The performance of ultrastable lasers sets the measurement speed of optical frequency standards.
In this talk I will present our efforts toward improving the fractional frequency instability of
ultrastable lasers below 6×10 -17 . I will talk about our laser at 6×10 -17 fractional instability, based on a
room-temperature 48.5 cm long cavity, whose performance has been assessed with a comparison
with another laser based on a cryogenic cavity at PTB, through a 2220 km long optical fibre link [1]. I
will present our work on an improved version of the 48.5 cm long cavity with larger radius of
curvature mirrors, reducing the thermal noise limit to 4×10 -17 . Finally, I will introduce preliminary
development work on a room-temperature 68.5 cm long cavity with an estimated thermal-noise-
limited instability at 2×10 -17 .
[1] M. Schioppo, et al., Comparing ultrastable lasers at 7×10 -17 fractional frequency instability through a 2220 km optical
fibre network, Nat. Commun. 13, 212 (2022).