JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 32, NO. 3, FEBRUARY 1, 2014 505 Wavelength Locking and Thermally Stabilizing Microring Resonators Using Dithering Signals Kishore Padmaraju, Student Member, IEEE, Dylan F. Logan, Takashi Shiraishi, Member, IEEE, Jason J. Ackert, Andrew P. Knights, and Keren Bergman,Fellow, IEEE, Fellow, OSA Abstract—The bandwidth bottleneck looming for traditional electronic interconnects has driven the consideration of optical communications technologies as realized through the complemen- tary metal-oxide-semiconductor-compatible silicon nanophotonic platform. Within the silicon photonics platform, silicon microring resonators have received a great deal of attention for their ability to implement the critical functionalities of an on-chip optical net- work while offering superior energy-efficiency and small footprint characteristics. However, silicon microring-based structures have a large susceptibility to fabrication errors and changes in temper- ature. Integrated heaters that provide local heating of individual microrings offer a method to correct for these effects, but no large- scale solution has been achieved to automate their tuning process. In this context, we present the use of dithering signals as a broad method for automatic wavelength tuning and thermal stabilization of microring resonators. We show that this technique can be man- ifested in low-speed analog and digital circuitry, lending credence to its ability to be scaled to a complete photonic interconnection network. Index Terms—Frequency locked loops, multi-processor inter- connection, optical interconnects, optical resonators. I. INTRODUCTION G ROWING bandwidth needs are motivating the replace- ment of traditionally electronic links with optical links for applications as diverse as data centers, supercomputers, em- bedded computing processor-memory interconnects, and fiber- optic access networks [1], [2]. For applications such as these, the silicon photonics platform has received significant atten- tion because of its ability to deliver the necessary bandwidth, Manuscript received July 11, 2013; revised November 6, 2013; accepted De- cember 6, 2013. Date of publication December 10, 2013; date of current version December 27, 2013. This work was supported in part by the National Sci- ence Foundation and Semiconductor Research Corporation under Grant ECCS- 0903406 SRC Task 2001. This work was also supported by the Natural Sciences and Engineering Research Council of Canada. The work of K. Padmaraju was supported by an IBM/SRC PhD fellowship. K. Padmaraju and K. Bergman are with the Department of Electrical En- gineering, Columbia University, New York, NY 10027 USA (e-mail: kpad- mara@ee.columbia.edu; bergman@ee.columbia.edu). D. F. Logan is with the Ranovus Inc., Ottawa, ON A6 K1A 0R6, Canada (e-mail: dylan@ranovus.com). T. Shiraishi is with the Department of Electrical Engineering, Columbia University, New York, NY 10027 USA, and also with the Photonics Labora- tory, Fujitsu Laboratories Ltd., Kanagawa 2430197, Japan (e-mail: ts2821@ columbia.edu). J. J. Ackert and A. P. Knights are with the Department of Engineering Physics, McMaster University, Hamilton, ON L8S 4L8, Canada (e-mail: ackertjj@ mcmaster.ca; aknight@mcmaster.ca). 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.2013.2294564 Fig. 1. An optical link composed of microring-based devices. A wavelength source (λ-source) is modulated by multiplexed microring modulators (doped in a diode configuration to enable fast carrier-induced resonance shifts). A microring switch can then route the entire set of signals appropriately before it is received by a demultiplexing microring array. and by leveraging its compatibility with complementary metal-oxide-semiconductor (CMOS) fabrication, at a poten- tial economy of scale. In particular, silicon microring res- onator based devices exhibit leading metrics on size density, energy-efficiency, and ease of wavelength-division-multiplexed (WDM) operation [3]. Fig. 1 illustrates a portion of an envisioned microring-based photonic network that would be used for transcribing electrical data signals into the optical domain, transmitting and routing them as necessary, and converting the optical signals back to the electrical domain at the termination of the link. The beginning of the link consists of a multi-wavelength laser source. These laser wavelengths are individually modulated by cascaded microring modulators in a multiplexed configuration. The entire set of signals can then be routed as necessary by microring-based switches. Finally, they are received by a microring array that demultiplexes the individual signals before receiving them on independent photodetectors. The basic configuration of Fig. 1 is but one of a myriad of pro- posed possibilities for photonic networks enabled by microring- based devices, with more complex network designs fully lever- aging the unique capabilities of microrings [4], [5]. However, these proposed microring-based photonic networks currently face severe challenges in the path towards commercial realiza- tion. Specifically, the relatively high thermo-optic coefficient of silicon combined with the wavelength selectivity of microring 0733-8724 © 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications standards/publications/rights/index.html for more information.