 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.
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}.
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