Characterization of carrier-envelope phase noise of passively phase-locked fiber-based frequency combs up to the nyquist frequency

Applications of optical frequency combs in high precision metrology [1] require low-noise stabilization of its carrier-envelope offset (CEO) frequency. This task is commonly achieved via active feedback. Fully passive elimination of the CEO frequency based on difference frequency generation (DFG) between two octave-separated comb sections followed by amplification of the DFG signal in the EDFA has been demonstrated recently in an all-fiber design [2]. In this work, we develop a novel broadband characterization method of carrier-envelope phase (CEP) noise and apply it to study the passively phase-locked 100 MHz Er:fiber comb [3].