  Narrow linewidth visible light lasers are critical for atomic, molecular and
optical (AMO) applications including atomic clocks, quantum computing, atomic
and molecular spectroscopy, and sensing. Historically, such lasers are
implemented at the tabletop scale, using semiconductor lasers stabilized to
large optical reference cavities. Photonic integration of high spectral-purity
visible light sources will enable experiments to increase in complexity and
scale. Stimulated Brillouin scattering (SBS) is a promising approach to realize
highly coherent on-chip visible light laser emission. While progress has been
made on integrated SBS lasers at telecommunications wavelengths, barriers have
existed to translate this performance to the visible, namely the realization of
Brillouin-active waveguides in ultra-low optical loss photonics. We have
overcome this barrier, demonstrating the first visible light photonic
integrated SBS laser, which operates at 674 nm to address the 88Sr+ optical
clock transition. To guide the laser design, we use a combination of
multi-physics simulation and Brillouin spectroscopy in a 2 meter spiral
waveguide to identify the 25.110 GHz first order Stokes frequency shift and 290
MHz gain bandwidth. The laser is implemented in an 8.9 mm radius silicon
nitride all-waveguide resonator with 1.09 dB per meter loss and Q of 55.4
Million. Lasing is demonstrated, with an on-chip 14.7 mW threshold, a 45% slope
efficiency, and linewidth narrowing as the pump is increased from below
threshold to 269 Hz. To illustrate the wavelength flexibility of this design,
we also demonstrate lasing at 698 nm, the wavelength for the optical clock
transition in neutral strontium. This demonstration of a waveguide-based,
photonic integrated SBS laser that operates in the visible, and the reduced
size and sensitivity to environmental disturbances, shows promise for diverse
AMO applications.
