PhD Defense: Dynamics of Optical Localized Structures in Passively Mode-Locked Lasers

Summary:

This thesis focuses on the theoretical analysis of passively mode-locked (PML) lasers and in particular on the dynamics of mutually independent pulses called temporal localized structures (TLSs). These can exist below the lasing threshold in the presence of a saturable absorber with low saturation intensity and sufficiently large modulation of the nonlinear absorption in relation to the amount of linear cavity losses.

These TLSs may be placed in arbitrary arrangements and naturally remain at their relative  positions if not moved by an additional perturbation such as a modulation of the gain. This renders them of great interest for applications like telecommunications because they could lead, for instance, to reconfigurable bit arrays.

In this thesis, models for four different setups are derived and analyzed both via direct numerical simulations and numerical path continuation using the Matlab package DDE- BIFTOOL. All of the models contain time-delay and the specific details of their numerical treatment are discussed. In particular, a novel functional mapping approach is introduced which significantly reduces the computational effort of simulating TLSs.

From the comparison of the results obtained in the different models it can be concluded that the standard ring laser model is not sufficient to accurately describe all aspects of  the dynamics in Vertical External Cavity Surface Emitting Lasers (VECSELs) setups. In particular, we identify Third order dispersion to be a dominant effect in pulse destabilization.  It affects short pulses more strongly, thus rendering the optimization of the pulse duration  a trade-off inherent to such systems.

Finally, a Gires-Tournois interferometer containing a Kerr nonlinear medium is considered.  This passive micro-cavity is arranged with an external cavity geometry where energy is supplied via optical injection by a continuous wave (CW) laser.  The coherent CW pump is converted into phase-locked pulses thus forming a Kerr frequency comb. They are made up  of interlocking fronts that connect two bistable CW background states  and can form complex patterns of TLSs.