Comparison of Optical Frequency Standards
Current optical frequency standards are complex research-oriented systems and cannot readily be transported. Comparison of standards in different laboratories therefore depends at present on making measurements of their frequency relative to local caesium primary frequency standards using femtosecond combs, with satellite time-transfer techniques being used to compare the caesium standards. However these techniques are only just adequate to support intercomparisons at the part in 1015 level. Optical frequency standards have already demonstrated similar accuracy and superior stability to caesium fountain primary frequency standards, with excellent prospects for further improvement by several orders of magnitude. This creates a pressing need for new methods of frequency transfer with unprecedented levels of stability to enable optical frequency standards in remote locations to be compared.
Optical fibres provide a promising alternative for stable distribution of either microwave or optical frequency references, since they are low loss, readily scalable, and can be environmentally isolated. Furthermore there is an extensive existing infrastructure in place in the form of optical telecommunications networks.
Several distinct methods for distribution of ultrastable reference frequencies have been demonstrated over the past few years. The most extensively studied technique is the transfer of a microwave frequency reference using an amplitude-modulated cw laser. More appropriate for remote comparison of optical frequency standards is to directly transfer an optical frequency using a cw laser. The improved timing resolution available from the higher frequency optical carrier offers the potential for several orders of magnitude reduction in instability.
The ability to compare remote optical frequency standards in this way will be of vital importance to the international metrology community in assessing their suitability for a potential future redefinition of the second. However techniques for high stability frequency transfer will also be beneficial for a variety of other applications including tests of fundamental physical theories, precision spectroscopy and remote synchronization.