The second-order interference of two independent single-mode He-Ne   lasers
  The second-order spatial and temporal interference patterns with two independent single-mode He-Ne lasers are observed in a Hong-Ou-Mandel interferometer. Two-photon interference in Feynman's path integral theory is employed to interpret the experimental results. The conditions to observe the second-order interference pattern with two independent single-mode continuous wave lasers are discussed. It is concluded that two-photon interference exists for not only identical photons, but also photons with different spectrums if the detection system can not distinguish them in principle. The second-order temporal beating with two independent lasers can be employed to measure the coherence time and frequency of one laser if the properties of the other laser were known. 
Jianbin Liu, Minglan Le, Bin Bai, Wentao Wang, Hui Chen, Yu Zhou,   Fu-Li Li, and Zhuo Xu
physics.optics quant-ph