The second-order interference of two independent single-mode He-Ne lasers

TL;DR

This paper demonstrates second-order interference patterns with two independent He-Ne lasers using a Hong-Ou-Mandel interferometer, revealing that two-photon interference can occur even with different spectra if detection cannot distinguish photons.

Contribution

It provides experimental evidence and theoretical interpretation of second-order interference with independent lasers, expanding understanding of photon indistinguishability.

Findings

Second-order interference observed with independent lasers.

Two-photon interference occurs even with different spectra.

Temporal beating can measure laser coherence and frequency.

Abstract

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.