Visible Lasers with Subhertz Linewidths

We report a visible laser with a subhertz linewidth for use in precision spectroscopy and as a local oscillator for an optical frequency standard. The laser derives its stability from a well-isolated, high-finesse, Fabry-P\'erot cavity. For a 563 nm laser beam locked to our stable cavity, we measure a linewidth of 0.6 Hz for averaging times up to 32 s. The fractional frequency instability for the light locked to the cavity is typically 3×10^-16 at 1 s. Both the linewidth and fractional frequency instability are approximately an order of magnitude less than previously published results for stabilized lasers.