Fiber loopback is designed to provide a media of return patch for a
fiber optic signal; typically it is used for
fiber testing applications or network restorations. For the testing applications, the loopback signal is used for diagnosing a problem.
A loopback cable is also known as loopback plug or loopback adapter,
which is a plug used to test physical ports to identify network problem. It
provides system test engineers a simple but effective way of testing the
transmission capability and receiver sensitivity of network equipment.