Frequency-Modulation Mode-Locked Laser with GHz Spectral Width Tunable in the 2-3 um Region

A narrow-bandwidth actively mode-locked laser using a Cr:ZnS gain medium has been successfully demonstrated. A free-space electro-optic phase modulator is employed in the solid-state laser resonator to achieve frequency-modulation (FM) mode-locking, which achieves a narrow spectral width of ~1 GHz and a pulse duration of ~500 ps over a wide tuning range of 1947-2445 nm. The operation frequency of the modulator determines the repetition rate of the mode-locked pulse train and can stabilize it to millihertz-level without any additional feedback loop systems. We also study the theoretical expression of pulse duration and spectral width in a FM mode-locking in a laser cavity that contains considerable group-delay dispersion. The results indicates that larger intracavity dispersion can only stabilize the laser operation by avoiding mode switching, but also narrow the spectral width and increase the pulse duration. The proposed laser features a narrow spectral width at a desired mid-infrared wavelength and a comb-like spectral structure with stabilized longitudinal mode spacing, providing a powerful tool for sensing and control of molecules.