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Connection between Optical Frequency Combs and Microwave Frequency Combs Produced by Active-Mode-Locked Lasers Subject to Timing Jitter

Optical frequency combs (OFCs) and microwave frequency combs (MFCs) are of intense interest in fields ranging from metrology to multifrequency light sources. MFCs in particular have attracted interest for applications in microwave photonics. Timing jitter is a key limiting factor of the quality of OFCs and MFCs, and despite the central importance of timing jitter, a clear generic account of its effect on OFCs and MFCs that provides a connection between the two is lacking. We consider the power spectral density (PSD) of the OFCs and MFCs produced by active and hybrid active-passive mode-locked lasers in which the active mode locking is effected by a high-spectral-purity oscillator, but with timing noise originating in both the active locking and the mode-locked oscillators. Based on an analytically tractable theoretical treatment, we account quantitatively for the characteristic comb line shape consisting of a narrow central peak lying atop a broad pedestal. The theory provides a quantitative comparison between characteristic OFC and MFC line shapes and identifies the origin of the difference.