Abstract: We propose and compare several approaches for ultrashort pulse generation in fiber laser with high pulse repetition rate. This includes erbium laser made of ionic liquid gated carbon nanotubes and integrated microring resonator nested in the fiber cavity. First approach implies harmonic mode locking, which allows to increase fundamental repetition rate of the laser tens or even hundreds of times, but suffers from low reproducibility and high pulse jitter. In contrast, lasers with nested resonators can operate on the repetition rates of the microcavities which reach 1 THz and provide reproducible pulse trains with low jitter.  

Ultrafast lasers with high repetition rates are paving the road towards real world application. Among them soliton information transmission and generation of entangled photons. For some applications it is not a pulse train that it is needed, but its representation in frequency domain – a frequency comb – an equidistant sequence of narrow coherent spectral lines. Frequency combs with GHz to THz spacing may find applications in spectroscopy, coherent data transmission, coherent lidars and many more. In the work we present two approaches for high repetition rate pulse generations.
First, we consider laser with a carbon nanotube saturable absorber deposited on a side polished fiber. We show that controlling the response of a saturating absorber with electrochemical gating allows to control the pulse generation of a fiber laser. It includes switching between mode locking and Q-switching, as well as switching between different orders of harmonic mode synchronization. It is known that for lasers without polarization conservation, the order of harmonic synchronization at a given pump can significantly depend on the position of the polarization controller, while generation in a laser with polarization conservation, a given generation mode is considered more reproducible. We show that when adding another degree of freedom, the state of the saturating absorber, the order of harmonic mode locking for a given pump and the state of the absorber may depend on the trajectory along which we arrived at a given point in the parameter space of the laser. This behavior opens up opportunities for using machine learning to find optimal trajectories corresponding to the maximum harmonic mode synchronization order [1]. With this approach we demonstrate pulse repetition rates up to hundreds of MHz with possibility for further increase to GHz scale.

To go for a much higher repetition rate it is necessary to use another technique – a so-called filter driven four wave mixing. In this technic a microring resonator is implemented into the cavity, that filters laser modes forcing the laser to generate pulses on a repetition rate of the microcavity. We suggest further development of this approach by utilizing the two-port integrated chip with microcavity working as a reflecting mirror in fiber laser resonator (see Fig. 1a). Here laser resonator is formed by the gold mirror on one side and microring resonator on the other. High quality factor microring resonator give rise to a comb formation through four wave mixing. In the ring microcavity directly propagating comb give rise to the counter propagating comb due to Rayleigh scattering, which goes back into the laser cavity forming an optical feedback. With this approach we show robust self-starting soliton comb generation with spectral width more than 500 nm, which greatly exceeds erbium amplification window (see Fig. 1b). In the time domain it gives a train of femtosecond pulses with repetition rate of the microring cavity reaching 1 THz in our experiment [2].
This work is supported by the Russian Science Foundation (Grant No. №25-22-00388)

References
[1] Kokhanovskiy, A., Kuprikov, E., Serebrennikov, K., Mkrtchyan, A., Davletkhanov, A., Bunkov, A., Krasnikov, D., Shashkov, M., Nasibulin, A. and Gladush, Y. “Multistability manipulation by reinforcement learning algorithm inside mode-locked fiber laser,” Nanophotonics, Vol. 13 (Issue 16), pp. 2891-2901 (2024)
[2] Mkrtchyan A, et al. “Microring resonator as a Rayleigh mirror for broadband laser cavity comb generation,” arXiv:2503.09166