Four-Channel WDM 640 Gb/s 256 QAM Transmission Utilizing Kramers-Kronig Receiver

Direct detection (DD) systems based on high-order quadrature amplitude modulation (QAM) is promising to provide high spectrum efficiency (SE) for future short-reach data-center applications with next generation standardization of 800 GbE or even 1.6 TbE considering bandwidth limitation. Kramers-Kronig (KK) receiver is recently proposed as an effective method to inherently remove signal-signal beat interference (SSBI) by reconstructing complex optical field in single-sideband (SSB) DD systems. In this paper, we make a detailed system performance assessment of applying a KK receiver in 128, 256 and 512 QAM systems through simulations and experimental demonstrations. 25 GHz-spaced 4 × 140 Gb/s wavelength division multiplexing (WDM) 128 QAM and 4 × 160 Gb/s WDM 256 QAM SSB KK transmissions over 20 km of standard single-mode fiber (SSMF) are successfully achieved. As far as we know, this is the first time that a constellation size as high as 256 QAM with a net SE of 5.12 b/s/Hz is experimentally demonstrated in a WDM system with KK receiver, and the effect of adjusting carrier-to-signal power ratio (CSPR) on the performance of 128, 256 and 512 QAM systems is also investigated with simulation and experimental results.