Nonlinear coupling of relative intensity noise from pump to a fiber ring laser mode-locked with carbon nanotubes.

Pump relative intensity noise (RIN) has been recognized as a major source of noise in mode-locked lasers. The coupling of RIN from the pump to the output of a passively mode-locked fiber laser (PMFL) is systematically investigated using a pump modulation technique. It is found that the linear RIN coupling ratio from pump to PMFL is decreased with an increase in modulation frequency and is independent of modulation power. Moreover, the nonlinear RIN coupling from pump to PMFL is clearly demonstrated with a square wave modulated pump. The nonlinear RIN coupling ratio is noise power dependent. An exponential decay model based on the view of gain modulation is proposed and explains well the behavior of the nonlinear coupling phenomena.