  We report an experiment of observation of classical double-slit interference
fringes of two-photon interference. In the experiment, a commercial
continuous-wave multi-mode F-P laser diode without either mode-locked or
frequency-locked is used as the light source, the density of photons is far
more than the single-photon level, and the path difference of the long and
short paths is far more than the longitudinal coherence length of the laser
diode. The temporal stable and clearly visible spatial-distributed pattern,
i.e. first-order interference fringes, was observed .Contradict to the
prediction of single-photon interference mechanism, the interference happened
far beyond the coherence length, the occurrence possibility and time duration
of the interference fringes decrease with the reduction of mode number of the
LD, and the time difference between the fringes disappearance and the light
vanish is equal to the time difference of the two paths. After discussion, we
came to the conclusion that the observed phenomena can be understood in
time-resolved two-photon interference mechanism. We reveal a new method to
perform two-photon first-order interference, and this help to understand the
nature of two-photon interference and also can be useful for quantum
information science.
