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Updated 11/21

Topics (Click on them to jump to that section)

Technology  

Applications   

Installation

Fiber/Cable   

Termination/Splicing   

Test & Measurement   

Education/Learning  

Where Are The Jobs In Fiber Optics?  

Technology  

Manufacturing Guide?
Q: Is there a guide published by FOA that provides insight as to the process of fiber optic manufacturing? It's my understanding that the guide stresses quality and controls to ensure performance and reduce product loss?
A: We do have a guide for manufacturers. It is mostly aimed at communications systems and components manufacture. Here is a link to download it.

Reflectance
Q: What is the importance of reflectance and all the other numbers in installing and trouble shooting a fiber circuit?
A: Reflectance has always been a secondary issue to connection loss but has some important issues that need consideration. There are two basic issues with reflectance, affecting with the output of laser transmitters and creating background “noise” in a fiber link.
Reflectance can interact with the laser chip itself, causing laser transmitters nonlinearities or random fluctuations in the output. The background noise is a secondary issue, but can be seen in ghosts in an OTDR trace. The light bouncing back and forth in the fiber that causes ghosts will be added to the signal at the receiver end, adding noise to the actual signal. Both these effects are more significant on shorter links, for example FTTH or LANs using PONs (passive optical networks). We always recommend using APC (angled physical contact) connectors on short SM links. And most short SM networks do use APC connectors.
FOA tries to stick to the definition that reflectance is the light reflected from a connection but some others call it “return loss.” Return loss has been defined generally as the combination of reflectance and backscatter from the fiber, and that’s how OTDRs measure return loss. Standards vary in the definition sometimes.
Here is a FOA Guide page on reflectance that gives the basics and explains how it is tested. https://foa.org/tech/ref/testing/test/reflectance.html

Electromagnetic Interference
Q: Is there and electromagnetic interference with optic cables?
A: The fiber is glass and the cable is plastic, neither of which are affected by electromagnetic interference. There is a cable used in electrical transmission lines called OPGW- optical power ground wire - that has fiber inside a wire conducting high voltage - doesn’t bother the fiber at all.

How Light travels In An Optical Fiber
Q: Is there a generalised ratio between the length of an optic fibre and the length of the path actually taken by a light pulse inside that fibre? If yes, do OTDRs factor in such differences in any way? or they such sown the length of the actual path of the light pulses?
A: Each optical fiber has an effective independent of refraction. The index of refraction is the ratio of the speed of light to the speed of light in the material: n=c/v where n=index of refraction, c=speed of light in a vacuum and v=speed of light in the fiber.
For an optical fiber, the manufacturer measures the index of refraction which is usually in the range of 1.47. Corning SMF-28 singlemdoe fiber for example is specified at 1.4670 @ 1310 nm and 1.4677 @ 1550 nm.
So if you use the equation above, the speed of light in SMF-28 fiber for a 1310nm pulse is c/n or 300,000 km/s divided by 1.4670 = 204,500 km/s.
When an OTDR measures length, it actually measures the time its test pulse takes to go to the end of the fiber and return, so the distance is 2X the actual fiber length. The distance is speed x time.
If a fiber is 1 km long and the speed is 204,500 km/s, the  time forlight to travel the 1km is 1/204500 = 0.00000489 seconds or about 5 milliseconds.
OTDR will measure that fiber as 10 ms becasue its pulse has to go both ways, and it would calculate the length as i km, using that effective index of refraction of 1.4670.
Back to your original question, the index of refraction is the generalized number based on how light travels in the fiber.

Applications  

Managing And Maintaining a Fiber Optic Cable Plant During Its Lifetime.
Q: Are there guides / recommendations for optic fibre cable life cycle management? (outside plant) including rehabilitation / replacement timelines together with factors that may alter those timelines ( such as seismic activity, extreme weather, human activity-induced fibre cuts etc) also including typical performance deterioration over the life cycle, and the performance levels at which replacement / rehabilitation happens. Or does it happen (and is it normally expected) that operators replace entire sections of fibre (say 400 km) as part of routine maintenance?

A: There is a saying here in the US that in fiber optics “the most common cause of failure is “backhoe fade” in underground cables and  “target practice” for aerial cables.” In other words, damage caused by humans. We know of many fiber optic cable plants that have survived natural disasters like earthquakes - in fact there is a lot of work today using regular cables used in communications to monitor for seismic activity. Fire can be a problem in remote areas, but often it’s because the poles are burned causing the cables to fall.

