Roman Shubochkin

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.

All-fiber mode-group-selective photonic lantern using graded-index multimode fibers

We fabricate mode-group-selective photonic lanterns using multimode graded-index fibers. The use of the multimode graded-index fibers in the taper can significantly relax the adiabaticity requirement in comparison with using single-mode fibers.

Mode-multiplexed transmission over conventional graded-index multimode fibers

We present experimental results for combined mode-multiplexed and wavelength multiplexed transmission over conventional graded-index multimode fibers. We use mode-selective photonic lanterns as mode couplers to precisely excite a subset of the modes of the multimode fiber and additionally to compensate for the differential group delay between the excited modes. Spatial mode filters are added to suppress undesired higher order modes. We transmit 30-Gbaud QPSK signals over 60 WDM channels, 3 spatial modes, and 2 polarizations, reaching a distance of 310 km based on a 44.3 km long span. We also report about transmission experiments over 6 spatial modes for a 17-km single-span experiment. The results indicate that multimode fibers support scalable mode-division multiplexing approaches, where modes can be added over time if desired. Also the results indicate that mode-multiplexed transmission distance over 300 km are possible in conventional multimode fibers.

50Gbit/s PAM-4 MMF Transmission Using 1060nm VCSELs with Reach beyond 200m

We experimentally demonstrate error-free transmission of 50Gbit/s PAM-4 signals over OM3 and prototype wideband fiber using 1060nm VCSELs. FEC-conformed performance is demonstrated over 200m. An analytic model allows identification of penalties and demonstrates negligible MPN.

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.