All multimode fiber types are
affected by modal dispersion.
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 sophisticated
signal processing technology in the receiver portion
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 centerline
with the multimode fiber centerline. This configuration
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 dispersion
is reduced in terms of differential mode delay and
inter-symbol interference, thereby allowing the maximum
distance over multimode optical fiber to be
increased at a high throughput. While mode conditioning
patch cables reduce the number of excited modes and
improve the odds of finding a favorable operating condition,
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 manufacturing
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 technique
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 multimode
fiber would no longer induce modal dispersion
as only one mode would propagate; this can be considered
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 singlemode
fiber. Essentially, MPLC enables flexible and