Planar-Waveguide External Cavity Laser Stabilization for an Optical Link   with 1E-19 Frequency Stability
  We stabilized the frequency of a compact planar-waveguide external cavity laser (ECL) on a Fabry-P\'erot cavity (FPC) through a Pound-Drever-Hall scheme. The residual frequency stability of the ECL is 1E-14, comparable to the stability achievable with a fiber laser (FL) locked to a FPC through the same scheme. We set up an optical link of 100 km, based on fiber spools, that reaches 1E-19 relative stability, and we show that its performances using the ECL or FL are comparable. Thus ECLs could serve as an excellent replacement for FLs in optical links where cost-effectiveness and robustness are important considerations. 
Cecilia Clivati, Alberto Mura, Davide Calonico, Filippo Levi, Giovanni   A. Costanzo, Claudio E. Calosso and Aldo Godone
physics.optics