In order to diagnose the problems of optical communication equipment, a fiber optic loopback is a must to provide an easy way to test the performance of the optical network devices. Generally, loopback cable and loopback module are both fiber optic loopbacks. Fiber optic loopback cable is the traditional fiber optic loopback with a visible cable. It is equipped with two fiber optic connectors on each end of the cable. When sticking the connectors together, the cable will shape like a loop. As for fiber optic loopback module,the biggest difference is that it has a enclosure to protect the inside cable.And the looped space is reduced for an easier usage and economic package.