Hello, everyone!
Today I will explain you test procedure in the FTTH PON networks.
FTTH PON networks are complicated for testing. Problems are bi-directional optical splitters, short links, wavelengths (for GPON: 1310nm for upstream and 1490nm for downstream), and multiple connections. Testing is required during and after building.
Cleaning optical connectors
First, if we are building a network with optical connectors, it is mandatory to clean the connectors. Dirty on the optical connectors can increase optical attenuation. Sometimes this optical attenuation can be about 4-5 dB and it can disconnect the optical links. Further, dirty can damage optical connectors. Cleaning is required before connection as well as before measurement. We can use a microscope to visually check the cleanliness of the connector surface. Sometimes cleaning several times is required to obtain acceptable connector cleanliness. This is a very big problem in optical networks. You can see details about this theme in my articles: Main instruments for measuring in optical networks (1)
OLT testing
After completing the FTTH PON network, we have to test IL (Insertion Loss). For this testing, we use OLTS (light source, optical power meter, and referent cables). GPON system uses 1310nm for upstream and 1490 nm for downstream. Because of that, we have to measure insertion loss at 1310 nm and 1490 nm incorrect direction. For other PON standards, we will use other wavelengths.
In headend or CO (Center Office) we will test downstream. GPON system uses 1490nm, because of that, we will use a test source and optical power meter with 1490nm. In the upstream (user side), the GPON system uses 1310nm, and we will use a test source and optical power meter with 1310nm.
If you want to read details about optical instruments for this testing, read my articles: Main instruments for measuring in optical networks (3)
OTDR testing
After OLTS testing, if we measured good values for optical power in both wavelengths, we can finish our testing. But if there is some problem, we have to use OTDR. This is very complicated. There are many problems: short links, bi-directional optical splitters, wavelengths, etc.
OTDR is the most important and very useful instrument in every optical network. With an OTDR instrument, the technician should find where is the problem (fiber bend, bad splice, bad optical splitter, etc) and fix it. For details about OTDR, you can read the next article: Main instruments for measuring in optical networks (4)
We have to measure the 1490nm and 1310nm, because the GPON system uses these wavelengths. The biggest problem during the measuring is the optical splitter.
When we measure in headend or CO (downstream test), we can see the next trace:
Figure 1. OTDR testing – downstream
(https://www.thefoa.org/tech/ref/appln/FTTH-test.html)
This result is very complicated to analyze. We can see a big drop in optical power, it's an optical splitter. But after the optical splitter, we can't understand the trace. There are many peaks from reflections.
Let's take an example from figure 1, it is an optical splitter 1:4. One fiber is divided into four branches or four traces. It is the problem. OTDR cannot well display events on each of these routes. A simplified measurement from OLT to ONUs is given in Figure 2.
Figure 2. OTDR testing – downstream
(https://www.thefoa.org/tech/ref/appln/FTTH-test.html)
If we have problem X with one of these branches, we can not see it this way. Because of that, we have to measure from users to OLT (upstream test). Trace analysis is very simple, we can easily see everything on the device display – all detailed for one branch: splices, optical splitter, problem X, etc (figures 3 and 4).
Figure 3. OTDR testing – upstream
(https://www.thefoa.org/tech/ref/appln/FTTH-test.html)
Figure 4. OTDR testing – upstream
(https://www.thefoa.org/tech/ref/appln/FTTH-test.html)
Problem with measurement in upstream may occur if GPON system has 2-input optical splitters (for example 2:2, 2:4, 2:8, etc). The second fiber is used for testing or downstream protection.
When measuring with an OTDR instrument, as well as when measuring IL, a reference cable is used. It is mandatory.
These were the most important and complicated measurements. In addition to these measurements, testing of optical components and devices before installation is required – optical splitters, OLT, optical transmitter 1550 nm, EDFA (Erbium Dropped Fiber Amplifier), etc.
The optical splitter is tested in both directions at 1310 and 1490 nm, to determine attenuation. OLTS is used for these measurements.
We measure output optical power for OLT, optical transmitter, and EDFA. An optical power meter or PON meter is used for this measurement.
Thank you!
Reference:
Fiber To The Home Handbook, Jim Hayes, The Fiber Optic Association
FOA Reference Guide To Fiber Optic Testing, Jim Hayes, The Fiber Optic Association
https://www.thefoa.org/tech/ref/appln/FTTH-test.html
