High-speed SDM Interconnects with Directly-Modulated 1.5-μm VCSEL enabled by Low-Complexity Signal Processing Techniques Xiaodan Pang 1,3 , Joris Van Kerrebrouck 2 , Oskars Ozolins 3 , Rui Lin 1,4 , Aleksejs Udalcovs 3 , Lu Zhang 1 , Silvia Spiga 5 , Markus C. Amann 5 , Geert Van Steenberge 6 , Lin Gan 4 , Ming Tang 4 , Songnian Fu 4 , Richard Schatz 1 , Gunnar Jacobsen 3 , Sergei Popov 1 , Deming Liu 4 , Weijun Tong 7 , Guy Torfs 2 , Johan Bauwelinck 2 , Xin Yin 2 and Jiajia Chen 1 1 KTH Royal Institute of Technology, Kista, Sweden 2 IDLab, INTEC, Ghent University – imec, Gent, Belgium 3 Networking and Transmission Laboratory, RISE Acreo AB, Kista, Sweden 4 Huazhong University of Science and Technology, Wuhan, China 5 Walter Schottky Institut, Technische Universität München, Garching, Germany 6 CMST, Ghent University – imec, Gent, Belgium 7 Yangtze Optical fiber and Cable Joint Stock Limited Company, Wuhan, China Abstract: We report on our recent work in supporting up to 100 Gbps/λ/core transmissions with a directly modulated 1.5-μm single mode VCSEL and multicore fiber, enabled by low-compleixty pre- and post- digital equalizations. OCIS codes: (250.7260) Vertical cavity surface emitting lasers; (200.4650) Optical interconnects Vertical cavity surface emitting laser (VCSEL)-based transceivers are considered as promising candidates to support future high-performance computing (HPC) and datacenter applications, by fulfilling stringent requirements on data rate, cost, reliability, power consumption, footprint and size [1]. Among them, the VCSEL array can seamlessly extend its potential of high-density integration to space division multiplexing (SDM) to scale up the lane count per fiber and reduce cabling complexity [2]. High speed single lane transmission with short wavelength multimode (MM)-VCSELs and advanced modulation formats, and multi-lane transmissions with coupled MM-VCSEL array with multicore fibers (MCFs) for high aggregate data rates, have been reported [3-7]. For applications requiring distances more than a few kilometers, longer wavelength single mode (SM)-VCSELs with single mode fiber (SMF) links are more feasible. A few recent high-speed transmission works with 1.5-μm SM-VCSELs show promising results towards implementation [8-11]. In this abstract, we present our recent work on high-speed transmissions using 1.5-μm SM-VCSEL over 7-core fiber links of different distances [12]. We demonstrate up to 70 Gbaud PAM- 4 signal generation, and 50 Gbaud PAM-4 transmissions over 1-km dispersion-uncompensated and 10-km dispersion-compensated single mode MCF links, with pre-equalization based on accurate end-to-end channel characterization, combined with low-complexity digital post-equalization. Figure 1 shows the experimental setup, the VCSEL L-I-V characteristic, the picture of the probe station, and the S21 response curves of the chip and the transmission system before and after fiber transmission. The maximum 3- dB modulation bandwidth of the VCSEL chip is ~ 23 GHz. The PAM-4 symbols were offline generated, and pre- AWG 92GSa/s Probed VCSEL 1543 nm Fan-in device Delay lines splitter Fan-out device PD DSO 160 GSa/s 1km / 10km 7-core fiber DCM Bias-T Offline DSP Offline DSP -159 ps/nm 0 2 4 6 8 10 I (mA) 0 0.4 0.8 1.2 1.6 Power (mW ) 0 0.4 0.8 1.2 1.6 Voltage (V) (a) VCSEL-LIV 0 10 20 30 40 Frequency(GHz) -50 -40 -30 -20 -10 0 10 I=2mA I=4mA I=6mA I=8mA I=10mA S21 (dB) (c) VCSEL B2B 0 20 40 60 80 Frequency(GHz) -50 -40 -30 -20 -10 0 10 core 1 core 2 core 3 core 4 core 5 core 6 core 7 S21 (dB) (d) 1-km MCF 0 20 40 60 80 Frequency(GHz) -50 -40 -30 -20 -10 0 10 S21 (dB) core 1 core 2 core 3 core 4 core 5 core 6 core 7 (e) 10-km MCF + DCM (b) Fig. 1. Experimental setup. Insets: (a) VCSEL L-I-V curves; (b) photos of probe station; S21 response of (c) the VCSEL chip, and after transmission of each core of the (d) 1-km MCF and (e) 10-km MCF + DCM