Intra-pulse Intensity Noise Shaping in a Mode-locked Fiber Oscillator

Saturable absorbers (SAs) are the key constituent for the generation of ultra-low noise optical pulse trains with mode-locked lasers, which are applied in a variety of cutting-edge scientific fields and industrial applications. In this work, we demonstrate a mechanism for intra-pulse intensity noise shaping in a Kerr-type fiber laser mode-locked with nonlinear polarization evolution (NPE). With direct measurement of SA intra-pulse intensity noise transfer functions, it is shown that the fast SA dynamics periodically re-shape the intra-pulse noise distribution (IPID) with significant noise suppression around the centre of the circulating intra-cavity pulse. This periodic suppression of the intra-cavity IPID via SA dynamics gives a possible explanation for the intrinsic ultra-low intensity noise of mode-locked lasers and can further be utilized as highly functional tool for system optimization.