Fiber Optic Update
year is another busy year for fiber optics. New technology,
components, applications and usually a few surprises.
On this page we've gathered some of the more important
topics, covering new technology and applications that FOA believes every tech needs to
know. Many of these articles are from the FOA
monthly newsletter, which you can subscribe
We also recommend the FOA "Fiber
FAQs" page with tech questions from customers originally
printed in the FOA Newsletter. We get lots of interesting
questions at FOA.
This page is part of a Fiber
U Tech Update Course.
questions? Try the FOA
Guide and use the site search.
ALL Got It All Wrong - And They Confuse A Lot Of People - YOU
CANNOT STRIP THE CLADDING OFF GLASS FIBER!!!
recently got this email from a student with field experience
taking a fiber optic class:""The instructors are telling us
that we are stripping the cladding from the core when prepping
to cleave MM and SM fiber. I learned from Lenny
Lightwave years ago, this is not correct. I do not want
to embarrass them, but I don't want my fellow techs to look
foolish when we graduate from this course."
share with you our answer to this student in a moment, but
first it seems important to understand where this
misinformation comes from. We did an image search on the
Internet for drawings of optical fiber. Here is what we found:
fiber drawing we found on the Internet search with one exception
(which we will show in a second) showed the same thing - the
core of the fiber separate -sticking out of the cladding and the
cladding sticking out of the primary buffer coating. Those
drawings are not all from websites that you might expect some
technical inaccuracies, several were from fiber or other fiber
optic component manufacturers and one was from a company
specializing in highly technical fiber research equipment.
The only drawing we found that does not show the core separate
from the cladding was -
really! - on the FOA
Guide page on optical fiber.
No wonder everyone is confused. Practically every drawing shows
the core and cladding being separate elements in an optical
So how did FOA help this student explain the facts to his
instructors? We thought about talking about how fiber is
manufactured by drawing fiber from a solid glass preform with
the same index profile as the final fiber. But we figured a
simpler way to explain the fiber core and cladding is one solid
piece of glass was to look at a completed connector or a fusion
We started with a video microscope view of the end of a
connector being inspected for cleaning.
Here you can see the fiber in the ceramic ferrule. The hole of
the connector is ~125 microns diameter (usually a micron or two
bigger to allow the fiber to fit in the ferrule with some
adhesive easily.) The illuminated core shows how the cladding
traps light in the core but carries little or no light itself.
This does not look like the cladding was stripped, does it?
Here is the same view with a singlemode fiber at higher
And no connector ferrules have 50, 62.5 or 9 micron holes so
that just the core would fit in the ferrule, do they?
What about stripping fiber for fusion splicing. Here is the view
of fiber in an EasySplicer ready to splice.
What do you see in the EasySplicer screen? Isn't that the core
in the middle and the cladding around it? In fact, isn't this a
"cladding alignment" splicer?
We rest our case. If that's not sufficient to convince everyone
that you do not strip the cladding when preparing fiber for
termination or splicing, we're not sure what is.
Special Request: To everyone in the fiber optic industry
who has a website with a drawing on it that shows the core
of optical fiber separate from the cladding, can you please
change the drawing or at the very least add a few words to
tell readers that in glass optical fiber the core and
cladding are all part of one strand of glass and when you
strip fiber, you strip the primary buffer coating down to
the 125 micron OD of the cladding?
diagrams of fiber construction are wrong - showing core
and cladding as separate - but they are one solid peice of
cannot strip the cladding from glass fibers.
Loss For Splice-On Connectors
received a call from a contractor working on a network. His
subcontractor doing termination presented data on terminations
using mechanical splice-on connectors where he claimed the TIA
standard for these connectors was 0.75dB for the connector
PLUS 0.3dB for the splice, for a total of 1.05dB. He wanted to
know if this were true.
is not true. These connectors have an internal splice to a
stub fiber already glued in the ferrule and factory polished.
The loss of the connector used to terminate a fiber must
include the splice since it is the termination method and
there is no way to test it separately from the connector
mechanical splice-on connector, also called a prepolished/splice
We noted the TIA loss value, 0.75dB was very high compared to
adhesive polish connectors which average around 0.3dB loss when
tested against a reference connector. In the standards it has
remained at 0.75dB to cover this type of connector and array
connectors like the MPO.
fiber optic cables have specifications that must not be
exceeded during installation to prevent irreparable damage
to the cable. This includes pulling tension, minimum bend
radius and crush loads. Installers must understand these
specifications and know how to pull cables without damaging
recommendation for fiber optic cable bend radius is the minimum
bend radius under tension during pulling is 20 times the
diameter of the cable. When not under tension, e.g. cable stored
in service loops, the minimum recommended long term bend
radius is 10 times the cable diameter.
Note: Always check the cable specifications for cables you
are installing as some cables such as the high fiber count
cables have different bend radius specifications from
And also note that some manufacturers are now quoting "bend
diameter" instead of or in addition to bend radius. Bend
diameter is more relevant when dealing with service loops or
storage loops, while ben radius is more aimed at bending
cable around corners. Remember the diameter is twice the
radius of a circle, so the minimum bend diameter of a
cable under pulling tension, e.g. the diameter of a
capstan used in pulling cables, would be 40 times the
diameter of the cable and the storage loop minimum diameter
would be 20 times the diameter of the cable.
tension (top) and after pulling (bottom)
Bend radius example: A cable 13mm (0.5") diameter would have a
minimum bend radius under tension of 20 X 13mm = 260mm (20 x
0.5" = 10") That means if you are pulling this cable over a
pulley, that pulley should have a minimum radius of 260mm/10" or
a diameter of 520mm/20" - don't get radius and diameter mixed
is it important? Not following bend radius guidelines can lead
to cable damage. If the cable is damaged in installation, the
manufacturer's warranty is voided. Here is what one
manufacturer's warranty says: "This
warranty does not apply to normal wear and tear or damage
caused by negligence, lack
of maintenance, accident, abnormal operation, improper
installation or service, unauthorized repair, fire,
floods, and acts of God." And their specifications call
our the minimum bend radius as "20 X OD-Installation, 10 X
When An Installer Gets it Wrong
There are two problems here, one visible and one hidden.
The visible one is the pulley mounted on the side of the
truck used to change the direction of the cable to allow
using the capstan mounted on the rear of the truck. The
cable is being bent about 120 degrees over a pulley that
appears to be about 120mm (5 inches) diameter. That's a
radius of 60mm or 2.5 inches. That pulley looks like a
stringing block uses for stringing ropes when pulling in
We believe the cable was a 864 fiber ribbon cable with a
diameter of 24mm (0.92") with a minimum bend radius of
360mm or 14". That means the pulley the cable is
being pulled over is ~1/6th the size it should be - shown
by the dotted red circle above.
