Flexible optical network components based on densely integrated microring resonators

This thesis addresses the design, realization and characterization of reconfigurable optical network components based on multiple microring resonators. Since thermally tunable microring resonators can be used as wavelength selective space switches, very compact devices with high complexity and flexibility can be created. In chapter 1 an introduction to this thesis is given by stating the context in which the work has been done. A brief overview of optical communication networks is given as well as a description of the projects in which the research has been carried out. Finally the basic properties of microring resonator filters are presented, like the Free Spectral Range and Finesse. In chapter 2 an application oriented top-down design approach for the microring resonator as wavelength filter is described. A scattering matrix model of a MR is used, that incorporates the coupling constants, radius and the losses. With this model the geometrical design parameters are investigated and chosen such that the network specification can be met. It comes out that for high bandwidth filtering applications the coupling constants need to be relatively large. Furthermore for a MR which drops as much power as possible, the coupling constants need to be equal. Physical layer simulations of the MR in a network environment are done that confirm the high bandwidth filtering application. In chapter 3 the thermal properties of a microring resonator are described. These properties allow the filter to be tuned to a specific wavelength. This way switching functions can be realized and fabrication errors and environmental temperature changes can be corrected. The basic properties of thermal tuning are explained, whereafter the design and characterization of a thermally tunable single ring is given. Several SiO2/Si3N4 MRs with chromium -shaped heaters were fabricated. The MRs show good reproducibility and the parameters extracted from measurements show good agreement with the designs. A thermal tunability between 11.3 pm/mW and 20 pm/mW for several different geometries is demonstrated. A method of increasing the switching speed is described, that uses an overshoot and bias of the driving signal. By using this method the rise-time of the MR response was improved by 42%, enabling modulation frequencies of 10 kHZ and switching speeds in the order of 0.1 ms. Chapter 4 is describing the use of devices built out of more than one single MR to create complex structures with enhanced functionality. Some examples of these complex structures are described, like an optical-cross-connect, a reconfigurable wavelength router and an optical network unit. The multiple MR structures are compared to their competing technologies and it is calculated that the used area can be reduced with a factor of 50 by using complex MR functions. Some examples of fabricated and measured structures based on multiple MRs are given. First a wavelength selective switch is demonstrated which has an ON/OFF ratio of 12 dB and a channel separation better than 20 dB. Second, a Vernier switch is shown which has a total Free Spectral Range of 28 nm, by combining the specifications of two rings. Finally a reconfigurable optical add-drop multiplexer is demonstrated which is made out of four MRs and which is pigtailed and packaged. It demonstrates a symmetric add and drop response with 17 dB resonance peaks. Since the tuning range of the rings is larger than the Free Spectral Range of the rings, any wavelength can be addressed. A single channel configuration could be reached by only 20 mW of driving power. A four channel configuration uses 446 mW of driving power. Chapter 5 gives system level measurements in order to assess the performance of multi MR based structures in optical networks. It describes the system level characterization methods, - setups and the measurements on both single MR as well as multiple MR structures for 10 and 40 Gbit/s datasignals. The results show that 10 Gbit/s non-return-to-zero datasignals can be filtered by the described MR as a clear eye diagram is measured. A measurement of the groupdelay confirms this also since the delay of 7 ps does not contribute significantly at this datarate. The 40 Gbit/s return-to-zero measurement results show that the reconfigurable add-drop multiplexer can filter these signals with clear eye diagrams to all add and drop ports. A power penalty of 1 dB was measured at a bit-error-rate of 10−9. The measurements also demonstrate the principle of multicasting the 40 Gbit/s signals to more than one output port at once. Finally, in chapter 6, a discussion is given of the results presented in this thesis and of the use of MRs in optical telecommunication components. This discussion will lead to the conclusion that it is possible to design and realize multiple MR devices allowing high bitrate optical network components with reconfigurable functions. The chapter also gives an outlook recommending future research for multi MR structures in optical telecommunications components. In this context also some brief comments on recent picosecond pulse measurements are given, where pulses with a length in the order of the round-trip time of the MR are filtered.