There are wavelength ranges where the generation of coherent radiation is more difficult than others and novel concepts of light sources need to be developed. In the mid-infrared (MIR) region, where direct laser generation is rather complicated, a good option is offered by nonlinear frequency down-conversion processes in optical parametric oscillators (OPOs). OPOs can be exploited to generate high-quality optical frequency combs, which are of much interest for the realization of high-resolution and high-speed systems for advanced spectroscopy, diagnostic and sensing. However, conventional OPOs are typically bulky and solutions for generating MIR-range combs in compact devices operated with a low threshold power are required.

In this work, X. Wang and coauthors demonstrate efficient generation of MIR frequency combs in a small-size lithium niobate resonator with a ultra-high quality factor (4.0 × 10⁷ ). Upon CW pumping at 1048 nm, highly-coherent comb generation at 2 μm is observed across a 40-nm-wide wavelength range, with a threshold power of only 80 mW, a maximum output power of 340 mW, and a repetition frequency of 1.426 GHz. Notably, this OPO concept can be flexibly shifted to longer wavelengths by optimizing the design of the resonator and can be also ported to other technology platforms, such as lithium niobate on insulator (LNOI), thus enabling integration with other optical functionalities. This advance is likely to open new avenues in many applications for gas sensing, metrology and spectroscopy.

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