The fiber optic adapter is also called a flange or fiber optic connector. Fiber optic connector is the most widely used optical passive device in fiber optic communication system. Also known as fiber adapter, optical fiber adapter, fiber coupler, fiber optic coupler, mating sleeve, or simply adapter, this. What is fiber optic adapter Fiber optic adapter (also known as fiber optic flanges, mating sleeves and couplers), are fiber optic active components.