Numerical simulation of a passive twin-core fibre nonlinear coupler ring laser

Passive modelocking of a passive nonlinear directional coupler ring laser employing a near-half-beat-length passive twin-core fibre is numerically simulated by taking into account Raman self frequency shift and gain saturation. Raman self frequency shift is reduced by inserting two bandpass filters. Modelocking threshold condition and steady-state characteristics are investigated in extensive numerical experiments. It is found that, in addition to solitonic modelocking operation regime predicted by previous studies, non-soliton and chirped soliton operation regimes also exist in such a laser. The solitonic modelocking operation requires a proper balance of saturable amplification, bandpass filtering, nonlinear coupling and output coupling. The generation of transform-limited solitons with durations as short as 0.44 ps and pedestal extinctions of better than 40 dB is numerically demonstrated