How do you know when and how to test fiber optic links?
The first testing can be carried out as soon as a bundle of fiber optic cables is installed. Essentially there are two test approaches. The first one that I recommend, as it is compliant with the industry standards, is to inspect the fiber end faces, clean if necessary and then do a link loss test using fiber optic sources and power meters. Either Certifiber Pro or SimpliFiber Pro can be used to perform a loss test for certification purposes. The MultiFiber Pro is the preferred tool for certifying MPO trunk cables.
The CertiFiber Pro "Qi" version (shown above) with 2 main units enables fast and easy end-face inspection and documentation at both ends of a fiber-optic link.
Some technicians prefer to shoot a trace with an optical time domain reflectometer, (OTDR) to see if anything strange is going on, before they do the official loss test. You could say this is doubling up on work but if you're in any way suspicious about the performance of the link, shooting a trace with an OTDR makes sense from an installation perspective, because it provides insight into what is happening in the whole link. An end to end loss test will only tell you about loss over the link, it cannot tell you about anything that’s happening in the middle of the channel.
In any case, if a loss test fails, the use of an OTDR is highly recommended as it can pinpoint the location of events that exceed loss or reflectance limits. Maybe you’ll need to replace a connector, clean a dirty connector, or maybe there’s a tight bend in the cable. Some people like to use tie wraps but if used incorrectly they can bend the fiber excessively. If you do that with a copper cable, electrons will still run through the cable but in fiber optics the light will exit the fiber and there will be major loss. The OptiFiber Pro EventMap greatly simplifies the interpretation of the measurement: here we see the OTDR at the bottom of the screen, connected to a designated launch fiber , shown in grey, then the installed fiber shown in black, with connectors identified as rectangular 'boxes' at each end, and finally the receive/tail fiber, again shown in grey.
When testing for loss you always need to test if it is within the limits that are required for an application to work. The reality is that every piece of fiber and connecting hardware has some level of loss, so testing is essential to see if the allowable loss levels have been breached. A list of compliant networks standards can be found on a CertiFiber Pro LinkWare PC Report. It is highly recommended to check if your intended application is listed there, as it is not unusual to exceed length limitations for specific applications. Likewise, loss limits continue to grow tighter, as application demands drive lower and lower end-to-end budgets.