When passively mode-locked fiber lasers are forced to operate in an extraordinary condition, they tend to exhibit a chaotic/stochastic nature of photon dynamics. Such behavior is observed since they are a complex nonlinear dissipative system of a multitude of cavity parameters. Recently, Quasi-mode-locked regimes that distinctively generate partially coherent or incoherent multiple optical pulses have been seen to be among such stochastic regimes of the passively mode-locked fiber lasers. Despite numerous studies being undertaken with a purpose to unveil the detailed chaotic/stochastic nature of Quasi-mode-locked regimes, the use of conventional measurement techniques has rendered these efforts futile. Fortunately, novel techniques such as the dispersive Fourier transform and spatio-temporal measurement methods have recently been implemented, providing real-time shot-to-shot information of pulse dynamics in Quasi-mode-locked regimes. Contrary to our expectations, the Quasi-mode-locked regimes have still remained obscured in many aspects, such as their shot-to-shot coherence properties and wave-packet formation mechanisms.
A team of researchers led by Professor Yoonchan Jeong at Seoul National University in South Korea systematically carried out dedicated experimental investigations on the shot-to-shot coherence and wave-packet formation in quasi-mode-locked regimes, including noise-like-pulse, symbiotic, and multi-soliton regimes in an anomalous-dispersion fiber ring cavity. Their work is now published in the research journal, Optics Express.
The main procedure of this work entailed the researchers making fine adjustments of the cavity saturation power. By doing this, they made the laser cavity operate in three distinct constitutional regimes of noise-like-pulse, symbiotic, and multiple-soliton regimes. The research team then investigated the individual regimes by empirically analyzing their optical spectra, autocorrelation traces, and the shot-to-shot spatio-temporal evolutions for every roundtrip up to 800-roundtrip times.
The authors observed that the individual regimes exhibited significantly different coherence characteristics, that not only depended on the amount of nonlinear phase shift accumulated per roundtrip, but also on the degree of soliton interaction, the latter of which was noted to crucially govern the bunching or anti-bunching mechanisms in the corresponding Quasi-mode-locked regimes. Additionally, the team realized that solitons with higher intensities underwent higher nonlinear phase shift and stronger soliton interactions. Moreover, it was seen that the intensified soliton interactions among the individual solitons in the multi-soliton-regime cavity triggered them to form a bunched soliton-group: a wave packet, thereby resulting in Quasi-mode-locked pulses in the noise-like pulse or symbiotic regime.
Yoonchan Jeong and his colleagues successfully presented empirical characterization of the three constitutional Quasi-mode-locked regimes of passively mode-locked fiber lasers of an anomalous-dispersion cavity, analyzing and detailing out their shot-to-shot coherence properties, wave-packet-forming/bunching processes, and correlations with soliton interactions. In their study, it has been verified that solitons with higher intra-cavity intensities not only experienced greater nonlinear phase shifts but also trigger stronger soliton interactions. In totality, the research work is in good agreement with previous studies and more so, it is hoped that it will provide a good link in the existing knowledge on the Quasi-mode-locked regimes of passively mode-locked fiber lasers, as well as act as a stepping stone to the further expansion of them.
Seungjong Lee, Luis Alonso Vazquez-Zuniga, Hyuntai Kim, Youngchul Kwon, Kyoungyoon Park, Hansol Kim, Yoonchan Jeong. Experimental spatio-temporal analysis on the shot-to-shot coherence and wave-packet formation in quasi-mode-locked regimes in an anomalous dispersion fiber ring cavity. Vol. 25, No. 23 | 13 Nov 2017 | Optics Express 28385
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