Next generation Active Integrated optic Subsystems

Innovations in the areas of material sciences and lithography, coupled with the availability of powerful microscopy tools initiated intensified efforts towards ``all-optical-processing'' systems. As one of the outcomes of these current developments, it is now possible to realize compact and high finesse dielectric microcavities. These optical microcavities have shown a great potential for many applications [9]. For quite some time, microcavity resonators are discussed as promising building blocks for passive and active components for integrated optical devices [10]. Recent advances opened a range of possibilities of using micro-ring/ disk resonators (a ``cavity'' coupled to single/dual waveguides) as lasers [11,12,13], logic gates [14,15], optical switches [16,17,18], sensors [19,20,21], and add/drop (multiplexing/demultiplexing) wavelength filters [22,23].

The performance of wavelength division multiplexing/demultiplexing filters plays a crucial role for the full utilization of the high bandwidth potential of optical fibers. At present, arrayed waveguide gratings (AWGs) are used for this purpose [24,25]. But due to their comparably large sizes, AWGs are not suitable for the use in densely integrated photonic circuits [26,27]. Alternatively, wavelength division filters based on photonic crystals are quite promising [28,29,30], but the corresponding technological issues are still challenging. On this background, due to their versatility, compactness and possibility of dense integration, resonators based on conventional microcavities became attractive candidates for add/drop wavelength filters. A recent overview of this field can be found in Ref. [31].

Microcavity based resonator filters can be configured in various ways [22]. In this work we are interested in resonators, in the form of dual waveguide coupled circular microcavities, as tunable add/drop wavelength filters. A wavelength filter is a device which, given several input signals of different wavelengths, selects one of these as the output wavelength. By an add/drop filter one means that not only one of the inputs can be dropped (i.e. extracted/demultiplexed), but also a new input can be added (i.e. inserted/multiplexed) to the outgoing signal. Tunability of a filter implies that one can tune (i.e. change) the output response of the filter to one of the desired input wavelengths.

The co-ordinated efforts to demonstrate the feasibility of microresonator based compact integrated optical subsystems resulted in the project ``Next generation Active Integrated optic Subsystems'' (NAIS), funded by the European Commission within the framework of the Information Society Technologies programme [32]. This project was centered around the realization of microresonator based tunable add/drop wavelength filters. The work presented in this thesis has been part of that project.

The project NAIS has covered several aspects like material research, design tools and techniques, technology/fabrication, testing and packaging, and system requirements. Naturally, there were lots of interdependencies among these different research topics. The project involved several disciplines like physics, mathematics, electrical engineering, material science, etc. The progress of such a time-bound multidisciplinary engineering project is advanced by a sound understanding of the dependence of the performance of the devices on material and design aspects. In this regard, a realistic model of the device and a reliable simulation tool based on such a model is essential. This is where the present work, as part of the design workpackage, contributed to the project NAIS.