Over the years we have questioned cable manufacturers about the lifetime of fiber optic cable. They don’t like to make definitive statements but we have been told that based on the cables installed in the past that 40 years is a probable lifetime for most cables. There are certainly cables in use today that are over 30 years old already. The glass fiber is not a problem, it’s the protection from the cables that will eventually fail. Installation techniques can have an effect on the longevity. For example splice closures should be sealed properly to prevent ingress of moisture or dirt. Cables should not be installed with bends below the rated bend radius or with excess tension.

FOA has always told users that fiber optic cables do not need maintenance (https://foa.org/tech/ref/user/maintain.html), a response to some people advocating periodic inspection and cleaning of connections, for example. That’s just more likely to cause damage.

When an accidental break in a cable occurs, we have guidelines for restoration (https://foa.org/tech/ref/restoration/rest.html), and planning for restoration when building the cable plant is very important.

Someday you will certainly want to replace cables, often well before the lifetime of the cable, but generally because you need more fiber or the older fiber will not support the network speeds you want for upgrades. Planning for more fiber by installing more cables can be eased by installing spare underground ducts when first installing cables - here in the US, we call this “Dig Once” (https://foa.org/tech/ref/OSP_Construction/Underground_Construction.html). Testing fibers for higher speeds is called "fiber Characterization” (https://foa.org/tech/ref/testing/test/CD_PMD.html) and is routinely done when speeds above 10G or certainly 100G are considered for older fibers.

Knowing that the lifetime of fiber optic cable plants are ~40 years, it makes sense to plan ahead for future applications, installing lots of fibers, leaving lots of open duct space and choosing network architectures that will not obstruct upgrades. See the article on Netly's network above.

Transmitting Multiple Data Types
Q: How do you integrate fiber optic digital communications with other sensing and control systems and platforms?
A: Fiber optic networks generally have lots of bandwidth and sensors and control systems generally do not require much bandwidth. The mixing of data streams is generally done by multiplexing the data using electronics on each end, but one can also do it with wavelength division multiplexing.
 

Single Fiber DWDM
Q: Can you do bidirectional links on a single fiber with DWDM? (Dense Wavelength Division Multiplexing)?
A: A company Called Edge Optical Solutions sells multiplexers for bi-directional DWDM on one fiber  by using adjacent wavelength channels for each direction. It is good to ~400km with coherent transceivers but cannot use fiber amplifiers for repeaters.

Identifying Users On A PON Network
Q: How or what testing tool or technique can I use to verify whether there is a live customer w/ONT working on any fiber i may select @ a splice enclosure prior to getting further down the cable and  to the MST service terminal. All our fibers have light on them leaving the CO so when we go into a splice enclosure to pick a fiber to connect a drop to, to service a home, they are usually all lit up in that enclosure.
A: The simple answer for a tool or technique that can tell you if a customer is connected on an output of a PON splitter is “documentation.” If you know where each fiber is connected going downstream. Then the IT person who programs users into the system can tell you if that fiber is connected to a customer. There is a possibility that there is a test solution. Have you ever heard of a “fiber identifier”? It’s a gadget that can tell if there is signal in a fiber and some can identify the direction it comes from. What I don’t know if the unit can somehow indicate bi-directional traffic. Nobody we contacted seems to know either.

• Building to building can more easily be done with indoor/outdoor cable to get past the 50’ code limit for OSP cable.
• Many campuses have ducts but they are often crowded. Microducts or fabric ducts are often the solution even if you have to pull out an older cable to pull in new microducts into one old duct.
• Microducts and blowing microcables are gaining lots of traction for their practicality.
• Microducts and microtrenching can be what we call “construction without disruption.”
• We are seeing more and more directional boring - works fine as long as you know where other buried utilities are! Many contractors need to learn more about underground location.
• People are finally getting the idea about singlemode fiber - now it’s probably cheaper than multimode.
• Passive optical LANs can save money. Biggest advantage is the upgrade from GPON to 10GPON is seamless - you can even run both simultaneously, e.g. for a student system and a faculty/research system.
• High fiber count cables are tempting, but require special handling and lots of manhole/handhole space.
• We’re working with a Corning “Pioneer” (retired engineer) on trying to educate installers about bend radius violations, esp when pulling large fiber count cables.
• Most installations shortchange manhole/handhole space.
• Aerial is sometimes used. Lashing to a messenger is probably best. We worked with a school in Canada last year trying to use short lengths of ADSS and it was not cost effective nor was their much applications support for short ADSS links.
• Line of sight wireless (RF or optical) works across highways and may be cheaper than construction for fiber.

