Passive optical network (PON) is a cost-effective way to deliver high-bandwidth broadband services to users and widely deployed all over the world. FTTH uses PON technology and provides high bandwidth from the central office (CO) to subscribers. FTTH PON system achieves network reliability and makes network testing, monitoring and measuring easier. This article will tell about FTTH PON testing from the four aspects below.
Connector inspection/cleaning plays an important role in network installation and maintenance. Typically an optical microscope is used for connector inspection. To prevent accidental eye damage when inspecting fibers potentially carrying live traffic, a video microscope images the connector end-face and displays the magnified image on a handheld display. In this way, the dirt, debris or damage on the connector could be easily detected. According to the study by NTT-Advanced Technology, connector contamination and damage are the key reason for poor optical network performance.
In telecommunications, the term “insertion loss” expressed in dB, refers to the loss of signal power resulting from the insertion of a device in a transmission line or in an optical fiber. An insertion loss test measures the end-to-end loss of the installed link by injecting light with a known power level and wavelength at one end, and then measures the received power level output from the other end. The measured difference between the transmitted and received power levels exactly indicates the very optical loss through the network. In some occasions, insertion loss is allowed and considered acceptable when the measured loss level is lower than the budget loss level.
In telecommunications, return loss expressed in dB, means the loss of power in the signal returned or reflected by a discontinuity in a transmission line which can be a mismatch with the terminating load or with a device inserted in the line. The optical return loss test injects light with known wavelength and power level into one end and measures the power level returned to that same end. Then the return loss is the difference between the injected power level and the measured return level. When the return loss is higher than the budgeted return loss target, it is considered acceptable.
Typically insertion loss test and return loss test are performed by using wavelengths at those which will be used during network operation. For FTTH PON system, 1310nm wavelength is used in the upstream direction, while 1490nm and 1550nm wavelengths are used in the downstream direction. So it is essential to have insertion and return loss testing at 1310nm, 1490nm and 1550nm wavelengths. The optical network is considered ready for activation when the insertion loss and return loss measured at each wavelength are within the budgeted levels for the link. However, in some cases, the network operator uses an optical time domain reflectometry (OTDR) as the following picture shows for more fully documented network.
OTDR scans a fiber from one end to measure the length, loss and optical return loss of an optical network. And it also locates and measures reflective or non-reflective events in the network caused by splices, connectors, splitters, faults, etc. OTDR operates like a radar by injecting narrow pulses of light into the fiber under test. As each pulse travels down the fiber, imperfections in the fiber scatter some of the light, with some of this Rayleigh-scattered light being guided back up the fiber.
From the above description, four tests are commonly used to verify optical links. Proper testing is critical for FTTH PON installing, activating and maintaining, because excess loss or reflectance can result in poor network performance if not detected and corrected. And over time, transmission errors will occur before the need for any maintenance activity.