Abstract

Rapid recent progress in passively mode-locked fibre lasers is closely linked to new nonlinear regimes of pulse generation [1,2]. Mode-locked fibre lasers are complex physical systems exhibiting a non-trivial interplay between the effects of gain, dispersion and nonlinearity. This makes such lasers interesting realizations of so-called dissipative nonlinear systems with a variety of possible pulse shaping mechanisms not yet fully explored. In this paper, we numerically demonstrate a new nonlinear regime using a laser cavity similar to that described in [1] as an example (Fig. 1). A segment of single-mode fibre (SMF) with normal group-velocity dispersion (GVD) forms the major part of the cavity, and is connected to a short length of gain fibre that provides pulse amplification. The gain fibre is followed by a saturable absorber (SA) element. The final element is a dispersive delay line (DDL) that provides anomalous GVD with negligible nonlinearity. The cavity is a ring and, thus, after the DDL the pulse returns to the SMF. Two practically tunable system parameters, namely the net cavity dispersion ${\beta }_{\text{net}}^{(2)}$ and the integrated gain of the gain fibre G, are varied to achieve different mode-locking regimes. We find that for normal net dispersion, formation of two distinct steady-state solutions of stable single pulses can be obtained in the laser cavity: the well-known self-similarly evolving pulse characterised by a parabolic shape and a linear frequency chirp, and a linearly chirped pulse that exhibits a triangular temporal form (Figs. 1, 2). To characterise the pulse shaping, we use the parameter of misfit M_S between the pulse intensity profile and a parabolic or triangular fit of the same energy and full-width at half-maximum duration: $M_S^2=\int \text{d}t{(|u{|}^2-|u_S{|}^2)}^2/\int \text{d}t|u{|}^4$, S = P, T, where u_P(t) and u_T(t) correspond to the respective intensity profiles $|u_P(t){|}^2=P_{0,P}(1-t^2/{\tau }_P^2)\theta ({\tau }_P-\left|t\right|)$ and $|u_T(t){|}^2=P_{0,T}(1-|t/{\tau }_T)\theta ({\tau }_T-|t|)$, with θ (x) being the Heavside function.

© 2011 IEEE