  Phase noise or frequency noise is a key metrics to evaluate the short term
stability of a laser. This property is of a great interest for the applications
but delicate to characterize, especially for narrow line-width lasers. In this
letter, we demonstrate a digital cross correlation scheme to characterize the
absolute phase noise of sub-hertz line-width lasers. Three 1,542 nm
ultra-stable lasers are used in this approach. For each measurement two lasers
act as references to characterize a third one. Phase noise power spectral
density from 0.5 Hz to 0.8 MHz Fourier frequencies can be derived for each
laser by a mere change in the configuration of the lasers. This is the first
time showing the phase noise of sub-hertz line-width lasers with no reference
limitation. We also present an analysis of the laser phase noise performance.
