Fiber
How High Performance Fiber
is Designed, Manufactured
and Tested
By John Kamino and George Oulundsen III
M
ultimode fiber systems have
to ensure the bandwidth of their OM4
pulse is known as differential mode delay
long been recognized as the
offerings.
(DMD). DMD is a direct measurement of
lowest cost solution for high-
speed transmission over short reach
optical communications. This advan-
tage continues even as transmission
rates evolve to 40 and 100 Gigabits
per second (Gb/s) speeds. The industry
is currently developing standards for a
new multimode fiber called OM4, specifically designed for more demanding
applications.
The key fiber performance charac-
Manufacturing Process
Designed to Maximize
Bandwidth
In digital terms, bandwidth is
expressed as a data rate at which signals
can be transmitted over a given distance
without one bit overlapping another,
making the signal unreadable. When a
pulse of light enters the fiber’s core, it
separates into different paths known as
the light transmission properties affecting
bandwidth.
Manufacturers strive to maximize
bandwidth by minimizing modal dispersion. This is done by controlling the
precision and accuracy with which the
fiber is designed and manufactured.
Several methods are available to manufacture optical fiber, including Inside
Vapor Deposition (IVD) and Outside
Light in different modes can
Vapor Deposition (OVD) processes. In
teristic for OM4 fiber is bandwidth, or
travel at different speeds through the
IVD processes, each layer of glass that
information-carrying capacity. Bandwidth
fiber. As the light pulse travels down the
forms the most important light-guiding
can be defined in a variety of ways
fiber, these modes spread out in time.
core region of the fiber is independently
modes.
depending on the application. This article
This effect, known as modal disper-
deposited and sintered in precise high-
explains how today’s fiber is manufac-
sion, can cause overlapping of bits and
purity glass tubes, resulting in precise
tured to optimize bandwidth performance
loss of signal integrity. The difference
control of the core refractive index profile.
and the methods that manufacturers use
in arrival time between modes within a
In OVD processes, the entire core
page 8 July 2009 www.cablingbusiness.com
How High Performance Fiber is Designed, Manufactured and Tested
Fiber
and clad regions are deposited as soot
particles on a target rod.
The resulting
soot boule must then be sintered (or consolidated) into a glass perform. During the
sintering process, diffusion takes place,
making it more difficult to precisely control the refractive index profile of the most
important light-guiding core region of the
fiber.
OFS uses a proprietary Modified
Chemical Vapor Deposition (MCVD) process (an IVD process), which takes advantage of the tight index profile control provided by the deposition and sintering of
Many of these layers are deposited on top
or preform, by increasing the temperature
of each other combining to form the wave-
of the flame, and decreasing the speed of
guide region of the fiber. Layer by layer,
the torch. The preform is then analyzed to
Ultra-pure chemicals
this process creates the precise wave-
measure its refractive index characteristics
are introduced into the tube and, as a spe-
guide profile (also known as the refractive
and core diameter.
cial torch along the lathe bed, white soot is
index profile) that is necessary for today’s
deposited on the inner tube wall.
high performance, laser-based communi-
individual layers in the critical fiber core.
The MCVD process begins when a
pure synthetic quartz tube is placed in a
glassworking lathe.
As the torch traverses over the depos-
cation systems.
Process Control is Key to OM4
Manufacturing
ited soot, the soot is fused into a thin layer
After the core deposition is complet-
The symmetry of the core relative to
of pure transparent glass inside the tube.
ed, the tube is collapsed into a solid rod,
the cladding (called core/clad offset) is crit-
Blog at www.cablingbusiness.com/wordpress July 2009 page 9
Fiber
How High Performance Fiber is Designed, Manufactured and Tested
ical, especially for high-speed (one - 100
process, to maintain uniformity along the
cise 1 micron steps across the diameter
Gb/s) Ethernet applications, where Vertical
entire fiber length.
of the fiber core and comparing the arrival
time for the pulse with a detector at the
Cavity Surface Emitting Lasers (VCSELs)
Throughout the draw process, the
are used as the light source. Precise core/
fiber diameter is controlled to 125 microns,
clad offset helps ensure proper coupling of
within an extremely tight tolerance of a
the VCSEL into the fiber core and reduces
few tenths of a micron.
A laser-based
at each step, and their arrival times are
splice losses. Tight process control of the
diameter gauge samples the fiber over a
recorded. The DMD of the fiber is the dif-
central regions and core/cladding sym-
thousand times a second, ensuring that
ference between the earliest and the latest
metry are essential steps in manufacturing
precise dimensions are maintained. Before
arrival times of all pulses. The lower the
fiber for laser-optimized applications at 10,
the fiber is put onto winding drums at the
DMD, the higher the bandwidth of the fiber.
40 and 100 Gb/s.
bottom of the draw tower, it is threaded
As with most specifications, the tighter
through dies that apply a protective acry-
the specification the better the perfor-
late coating.
mance. Within the zero to five µm core
In the next manufacturing step, called
fiber draw, the preform is converted into
opposite end of the fiber.
Only a few mode groups are excited
First, the preform is
Each spool of fiber is measured to
region, some manufacturers’ fibers provide
lowered into a high purity graphite fur-
validate that the product will meet stringent
even tighter DMD than is required by the
nace where tightly controlled temperatures
industry and internal specifications. These
standard. This results in higher reliability
approaching 2200 degrees Celsius are
tests measure mechanical strength, geo-
margins and improved performance with
used to soften its tip. Then gravity takes
metric properties and optical properties.
center-launch light sources.
