A phase-locked frequency divide-by-3 optical parametric oscillator
  Accurate phase-locked 3:1 division of an optical frequency was achieved, by using a continuous-wave (cw) doubly resonant optical parametric oscillator. A fractional frequency stability of 2*10^(-17) of the division process has been achieved for 100s integration time. The technique developed in this work can be generalized to the accurate phase and frequency control of any cw optical parametric oscillator. 
A. Douillet, J.-J. Zondy, G. Santarelli, A. Makdissi, A. Clairon   (BNM-LPTF/Observatoire de Paris)