Supermode noise of harmonically mode-locked erbium fiber lasers with composite cavity

The supermode noise of a harmonically mode-locked erbium-doped fiber laser with composite cavity is investigated both theoretically and experimentally. We propose a simple model based on the transfer function of the composite cavity. From this model, the frequencies of the cavity modes and their frequency-dependent losses are determined. A comprehensive experimental study of the composite cavity laser is carried out, covering a wide range of path-length differences between both arms of the fiber interferometer that form the composite cavity. Experimental results are in good agreement with our model. In particular, the path length difference determines periodic frequency windows within which supermodes of the composite cavity are observed. Outside these windows, supermodes are not detectable. Our results show quite remarkably that, although the number of supermodes is reduced with respect to the simple cavity, no measurable reduction of the overall supermode noise is obtained, contrary to what has recently been previously suggested.