Multimode dynamics of semiconductor quantum-dot lasers
Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics
Final Report Abstract
In this project the multi-mode dynamics within laser cavities has been investigated. In Fabrye-Pérot cavities, which support multiple longitudinal laser modes, the interference pattern generated by the counter-propagating optical fields induces a spatial grating of the available gain. The scattering of the laser modes on this grating destabilises the single-mode laser emission and promotes multi-mode lasing. This effect is especially pronounced in long laser cavities where the frequency spacing between lasing modes is small, and the nonlinear interaction between lasing modes is amplified. The nonlinear interaction is particularly strong in semiconductor quantumdot lasers, due to the strong localisation of electrons and holes in the active medium, leading to the build-up of a pronounced gain grating. A numerical model based on a delay-algebraic-equation approach has been derived to describe the multi-mode dynamics of long-cavity quantum-dot lasers. This model allows a consistent description of the mode structure arising from the cavity boundary conditions and the interaction with the gain grating, while maintaining a relatively high numerical efficiency. This model has been used to explain experimental measurements of the dynamics of a quantum-dot multi-mode laser under optical single-mode injection. Depending on the frequency of the injected laser light, individual lasing modes can be locked to the driving signal. The laser unlocks when the injected signal is too far detuned from a given laser mode, characterised by the onset of strong multi-mode dynamics upon leaving the locking range. At sufficiently high injection strengths, a single locking range extends over nearly the whole gain bandwidth of the multi-mode laser, allowing for a tuning of the output wavelength over an ultra-wide bandwidth. The dynamics can be explained using the derived numerical model, which allows an insight into the temporal and spatial behaviour of the induced gain grating and the resulting multi-mode dynamics. The excellent agreement between the numerical predictions and experimental data shows that this model can be applied to accurately predict the behaviour of multi-mode lasers in different setups. The second part of the project involved the investigation of the dynamics of a single-mode laser under injection of a multi-mode optical signal. This can be interpreted as the inverse setup of the first half of the project, in which the interaction of a multi-mode laser and a single-mode injected signal had been discussed. The dynamics of this second setup is again determined by the nonlinear interaction of neighbouring locking regions. In between the locking regions where the injected laser is entrained by one of the injected lines, additional regions of harmonic locking were found. There, the laser output intensity is modulated with a frequency equal to an integer fraction of the original spacing of the injected modes. In these regions, the laser output effectively produces a dense optical frequency comb, with additional optical lines generated in between the original injected modes. This setup could thus potentially be used for an all-optical generation of frequency combs from a single seed signal, enabling the reduction of the comb spacing over a large range.
Publications
- “Harmonic frequency locking and tuning of comb frequency spacing through optical injection”, Optics Express (2019)
K. J. Shortiss, B. Lingnau, F. M. Dubois, B. Kelleher, and F. H. Peters
(See online at https://doi.org/10.1364/OE.27.036976) - “Multi-mode Dynamics and Modelling of Free-Running and Optically Injected Fabry-Pérot Quantum-Dot Lasers”, Physical Review A (2019)
B. Lingnau, M. Dillane, J. O’Callaghan, B. Corbett, B. Kelleher
(See online at https://doi.org/10.1103/PhysRevA.100.063837)
