Scaling multimode fibre IM/DD transmission capacity through spatial-spectral multiplexing

Abstract

Data centre connections can greatly benefit from parallel transmission channels on one multimode fibre (MMF). Shortwave wavelength division multiplexing (SWDM) achieves parallel transmission through spectral multiplexing. Furthermore, MMFs offer a spatial dimension that should be exploited to increase parallel transmission, albeit in a cost-effective way. In this paper, it is shown that SWDM and spatial multiplexing can be combined in intensity modulation and direct detection MMF transmission systems that use selective offset excitation and mode-selective spatial filtering. Keywords: space division multiplexing, wavelength division multiplexing, multimode fibre, data centre network INTRODUCTIONIntra-data-centre optical connections are extensively employing graded-index (GI) multimode fibres (MMFs). Such MMF connections do not normally exceed a 500-m reach and should support the Gbps needs of the massive data centre networks. To achieve transmission of 100 Gbps and beyond, the following approaches have been considered: parallel optics in terms of parallel MMFs and MPO connectors, using advanced modulation formats such as pulse amplitude modulation and discrete multitone, and introducing shortwave wavelength division multiplexing (SWDM) by extending the 850-nm window and employing wideband MMFs [1], [2]. These approaches lead to solutions based on intensity modulation and direct detection (IM/DD). However, it is also the case that the MMF modes can be exploited by applying space division multiplexing (SDM) techniques. This can have significant advantages such as increasing the bit-rate-distance product, or reducing the number of parallel MMFs required. Applying SDM with IM/DD on GI-MMFs is possible and can be realized with selective offset excitation and mode-selective spatial filtering (MSSF) [3], [4].In this paper, we explore the application of IM/DD SDM with offset excitation and MSSF on 50/125 μm GIMMFs in a SWDM scenario. This scenario would require wavelength-selective optical filters that do not impact the mode power distribution that forms each spatial channel. This requirement can be met by WDM filters based on free-space optics and thin film filters [5]. It remains, though, crucial to investigate the application of SDM in terms of offset excitation and MSSF over the wavelength range proposed for SWDM. It is shown here that this IM/DD SDM approach can apply similarly on each SWDM channel, although the mode volume of the GI-MMF varies significantly within the SWDM wavelength range. Furthermore, the case of two IM/DD SDM channels can be realized without any additional signal processing requirements, presenting a very simple and straightforward scenario.