Development of a high-Doppler shift optical simulator to test femtosecond-level optical time–frequency transfer

We have developed an optical simulator to test real-time optical two-way time–frequency transfer at the femtosecond level capable of simulating relative motion between two linked optical clock nodes up to Mach 1.8 with no moving parts. The technique is enabled by artificially Doppler shifting femtosecond pulses from auxiliary stabilized optical frequency combs. These pulses are exchanged between the nodes to simulate the Doppler shifts observed from a changing optical path length. We can continuously scan the simulated velocity from 14 to 620 m s −1 while simultaneously measuring velocity-dependent clock shifts at much higher velocities than previously recorded. This system provides an effective testbed that allows us to explore issues and solutions to enable femtosecond-level optical time transfer between optical clocks moving at high relative velocities.