Optical Return Loss -An Important Test

When talking about fiber, Optical Return Loss (ORL) is one of the number one things that is tested. This is the ratio of light that is reflected back to the source of the light over the entire fiber link. This is always measured in dB (decibels) and will typically be a negative number. The closer the number is to zero, the higher the reflectance (a poor connection). There are many different reasons that can cause this in a fiber optic system. We will look at some of these and give you a better idea of what to do to help you have the least amount of return loss in your network. We will touch base on what tool is used to measure this, as well as some different types of loss such as insertion loss and what is reflectance in a link.

What contributes to Optical Return Loss?

Here is where we discuss several of the factors that contribute to high return loss. In fiber optics, it is imperative that you make sure you are always cleaning the fiber optic connectors before you mate them together. Dirt in a network is the number one cause of return loss, and it is the easiest to prevent if done properly. We all know that wiping a fiber connector on a shirt may seem like a viable and quick option; however, it is not the proper or correct way to clean your connector. With how small the core is of singlemode fiber, even the tiniest dust particle can wreak havoc on your optical signal. Using 99% reagent grade isopropyl alcohol and lint free tissues is one way to clean properly. There are also one click cleaners that assist in cleaning the end face of a connector.

Another thing that will have high return loss is a broken piece of fiber. Some pieces of fiber can have a small crack that will give you very little insertion loss. This can actually show on return loss and help to avoid a bigger headache down the line.

Poorly mated connectors are another cause of high return loss that can lead to other problems if not corrected. If the connectors are not fully inserted into the mating sleeve, this can lead to air between the two end faces, which in turn will lead to higher loss. This type of problem can have low insertion loss but will be a problem if the loose connectors become misaligned or even disconnected. If the mated connectors become disconnected, it can cause a complete loss of signal that will take your network down. So when you see a high return loss at a pair of mated connectors this should be checked out.

What is Insertion Loss?

Insertion loss is the measurement of the light that is lost between two fixed points in the fiber. This can occur when optical fibers are spliced together, mated, or when they are sent through other passive components. You can have low insertion loss but high return loss. This is important in a network since most networks have several points that may have both insertion loss and return loss. One main spec that is always asked when dealing with optical connectors is what is the typical insertion loss of the connector? This refers to how much light should you lose when two connectors are mated together using a coupler. By knowing this, it will assist you in putting your link budget together. The link budget is the amount of loss that is allowed over a run of fiber including splice points, mated connectors, and the loss in the fiber itself. Knowing that the typical insertion loss on a mated pair is 0.5 dB, allows you to incorporate that into a budget. So if you have three mated pairs in your link then you know that you will have at least 1.5dB of loss just in the connection points alone.

Optical Reflectance

When looking at your overall Optical Return Loss, you have several events that have happened in a link. These events as individual occurrences are known as the reflectances. So a mated pair of connectors is a reflectance that happens in a network. The type of polish that a connector end face has will affect the amount of return loss and insertion loss for that particular event. When you are looking at these, the closer to zero your measured test result, the poorer the connection is, which results in more insertion loss and higher return loss. Below are some different types of reflectance and the amount of reflection in the negative dB levels.

   • Fiber end with flat cleave: -14 dB

   • Good multimode PC (Physical Contact) connection: -35 dB or lower

   • Good single mode UPC (Ultra Physical Contact) connection: -55 dB or lower

   • Good APC (Angled Physical Contact) connection: -60 dB or lower

   • Good fusion splice: -60 dB or lower

So we know that the closer to zero a reflectance event is, the worse it is for return loss. Let's look at angle polish connectors as compared to a fusion splice. When doing single mode long haul applications, having low return loss is very important so the fiber link will be able to cover the distance that is needed. So looking at the APC connector we see that typical reflection is -60db, which is the furthest away from zero, meaning that it has very little reflection causing the amount of loss to be less. Compared to the fusion splice which has the same value as the angled polish connector which is why in a long distance application splicing and APC connectors are used.

Importance of Measuring Return Loss

Why is all the information above important and relevant to your network and fiber optic links? In all networks the end goal is to be able to carry your signal by transmitting light over fiber. If you have high return loss, this may cause distortion of your signal, interruption of transmission or even worse - shut down your network completely. The tool that everyone should have to take optical return loss measurements is an Optical Time Domain Reflectometer (OTDR). An OTDR allows you to measure your entire link, and will even give you a map that will tell you at what distance is the pair of connectors that need to be cleaned or just to make sure they are not loosely mated together. It is a key piece of equipment and will be discussed in detail in a couple of our other blogs.