An OTDR is an extremely valuable tool when you are trying to characterize optical fibers. OTDR (optical time domain reflectometers) units are used to locate any events in your fiber cable and the amount of attenuation of the fiber. An OTDR uses the effects of Rayleigh Scattering and Fresnel Reflections to measure characteristics in your fiber. The OTDR will send a light pulse down your fiber and by measuring the time it takes for reflections to return, as well as the power of light being reflected, the OTDR will make a trace profile of your fiber.
The OTDR will use two different methods to perform these tests. Rayleigh Scattering is the first method. When the OTDR pulses light down the fiber, the light will collide with particles that are in the fiber. When the light hits these impurities, the light will be scattered in every direction. Some of this scattered light (typically around 0.0001%) is shot down the glass in the opposite direction back to the OTDR. Rayleigh Scattering occurs along the entire length of the fiber and it a huge loss factor in fiber.
The second method the OTDR will use is called Fresnel Reflections. These types of reflections occur when light traveling through the glass encounters a different density which will change the light speed. Some of the light (typically around 4%) is reflected back to the light source. Fresnel Reflections can have power up to 40,000 times as much as backscatter. Fresnel reflections will be shown on your OTDR at connector points as well as mechanical splices that were put in the line.
The typical reflectance for some common applications can be seen below. This is what you can expect to see as a loss on your OTDR when you encounter these characteristics of your fiber.
- Fiber end with flat cleave: -14 dB
- Good multi-mode PC connection: -35 dB or lower
- Good single mode PC connection: -50 dB or lower
- Good angle-polish connection: -60 dB or lower
- Good fusion splice: -60 dB or lower
The two types of attenuation can basically be broken down into non reflective or reflective attenuation. We will briefly touch on the differences in these two below.
- Non-Reflective Attenuation
Sometimes when viewing the OTDR readout you will notice a sudden drop in the graph. This drop represents an event that causes attenuation. When there is attenuation and no previous spike, a non-reflective event is now identified. That attenuation with no reflection is typically indicative of a kink, bend or fusion splice somewhere in the fiber cable.
- Reflective Attenuation
Reflective attenuation looks different on the OTDR than a non-reflective attenuation. The most distinct difference is that a reflective event shows a spike before the attenuation. This may be cause by a connector on the fiber or a mechanical splice that has been added to the fiber. The reflection is created when light passes between substances that have different indexed of reflection (glass to air). Reflective attenuation (same as Fresnel Reflection we reviewed on the prior page) occurs when light leaving the glass core of the fiber encounters an air gap within a fiber optic connector or when light travels from glass to index matching gel in a mechanical splice. The height of the spike represents the amount of power that is being wasted through reflection.
When using your OTDR you will come across Dead Zones in your tracing. Dead Zones refer to the amount of time (distance) that is lost on a fiber trace that follows a reflective attenuation. This trace section will be in correlation the portion of the fiber test that is closest to your OTDR unit. The Dead Zone part of a trace has no way to be reliably tested. This problem occurs on account of the sensor on the OTDR measuring backscatter levels in a fiber. Essentially, the OTDR becomes blinded when it encounters a much larger Fresnel Reflection. This blind period will last as long as you have the light pulse duration set for on the OTDR. When the light comes back to the OTDR as a high level of light, the OTDR becomes over saturated and can’t properly measure the lower levels of backscatter that follow this event.
There are a couple things you can do to alleviate these problems presented by dead zones and JM test is here to offer you those items. Since the Dead Zone is 100% correlated to pulse width, its distance can be reduced by decreasing the pulse width setting on your OTDR. Decreasing your pulse width will also lower your measurable width so you will need to experiment to find your sweet spot in the settings to minimize this distance. The other way you can skirt this problem is by using a launch box at the beginning of your trace. This will push the initial dead zone away from the origin of your trace. This will allow you to determine the attenuation of the initial spike where the connector is.
In the previous dead zone section we learned that the pulse rate can be adjusted on your OTDR. Now we will review the difference that a pulse width setting can make on your OTDR. The pulse width will control the duration of the testing pulse. The pulse width setting will have a direct correlation to dynamic range as well as dead zones. When the pulse is increased the dynamic range (measurable distance) will increase but on the other side of the coin, your dead zones will grow as well. When you select a pulse setting, a good rule of thumb is that a long pulse is good for long fibers and a short pulse for shorter fibers.
