AEN 134 details the basics of OTDRs, including how to use them, why they are effective and insight on interpreting traces. The use of an optical time domain reflectometer (OTDR) for system troubleshooting, verification and documentation has always been an important step of the system installation process. Despite the OTDR’s importance, the ability to read and interpret the information gathered froman OTDR trace is known by very few, and due to the recent decline in OTDR prices, many more technicians are using OTDRs, most with no training or with just the user manual. To help alleviate the lack of training, AEN 134 provides basic information on how an OTDR works and a brief instruction on interpreting and obtaining useful OTDR traces.
There are four main reasons one uses an OTDR:
- Product acceptance
- The assumption that all product received from a vendor is ingood condition assumes nothing happens during transportation and that the cable wasnot spooled onto a smaller reel. Both actions can cause damage to the cable if doneimproperly. If the cable is tested prior to installation via a bare fiber OTDR test andfound to be damaged, then the supplier must accept the unsatisfactory product. If acable is installed, the supplier will not accept a return on that cable because the supplierwill claim the installer assumes liability upon installation.
- Troubleshooting
- The OTDR trace provides an as-built drawing or map of a system,and as such it is the perfect tool for finding problems. Using this map, an installer canlocate and repair each system component that is out of specification.
- System verification
- Again, because the OTDR trace provides a map of a system, thetrace confirms that the system meets specification by measuring each component. It isimportant to note that only the OTDR can measure individual components, whereasother test equipment (Link-loss test sets, VFLs, etc.) cannot.
- Documentation
- The OTDR is a key instrument in compiling a final documentationpackage to the customer because its traces show the status of the system when oneleaves the job site. More often than not, the customer will call back sometimes severalmonths after the project is finished claiming that the system never worked. The OTDRtraces provide physical evidence that the system had no problems upon completion ofthe project.
OTDRs can calculate fiber attenuation, uniformity, splice and connector losses, then provides pictorial trace signatures (a graph of optical power in dB versus the length of the fiber). Its ability to locate and measure reflectance and loss makes OTDR the troubleshooting and fault locating equipment of choice.
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