The second problem is the angle of the cable coming out of
the manhole. It is exiting a conduit and being pulled
almost straight up out of the manhole. If there is no
hardware in the manhole, the cable is being pulled over an
edge exiting the conduit or the manhole, bending with a
very, very small radius.
can only speculate about the possible damage to a
cable when treated like this. What comes to mind
first is broken fibers, and that is a possibility.
But bending this tightly can also damage the cable
structure, including the fiberglass stiffeners,
strength members and jacket. Compromising the
integrity of the cable reduces its protection for
the fibers. Even the fiber ribbons can be
delaminated and fibers put under stress. A cable
pulled under these circumstances can have damage
along the entire length, not just a point where it
What should have been done on this pull? The
120mm/5" pulley should have been replaced with one
at least 6 times larger. The truck could have been
further from the manhole (and maybe turned to be
inline with the pull) so the angle of the cable
exiting the conduit was less. Hardware should be
attached to the conduit to provide a proper bend
radius for the cable as it exited the conduit and
the cable should have been protected if it contacted
the edges of the manhole..
more about bend radius.
cables have specifications for minimum bend radius
this spec may permanently damage the cable
radius is generally 20X cable diameter under tension - 10X
Loss: Are You Positive It’s Positive?
7/2020: Mystery solved! Investigations into ISO standards
showed the international standards committees changed the
definition of loss in a way that changes the sign for loss
but makes it violate all scientific convention on the use of
dB. This is documented below.
recent post on a company’s
article on the CI&M website discussed the topic of the
polarity (meaning “+” or “-“ numbers) of measurements of optical
loss, claiming loss was a positive number. The implication was
that some people failed fourth grade math and did not understand
positive and negative numbers. The claim is that insertion loss
is always a positive number.
Is that right?
The asnwer is no - loss is a negative number, but instruments
that only measure loss - OLTS and OTDRs - display loss as a
Suppose we set up a test. Let's measure power out of a cable
with a power meter and then attenuate the power by stressing the
cable. What happens?
created this short movie on the FOA
Guide page explaining dB showing how a power meter shows
loss when a cable is stressed to induce loss:
As the fiber is stressed, inducing loss, the power level goes
from -20.0 dBm to --22.3 dBm.That's a more negative number.
(-22.3dB) - (-20.0dB) = -2.3dB That's basically 4th grade
No question – loss means a more negative power reading in dB
and a negative number in dB indicates loss.
If you want to calculate this yourself, FOA
has a XLS spreadsheet you can download that will
calculate the equations for optical power for you.
But if you are a manufacturer of fiber optic test instruments
that offers optical power meters and sources to test loss, why
would this confuse you? Well, it seems they think when
we talk about loss, we do not give it a "+ or -" sign, we just
say loss, so they just display it as a number without sign,
In IEC (and TIA documents adopted from IEC documents, the
definition of attenuation in Sec. 3.1 is written to have
attenuation calculated based on
Power(reference)/Power (after attenuation). This
definition leads to attenuation being a positive number as
it is normally displayed by an OLTS or OTDR. However if
one uses a fiber optic power meter calibrated in
dBm, the result will be a negative number, since dBm is
defined as Power(measured)/Power(1mw) (see FOTP-95, Sec.
6.2). If dBm were defined in this manner, power levels
below 1mW would be positive numbers, not negative as they
are now, and power levels above 1mW would be negative!
Bottom Line: Confusion
in dB is a negative number
that measure loss do not display negative signs with
are displayed with a negative sign
or dBm -Still Confusing 4/2020 -
second most missed question on FOA/Fiber U online tests
concerns dB, that strange logarithmic method we use to measure
power in fiber optics (and radio and electronics and acoustics
and more...). We've covered the topic several times in our
Newsletter but there still seems to be confusion. So we're
going to give you a clue to the answers and hopefully help you
understand dB better.
These are all correct statements with the percentage
of test takers who know the answer is correct.
The most answered correctly: dBm is absolute power
relative to 1mw of power (78.8% correct. Does "absolute"
confuse people? It's just "power" but absolute in contrast
to "relative power" which is loss or gain measured in dB.)
This one is answered correctly less than half the time: dBm
is absolute power like the output of a transmitter. (41.5%
correct, see comment above.)
This one does often get answered correctly: The difference
between 2 measurements in dBm is expressed in dB. (23.8%
Here is an example of a power meter measuring in dBm and
microwatts (a microwatt is 1/1000th of a milliwatt.)
is an example
watts to dBm.
This meter is
If we convert
to dBm, it
out using dB
-10dBm is 1/10
of a milliwatt
that is a
factor of 0.25
so 0.1mW X
0.25 = 0.025mW
The other way
to figure it
is -10dB is
1/10 and -6dB
is 0.25 or
= 1/2, so 6dB
= 3dB + 3dB =
1/2 = 1/4) so
Read a more
comprehensive explanation of dB here in the FOA Guide.
A FOA Newsletter reader
sent FOA these microscope photos of two MM (multimode) fibers,
asking what was the difference with the one on the right. It is
a bend-insensitive (BI) fiber and compared to the regular
graded-index MM fiber you readily notice the index "trench"
around the core that reflects light lost in stress bends right
back into the core. You can read
more about bend-insensitive fiber in the FOA Guide.
does Bend-Insensitive Fiber Look Like?
researching the answers to the question above, we talked to Phil
Irwin at Panduit. He mentioned that you could see the structure
of BI fiber and sent along this photo:
At the left, you can see the gray area surrounding the core,
shown in the drawing in the right as the yellow depressed
If you want to try to see it yourself, it's not easy. Phil tells
us that OFS fiber is the easiest to see, Corning a bit more
difficult. You need a good video microscope. You may need to
vary the lighting and illuminate the core with low level light.
Today most multimode (MM) fibers are bend insensitive fibers. If
you buy a MM cable or patchcord, it is probably made with
bend-insensitive fibers. That's generally good because thee
fibers are less sensitive to bending or stress losses which can
cause attenuation in regular fibers.
Many singlemode fibers are bend-insensitive also, especially
those used with smaller coatings to pack more fibers into
microcables or high fiber count cables.
Perils Of 2-Cable Referencing
received an inquiry about fluctuations in insertion loss
testing. The installer was using a two cable reference
method for setting a "0dB" reference where you attach one
reference cable to the source, another to the meter and
connect them to set the "0dB" reference. The 2-cable
reference method is allowed by most insertion loss testing
standards, along with the 1- and 3- cable reference
methods, although each gives a different loss value.
