LC, SC, FC, ST, MPO/MTP compared: ferrule sizes, polishing types, insertion loss, and a decision flowchart to choose the right fiber connector for your application. A fiber-optic adapter — sometimes called a coupler or bulkhead coupler — is a passive mechanical interface that mates and aligns two terminated optical fibers (i., two fiber connectors) such that light can reliably pass from one to the other with minimal insertion loss and maximum return loss. Fiber optic adapters play a critical role in ensuring stable and low-loss fiber connections.