LAUSR

A 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 $$\gamma $$. 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 $$\gamma _{01}$$ and $$\gamma _{11}$$, respectively, are the complex degrees of coherence for the interferences between the $$\hbox {LP}_{01}$$ and $$\hbox {LP}_{11}$$ modes propagating through a TMF and a wave traversing air-path, the interference effect can be verified using the relationship $$|\gamma |^2=|\gamma _{01}+\gamma _{11}|^2$$. 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 $$|\gamma |^2$$-response waveforms have oscillatory structures due to the interference between $$\gamma _{01}$$ and $$\gamma _{11}$$. A $$|\gamma |^2$$-response measurement employing the TBI reveals that the theoretically obtained $$|\gamma |^2$$-curves well reflect $$|\gamma |^2$$-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.