  Narrow-linewidth lasers are important to many applications spanning precision
metrology to sensing systems. Characterization of these lasers requires precise
measurements of their frequency noise spectra. Here we demonstrate a correlated
self-heterodyne (COSH) method capable of measuring frequency noise as low as
0.01 Hz$^2$/Hz at 1 MHz offset frequency. The measurement setup is
characterized by both commercial and lab-built lasers, and features low optical
power requirements, fast acquisition time and high intensity noise rejection.
