Hello, everyone!
Today I will continue to write about FTTA testing. In this article, I will explain OTDR testing. Let’s get started.
Briefly about OTDR. It is one of the most important instruments during building, maintenance, and troubleshooting. It allows measuring the length, total attenuation and reflection, as well as attenuation and reflection in characteristic events, etc. There are for MM and SM optical fibers, with different wavelengths. They are usually 850 nm and 1300 nm for MM fibers and 1310 nm and 1490 nm for SM fibers. Modern OTDRs have additional functionalities such as integrated OPM, VFL, etc. Most OTDRs work with dark fibers, special OTDRs using a filtered wavelength of 1625 nm is used to work with live fibers. When doing OTDR testing, we always use a reference optical cable and select the wavelengths that the system uses. OTDR testing in both directions is required.
The problem with OTDR testing is the short FTTA cables for typical OTDR resolution. It is necessary to use OTDRs with high resolution and set short range and short pulse.
Generally, OTDR testing is done with loopback, as in the case of IL testing. OTDR is always used with the reference cable, and a reference cable is mounted on the other cable during testing. We can see it in the next figure.
In the following figure, we see the schematic of the OTDR trace. With these measurements, we measure the end-to-end loss of the cable. The result is attenuation of the optical fiber with all splices and optical connectors. FTTA cables are small in length, so the attenuation of the optical fiber is very small. We can use markers for more precise measurements of attention and reflection. There is another way to measure more accurately, and that is to use the LSA (Least Squares) function.
Measurements can be made without loopback. And the reference optical cable used for OTDR is longer than the FTTA cable we are testing.
This is the end of this article. In the next article, I will explain OTDR testing >>Testing FTTA (4): Continuity testing and optical power testing equipment.
Thank you!