OM4 optical fiber.
over, allowing the molten tip to freefall,
One key test for OM4 and other laser-
DMD measurement is currently the
drawing a thin strand of glass from the pre-
optimized multimode fibers is DMD. This
most reliable method for verifying band-
form. The tip of the fiber strand is threaded
measurement compares the difference in
width required for 10 Gb/s or beyond,
through tension pulleys and attached to
arrival times of principal mode groups
because it is the only method that checks
a draw spool.
The tension on the fiber
traveling down the fiber. The measurement
the modal dispersion across the fiber core
is then controlled during the entire draw
is done by launching pulses of light at pre-
independently. For that reason, industry
page 10 July 2009 www.cablingbusiness.com
How High Performance Fiber is Designed, Manufactured and Tested
associations such as TIA/EIA and ISO/
IEC have published standards for DMD
measurement and specifications for laser-
Measuring Bandwidth of OM4
Multimode Fibers
Bandwidth measurement and assur-
optimized multimode fiber.
ance of OM4 performance will be done
The Development of OM Fibers
using the same methods (albeit with tighter
As data rates in enterprise applications have exploded, OM4 fiber becomes
the latest in a line of ever more capable multimode product types, which are
identified by the OM (“optical mode”)
designation as outlined in the ISO/IEC
11801 standard.
It follows on the heels of OM3 fiber,
now familiar to the industry as a laseroptimized 50 µm fiber having 2000 MHzkm Effective Modal Bandwidth (laser
bandwidth), designed for 10 Gb/s transmission. In order to transmit 300 meters
at 10 Gb/s on OM3 fiber at 850 nm, the
IEEE 802.3 standards body determined
that 2000 MHz-km of Effective Modal
Bandwidth (EMB) is necessary for the link
to function properly.
EMB is the combined bandwidth of
the fiber and the VCSEL light source.
EMB is a function of the interaction
between the fiber’s modal structure (as
measured by DMD) and the VCSEL’s
mode power distribution.
specifications) as OM3. According to the
standard now in development, OM4 fibers
will have an EMB of 4700 MHz-km.
To
achieve this higher level of bandwidth, the
DMD Mask specifications will be tightened
proportionately, as will the OFL bandwidth
specification.
Finally, a 1300 nm OFL
bandwidth spec of > 500 MHz-km will be
maintained for backward compatibility.
A recent study conducted by OFS demonstrates that DMD provides more rigorous
screening than other methods. The study
Fiber
“As data rates
in enterprise
applications have
exploded, OM4
fiber becomes the
latest in a line
of ever more
capable multimode
product types ...”
was conducted with commercially obtained
“Extended Reach OM3” (OM4) fibers and
cables from various vendors to determine
their ability to support claims of extended link
distances for 10 Gb/s transmission.
In the study, fibers that failed DMD
and passed EMB requirements show significantly poorer performance, and in several cases failed system testing.
Also,
it should be noted that one of the failing
fibers passed EMB, but failed DMD and
OFL requirements. This indicates that if
Since the
EMB calculations are used, OFL should be
power output of VCSEL light sources dif-
used as a further screen to determine fiber
fers from one to the next, a given VCSEL
performance.
can produce different EMB on different
With this as background, the job of
fibers, and different VCSELs can produce
specifying the bandwidth of the multimode
different EMB on the same fiber.
John Kamino is the multimode optical fiber
fiber is actually quite simple. For fiber used
In order to ensure a minimum of 2000
product manager for OFS (www.ofsoptics.com).
in legacy LED based systems, the OFL
MHz-km EMB, the Telecommunications
A 27-year veteran of the company,
method is perfectly valid as a measure
Industry Association (TIA) established
he has also held positions in optical connectivity
of bandwidth. However, for fiber being
specifications for the VCSELs and fiber
product management, offer management, product
installed in networks designed for one Gb/s
marketing management, sales, and engineering
(reference Detail Specification TIA/EIA-
speeds, the fiber must be laser-certified
with OFS. He can be contacted at
492AAAC or international equivalent
or DMD controlled to provide reliable per-
jkamino@ofsoptics.com.
IEC 60793-2-10).
Any combination of
formance in accordance with applicable
VCSELs meeting the specs and fiber
specifications. For 10 Gb/s applications
George E. Oulundsen III is a Distinguished Member
meeting the specs would produce 2000
and beyond, a fiber whose refractive index
of Technical Staff in the R&D Group at OFS
MHz-km EMB.
profile has been precisely controlled using
(www.ofsoptics.com). He has developed many of
These standards allow two ways to
an advanced IVD process, and whose
the various multimode processes and products
disposition the fiber: the DMD Mask meth-
laser bandwidth is assured using the DMD
at OFS over the past 10 years, and has spent
od, and the EMBc method. Both methods
method and specifications will provide the
the last several years improving multimode
require DMD testing - the difference lies in
best, most reliable performance for the
fiber measurements. He can be contacted at
how the DMD data is used and interpreted.
most demanding network.
goulundsen@ofsoptics.com.
Blog at www.cablingbusiness.com/wordpress July 2009 page 11