In an optical fiber, the propagation constant evaluated along the optical axis of the fiber in the direction of transmission.
Note: The real part of the axial propagation constant is the attenuation constant. The imaginary part is the phase constant. [After 2196]

Of an electromagnetic wave having elliptical polarization, the ratio of the magnitudes of the major axis and the minor axis of the ellipse described by the electric field vector.

A light ray that travels along the optical axis.