JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 32, NO. 4, FEBRUARY 15, 2014 729 Terabit+ (192 × 10 Gb/s) Nyquist Shaped UDWDM Coherent PON With Upstream and Downstream Over a 12.8 nm Band Jacklyn D. Reis, Member, IEEE, Member, OSA, Ali Shahpari, Ricardo Ferreira, Somayeh Ziaie, Darlene M. Neves, Member, IEEE, M´ ario Lima, Member, IEEE, and Ant´ onio L. Teixeira, Member, IEEE, Member, OSA Abstract—In this paper, we numerically and experimentally demonstrate a bidirectional Terabit+ ultradense wavelength- division multiplexing (UDWDM) coherent passive optical network with Nyquist shaped 16-ary quadrature amplitude modulation, of- fering up to 10 Gb/s service capabilities per user/wavelength in a total spectrum of 12.8 nm over 40 km of standard single-mode fiber. This paper first demonstrates the capability of Nyquist pulse shap- ing to mitigate crosstalk arising from back-reflections and nonlin- ear effects in UDWDM networks with coherent transceivers. The latter part of the paper experimentally investigates the bidirec- tional transmission in terms of receiver sensitivity and nonlinear tolerance under different network transmission capacity condi- tions, e.g., number of users and bit rate per users. Index Terms—All-optical networks, coherent communications, networks, Nyquist pulse shaping, ultradense wavelength-division multiplexing. I. INTRODUCTION T HE increased demand for broadband services in optical access networks (OAN) has pushed engineers and scien- tists around the world to develop new technologies to better exploit the bandwidth capabilities of passive optical networks (PONs). Most of the research works focuses on how to max- imize the number of users, capacity, reach, and flexibility at minimal cost, complexity, and occupied bandwidth (e.g., from 1538 nm up to 1551 nm). Recently, particular attention has been given to orthogonal frequency-division multiplexing/multiple access (OFDM/OFDMA) and ultradense wavelength-division Manuscript received May 30, 2013; revised September 11, 2013; accepted September 18, 2013. Date of publication September 20, 2013; date of cur- rent version January 10, 2014. This work was supported in part by FCT– Fundac¸˜ ao para a Ciˆ encia e a Tecnologia, under Grants SFRH/BD/71667/2010, SFRH/BPD/84183/2012, TOMAR-PON, PTDC/EEA-TEL/108412/2008, ADI 30370 NG-PON2 and in part by Tektronix. The first two authors contributed equally to this work. J. D. Reis, A. Shahpari, R. Ferreira, S. Ziaie, D. M. Neves, and M. Lima are with the Department of Electronics, Telecommunications and Informat- ics (DETI), University of Aveiro, Instituto de Telecomunicac¸˜ oes, 3810-193 Aveiro, Portugal (e-mail: jacklyn@av.it.pt; ali@ua.pt; ricardomferreira@ua.pt; sziaie@ua.pt; darlene@ua.pt; mlima@ua.pt). A. L. Teixeira is with the Department of Electronics, Telecommunications and Informatics (DETI), University of Aveiro, Instituto de Telecomunicac¸˜ oes, 3810-193 Aveiro, Portugal and also with the Coriant, 81541 Munich, Germany (e-mail: teixeira@ua.pt). 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.2013.2283017 multiplexing (UDWDM) [1]–[3] technologies as solutions for enhancing spectral efficiency, capacity, and flexibility of future optical access networks (F-OANs. Furthermore, these technolo- gies are the enablers of a flexible OAN, capable of supporting users with different bandwidth demands, i.e., from residential to business customers and possibly mobile back/front hauling applications. Both UDWDM and OFDM technologies share similar con- cepts such as providing very high wavelength channel granular- ities, i.e., allocating narrow optical bands per user (e.g., 5 GHz) for several users (e.g., 1000 users) in the same optical distri- bution network (ODN). The referred technologies can be en- hanced by resorting to coherent detection along with advanced modulation formats in the network transceivers. The latter, con- sequently, will improve receiver sensitivity, wavelength tuning range, and provide means of simple data rate upgrade without changing optoelectronic components and simply by updating digital signal processing (DSP) techniques in the transceivers. Hence, the DSP resources required in both optical line terminal and optical network unit (ONU) sides play a major role for both network operation and flexibility in high-capacity PONs. For ex- ample, the work in [4] demonstrated symmetric 1 Gb/s per user for 1000 users using dense WDM with OFDMA subband allo- cation in a total wavelength spectrum of 16 nm. A total capacity of 1.92 Tb/s (40×48 Gb/s spaced by 50 GHz) has been demon- strated with ONUs electronics operating at 5 GHz and requiring two laser sources, i.e., one for coherent detection of downstream data and the other for upstream data transmission. Another co- herent PON concept not based on OFDM has been proposed in [5] using Nyquist shaped UDWDM over dense WDM allo- cation in a total bandwidth of 12.8 nm. In that case, the same total capacity of 1.92 Tb/s has been demonstrated with sym- metric 10 Gb/s per user for 192 users. Due to heterodyning [6], only one laser source is required in the ONU, i.e., the laser source can be split for upstream transmission and downstream. Also, the electronics in the ONU may operate at the symbol rate as very simple DSP is required for recovering the data infor- mation. For instance, if 2.5 Gbaud 16-ary quadrature amplitude modulation (16QAM) (10 Gb/s) with Nyquist pulse shaping is employed, sampling at 2.5 GSa/s (one sample per symbol) is sufficient for performing all the DSP functionalities needed in the receiver: clock recovery, phase estimation, and frequency offset estimation [5]. Extra DSP blocks such as fast Fourier transform (FFT)/inverse FFT (IFFT), cyclic prefix, channel 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.