While considerable progress has been made regarding photonic signal transmission technology over the last decade, the problem of efficient and convenient all-optical signal processing remains a major challenge in photonics. Within this project we theoretically investigate the possibilities offered by mutually coupled Discrete Mode Lasers (DMLs).
This project is funded by Science Foundation Ireland via a Starting Investigator Research Grant.
The Discrete Mode Laser principle and technology has been recently developed by the Photonics Theory Group at Tyndall National Institute, Cork, in collaboration with the Dublin-based SME Eblana Photonics. Discrete Mode Lasers are a family of edge-emitting lasers which allow the almost arbitrary selection of a pair of primary Fabry-Pérot lasing modes with a high side mode suppression ratio (> 35dB). These unique two-colour lasers offer many new possibilities, including an all-optical memory element, which we have recently demonstrated for a single device with optical injection. Furthermore we have been able to theoretically model the dynamical properties of an optically injected Discrete Mode Laser in remarkable agreement with experiment. By coupling two or more Discrete Mode Lasers, even more complex all-optical functionality is expected.
Fig 1: Bifurcation diagram of an optically injected two mode laser. For further explanations of the individual bifurcations see 
Networks in Frequency Space
A crucial property of Discrete Mode Lasers is that they allow in principle the realisation of a complex network topology, even if the physical coupling between the lasers is a simple all-to-all fibre coupling. The reason for this is demonstrated in Fig. 2. In Fig. 2(a) the physical setup is shown for four two-mode lasers, which are mutually coupled via a wavelength multiplexer, which at first sight seems to give rise to a trivial coupling. However if we take into account the fact that in principle the active lasing modes on each laser can be selected differently (for example as in Fig. 2(b)), we arrive at a schematic coupling scheme as in Fig. 2(c). This method avoids a cumbersome implementation of a network topology at the optical fibre level. We propose instead the realisation of an abstract network topology by selecting the appropriate lasing modes on the individual Discrete Mode Lasers. Such an approach will be crucial for the possible exploitation of Discrete Mode Lasers in the context of signal processing. For two and three Discrete Mode Lasers, all possible nontrivial topologies are shown in Fig. 2(d).
The objective of this project is to explore new concepts in all-optical signal processing based on optically interacting DMLs, coupled in various topological configurations.
Fig. 2(a) Physical coupling scheme of four Discrete Mode Lasers. This single waveguide is terminated by a reflecting mirror (orange). (b) Mode scheme of the Discrete Mode Lasers. Each diode is operating on two individually selected modes. (c) Abstract network topology arising from the simple physical all-to-all coupling as in panel (a) and the mode spectrum of panel (b). (d) Topologies for two and three interacting Discrete Mode Lasers.