Long-haul terabit transmission (2272km) employing digitally pre-distorted quad-carrier PM-16QAM super-channel

Abstract

We experimentally demonstrate long-haul WDM transmission of 36GBaud four-subcarrier Terabit PM-16QAM super-channel. Digital pre-distortion enables ~50% reach improvement for both LAPSCF and SSMF fiber-types, with maximum recorded reach of 2272km and 949km, respectively. Introduction Next generation optical networks are expected to evolve from fixed channel-grid to a flexible channel-grid, capable of supporting variable data rates and modulation formats. Consequently, ~30% improvement in spectral efficiency (SE) is expected: sufficient to satisfy the forecasts of traffic growth in medium term. The flex-grid networks allow for increase in SE by packing different channels closely together forming a super-channel structure. Increase in SE can also be achieved by using both high-order modulation formats and high baud-rate (>25GBd), leading to reduced system costs. In order to support such network upgrades, next generation optical systems will rely on digital pulse shaping and digital to analog converters (DACs) IQ-modulator drive signals. Polarization multiplexed (PM) 16QAM based transmission of baud-rates above conventional 30GBd (net baud-rate) is severely limited by the finite RF bandwidth and effective number of bits (ENOB) of DAC, and nonlinearities of MachZehnderModulator (MZM) based IQ-modulators and driver amplifiers. It is thus critical to pre-compensate these effects at the transmitter, in order to realize feasible higher baud rate transmission. Recently, high baud rate transmission experiments have been reported using electronic time division multiplexing, wide bandwidth photodiodes, etc.. In this paper, we employ pre-distortion of an integrated commercial DAC (3-dB bandwidth of ~16GHz), and an IQ-modulator, using 1.67samples/symbol. We report WDM transmission of 36GBd four-subcarrier 1.152Tb/s super-channel over standard single mode fiber (SSMF) and large area pure silica core fiber (LAPSCF), enabling transmission reach of 949km and 2272km, respectively. DAC and Driver Amplifier Transfer Function The overall transfer function of the DAC and driver was determined by generating sinusoids of varying frequency, and measuring the root mean square voltage at the output of the driver amplifer. The relative RF output power of the amplifier versus frequency for all of the four DAC ports are plotted in the Fig. 1. It can be seen that the combined 3-dB bandwidth of DAC and driver is ~16 GHz for all four channels. Fig. 1: Transfer function of DAC and driver. In addition to the DAC induced performance degradations, IQ-modulator also has a frequency dependent response which is reduced to half at the frequency of ~15GHz compared to DC which further deteriorates the performance. Terabit Super-channel: Digital Pre-distortion and Generation A pseudo-random-bit-sequence (PRBS) of order 13 was chosen as transmit data. The transmit data was first mapped to 16-QAM symbols followed by upsampling and rootraised-cosine (RRC) filtering with a roll-off factor (RO) of 0.2 to limit bandwidth and minimize inter-subcarrier crosstalk. Using the experimentally measured joint transfer function of the DAC and driver, we applied pre-distortion of low pass filtering effects in frequency domain. In addition to the DAC and driver pre-distortion, additional correction for the IQ-modulator’s effects also in frequency domain was employed. Using only 1.67samples/symbol, pre-distortion was applied through Eq. 1: similar to zeroforcing equalizer. 0 4 8 12 16 20 24 28 32 -18 -15 -12 -9 -6 -3 0 Frequency [GHz] N o rm al iz ed p o w er [d B ] HI