  Laser with high spectral purity plays a crucial role in high-precision
optical metrology and coherent communication. Thanks to the rapid development
of laser frequency stabilization, the laser phase noise can be remarkably
compensated, allowing its ultra-narrow linewidth subject to mostly quantum
limit. Nevertheless, the accurate characterization of phase noise dynamics and
its intrinsic linewidth of a highly coherent laser remains ambiguous and
challenging. Here, we present an approach capable of revealing
delay-time-resolved phase noise dynamics of a coherent laser based on coherent
optical time domain reflectometry (COTDR), in which distributed Rayleigh
scattering along a delay fibre essentially allows a time-of-flight mapping of a
heterodyne beating signal associated with delay-time-dependent phase
information from a single laser source. Ultimately, this novel technique
facilitates a precise measurement of ultra-narrow laser linewidth by exploiting
its delay-time-resolved phase jitter statistics, confirmed with the analytical
modelling and numerical simulations.
