One the primary objective of Freetwm is the study of Passive Mode-Locking in the presence of reverse biased saturable absorber, like e.g. in [5, 9]. Doing so is therefore very easy and can be achieved by imposing different optical properties in different sections of the laser cavity, like e.g. a fast voltage induced sweep-out rate of the optically generated carriers or an offset of the semiconductor band-gap wavelength due to the Quantum confined Stark effect. For appropriate parameters, one can trigger the onset of pulsating mode-locked regime.
|One can see on the right picture the pulsating amplitude of the field at both output facets as well as the characteristic evolution of the population inversion (D0) in the gain and within the absorber sections. The strong peak in the RF power spectrum at the modal separation signal fundamental Mode-Locking as well as the residual contribution of the relaxation oscillations around 5 GHz.|
The setup of such a photonic device can be achieved in several ways, e.g. by assuming spatially dependent optical properties or by defining a device composed of several sections, each one possessing uniform optical properties. These sections are then coupled only at the level of the boundary conditions which in the simplest case is a transmission matrix but can also contain the effect of some residual reflectivities at the gain-absorber junction.
NB: the animation from the starting page was generated from the final state of this simulation.