Mode division multiplexing using an orbital angular momentum mode sorter and MIMO-DSP over a graded-index few-mode optical fibre

TLDR

Mode division multiplexing uses the spatial modes of a multimode fiber as independent data channels to potentially increase capacity, but requires high mode selectivity and avoidance of the 1/N insertion loss inherent to cascaded beam splitters, and orbital angular momentum modes are a promising spatial basis. The authors aim to demonstrate that OAM modes can be demultiplexed over a multimode fiber with high selectivity and without cascaded beam splitting loss using an OAM mode sorter. They employ an OAM mode sorter to achieve greater than –15 dB mode selectivity and eliminate the 1/N loss associated with cascaded beam splitters. As a proof of concept, 20‑Gbit/s polarization‑division‑multiplexed quadrature phase‑shift‑keyed OAM modes of the LP 11 group were transmitted 5 km over a graded‑index few‑mode fiber with channel crosstalk mitigated by 4×4 MIMO DSP, yielding less than 1.5 dB power penalty at a BER of 2×10⁻³.

Abstract

Abstract Mode division multiplexing (MDM)– using a multimode optical fiber’s N spatial modes as data channels to transmit N independent data streams – has received interest as it can potentially increase optical fiber data transmission capacity N-times with respect to single mode optical fibers. Two challenges of MDM are (1) designing mode (de)multiplexers with high mode selectivity (2) designing mode (de)multiplexers without cascaded beam splitting’s 1/N insertion loss. One spatial mode basis that has received interest is that of orbital angular momentum (OAM) modes. In this paper, using a device referred to as an OAM mode sorter, we show that OAM modes can be (de)multiplexed over a multimode optical fiber with higher than −15 dB mode selectivity and without cascaded beam splitting’s 1/N insertion loss. As a proof of concept, the OAM modes of the LP 11 mode group (OAM −1,0 "Equation missing"<!-- image only, no MathML or LaTex --> and OAM +1,0 ), each carrying 20-Gbit/s polarization division multiplexed and quadrature phase shift keyed data streams, are transmitted 5km over a graded-index, few-mode optical fibre. Channel crosstalk is mitigated using 4 × 4 multiple-input-multiple-output digital-signal-processing with &lt;1.5 dB power penalties at a bit-error-rate of 2 × 10 −3 .