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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.

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}|.