Receiver design for Modal Receiver design for Modal Multiplexing in Multimode Fiber Multiplexing in Multimode Fiber

Communication SystemsCommunication Systems

Alan Pak Tao LauStanford UniversitySupervisor: Dr. Lei Xu, Dr. Ting Wang

RoadMap for 10Gb/s Ethernet and RoadMap for 10Gb/s Ethernet and BeyondBeyond

IEEE 802.3 Ethernet standards – 100 IEEE 802.3 Ethernet standards – 100 Mb/s, 1Gb/s, 10Gb/s,….Mb/s, 1Gb/s, 10Gb/s,….

New PCs already have Gb Ethernet port New PCs already have Gb Ethernet port installedinstalled

Fiber is the only medium that can support Fiber is the only medium that can support 10Gb/s and beyond10Gb/s and beyond

Enterprise LAN backbone pre-dominantly Enterprise LAN backbone pre-dominantly multimode fiber (MMF)multimode fiber (MMF)

Single mode vs. Multi-modeSingle 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 mode

zjjl eeruzr )(),,(

SMFSMF MMFMMF

High CapacityHigh CapacityLow attenuationLow attenuationHigh CostHigh CostRequire skilled laborRequire skilled labor

Low CostLow CostDoes not require skilled Does not require skilled laborlaborInter-modal dispersionInter-modal dispersion

• Inter-modal dispersion limit for MMF systems -- BL product 500 MHz-km for 1330nm source.

Single mode vs. Multi-modeSingle 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

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Linear Detection AlgorithmsLinear Detection Algorithms Consider 2 photo-detector structuresConsider 2 photo-detector structures

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•For both detectors, we can pass received signal into a bank of matched filters (MF) matched to the individual spatial profiles.

Linear Detection AlgorithmsLinear Detection Algorithms

thi•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 )(iygr

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2

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noise,

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g(2)

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•Zero-forcing: to recover the bits

•Crossterms

•Less restrictions on mode choices for concentric array detector

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Results Results

Step index fiber. 62.5 um. V=80. 5 channelsStep index fiber. 62.5 um. V=80. 5 channels Using modes closer together results in worse performance – too Using modes closer together results in worse performance – too

much spatial correlation much spatial correlation Inherent signal summation over in circular detector mitigate some Inherent signal summation over in circular detector mitigate some

cross terms produced in the photo-detector square operationcross terms produced in the photo-detector square operation

Results(2)Results(2) Real system: individual bit stream excites multiple modes Real system: individual bit stream excites multiple modes

with different delays such that each detected mode with different delays such that each detected mode contains information from all the users contains information from all the users

Mode coupling effectsMode coupling effects

The weight is time varying but can be tracked. To The weight is time varying but can be tracked. To recover the bits, recover the bits,

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Results (3)Results (3)

Weak coupling Strong coupling

•Linear zero-forcing detector does not work well with strong coupling, due to the cross terms in

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ConclusionsConclusions Practical methods do exist to exploit modal multiplexing Practical methods do exist to exploit modal multiplexing

in MMF which provides capacity gainin MMF which provides capacity gain

Error floors resulting from cross term generation at Error floors resulting from cross term generation at photodetectorsphotodetectors

Concentric Array detector – easier to eliminate error floorConcentric Array detector – easier to eliminate error floor

Multimode excitation by single bit stream and loss of Multimode excitation by single bit stream and loss of phase information at the detector leads to further error phase information at the detector leads to further error floors. floors.