Issue link: https://www.e-digitaleditions.com/i/1327492
40 I ICT TODAY Another way to improve density is to increase the number of transmission lanes within a single optical fiber. This is called space division multiplexing (SDM) and can be done with multi-core fiber or few-mode fiber (Figure 4). Multi-core fiber has multiple cores packed inside a single 125 µm diameter fiber. Each core carries the signal, just like an individual optical fiber. Graded index multimode cores, combined with low cost VCSEL sources, show promise for economical, short-reach component connectivity in data center and high-performance compute environments. This can be achieved because the larger multimode cores allow for easier coupling and alignment compared to tiny singlemode cores. Few-mode fibers are designed to carry a few guided modes of light; each individual mode carries a signal. While few-mode fiber certainly has promise, the draw- back is the complex and potentially expensive electronics required to sort out and interpret the signal. MULTIMODE FIBER: THE ROAD TO 400G While some network managers choose to use SMF to support 400G, there are multiple ways to get to 400G using the MMF that exists today. Four tools to enable MMF-based transceivers to function at 400G include modulation rate, more fiber lanes, multiple wavelengths, and encoding. Modulating the VCSEL has some limitations, although in the IEEE there are currently proposals to achieve 200G using 100G at a single wavelength, encoded up to 200G, or using 50G encoded up to 100G with double the number of lanes. This scenario uses short reach transceivers, such as SR4 and SR8. Increasing the number of lanes, while simpler and lower risk for the transceiver manufacturer, can present challenges for the user. Using multi-core fiber would be a more elegant solution, but it will not be possible until several interconnect issues have been solved. Multi-Core Fiber (MCF) Few-Mode Fiber (FMF) 3 Spatial modes x 2 polarizations = 6 modes FIGURE 4: Comparison of multi-core and few-mode fiber technologies. Future Fiber Technology Considerations The second option involves multiple wavelengths. This has been used in the carrier space for many years. It has been most recently introduced to the enterprise space as SWDM4: 40G and 100G BiDi transceivers that use two different wavelengths to achieve transmission in both directions on a single fiber. The 100G version of this sends 25G pulse amplitude modulation (PAM) 4 encoded data, doubled to 50G, by sending two bits per clock cycle. It is already possible to achieve 400G over SMF using SR4 (single wavelength, 4 lanes of 100G). To encourage development of a solution for multi- mode, the IEEE and the Ethernet Alliance are offering a prize to the first company that demonstrates <3.5 W QSFP28+ 100Gb/s transceiver module with CAUI-4 electrical interface using a single wavelength