Minimum Link Length
Q: I have a question regarding minimum fiber optic distances for horizontal runs. Is there a minimum distance for a horizontal fiber optic run? Any information regarding this would be greatly appreciated. Thank you for you help!
A: The answer to your question is for the most part no, there is no minimum distance for a fiber optic link. For example, fiber is used in offices, data centers, etc. sometimes connecting equipment on a single rack. And there are many fiber optic links used on platforms - aircraft, helicopters, ships, etc. - and in command posts.
Most of the Ethernet standards are based on a 2m minimum, but also most are defined by a maximum length. For multimode systems, the max length is mainly a bandwidth issue, so shorter links are no problem.
For singlemode links, the bandwidth is not an issue, it’s the power budget, limited by the transmitter power and receiver sensitivity, translated into the loss of the cable plant. But for receivers, often they have not only a minimum input power limited by their baseline noise but also a maximum power they can have before saturating and causing high bit error rates. See “Power Budget” on this page in the FOA Guide. So if a singlemode link is short, the receiver can be overloaded so an attenuator is used at the receiver.
There is a secondary problem with singlemode systems, reflectance. Reflections from connections can cause problems with both transmitters and receivers, a topic covered in the link given above. The reflectance problem can be solved with APC connectors.
The FOA Guide has many pages on links, networks, reflectance, testing, etc. that you may find helpful.

What Is A Ring Network?
Q: If according to the TIA or ISO structured cabling standards the fiber optic campus backbone must be star-hierarchical type, how should a fiber optic "ring" be built? to always ensure connectivity on a LAN?
A: A “ring” network consists of a series of links connecting equipment (nodes) in series until the last one connects back to the first. Since the links are communicate in both directions, the network can still operate if any one cabling link or equipment fails. Today, survivability is usually ensured by using a “mesh” network; the architecture of data centers, the Internet or phones. In addition to having a series connection of nodes, there are other interconnections that provide for multiple alternative paths. See Networks in the FOA Guide.

Crossed Connections
Q: If a FO connector is crossed connected i.e Rx connected Rx and Tx to Tx at both end, will it works?
I know in theory it will not due to light circuits arrangement, but is there SFP in the market can tolerate that? 
A: We do not know how a SFP could sense and change polarity unless it had an optical switch inside the module. A transmitter is a laser or LED and a receiver has a photodetector. Unless one could have the devices change function, changing polarity would be impossible.

FTTH GPON
Q: Can you guide me some websites or pages where I can learn more about GPON Technology please?
A: FOA Guide has a big section on FTTH and OLANs using GPON technology. Follow those links, Also FOA has a new book, The FOA FTTH Handbook you can order from Amazon.

Cheating On Link Length
Q: I have a fiber run for a camera starts at location A to location B it is 467 feet.  Location B jumpers through to location C which is 2060 ft at location C. Transceivers areSFPs ONLY GOOD UP TO 1800 ft, but this company only has a multimode system. Is there something i can do to make this work?
A: It might work as is, since electronics are usually quoted with conservative specs and will work farther than specified most of the time. If you have several SFPs, test the output power to see if it exceeds specs and choose the 2 ones with highest power. If that still doesn’t work, contact SFP manufacturers for higher power units.

Errors In A Data Link
Q: What is the significance of bit error, and what is the acceptable rate for communications and submersible vehicles?
A: On any data link, there is an acceptable amount of error that can be tolerated. If it’s a digital voice link, a BER 10E-6 (1 error in 1million bits) is acceptable without affecting voice quality. If it’s a link to your bank, the typical standard is a million times higher (10E-12). Link protocols usually have ways to determine BER, like attaching a checksum to the end of a data packet and having it checked at the receiving end. If a error is suspected, the packet will be discarded and retransmitted. Here is a tutorial on BER and an explanation of errors in a a fiber optic link.

GPON
Q What is normal Range for good power in an FTTH fiber?
A: The GPON specification for downstream power from the OLT is OLT transmitter power should be 0 to +6dBm and link attenuation in the range of 13 to 28dB, which says receiver power the ONT must be a maximum of 13 dB less than +6dBm or -7dBm and a minimum of 28 dB less than 0dBm or -28dBm, so -7 to -28dBm at the receiver.
Upstream, the similar calculation is ONT transmitter -4 to +2dBm  and the receive power at  theOLT is -11 to -32dBm.
See http://thefoa.org/tech/ref/appln/FTTH-PON.html for the full specifications for GPON.

