With the demand of ever growing bandwidth, FTTx PON (passive optical network) is being deployed to provide optical fiber’s advantages at a lower cost than point-to-point architecture affords. But there are some unique test challenges when installing and maintaining the FTTx PON network. How to solve it? This article will provide some information about FTTx PON testing for your reference.
Proper testing is a critical part of installing, activating and maintaining a PON. Most components are tested again after splicing, and installation of splitters and access terminals although they are tested during manufacturing process. Field testing is required to ensure no excess loss or reflectance has been introduced due to micro-bends in installed fiber, poor splices, macro-bends in splice closures or access terminals, or dirty, damaged, or improperly seated connectors. If not detected and corrected, excess loss or reflectance often results in poor network performance. Initially, performance may seem acceptable, but over time, transmission errors may begin to increase long before the need for any maintenance activity would normally be expected. Usually, four tests are taken to verify optical links, which are connector inspection, insertion loss test, optical return loss test and OTDR (optical time domain reflectometry) test.
Connector inspection and cleaning during installation and maintenance are one of the most effective methods to ensure expected performance of an optical network. Typically, an optical microscope is used to inspect the connectors as shown in the following picture. 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. Thus it’s easy to detect the dirt, debris or damage. Images may be captured before and after cleaning, then compared for any variation. Connector contamination and damage are the most common causes of poor optical network performance.
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 measuring the received power level output from the other end. The measured difference between the transmitted and received power levels indicates the optical loss through the network. Insertion loss is considered acceptable when the measured loss level is lower than the budgeted loss level.
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. The difference between the injected power level and the measured return level is the return loss. Return loss is considered acceptable when it is higher than the budgeted return loss target. A low return loss value (below 35 dB) is often an indication of one or more sources of excess reflection in the network under test, typically due to dirty or damaged connectors or a fiber break.
As optical network loss is wavelength dependent, insertion and return loss testing is typically performed using wavelengths at or near those which will be used during network operation. Regarding FTTx PON, upstream wavelength of 1310 nm may be used, while 1490 nm and 1550 nm are used in the downstream direction. Consequently, insertion and return loss testing at 1310 nm, 1490 nm and 1550 nm may be required. If the insertion and return loss measured at each wavelength are within the levels budgeted for the link, then the optical network may be considered ready for activation. But in many cases, the network operators require the network to be more fully documented using an OTDR.
OTDR scans a fiber from one end to measure the length, loss and optical return loss of an optical network. It also locates and measures reflective and non-reflective events in the network due to splices, connectors, splitters or faults. The OTDR measures the level of returned back scatter and reflections. Since the speed of light through the optical fiber is known, the OTDR is able to convert time-of-flight into distance, creating a trace which plots changes in back scattered and reflected light levels. Losses due to connectors or macro-bends appear as abrupt changes in the back scatter signal level. Reflections due to connectors, air gaps and open ends appear as spikes in the OTDR trace. The following picture shows the OTDR testing.
Optical testing is typically performed at various points in a network’s lifetime. Installation verification testing occurs as the network is being constructed or after network installation is complete, but before the network is activated. This is usually when the most complete testing is performed, and may include insertion and return loss testing as well as OTDR testing. Maintenance troubleshooting is performed when service outages occur, and typically requires rapid response to restore service as quickly as possible.
Insertion loss tests are primarily used to test FTTx PON during installation. Insertion loss testing may be performed on individual fiber segments as they are installed. An end-to-end insertion loss test may also be performed on the FTTx PON after is partially or fully installed. A stable optical light source and an optical power meter are required to measure insertion loss. Access to both ends of the fiber-under-test is required. Consequently, this is typically an out-of-service test.
OTDR testing is typically completed as the FTTx PON is being deployed. Using an OTDR, distribution fibers are typically tested after installation and connection to the splitter. Once attached to the splitter, these fibers may only be tested from the downstream access point or subscriber premise. OTDR testing during FTTx PON installation testing is usually performed only at 1310 and 1550 nm. During operation, the FTTx PON always utilizes 1490 nm in the downstream direction and 1310 nm in the upstream direction. It may additionally utilize 1550 nm as a second downstream wavelength. Fiber loss is highest at 1310 nm and lowest at 1550 nm, while bending-induced loss is highest at 1550 nm.
PON is being deployed worldwide to more cost-effectively deliver higher bandwidth broadband services to subscribers. FTTx PON presents some unique testing challenges. The testing methods mentioned in this article are recommended to verify or troubleshoot FTTx PON. Hope it can help when needed.
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