The most common operating principle of a directional fiber coupler is evanescent wave coupling in a configuration where two fiber cores come close to each other. The device allows the transmission of light waves through multiple paths. It was developed by Nippon Telegraph and Telephone (NTT) company. SC is a snap (push-pull coupling) connector with a 2. There are two main types of fiber couplers: those that distribute light between. This paper describes the design principles of a fiber-optic directional coupler, including the intracellular photoelectric field equations, field amplitude equations, and propagation constants derived from Maxwell's set of equations for single-mode fiber.