1372 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 16, AUGUST 15, 2012 Colorless Optical Network Unit Based on Silicon Photonic Components for WDM PON Lin Xu, Qi Li, Noam Ophir, Kishore Padmaraju, Lian-Wee Luo, Long Chen, Member, IEEE, Michal Lipson, Senior Member, IEEE, and Keren Bergman, Fellow, IEEE Abstract— We demonstrate a low-cost colorless optical network unit (ONU) utilizing silicon photonic components for wavelength division multiplexed passive-optical-networks. At the ONU, a waveguide-coupled microring works as a demultiplexer for separating the downstream signal from the centrally dis- tributed continuous-wave (CW) light. The 10-Gb/s downstream signal is received using a waveguide-integrated germanium photodetector while the CW light is further modulated at 5 Gb/s using a silicon microring modulator for upstream signal gener- ation. Error-free transmission over 25-km single mode fiber is achieved with 0.2- and 0.4-dB power penalties for the downstream and upstream signals, respectively. Complementary metal-oxide semiconductor-compatible silicon photonic technology offers the potential for monolithic integration and mass production. Index Terms—Colorless ONU, silicon photonics, WDM-PON. I. I NTRODUCTION T O COPE with the ever-increasing bandwidth demands in access networks, wavelength-division-multiplexed (WDM) passive-optical-networks (PONs) have been consid- ered as a promising solution for the next generation broadband access network technology. Reducing the cost of WDM PONs will be the key challenge toward realizing broad deployment in the highly cost-sensitive optical network units (ONUs). Recent advancements in silicon photonics have introduced photonic devices including waveguides, modulators, switches and receivers [1]–[4]. CMOS-compatible silicon photonic technology offers the potential for compact, high-performance, energy-efficient and low-cost optical transceivers, rendering them attractive for access networks [5]. An ONU transceiver based on silicon photonics has been recently proposed, which envisions WDM filters, modula- tors and germanium photodetectors integrated on a single chip [6]. However, the lack of a silicon light source prevents the realization of fully integrated on-chip solution. A colorless ONU design has been suggested to overcome this problem Manuscript received February 10, 2012; revised May 15, 2012; accepted June 6, 2012. Date of publication June 15, 2012; date of current version July 17, 2012. This work was supported in part by the NSF Engineer- ing Research Center for Integrated Access Networks (CIAN) (subaward Y503160). L. Xu, Q. Li, N. Ophir, K. Padmaraju, and K. Bergman are with the Department of Electrical Engineering, Columbia University, New York, NY 10027 USA (e-mail: lx2140@columbia.edu; ql2163@columbia.edu; ophir@ee.columbia.edu; kp2362@columbia.edu; bergman@ee.columbia.edu). L.-W. Luo, L. Chen, and M. Lipson are with the School of Electrical and Computer Engineering, Cornell University, Ithaca, NY 14853 USA (e-mail: ll399@cornell.edu; lc286@cornell.edu; ml292@cornell.edu). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2012.2204241 Fig. 1. (a) Architecture of the proposed WDM PON. (b) Configuration of the individual colorless ONU. (c) Transmission spectra of the AWG, microring demultiplexer (Demux), and the microring modulator (Si Mod), respectively. and allow the same physical unit to be used irrespective of the local wavelength [7]. Following this path toward realizing a single chip silicon ONU, a transceiver based on Mach–Zehnder Interferometer (MZI) modulator has been demonstrated recently [8]. Compared to that device, micror- ing based devices are more compact, power efficient, and require only CMOS-level driving voltages. In this letter, we demonstrate a colorless ONU transceiver based entirely on silicon photonic components, including a microring modulator, a germanium photodetector and a microring based WDM demultiplexer. The potential for single chip integration of this colorless ONU transceiver places it as a potentially attractive solution capable of meeting the stringent cost requirements for WDM PON. II. ARCHITECTURE AND DEVICE In the proposed architecture Fig. 1(a), a dual-fiber link connects the transceivers located at the central office with the corresponding ONUs. The downstream fiber link carries downstream signals and centrally distributed continuous wave (CW) carriers for upstream modulations; the upstream fiber link carries upstream signals only. Each ONU Fig. 1(b) consists of a microring based demultiplexer for separating the downstream signal and the CW light, a photodetector for downstream signal detection, and a microring modula- tor for upstream signal generation. The transmission spectra Fig. 1(c) of the arrayed waveguide grating (AWG), microring based demultiplexer, and modulator depict the WDM channel positioning. The downstream signal (λ 1 ), which is on one resonance of the microring demultiplexer, is sent to the drop port and received by the photodetector. In parallel, the CW light (λ 2 ), which is off resonance of the microring demul- tiplexer while on the resonance of the microring modulator, 1041–1135/$31.00 © 2012 IEEE