Contents 1. Introduction
In the quest to reduce the cost per bit in wavelength-division multiplexed (WDM) transmission systems, it is attractive to reduce the number of wavelength channels while increasing the total capacity per fiber. This is achieved by increasing the symbol rate and at the same time the spectral efficiency through the use of polarization-division multiplexed (PDM) higher-order quadrature amplitude modulation (QAM). Today's commercial systems typically run on a 50-GHz grid with symbol rates on the order of 32GBd. Recent demonstrations of electrically multiplexed systems at high symbol rates include 107-GBd PDM-16-QAM at a line rate of 856 Gb/s within an optical bandwidth of 130GHz [1], 41.4-GBd PDM-64-QAM at a line rate of 493 Gb/s [2], 43-GBd PDM-64-QAM at a line rate of 516Gb/s using a SiGe digital-to-analog converter (DAC) [3], and 46-GBd PDM-64-QAM at a line rate of 552 Gb/s, generated using 65-GS/s DACs in CMOS technology and transmitted over 2000 km [4]. Recently, we presented a novel arbitrary-waveform generator (AWG) based on high-speed SiGe DACs (MICRAM VEGA DAC3) and used it to generate a digitally-shaped single-polarization 55-GBd 64-QAM signal [5]. In this work, we demonstrate the generation and detection of an optical 72-GBd PDM-64-QAM signal at a record-high single-carrier line rate of 864 Gb/s. Placing five such channels on a 100-GHz optical grid, we show transmission over a 400-km fiber link. We achieve this result by applying advanced digital-signal processing (DSP) at the transmitter and at the receiver.
