Dynamics of passively mode-locked lasers with saturable absorber and saturable nonlinearity
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Dynamics of passively mode‑locked lasers with saturable absorber and saturable nonlinearity P. Achankeng Leke1 · Alain M. Dikandé1 Received: 15 April 2020 / Accepted: 30 August 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The dynamics of a model of passively mode-locked laser with saturable absorber, the optical amplifier of which possesses a saturable nonlinearity, is considered with interest in both continuous wave and pulse regimes of operation. The study rests on the laser self-starting picture which assumes that the laser should operate instantly in the pulse regime, when continuous waves become unstable in the system. Within the framework of the modulational-instability analysis, a global map for the laser self-starting conditions is constructed in terms of a two-dimensional complex parameter space, mapped by the real and imaginary parts of the modulation gain over a broad range of values of the modulation frequency. The map suggests that the saturable nonlinearity lowers the threshold value of the input intensity required for laser self-starting, analytical expression for this threshold input field is derived in the particular case of a zero modulation frequency. Treating the system dynamics in the full nonlinear regime using numerical simulations, time series of the laser amplitude and instantaneous phase, as well as of the gain, are obtained and their changes with the variation of the homogeneous gain are examined. It is found that a relatively small value of the equilibrium gain will favor gain decrease, and pulse generation and buildup into either simpleperiodic or multi-periodic pulse trains, depending on the magnitude of the saturable nonlinearity coefficient.
1 Introduction Mode-locked fiber lasers with saturable absorbers have attracted a great deal of attention in recent years [1–14], because they stand for ideal sources of high-intensity short pulses used in a vast area of modern communication technology. In these devices, an intensity fluctuation acts in conjunction with nonlinearity of the optical amplifier to modulate the cavity loss, without need for some external control [8, 9, 11–13]. Though several mode-locking techniques are reported in the literature, for applications requiring ultrashort pulses produced at finite repetition rates passive mode-locking remains the most preferred technique [1–3, 13]. Passive mode-locking rests mainly on an appropriate choice of the optical gain medium, to this last point passively mode-locked fiber lasers with rare-earth-doped optical amplifiers have demonstrated high efficiency in femtosecond * Alain M. Dikandé [email protected] 1
Laboratory of Research on Advanced Materials and Nonlinear Science (LaRAMaNS), Department of Physics, Faculty of Science, University of Buea, P.O. Box 63, Buea, Cameroon
pulse generation. In particular, they play a relevant role in pulse multiplexing and applications involving soliton-train structures [15–18], owing to their optical nonlinearity which can be scaled up (i.e., increased) or
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