Tunable mode-locked laser in the 2.0–2.6 <b> <i>μ</i> </b>m range with GHz-level spectral width and sub-nanosecond pulse duration

Mode-locked lasers with narrow spectral bandwidths and high power spectral density are valuable for various applications that require selective and efficient light–matter interactions. However, mode-locked lasers in the mid-infrared (MIR) region so far face significant challenges in both narrowing the spectral width and expanding the tunable range. In this work, we demonstrate a Fourier transform-limited pulsed laser with ultra-narrow bandwidth by utilizing the frequency-modulated (FM) mode-locking technique in a Cr:ZnS laser. The proposed laser achieves a spectral width of ∼2 GHz (∼40 pm) and a pulse duration of ∼200 ps within a wide tunable range from 1966 to 2572 nm. By incorporating an etalon into the cavity, the spectral width is further narrowed to ∼500 MHz (∼8 pm), and the pulse duration is extended to ∼1 ns. We also find unique detuning characteristics of the FM mode-locked laser in that the narrowband mode-locked state is maintained over a large detuning frequency of ∼20 kHz, while its operating wavelength varies linearly with detuning. This property indicates a significantly better resistance to cavity length fluctuations compared to other active mode-locking schemes as well as a promising way to achieve precise wavelength fine-tuning through electronic control. The demonstrated laser, featuring an ultra-narrow spectral width at a desired mid-infrared wavelength, offers a powerful tool for high-resolution and high-sensitivity spectroscopy and coherent control of molecular vibrations.