Issue link: https://www.e-digitaleditions.com/i/1192126
January/February/March 2020 I 21 faces transmit within the range of 1300 nm and are capable of the same transmission distance over OM1 and OM2 fibers, up to 220 meters (240 yards). The key to the longer reach of the 10GBASE–LRM standard on conventional multimode fibers is the use of a sophisti- cated signal processing technology in the receiver por- tion of these devices. The specialized signal processing is referred to as electronic dispersion compensation (EDC). EDC is deployed as an integrated circuit that acts as a complex continuous adaptive filter on the received signal from the optical fiber in order to reduce inter- symbol interference. To achieve the maximum distances of 220 meters at 10GB/s, 10GBASE-LRM transceivers must be used with mode conditioning patch cables (MCPCs) at both ends of the fiber link. The standard relies on specific launch conditions. The mode conditioning patch cord is a special assembly that precisely offsets the singlemode fiber center- line with the multimode fiber centerline. This configura- tion is referred to as the offset launch technique. Therefore, mode conditioning patch cord partially controls the light launch location of the singlemode fiber from the LRM source into the multimode fiber. More precisely, this type of patch cable produces a launch that only excites some of the modes within the multimode fiber optical core, mainly higher order modes. By reducing the number of modes excited by this offset launching technique, the impact of modal disper- sion is reduced in terms of differential mode delay and inter-symbol interference, thereby allowing the maxi- mum distance over multimode optical fiber to be increased at a high throughput. While mode condition- ing patch cables reduce the number of excited modes and improve the odds of finding a favorable operating condi- tion, they also provide diversity in the modes it excites; there is no fiber or connector specification that ensures a minimum bandwidth level for this launch condition. Thus, depending on the quality of the optical cable and the eccentricity of field connectors, it is a hit-or-miss situation, leaving users to experiment on-site with their own optical fibers. Furthermore, the maximum distance of 220 meters, supported by 10GBASE-LRM equipment, is unfortunately often too short for LAN backbone cabling networks for which typical lengths equal or exceed 300 meters (330 yards). Cabling surveys that list the needs for distance capabilities of existing 10 Gb/s multimode solutions indicate that supporting only 220 meters addresses only 60 percent of multimode links in local networks. Many studies have been carried out within the IEEE 802.3aq workgroups, and one of them focused on the launching offset range needed to maximize bandwidth increase. Theoretical results showed a significant gain in bandwidth within a range of 15-to-20 μm offset launch. This range is what is being targeted today in the manufac- turing of MCPCs. However, this offset range corresponds to a secondary maximum for the bandwidth gain. The maximum gain is obtained for a zero offset. Such a tech- nique is called center launching. The best-case scenario would be to perform mode conditioning, thereby enabling a launching condition that accurately excites the fundamental center mode into the multimode fiber core. In this way, the light transmission within the multi- mode fiber would no longer induce modal dispersion as only one mode would propagate; this can be consid- ered a kind of quasi-singlemode transmission over the multimode fiber. By reducing the number of modes excited by this center launching technique to one, the impact of modal dispersion moves towards zero in terms of differential mode delay and inter-symbol interference, thereby allowing the maximum distance over multimode fibers to be increased at a high throughput. A MORE EFFICIENT INNOVATIVE MODAL ADAPTING PASSIVE TECHNIQUE One way to overcome modal dispersion is to carry the information in just one mode over multimode fibers. By properly exciting a single fiber spatial mode, the coupling of the signal to different group modes and propagation speeds is negligible. Consequently, the spatial mode can then be used as an independent high-speed transmission channel with the same transmission properties as single- mode fiber. Essentially, MPLC enables flexible and All multimode fiber types are affected by modal dispersion.