Narrow-linewidth, piezoelectrically tunable photonic integrated blue laser

Frequency-agile lasers operating in the ultraviolet–blue spectral range (360–480 nm) are necessary for a wide range of technologies, including free-space and underwater optical communications, optical atomic clocks, and Rydberg-atom-based quantum computing. Integrated lasers offer a compelling platform for these applications in a compact, robust form factor but remain challenging to realize in this wavelength range. Here we report a photonic integrated blue laser operating at 461 nm, which simultaneously achieves frequency agility and low phase noise. This implementation is based on the hybrid integration of a gallium-nitride-based laser diode, which is self-injection locked to a high- Q microresonator fabricated on a low-loss silicon nitride photonic platform with a propagation loss of ~0.4 dB cm −1 . The laser exhibits a sub-30-kHz linewidth and delivers over 1 mW of optical output power. In addition, aluminium nitride piezoelectric actuators are monolithically integrated onto the photonic circuitry to enable the high-speed modulation of refractive index and allow laser frequency tuning. This enables mode-hop-free laser linear frequency chirps up to 900 MHz at repetition rates up to 1 MHz, with tuning nonlinearity below 2%. We showcase the potential applications of this integrated laser in underwater communication and coherent aerosol sensing experiments.