Plenary Speakers

Recent advances in mode-locked optical frequency combs in the gigahertz regime based on ion-doped solid-state lasers and optically pumped semiconductor disk lasers are presented. These sources deliver high average power and low noise with repetition rates between 1 and 10 GHz and are scalable beyond 100 GHz. A major development is the single-cavity dual-comb laser, which generates two frequency combs with a precisely controlled, small difference in repetition rate. This architecture enables rapid pump–probe measurements, high-resolution spectroscopy, and sub-micrometer long-distance LiDAR without mechanical delay lines or active cavity stabilization. Single-cavity dual-comb systems based on Yb-doped solid-state lasers are now commercially available.

In the few-gigahertz regime, the peak power is sufficient for low-noise nonlinear frequency conversion, including synchronously pumped optical parametric oscillators and supercontinuum generation. The shared cavity design reduces system complexity and induces strong correlation between the combs with respect to mechanical and environmental perturbations, thereby suppressing uncorrelated noise across most spectral bands in the acoustic (Hz–kHz) range. At higher offset frequencies, noise is predominantly driven by pump intensity fluctuations and can be mitigated through low group-delay dispersion. Optimal performance requires identical transfer functions from pump noise to both relative intensity noise and phase noise in the two combs.

Strong noise correlation permits coherent averaging at small repetition-rate differences, extending usable optical bandwidth prior to spectral aliasing even without any stabilization. A recent hybrid implementation actively controls and sweeps one comb while the second passively tracks via intrinsic cavity correlation. Spectral interleaving then fills the comb-line gaps, enabling resolution of narrow spectral features, such as those encountered in low-pressure gas spectroscopy.

The use of mechanics to set and shape signal frequency content is ever-present in applications that permeate society, from the oscillators that tell time and synchronize communications to the front- end filters that outright enable our smartphones. Microelectromechanical systems (MEMS) have played no small role in the advancement of these capabilities, and this technology continues to shape what’s to come. Specifically, MEMS-based oscillators using nano-scale transducer gaps have come a long way, from early days when smaller was simply deemed less stable, to today’s devices that sport
frequency stabilities capable of challenging atomic clocks in certain application spaces. Since good frequency stability generally permits excellent  sensors, it is not surprising that sensors have recently taken center stage for this technology. Here, nano-scale approaches to suppressing environmental interference, e.g., due to temperature changes, may soon enable leaps in capabilities, such as faster brake response and hydrogen tank health monitoring for future fuel cell vehicles, both of which benefit from sensors that can operate over wide temperature ranges. Meanwhile, on the signal processing front, mechanical circuit approaches employing periodic switching over nanometer-scale gaps have lowered communication dynamic range requirements to levels that now permit low-bit-rate all-mechanical radios that can listen continuously with no battery drain, only consuming power when valid bits arrive. This talk will use examples like the above to chronicle how small-gapped MEMS-based frequency control technology has and continues to transform intelligent system capabilities.