Characterization of the frequency stability of a multibranch optical frequency comb

Presently, ultra-precise optical clocks' comparisons can be performed easily using the optical frequency combs (OFCs) as a transfer oscillator. In this scheme, different OFC branches are usually used to enable optical clocks' comparison at different wavelengths. Some branches may involve processes such as optical amplification, second-harmonic generation, self-phase modulation, and propagation through fibers. Such processes can degrade the stability of the comparison if they are uncommon between branches. In this article, a technique is introduced to investigate the applicability of multibranch OFCs to perform ultra-precise optical frequency comparisons. This technique is based on generating a software-transfer beat (STB) between the OFC branches under investigation and the fundamental or the second-harmonic outputs from a frequency-stabilized laser to a hyperfine component from the two-photon transition ⁵ S _1/2 → ⁵ D _5/2 in rubidium. In addition, a simple compensation scheme is proposed to enhance the relative stability between OFC branches to enable the comparison of the best available optical clocks.