Contents I. Introduction
Commercial 100-Gb/s transmission systems available and in development today are largely based on single-carrier 28-Gbaud polarization-division-multiplexed (PDM) quadrature phase-shift keying (QPSK) [1]. Such signals can be operated on a 50-GHz frequency grid in wavelength-division-multiplexed (WDM) systems (i.e., at a spectral efficiency, SE, of 2 b/s/Hz), while still providing spectral margin for several reconfigurable optical add/drop multiplexers (ROADMs). In order to increase transport capacity, greater SE can be achieved using higher-level quadrature amplitude modulation (QAM). PDM 64-QAM has recently been demonstrated in WDM systems at line rates consistent with 100-Gb/s operation. Operating on a 12.5-GHz frequency grid, an SE of 8.4 b/s/Hz (assuming 7% overhead for forward error correction, FEC) was reached at a per-channel line rate of 9.4 Gbaud (112.8 Gb/s) [2], and an SE of 9.0 b/s/Hz was achieved at 10.03 Gbaud (120.4 Gb/s) [3]. These experiments used in-phase/quadrature (I/Q) modulators driven with 8-level electrical signals derived either from an opto-electronic scheme [2] or from a commercial arbitrary waveform generator [3]. Scaling per-channel transport bit rates beyond 100 Gb/s is important to accommodate the continuing increase in router interface rates, and to reduce the channel count in high-capacity transport systems. Unfortunately, increasing the channel rate at high SE such as provided by 64-QAM has proven challenging. A complex integrated modulator, comprised of three nested Mach–Zehnder modulators, was used to generate 20-Gbaud 64-QAM using six binary electrical drive signals [4]. Using this transmitter, back-to-back PDM operation with a coherent intradyne receiver resulted in a bit-error-rate (BER) floor of . Another experiment used a commercial digital-to-analog converter (DAC) to drive an I/Q modulator with 8-level electrical signals to produce 28-Gbaud 64-QAM [5]. BER measurements were not performed, but the obtained constellation suggests a similarly high BER floor. We have recently reported [6] 21.4-Gbaud 64-QAM generation using a novel 3-bit electronic high-power DAC circuit [7] driving an I/Q modulator with 8 electrical levels. In PDM operation this equates to a line rate of 256.8 Gb/s, or 240 Gb/s after accounting for 7% FEC overhead. Using a low-noise real-time oscilloscope, we obtained intradyne-detection performance only 4.6-dB off the theoretical limit at a BER of . Transmission over 400 km of ultra-large-area fiber (ULAF) was demonstrated, and a digital nonlinearity-compensation scheme, implemented in the digital signal processing algorithm at the receiver, was shown to improve the results. In this paper, we expand upon the work reported in [6], using the DAC to also obtain excellent results with QPSK and 16-QAM signals (requiring 2- and 4-level electrical drives, respectively).
