Fully stabilized Er fiber comb at 1 GHz by harmonic modelocking

Modelocked frequency comb lasers have always operated with a single pulse circulating in the laser cavity. This meant that each laser technology had an associated limit on pulse repetition rate. Achieving higher rates required different technology, for example exchanging optical fiber for solid state gain media. However, this is a perceived, rather than a fundamental limit. We demonstrate a new fiber laser design with multiple pulses circulating in the fiber gain cavity with the same high precision as a conventional fiber frequency comb. This has the immediate benefit of bringing mature fiber technology to higher repetition rate frequency combs. More generally, it adds great design freedom to laser engineering, where the laser can be separated into an optical cavity and a gain medium that are combined using standard frequency comb techniques. Fundamental frequency comb performance improvements may even be possible from the filtering intrinsic to our design, or by incorporating high stability cavities directly into the laser itself.