  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.
