A passive quadrature phase detection system for coherent fiber systems includes first and second optical detectors positioned to receive an output signal from the output end of an optical signal apparatus such as an interferometer, or the like. The optical signal from the apparatus includes light propagating in two propagation modes. As the light propagates away from the output end of the apparatus from the near field to the far field, the light in the two modes undergoes a relative phase shift of .pi./2 in accordance with the Guoy effect. The two detectors are positioned such that the first detector detects the intensity of light resulting from the interference between the two modes in the near field of the output signal, and such that the second detector detects the intensity of light resulting from the interference between the two modes in the far field of the output signal. The additional .pi./2 phase difference introduced between the two modes as the light propagates from the near field light to the far field causes the detected light intensities to be in phase quadrature. Electrical signals proportional to the detected light intensities can be processed to determine changes in the phase difference between the two modes within the apparatus. In preferred embodiments, the detection of the near field light intensity is accomplished at a position optically displaced from the output end of the apparatus.