Reflective Events Causing Transmission Problems
Q: I have a technical question about reflective events. I recently assisted to troubleshoot an intermittent SM fiber link for a customer. The cable was dug up a few years ago and a fiber contractor has (fusion spiced) a different chunk of cable into the link to repair it. When troubleshooting the link, I checked the cable with the otdr. I found that each of the 12 fibers had a reflective event at the fusion splice. This was only at the splice tube closest to me. The other fusion splices in the other tube were virtually invisible (as they should be). I'm a little puzzled as to why there are reflections at the fusion splices. I did a little research, but couldn't come up with a good answer as to what is a possible cause of the reflections. (The OTDR also showed a lot of ghosting on every fiber tested) (in some cases it recorded over 40 ghost events) Although I haven't been able to confirm that there is high Bit error rate due to the transceiver not providing these statistics, (except for 3 out of 10 pings fail) I am suspecting that High reflectance is possibly the cause of their unreliable fiber link.
A: Reflectance is a big problem in SM links, especially short links. If you are seeing lots of ghosts, I suspect the link is very short. Fusion splices can have reflectance if the splicer is improperly set and the fusion is incomplete or has bubbles. Those splices should have not only have reflectance but higher loss. The solution is to open up the closure, use a VFL to find the reflective events and redo the splices.

POTS over Fiber
Q: I would like to know if there is information on your website that explains "POTS OVER FIBER"?
A: POTS - the acronym for “plain old telephone service” - is digitized to transmit over fiber. In the early days (late 70s and 80s) it was simply T-carrier with a fiber converter. By the end of hte 80s it was ATM and SONET. More recently, it’s all going to carrier Ethernet since 99%+ of the traffic is data not voice or PONs (passive optical networks) for fiber to the home.

Can I Build A GPON Network With "Taps"?
Q: Can I build a GPON network where I do a drop to one subscriber then continue to the next subscriber for another drop and so on?
A: There have been examples of this type of “tap” drop proposed, for example in rural areas for drops to  widespread subscribers on a longer network than is typical for FTTH. It’s just a version of a cascaded splitter network. with taps that just do a 2 way split. The taps used are typically 90/10 taps, where 10% of the power is tapped off for the drop.
There are some important issues to consider - Since you are dropping 10% of the power at each tap, you are limited by how many drops you can have.   If you calculate the loss budget - after the first tap, you have 90% power left less the excess loss of the splitter (~0.3-0.5dB). The tap power is down ~10.3 dB and the through power is down ~0.6 dB. At the next tap, you  use the same formula plus you add the loss of the fiber to that tap and so on until you reach the GPON limit.  It’s a pretty complicated process to design, but you can see that with these power losses you will not get a large number of drops in a GPON network with 28dB max power budget. We did a rough calculation and 20-24 drops may be possible depending on the fiber lengths.
This network will probably be much more expensive and more distance limited than simply running a cable with many fibers and dropping fibers from that cable with midspan entry. Couplers are expensive, fiber is cheap. We also do not know the issues with the large differences in transmission times between the first connections and the last ones, which depends on the length of fiber in the systems. That may require some programming at the OLT.

Do We Need Repeaters For 30 Mile Link?
Q: I need to design a 30 mile (~50km) link. Will regeneration like a fiber amplifier be necessary?
A: It depends on the comms equipment but I doubt you need regeneration. 30 miles is 50km, only 10dB of loss for the fiber at 1550nm, maybe 10 splices at <0.1dB adds only 1dB loss and another dB for connectors on each end. I think you probably can find equipment that runs on 12dB loss budget. That said, most new high speed systems (>10G) have 20km versions then go to expensive long haul coherent systems. So talk to the communications equipment manufacturers and see what they say. If you do need a EDFA, they are not that expensive but the site is expensive and requires power (+ backup). See if it’s possible to put the EDFA in the end facilities to get enough power for the whole run.

Fiber Ports Or Media Converters?
Q: Should I Buy A Switch With Fiber Ports Or Use Media Converters?
A: I’m assuming you are thinking of using a switch with copper Ethernet ports and a media converter instead of a switch with fiber ports. The downside is that it adds complexity and increases the chance of failure. My analogy is something my primary flight instructor told me many years ago - multiengine planes are not safer because having two engines doubles your chance of having an engine failure. IBM still says that most network problems are cabling problems. Using media converters adds more electronics, more power supples and more cabling connections.

