Brillouin Assisted Optoelectronic Self-Narrowing of Laser Linewidth

We propose and demonstrate the possible self-narrowing of a laser line using optoelectronic and Brillouin approaches. Part of the laser is used to feed a long fiber Brillouin resonator whose Stokes wave is phase locked to the laser. The phase-locked loop which first forbids the multimode operation of the Brillouin resonator enables in a second time a dramatic reduction in the optical phase noise of the laser 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 reduction to 2 Hz. The method being independent of the laser wavelength, it can be implemented to almost any laser.