Maximum distance
of link
TABLE 1: Performance of multimode optical fibers: throughput versus distance.
20 I ICT TODAY
100 Mb/s
(100BASE-FX)
1 Gb/
(1000BASE-SX)
10 Gb/s
(10GBASE-SR)
40 Gb/s
(40GBASE-SR4)
OM1 (62.5/125) 2000 m 400 m 33 m N/A
OM2 (50/125) 2000 m 550 m 82 m N/A
OM3 (50/125) 2000 m 575 m 315 m 100 m
OM4/OM5 (50/125) 2000 m 600 m 500 m 150 m
FIGURE 3: The problem of multimode fiber transmission
capacity has led to the creation of technologies that upgrade
existing cabling infrastructure.
This leads to inter-symbol interference and the
inability to retrieve the data that was transmitted. The
higher the transmitted throughput, the greater the distortion.
All multimode fibers are affected by modal dispersion,
including OM5. Certain optical fiber properties,
especially the refractive index profile of the fiber, strongly
impact the velocity of the different modes of propagation,
thereby causing modal dispersion. As glass transformation
techniques have evolved over the years, there are different
generations of graded-index multimode fibers (OMx,
x = 1, 2, 3, 4 or 5) that decrease the differential mode
delay between modes according to the fiber type and,
therefore, the impact of modal dispersion on bandwidth.
To sum up, modal dispersion degrades the width of the
bandwidth capacity of the multimode optical fiber. For
a given throughput, it reduces the distance that can be
reliably attained between the transmitter and receiver.
As a result, standard transmission over multimode fiber
within 850 nm operating wavelength with 10GBASE-SR
is limited to distances of up to 33 meters (36 yards)
on conventional 62.5 μm OM1 fibers and 82 meters
(90 yards) over conventional 50 μm OM2 fibers. Table 1
provides an overview of the performances of multimode
optical fibers types.
A typical solution that prevents throughput and
distance limitations due to modal dispersion in legacy
multimode fiber cabling is to upgrade to a newer-generation
multimode fiber type or to singlemode optical fiber,
which does not experience modal dispersion. However,
redeploying the fiber cabling is frequently a difficult task.
It depends upon the distances involved and the difficulties
in drawing the fibers. An audit of the cabling infrastructure
is necessary in order to verify the availability of
the cable duct as well as its condition. Depending on the
case, civil engineering may be required to deploy new
cable sheaths. For these reasons, the complexity and costs
of redeploying new cable can be exorbitant. Following
this approach may result in a complex installation with
a significant impact on the business and its budget.
SPECIFIC LIGHT LAUNCHING CONDITIONS
TO OVERCOME OPTICAL MULTIMODE FIBER
BANDWIDTH LIMITATIONS
The problem of multimode fiber transmission capacity
as it relates to the need to increase the throughput was
discussed as far back as the late 1990s. To try and overcome
the multimode bandwidth limitation, a new type of
10 Gb/s interface was introduced within the IEEE 802.3aq
standard, referred to as long reach multimode (LRM).
These interfaces are specifically designed for backward
compatibility with OM1 and OM2 fibers. The LRM inter