Passive OLANs in Hotels And Resorts
Q: Are passive OLANs a good choice for hotels or resorts?
A: Passive Optical LANs are enterprise networks based on fiber to the home (FTTH) technology not Ethernet over structured cabling. The FTTH network is usually using GPON standard equipment over one singlemode fiber with passive optical splitters that provides basic Level 1 and 2 network functionality. This is not Ethernet but carries Ethernet over the GPON protocols at 2.5G downstream and 1.25G upstream.
Passive OLANs offer several advantages over conventional Ethernet switches and structured cabling, including much less cost  (~50% capital expense and ~20% operating expense), much lower space requirements (see the link to the library photos below and note the two racks of equipment that support 4000 drops), longer distance requirements (to 20km), easy expansion (these are systems designed for hundreds of thousands of users) and easy management (when you have hundreds of thousands of users, that’s important.)
For hotels, convention centers and similar facilities, the ease of upgrading to a passive OLAN is a big advantage - one fiber goes from the computer room to a splitter where it can serve 32 switches of 4 ports each. That’s right, one fiber can support 128 users! It can support anything that a network can - wireless access points, security cameras, secure entry systems, VoIP phones or POTS phones - anything that will run over a conventional network.

Duplex Communications Over One Fiber
Q: Is true duplex over a single fiber possible, or is more like a shared time-domain technique in a quasi-duplex mode? I would guess that true duplex would lead to interference problems.
A. Bidirectional links are widely used - that’s how FTTH PONs work. They use splitters to combine/split the signals and one wavelength downstream and another upstream. See Fiber Optic Datalinks and for FTTH FTTH Architectures.

Installation

Fiber Optic Color Codes Reference Chart
Q: Has anyone made a fiber optic pocket reference chart that has cable color orders, frequencies, or other commonly used info on it?
A: The FOA has a page on its Online Guide that covers color codes (https://foa.org/tech/ColCodes.htm). It is the most popular page in the FOA Guide! It works great with a smartphone.

Where In The US Do Contractors Need Licenses For Fiber Optics?

Digging Safely (Read the FOA Tech Topic)

• There is a toll-free "call before you dig" number in the USA: 811
• See www.call811.com for more information
• The Common Ground Alliance has an excellent "Best Practices Guide" online
  
• Fiber Optic Asset Protection Summit by the "811" group.
  
• The US Department of Transportation has a website called "National Pipeline Mapping System" that allows one to search for buried pipelines.  
  

Markers Required For Underground Fiber Optic Cables?
Q: Are signs required for underground cables like fiber optic cables? Are they required to have signage so people don’t dig them up or damage them?
A: In the US the answer is NO. There is no Federal or State law which requires marking anything other than hazardous liquids and gases. It is purely a business decision or a moral decision to invest in signs/markers to protect buried fiber. If a fiber gets cut it can disrupt 911 service and all kinds of vital communication related to hospitals, air traffic control, etc.

Distances Between Manholes
Q: What is the standard or max distance between manholes and handholes for fiber optic cable?
A: There are no hard rules, but the distances are determined by a number of factors. In populated areas, the manholes or handholes would be situated where you need drops line in front of a building or a splitter pint for FTTH or conversion from underground to aerial or underwater cables. From a viewpoint of how far you can go, it’s determined by: 1. The length of cable on the reel (typically ~5km max, maybe further for smaller cables, shorter for higher fiber count cables.  2. The type of the duct, cable and method of installation for underground. That includes the type of duct, lubricant used, the number of corners passed, pulling equipment (pulled or blown)  and the tupe of cable - most limited to 600 pound tension. Cable manufacturers and American Polywater (lubricants) are good sources of information here. 3. Aerial cable can have quite long spans, esp. using the moving reel method, which can be limited by the length on the spool.

Gloves for Fiber Techs
Q: I was wondering if as part of the safety rules, in addition to glasses, if it is recommended to use gloves.
If that the case, would you recommend a specific type of gloves.
A: FOA emphasizes the need for safety glasses because of the problem with fiber scraps flying around, especially when students in class are learning to strip fibers. Proper safety glasses have side shields that provide more protection than regular eyeglasses. For eyeglass wearers, prescription safety glasses are available at very reasonable costs that are much more comfortable to wear than wearing safety glasses over the user’s prescription eyeglasses.
We only recommend gloves when working with cables that have sharp metallic armor in them or some heavy outside plant cable. The metallic armor can cause serious cuts if one slips when splitting or removing it. The gloves to use are the kevlar gloves used to prevent cuts (they are also used for chefs working with sharp knives.)
Once the cable is opened and you are dealing with buffer tubes or bare fibers, gloves like the ones used for cables can make the work difficult because gloved hands are clumsy. Tight surgical rubber gloves might work for some, but still make working with bare fiber difficult and provide limited protection.  There we recommend bare hands and being very cautious. 

