Second-order interference of two independent lasers. Two independent laser light beams are incident to two adjacent input ports of a 1:1 non-polarizing beam splitter (BS), respectively. L₁ and L₂ are two single-mode continuous-wave lasers. D₁ and D₂ are two single-photon detectors. CC is two-photon coincidence count detection system. The mean frequencies of photons emitted by L₁ and L₂ are ν₁ and ν₂, respectively. The distance between the laser and detection planes are all equal. For simplicity, the polarizations and intensities of these two light beams are assumed to be identical, respectively.