  Widely-tunable and narrow-linewidth integrated lasers across all visible
wavelengths are necessary to enable on-chip technologies such as quantum
photonics, optical trapping, and biophotonics. However, such lasers have not
been realized due to the strong wavelength sensitivity and high loss of
integrated optical devices at short wavelengths. Here we overcome these
challenges and demonstrate a chip-scale visible lasers platform by using
tightly-confined, micrometer-scale silicon nitride resonators and commercial
Fabry-Perot laser diodes. We achieve tunable and narrow-linewidth lasing in the
deep blue (450 nm), blue (488 nm), green (520 nm), red (660 nm) and near-IR
(785 nm) with coarse tuning up to 12 nm, fine tuning up to 33.9 GHz, linewidth
down to sub-kHz, side-mode suppression ratio > 35 dB, and fiber-coupled power
up to 10 mW. These specifications of our chip-scale lasers are comparable to
those of state-of-the-art commercial laser systems, making them stand out as
powerful tools for the next generation of visible-light technologies.