Height Of Aerial Fiber Optic Cables
Q: Is there a code standard for how high from the ground a for a fiber optic cable running through a residential yard? if yes, please provide the standard or point me to the standard.
A: If we go by NEC 2020, the height is 8 feet,above roofs. with this qualifier. No driveways  just over grass. Art /section 770.44 B. Also 800.44 A 4 states 12 inches between electric service and Fiber optic cable. But service has to be 12 feet at house so I would say 11 feet above grass. If driveway is there, Residential 15 feet for service, electrical, so fiber at 14 feet.

Seal End Of Cable
Q: For aerial OSP cable, are there any problems with leaving the end of the cable open or should it always be put into a closure of some kind?
A: The open end of the cable allows moisture to get into the cable and can be a problem.
I see several scenarios here. If the cable is installed and waiting for splicing, it could be a matter of time. If the work is to be done soon - a week or two - leaving it open is OK, but if the time is longer or you prefer being careful, just seal the end of the cable by wrapping it with plastic electrical tape. The end will be opened up for splicing;  about 2m of cable needs to be stripped to splice it, so a few days exposure is OK, but long term we’d recommend a simple tape seal, the way manufacturers do when shipping cable on a reel.


Installing Cable
Q: Below are specs for an installation. We’ve never installed a Fiber Optic run this long. Please see below questions and info.
-Fiber Optic cable to be used is a 24 strand Single Mode application
-Length of run is 7200 m long
-Appears that all the Fiber is on one reel. However do you recommend having some junction points on pedestals along the way for testing-maintenance purposes or just one continuous run if possible?
A: FOA has lots of information to help answer your questions:
Re underground installation. See https://foa.org/tech/ref/OSP_Construction/Underground_Construction.html and https://foa.org/tech/ref/OSP_Construction/Underground_Installation.html in the FOA Guide.
There are other questions you need to ask:
Are there no intermediate connections or drops required? It’s just one straight fiber run? You should be able to install it continuously.
What is the installation type? Pulled in conduit or direct burial?
If pulled in conduit and you can pull in one try, that’s best. You should use a pulling capstan to limit tension, attached to the cable with a breakaway swivel pulling eye and use lubrication. Use the American Polywater guides (https://www.polywater.com/product/polywater-f-fiber-optic-pulling-lubricant/) for choosing lubricant and decide if you need an intermediate pull.
Direct burial is simple for a long run, just ensure you have the proper equipment.

Preparing Cable For Splicing
Q: Is there any standard on the preparation length of strip jacket upto the splice tray. Ideally its better to have a loop of buffer before getting into the tray if ever the closure has enough space for slack.. its also nice to put some hose to the buffer to add on protection. So far, i don't see any standard and can't support the remarks on what to follow. The practice was to take note on macrobend and have enough length of fiber to reach the machine.
A: There is a lot of variation in the size, shape and design of splice closures, so the length varies according to the closure and trays. For loose tube cable, the length of buffer tube from the entrance to the splice tray and the length of fiber needed in the tray are given in the directions for that splice tray. Similarly for ribbon cable, but the variations in ribbon cable designs often requires special handling and sleeving for the ribbons. Most manufacturers have specs available online.

Fiber Splicing Cost
Q: What is the standard of costing for fiber splicing and terminations? Is it per core / per splice or per each cable end irrespective of the number of cores?
A: That is a very hard question to answer, other than to say ”it depends. ” The number of fibers is definitely a factor because each fiber must be stripped, cleaned, cleaved and spliced then placed in the splice tray.
It also depends on:

• Single fiber or ribbon splicing?
• Type of splice closure
• Type of cable (loose tube, ribbon, flexible ribbon, high density, armored, ADSS, etc.)
• Installation: aerial or underground
• Location: urban or rural
• Set up time (same for low fiber count cable as high fiber count cable)

Lashing Aerial Cable With Cable Ties?
Q: I am considering an electrical job installing fiber optic aerially on a messenger cable.
I have seen the cable tie method of lashing the fiber to the messenger. Would you recommend this method considering the cost of a lashing machine for a single project and if so what would be a good distance between ties for the proper support of the fiber to the cable.
A: The normal way to attach an aerial cable to a messenger is lashing the cable with stainless steel wire. If you use cable ties, you would need ensure the cable doesn’t droop and the cable ties are designed for outdoor use in the sun over a long time (stainless steel ones are available). How long is the span? If it’s more than 100 feet, I think I would go with lashing. If you don’t have a lasher, you can rent one. You will need a bucket truck anyway.

