Basic Principles of Fiber Optics Series: Optical Return Loss/Reflection

Written by Ben Hamlitsch, trueCABLE Technical and Product Innovation Manager RCDD, FOI

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

What contributes to Optical Return Loss?

Here is where we discuss several of the factors that contribute to high return losses. In fiber optics, it is imperative that you make sure you are always inspecting and cleaning the fiber optic connectors before you mate them together. Dirt, dust, grease, and smudges on the connector face is the number one cause of high return loss, but can be 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 of single mode fiber is (9um), 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 fiber click cleaners that assist the mating sleeve in cleaning the face of a connector.

Another thing that will cause a high return loss is a broken or cracked piece of fiber. Some pieces of fiber can have a small crack that will go somewhat undetected when preforming a insertion loss test. This is because insertion loss tests give you a loss value of the overall cable link and not at each individual component. The advantage of testing that implements return loss and reflection can really 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, this can lead to air gaps between the two mated connectors, which in turn will lead to high 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 a red flag and some troubleshooting should be in order.

Optical Reflectance

When looking at your overall Optical Return Loss, you have several events that happen in a link. These events as individual occurrences are known as reflectance events. A mated pair of connectors is a reflectance event that happens in a fiber link. 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 reflectance events, the closer to zero your measured event result is, the poorer the connection will be, which results in more insertion loss and higher return loss. Below are some different types of reflectance figures and the average amount of reflection you should see when tested at these different polish types. The reflectance value is always displayed as a negative dB figure.

• 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

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 (APC) 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 with the minimal amount of light loss. Looking at the APC connector, we see that the typical reflection is -60dB (the furthest away from zero), meaning it has very little reflection, causing the amount of loss to be less or better. We can compare this to the fusion splice, which has the same value as the angled polish connector, this is the reason why, in a long-distance application, splicing is used and APC connectors are the preferred choice in these fiber applications.

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 from point A to B with minimal loss over fiber link. 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 the pair of connectors are that need to be cleaned or just checked to make sure they are not loosely mated together. It is a key piece of equipment and will be discussed in more detail in this article.

As we mentioned earlier reflection refers to the phenomenon where a portion of the light that is transmitted through the fiber is reflected back into the fiber. This can occur at the interfaces between different materials with different refractive indices, such as at the interface between the core and cladding of an optical fiber, or where two connectors are mated together.

Reflection is an important consideration in fiber optics because it can cause signal loss and degradation of the fiber link. When light is reflected back into the fiber, it travels in the opposite direction and can interfere with the forward-traveling signal, causing attenuation. In addition, reflections can cause standing waves, which can lead to signal distortions.

To minimize reflection in fiber optics systems, it is important to use fiber optic cables with low reflection loss and to properly terminate the fibers to reduce reflection at the connectors. This can be done using techniques such as angle polishing and anti-reflection coatings.

There are two ways to measure reflection in fiber optics:

1. One method uses a source and power meter with some accessories, or an instrument called an optical CW reflectometer (OCWR) The OCWR technique is really designed to work on patch cords. Since the natural backscatter of the fiber adds to the measured reflectance, longer cable runs will include a significant amount of backscattered light.
2. Optical time-domain reflectometer (OTDR) This method uses a pulse of light to measure the reflection and transmission characteristics of the fiber. The OTDR can measure the amount of light that's returned from both the backscatter of the fiber and reflected from a connector or splice, leading to two independent tests, reflectance and optical return loss. The OTDR sends a pulse of light down the fiber and measures the time it takes for the reflected pulse to return. From this information, it is possible to calculate the reflection and transmission characteristics of the fiber. The OTDR is very similar to the TDR, but it uses a longer pulse of light and can measure reflections along the entire length of the fiber.

The OTDR gives reflectance measurements at any event location along the fiber link. This could be a splice event or a connection between two mated connectors.

It is important to note that when ORL/Reflection is tested, the test will show a negative value. In the chart below, you can see reflection values. The higher the negative number, the better the reflectance. So when it comes to low reflectance, the APC connector has the best performance of all connector types.

The amount of light reflected at a connection between two fibers is determined by the differences in the index of refraction of the two fibers connected, a function of the composition of the glass in the fiber or air in the gap between the fiber connectors, as is common with connector terminations and mechanical splices. Connectors will show reflectance peaks on OTDR traces; mechanical splices typically will also show reflectance peaks; and fusion splices will most often show no reflectance or sometimes with just a small dip in the location of the fusion splice.

In an OTDR, the peak that identifies a reflective event is measured and reflectance calculated. Higher peaks indicate higher reflectance. In order to measure reflectance, the peak must not saturate the OTDR receiver, as indicated by a flat-topped reflectance peak (below). For instance, this is an OTDR trace where reflectance cannot be accurately measured. It will only indicate a value less than the actual. However if this is the case you can always guarantee that the reflection at this event needs to be looked and there will need to be some investigation into what is causing this high reflectance.

Calculating reflectance in an OTDR is a complicated process involving the baseline of the OTDR, the backscatter level, and the power in the reflected peak, as shown in the diagram below. Since reflectance is defined as a fraction of the power in the test signal, the OTDR must calculate the test power from the backscatter level of the fiber, based on the typical backscatter coefficient of the fiber being tested. In other words it uses lots of fancy calculations to make these detailed and precise measurements.

Which method to use will depend on the specific needs of the application and the type of information that is desired.

There are several ways to reduce reflection in fiber optic cables:

1. Proper termination: Using the right connectors and properly terminating the fibers can help reduce reflection.
2. Cleaning and Inspection: Cleaning the fiber end faces and inspecting them before connecting them can help reduce reflection.
3. Angle polishing: Using an APC connector if the application requires can help reduce reflection because of the design of these connectors.
4. Using fusion splices instead of mechanical splices. Fusion splices work exactly as they sound they melt or fuse the fiber ends together. This ensures very low reflection.
5. Maintaining good bend radius, never over bend or put excess stress on the fiber cable. These things can cause excess reflectance in your fiber link.
6. Always verify that your connecting multimode cable with multimode and singlemode with singlemode. It also helps to use the same cable manufacture type this can help with the overall results of your fiber link.
7. Use an OTDR to test your fiber link, this ensures that your link is preforming as it should at each connection point and splice along the link under test.

Optical Return Loss and reflective events, are a very important measurement in fiber optic cabling systems. This measurement parameter can tell the technician a lot about what is wrong with a fiber optic network, because it breaks it down to the individual connector and splice giving the technician much more detail to troubleshoot an issue. Modern day OTDR’s are designed to test for reflection loss and give a user-friendly graphical interface where the technician can easily and quickly troubleshoot the fiber link. Good installation practices can ensure that the Optical Return Loss/Reflectance is as low as possible for any given fiber cable link. 

HAPPY NETWORKING!

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