Why Use Launch Cable in OTDR Measurement?
It also helps minimize the effect of a dead zone that can occur when OTDRs are used to test cable plant. This dead zone happens due to high launch power or faults at the end of the fiber being tested.
It Allows Time for the OTDR Trace to Settle Down
OTDRs send out test pulses that can overwhelm the instrument’s receiver circuit and cause problems in interpreting the traces. The trace takes time to settle down, so a launch cable is used to let the OTDR trace settle down before analyzing it. This gives OTDR technicians more time to interpret their results.
During a trace, OTDRs measure the percentage of light that returns to them after it passes through the fiber. This is very sensitive, so big reflections can saturate the OTDR receiver and interfere with measurement. This can cause problems in tracing events such as splices and connectors, and can result in loss errors on a trace.
A long launch cable helps to reduce this problem because it provides a reference for the first connector of the cable under test. This allows the OTDR to be more accurate in testing the connector on the first cable of the fiber under test. It also prevents a lot of repeating reflections from showing up in the trace.
This is because the OTDR only sees features in the fiber that are close enough to it for the test pulse to pass through them. This means that if a connector on the first fiber has more loss than the connector on the second fiber, the OTDR will show both as a single event. This causes a lot of confusion for new users, and can lead to over- or under-displayed losses in the trace.
For this reason, OTDRs are best used to detect events in the middle of the fiber link under test. These include splices and connectors, but may also involve splitters, pigtails and other events that aren’t visible at the end of the link.
To avoid this problem, the length of the launch cable should be at least twice as long as the end of the fiber under test, so that it can be seen in the OTDR trace. For example, if you are measuring the loss of a 1000 meter cable plant, the launch cable should be at least 500 meters long.
Another important factor in determining the length of a launch cable is the wavelength. Different OTDRs are more wavelength sensitive than others. This can make a difference in the amount of time it takes to recover from a reflectance overload, which can affect the slope of the measured trace.
It Provides a Reference to Push Out the First Connector of the Cable Under Test
When you test a fiber optical cable, you may find that the first connector along the path has a very large event on the OTDR trace caused by reflection from the connector on the OTDR itself. This causes an overload that can take the OTDR some time to recover from, depending on the OTDR design, wavelength and magnitude of the reflection.
This can make it difficult to analyze the first connector along the optical path and minimize dead zones. This is why launch cables are used in otdr testing.
The launch cable provides a reference to push out the first connector of the cable under test, which can help determine the loss. It also allows you to see the condition of the first connector on the cable plant that you are testing and can help reduce multiple reflections in your Palm OTDR trace.
A launch cable is long and can be at least 500 to 6000 meters in length. It should match the connectors on the test fiber to reduce multiple reflections and be manufactured from the highest quality materials possible to prevent insertion losses.
Using a launch cable in OTDR testing also helps to minimize Dead Zones, which occur when there is high launch power or faults near the end of the fiber. These problems can be found in splice closures or other places on the fiber that you cannot see when looking at an OTDR trace.
These dead zones can affect the measurement uncertainty of the OTDR trace and can cause errors in loss measurements. Using launch cables in OTDR testing can help to reduce these problems and improve your results.
Launch cables are available in several different shapes and sizes with robust outer casings to make them more durable. They can be used with single-mode or multimode OTDRs.
They are usually attached to the OTDR using a patch cord or plug and socket connection, but can also be used with a small fiber ring. These cables have a single pigtail on one end that is terminated with the OTDR and a pigtail on the other end that is terminated with the link under test.
image source https://www.pinterest.ph/
It Reduces Multiple Reflections
OTDRs are highly sensitive and if the receiver circuit is saturated by large reflections, it will take time for it to recover fully. As a result, you might see a trace with multiple reflections called ghosts.
A ghost is an event where light bounces back and forth in the fiber optic cable between a remote connector and the connector at the end of the fiber. This produces repetitions of the trace and is usually a sign of a bad connector or other issue.
Ghosts are a big source of confusion to new users. It is important to understand the difference between a ghost and a loss, as one can be almost identical to the other.
Reflective events, like connectors, splices or breaks are recognized by a spike in the OTDR trace. They are also recognized by a discrete drop in the backscatter level.
Nonreflective events are not as easily identified by an Mini OTDR. They are typically caused by fusion splices or bending, which cause stress to the fiber and cause it to lose its ability to absorb light.
In order to recognize a splice, an OTDR must know the exact loss of the fiber and the backscatter level at the time of the event. Fortunately, the length of the fiber is known and can be used to calculate a cable-specific index of refraction. This value, along with the splice loss, determines the backscatter level and is therefore more accurate than a generic backscatter value.
Moreover, the OTDR can measure the distance between two markers placed on the launch and receive cable. This is an essential step for testing the cable plant to ensure it is installed properly and does not have any defects in its design.
The OTDR can then measure the loss between the two markers and calculate the attenuation coefficient of the fiber. This is a complex process that requires a knowledge of the length of the fiber, and is based on a number of calculations. It is therefore important that the OTDR be calibrated for the specific length of the fiber being tested, and the meter be able to read the loss accurately.
image source https://www.pinterest.ph/
It Prevents Dead Zones
The OTDR is a crucial tool in testing fiber optic network. It helps to find breaks, splices and connectors in the network and can also measure light loss. However, there is a phenomenon that occurs when using the OTDR called 'dead zone' which limits the test results. This dead zone is a result of the reflection from the OTDR connector and takes 50 meters to one kilometer to recover fully, depending on the design of the OTDR, wavelength and magnitude of the reflection.
To avoid this problem, technicians use a launch cable in OTDR tests. These cables are much longer than the standard patchcords used in networks and are terminated with high quality connectors that will minimize multiple reflections and make OTDR measurements easier.
Launch cables are available in different lengths and configurations and can be used as an OTDR transmitting fiber or an OTDR receiving fiber, depending on the specific application. They are also a good choice for certification measurements, because they allow the technician to analyze the first connector along the path and minimize dead zones when measuring insertion loss or reflectivity of near-end connections.
In addition, they are also a great help when troubleshooting. If the launch cable is long enough to cover all of the 'dead zone', it will enable technicians to determine whether or not the OTDR was able to compensate for the dead zone and get accurate test results.
OTDR Launch Cables can be purchased in a variety of lengths and configurations, including 100-meter, 300-meter, and 5000-meter multimode and singlemode types. Each launch cable includes a simplex fiber assembly with both an OTDR transmitting fiber and an 7 inch multifunction OTDR receiving fiber, both of which are terminated on connectors matching the type of coupler that will be used when connecting to the OTDR.
A quality launch cable is essential to minimizing the dead zone that can be caused by a high launch power or faults that occur in the connector on the far end of the fiber. Ideally, the cable should be long enough to see all of the "A" & "B" locations and it should be terminated with connectors that have been carefully polished.
