Very narrow-linewidth (hertz-level) lasers are at the heart of instruments such as atomic clocks and some laser gyroscopes; linewidths this narrow could also have wider benefits in other systems—for example, lidar, spectroscopy, and communications. But hertz-linewidth lasers are typically large, expensive, and finicky. Now, researchers at the California Institute of Technology (Pasadena, CA) and Anello Photonics (Santa Clara, CA) are developing CMOS-foundry-fabricated microresonators with extremely high Q factors, leading to the fabrication of an integrated laser with fundamental noise level below 1 Hz2 Hz-1, or equivalently, a 3 Hz linewidth. Microresonators with a Q of more than 2.6 × 108 and a finesse of more than 42,000 have been fabricated with silicon nitride as the waveguide, silica as the cladding, and silicon as the substrate.
The researchers fabricated ring resonators with a 30 GHz free spectral range (FSR) and racetrack resonators with 5 and 10 GHz FSR, all on one wafer; the ring resonator had the highest Q. To demonstrate very narrow linewidth, a commercial thermoelectric-cooler-controlled 30 mW, 1556-nm-wavelength distributed-feedback (DFB) laser was butt-coupled to the bus waveguide of the resonator chip, with backward optical scattering providing enough feedback into the laser to lock the laser frequency to a resonator mode. The setup allowed the different resonators to be characterized. Turnkey Kerr frequency-comb formation was observed.
Previous very high-Q waveguide resonators were based on challenging geometries such as suspended structures or very large (centimeter-scale) bend radii. In contrast, the new very high-Q resonators are straightforward to fabricate and have a common geometry. The researchers say that even lower propagation losses could be reached; eventually, hybrid integration of III-V lasers with these silicon-based resonators would result in a potential for large-volume production of these hertz-linewidth lasers. Reference: W. Jin et al., arXiv:2009.07390v1 [physics.optics] (Sep. 15, 2020).