Cable Installation Guidelines
Q: I am trying to find information on the recommendations regarding fiber underground in conduit. I am looking for industry specific verbiage on the cumulative turn degrees before you need a handhole or manhole. I believe it is 180 degree cumulative but I can’t find it anywhere.
A: We’ve heard the 180 degree limit mentioned on some conduit but not for fiber optics. For any fiber optic cable pulling, the relevant issues are pulling tension and bend radius.
We know of no specific standards or guidelines on conduit bends for fiber optics. It has many factors, including conduit size and type - there are many types, length of the pull, radius of the bends, type of fiber optic cable and lubricants used, if any. For the cable, there are thousands of fiber optic cable designs that vary in diameter from ~3mm to ~30mm depending on the type of cable and number of fibers, the stiffness of the cable and the location and type of stiffer/strength members and the method of installation - pulling or blowing/jetting. And for locations as far North as you are, temperature can be an issue as cable gets stiffer when colder!
For any given installation, corners are generally accommodated by handholes/manholes and pulling done from handhole to handhole with figure-8ed cable pulling techniques to prevent cable damage by excessive tension or bending.
FOA has a section of our Guide on OSP construction: Outside Plant Fiber Optic Cable Plant Construction and in that is a section on OSP installation. For specific cables or conduit runs, we’d suggest talking to the application engineers at cable manufacturers who can give specific advice.

Construction Near Underground Fiber
Q: What is the recommended distance for any new building construction to build near underground fiber duct channel?
A: We do not know of any standards or codes related to construction near fiber or other underground utilities. Common sense dictates that one stay far enough away to prevent accidental damage, so adding 5-6 meters(15-20ft) from the areas of construction makes sense.

Pulling Cable
Q: I’m having trouble finding much information on the matter. What type of swivel should be used to pull fiber and what would be the correct way to pull armored fiber.
A: Start  on the FOA Guide here and go here for types of swivel pulling eyes. with https://foa.org/tech/ref/OSP_Construction/Underground_Installation.html and here are sources https://www.comstarsupply.com/cable-pulling/swivels.html It’s not common to “pull” armored cable since it’s designed for direct burial, but a kellums grip on the jacket will generally work.

Communications Cables on Utiity Poles
Q: Is there a standard that service providers such as ISP, FTTH or cable TV should follow when installing their cables on existing electric poles. For necessary clearances etc. ?
A: The location of comms cables is in the “Communications Space.” At the top of the pole is the “Supply Space” for power conductors and between the two is a “Safety Zone Space.” It is There are guidelines of various types, mostly referring to NESC Rule 235. One of the best documents on this is from Nashville Electrical Service.  This presentation from Finley Engineering offers a good summary.

"Snowshoes" On Aerial Cable
Q: For overhead installation, can snow shoes, or other service loop devices, hold two separate cables? 
A:  Snowshoes are sized for different cable sizes and types. Some snowshoes are big enough for several cables, that’s no problems.

Fiber and Power Sharing Conduit
Q: We are working on a project that has miles of underground 7 cell innerduct conduit with existing fiber already running through one of the cells. Is it possible to run electrical conductors through the 2 of the other cells? The conductors would be no larger than 1/0 AWG at 480V or 600V. Both the fiber and the electrical are being installed for the same use.
A:We questioned several people in the electrical side that also do fiber work. The opinion is that the electrical may use the other ducts. If the fiber cable has conductive members, e.g. armor, it must be properly grounded. And any cables spliced in manholes need separation and marking. The concern is over what happens with a dig up, but as long as the electrical is turned off before restoration begins, there should be no problem.

Markers For Underground Fiber Optic Cables
Q: I have a general question about above ground markers for fiber optic cable in conduit.  Is there a recommended spacing for the markers?  Is there a standard to reference for this?
A: We asked some people who make them and they said the guideline is “line of sight.”  The rules for markers are mainly what information needs to be on them. Of course we also recommend adding marker tape about a foot above the conduit. I was curious if there were any legal issues and I found this interesting page from Cornell Law School:  https://www.law.cornell.edu/cfr/text/49/192.707
So I might  add to line of sight any crossings of roadways, rail ways and some markers for bridge crossings.
We have a new section on the FOA Guide: Outside Plant Fiber Optic Cable Plant Construction that may be useful.

