Broadband difference frequency generation around 4.2 μm at overlapped phase-match conditions

Broadband difference frequency generation is theoretically and experimentally confirmed. It is shown that a wide tunable range of greater than 220 nm (FWHM) around 4.2 μm can be obtained in a 40 mm long periodically poled lithium niobate crystal with a single quasi-phase-matched period at a certain temperature. The broad bandwidth can be explained by means of the group-velocity matching or phase-mismatch minimization at overlapped phase-match conditions. The result shows that the broadband mid-infrared (mid-IR) laser source may find profound applications in trace gases detection of multiple atmospheric species and high resolution spectroscopy.