Receiver design for Modal Multiplexing in Multimode Fiber Communication Systems

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

2. RoadMap for 10Gb/s Ethernet and Beyond • IEEE 802.3 Ethernet standards – 100 Mb/s, 1Gb/s, 10Gb/s,…. • New PCs already have Gb Ethernet port installed • Fiber is the only medium that can support 10Gb/s and beyond • Enterprise LAN backbone pre-dominantly multimode fiber (MMF)

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

4. 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

5. Linear Detection Algorithms • 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.

6. 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

7. Results • Step index fiber. 62.5 um. V=80. 5 channels • 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

8. Results(2) • 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 • The weight is time varying but can be tracked. To recover the bits,

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

10. Conclusions • Practical methods do exist to exploit modal multiplexing in MMF which provides capacity gain • Error floors resulting from cross term generation at photodetectors • 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.