Insertion Loss

What is Fiber Insertion Loss?

Insertion loss is the amount of energy that a signal loses as it travels along a fiber cable link. It is a natural phenomenon that occurs for any type of transmission—whether it's electricity or data. This reduction of signal, also called attenuation, is directly related to the length of a cable—the longer the cable, the greater the insertion loss. Insertion loss is also caused by any connection points along a cable link (i.e., connectors and splices).

Causes for Insertion Loss

Exceeding the insertion loss for a given application can be a result of inferior quality components or poor field termination such as connector misalignment or dirty fiber end faces. In fact, end-face contamination remains the primary cause of insertion loss. If the system has undergone moves, adds and changes without properly cleaning and inspecting the fiber end faces, what may have been clean at the time of installation may now be dirty and add insertion loss to the channel.

Other causes can include miscalculations when determining the insertion loss budget or the chance that something changed during installation, such as a connection being added or the link being longer than planned. Post-installation issues can also be caused by the customer upgrading to a higher-speed application that has more stringent insertion loss requirements than what was used in the original insertion loss budget calculations.

Insertion Loss Testing

Because insertion loss is the primary performance parameter that impacts the ability of a fiber link to support a given application, it is required for fiber certification testing per industry standards. Optical Loss Test Set (OLTS) provides the most accurate insertion loss measurement on a link by using a light source on one end and a power meter at the other to measure exactly how much light is coming out at the opposite end. Both TIA and ISO standards use the term “Tier 1” to describe testing with an OLTS.

Some projects will also specify extended “Tier 2” testing that requires an Optical Time Domain Reflectometer (OTDR) to characterize the loss of individual splices and connectors. An OTDR can measure the loss of specific events such as breaks, bends, splices, and connectors. This can allow technicians to determine the cause and exact location of the loss event. An OTDR provides a graphical trace that characterizes each loss even along a fiber link. It achieves this by transmitting light pulses into a fiber and measuring the amount of light reflected from each pulse. An OTDR is also typically required for calculating the return loss of specific connectors and is therefore essential for applications like short-reach single-mode where maximum insertion loss is based on the number and return loss of connectors in a channel.

It’s important to note, that the use of an OTDR in Tier 2 testing does not replace the OLTS because the total insertion loss measurement achieved with an OTDR is an inferred calculation that does not necessarily depict the total loss that will occur on a link once it is live. So even if a specification requires extended Tier 2 testing, Tier 1 insertion loss testing via an OLTS is still required. In these scenarios, it is also recommended that OTDR characterization be done before OLTS insertion loss testing since it is required for final proof of compliance.