What optical effect is produced by all mechanical interconnections, whether mechanical splices or mated connector pairs?

Fresnel reflections occur when light transitions between media with different refractive indices, such as when fiber optic cables are connected through mechanical splices or mated connectors. This phenomenon is a fundamental optical effect associated with any interface in optical systems, including connections in fiber optics.

When light encounters an interface, part of it reflects back while another part transmits into the next medium. In the context of fiber optic connectors and splices, the changes in refractive indices at the junctions of the fibers and the surrounding air or optical gel lead to these reflections. Fresnel reflections can cause signal loss and degradation in optical systems but are an inherent characteristic of how light behaves at boundaries.

Other optical effects mentioned in the options, like concentricity loss, ellipticity loss, and Rayleigh scattering, do not describe the fundamental phenomenon that results from mechanical interconnections in optical fibers in the same direct manner as Fresnel reflections do. Concentricity loss refers to misalignment issues, ellipticity loss involves distortion of light modes, and Rayleigh scattering relates to light scattering due to microscopic inhomogeneities within the fiber material rather than interface reflections. Hence, the defining optical effect produced during mechanical interconnections is indeed Fresnel reflections.