Development of an ultra-stable laser in the 1.5 µm band for optical frequency transfer over optical fibre

Comparison of microwave frequency standards by satellite techniques is currently limited in instability at the 10^-15 level at one day [1]. However, the best optical frequency standards have already demonstrated an instability of ~ 5 × 10^-17 at 2000 s and an estimated uncertainty of 9 × 10^-18 [2]. The predicted improvement in optical frequency standards motivates the need for a new transfer method capable of performing below these predicted levels of instability. The transmission of cw optical frequencies through optical fibre networks is the most promising alternative to satellite methods when it comes to comparing remote optical frequency standards. Intra-lab comparison of optical frequency standards can already be carried out with the use of femtosecond optical frequency combs, which can easily bridge the frequency gap between the different standards under test. However, a particularly more challenging task is to compare different standards at remote European laboratories. The comparison of remote standards, particularly like-for-like systems (such as the Yb⁺ clocks at NPL and PTB), will be important in determining the relative uncertainty and reproducibility of these clocks. For this remote comparison method to operate below the instability level of the optical standards, a highly stable phase-coherent local oscillator is needed so as not to degrade the stability of the atomic reference being compared.