Phase Noise Reduction of an Electro-Optic Dual Comb Source with a Twisted Multicore Fiber

Dual comb spectroscopy (DCS) is an advanced broadband spectroscopic technique that uses two combs with slightly different repetition rates to downconvert the spectroscopic information from the optical domain to the RF domain. Recent developments in DCS has led to an interesting DCS scheme that uses electro-optic modulators (EOM) for the comb generation (providing frequency agility) and subsequent spectral broadening with a nonlinear fiber (providing ample bandwidth). In such a system, the detection fidelity is highly sensitive to the mutual coherence between the beating combs. As a result, often different cores of a single nonlinear multicore fiber is used for the spectral broadening of the EOM combs, such that they experience almost the same phase fluctuation [1]. Albeit such efforts, the large intercore separation in the multicore fiber (to ensure minimal intercore crosstalk), can lead to a significant degradation of the mutual coherence of the combs. To overcome this limitation, in this work we report the phase noise improvement of a dual comb RF spectrum by using a twisted multicore fiber for the spectral broadening. The twisting of the multicore fiber with a small pitch (3 mm in our case) could significantly average out the differential fluctuations experienced by the combs propagating in adjacent cores of the fiber [2] leading to an improvement of the phase noise in the RF beating comb.