The Brillouin fibre laser has experimentally demonstrated its suitability for the measurement of laser linewidth in the 1 kHz to 500 kHz frequency range. A major advantage of this system is to be widely independent of the source wavelength by generating the uncorrelated reference signal at a fixed frequency below the source under test. This solution is simple and affordable, since it only requires widely available standard optical supplies. The measured spectra of a compact diode-pumped Nd:YAG laser and a spectrally-narrowed semiconductor laser are shown