A method may include generating first and second optical signals and modulating the first and second optical signals simultaneously to synchronize the signals. The method may include varying the polarization of the second optical signal and transmitting the first and second optical signals. The method may include receiving the first and second optical signals, wherein the signals traveled along a same optical path. The method may include determining a plurality of differential travel delays between the first and second optical signals over a period of varying polarizations and determining a differential group delay based on the maximum and the minimum differential travel delays.