80 Gbit/s/λ̣ polarization multiplexed star-16QAM WDM transmission over 720 km SSMF with electronic distortion equalization

Abstract

For the first time, 80/40 Gbit/s/O Star-16QAM WDM transmission with/without polarization multiplexing has been demonstrated over 720/1200 km SSMF on a 50 GHz frequency grid applying enhanced techniques for electronic distortion equalization. IntroductionHigher-order optical modulation formats in combination with coherent detection and electronic distortion equalization (EDE) are key emerging technologies paving the way to 100 Gb-Ethernet and highest spectral efficiency as they lead to a reduction of symbol rate and spectral width per WDM channel. Transmission over long distances using higher-order modulation and coherent detection was already demonstrated for quadrature phase shift keying (QPSK) [1] and 8-ary phase shift keying (8PSK) [2,4]. In this paper, long-haul WDM transmission of 16-ary quadrature amplitude modulation in star constellation, Star-16QAM, is investigated for transmission spans without inline dispersion compensation. When applying Star-16QAM, the spectral efficiency can theoretically be increased 4-fold without, and 8-fold with polarization multiplexing (PM), respectively. Data rates of 80 Gbit/s per wavelength can be generated by using PM Star-16QAM requiring a symbol rate of only 10 Gbaud. Generation and transmission of the Square-16QAM format have been already demonstrated by using multilevel driving signals and a single stage IQmodulator [5] or by using an integrated solution of a quad-parallel Mach-Zehnder modulator (MZM) [6]. At this stage, main challenges of these solutions are the generation of the high quality driving signals for the IQmodulator as well as the performance of the integrated quad-parallel MZM. In this paper we demonstrate generation of Star-16QAM by using binary driving signals only based on state of the art modulators as well as WDM transmission over 720/1200 km SSMF on a 50 GHz frequency grid with/without polarization multiplexing. Using EDE after coherent detection, chromatic dispersion can effectively be compensated for in the frequency domain [3], while other transmission impairments (such as polarization cross-talk) are usually compensated in the time domain using blind channel equalization (BCE) techniques, such as the constant modulus algorithm (CMA). In this paper,