Ultra-Low Repetition Rate Frequency Comb for Precision Spectroscopy

Optical frequency combs (OFCs) are a powerful tool for optical frequency metrology [1]. In the frequency domain, OFCs consist of spectral modes equally spaced by the repetition rate, which typically lies in the MHz-GHz range. Frequency conversion using non-linear processes can extend optical frequency metrology to exotic wavelengths. For example, our planned experiment for precision spectroscopy of the 1S-2S transition in He + ions requires an OFC in the extreme ultraviolet (XUV) at 60.8 nm [2]. XUV combs are often generated by intracavity high-order harmonic generation, where high average power laser pulses are stored in an enhancement cavity and HHG takes place at the intracavity focus [3]. Alternatively, OFCs can be operated at a lower repetition rate to achieve similarly high pulse energy without using a high average power laser system and enhancement cavities. OFCs with a repetition rate of several tens of kHz may have sufficiently large mode spacing for precision spectroscopy of transitions with very narrow natural linewidths, e.g. the nuclear transition in 229m Th at ~150 nm (natural linewidth of ∼100 μHz )[4] and the 1S-2S transition in He + ions (natural linewidth of 84 Hz) [2]. Although high-intensity CEP-stabilised low-repetition rate laser systems based on the chirped pulse amplification (CPA) scheme have been shown to be suitable for studying ultrafast phenomena in the time domain[5], the existence of a low-noise comb structure in the frequency domain remains unproven.