Power-stabilized 3-W blue laser locked to the 420-nm transition in rubidium

Using modulation transfer spectroscopy, we achieve the frequency stabilization of a high-power 3-W blue laser at the wavelength of 420 nm to the $\mathrm{Rb}$ $5S_{1/2}$–$6P_{3/2}$ transition. The in-loop frequency stability of this laser is $1.8\times {10}^{-11}/\sqrt{\tau }$, reaching $1.7\times {10}^{-12}$ at 100 s and $7\times {10}^{-13}$ at 1000 s. An external power feedback loop is established using an acousto-optic modulator, employing the zeroth-order diffracted light for power stabilization, achieving an in-loop power stability of $1.0\times {10}^{-6}$ at 1 s. Moreover, the continuous mode-hop free interval of this high-power laser can simultaneously cover the $5S_{1/2}$–$6P_{3/2}$ transitions of ${}^{85}\text{Rb}$ and ${}^{87}\text{Rb}$, with successful locking achieved for both isotopes, providing a comprehensive analysis of the $\mathrm{Rb}$ atomic transitions in the blue spectral region. As an application, this 3-W 420-nm laser with excellent power and frequency stabilities is used as a repumping source for diffuse laser cooling of ${}^{87}\text{Rb}$ atoms, realizing a one-meter-long cold-atom cloud. This paves the way for using blue-light cooling to realize a cold-$\mathrm{Rb}$-atom active optical clock. In addition, the ultrahigh-stability 420-nm laser also finds applications in various fields such as the Rydberg atom experiment, Bose-Einstein condensation, ultranarrow-bandwidth Faraday atomic filter, and so on.