Kasyapa Balemarthy

VCSEL-Based Interconnects for Current and Future Data Centers

The vast majority of optical links within the data center are based on vertical cavity surface emitting lasers (VCSELs) operating at 850 nm over multimode optical fiber. Deployable links have evolved in speed from 1 Gb/s in 1996 to 28 Gb/s in 2014. Serial data links at 40 and 56 Gb/s are now under development and place even more demand on the VCSEL and photodiodes. In this paper, we present the characteristics of VCSELs and photodiodes used in current generation 28 Gb/s links and present several methods to extend link distances using more advanced data encoding schemes. Finally, we will present results on wavelength division multiplexing on multimode optical fiber that demonstrate 40 Gb/s Ethernet connections up to 300 m on duplex OM3 optical fiber, and present results on fiber optimized for modal bandwidth in the 850 to 980 nm range.

Electronic Equalization of Multikilometer 10-Gb/s Multimode Fiber Links: Mode-Coupling Effects

This paper investigates the ability of electronic equalization to compensate for modal dispersion in the presence of mode coupling in multimode fibers (MMFs) at 10 Gb/s. Using a new time-domain experimental method, mode coupling is quantified in MMF. These results, together with a comprehensive link model, allow to determine the impact of mode coupling on the performance of MMF. The equalizer performance on links from 300 m to 8 km is quantified with and without modal coupling. It is shown that the mode-coupling effects are influenced by the specific index profile and increase the equalizer penalty by as much as 1 dBo for 1-km links and 2.3 dBo for 2-km links when using a standard model of fiber profiles at 1310 nm

Spatially resolved detection and equalization of modal dispersion limited multimode fiber links

The authors present theoretical and experimental results of an optoelectronic-equalization technique that mitigates intersymbol interference caused by differential modal delay in multimode fiber. By exploiting the spatial diversity of the transverse optical-fiber modes, the authors are able to provide a sufficient additional information in the form of mode-dependent photocurrents to enhance total signal integrity. A fabricated two-segment photodetector is demonstrated with a routinely achievable two-times improvement in bandwidth-distance product. They also show the robustness of the technique to the expected variations in graded-index profiles.

10 Gb/s transmission over 300 m OM3 fiber from 990-1080 nm with electronic dispersion compensation

We demonstrate numerically and experimentally the transmission of 10 Gb/s over OM3 fibers up to 300 m, under low-cost VCSEL launch conditions from 990-1080 nm with electronic dispersion compensation (EDC), showing a path to low cost 40 Gbps links.

Electronic dispersion compensation of non-ideal multimode fiber links

We study the capability of electronic filters to equalize challenging examples of 500 MHz-km compliant 62.5 micron multimode fibers representative of the installed base. Most cannot be equalized via conventional EDC with acceptable power penalty.

MIMO Processing of Multi-mode Fiber Links

Multiple-input multiple-output (MIMO) processing of multi-mode fibers using dual launches, a two-segment photo-detector and an electronic MIMO-DFE is proposed. The potential benefits are demonstrated using capacity for the 108 fiber model as the performance metric

Next-generation wideband multimode fibers for data centers

Short-reach optical links such as those used in data centers pre-dominantly employ VCSELs together with laser- optimized OM4 and OM3 multimode fiber (MMF), mainly due to their reliability, energy-efficiency and low end-to-end system cost. The IEEE 802.3bm specification for 100Gbps Ethernet utilizes four parallel MMFs each operating at a serial data rate of 25Gbps. Due to the rapidly increasing internet traffic, the IEEE P802.3bs Task Force is working towards a 400Gbps Ethernet standard requiring a commensurate increase in the number of parallel fibers deployed. Using 16 parallel lanes, while feasible, is not the most efficient use of cabling. One solution to the data rate - cable density problem is the use of shortwave wavelength division multiplexing (SWDM) near 850nm. For example, employing four wavelengths separated by ~30nm (with an operational window of ~840-950nm) results in a four-fold increase in the per-fiber data rate. Furthermore, SWDM can be combined with the parallel solution to support 400Gbps with the same cable density as the current 100Gbps Ethernet solution using OM4 fiber. Conventional laser-optimized OM4 gives diminished performance at the longer wavelengths compared to 850nm. Shifting the OM4 optimization wavelength to longer wavelengths sacrifices the 850nm performance. In this paper, we present next-generation wideband multimode fibers (NG-WBMMF) that are optimized for SWDM operation using a novel design approach employing multiple dopants. We have fabricated and characterized a wideband MMF that is OM4 compliant over the 850-950nm wavelength window. BER measurements demonstrate that this next-generation WB MMF satisfies the pre-FEC requirement of 5 × 10-5 even after transmission over 300m.

Mode coupling: Why POF Supports 40Gbps

We demonstrate experimentally and numerically that mode-coupling in graded index plastic optical fiber enables 40 Gbps over 200 m in the presence of dramatic refractive index errors.

Low-complexity hybrid MLSE equalizers for 10 Gb/s Ethernet multi-mode fiber links

We demonstrate a new class of electronic equalizers for 10 Gb/s multi-mode fiber links that combine the advantages of Viterbi equalizers and decision feedback equalizers (DFE) yielding a low complexity, readily implemented architecture with performance /spl sim/0.5-0.75 dBo better than an infinite-length DFE.

Group Delay Ripple in Fiber Bragg Gratings: Electronic Equalization

We investigate the impact of electronic equalization on fiber Bragg gratings (FBG) with group delay ripple at 40 Gbit/s. Correlating with measured FBGs, we report that the associated penalty reduction may be limited in practice.