0733-8724 (c) 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JLT.2016.2647203, Journal of Lightwave Technology > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 Abstract — We present an analysis of PAM-4 based multimode fiber (MMF) links using VCSELs at 850 nm and 1050 nm across a wide range of fiber profiles with Effective modal bandwidth (EMBc) spanning from 2 GHz∙km to 10 GHz∙km. We demonstrate speeds > 51.56 Gbit/s over a set of standard OM3 and OM4 fibers with BER < 10 -12 using PRBS-7, up to reaches of 150 m and BER < 2∙10 -4 up to 300 m using 850 nm and 1050 nm VCSELs. Further, using a prototype wide band multimode fiber (WB-MMF) that is specially designed to operate between a wide band of 850 nm and 1100 nm, we demonstrate transmission at 62 Gbit/s with BER < 10 -12 up to 100 m reach and back-to-back (btb) at 66 Gbit/s. Fiber penalties are extracted at BER ~ 10 -12 for the set of OM4, OM3 and the WB-MMF tested. Results are compared for 850 nm and 1050 nm links, providing insights into the advantages and challenges in deploying longer wavelengths for bitrates > 50 Gbit/s using PAM-4. We perform VCSEL characterization and through a comparison with our analytic link model, identify various link penalties including those due to the relative intensity noise (RIN) and mode partition noise (MPN) of the VCSEL. Index Terms— 50 Gbit/s MMF link, MMF, optical communication, VCSEL-based MMF link, optical noise in VCSELs, shortwave wavelength division multiplexing (SWDM), short reach optical links, relative intensity noise (RIN), PAM-4 based VCSEL-MMF links, WB-MMF performance, 1050 nm based optical links. I. INTRODUCTION ith the recent efforts in standardizing the deployment of 400 GbE short reach links by the IEEE 802.3bs work group, focus on solutions for increasing throughput per lambda has increased significantly [1]-[3]. VCSEL based links offer low power and high capacity, a primary requirement to scale data centers. Indeed, volume manufacturing is now in place for VCSELs that support the deployment of links up to 25 Gbit/s using OOK modulation [4], [5], [6] and VCSELs have been reported with OOK based 50 Gbit/s links [7], [8]. However, future high speed short reach solutions require much higher data rates per lambda which will be difficult to achieve using OOK-based systems without significant improvement in S.Kota Pavan was with the Georgia Institute of Technology, Atlanta, GA 30318 USA. He is now with Texas Instruments, Dallas, TX 75243 USA ( e-mail: skotapavan@ti.com) Justin Lavrencik and S. E. Ralph are with the Georgia Institute of Technology, Atlanta, GA 30318 USA (404-894-5168; e-mail: stephen.ralph@ece.gatech.edu). Copyright (c) 2016 IEEE. Personal use of this material is permitted. However, permission to use this material for any other purposes must be obtained from the IEEE by sending a request to pubs-permissions@ieee.org. VCSEL modulation bandwidth. Multilevel modulation schemes such as PAM-4 based links offer reliable solution to this challenge by doubling the throughput while requiring similar bandwidth as that of OOK. In fact, recently IEEE 802.3bs adopted the PAM-4 modulation as a standard for short reach applications using single-mode fiber (SMF) [1]. With successful PAM-4 based VCSEL-MMF link demonstrations reported recently, efforts to standardize the 50 Gbit/s PAM-4 solutions even for the VCSEL-MMF based applications are underway [1]. There have been a few high speed demonstrations using PAM-4 at 50 Gbit/s and higher recently. In particular, reaches up to 200 m at 50 Gbit/s and higher [3], [6], [9], [10], [11] and up to 100 m at 100 Gbit/s [12] have been shown to be possible at 850 nm. However, none of them offer a comprehensive demonstration of the error-free performance over a wide range of fiber profiles. The nature of the MMF is such that slight variations of the fiber index profile can result in significant penalties even for the same effective modal bandwidth (EMBc) as a result of the complex modal and chromatic dispersion interactions (MCDI) which are not seen in SMF-based links. The MCDI effects were first modeled by [13], taking into account the actual spectral-dependent coupling of each VCSEL-MMF pair inside transmit optical sub-assembly (TOSA). The MCDI can be understood from the DMD plot of an MMF, which represents the relative delays between the different fiber modes caused by the non-ideal fiber alpha profile of a graded-index MMF [16]. Depending on the slope of the DMD plot and the spectral-coupling between the VCSEL and MMF, the modal delays can either exacerbate the differential delays due to CD or compensate for it. In the first case, the MMF exhibits a right-sloped DMD plot (R-MMF) and results in an increased net fiber penalty compared to the case when CD and MD are treated independently. For a left-sloped DMD plot (L-MMF), the net fiber penalty is lower than the individual CD and MD penalties added together. The extent of the MCDI depends on the exact coupling between the VCSEL-MMF pair [13], [14]. Thus, for a reliable assessment of deployable MMF-based links, a statistical analysis across a range of OM4 and OM3 fibers with different fiber profiles and EMBc. The primary limitations in VCSEL-MMF links include both chromatic dispersion (CD) and modal dispersion (MD) caused by the differential modal delay (DMD) in the MMF. However, the new OM4 fibers exhibit very low DMD and the primary penalty is typically CD when using 850 nm 25 Gbit/s capable VCSELs. For silica based MMF, CD decreases from ~ -100 ps/nm∙km at 850 nm to ~ -40 ps/nm∙km at 1050 nm. Thus, longer wavelength solutions have been proposed using VCSEL-based PAM-4 Links up to 62 Gbit/s over OM3, OM4 and WB-MMF: Performance Comparison at 850 nm and 1050 nm Sriharsha Kota Pavan, Member, IEEE, Justin Lavrencik, Student Member, IEEE, and Stephen E. Ralph, Senior Member, IEEE W