Frequency splitting in repetition-rate doubled rational harmonic actively mode-locked pulse train

Summary form only given. Rational harmonic mode-locking (RHML) is a very simple technique for repetition-rate multiplication of the pulse train generated by actively mode-locked fiber lasers. In general, direct output pulse train of RHML laser suffers from strong pulse-to-pulse amplitude fluctuations. Important exception is repetition-rate doubling (RRD) by RHML. For this reason RRD-RHML is the most attractive for applications. In this paper, an investigation of the RRD-RHML Er-doped fiber laser is presented. For the first time to our best knowledge, we demonstrate that nonzero dispersion of the laser cavity causes frequency splitting in the output pulse train. It appears as periodical alternation of the central frequencies of sequential pulses in the train. We consider an actively mode-locked fiber laser in unidirectional ring configuration formed by Mach-Zehnder modulator followed by output coupler, Fabry-Perot filter and Er-doped fiber as amplification medium. The saturated gain of the Er-doped fiber is assumed to be equal to intracavity loss, spectrally flat and time independent. All elements are connected by optical fiber to form a ring.