The second- and the fourth-order interferences observed in a two-photon interferometer

We design a two-photon interferometer in which the second-order interference of ‘two-photon’ can be observed. This is realized by introducing a relative phase shift between different path combinations which the photon pairs choose to take. In this interferometer, both the second- and the fourth-order interferences occur when we adjust the time delay continuously. This phenomenon can help us get a better understanding of the second- and the fourth-order interferences.