Nature Phys. 10, 30–33 (2014)

Optical frequency combs with high-intensity pulses are desirable for realizing a much wider range of capabilities and applications for ultrahigh precision, direct frequency comb spectroscopy. Now, Jonas Morgenweg and colleagues from VU University in the Netherlands have presented an approach that can achieve not only millijoule pulse energies, but also improved frequency comb resolution and accuracy. Their method of Ramsey-comb spectroscopy employs a series of excitations with two selectively coherently amplified pulses from a frequency comb laser. The delay between these pulses can be varied over a wide range without affecting the optical phase relationship. The coarse delay of the pulse pairs can be changed in steps of the frequency comb repetition time, whereas fine-tuning for a Ramsey scan is achieved by small adjustments of the repetition time. The capabilities of Ramsey-comb spectroscopy were demonstrated by investigating weak two-photon transitions in atomic rubidium and caesium. The team achieved frequency comb pulses with an energy of 5 mJ, which is orders of magnitude more energetic than previous studies. Furthermore, they realized a frequency accuracy that is up to 30 times better than that of traditional frequency comb spectroscopy.