Fiber Optic Loopback modules are also called optical loopback adapters. They are designed for testing, engineering and the burning stage of boards or other equipment. Fiber loopback provide system test engineers a simple but effective way of testing the transmission capability and the receive r sensitivity of network equipment. Fiber Optic Loopback is providing a media of return patch for a fiber optic signal. Typically, it is used for fiber optic testing applications or network restorations. When used in testing applications, loopback signals are used for diagnosing problems. The best practice is to send a loopback test to network equipment, one at a time for isolating the problem.