Ultrafine-spaced InGaN-based DFB laser arrays enabled by continuous phase modulation

High-performance blue-violet lasers are essential for compact quantum sensing and metrology. However, light sources in this spectral range currently rely on bulky external cavities or complex nonlinear frequency conversion, limiting their robustness and field deployment. While monolithic InGaN-based distributed feedback laser diodes (DFB LDs) offer a path toward miniaturization, precisely targeting narrow atomic transitions remains challenging due to fixed grating periods and fabrication-induced wavelength deviations. In this work, we demonstrate a 420-nm DFB laser array with an ultrafine 0.2-nm channel spacing, enabled by a continuously phase-shifted grating design. This approach allows for precise wavelength control without compromising coupling efficiency, while significantly enhancing the array's robustness against fabrication variations. By integrating ten devices, the array spans a 2-nm spectral range around 420 nm, enabling coarse wavelength selection through individual device addressing. This strategy minimizes the required thermal or electrical tuning range, thereby keeping the LD within its optimal operating regime while reaching the target atomic transitions. The fabricated LDs exhibit a 40 mA threshold current, 1 W/A slope efficiency, and up to 40 mW single-mode output power, with tuning coefficients of 1.6 pm/mA and 20 pm/^∘C for current and temperature, respectively. Optical heterodyne measurements reveal a Lorentzian linewidth of 9.8 MHz, highlighting a practical and manufacturable path toward robust blue-violet light sources for next-generation chip-scale atomic and quantum photonic systems.