Femtosecond Frequency Combs with Few‐kHz Passive Stability over an Ultrabroadband Spectral Range

Abstract

Femtosecond frequency combs are among the most precise measurement tools in existence. They have applications ranging from high-precision spectroscopy and metrology to time-domain quantum physics. Maximizing the passive stability of these instruments is essential to achieve their full potential in fundamental science and high-tech industry. However, the noise mechanisms across the entire operating space of these devices have not been fully characterized. Here the noise properties of fiber-based frequency combs are studied as a function of intracavity dispersion, pump power, and repetition rate. Distinct minima are discovered in this parameter space where the free-running linewidth of the carrier-envelope offset (CEO) frequency fCEO drops below 1 kHz. The individual comb lines are analyzed spread over a wide spectral range producing a complete understanding of the particular contributions to the phase noise and their interplay. Exploiting these findings, combs featuring sharp teeth at specific frequency positions and over the entire spectrum from fCEO to 300 THz are demonstrated. The ultrabroadband stability offered by these compact systems provides a new level of quality for front-end measurement tasks in both time and frequency domains.