Ultra-stable optical frequencies for space

Attracted by the prospect of vastly increased bandwidth, a move from the microwave to the optical for space-based communications is now underway. It is important to consider the associated optical frequency source requirements. "Direct detection" systems have been considered for the first phase of commercial intersatellite optical-communication links. "Coherent communication", which offers at least an order of magnitude improvement in capacity, requires spectrally narrow sources, and is more difficult to implement. Ultra-narrow lasers will be needed for low data-rate applications such as the Deep-Space Network (DSN). The required stability and accuracy of the lasers will also depend on the additional demands of any navigation, tracking and "radio/light science" functions served. Cavity-stabilised narrow lasers are expected to be the basis of the best laser oscillators operating in the optical for space applications.