  We stabilized the frequencies of two independent Nd:YAG lasers to two
adjacent longitudinal modes of a high-finesse Fabry-P\'erot resonator and
obtained a beat frequency instability of 6.3 mHz at an integration time of 40
s. Referred to a single laser, this is $1.6\times10^{-17}$ relative to the
laser frequency, and $1.3\times10^{-6}$ relative to the full width at half
maximum of the cavity resonance. The amplitude spectrum of the beat signal had
a FWHM of 7.8 mHz. This stable frequency locking is of importance for
next-generation optical clock interrogation lasers and fundamental physics
tests.
