Interference phenomenon on the complex degree of coherence

two-beam interferometer (TBI) with a two-mode fiber (TMF) as a dispersive medium in one arm and air in the other provides evidence of interference related to the complex degree of coherence γ . A TMF with a low differential group delay (DGD) is used to measure the interference effect, since the overlapping of each individual mode at the output of a TMF is easily and artificially created in the vicinity of the zero DGD wavelength. When γ _01 and γ _11 , respectively, are the complex degrees of coherence for the interferences between the LP_01 and LP_11 modes propagating through a TMF and a wave traversing air-path, the interference effect can be verified using the relationship |γ |^2=|γ _01+γ _11|^2 . The experiments are carried out using a low coherence source and three kinds of TMFs with zero DGD wavelengths around 1550 nm. It is clearly observed that |γ |^2 -response waveforms have oscillatory structures due to the interference between γ _01 and γ _11 . A |γ |^2 -response measurement employing the TBI reveals that the theoretically obtained |γ |^2 -curves well reflect |γ |^2 -curve obtained experimentally for a temporally overlapped wave composed of the two modes. To the best of our knowledge, this is the first experimental demonstration of the interference between complex degrees of coherence.