Two independent lasers, each with their own silicon Fabry-Perot cavities and with laser linewidths of just 0.01 Hz, have been created by researchers at Physikalisch-Technische Bundesanstalt (PTB; Braunschweig, Germany) and JILA, a joint institute of the National Institute of Standards and Technology and the University of Colorado (Boulder, CO).1 This is a world record, and far smaller than the more-usual kilohertz- or megahertz-scale laser linewidths. The finesse of each Fabry-Perot cavity is close to 500,000. When cooled to 124 K, the commercial erbium-doped distributed feedback (DFB) fiber lasers, when stabilized by the silicon cavities, have a beat note between them of as small as 5 mHz at their 194 THz operating frequency (1542 nm wavelength). The mutual phase coherence times for these two lasers range up to 55 s. The thermal Brownian noise of the cavity mirrors is what limits the linewidth of these lasers from being even narrower.
Since there was no other comparably precise laser in the world, the scientists working on this collaboration had to set up two such laser systems. Only by comparing these two lasers to each other was it possible to prove the outstanding properties of the emitted light. The core piece of each of the lasers is a 21-cm long Fabry-Perot silicon resonator. The two resonator mirrors are kept at a fixed distance using a double silicon cone. Stabilization electronics ensure that the light frequency of the laser constantly follows the natural frequency of the resonator. The new lasers are now being used both at PTB and at JILA to further improve the quality of optical atomic clocks and to carry out new precision measurements on ultracold atoms. At PTB, the ultrastable light from these lasers is already being distributed via optical waveguides and is then used by the optical clocks in Braunschweig. The scientists from this collaboration see further optimization possibilities. With novel crystalline mirror layers and lower temperatures, the disturbing thermal noise can be further reduced. The linewidth could then even become smaller than 1 mHz. Reference: D. G. Matei et al., Phys. Rev. Lett., 118, 2632202 (2017); https://doi.org/10.1103/physrevlett.118.263202.