LC Single Mode Fiber Optic Loopback Adapter High Quality and Brand New Item - Lowest Price Fiber Optic Loopback is designed to provide a media of return patch for a fiber optic signal; typically it is used for fiber testing applicons or network restorons. For the testing applicons, the loopback signal is used for diagnosing a problem. Sending a loopback test to network equipment, one at a time, is a technique for isolng a problem.