Passively phase-locked and self-controlled femtosecond frequency combs

Advanced applications of femtosecond frequency combs such as front-end time standards or high-resolution spectroscopy still demand improvements of their overall performance. Especially in harsh environments, e.g., outer space, robust and compact table-top sources are desired. To address all of these requirements, we have developed a fully self-controlled frequency comb in the near infrared based on Er:fiber technology. This scheme combines i) cancellation of the carrier-envelope offset frequency via difference frequency generation, ii) extreme phase noise reduction and iii) absolute stabilization of all comb modes. The second and third tasks are achieved by means of direct transfer of the quantum-limited phase jitter of our fundamental mode-locked oscillator to the offset-free comb. At the moment, long-term drifts of the pulse repetition rate are suppressed by direct Doppler-free locking to a two-photon transition in ⁸⁵Rb. We demonstrate both the inherent shot-to-shot and long-term carrier-envelope phase stability as well as measurement-limited timing jitter of the fully self-controlled comb. No radio frequency or cavity-stabilized optical references are required to lock its longitudinal modes, resulting in fully autonomous operation.