High Bandwidth, Multi Mode Fiber Systems G.Giaretta, R.Michalzik, M.Wegmueller, S.Hunsche, M.C.Nuss, OPTICAL LAN RESEARCH DEPARTMENT,HOLMDEL S.Golowich, W.Reed, OPTICAL FIBER RESEARCH, MURRAY HILL P.Kolesar, J.Ritger, S.Jones, NETWORK CABLE SYSTEMS, ATLANTA
Why Multi Mode Fiber Systems at 10 Gb / s? Backward compatibility with previous LAN standards (FDDI, Ethernet, ATM) and sources (LED, VCSELs, FP Lasers) Extend the existing lowest cost solution with minimal changes Uniform solution from 10 Mb / s to 10 Gb / s for in building applications
Systems within buildings, How far? Previous surveys have shown that 300 m links cover the great majority of building interconnects Previous successful Gb Ethernet solutions run for less than 300m (220 m at 0.85 µm) Standards (and especially initial objectives) define the minimal system requirements, vendors improve to differentiate and cover niche markets Objective: 300 m MMF links for building links.
Multi Mode fiber solutions Three approaches have been proposed for ~10 Gb / s multi mode fiber optic links: Serial solution at 0.85 µm on new MMF Coarse WDM at 1.3µm over any fiber Serial solution using multi level coding over any fiber
What do they have in common? Use of single mode sources over MMF Serial solution over new MMF Chromatic Dispersion => Single mode VCSELs WDM solution Temperature stability => Single mode DFBs Multi level coding Linearity and noise => Single mode lasers
Modal Noise Key ingredients to generate modal noise are: Coherent sources => Single mode sources Multi path links => Multi mode fibers Spatial filtering => Lossy connectors Fraction of coupled power is time dependent Noise
Not a problem in the past, why now? All the sources for Gb Ethernet were multi mode sources Low coherence VCSELs had multi spatial modes FP sources had multi longitudinal modes LED MM-VCSEL FP-LASER Coherence SM-VCSEL DFB-LASER
What to do about it? Extensive publications exist that confirm the seriousness of the problem Bates et al., IBM Watson Research Center, Improved multi mode fiber link.., Opt.Quan.Elect. 1995. Cunningham et al., HP Labs, Modal noise penalties..., Elect.Letters 95. Some general solutions have been proposed but are expensive and unpractical out of band modulation (100 GHz??) self pulsating lasers (100 GHz??)
Restricted center launch Restricted center launch can be used to mitigate modal noise in a system through reduced loss First connector Second connector Third connector Requires new fiber with high restricted launch bandwidth close to the center Not applicable if overfilled launch is needed
Traditional MMF 0 1 Pulse shapes at different offsets Time (ns) 0.5 1 Pulse amplitude (a.u.) 0.8 0.6 0.4 0.2 1.5-30 -20-10 0 10 20 30 Lateral offset (µm) 0 0.2 0.4 0.6 0.8 1 1.2 Time (ns) The traditional MMF can not support robust 10Gb/s transmission
Installed base MMF systems Coarse WDM (and multi level coding) require single mode tolerances if they have to run on any fiber Coarse WDM (and multi level coding) systems as proposed today require offset patch cords to run on installed MMF Modal noise can not be mitigated in high bit rate systems using restricted center launch on installed MMF Further investigation is needed to show robust transmission on installed MMF at 10 Gb / s
ZETA multi-mode fiber 0 1 Pulse shapes at different offsets Time (ns) 0.5 1 Pulse amplitude (a.u.) 0.8 0.6 0.4 0.2 1.5-30 -20-10 0 10 20 30 Lateral offset (µm) 0 0.2 0.4 0.6 0.8 1 1.2 Time (ns) ZETA Multimode fiber has a flat and narrow DMD
Advantages of the MM serial approach Simple evolutionary extension, of current least expensive Gb Ethernet solution made possible from improvements in fiber manufacturing and speed advancements of electronics and receivers Backward compatibility with previous LAN standards (FDDI, Ethernet, ATM) and sources (LED, VCSELs, FP Lasers) Less expensive than SM serial due to the larger tolerances in alignment and simplified source packaging Less expensive than WDM since it does not require optical multiplexing and de-multiplexing, wavelength control, single mode tolerances ( any fiber solution), and higher reliability.
System requirements for 300m MMF solution at 0.85µm Dispersion Modal Chromatic Modal Noise New high bandwidth fiber (ZETA) (>2.2GHz km) Single mode VCSELs (<0.2nm) Center restricted launch Fiber with flat and narrow DMD around the center of the fiber
ZETA stressed system configuration ±5µm XYZ offset Fiber Shakers 300m ZETA Fiber Offset connectors Narrow linewidth Mode Scrambler (2dB mode selective loss) VCSEL (0.2dB micro-bending loss) BERT Pin receiver Attenuator
ZETA stressed system demonstration 1.0e-03 1.0e-05 Back to back 300m of Zeta MMF 300m of stressed Zeta MMF BER 1.0e-07 1.0e-09 1.0e-11 1.0e-13 2.3dB 5dB -22-20 -18-16 -14-12 -10-8 -6 Receiver Power (dbm) ZETA fiber can support robust 10 Gb/s transmission even under stressed conditions
10 Gb / s Ethernet power budget (12.5 Gbauds at 0.85µm) 9 8 7 6 5 4 3 2 1 0 0.100 0.110 0.120 0.130 0.140 0.150 0.160 0.170 0.180 0.190 0.200 0.210 0.220 0.230 0.240 0.250 0.260 0.270 Optical Penalties and Power Budget ( db) 0.280 0.290 0.300 Pbudget Ptotal Pisi Patt Prin R. Eye Pmn Pmpn Link Length (km) Minor modifications to the power budget compared to 1 Gb/s Ethernet
Options to accommodate the power budget The 8dB power budget can be accommodated without significant cost increases using either: more sensitive receivers (-16,-18dBm) Increased eye safety limits (-1.5dBm) More strict laser source power control (4.3dB) Forward Error Correction Coding Open loop fiber control
Conclusions Modal bandwidth and Modal noise can be very serious problems for MMF solutions that don t rely on a new fiber design specifically tailored to mitigate them Objectives mentioning explicitly installed MMF are premature since modal noise issues have not been sufficiently studied (published experimental evidence supports the existence and seriousness of such problems) The new MMF serial solution has been demonstrated: to be robust at 10 Gb / s over 300 m (Lucent Technologies) to operate at 10 Gb / s over 400 m (Lucent Technologies, Gore Photonics) to operate at 12.5 Gb / s over 300 m (Lucent Technologies, Gore Photonics)