Improved optical filter responses in cascaded InGaAsP/InP microdisk resonators

Waveguide-coupled microdisk and microring resonators as optical filters have been of great interest due to their high wavelength selectivity and ultra-compact sizes [1 – 4]. Recently, higher-order filters based on microring resonators with improved performances have been demonstrated in compound glass and AlGaAs/GaAs material systems [3, 4]. These higher-order filters show steeper rolloffs and flatter passbands than single resonator filters. Basically, higher-order filters can be realised by using multiple microresonators in the arrangement of either latticed (series coupling) or cascaded (parrallel coupling) structures. In the former case, each resonator is mutually coupled, and a signal from the input port must go through each resonator sequentially to be dropped; in the latter case, each resonator is equally spaced and only coupled to the input and output waveguides. Therefore, resonators interact with each other not directly but through the coupling waveguide. It has been pointed out that although the latticed structure is supposed to have better filter performance than the cascaded structure it is more sensitive to fabrication error and loss [3]. In this Letter, we show experimental results on cascaded third-order and fifth-order microdisk resonator filters based on an InGaAsP/InP material system. An InGaAsP/InP material system is of particular interest since it provides the possibility of the monolithic integration of passive and active devices, which is crucial for integrated optical networks. We use microdisk instead of microring geometry in our devices since microdisk resonators suffer much lower scattering losses than microring resonators.