JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 31, NO. 24, DECEMBER 15, 2013 3943 Analysis and Design of Microring-Based Switching Elements in a Silicon Photonic Integrated Transponder Aggregator Paolo Pintus, Member, IEEE, Pietro Contu, Nicola Andriolli, Antonio D’Errico, Fabrizio Di Pasquale, Member, IEEE, and Francesco Testa Abstract—In this paper, we present and investigate a new archi- tecture of a silicon photonic transponder aggregator as a new in- terconnect subsystem enabling the implementation of colorless, di- rectionless, and contentionless ROADMs. Such subsystem is based on a microring resonator switching fabric integrated in a silicon photonics platform to achieve high functional integration together with reduction of cost, footprint, and power consumption. In the proposed device, microring resonators perform simultaneous add and drop of wavelength channels which suffer from two detrimen- tal effects: residual dropped signal crosstalk and residual added signal crosstalk, respectively. Considering three microring-based switching elements, the transfer matrix method has been used to compute the add/drop transfer functions of the switches as a func- tion of their geometrical parameters. The two crosstalk effects have been evaluated jointly with other important transmission param- eters, such as bandwidth, insertion losses, side lobe suppression, adjacent channel rejection, extinction ratio, and group dispersion. In addition, device sensitivity with respect to the ring-waveguide coupling coefficients has been calculated. Finally, the performance of the different switches has been assessed to demonstrate that, by a proper design, the proposed transponder aggregator can support 100 Gb/s DP-QPSK modulated signal transmission. Index Terms—Integrated optics, optical switches, ring resonators. I. INTRODUCTION D YNAMIC rearrangement of capacity while optimizing transport resources utilization and further lowering capital cost and power consumption is a fundamental requirement for next generation optical transport networks. These features can be ensured by implementing highly flexible transport nodes in conjunction with an intelligent control and management plane empowering software defined networking (SDN) [1]. Conventional reconfigurable optical add and drop multiplexer (ROADM) nodes are implemented using broadcast-and-select architecture. Fig. 1 shows a four-directional ROADM based on Manuscript received June 14, 2013; revised July 31, 2013; accepted August 1, 2013. Date of publication August 5, 2013; date of current version November 27, 2013. P. Pintus, P. Contu, N. Andriolli, and F. Di Pasquale are with the Scuola Superiore Sant’Anna, 56124 Pisa, Italy (e-mail: paolo.pintus@sssup.it; pietro.contu@sssup.it; nick@sssup.it; f.dipasquale@sssup.it). A. D’Errico and F. Testa are with the Ericsson S.p.A, 56124 Pisa, Italy (e-mail: antonio.d.errico@ericsson.com; francesco.testa@ericsson.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2013.2276852 Fig. 1. Conventional ROADM architecture. two main system blocks: the optical line switching section and the local add and drop section. The former comprises, for each direction, a N × 1 (with N equal to the node directions) wave- length selective switching (WSS) module to selectively combine onto the output fiber the wavelengths distributed by the differ- ent power splitters (PS). Local add and drop section includes arrayed waveguide gratings (AWG) to interconnect the optical line switch to fixed wavelength transponders (Tx modules in Fig. 1). For signal dropping, an array of AWG demultiplexers is used to separate the signals distributed by the power splitter in each network direction before reception at the receivers (Rx modules). Similarly, another AWG array is used to multiplex the locally generated signals to be added to each network direction. In such conventional nodes, full switching flexibility is only en- sured for wavelength channels in transit across the node toward other nodes. Actually these channels can be routed from any to any direction. On the contrary, added and dropped wavelengths are rigidly assigned to a fixed direction and to a fixed color (by means of WDM multiplexer and demultiplexer array) and any change in these configurations must be performed by manual rewiring [2]. Next generation ROADM should provide higher flexibil- ity with respect to currently deployed optical nodes. In such new nodes, the routing of added/dropped wavelength channels to/from any direction should be guaranteed without any manual intervention (directionless operation), independently from the transponder wavelength (colorless operation) and by allowing 0733-8724 © 2013 IEEE