1041-1135 (c) 2015 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. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LPT.2015.2439571, IEEE Photonics Technology Letters Abstract—We experimentally demonstrated 140Gbit/s transmission over 20km standard single mode fiber (SSMF) employing PAM-4 and direct detection at 1.3 μm. Direct detection faster than Nyquist (DD-FTN) is employed to compensate for channel impairments. The optimal length of taps of DD-LMS and the optimal tap coefficient for digital post filter are investigated. A receiver sensitivity of -5.5dBm at BER of 3.8x10 -3 is realized for 140Gbit/s PAM-4 signal after 20km transmission. To the best of our knowledge, this is the highest reported baud rate (70 Gbaud) of direct detected PAM-4 signal and the highest per channel bit rate with single polarization and direct detection for short reach communications. Index Terms—Pulse-amplitude modulation, direct detection, optical modulation. I. INTRODUCTION HE increasing bandwidth demand of data center and other optical inter-connect applications requires higher data rate short reach optical communication systems. 100Gbit/s transmission of Ethernet frames has been standardized using four parallel streams of 25Gbit/s transmission with OOK modulation. Research interest has now moved towards 400Gbit/s and above. 4x100Gbit/s scheme based on existing 100GbE inexpensive and power efficient components is a promising way to realize 400Gbit/s or above in terms of cost, footprint and power consumption [1-4]. This means a bit rate of 100Gbit/s per lane is essential. This work was supported by project G-YJ84 and G-YL24 of the Hong Kong Polytechnic University, H-ZG46 of Huawei Technologies Co. Ltd, The Hong Kong Scholar Program (XJ2013026), National Natural Science Foundation of China (61401020,61377093,61435006), and Beijing Natural Science Foundation (4154080). K. P. Zhong and C. Lu are with Photonics Research Center, Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong (e-mail:zhongkangping1987@gmail.com, enluchao@polyu.edu.hk). X. Zhou is both with University of Science and Technology Beijing (USTB), Beijing, China, and Photonics Research Center, Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong ( e-mail: zhouxian219@gmail.com). Y. L. Gao and A. P. T. Lau are with Photonics Research Center, Department of Electrical Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong (e-mail: ryancoolg@gmail.com, eeaptlau@polyu.edu.hk). W. Chen, J. Man and L. Zeng are with Fixed Network R&D Department, Huawei Technologies Co,. Ltd., Shenzhen, China (micro.chen@huawei.com, manjiangwei@huawei.com, zengli@huawei.com) Copyright (c) 2015 IEEE. Personal use of this material is permitted. Recent reported short reach transmission experiment to achieve 100Gb/s per lane or above include 112Gb/s half cycle 16QAM Nyquist subcarrier modulation (SCM) with polarization division multiplexing or dual wavelength scheme [5], multi-band carrierless amplitude and phase modulation (Multi-CAP) [6,7], discrete multi-tone modulation (DMT) [8,9]. Besides half-cycle Nyquist subcarrier modulation, Multi-CAP and DMT, PAM-4 modulation is also an attractive modulation format for optical interconnection, PON and metro networks. 107Gbit/s short reach transmission using single polarization PAM-4 signal was demonstrated using SiP Mach-Zehnder modulator [10]. A 112Gbit/s doubinary-4-PAM signal was experimentally demonstrated for short reach transmission system [11]. In our previous work, we proposed a direct detection faster than Nyquist technique (DD-FTN) for channel equalization and symbol detection and experimentally demonstrated single channel bit rate from 112Gb/s to 128Gb/s and four lanes transmission with a total capacity of 500Gb/s employing PAM-4 and direct detection [12-14]. In this paper, we further extend the bit rate and using DD-FTN technique for channel equalization. The optimal length of taps of DD-LMS and the optimal tap coefficient for digital post filter are investigated. We experimentally demonstrated a 140Gbit/s PAM-4 signal transmission over 20km of SSMF with a receiver sensitivity of -5.5dBm at BER of 3.8x10 -3 . To the best of our knowledge, this is the highest reported baud rate of direct detected PAM-4 signal and the highest per channel bit rate with, single polarization and direct detection optical link for short reach communications. II. EXPERIMENTAL SETUP Fig.1. shows the experimental setup of PAM-4 transmission system. A 2 16 de Bruijn bit sequence is used for bit to symbol mapping and generation of PAM-4 signal. In the experimental setup, the sampling rate of AWG (arbitrary waveform generator, Micram AWG6010) is set to be 70GS/s, which means the digital PAM-4 signal has 1 sample per symbol. The signal is then pre-emphasized by an inversed 4 th order Gaussian filter with a 3dB bandwidth of 24GHz in order to compensate for the limited bandwidth of transmitter and receiver. Then, the electric signal is amplified to a peak-to-peak voltage of 1.7V by a linear EA (electric amplifier) and used to drive a 20GHz EML (externally modulated laser). The 3 dB bandwidth of the EML is 20GHz.The bias voltage is optimized to be -2.3V. The center 140Gbit/s 20km Transmission of PAM-4 Signal at 1.3 μm for Short Reach Communications Kangping Zhong, Xian Zhou, Yuliang Gao, Wei Chen, Jiangwei Man, Li Zeng, Alan Pak Tao Lau and Chao Lu T