The rate of change of the total phase shift with respect to angular frequency, D /d , through a device or Transmission medium, where is the total phase shift, and is the angular Frequency equal to 2f , where F is the frequency.

In an optical fiber, the transit time required for optical power, traveling at a given mode’s group velocity, to travel a given distance. Note: For optical Fiber Dispersion measurement purposes, the quantity of interest is group delay per unit length, which is the reciprocal of the Group Velocity of a particular mode. The measured group delay of a signal through an Optical Fiber exhibits a Wavelength dependence due to the various dispersion mechanisms present in the fiber.