@inproceedings{bee3794c2a9443a0b7a933505ee13edc,
title = "Stable harmonic mode locking in soliton fiber laser with frequency shift: Theory and experiment",
abstract = "Laser sources delivering high-repetition-rate ultrashort pulses are highly demanded in numerous photonic applications. Among them, harmonically mode-locked soliton fiber lasers exhibiting advantageous consumer properties, such as compactness, reliability, low cost and maintaining convenient output are mostly demanded. Drawback of these lasers is relatively large timing jitter which is significantly higher than the timing jitter in lasers operating at fundamental frequency. We report the stabilizing frequency shift effect in harmonic mode-locking ring soliton fiber laser that is studied theoretically and numerically. It is known that a harmonic mode-locking regime in a fiber laser occurs due to interpulse repulsion, which leads to a uniform distribution of pulses in the cavity. We demonstrate that the frequency shift contributes to an increase in the hardness of interpulse interactions and can led to stabilization of the periodic arrangement of pulses. The experiment carried out confirms the theoretical predictions and the results of numerical simulation. ",
keywords = "Fiber lasers, Frequency shift, Harmonic mode-locking, Solitons, Timing jitter",
author = "Korobko, {Dmitry A.} and Stoliarov, {Dmitry A.} and Itrin, {Pavel A.} and Ribenek, {Valeria A.} and Fotiadi, {Andrei A.} and Gumenyuk, {Regina V.}",
note = "JUFOID=71479 Funding Information: Theoretical calculations based on the generalized NLSe showed that multi-soliton dynamic system with interpulse repulsion through the GDR effect and frequency up-shift came to the equilibrium point much faster than a similar system without frequency shift. Physically, the acceleration of the system evolution to the equilibrium point is equivalent to increasing in repulsion strength or, in other words, deepening of {"}potential wells{"}, which lead to periodic pulse distribution in the cavity. The numerical model of the ring soliton laser with interpulse repulsion induced by gain depletion and recovery agrees with the theoretical predictions. The frequency up-shift implemented into the model allowed reducing the time of the HML formation by orders of magnitude. Numerical simulations of HML arrangement also confirmed these conclusions after induced noise perturbations in the system. The simulations showed that the frequency shifting mechanism might be responsible both for mode-locking and for simultaneous stabilization of the periodic pulse distribution. It is important to note that the results obtained theoretically and numerically for the two pulses can be quite easily generalized for an arbitrary number of pulses propagating in the cavity [52]. Finally, the frequency-shifted HML ring fiber laser was experimentally realized and compared with a similar laser without frequency shift. The pulse parameters of two soliton lasers are close to each other, so we can assume that the mode-locking in both lasers was initiated by the same mechanism. The HML state achieved in both cases indicated the existence of pulse repulsion arising from some interaction mechanism. The GDR and acoustic interactions remain the main candidates for the role of the most intense interaction [52]; however, this issue requires additional research. It is important to note that the maximal pulse repetition rates obtained in both cases (1 GHz and 2 GHz) did not differ drastically, so the interaction mechanism did not depend on the presence of frequency shift. However, there were several principal differences between the frequency-shifted laser and the laser mode-locked by NPR only. One was the limited wavelength range where non-shifted laser could be mode-locked by NPR only, at the same time a similar laser with frequency up-shift possessed higher nonlinear losses and it could be mode-locked in the extended wavelength range. The main differences between two cases concerned the stability of the generated pulse train to the noise-induced perturbations. In the laser configuration without frequency shift, the RF spectrum data indicated high fluctuations of the uniform pulse distribution, so the quality of the HML operation was rather poor. In its turn, both the low-and high-frequency timing jitter of the frequency-shifted laser were significantly smaller than the same characteristics for the non-shifted laser demonstrating quite stable periodic pulse arrangement within the cavity. Moreover, the stabilizing effect of the frequency shift is further confirmed by the fact that the low-frequency timing jitter of the frequency-shifted laser corresponding to a longer correlation time is substantially less than the high-frequency jitter. Thus, the experiment confirmed the theoretical predictions and the results of numerical simulation. The main result of this study is the improved stability of high-repetition-rate pulse trains generated by the frequency upshifted HML soliton laser. This outcome can enhance the physics of hybrid HML lasers and be extended to the laser cavities with saturable absorbers based on SESAM or carbon nanotubes. Undoubtedly, the results of this research can be interesting to a wide range of laser physics specialists engaged in different problems, from studying the nature of the laser pulses interaction to the generation of frequency combs and ultrahigh repetition rate pulse trains. This work was supported by the Russian Science Foundation (grant no. 19-72-10037), the Russian Foundation for Basic Research (grant no. 19-42-730009) and the Ministry of Higher Education and Science of the Russian Federation (Megagrant Program, project # 2020-220-08-1369). Publisher Copyright: {\textcopyright} 2021 SPIE.; Nonlinear Optics and Applications ; Conference date: 19-04-2021 Through 23-04-2021",
year = "2021",
doi = "10.1117/12.2589124",
language = "English",
series = "Proceedings of SPIE - The International Society for Optical Engineering",
publisher = "SPIE",
editor = "Mario Bertolotti and Zayats, {Anatoly V.} and Zheltikov, {Alexei M.}",
booktitle = "Nonlinear Optics and Applications XII",
address = "United States",
}