7×149 Gbit/s PAM4 Transmission over 1 km Multicore Fiber for Short-Reach Optical Interconnects Oskars Ozolins 1 , Xiaodan Pang 2,1 , Aleksejs Udalcovs 1 , Rui Lin 2,4 , Joris Van Kerrebrouck 3 , Lin Gan 4 , Lu Zhang 2 , Ming Tang 4 , Songnian Fu 4 , Richard Schatz 2 , Urban Westergren 2 , Gunnar Jacobsen 1 , Deming Liu 2 , Weijun Tong 5 , Guy Torfs 3 , Johan Bauwelinck 3 , Jiajia Chen 2 , Sergei Popov 2 , and Xin Yin 3 1 Networking and Transmission Laboratory, RISE Acreo AB, Kista, Sweden 2 KTH Royal Institute of Technology, Kista, Sweden 3 IDLab, INTEC, Ghent University – imec, Gent, Belgium 4 Huazhong University of Science and Technology, Wuhan, China 5 Yangtze Optical fiber and Cable Joint Stock Limited Company, Wuhan, China Author e-mail address: oskars.ozolins@ri.se Abstract: We transmit 80 Gbaud/λ/core PAM4 signal enabled by 1.55 μm EML over 1 km 7-core fiber. The solution achieves single-wavelength and single-fiber 1.04 Tbit/s post-FEC transmission enhancing bandwidth-density for short-reach optical interconnects. OCIS codes: (200.4650) Optical interconnects; (060.2360) Fiber optics links and subsystems 1. Introduction Intra-datacenter interconnects have been experiencing a tremendous growth of capacity to enable web-based high- performance applications [1-3]. High capacity demand from datacenter applications has become the technology driver for high bandwidth (BW) short-reach communications [4]. Earlier demonstrations, e.g. [5-7], include two- lanes 400 GbE client-side links interconnecting the high bandwidth systems and providing a smaller footprint while consuming less power. Pulse amplitude modulation with four levels (PAM4) is the modulation format of choice for physical layer specifications of 400 GbE interfaces [3]. Line side optics need to scale on faster pace to support tremendous bandwidth growth. Therefore, digital signal processing enhancements are critical factors for the optical devices to enable higher linearity, extinction ratio, and tighter integration with electronics [1]. Space-division multiplexing (SDM) used today in datacenters is in the form of parallel single mode fibers thanks to less dominant fiber cost for short-reach links [8]. Meanwhile, SDM fibers are actively explored to scale up system capacity through spatial efficiency [2]. A combination of high bandwidth devices and spatial efficiency through multicore fibers (MCFs) seems to be a viable solution to enable high bandwidth-density, having enormous potential to increase the throughput per single-wavelength and single-fiber to 1 Tb/s for short-reach optical interconnects [4, 9,10]. In this paper, we demonstrate 80 Gbaud/λ/core PAM4 signal transmission using a monolithically integrated externally modulated laser (EML). We achieve below 7% overhead (OH) hard-decision forward error correction (HD-FEC) performance over 1 km single mode 7-core MCF with a negligible inter-core crosstalk and a low-loss fan-in/fan-out device. The solution enables terabit scale (7×149 Gbit/s) post-FEC transmission speed over 1 km single-wavelength and single-fiber, greatly enhancing bandwidth-density for short-reach optical interconnects. 2. Experiment setup Figure 1(a) shows the experimental setup for signal generation, transmission and direct detection and includes the 7- core fiber cross section as inset. The 80 Gbaud PAM4 signal is generated offline using a 2 15 -1 long pseudorandom binary sequence. Then it is up-sampled and filtered with a root-raised-cosine filter having 0.15 roll-off factor. Frequency domain channel pre-equalization is applied for channel frequency response calibration (up to 46 GHz due to the limitation of the arbitrary waveform generator (AWG)). The calibration is based on the end-to-end system measurement as shown in Fig. 1 (b). After pre-equalization 80 Gbaud PAM4 signal is loaded to a 92 GSa/s AWG. Then it is amplified in a 65 GHz electrical amplifier with 11 dB gain. A monolithically integrated 1.55 μm EML (3 dB BW >100 GHz [11]) is used to generate optical the PAM4 signal. DSO Clock recovery BER counting FFE, DFE LPF (0.75·baudr.) 160 GSa/s, 63 GHz VOA EDFA AWG EML 65 GHz 65 GHz DSP 1 km 7-core fiber EDFA 1x8 ... ... 100 GHz 90 GHz 92 GSa/s, 32 GHz τ n (a) FI FO Receiver Transmitter Link (b) Fig. 1. Experimental setup (a), 7-core fiber cross section (b), channel magnitude calibration frequency response (c)