Characterization of flicker noise contributions to the carrier-envelope phase stabilization of femtosecond lasers

In the last decade, carrier-envelope phase (CEP) stabilized passively mode-locked lasers have become an indispensable tool in a variety of fields ranging from optical frequency metrology to attosecond physics. This method enables counting of optical frequencies in the 100 THz range and is currently discussed for the definition of a new frequency standard to deduce the SI unit of the second. The relative precision of these measurements has reached a value of <10⁻¹⁵, which makes the second the most precisely determined unit in the SI system. Moreover, the residual timing jitter between carrier and envelope amounts to < 10 attoseconds and constitutes the best temporal synchronization ever obtained. While methods for CEP stabilization have been substantially refined over the years, careful verification measurements of the obtained phase stability with a second independent interferometer always revealed a flicker noise contribution, which amounts to about 1 mHz/Hz^1/2 for Ti:sapphire based frequency combs and is about 100–1000 times larger for fiber-laser based combs. While this noise contribution does seem to play a role at the 10⁻¹⁵ precision level, it will eventually hinder further improvement in frequency metrology.