Hello everyone!
Today I want to explain how to adjust the OTDR parameters to make the measurement more accurate. I mentioned this in the first article on this topic.
The OTDR instrument can measure using the “auto test” function (automatically set parameters). However, these measurements are not accurate enough, and errors may occur. Now I will repeat OTDR parameters and I will explain them in detail.
OTDR parameters are:
fiber type and application,
wavelength,
range,
pulse width,
duration,
index of refraction.
Fiber type and application
There are OTDR instruments that have modules for type of optical fiber – SM or MM. After that, some instruments have multiple modules for SM fibers (special measurement for metropolitan areas, long distance networks or short distance networks). This is the first parameter that we have to choose before measurement.
Wavelengths
This is the second important parameter. Measurements are performed at specific wavelengths or at multiple wavelengths. These are the wavelengths that will be used in the system or that are already used in the system. These are usually 1310 nm and 1550 nm for SM systems, while for MM systems it is 850 nm and 1300 nm. The wavelength of 1625 nm is used when it is live fiber or when we want to measure macro bends. Then, for PON systems, there are OTDR instruments that can measure on the 1490 nm, etc.
I want to say a few sentences here about out of band testing. There are specialized OTDR instruments that, in addition to standard wavelengths such as 1310 nm and 1550 nm, also have a filtered wavelength of 1625 nm. These OTDR instruments are used for two types of measurements, as I mentioned.
The wavelength range 1625 - 1650 nm is used to measure existing optical links, where the fibers have traffic - live fibers. This is very useful for troubleshooting as otherwise larger interruptions would have to be made. On the OTDR, the reference cable is connected to the correct port - live fiber, and a wavelength of 1625 nm is selected. After that, we have standard OTDR settings and measurements.
Another reason to use this wavelength is to measure macro bends. As I have said many times, macro bends are easy to see at higher wavelengths, which allows us to quickly detect and fix problems.
Continued in the next article >> OTDR trace analysis (7)
Thank you!
