Abstract
Kerr microresonators are widely recognised as a powerful platform to generate compact and efficient optical frequency combs (micro-combs) [1]. Micro-combs offer low noise stable operation when configured to generate localized dissipative structures known as temporal cavity solitons (CS). For spectroscopy, imaging, and ranging applications, the generation of two frequency combs with different comb line-spacings is often highly desirable [2], [3]. In this configuration, the beat signals of consecutive comb line pairs can be mapped from the optical domain to the radio-frequency (RF) domain as an RF comb enabling high-speed, high-resolution measurements. Control over the RF comb spacing (\delta v) allows users to optimise between measurement resolution (which scales with \delta v ) and acquisition time (which scales with \delta v^{-1} ). In this work, we realise a dual micro-comb source where two counter-propagating combs are pumped by two lasers in same spatial resonator mode at different azimuthal mode frequencies. We show that by changing a pumped azimuthal mode, through adjusting one of the frequencies used, the micro-combs' RF line-spacing can be discretely tuned. Moreover, varying the relative detuning of the two driving pumps provides an additional continuous nonlinear tuning due to differential self-phase-modulation (SPM) [4].