Contents 1. Introduction
The single channel bit rate of coherent optical transmission systems is continuously increasing with advances in digital signal processing (DSP), modulation formats, and high-speed electrical and electro-optical components. A key factor in achieving higher bit rates are increases in symbol rates, which require innovations in high-speed electronics and optoelectronic components, with an increase only of around 10% per year. Currently, symbol rates in commercial coherent optical systems range from 28 GBaud to 56 GBaud, enabling single-carrier systems to carry bit rates of 400 Gb/s using polarization-division multiplexed (PDM) quadrature amplitude modulation (QAM) [1]. There are clear indications that commercial systems will soon be pushing to 65–70 GBaud [2]. In research, the highest-symbol-rate electronically time division multiplexed (ETDM) systems without an integrated digital-to-analog converter (DAC) use 138.4-Gbaud PDM-QPSK (553.6 Gb/s line rate) [3]. Using high-speed DACs, 90-GBaud and 100-GBaud PDM-64-QAM for a line rate of 1.08 Tb/s [4] and 1.2 Tb/s [5] has been achieved. Using optical laboratory techniques such as spectral synthesis through electrical or optical digital band interleaving (DBI), optical symbol rates as high as 180 GBaud (BPSK/optically emulated QPSK [6]) and 127.9 GBaud [7], as well as optical time division multiplexing (OTDM) up to 1.28 TBaud [8] has been reported. However, the preferred approach for commercial viability remains ETDM, and in this paper, we extend previous 138.4-GBaud record by 30%, to 180 GBaud. We report the generation of a 180-GBaud (720-Gb/s line rate) PDM-QPSK single-carrier signal using all-ETDM with new Indium Phosphide (InP) Double Heterojunction Bipolar Transistor (DHBT) selectors to generate 180-Gb/s electrical signals. The signals are applied to a conventional LiNbO3 in-phase/quadrature (I/Q) modulator without any transmitter digital signal processing applied. Single-channel transmission over 4480 km of standard single mode fiber (SSMF) using erbium-doped fiber amplifiers is achieved. The signals are detected using ~100-GHz bandwidth balanced photodiodes connected to a >110-GHz (256-GS/s) Keysight oscilloscope.
