Frequency comparisons of optical frequency standards and new results on a long-distance carrier-phase optical fiber link

The transmission of frequency information via fiber optic telecommunication networks offers an attractive option for a long distance frequency comparison and future public dissemination of ultra-precise frequencies. Here, mode locked erbium-doped fiber lasers that combine the field of frequency metrology with that of optical telecommunication, enable a new and attractive opportunity for a direct frequency comparison of distant optical clocks which circumvents a down-conversion of the optical frequency into the microwave domain.