Differential group delay

In optics, differential group delay is the difference in propagation time between the two eigenmodes X and Y polarizations. Consider two eigenmodes that are the 0° and 90° linear polarization states. If the state of polarization of the input signal is the linear state at 45° between the two eigenmodes, the input signal is divided equally into the two eigenmodes. The power of the transmitted signal E_T,total is the combination of the transmitted signals of both x and y modes.

$E_T = (E_{i,x} \cdot t_x)^2 + (E_{i,y} \cdot t_y)^2 \,$

The differential group delay D_t is defined as the difference in propagation time between the eigenmodes: D_t = |t_t,x − t_t,y|.