Comparative noise analysis of nonlinearly broadened dual frequency combs in different fiber propagation schemes

We make a comparative study of the impact of fiber configuration on relative phase noise of a nonlinearly broadened electro-optic dual-comb light source. In particular, we considered two all-fiber platforms for the comb broadening, where the two combs were broadened (a) in the same core (counter-propagation configuration) and (b) in different cores of a multicore fiber (same facet input). Although far from the comb center, the RF synthesizer noise is dominant, leading to the same relative phase noise for different configurations, near the comb center, using two distinct cores in a multicore fiber allows a significant reduction of the residual phase noise compared to counter-propagation in the same core. These results are also verified using a continuous wave in a simple traditional interferometric setup, where better mutual coherence is obtained when propagating through two different cores of a multicore fiber compared to two different single-core fibers, or same core with counter-propagation. These results highlight the advantage of multicore fibers for nonlinear broadening of two mutually coherent frequency combs, essential for dual-comb or multi-comb spectroscopic techniques.