Receiver design for Modal Multiplexing in Multimode Fiber Communication Systems Alan Pak Tao Lau Stanford University Supervisor: Dr. Lei Xu, Dr. Ting Wang

RoadMap for 10Gb/s Ethernet and Beyond IEEE Ethernet standards – 100 Mb/s, 1Gb/s, 10Gb/s,…. IEEE Ethernet standards – 100 Mb/s, 1Gb/s, 10Gb/s,…. New PCs already have Gb Ethernet port installed New PCs already have Gb Ethernet port installed Fiber is the only medium that can support 10Gb/s and beyond Fiber is the only medium that can support 10Gb/s and beyond Enterprise LAN backbone pre-dominantly multimode fiber (MMF) Enterprise LAN backbone pre-dominantly multimode fiber (MMF)

Single mode vs. Multi-mode For a given polarization, a mode is a spatial electric field profile whose shape is preserved after propagation through the core of the fiber (Eigenfunctions of the system). Mathematically, a modeSMFMMF High Capacity High Capacity Low attenuation Low attenuation High Cost High Cost Require skilled labor Require skilled labor Low Cost Low Cost Does not require skilled labor Does not require skilled labor Inter-modal dispersion Inter-modal dispersion Inter-modal dispersion limit for MMF systems -- BL product 500 MHz-km for 1330nm source.

Single mode vs. Multi-mode From Information Theory point of view, multi-mode fiber should provide higher information capacity since for the same wavelength, one can multiplex independent bit streams onto different modes, Need multiple Tx and Rx. Set up conceptually similar to Wireless Multiple Input Multiple Output (MIMO) systems but very different in reality due to various fiber optic specific constraints We study the performance of various detection algorithms with different photo-detector structures

Linear Detection Algorithms Consider 2 photo-detector structures Consider 2 photo-detector structures For both detectors, we can pass received signal into a bank of matched filters (MF) matched to the individual spatial profiles.

Linear Detection Algorithms We can pass received signal into a bank of matched filters (MF) matched to the individual spatial profiles. Denote the output of MF output as and. Then Zero-forcing: to recover the bits Crossterms Less restrictions on mode choices for concentric array detector

Results Step index fiber um. V=80. 5 channels Step index fiber um. V=80. 5 channels Using modes closer together results in worse performance – too much spatial correlation Using modes closer together results in worse performance – too much spatial correlation Inherent signal summation over in circular detector mitigate some cross terms produced in the photo-detector square operation Inherent signal summation over in circular detector mitigate some cross terms produced in the photo-detector square operation

Results(2) Real system: individual bit stream excites multiple modes with different delays such that each detected mode contains information from all the users Real system: individual bit stream excites multiple modes with different delays such that each detected mode contains information from all the users Mode coupling effects Mode coupling effects The weight is time varying but can be tracked. To recover the bits, The weight is time varying but can be tracked. To recover the bits,

Results (3) Weak coupling Strong coupling Linear zero-forcing detector does not work well with strong coupling, due to the cross terms in

Conclusions Practical methods do exist to exploit modal multiplexing in MMF which provides capacity gain Practical methods do exist to exploit modal multiplexing in MMF which provides capacity gain Error floors resulting from cross term generation at photodetectors Error floors resulting from cross term generation at photodetectors Concentric Array detector – easier to eliminate error floor Concentric Array detector – easier to eliminate error floor Multimode excitation by single bit stream and loss of phase information at the detector leads to further error floors. Multimode excitation by single bit stream and loss of phase information at the detector leads to further error floors.