The term coupling angle is used for optical waveguides.
The coupling angle is the angle between the propagation direction of the incident light and the optical axis of the optical waveguide. In order for the incident light to be coupled in, the coupling angle must be between 0 degrees and a maximum value, which depends on the location on the fiber face, or its refractive index relative to the cladding glass. An increase in the coupling angle leads to the critical angle and thus to the acceptance angle, i.e. the angle at which the light does not exit the optical waveguide.