Transmission Improvement through Dual-Carrier FEC Gain Sharing Jeffrey Rahn 1 , Kevin Croussore 1 , Gilad Goldfarb 1 , Saurabh Kumar 1 , Matthew Mitchell 1 , Vince Dominic 1 , Brian Taylor 1 , Yeongho Park 1 , Vinay Adavani 1 , Veena Shyamsundar 1 , Mandar Patil 1 , Song Yu 1 , Paul F. Freeman 1 , Atul Mathur 1 , Mehrdad Ziari 1 , David Welch 1 1. Infinera, 140 Caspian Ct., Sunnyvale, CA 94089 USA Author e-mail address: jrahn@infinera.com Abstract: A coherent MODEM is implemented with FEC payload and checksum distributed between two PM-QPSK optical channels spaced at 200 GHz. Real-time experiments verify PDL and PMD penalties are 40-50% higher without FEC gain sharing. 2013 Optical Society of America OCIS codes: (060.1660) Coherent communications; (060.2330) Fiber optics communications. 1. Introduction As carriers respond to pressure to deliver higher data rates, system vendors must respond by continuous improvement in the long-reach optical performance to deliver the reach and capacity required. A critical improvement has been the introduction of forward error correction (FEC) algorithms. FEC can guarantee an error- free output provided the input error rate is better than a threshold value (FEC threshold). This paper explores the ability to stripe transmitted bits across two wavelengths and two polarizations of PM-QPSK signals, such that the FEC algorithm sees an average bit-error rate of those two wavelengths (“gain sharing”). Previous work posits a benefit for PMD tolerance in this configuration [1]. Here we consider the benefit across a range of impairments. 2. System Description a) Superchannel TX PIC & Module TX Retimer Superchannel RX PIC & Module 40 40 FEC Ch 9/10 Modulator Driver ASIC Transimpedance Amplifier ASIC Laser 1 λ2 λ1 λ10 ... Laser 2 ... Laser 10 90˚ Hybrid λ2 λ10 ... Laser 1 Bal. PD Bal. PD 90˚ Hybrid Bal. PD Bal. PD FEC Ch 7/8 FEC Ch 5/6 FEC Ch 3/4 FEC Ch 1/2 RX DSP X Pol VOA Y Pol VOA b) Data bits Row checksum Column checksum Columns Rows Figure 1. a) Optical/RF layout of card. b) Design of FEC algorithm. The coherent MODEM uses a Polarization-Multiplexed QPSK transmitter and receiver PICs implemented at 14.24 GBaud on 10 optical waves spaced 200 GHz, described in [2]. The FEC algorithm uses an iterative, concatenated hard-decision code, shown in Figure 1b. After appending parity check bits to the frame, the contents are distributed between two different wavelengths on the PIC. Received signals are identified using frame header bits to allow the received frame to be deskewed and reconstructed from the separate optical paths. FEC gain sharing benefits the system in a number of ways. In a typical installation, 0.2-0.3 dB benefit is observed from distributed link effects such as gain flatness, fiber nonlinearities, and other effects. System margin allocated for changes over life can be reduced as this variability often has low correlation between channels. However, in this paper we will primarily focus on the reduction of polarization impairments, which tend to change quickly in time, and for which budgeting normally has to consider the worst-case penalties.