  We demonstrate that the implementation of phase-locked loop forbidding
multimode operation of a long Brillouin resonator also leads to a dramatic
reduction of the optical phase noise of the pump itself. In the case of a
continuous Er,Yb:glass laser, a reduction by more than 90 dB at 100 Hz of the
carrier is observed. This yields an optical linewidth estimated narrower than 2
Hz for the pump laser. The method being independent of the laser wavelength, it
can be applied to almost any laser.