3 different ways to set a 0dB reference for loss testing
When a 1-cable reference is used, one sets a reference
value at the output of the launch cable and measures the
total loss. With a 2-cable reference, a connection between
the launch and receive reference cables is included in
making the reference, so the loss value measured will be
lower by the amount of that connection loss. The 3-cable
reference includes two connection losses so the loss will
be lower still.
The problem with the two cable reference is the
uncertainty added by including the connection between the
two reference cables when setting the "0dB"
Unless you carefully inspect and clean the two connectors
and check the loss of that connection before setting the "0dB"
reference, you add a large amount of uncertainty to
measurements of loss. The best way to use a 2-cable
reference is to set up the source and reference cable
(with inspected and cleaned connectors), measure the
output of the launch cable, attach the receive cable (with
inspected and cleaned connectors) and measure the loss of
the connection before setting the "0dB"
reference. If the connection loss is not less than 0.5dB,
you have connectors that should not be used for testing
other cables. Find better reference cables.
The two cable reference is often used when the connectors
on the cables or cable plant being tested are not
compatible with the connectors on the test equipment, so
you must use hybrid launch and receive cables. Then you
can only reference the cable when connected to each other.
In that case, you need the 2-cable reference but should
expect lower loss and higher measurement uncertainty.
Experiments have shown that the uncertainty with a 1-cable
reference is around +/-0.05dB while the 2-cable has an
uncertainty of around +/-0.2 to 0.25dB caused by the
mating connection between the two reference cables. Those
experiments also showed the uncertainty of the 3-cable
reference was not significantly larger than the 2-cable
When possible, use a 1-cable reference. When you must use
the 2- or 3-cable reference, inspect and clean all
connectors carefully before making connections for the
reference or test.
value of loss you measure depends on how you set
your "0dB" reference - more reference cables means
between reference cables when setting a 0dB loss add
uncertainty to measurements
With A VFL: Fibers
Damaged In Splice Trays
a trend? Twice in one week, we have inquiries from readers
with problems and both were traced to fibers cracked when
inserted in splice trays. The photo below shows one of them
illuminated with a VFL. This was the same issue we found in
the first field trial of a VFL more than 30 years ago that led
to its popularity in field troubleshooting.
are invaluable troubleshooting tools for finding cable
only work close by - 3-4km range max
"Fast" Is Fiber?
probably all heard the comment that fiber optics sends
signals at the speed of light. But have you ever thought
about what that speed really is? The speed of light most
people think about is C = speed of light in a vacuum =
300,000 km/s = 186,000 miles/sec. But in glass, the speed is
reduced by about 1/3 caused by the material in the glass.
The light is slowed down and the amount is defined as the
index of refraction of the glass. V=
speed of light in a fiber = c/index of refraction of
(~1.46) = 205,000 km/s or 127,000 miles/sec. So in
the "speed of light" is about 2/3 C, the speed of
light in a
vacuum. And the difference in speed in
different materials is what makes fiber work - causing "total internal
of the FOA instructors sent us this question: "I work
with at Washington Univ with an engineer who works for an
electrical utility. He asked a question about the speed of
signal transmission over fiber optics, single mode, at top of
towers. They need signal to be sent in 18 millisecs for relays
to function properly. Is there a problem over a distance of
Let’s do a calculation:
C = speed of light in a vacuum = 300,000 km/s = 186,000
V= speed of light in a fiber = c/index of refraction of fiber
(~1.46) = 205,000 km/s or 127,000 miles/sec
150 miles / 127,000 miles/sec = 0.00118 seconds or ~1.2
Another way to look at it is 127,000 miles/sec X 0.018 seconds
(18ms) = 2,286 miles
So the fiber transit time is not an issue. The electronics
conversion times might be larger than that.
I used to explain to classes that light travels about this fast:
300,000 km / sec
300 km / millisecond
0.3km /microsecond or 300m / microsecond
0.3 m per nanosecond - so in a billionth of a second, light
travels about 30cm or 12 inches
Since it travels slower by the ration of the index of
refraction, 1.46, that becomes about 20cm or 8 inches per
That is useful to know since an OTDR pulse 10ns wide translates
to about 200cm or 2 m pr 80 inches (6 feet and 8 inches), giving
you an idea of the pulse width in distance in the fiber or an
idea of the best resolution of the OTDR with that pulse
is "fast" because of its bandwidth capability
travels in fiber at the speed of light
the speed of light in glass is only 2/3 as fast as the
speed of light in air or a vacuum
Does A FTTH ONT Look Like Today?
That's all there is to the ONT that goes into the home. The
arrow points to the 1310 TX/1490 RX transceiver for SC-APC
Note: That device is now priced at $10-20US. The incredibly
high volume of FTTH components has drive costs of these
devices down to incredibly low levels. Today media converters
for SM fiber are as priced the same as MM. Multimode fiber is
beginning to look obsolete.
several technologies that have continued growing in importance
in the fiber optic marketplace - components that
every tech needs to learn about and become familiar with their
Many Fibers? - What's The Optimal Cable Size?
often arises to reduce the number of fibers in a cable and
therefore reduce the cable cost, assumed to be important on long
cable runs. But is the cost of fiber such a big part of the cost
of the cable plant? We decided to analyze cable costs for
standard loose tube cable capable of being pulled into conduit
for underground or lashed to a messenger for aerial
Gathering data was not easy, but we found several large,
reputable US distributors who listed prices for several types of
loose tube singlemode OSP cables from top cable makers. All
prices are for small quantities (km, not 10s or 100s of
km). Prices are how they were quoted, in $US per foot, so
our readers outside the US should feel free to convert into
another currency and meters.
This graph shows what we found:
looks reasonable above 24 fibers, but unpredictable below that,
so we analyzed the data by cost per fiber per foot and got the
The cost per fiber per foot increases rapidly below 24 fibers,
probably because the cost of making cable doesn't change much
with fewer fibers; it's the cost of the plastics, strength
members and manufacturing process that dominates the cost.
However, after 24 fibers, the cost settles down and slowly
decreases for higher fiber counts, reflecting then the cost of
the added fibers.
Another way to think of this is that below 24 fibers, you are
paying for the cable; above 24 fibers you pay for the fibers.
The thing to note of course is the cost of each fiber is less
than 2 cents per foot for any cable above 24 fibers. When OSP
construction costs are $5-25 or more per foot, the cost of fiber
seems to be quite cheap. Certainly installing cable with
additional fibers is very cost effective if it means having
fibers to expand the network without having to install another
cable. And, of course, that applies to urban and suburban
networks, not just rural.
Microcables, Microducts and Microtrenching
MiniXtend cable is smaller than a pencil
microducts and microtrenching - three technologies that have
more in common than the prefix "micro" are gaining in
acceptance along with blown cable, the obvious method of
installation using them. Smaller is always better when it
comes to crowded ducts, especially in cities where duct
congestion is a problem in practically every city we have
everything else, cables keep getting smaller
well with microducts and microtrenching
need to become familiar with "blown cable" technology
are already accepted in the marketplace
demand for more fiber for smart cities services like small
cells and smart traffic signals, not to mention a ton of other
smart cities services, installing more cables in current ducts
- without digging up streets - is a major interest. Sometimes
it's possible to install microducts in current ducts with a
cable and blow in a new microcable. Sometimes it's worth it to
pull an older cable out and install a new microduct that will
accommodate 6 cables, making room for future expansion. The
makers of the fabric ducts, Maxcell, can even show you how to
remove the ducts in conduit without disturbing the current
cables and pull in fabric ducts to install more cable.
Comparison of MaxCell ducts to rigid plastic duct
Microducts are small ducts for blowing in cable. In the size of
a traditional fiber duct, you can get 6 microducts for 6 288
Microducts And Microtrenching
Nearly invisible microtrenching
have to trench, microtrenching is probably the best choice for
cities and suburbs. Rather than digging wide trenches or using
directional boring (remember the story about the contractor in
Nashville, TN using boring to install fiber who punctured 7
water mains in 6 months?), microtrenching is cheaper, faster
and much less disruptive.
this implies that contractors are willing to invest in new
machinery and training, sometimes an optimistic assumption.
Microtrenching machines and cable blowing machines are
available for rent, but personnel must be trained in the
design of networks using these technologies and operating the
actual machinery in the field. That's still a considerable
and ducts are getting smaller allowing more and more
fibers in the same space
allows "construction without disruption"
Fiber Count Cables
More manufacturers are introducing high fiber count cables -
864, 1728, 3456 or even 6,912 fibers. Like this one from
Prysmian with 1728 fibers: The applications were first in
large-scale data centers but are also seeing use in dense
urban centers to support FTTH and cellular small cell systems.
These cables use bend-insensitive fibers to allow high density
of fibers without worrying about crushing loads affecting
attenuation. Most also use fibers with 200 micron buffer
coatings instead of 250 micron
buffer coatings to allow even higher density. Many, or even
most, use ribbons of fiber, either the conventional hard
ribbons or the newer flexible ribbons, since, as we show
below, the time to splice even a 1728 fiber cable is
extremely long unless ribbon splicing is used.
High Fiber Count Cables may not be for everyone. Maybe only
for a very few. A single cable that has as many fibers as
12-144 fiber cables (1728 fibers) in a cable with a diameter
of only twice that of a conventional 144 fiber cable can
of all, the cost - it's high. You do not want to waste cable
at this price. Engineering the cable length precisely will
save lots of money.And it's worse for higher fiber counts.
making mistakes when preparing the cable for termination can
cable may require special preparation procedures to separate
fibers for termination. Most use new methods of identifying
cables and bundles.
skill, the tech working with high fiber count cables needs
lots of patience.
multiple cables at a joint can get complicated keeping all
cables will generally use 200 micron buffered fiber and
often a flexible ribbon instead of a typical rigid ribbon
structure to reduce fiber sizes. This may complicate
splicing as the methodology to splice the flexible fibers
and splice 200 micron fibers to regular 250 micron fibers is
a work in progress.
200 to 250 micron fibers may be easier with the flexible
ribbon designs which make it easier to spread fibers to the
heard the splicing time for flexible ribbons is about 50-100%
longer than that of conventional rigid ribbons. So
if you use that table below, you may need to increase your
ribbon splicing estimates when working with flexible
We've been looking for directions on how to deal with high fiber
count cables. Several contractors tell us ribbon splicing is the
way to go, and most of these cables now use a version of the new
ribbon types that are flexible. We've put together this
table from some articles on splicing ribbons:
generously sent FOA some samples of 1728 and 3456 "RocketRibbonTM"
cable. We took some photos and must admit that these cables are
fascinating updates on the traditional fiber optic cables.
Here are Corning RocketRibbon 1728 fiber (bottom) and 3456 fiber
(top) cables. To get an idea of these cables size, look at this
fiber cable is 32mm diameter, 1.3 inches. The 1728 fiber cable
is 25mm, 1 inch diameter.
These are cables made from conventional "hard" ribbons, not the
"flexible" ribbons used on some cable designs. As a result of
using hard ribbons, the fibers are arranged in regular patterns
to get high density.
the tubes of ribbons from these cables. Each of those tubes of
ribbons has the equivalent of 24 ribbons of 12 fibers each
(actually 8 X 12 fibers and 8 by 24 fibers stacked up) for 288
fibers total. The 1728 fiber cable has 6 tubes and a center foam
spacer, with 144 ribbons total. The 3456 fiber version has 12
tubes and no spacers, 288 fiber ribbons total.
What amazes us is the density of fibers.
We calculated the "fiber density" of this 3456 fiber cable based
on 200 micron buffered fibers and determined that 54% of the
cable is fiber. Compare that to a typical 144 fiber loose tube
cable, which is about 14% fiber or a 144 fiber microcable which
is about 36% fiber.
Looking at the end of this cable reminded us of nothing so much
as this PR photo from AT&T from their intro of fiber in
Not the fiber, the dense cable of copper pairs!
Of course the cable is much lighter than copper but much heaver
than you are used to with fiber - it weighs 752 kg/km or about
1/2 pound per foot. And it's stiff. Very stiff. The minimum bend
radius is 15 times the cable diameter or 480mm (~19 inches),
about a meter or yard in diameter.
As we noted in the photo above, Ian Gordon Fudge of FIBERDK
taught some data center techs how to handle a 1728 fiber
Sumitomo cable with a slotted core. Ian sent FOA this photo to
illustrate the number of fibers in the cable he was using for
Here is the slotted core that separates the flexible fiber
in the Sumitomo cable:
More on high
fiber count cables and our continuing coverage.
High fiber count cables are all ribbon cables, some with hard
ribbons and some with flexible ribbons, All require ribbon
splicing because of the construction and the time it would take
to terminate them. This is a table of estimated termination
times. Is that realistic? We've heard the flexible ribbons may
longer than conventional ribbons due to the need to
carefully arrange and handle fibers.
Fiber Count Cables - Continued Updates - Installation
our ongoing research on high fiber count cables, last month we
were invited to visit Corning's OSP test and training facility
to experience the processes of installing these cables for
ourselves. We had the opportunity to handle some of these cables
ourselves and see how experienced techs worked with this cable.
Once you get a chance to handle this cable and see how big,
stiff and heavy it really is, you understand that it's quite
different from any fiber optic cable you have worked with, with
the possible exception of some hefty 144/288 fiber loose tube
cable that's armored and double jacketed. With a bend radius of
15X the diameter of the cable, the minimum bend radius of a 1728
fiber cable is 15" (375mm) and that's a 30" (750mm - 3/4 of a
meter) diameter. Just the reel it's shipped on is outsized - it
should have a ~750mm (30 inch) core and will be probably ~1.8m
(6 feet ) in overall diameter. 3300 feet (1km) of this cable
will weigh 550-750kg (1200-1700 pounds.) and the reel will weigh
another ~300-400kg (700-900 pounds). Will that fit on your
loading dock? Can you handle a ton of cable? (Metric or English)
I tried bending one of the 1728 fiber cables and (with the
manufacturer’s OK) tried to break it. The 1728 fiber cable I was
bending took an enormous amount of muscle to bend, and when I
got down to about an 8 inch radius, it broke, with a sound like
a tree limb of similar diameter cracking. In the field, that
would have been an expensive incident.
The stiffness of these cables affects the choice of other
components and hardware. You will not fit service loops into a
typical handhole, you need a large vault like the one shown in
the photos taken at Corning. You will also need close to 100
feet (30m) of cable for a service loop. You may need to figure 8
the cable on an intermediate pull and that will require lots of
space and a crew to lift the cable to flip it over.
This 1728 fiber cable is stiff, does not easily twist and only
bends in one direction because there are stiff strength members
on opposite sides of the cable. Placing it into a manhole or
vault and fitting service loops into it is not easy. In this
case, it helped to have two people and one was the trainer. You
need to have a "feel" for the cable - how it bends and twists -
to make it fit. The limits of bend radius, stiffness and
unidirectional bending makes it necessary to work carefully with
the cable to fit loops into the vault. Sometimes it's necessary
to pull a loop out and try in a different way to get it to fit.
But it can be done as you see at the right.
the cable out of conduit in the vault without damaging it also
requires care. You can see in the back the orange duct coming
into this vault. When pulling the cable, it's important to not
kink the cable while pulling it out of a duct. A length of
stiff duct can be attached to the incoming duct to limit bend
radius. Capstans, sheeves and radius cable sheaves need to be
chosen to fit the required cable bend radius. A a radius cable
sheave with small rollers can damage the cable under tension
and are bot a good choice unless the rollers are used with a
piece of conduit to just set the bend radius.
Corning also showed us a new feature of their RocketRibbon
Cables. A high fiber count cable has a lot of fibers, even a
lot of ribbons, so identifying ribbons can be a problem. In
addition to printing data on each ribbon, Corning now tints
the ribbons with color codes to simplify identification. Great
Fit: 6912 Fiber Cable Pulled in 1.25 inch Conduit
Electric Co., Ltd. (FEC) conducted an experiment in its Mie,
Japan facility to demonstrate the installation of a 6912-fiber
optic cable with an outer diameter of 1.14 inches (29 mm) in a
696 foot (200m) long conduit with three 90 degree curves and an
inner diameter of 32mm. The conduit used was a standard product
installed in conventional data center campuses. Engineers
confirmed a maximum pulling tension of 84 pounds (372N), well
below the maximum pulling tension of 600 pounds (2700N)
specified for the cable.
The cable was installed in a 1.25 inch (32mm) conduit with a
maximum length of 1,411 feet (430m) in a North American data
center campus in 2020 to support live traffic. The high fill
ratio in this application is not typically recommended for
Outside Plant (OSP) cable installation. However, in this
application, the end-user was willing to accept the installation
risk in return for maximum fiber density. The installation
demonstrated that FEC’s 6912 fiber optic cable can be
successfully installed into 1.25 inch (32mm) conduit using
appropriate tools, work procedures, and optimum installation
“The FEC 6912 fiber optic cable at least doubled the fiber count
possible in a 1.25 inch conduit, compared to competing available
designs,” said Ichiro Kobayashi, General Manager of optical
fiber & cable engineering department, FEC.
PR also on OFS
Website. OFS is a FEC company.
fiber count cables allow extremely high fiber counts in
small cable sizes, perfect for dense applications in data
centers and metro areas
so many fibers, ribbon splicing is the only sensible way
to splice them
you splicing machines can handle 200micron buffer fibers
bend radius limits are so high, they require special
consideration for installation and storage - BIG manholes
You are all
familiar with the information printed on a typical fiber optic
cable which includes the manufacturer, how many fibers in the
cable and distance markings, plus sometimes other information
like the manufacturer's part number.
But recently two people made reference to the small symbol that
looks like an old-style telephone handset. One thought the
manufacturer of the cable used that symbol to show where the
helical winding of the buffer tubes reversed, a reference point
for preparing the cable for midspan access. Another thought it
was to indicate this was a telecom cable not a power cable.
FOA has been reaching out to people at cable companies to see if
anyone has a definitive answer as to what this symbol means, and
the answer comes from Rodney Casteel and his engineers at
"The handset symbol is mandatory for cables “suitable for direct
burial applications” per ICEA 640 and Telcordia GR-20. I
think this handset symbol started a long time ago so data cables
could be identified if they were dug up. My guess is the first
standard to mandate this was Telcordia GR-20 Issue 1 back in the
And Bellcore/Telcordia GR-20:
seeing some interesting new connectors being introduced. 3M
announced a multifiber array connector using expanded beam
technology and several new ideas of making a duplex connector
Expanded Beam Connector
sketchy but from the video on the 3M website, the connection is
made by a small plastic fixture that is shown by the arrow in
the top photo. The plastic seems to turn the beam 90 degrees so
the connection is made when two pieces overlap., in the
direction of the arrow in the lower photo. The connectors are
hermaphroditic - that is two identical connectors can mate.
There are models for singlemode and multimode fibers and you can
stack the connection modules to handle up to 144 fibers. We
understand this was not part of the 3M fiber optic product line
recently acquired by Corning. 3M
Expanded Beam Connector.
For more information on expanded beam connectors, see the FOA
Newsletter for October 2018 that discusses the R&M
QXB, another multifiber expanded beam connector announced last
CS and SN
In the FOA Newsletter for January 2018, we featured
the SENKO CS connector, a miniature duplex connector using two
1.25mm ferrules, but much smaller than a duplex LC. The CS is
sell on its way to becoming standardized with a FOCIS (fiber
optic connector intermateabliity standard), but on the SENKO web
page, there is another new connector, the SN, that makes the SC
look huge! The big difference is the vertical format that allows
stacking connectors very close. That can allow transceivers to
have more channels, a big plus for data centers. Here
is more information on the SENKO CS and SN connectors.
Comparison of SENKO CS (L) and SN (R) connectors with duplex LC.
US Conec MXC
The R&M and 3M expanded beam multifiber connectors
reminded us that US Conec introduced the
connector over 5 years ago, using similar technology for up to
64 fibers per connector. The MXC is on the US Conec website, but
seems to be aimed at board level connections, not far off its
original purpose as a connector for silicon photonic circuits.
But when we checked the US Conec website, there was a connector
name we dis not recognize, the MDC. The MDC (below) is a
vertical format duplex connector using 1.25mm ferrules that
looks similar to the SENKO SN above. Here
is information on the US Conec MDC duplex connector.
Its All About The Data Center
Just like the high fiber count cables discussed above, the CS,
SN and MDC connectors are aimed at high density cabling and
transceivers for data centers. All three are specified for the
pluggable transceiver multi-source agreement.
everything else, connectors keep getting smaller
early to determine if they will be accepted in the
marketplace and can compete with LCs
with SC SOC in EasySplicer
has been seeing greater acceptance of the SOC - splice-on
connector - using fusion splicers. It's popularity started in
data centers for singlemode fiber where the number of
connections is very large so the cost of a fusion splicer is
readily amortized and the speed of making connections is the
real cost advantage. The performance of SOCs is much better
(mechanical splice) connectors simply because of the
superiority of a fusion splice and the cost of the SOCs are
much less since they do not have the complex mechanical splice
in the connector.
used SOCs in training and the techs take to them readily. In
classes you can combine splicing and termination in one session.
The cost of fusion splicers has been dropping to near the cost
of a prepolished/splice (mechanical splice) connector kit so the
financial decision to use SOCs is easier to make.
Connectors (SOCs) are easy to install, low loss and low
hardware than pigtail splicing
Connector Manufacturers and Tradenames
FOA Master Instructor Eric Pearson of Pearson
Technologies shared a list he has researched of
prepolished splice connectors with mechanical splices and SOC -
splice-on connectors for fusion splicing. This list shows how
widepread the availability of these connectors has become,
especially the SOCs and low cost fusion splicers.
1. Corning Unicam® (50, 62.5, SM)
1. FIS Cheetah (???)
2. Panduit OptiCam® (50, 62.5, SM)
3. Commscope Quik II (50, 62.5, SM)
4. Cleerline SSF™ (50, SM)
5. LeGrand/Ortronics Infinium® (50, 62.5, SM)
6. 3M/Corning CrimpLok (50, 62.5, SM)
7. Leviton FastCam© (50, 62.5, SM)
2. Inno (50, 62.5, SM)
3. Corning FuseLite® (50, SM)
4. FORC (50, 62.5, SM)
5. Siemon OptiFuse ™ (SM, MM)
6. Belden OptiMax?? FiberExpress (SM, MM)
7. AFL FuseConnect® (SM, MM)
8. OFS optics EZ!Fuse ™ (50, 62.5, SM)
9. Sumitomo Lynx2 Custom Fit® (50, 62.5, SM)
10. Commscope Quik-Fuse (50, SM)
11. Ilsintech Pro, Swift® (50, 62.5, SM)
12. LeGrand/Ortronics Infinium® (50, 62.5, SM)
13. Greenlee (50, 62.5, SM)
14. Hubbell Pro (50, SM)
15. Easysplicer (SM)
Note: There are additional manufacturers from the Peoples
Republic of China, which advertise on Amazon and eBay.
Access - Simplifying Installation Of Drops
installations involve dropping a small fiber count cable from a
large backbone cable. Backbone cables of 144-288 fibers are
common and larger ones are becoming more common too. Drop cables
are often only 2-14 fibers, meaning most fibers are continuing
straight through the drop point. Midspan access involves opening
the cable by removing the jacket and strength members, opening
the buffer tube and splicing only the fibers being dropped at
that point. The untouched buffer tubes from the opened cable are
carefully rolled up and stored in the same splice closure as the
fibers that will be separated and spliced to a drop cable.
If there is a method of splicing only the 4 drop fibers instead
of the 144 fibers, we will only have 4 splices instead of 144 or
146 depending on the architecture of our system. The difference
is according to how the drop is configured.
If you are building a star network where every drop links back
to the origin of the network, you will splice 4 fibers in the
cable to the drop cable, leaving 4 splices on 4 fibers (instead
of 144 splices if the backbone cable is cut and respliced.
If you are building a ring network, you may only be splicing two
fibers going to the drop and two others that are continuing
along the ring network.
All this may seem obvious but in actual practice requires some
knowledge, skills and careful workmanship. To do a proper job.
Fortunately, manufacturers of cables and tools have good
information available online on how to do it, and FOA Master
Instructor Joe Botha has provided FOA with a application
note on how midspan access is done in his classes also.
The basic process is simple. We will look at a loose tube cable
but processes exist for ribbon cables also. You remove the
jacket of the cable for a specified length according to the
cable type and splice closures used. After removing the cable
jacket, you remove unnecessary strength members, leaving enough
of the stiff central member on both ends to attach to the splice
closure. Identify the tube with the fibers to be spliced to the
drop cable and set aside while carefully coiling the other tubes
for storage in the closure.
To open the buffer tube, you need a midspan access tube that
shaves off a section of the tube to allow removal of the fibers
without damaging them. Here two types of Miller tools that shave
shaving the tube and removing the fibers - count carefully to
ensure you remove all the fibers! - you can cut the tube off to
have bare fibers only for the length you need to splice on the
drop cable. All these fibers will be placed in a splice tray for
safe storage but only the fibers being dropped will be cut and
spliced to the drop cable. This is what the closure will look
like, ready for splicing the drop cable.
In the case
of the particular user who contacted us, not every drop would
use midspan access. His cable plant was 15miles (25km) long with
roughly 17 locations where cable drops were needed. The cable he
was using could only be made in 5km lengths, so there would have
to be several locations where the cable would be spliced in the
The design would need to carefully determine how much cable was
needed along each section of the route, including lengths for
service loops and midspan access or splicing, to determine which
drop points would be using midspan access ans which would be
used as splice points for the entire cable.
That's why fiber optic network design is important but sometimes
Search online for "midspan access" to find lots of application
notes and videos on the subject. Or talk to your fiber optic
Guide Page on Midspan Access
Failure In Louisville, KY
FIber tried a new way to install cable in Louisville, KY,
that turned out to be a very expensive failure.
Nanotrenching is what some call very shallow trenching for
installing fiber optic cable - see the photo below - and
filling with rubber cement. It did not work.
Otts, WDRB Louisville,
Feb 7, 2019
LOUISVILLE, Ky. (WDRB) – Google Fiber is leaving Louisville
only about a year after it began offering its superfast
Internet service to a few neighborhoods, citing problems
with the method it used to build the network through shallow
trenches in city streets.
The shut off will happen April 15, said Google Fiber, a unit
of Silicon Valley tech giant Alphabet, in a blog post
Google Fiber has served about a dozen cities, and Louisville
is the first it has abandoned."
Shortly after Google announced Louisville as a possible
location in 2015, the Metro Council passed a utility pole
ordinance at Google’s behest, then spent $382,328 on outside
lawyers to defend the ordinance in lawsuits from AT&T
and the cable company now called Spectrum.
Mayor Greg Fischer said in early 2016 that Louisville’s
landing Google Fiber was “huge signal to the world.”
Louisville’s public works department allowed Google Fiber to
try a new approach to running fiber – cutting shallow
trenches into the pavement of city streets to bury cables.
It led to a lot of problems, including sealant that popped
out of the trenches and snaked over the roadways.
Louisville street, Copyright
2019 WDRB Media. Reproduced with permission.
It feels like you are using us for a
science-fair experiment,” Greg Winn, an architect who lives
on Boulevard Napolean, told Google Fiber representatives
during a Belknap Neighborhood Association meeting last year.
“…Our streets look awful.”
Google Fiber would go on to fill in the trenches with
asphalt, what company executives said was like filling a
60-mile long pothole.
Google Fiber never ended up using the utility pole law -- a
policy called One Touch Make Ready -- that Louisville passed
at its behest, as the company only buried its wires instead
of attaching them to poles.
A public relations representative for Google Fiber said no
one was available for an interview.
In written responses, the spokesman said Google Fiber
initially chose not to use the utility pole access because
of "uncertainty" about whether the ordinance would hold up.
Now that it has cut trenches in the streets, the company has
no desire to start over.
Even using (One Touch Make Ready), we’d need to start from
scratch, and that’s just not feasible as a business
decision," the spokesman said.
Be sure to watch the video from WDRB.
Copyright 2019 WDRB Media. Reproduced with permission.
you try some new idea, ask some experienced installers
what they think
Q: What you
recommend when it comes to manjole/handhole sizing. If
they are being used for splicing, do you have a general formula
of length of splice closure plus X factor more for cables in/out
of closure and slack storage?
FOA has been doing some research on underground construction to
expand our section in the FOA
Guide. We are looking at what people are specifying on
some projects since we do not know of any industry standards.
There are links of some interesting/useful information below.
From our standpoint, the minimum size would be determined by the
bend radius of the fiber optic cable (see
article above), how much slack (service loops) would be
stored (slack from how many cables - see photo below) size of
the splice closures, and how many ducts and cables would be
served. Generally you will have 20-30 feet in service loops to
allow for splicing, Typical cable up to 1/2” needs a loop
>20” but I don’t know how you would ever get a loop that
small for that much cable, so you probably have minimum 2’
loops, at least 5 coils. Add a closure and you probably need a
2’X4’ handhole, at least 2’ deep, as a minimum - see the “good”
We were at Corning training last Spring on high fiber count
cables and those cables require ~6’ X 4” min manholes just to
fit the loops of cable. Handholes can be smaller, depending on
the type of splice or drop, midspan access, etc.
Guide pages on OSP Construction created by Joe Botha for
his course in South Africa talks about manholes and handholes on
page near then end.
web page shows the number of different designs and sizes.
Detail from Central
FL Expressway Design Standards offers several sizes: 4' X
4' X 4', 4' X 6.5' X 6.5', 4' X 6.5' X 6.5' and specifies
a duct organizer.
Here is a Wisconsin
DOT spec for a 4’ diameter manhole.
Broadband General Network Specifications: see page
need to be big enough for the cables they must contain
recently had a message with this thought:
"It is time for spring cleaning, and we don't mean just at
home. Contaminated fiber end faces remain the number one cause
of fiber related problems and test failures. With more
stringent loss budgets, higher data speeds and new multifiber
connectors, proactively inspecting and cleaning will help you
ensure network uptime, performance, and reliability. Despite
"everyone" knowing this, fiber contamination and cleaning
generates a lot of failed test results."
Well, experience tells us that "proactively inspecting and
cleaning" can generate a lot of damage to operating fiber
Inspection and cleaning should be done whenever a fiber optic
connection is opened or made, of course. But the act of opening
the connection exposes it to airborne dirt and the possibility
of damage if the tech is not experienced in proper cleaning.
Fiber optic connections are well sealed and if they are clean
when connected, they will not get dirty sitting there. Fiber
optic connections do not accumulate unseen dirt like under your
bed or sofa, requiring periodic cleaning, as implied in this
Clean 'em, inspect 'em to ensure proper cleaning, connect 'em
and LEAVE THEM ALONE!!!
And, duh, remember to put dust caps on connectors AND
receptacles on patch panels when no connections are made
this perhaps another early April Fools' joke...like this one
we ran several years ago about the wrong way to clean
Clean Connectors Before You Make Connections
Teague of Microcare/Sticklers
send us this series of photos showing what happens when you make
connections with dirty connectors. It speak for itself!
Backfill A Trench For Underground Construction
answer to a question we've gotten. Where did we find the answer?
In the new FOA
Guide section on OSP Construction developed using Joe
Botha's OSP Construction Guide which is published by the FOA.
Joe's book covers underground and aerial installation from a
construction point of view, covering material after the FOA's
design material and before you get into the FOA's information on
splicing, termination and testing.
DO NOT FORGET THE MARKER TAPE! It makes the cable easy to locate
and hopefully prevent a dig-up.
The 2019 update of the FOA
Reference Guide To Outside Plant Fiber Optics contains
this and lots of other new material on OSP construction.
Adopts One Touch Make Ready (OTMR) Rules For Utility Poles
3, The US Federal Communications Communications Commission
adopted a new rule that allows "one-touch make-ready" (OTMR) for
the attachment of new aerial cables to utility poles. From the FCC
explanation of the rule, "the new attacher (sic) may opt
to perform all work to prepare a pole for a new attachment. OTMR
should accelerate broadband deployment and reduce costs by
allowing the party with the strongest incentive to prepare the
pole to efficiently perform the work itself."
You may remember that FOA has reported on the "Pole
Wars" for several years. Battles over making poles
available and/or ready for additional cable installation has
been slowing broadband installations for years and now threatens
upgrading cellular service to small cells and 5G in many areas.
A Good Idea?
the potential to speed deployment of new communications networks
if handled properly. However, one hopes the installers doing
OTMR know what they are doing. We've heard so many horror
stories about botched installations, cut fiber and power cables,
punctured water mains and gas lines done by inept contractors
that we just hope this doesn't cause even more trouble.
For example, here are 2 poles in the LA area where small cells
are being installed. Can just any contractor handle OTMR on
may be problematic if contractors doing installation are
Sources For Multimode Fiber Testing
our schools recently asked up for recommendations on test
sources for multimode fiber, wondering if the sources should
be a LED or laser. Multimode test sources are always LEDs and
these sources should be always used with a mode conditioner,
usually a mandrel wrap. See here.
This is how all standards for testing multimode fiber require
ago, as systems got faster and LEDs were too slow at speeds
above a few hundred Mb/s. Fortunately 850nm VCSELs were
invented to provide the solution for faster transmitters. But
VCSELs were not good for test sources. They had variable mode
fill and modal noise, so testers continued using LEDs for test
sources, but with mode conditioners like the mandrel wrap that
filtered out higher order modes to simulate the mode fill of
an ideal VCSEL
The bigger issue with MM fiber is whether to test at both 850
and 1300nm. In the past, we did both because there were
systems that used 1300nm LEDs or Fabry-Perot lasers for
sources because the fiber attenuation was lower at 1300nm than
850nm. As network speeds increased to 1Gb/s and above,
bandwidth became the limiting factor for distance, not
attenuation. VCSELs only worked at 850nm and all systems
in MM basically have been switched to 850nm VCSELs.
We also used to test at both wavelengths because if a fiber
was stressed, the bending losses were higher at 1300nm, so you
could determine if a fiber had problems with stress. But since
MM fiber has all gone to bend-insensitive fiber, that no
longer works and the need or reason to test at 1300nm went
away. It has not been purged from all standards yet however.
To complicate things, standards say that you should not use
bend-insensitive fiber for test cables (launch or receiver
reference cables) because they modify modal distribution, but
it’s a moot point - practically all MM fiber is
bend-insensitive so you have no choice but to use it. And most
links will have BI to BI connections that should be tested.
But we checked with some technical contacts and there are no
specifications for BI fiber mandrels as mode conditioners.
solution - 850 LED with a mode conditioner on non-BI fiber (if
you can find it - see above).
fiber needs testing with a 850nm LED source
Budgets and Loss Budgets
was this topic a long discussion with our new instructors but
it's a common question asked of the FOA - we received two
inquiries on loss budgets in the last month alone. The confusion
starts with the difference between a power budget and a loss
budget, so we'll start there. and we'll include the points where
we were stopped to explain things.
What's The Difference Between Power Budget And Loss Budget?
power budget is the amount of loss the link electronics can
tolerate - transmitter to receiver. You use this to compare
to the cable plant link loss budget when designing a cable
plant to ensure the link will work on the cable plant
link loss budget is the estimated loss of the fiber optic
cable plant including the loss of the fiber, splices and
connections. You compare that to the power budget to ensure
the link will work on the cable plant being designed, then
again after installation to compare to test results.
Consider this diagram:
At the top of the diagram above is a fiber optic link with a
transmitter connected to a cable plant with a patchcord. The
cable plant has 1 intermediate connection and 1 splice plus, of
course, "connectors" on each end which become "connections" when
the transmitter and receiver patchcords are connected. At the
receiver end, a patchcord connects the cable plant to the
Definition: Connection: A connector is the hardware attached
to the end of a fiber which allows it to be connected to
another fiber or a transmitter or receiver. When two
connectors are mated to join two fibers, usually requiring a
mating adapter, it is called a connection. Connections have
losses - connectors do not.
Below the drawing of the fiber optic link is a graph of the
power in the link over the length of the link. The
vertical scale (Y) is optical power at the distance from the
transmitter shown in the horizontal (X) scale. As optical signal
from the transmitter travels down the fiber, the fiber
attenuation and losses in connections and splice reduces the
power as shown in the green graph of the power.
Comment: That looks like an OTDR trace. Of course. The
OTDR sends a test pulse down the fiber and backscatter allows
the OTDR to convert that into a snapshot of what happens to a
pulse going down the fiber. The power in the test pulse is
diminished by the attenuation of the fiber and the loss in
connectors and splices. In our drawing, we don't see reflectance
peaks but that additional loss is included in the loss of the
Power Budget: On the left side of the graph, we show the
power coupled from the transmitter into its patchcord, measured
at point #1 and the attenuated signal at the end of the
patchcord connected to the receiver shown at point #2. We also
show the receiver sensitivity, the minimum power required for
the transmitter and receiver to send error-free data. The
difference between the transmitter output and the receiver
sensitivity is the Power Budget. Expressed in dB, the
power budget is the amount of loss the link can tolerate and
still work properly - to
send error-free data.
Link Loss: The difference between the transmitter
output (point #1) and the receiver power at its input (point
#2) is the actual loss of the cable plant experienced by the
fiber optic data link.
Comment: That sounds like what was called "insertion
loss" with a test source and power meter. Exactly! Replace
"transmitter" with test source, "receiver" with power meter
and "patchcords" with reference test cables and you have the
diagram for insertion loss testing which we do on every
loss of the cable plant is what we estimate when we
calculate a "Link Loss Budget" for the cable plant, adding
up losses due to fiber attenuation, splice losses and
connector losses. And sometimes we add splitters or other
Margin: The margin of a link is the difference between
the Power Budget and the Loss of the cable plant.
Determining The Power Budget For A Link
Question: How is the power budget determined? Well, you
test the link under operating conditions and insert loss while
watching the data transmission quality. The test setup is like
Connect the transmitter and receiver with patchcords to a
variable attenuator. Increase attenuation until you see the link
has a high bit-error rate (BER for digital links) or poor
signal-to-noise ratio (SNR for analog links). By measuring the
output of the transmitter patchcord (point #1) and the output of
the receiver patchcord (point #2), you can determine the maximum
loss of the link and the maximum power the receiver can tolerate.
test you can generate a graph that looks like this:
A receiver must have enough power to have a low BER (or high
SNR, the inverse of BER) but not so much it overloads and signal
distortion affects transmission. We show it as a function of
receiver power here but knowing transmitter output, this curve
can be translated to loss - you need low enough loss in the
cable plant to have good transmission but with low loss the
receiver may overload, so you add an attenuator at the receiver
to get the loss up to an acceptable level.
You must realize that not all transmitters have the same power
output nor do receivers have the same sensitivity, so you test
several (often many) to get an idea of the variability of the
devices. Depending on the point of view of the manufacturer, you
generally error on the conservative side so that your likelihood
of providing a customer with a pair of devices that do not work
is low. It's easier that way.
On The Job
the most important part of any job. Installers need to
understand the safety issues to be safe. An excellent guide to
analyzing job hazards is from OSHA, the US Occupational Safety
and Health Administration. Here
is a link to their guide for job hazard analysis.