Broadband analysis and self-control of spectral fluctuations in a passively phase-stable Er-doped fiber frequency comb

Abstract

Carrier-envelope and optical phase noise of a femtosecond frequency comb based on Er-doped fiber technology is investigated and minimized without exploiting active external references. Ultrabroadband, coherent, and tailorable supercontinua are generated in a highly nonlinear germanosilicate fiber assembly. Difference frequency mixing between comb modes in their spectral extrema passively eliminates the carrier-envelope phase slip. This step generates an inherently offset-free comb with a relative frequency stability better than 10−21. In contrast, the phase fluctuations at the carrier frequency of 193 THz are increased as compared to the fundamental comb. Their level matches the value found by parabolic extrapolation of the phase noise of the fundamental comb to zero frequency. The latter is unambiguously accessible by means of any beat note centered at the carrier-envelope offset frequency. All these findings rely on strong correlations between the comb modes that are quantitatively described by an elastic tape model, underlining the deterministic character of the processes involved. The superior optical phase noise of the fundamental comb is transferred to the difference-frequency comb while not compromising the inherent cancellation of the carrier-envelope offset frequency. In this way, the optical linewidth of the passively phase-locked comb is reduced from 100 kHz to a measured value of 5 kHz, which is limited by the cw laser reference used for out-of-loop characterization.