Multi-delay, phase-coherent pulse pair generation for precision Ramsey-comb spectroscopy

The realization of optical frequency combs has revolutionized the field of precision metrology [1]. However, these combs typically operate in the near-infrared, while many interesting atomic transitions require wavelengths shorter than 100 nm. Hence high peak power (frequency comb) pulses are required for efficient nonlinear up-conversion of comb lasers. Our approach to realize this goal is based on selective amplification of two frequency comb pulses in a parametric amplifier, and we demonstrated MHz-level extreme ultra-violet frequency comb spectroscopy [2]. The fixed delay of the amplified pulse pairs (maximum ~10 ns) used in [2] limited the achievable accuracy and restricted the method to single-transition recording, as in traditional Ramsey spectroscopy [3]. In addition, the phase-shift introduced by the parametric amplifier had to be monitored carefully to correct the Ramsey-like signals.