Comparing ultrastable lasers at 7 × 10−17 fractional frequency instability through a 2220 km optical fibre network.

Ultrastable lasers are essential tools in optical frequency metrology enabling unprecedented measurement precision that impacts on fields such as atomic timekeeping, tests of fundamental physics, and geodesy. To characterise an ultrastable laser it needs to be compared with a laser of similar performance, but a suitable system may not be available locally. Here, we report a comparison of two geographically separated lasers, over the longest ever reported metrological optical fibre link network, measuring 2220 km in length, at a state-of-the-art fractional-frequency instability of 7 × 10⁻¹⁷ for averaging times between 30 s and 200 s. The measurements also allow the short-term instability of the complete optical fibre link network to be directly observed without using a loop-back fibre. Based on the characterisation of the noise in the lasers and optical fibre link network over different timescales, we investigate the potential for disseminating ultrastable light to improve the performance of remote optical clocks. Precision measurement plays an important role in frequency metrology and optical communications. Here the authors compare two geographically separate ultrastable lasers at 7 × 10⁻¹⁷ fractional frequency instability over a 2220 km optical fibre link and these measurements can be useful for dissemination of ultrastable light to distant optical clocks. [ABSTRACT FROM AUTHOR]