a fiber optic connector in such as a male mt - rj connector has a ferrule fixed to the housing . ferrule - to - ferrule alignment is provided by the floating ferrule in the mating female mt - rj connector . a field - installable connector includes a field termination unit with fiber stubs and index matching gel for receiving the cut ends of optical fibers . the fiber stubs extend into the ferrule of the connector . a loopback test unit includes a ferrule and housing like those of a standard connector . a fiber loop has both ends lodged in the fiber bores of the ferrule so that the loop will connect the transmitter of a transceiver to the receiver of the same transceiver when the unit is engaged in the connector of the transceiver . the fiber loop is held within a boot attached to the housing or ferrule so that the entire unit can be conveniently installed and removed , and so that numerous loopback testers can be used simultaneously even when the device under test has numerous closely - spaced transceivers .