IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 6, MARCH 15, 2008 407 Repeaterless 10.7-Gb/s DPSK Transmission Over 304 km of SSMF Using a Coherent Receiver and Electronic Dispersion Compensation Christoph Wree, Member, IEEE, Suhas Bhandare, Member, IEEE, Donald Becker, Member, IEEE, Daniel Mohr, and Abhay Joshi Abstract—Record repeaterless transmission of differential phase-shift keying (DPSK) at 10.7 Gb/s over 304 km of standard single-mode fiber (SSMF) is demonstrated using a coherent optical receiver and electronic dispersion compensation. This is the longest repeaterless 10-Gb/s transmission over SSMF in the absence of Raman amplifiers. The high receiver sensitivity and the high tolerance to nonlinearities of DPSK allow us to overcome a total link loss of 58 dB with a 3-dB system margin. Coherent detection enables linear electrical dispersion compensation and avoids the use of optical dispersion compensation. Index Terms—Coherent detection, differential phase-shift keying (DPSK), electronic dispersion compensation, repeaterless. I. INTRODUCTION R EPEATERLESS transmission enables less complex sys- tems not only for submarine [1] but also for terrestrial [2] fiber-optical communications. The fiber loss, fiber nonlinear- ities, and chromatic dispersion (CD) of the widely deployed standard single-mode fiber (SSMF) are the biggest hurdles to design and implement long-haul repeaterless terrestrial trans- mission systems operating at 1550 nm. Previously, a record 10.66-Gb/s repeaterless transmission distance of 285 km over SSMF without the use of Raman pumping was reported using a chirped managed laser with low extinction ratio [2]. However, this required the right combination of dispersion-compensating fiber, tunable optical dispersion compensator, and electronic dispersion compensation. In this letter, we extend the 10-Gb/s record repeaterless transmission demonstrated in [2] to 304 km of SSMF. This is achieved by using differential phase-shift keying (DPSK) in combination with coherent detection [3] and electronic dispersion compensation. DPSK provides high sensitivity and allows high fiber launch power to overcome link losses of up to 60 dB. No in-line amplifiers, remotely pumped erbium-doped fiber amplifiers (EDFAs), or Raman amplifiers are necessary. The heterodyne receiver used in the experiments preserves the optical phase distortions due to CD and enables linear microwave dispersion compensation in the electrical domain as previously demonstrated [4]. Such a compensation scheme eliminates any additional link loss Manuscript received November 2, 2007; revised December 6, 2007. The authors are with Discovery Semiconductors, Ewing, NJ 08628 USA (e-mail: cwree@chipsat.com). 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.2008.916922 Fig. 1. Experimental setup for 10.7-Gb/s NRZ-DPSK transmission over vari- able lengths of SSMF (0–304 km) with heterodyne detection and electronic dis- persion compensation using an optional EDC for distances of above 200 km. associated with the use of optical dispersion-compensating modules. At a transmission distance over 304 km of SSMF, we demonstrate a system margin of 3 dB considering the most simple Reed–Solomon forward-error correction (FEC) code RS(255,239) which requires a minimum bit-error ratio (BER) of 1.8 10 ( -factor of 11 dB) [5]. II. MEASUREMENT SETUP The experimental setup is shown in Fig. 1. The signal laser is a distributed-feedback (DFB) laser operating at an optical fre- quency of 193.4 THz. The following LiNbO Mach–Zehnder modulator (MZM) is biased in the minimum and is driven by a 10.7-Gb/s electrical nonreturn-to-zero (NRZ) pseudorandom bit sequence having a length of with a peak-to-peak voltage of . The output power of the MZM is amplified by an EDFA to an average power of 15 dBm. The signal is launched over 0, 51, 101, 152, 203, 254, and 304 km of SSMF (G.652), respectively. The dispersion parameter at 1550 nm is 17 ps/nm/km. The average fiber loss is 0.19 dB/km. The optical signal after transmission passes through a variable optical at- tenuator (VOA) to change the received optical signal-to-noise ratio (OSNR). The received optical signal is amplified with a commercial EDFA having a gain of 40 dB and a noise figure of 3.2 dB. A 0.3-nm-wide optical bandpass filter is used to re- duce the amplified spontaneous emission (ASE) noise. An auto- matic polarization controller (APC) aligns the state of polariza- tion of the input signal before combining it with the local oscil- lator (LO) signal. The balanced photodetector has a bandwidth 1041-1135/$25.00 © 2008 IEEE