Mode vector modulation: receiver architectures and DSP algorithms for polarization tracking

The emergence of modern hyperscale datacenters and artificial general intelligence (AGI) supercomputers creates an unprecedented demand for high-speed, scalable optical interconnects. Contemporary datacenter optical networks rely on pluggable transponders using <i>M</i>-ary pulse amplitude modulation (<i>M</i>-PAM) and direct detection (DD). To scale capacity, the prevailing strategy is to use parallel fiber strands that carry independent <i>M</i>-PAM/DD tributaries, a form of space-division multiplexing (SDM). Meanwhile, multicore fibers (MCFs) with nominally uncoupled, homogeneous, single-mode cores are intended to replace fiber strands in the future to reduce fiber count and simplify data center connectivity. Rather than treating each MCF core as an independent lightpath, we can exploit all MCF spatial and polarization degrees of freedom jointly to transmit multidimensional symbols via space modulation-a format we term mode vector modulation (MVM). This paper presents several novel MVM DD receiver architectures, each offering a different balance between hardware complexity and performance. In parallel, we introduce and evaluate a suite of digital signal processing (DSP) algorithms for polarization tracking, including both blind and pilot-assisted methods. Together, these innovations enhance the practicality of MVM as a high-sensitivity, bandwidth-efficient alternative to <i>M</i>-PAM for future direct-detection-based optical interconnects.