fiber seemingly exhibiting high IL in fiber to the home (FTTH) networks is the route of the cable itself.  Reducing the number of components within the network also logically lowers the insertion loss, so consider this if you’re simply using your splice machine to fuse poor quality components together.

Return loss

Return loss also impacts the network’s performance but in a different way. It is the amount of signal reflected back towards the source due to an impedance mismatch – effectively, if this is too high, the laser within the network may stop transmitting correctly.

As with IL, when planning a network you need to factor in RL, and all equipment should be supplied with a certificate of specification stating both. It is important that networks are tested to ensure that there aren’t any unexpectedly high RL figures that indicate problems with equipment or fibers. Cable, specifically, can show high RL if a gap exists (such as fiber undercut) or if the fiber is broken. Contamination, torsion, strain or poorly seated connectors can also lead to high return losses.

Tips for reducing loss

Minimize tight bends that cause light to refract through the fiber cladding. If you need to coil fiber, keep the radius as large as possible.

Clean connector ferrules little and often - especially before and after testing - and always use the right tools and consumables.

Decide which is higher: your "power loss" budget or your cable inventory budget. Buying cheap fiber can create larger costs further down the line.

Avoid any undue stress on the fiber, particularly during installation. Push where possible and if a cable needs pulling, do not exceed the cable’s maximum tensile load.

Minimize the number of splices or connections in your network; if it means better planning or more innovative drop cables, the investment is probably well worth it.