Ultrastable lasers are essential for the realization and study of optical clocks. Today’s most stable and spectrally narrow laser sources base on ultrastable cavities. Within a joint project of PTB and JILA we have developed a single-crystal silicon cavity cooled to 124 K with an expected thermal noise limit of mod σy ≈ 4×10-17. A first system presented in 2012 already realized a fractional frequency instability of mod σy ≈ 1×10-16.
Improving the design of the first-generation system, we have set up two independent laser systems employing nearly identical silicon cavities. All technical noise sources, such as temperature fluctuations, seismic and acoustic vibrations as well as residual amplitude modulation, have been carefully studied and suppressed to levels well below the expected thermal noise limit.
We confirmed the individual frequency stabilities of the two systems by a three-cornered hat comparison with a third ultra-stable laser working at 698 nm. This laser is stabilized on a
48 cm long resonator, made from ultra low expansion glass. A femtosecond frequency comb is used to bridge the wavelength gap between the 1.5 µm systems and the 698 nm laser. The modified Allan deviation of both silicon based laser systems show a thermal noise limited flicker floor at mod σy ≈
4×10-17 for averaging times between 1 s and 100 s.
A careful characterization of the phase noise and direct beat measurements indicate an individual linewidth of the lasers around 10 mHz. I will discuss the progress of the current project and the characterization and performance of the systems.