Radio interview with opticlock researchers, broadcast on 10.01.2020.
Video on opticlock at the Laser. World of Photonics:
...can be found on these pages from the PTB.
Article "Tausendmal genauer" on optical clocks by Christian Tamm and Ekkehard Peik in "Physik in unserer Zeit" (in German).
Youtube-videos [german only]: