The Fiber Optic Association
- Tech Topics
The FOA Online Reference Guide has a complete section on cabling, fiber and copper, plus wireless.
- Frequently Asked Questions
- We get questions all the time,
so we have started putting those we feel have general interest
on this web page. As it grows larger, we'll index it for convenience.
Here's more testing FAQs too.
- Looking for more information?
Try the FOA Online Reference Guide or the Tech
- The Questions We Get Most
- Can you explain to me in
simple terms what the difference in fiber optics and traditional
- We get this question so often,
and it is such a complex issue, we
created a whole web page to answer it!
- And do signals really travel
faster in fiber optics?
- You know that "sending
communications at the speed of light" means the speed of
light in glass (about 2/3 C), but you might be surprised to know
that signals in UTP (unshielded twisted pair) cables like Cat
5e travel at about the same speed (2/3 C). Coax, meanwhile, has
a faster NVP (nominal velocity of propogation), about 0.9C, due
to it's design. Fiber's "speed" is not referring to
the speed of the signal in the fiber, but the bandwidth potential
of the fiber.
- Can you give me a definition
of structured cabling?
- "Structured Cabling"
refers to a standardized cabling architecture, specified by EIA/TIA
568 in the US and ISO 11801 internationally. It uses twisted
pair and fiber optic cables to create a standardized cabling
system designed for telephones and LANs built by many manufacturers.
The nomenclature here is even less precise. Vendors also refer
to this as "structured cabling", data-voice cabling,
low-voltage cabling and limited-energy cabling.
- What is a better way of communication,wire
or fiber optics?
- The telcos and CATV companies
use fiber optics because of economics. It's greater bandwidth
and lower attenuation allow longer distances and more channels
(voice or video) per fiber pair. Typical fiber specs are more
than 100 times farther and 1000+ times faster! Try this
for some more reading.
- I'm a tech coordinator at
a k-12 school district. Depending on what you read and who you
talk too and which way the wind is blowing, the decision on whether
to wire CAT-5 or fiber is a toss-up? In your opinion, which is
more cost-effective for a building that is trying to set up a
solid switched backbone that will be usable for 5-10 years?
- I've headed a school tech committee
myself and work with local schools (had kids in school locally
- both now in college) and they're asking the same question.
Many are looking very seriously at fiber.
- Many schools are now wired or
wiring with a fiber optic backbone and Cat 5 to the desktop.
That will support Fast Ethernet, which will probably be good
for 5 years. It requires local hubs (which require conditioned
power and UPSes, and the total cost is probably more than an
all fiber network today (or equal).
WIth fiber, you get more potential bandwidth, but you also get
rid of the hubs (and all their additional needs, like power,
space, management, etc,) You can buy 100Base-SX NIC cards for
jsut over $100 and inexpensive hubs in fiber from several companies.
including Gemflex (www.gemflex.com ) which uses the new low cost
3M Volition connectors which save $$ on the cable plant. And
you can upgrade at least to GB Ethernet.
- With copper, I'm afraid you
buy into a cable of the year club. The industry has already written
off Cat 5, has been selling Cat 5E (extended) which has finally
become a standard, now is pushing Cat 6A (a year or more away
from being standard) and talking about Cat 7!
- On General Fiber Optics:
What are some of the uses of
fiber optic cabling in the business world? The biggest use is telephony, followed by CATV,
then LAN backbones, connecting hubs. Next is connecting remote
video cameras for security systems. The building management and
security systems are switching to fiber in many buildings due
to distance and EMI requirements. Fiber is not often used to the
desk because it is perceived to be too expensive, but it allows
a system without wiring closets, making the cost less in most
instances. Gigabit Ethernet will drive even more fiber into networks,
since UTP applications will be too difficult to install.
- Will "intelligent buildings"
use fiber optics or copper wiring to carry voice/data/video throughout
- Both. Fiber will be used when
the distances are longer than 90 meters or data rates are higher
(e.g. Gigabit Ethernet). Most backbones will be fiber. Desktop
connections to telecom closets will be copper for the near future,
until network managers find out what a telecom closet really
costs! Phones will continue to use copper until we all go to
voice over IP. Video (CCTV) uses fiber for distances over about
- Can you please tell me what
the defference between, dB and dBm when you are trying to test
fiber optic cable.
- Fiber optic power measurements
are generally made in a log scale of "decibells" or
"dB" (actually named after Alexander Graham Bell) that
has a scale of 10 dB for every factor of 10 in power. The equation
- dB=10 log (power 1/power
- dB is therefore a ratio measurement
- 10 times more power is +10 dB and 100 times less is -20 dB,
- For ABSOLUTE measurements, you
must have a reference point. If we use 1 milliwatt of power as
our reference, our equation becomes
- dB= 10 log (power/1
- So now 1 mW is 0 dB, 10 mW is
10 dB, 0.1 mW is -10 dB, etc.
more information on testing including dB too.
- I need to be able to measure
the "true" or "useful" power of a VCSEL Laser
Diode. To do this, I believe I need to measure the Peak-to-Peak
power (the extinction ratio?).
- All FO power meters measure
average power. This is simply peak power diluted by duty cycle.
If you know the duty cycle of the signal and the average power,
you can calculate peak power as (Avg pwr/duty cycle). If you
are measuring a signal with a clock of 50% duty cycle (1-0-1-0,
etc), the meter will read half the peak power. Most high speed
networks are sending random data, so the duty cycle can probably
be assumed to be 50%. Many systems have a test mode that transmits
50% duty cycle just for optical testing.
- I am confused by the resolution
choices in fiber optic power meters. If accuracy is +/-0.2dB,
then it doesn't seem worthwhile to have resolution out to the
hundredths & thousandths of a dB, when accuracy is only in
the tenths (one decimal place).
- Resolution for power meters
is an interesting subject. If you measure power, and the measurement
is accurate as (or as NIST prefers - has a measurement uncertainty
of ) 0.2 dB, a measurement of 0.00 dB +/- 0.2 dB is confusing.
The 1/100 th dB resolution is in fact meaningless. If the uncertainty
was 0.02 dB, a hundreth resolution would make sense. Now remember
we are talking "absolute power" measurements, calibrated
relative to NIST standards.
If we are looking at loss measurements, things changed considerably.
The loss of a LC connector, about 0.1 dB, is measured relatively,
eg. -15.00 dBm to -15.10, and the measurement uncertainty now
has nothing to do with the absolute power levels, but the RELATIVE
difference between the two readings. That difference is as precise
as the linearity of the power meter ( better than 0.01 dB) and
the uncertainty of the mating of the connectors ( a few hundreths
too?). So you certainly want a meter with 0.01 dB resolution
to test connectors!
If you are testing an installed cable plant with say 3-10 dB
loss, the uncertainty is probably 0.5 dB, so 0.1 dB is adequate.
If it's a long haul network with 30 dB loss, the uncertainty
can be over 1 dB, so 0.1 dB is much more than adequate.
- How do you classify fiber
- Broad question:
- By NEC - UL - flame retardancy
By cable types (tight buffer/distribution/breakout/loose tube)
By fiber types (multimode/singlemode/hybrid)
Whether it has fiber and wire (composite cable)
- See the cable section of our Online Reference Guide or Lennie
Lightwave for more information.
- What type of fiber is required
to run at gigabit speed?
- Depends on how far you want
to go. Plain old FDDI fiber (160 MHz-km bandwidth @ 850 nm and
500 MHz-km @ 1300 nm ) will go ~240 m with a 850 VCSEL or 500
m with a 1300 laser. Practically every fiber manufacturer has
50/125 laser-optimized premium fiber (OM2/OM3/OM4) that will go a lot further
-as far as 2 km - and while it's more expensive, we recommend
it for any backbone applications.
I am just starting to plan
for a fiber installation for our LAN. I am confused as to what
cable to buy. Is it determined by our network hubs? First, every fiber link uses two fibers,
each transmitting in opposite directions, so you need at least
two fibers. Most of these hubs in premises applications use multimode
fiber of the standard 62.5/125 micron (core/cladding) dimensions.
Most users specify "FDDI Grade" fiber, although there
is some new higher bandwidth fiber available that
is better if you expect to use it for Gigabit Ethernet someday.
At a minimum, you should use a two fiber "zip cord"
with a jacket UL rated for flame retardancy appropriate to the
installation. If it is in air handling areas above ceilings, it
should be plenum rated, otherwise Riser or General ratings will
be OK. Most installs put in larger fiber count cables since the
cost of installing is higher than the cost of cable. Extra fibers
in a cable are like memory in computers - cheap and put in all
you can justify! THis is especially important if you are installing
between locations that have lots of equipment that may use the
fiber in the future. For larger fiber count cables, use breakout
or distribution cable types - see Lennie
Lightwave for descriptions of the cable types. Connectors
should be chosen to match the connectors on the equipment (usually
ST, LC or SC.) A zip cord could be installed with connectors on
each end ready to go into the equipment. Mutlifiber cables would
be terminated in rack mounted patch panels or wall mounted boxes
and connected with patchcords.
- I have a client who wants
to connect his home computer to his office computer with a direct
link. The distance between buildings is approx. 500 yards. I
have told him he will need to use fiber cable between the buildings
due to the long distance.
- Buy a couple of FO media converters
and some fiber optic cable (needs to be outside plant cable
which is waterproof.) The cable should be multimode (62.5/125
or 50/125 micron size). Install the cable and terminate it. You
can get wall plugs for fiber optics from many sources including
many local distributors and computer stores. Attach the fiber
cable to media converters. Connect one computer to a media converter
with a regular Cat 5 cable and the other to a media converter
with a Cat 5 CROSSOVER cable. Plug and play!
- I am running a secure fiber
optic cable through a conduit is there special tools to pull
fiber optic cable vs. copper cable, if so where can i find these
- Fiber pulling gear is similar
to all cable pulling gear, with one big caveat: ALL fiber optic
cables must be pulled by the strength members, which are usually
kevlar fibers. You should not simply put a kellums grip on the
jacket and pull - that will ruin most cables.
- What is the difference between
indoor and outside cables? Generally,
outside cables are designed to resist water penetration by using
a gell fill or dry water-blocking compounds and a polyethelene
jacket. The new dry cables are getting very popular, since they
can be made as distribution types which are easier to terminate.
Many also have a PE jacket over a UL-rated PVC jacket so you
can bring the cable into the building, strip off the outside
jacket and run it anywhere in the building (not the 50 feet limit
of PE.) Indoor or premises cable must be rated for flame retardance
for safety and to meet code.
- When is it viable to string
fiber cable overhead?
There are two solutions: self-supporting aerial cable or regular
cable lashed to a messenger (maybe even the old telephone wire!)
Most cablers can help you with a suggestion of the proper cable
- How soon will it be, until
we are able to communicate via the telephone/internet/television
by means of fiber optics? And how many fibre optics would be
required for a small town?
- The answer to this question
is complicated. The Internet is all fiber optics today, as is
most of the phone and CATV systems. It's only the final connections
to the home that is still copper and activity in that area is
very high in 2005. The telphone companies have been pushing DSL,
but it is a flawed concept - bandwidth is heavily dependent on
the length of the lines, so generally it's not much better than
a telephone modem. CATV companies are happy with coax cable,
as it has
gigibit capability. Both complain fiber to the home is too expensive,
but the alternatives are not many, forcing the issue from a competitive
- As to how much fiber is needed,
that depends on the system used. Two fibers to the home are probably
adequate (one transmits, the other receives.) Backbone cables
are usually 72-288 fibers, since it is more economical to install
large fiber count cables now and leave them dark. Several techniques
exist to multiplex signals on the fibers, including frequency-division,
time-division and wavelength-division multiplexing, so one pair
of backbone fibers can serve thousands of connections. No one
- I need to connect an IBM
3174 controller (coax or balun to RJ45) to single mode fiber.
Then connect the other end of the fiber to a printer (coax again).
Local printers work easy with coax to balun to RJ45...then CAT
5 copper to balun to coax.
- Whatever you need to convert
to fiber, you can get a media converter somewhere.
- We have a small network with
a single server driving about 45 desktop pcs. 8 of the pcs are
at the far end of the shop from the server. (approx 300 ft) The
shop is filled with cnc equipment generating lots of "noise"
I was thinking to come from one of the 10/100 hubs (3com office
connect) at the server, through a cat5-fiber converter along
a fiber line to a fiber-cat5 converter, into a 8 port, 10/100
hub and cat5 down to the pcs'. The pcs' don't need much bandwidth,
just the cable length and interference problem. What do you two
- Good idea. A pair of media
onverters is only about $2-300 and the cable would be inexpensive.
- We currently have a central
computer room with several distribution closets (11) over 3 floors.
We have fibre backbone to some of the rooms (18 multimode fibres)
but some of the rooms do not have any fibre. We have a quote
which includes running both multi-mode and single-mode fibre
to these rooms. What benefit or purpose is there to the single
mode, I thought this was only for excessive distances and probably
not necessary "in-doors?" Are hubs and switches now
available that use single-mode rather than multi-mode?
- Most electronics are available
for both multimode and singlemode fiber, but today singlemode
is usually for long distances. However, singlemode fiber is so
inexpensive, that adding some singlemode fiber to every cable
you install can make sense and someday you may need its bandwidth.
(Remember 10 GB Ethernet is coming!)
I have been recommending installing hybrid (MM+SM) cables for
ten years as a hedge on future applications. Don't terminate
it now, but have it available for future networks that may require
it. Instead of a 18 fiber MM cable, price a 24 fiber cable with
18 MM and 6 SM. You will find the cost differential quite small
and if it saves installing another cable in the future, it will
have enormous payback!
What is modal bandwidth, and
how does it effect what distances gigabit ethernet can travel
over fiber? Modal bandwidth
is caused by the fact that light in multimode fiber travels in
rays or "modes" that take different times to get to
through the fiber, causing dispersion. The longer the fiber, the
greater the effect. This is a major factor in the distance limitation
of GBE and the incentive for fiber manufacturers to develop better
While the worst case distance for 62.5/125 FDDI-spec fiber using
a 850 nm VCSEL source is only 220 m, laser-optimized 50/125 fiber
capable of 1 km is now available.
Here is a page of specs on most current and legacy fiber optic systems.
- How can a coherent laser
single mode source be correctly coupled to a long length of multimode
fibre and multimode receiver ? This
is a difficult issue to address. Coherent lasers and multimode
fiber have always had problems - that's why the telcos switched
to SM fiber. The problem is the SM cable launches into the center
of the MM fiber and you get differential modal dispersion (DMD)
problems. The center of the MM fiber is somewhat unpredictable.
Plus every connector and splice changes the modal distribution
adding to the uncertainty. For GBE, companies are offering offset
launch cables that couple the SM fiber with an offset of about
15 microns. That seems to help. Laser-optimized 50/125 fiber
should be a good solution. If you signal is analog, I don't think
you will find a good solution, due to the distortion induced.
If it's digital, you will have more luck, but it's still hard
- I am distributing Satellite
signal via single mode fiber. Can I patch into my multi-mode
fiber data distribution system somehow using an adapter of some
- The connection from singlemode
to multimode fiber is OK - the big multimode fiber catches all
the light, but the other way is a problem - MM to SM gives 16-20
dB loss. Connectors (esp APC) must be matched. You can get adapters
called "media converters" that will convert from singlemode
to multimode and vice versa, within certain bandwidth limitations.
- Do you see any real serious
problems in splicing together fibre cables from different manufacturers,
as long as the cable is manufactured to the same specifications? No, not as long as they are the same
type and size, eg multimode 62.5/125 or 50/125 and singlemode
should be "normal" (non-dispersion shifted) or dispersion
shifted. Some singlemode fibers are made for 1300 nm only, 1550
nm only or both, and they should not be mixed. There are some
other singlemode fibers that have special coatings that cannot
be mixed with others. For singlemode, ask your fiber vendors,
splicer supplier or try it first before going into the field!
- What is the difference in
connectorization in tight buffered and loose tube type of cables?
- A tight buffer cable can be
terminated directly. THe 900 micron coating on the fiber is rugged
enough to allow the connnector to be connected directly and if
there is a 3 mm jacket, it is crimped to the connector for strength.
A loose tube cable has 250 micron buffer on the fiber in it and
is too fragile to attach a connector directly. It has be be used
with a breakout kit that sleeves the fiber in a protective tube
- What is the distance Limitation
on 100Mbps Fiber for Single Mode and Multi Mode. Also what are
the sizes of fiber for each distance limitation.
- Multimode 100 Mb/s (FDDI or
Fast Ethernet) is limited to 2 km (1.2 miles) by the dispersion
of the fiber and the chromatic dispersion limits of the LED in
the fiber. This holds for both 62.5/125 micron fiber and 50/125
which exists in some older installations (and is being used for
gigabit networks now, perhaps in error.) Singlemode links are
avaialble for over 20 km (12 miles) or more. All singlemode fiber
for 1310 nm is the same, with a core diameter of about 8-9 microns
and an cladding diameter of 125 microns.
- Can I splice 62.5/125 fiber
to 50/125 fiber? If so what type of nominal loss would I be looking
at at for my loss budget?
- If you splice it, you will get
directional losses. Transmitting from 50 to 62.5 fiber, you'll
get virtually no losses but from 62.5 to 50, you will get a minimum
of 1.6-1.9 dB loss due to the size and NA mismatch. (50 micron
fiber has a lower numerical aperture (NA) than 62.5.) See The Fiber Optic Technicians
Manual, Chapter 17 for a table of interconnection losses
with different size fibers.
- We have some basic R&D data
on this topic, posted in this article connecting
50/125 to 62.5/125 fiber.
- Can you tell me what the
unrepeatered length limitations are for 62.5/125 multimode fiber
10BaseFX : 12.5 dB loss
@ 850 nm, could equal 4 km
100BaseFX: 2 km, bandwidth, not loss limited, depending
on LED source
GBit Ethernet: 220 m @ 850 nm (bandwidth limited, and
this distance is likely to get longer) or 500 m at 1300 nm with
a singlemode pigtailled laser.
- These are on FDDI grade 62.5/125
multimode fiber, which has a bandwidth of 160 MH-km at 850 nm
and 500 MHz-km at 1300 nm. 62.5/125 fiber with 500 MHz-km bw
at 850 nm and 50/125 laser-optimized fibers are available, specifically
designed for premises applications of GBE.
Here is more information on installing fiber optic cable.
- I am wondering what the proper
pulling procedure is for premises type fiber cable. I have been
told to use the kevlar to pull with, and I have also been told
that pulling the cable by the kevlar can be damaging to the fibers.
What do you recommend?
- The cable has been designed
to be pulled by the Kevlar and ONLY by the
Kevlar. One of our demos in our Fiber U classes is to have students
a zipcord hard by the jacket and see how it destroys the cable.
Unless the cable has been specifically designed to pull by the
(which is usually a double jacket design with Kevlar between
layers,) you must strip back the jacket, cut off the fibers,
Kevlar, tie it off and use it to pull the cable.
- Kevlar, by the way, is a
duPont trade name for aramid fibers.
Here is more information on terminating fiber optic cable.
- Does this still hold true
with six fiber or 12 fiber premise style cable?
- Absolutely. Remove the jacket,
cut off the fibers and central strength member and tie a swivel
onto the kevlar. You'll need about 6 inches of kevlar, tie with
a double knot on the swivel then tape the kevlar back along the
cable, esp. covering the end of the jacket, to prevent the kevlar
from pulling loose or the cable snagging while pulling.
- I am interested in learning
how to terminate fiber optic cable.
- Take the free "Virtual
hands-on" tutorial on Lennie
Lightwave for hands-on details.
I have been informed
by a company that they now have a small form fibre optic connector
(LC) which has reduced loss to cater for the demands of Gigabit
Ethernet along with other benefits. The connector has not been
ratified by any International Standards body yet. Also, they
inform me that the connector has been included into the IBM active
equipment product range as well as their own and another hub
suppliers range of products (not one of the big players).
As there may be numerous cross connections to interlink hubs,
this may exceed the Db loss allowed for Gigabit Ethernet. The
LC connector with a lower loss would allow for a greater number
of cross connects.
What is your view on using proprietary fibre optic connectors.
- There is little risk in using
the LC connector. Of all the SFF (small form factor connectors)
it is the one that has become the most popular - in fact, it
is the de facto standard connector for gigabit and 10 gigabit
networks. Indeed the design is very well thought out. The smaller
ferrule is easy to polish well and has excellent mating performance
- which leads to low loss and back reflection. It is also easy
to terminante and test.
- 1. Is there any independent
testing procedures on "Rodent Proof" optical cables
and what sort of qualification/s being used to justify?
- There is a test referenced in
GR-20 issue 2. I have heard that there are facilities which still
perform the test, but I am not familiar with any of them. I know
of no-one who is trying to justify the test.
- 2. What measurements are
manufacturers using when they claimed that their products are
of "rodent resistance", "rodent proof" and
- The test referenced above did
not use any of these terms, but just gave a damage rating. There
is anecdotal information that some cable / duct types have less
rodent damage in the field than other cable / duct types.
- 3. How are these three terms
differentiate in terms of classification with respect to the
- There is no organization, of
which I am aware, which gives any formal definition to the terms.
- Rodent resistance" and
"rodent protection" are currently used by some organizations,
for cable designs which they feel have experienced less damage
in the field. "Rodent proof" , to the best of my knowledge,
has only been used by a company which makes ductwork .
- Can you give me a definition
of Fiber optics?
- What we call "fiber optics"
is communications by modulated light guided through a transparent
optical fiber. As a realtively young technology, the nomenclature
can be quite varied among users. In the UK, it's fibre optics,
sometimes its fiberoptics or fibreoptics (as one word). Within
the business, we generally say "fiber" when we refer
to the optical fiber itself, although some use it to mean a cable
of optical fiber. Lennie Lightwave has a fiber
optic glossary on the web.
- What type of components are
others using to facilitate fiber reel acceptance testing? I am
looking for an inexpensive, fast and reliable method to use with
an OTDR or power meter.
- The normal way to test fiber
on the reel is:
1. Physical examination of the reel and content. Any sign of
physical damage means you should test very carefully to insure
the cable or fiber has not been damaged. You can also read the
distance numbers off the cable to determine the length of cable
on the reel.
- 2. Continuity test using a visible
light source. MM can use a flashlight fiber tracer or a 650 nm
(red) LED source. Cleave the fiber and use a bare fiber adapter
or unterminated connector on the end of the fiber. SM fiber
should use a visible laser fault locator .
3. If you really need to do a loss test (usually a contractual
issue, not a technical issue, as the cable was tested thoroughly
at the manufacturer before shipment) you can do a cutback test
with a meter and source or a OTDR test.
3a. Source and meter: Use a launch cable with the source and
a bare fiber adapter on the fiber off the reel to connect to
the fiber. Measure the power at the far end of the cable again
with a bare fiber adapter, then come back to the source end and
cut off the fiber on the cable about 2-3 feet from the launch
cable connection. Measure the power there - without touching
the connection of the fiber under test to the launch cable !
The difference in the power measurements is the loss of the cable.
Divide by the length to get the attenuation in dB/km.
3b. OTDR: Use a long launch cable on the OTDR and a bare fiber
adapter on the fiber to test. Using a little index matching fluid
will reduce the reflection and loss of the connection. Read the
signature for attenuation and any localized stress losses.
- Other references: The
Fiber Optic Technicians Manual
has a chapter (17) on
testing and Lennie Lightwave has a section on testing
and a full
article on OTDRs.
- Please tell me how an optical
power meter differs from an OTDR.
- A fiber optic power meter measures
the amount of power coming out of a fiber, just like a voltmeter
measures voltage. See FOTP- 95, the standard test procedure for
power measurement . When used with a test source, it can be used
to measure the end-to-end loss of a fiber optic cable or installed
cable plant. FOTP-171, OFSTP-14 and OFSTP-7 cover this test .
OTDRs work like "optical radar" to find faults in cables,
measure length or test loss of splices in cables. They are used
for troubleshooting or for documentation of outside plant cables
that have splices (splicing is not often used in premises cabling.) Here is a more detailed explanation of all the options in cable testing.
See this page
for more info on OTDRs.
- What should the length of
the launch cable be from the source to the cable being tested?
Also I need a receive cable from the meter end to the connector.
should it be the same length?
- Any cable 1-2 meters length
Here is more information on Reflectacne and optical return loss.
- What do I need for connecting
Optic Fibre Cable to a Cat 5 Cable?
- You need a device called a "media
converter" avialable from a number of companies for $100-200.
- Is it possible to send a
forward and reverse signal along the same fiber?
WDM is only for sending two frequencies on the fiber in the same
- Nope, the direction is unimportant.
You can send upstream on one wavelength and downstream on the
other. It's been done for years using 1300 and 1550 nm on singlemode
or 850 and 1300 nm on multimode. For example you could use a
1550 transmitter with a fiber amp to broadcast out to numerous
locations then use 1300 coming back upstream with 1300/1550 WDMs.
- In fiber what is a "mode"
actually. "Multi" mode seems to imply they are discrete
separate paths. I see no way discontinuities occur in glass.
Is it a frequency thing?
Actually multimode fiber is made up of many discrete layers of
glass to create the graded index profile. There may be 150-2000
layers in the core. Modes are the equivalent of "standing
waves" in a fiber - paths for modes are reinforced by electromagnetic
fields. You can actually see the effects of modes by transmitting
a coherent source (laser) down a multimode fiber. You see a "speckle
pattern" which is the result of interference between discrete
modes. A good explanation is in Jeff Hecht's book Understanding
- Will a single mode connector
work on multi-mode cable?
The answer is maybe you can use SM connectors on MM but NOT the
reverse. SM connectors are made to tighter tolerances - as is
SM fiber - so the ferrule hole may be too small for some MM fibers. MM connectors have bigger holes for the fiber and will
have high loss (>1dB) with SM. Also MM connectors may not
be PC (physical contact) polish - terrible for return loss. MM
fiber may not fit the smaller hole in SM connectors.
- Could you plaese explain
to me what optical return loss is?
It's simply a reflection at a connector or splice caused by imperfect
mating of the fibers. The reflected light may cause a laser transmitter
to have problems with linearity or create background noise that
affects transmission. It does not affect LED systems.
SM cabling in a premises environment can have big ORL problems,
as the short cables allow multiple reflections that cause optical
"background noise." A good singlemode ORL is 30-40
dB for PC connectors, 40-50 for "Super PC" and >50
for APC connectors.
Multimode Gigabit systems are now concerned with ORL (they use
VCSELS) and specify something better than -20 to -30 dB depending
on the system.
- If you have a 50 micronfiber
backbone, can you use 62.5 fiber jumpers on each end?
On the receiver end it is OK, but on the transmitter end, the
core of 62.5 into smaller 50 micron fiber will have losses of
If you have 62.5 fiber backbone, can you use 50 micron patch
Same as above, except the excess loss is at the receiver end.
- In both cases, the losses depend
on the modal distribution in the fiber,
a result of the source output and the number of connections.
Information on mismatched fibers is here.
- If accuracy of a power meter
is +/-0.2dB, then it doesn't seem worthwhile to have resolution
out to the hundredths & thousandths of a dB, when accuracy
is only in the tenths (one decimal place).
- Resolution for power meters
is an interesting subject. Consider what happened when the first
handheld 8 digit calculators became available in the early 70s.
People would divide a two digit number by a two digit number
and report the results to 8 digits! Of course, the precision
of the answer was still two digits, not this "calculator
If you measure power, and the measurement is accurate as(or as
NIST prefers - has a measurement uncertainty of ) 0.2 dB, a measurement
of 0.00 dB +/- 0.2 dB is confusing. The 1/100 th dB resolution
is in fact meaningless. If the uncertainty was 0.02 dB, a hundreth
resolution would make sense.
Now remember we are talking "absolute power" measurements,
relative to NIST standards. If we are looking at loss measurements,
things changed considerably. The loss of a LC connector, about
0.1 dB, is measured relatively, eg. -15.00 dBm to -15.10, and
the measurement uncertainty now has nothing to do with the absolute
power levels, but the RELATIVE difference between the two readings.
That difference is as precise as the linearity of the power meter
( better than 0.01 dB) and the uncertainty of the mating of the
connectors ( a few hundreths too?). So you certainly want a meter
with 0.01 dB resolution to test connectors!
If you are testing an installed cable plant with say 3-10 dB
loss, the uncertainty is probably 0.5 dB, so 0.1 dB is adequate.
If it's a long haul network with 30 dB loss, the uncertainty
can be over 1 dB, so 0.1 dB is much more than adequate.
- Why do some power meters
have calibration at 1300 nm while others are 1310 nm?
- Convention. The "official"
laser center wavelength is 1310 nm, but vary between 1290 and
1330 nm. LEDs are broad spectral output devices that have outputs
over a broad range of wavelengths, roughly centered around 1300
nm. We prefer to just say 1300 nm, and so does NIST, who calibrate
at this wavelength with a 1300 nm YAG laser.
- I am a Safety officer involved
in the construction business for a major Manufacturuer. Our own
Construction forces are now installing and connecting fiber optic
cable. I am having difficulting finding any information on safety
procedures for this activity. Any information you have regarding
safety and disposal of "waste ends" would be appreciated.
- We have added a page
on safety to the FOA website and all our books and reference guide covers safety.
- Where do I find the best
information on fiber optics for lighting?
- We're not into fiber optic lighting but we have a tutorial on lighting on our website.
(C) 2002-11, The Fiber Optic Association,
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