Abstract

Contrary to lasers with population inversion, the Brillouin fiber laser is a light source which is very sensitive to the phase noise of the pump laser. This property is attributable to the strong nonlinear coupling between the two optical waves and the acoustic wave interacting inside the fiber. From the usual three-wave model of stimulated Brillouin scattering, we first derive an expression connecting the phase of the incident pump field to that of the emitted Stokes wave [1]. It shows that the phase noise of the pump laser is transferred to the Stokes wave after being strongly reduced and smoothed under the combined influence of the acoustic damping and the cavity feedback (see Figure). The magnitude of the resulting linewidth narrowing is given by the ratio K² between the linewidth of the pump laser and that of the Brillouin laser. The coefficient K² simply depends on the acoustic damping rate and on the cavity loss rate; its value can be greater than 10⁴ in all-fiber Brillouin lasers. Brillouin lasers are thus highly coherent light sources presenting potential interest in particular for metrological applications [2]. Finally, our theoretical prediction is quantitatively validated by experiments in which we record the response of a Brillouin fiber ring laser to a frequency modulation of the pump field.

© 2000 IEEE