Mechanism of intensity noise suppression in Yb-doped phase-biased mode-locked NALM lasers

Emerging applications in quantum spectroscopy, information processing, and time synchronization depend heavily on low-noise mode-locked lasers, as do microwave photonics and laser microscopy. Mode-locked fiber lasers have become a staple in this regard, most recently, the robust and self-starting phase-biased nonlinear amplifying loop mirror (NALM) lasers built with polarization-maintaining fiber. These lasers produce very low relative intensity noise (RIN), which has been further reduced by intracavity and extracavity filtering of amplified spontaneous emission (ASE) or via phase-bias variation, where in both cases the laser output's center wavelength was longer than the gain medium's peak emission wavelength. Here, for the first time for such lasers, we combine noise and power measurements with intracavity spectral filtering and measurement of pump-to-output transfer function. This novel approach reveals that filtering of pump noise via saturation of self-amplitude modulation (SAM) dominates RIN suppression across the board, where ASE plays a secondary role. We utilize this understanding to suppress RIN below the noise floor of our measurement system (-137 dBc/Hz over 100 Hz-1 MHz) by maximizing the saturation of SAM through adjustments of pump power and phase bias. Our findings make for a systematic and passive method for suppressing RIN.