We just mentioned dynamic range so let’s review that topic. The dynamic range is the difference of the amount of power that can be sent down the fiber and the weakest level of light that the detector can measure before the signal just gets lost in noise. Dynamic Range is typically listed in dB. The larger your dynamic range, the longer distance the OTDR can measure. To determine the OTDR’s dynamic range, the total pulse power of the laser and the sensitivity of the detector need to be factored in.
Gainers and Losers
Sometimes when you test a fiber with an OTDR unit, your trace will reveal what is referred to as a gainer. The gainer appears to have amplified the power of the laser; however, we know that that can not be correct. Gainers typically appear on the trace at a spot where a fusion splice has been installed on the fiber. When two different backscatter levels are spliced, this will present this false reading. When two fibers are mated and the receiving fiber has a backscatter level that is higher, the amount of light that shoots back to your OTDR increases. On your trace, this is interpreted as a gainer. On the other hand, if you measure from the opposite end of the fiber you will see a large power loss. This will make a spot on the trace that shows a very large level of sudden attenuation in your fiber. This is what is referred to as a loser.
The most acceptable way to accurately determine a splice loss is to shoot the power from both ends of your fiber connection. If you shoot the fiber from both ends and then average the splice results, the measurement error will be eliminated from your readings.
A ghost is a term that is used for a false or a double reflection that may occur on your OTDR trace. When the light pulse is sent down the fiber, some light will be reflected back to the OTDR’s detector. This outcome is desirable when testing as the returning light provides a baseline for your measurements. However, when the light reflection is large it reflects off of the OTDR connector and goes back down the fiber a second time. When the second light pulse gets reflected back to the OTDR it is recorded as a second trace. One big issue with ghosting is that a false trace that is shown can obscure events that are real and causing real issues. A technician can easily identify a ghost by the fact that they have no attenuation value and they occur in multiple distances from the big reflection that took place. To facilitate eliminating or reducing ghosts you probably need to clean or replace the connector on the OTDR. You may use index matching gel to further eliminate ghosting. It is very important to remember to wipe off all of the index matching gel from the OTDR connector once your test has been completed.
In the final section of our OTDR blog post we will review a screenshot of an OTDR trace and describe a few of the events that are seen.
- In reviewing the above OTDR trace, the first event that we see is the connector
- This is a reflective event and should be present on any fiber trace. This is a Fresnel Reflection. If you look through a plate of glass at night, you can see your reflection in the glass as well as see what is on the other side of the glass. Just like looking through that plate of glass, if you have too much light reflection then you will not be able to see through the glass and only your reflection. The OTDR is exactly the same theory, it is imperative that your OTDR fiber port is kept clean to alleviate this issue.
- The second event shown is your launch lead
- The trace tail after the connector can be quite long sometimes. Using a launch cable will push everything out to ensure that you can see the first event.
- At the top of the screenshot, you will see 5 connector spikes
- These are identified because they are reflective events with a significant amount of Fresnel reflection and immediately followed by a loss.
- Beneath the connector spikes, you will see the mechanical splice event
- This event will cause a reflection but it won’t be a large spike. The reflection on this spike is caused by the index matching gel that is in the mechanical splice.
- The next event we see is a fusion splice
- This event shows no reflection but only a loss. Fusion splices are the most reliable method for an almost lossless connection but you need to be sure to use a high-quality fusion splice to achieve this.
- Moving on down the trace, we see mismatched fibers.
- This is also referred to as a gainer, which we reviewed above. In this screenshot, this is due to a 50 μm that is mated to a 62.5μm fiber. This is caused by having a higher level of scattered signal that is coming from the larger fiber core.
- At the top right, we will see the end of link
- This is a highly reflective event with a tail that drops off at the end of the tracing. This can also be a spot where the fiber is broken. This is why it is imperative to shoot the fiber from both ends with the OTDR. If one direction reads 80 meters and the other direction reads 65 meters then you can easily tell that there is a break in the cable and this is not the end of link.
- The last event that is present in the above screenshot if a ghost
- Always remember that these are not real events and should be ignored. It will become easy to anticipate where the ghosts will be on your trace once you have experience with OTDR tracing. Also, ghosts will not have a step down on the trace level since there is no actual loss. This will help you to identify it as a ghost.
JM Test Systems is here for you from rental, sales, or service on your OTDR units or any other fiber optic equipment needs. We have a very knowledgeable staff that can help you pinpoint exactly what you need to procure to get your job done as efficiently and thoroughly as possible. When you rent an OTDR from JM Test we will offer an added benefit of being able to select the launch cables for your application.
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