lightpath

Lightpath is a term used in both physics and telecommunications to describe the path that light takes, though the meanings differ by context. In optics, a lightpath refers to the trajectory of a light ray as it propagates through media, governed by principles such as Fermat’s principle, Snell’s law, and the refractive index. The concept is often analyzed using geometric optics, where lightpaths are approximated as straight lines in uniform media and are bent at interfaces between different media. The optical path length, defined as the integral of the refractive index along the path, is a key quantity in calculating the actual route of light.

In telecommunications and optical networking, a lightpath denotes an end-to-end optical channel established across an optical network. It carries a single wavelength (in dense wavelength-division multiplexing, or WDM, systems) or a defined optical channel, and remains in the optical domain from source to destination, potentially traversing components such as ROADMs and optical switches. Lightpaths can be static or provisioned on demand and are used to create dedicated, circuit-like connections over fiber.

Provisioning a lightpath involves routing a path through the network and assigning a wavelength, while accounting for wavelength continuity, guard bands, dispersion, and, in some cases, optical-electronic regeneration. Control planes such as GMPLS automate lightpath setup in many modern networks. Networks can be transparent, allowing signals to pass without intermediate electrical regeneration, or opaque, in which regeneration occurs at intermediate nodes. The term is commonly encountered in fiber-optic communications and WDM architectures supporting multiple concurrent lightpaths on a single fiber.