A regeneratively mode-locked optoelectronic oscillator is proposed to realize multi-mode phase locking in optoelectronic oscillators by coupling a modulating signal generation loop. To verify the feasibility of the scheme, a numerical simulation model is established. In addition, a proof-of-concept experiment is demonstrated in a single-loop optoelectronic oscillator with a 2.2 km polarization-maintaining single-mode fiber, using an electric amplitude modulator/Mach-Zehnder modulator as an active mode-locking device. The generated microwave pulse signal has a center frequency of 10 GHz and a repetition rate of 95 kHz. We believe this scheme can provide a new approach to overcome the problem of detuning between the modulating frequency and the mode spacing during long-term operation.

Rational harmonic mode-locking refers to a mode-locking state achieved at the modulation frequency that doesn’t match the fundamental frequency. In this paper, we investigated and experimentally achieved rational harmonic mode-locking in optoelectronic oscillators (OEO) for the first time through three schemes based on electric amplitude modulator (AM), electric phase modulator (PM), and Mach-Zehnder modulator (MZM), respectively. In the experiment, the fundamental frequency mode-locking as well as the 2nd-order, 3rd-order, and 4th-order rational harmonic mode-locking were obtained, all generating ultrashort microwave pulses with a repetition rate of 95 kHz and a carrier frequency of 10 GHz. Subsequently, the characteristics of the pulse signals generated by different schemes, such as pulse width, pulse amplitude, and spectral width, were systematically investigated. By comparison, we found that the AM-based mode-locked OEO generates microwave pulse signals with higher stability and narrower pulse width; the PM-based mode-locked OEO can excite more longitudinal modes in the cavity but generates signals with more spurious noise; the MZM-based mode-locked OEO has a simple structure and requires lower power of the modulation signal. We believe this paper could provide some reference for the research on the physical mechanism of the mode-locking phenomenon generated in the OEO when the modulation frequency is mismatched.

We numerically study on the key factors and their influences which affected the generation and interaction of solitons in the figure-9 fiber laser based on nonlinear amplifying loop mirror (NALM) mode-locking mechanism. Our simulation used a more authentic approach, which tracing the pulse propagation within the loop cavity. It could act like a saturable absorber which exactly followed the working principle of the NALM. For this reason, the study on the self-starting of mode-locked operation could be carried out systematacially. The simulation results showed that the phase shift difference of the non-reciprocal phase shifter, the splitting ratio of the coupler in the Sagnac loop, and the strength of the pump power all played a role in enabling the mode-locking establishment. Further, multipulse generation were also studied by changing the pump power, and the final steady state presented that all solitons had the same pulse properties. Soliton interactions including attraction, repelling, collision and annihilation could also be observed. In addition, we explored the pump hysteresis phenomena in our simulations. The numerical results verified that the parameters affecting the self-starting of mode-locked operation, the mechanism of multisoliton formation and the interactions between solitons in figure-9 fiber laser, which would enlighten more various experiments and study of solitons dynamics.

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