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 $\sim 100\ \mu\text{Hz}$ )[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.