Terminate All Fibers Or Just Some?
Q:We are currently running fiber which will be 12/24, my question is do we need to terminate every pair even if we aren't going to be using them or is there an alternative?
A: No you do not need to terminate all of them and leaving some bare fibers is often done when there is no planned use for the fibers or to save money. However, there are some other issues to consider. You do need some spare fibers ready to use, either in case of problems or for upgrades. For small fiber counts, the cost of terminating them all at once will be cheaper than having to come back to the site and doing it in the future. The economics are quite different if you have 144 or more fibers, of course. If you leave bare fibers, be sure to leave enough length to terminate or splice later - about a meter for termination and 2m for splicing. And protect them from damage so they can be used in the future.

Removing Old Fiber
Q: I have several 1000 feet of old 62.5/125 armored fiber optic trunks under a raised floor that I am replacing/upgrading to 50u MM and SM trunks. Is there any guidance on ‘Best Practices’ to follow when cutting these trunks into more manageable lengths for removal?
A: Use a jaws-type cutter to cut the cable into reasonable lengths and remove it. There should be no danger in cutting the cable up as long as your workers only cut the right cable. Raised floors often have large numbers of cables - often including power cables - so its important to ensure the proper cables are being cut an removed.

Re-routing Old Fiber Optic Cables
Q; I have a questions about the re-routing of fiber optic lines that have been in place for a number of years.  Is it a standard transaction in the fiber optic business to have to re-route fiber that has been in service for a long period of time.  (e.g. >20 years)  If so, is there a best practice for removal from conduit for re-rerouting?
A; There is no way we would recommend removing and reinstalling 20-year old fiber cable. First of all, old cable may be damaged in removal. Then cable and fiber technology has improved over the years so you can get much better components today at greatly lower prices. (One industry analyst I know likes to say that fiber is cheaper than kite string and fishing line!) Today’s cable designs allow for much smaller cables with many more fibers (288 fibers in 9.7mm - just over 3/8”) and new conduit designs allow for more cables in a conduit (microducts and cloth ducts) and easier installation - blowing in cables and microtrenching are perfect for metro areas.
More fibers, especially in a big city, is a must. Smart cities, small cells, FTTH (fiber to the home), ITS (intelligent traffic systems), V2X (vehicle to vehicle, infrastructure, etc.) and many other services need lots of fibers.
Our recommendation is to pull it out and dump it. Install new ducts and the fiber you need (x10 maybe?) and have new ducts for future use. Are you familiar with “Dig Once”?

Why A Figure 8 Cables?
Q: What is the reason for wrapping the cable in a figure eight?
A: When you need to do an intermediate pull, you have to pull the fiber and coil it on the ground. A simple coil will put a twist in the cable. Figure-8 coils put in twists of opposite directions on each side of the 8 making for no overall twist in the cable. See How To "Figure 8" Cable For Intermediate Pulls in the FOA Online Guide.

Maintenance of Fiber Networks
Q: Can you guide me how to prepare Optical Fiber Cable Annual Maintenance Proposal?
A: Basically, the network needs to be installed properly, fully tested and everything carefully documented. Then no routine maintenance is required. Most problems with fiber optic networks occurs when techs are working with it, e.g. damaging cables or getting connectors dirty when testing, so leaving it alone is the best plan.
Electronic transmission equipment can be tested anytime to ensure proper data transmission, but that does not involve accessing the fiber.
We have several things which may be of help:
You Tube Video: FOA Lecture 39 Maintaining Fiber Optic Networks
Web page: Maintenance

Getting Old Cables Out Of Conduit
Q: How do you get old cables out of a conduit when they are stuck?
A: Usually we are concerned about reducing friction when pulling cables through conduit, but sometimes you need to get them out. Here is a page from American Polywater the leading lubricant company with advice on the subject.

Designations For Fibers
Q: I'm currently working on a project involving optical fiber with VMS signs, CCTV cameras and other ITS equipment. I was wondering if there is a standard or a good practice which describe the typical assignation for each optical fiber on a 6 fibers cable for example?  By that, I mean :
- fibers number 1 and 2 : VMS;
- fibers number 3 and 4 : CCTV;
- fibers number 5 and 6 : spares
A: There are several ways people assign polarity on duplex links. The way you suggest is the most common I believe. Use the odd number fibers to transmit in one direction, even number fibers to transmit in the other direction and document the color codes.

"Mining" Cables In Data Centers (Cable Removal)
Q: Is there any documentation out there on best practices for data center cable mining? Any help would be greatly appreciated.
A: This has been a topic in premises cabling since first proposed by NFPA  for the NEC twenty years ago. FOA has gotten this kind of question before for many different circumstances, but I must admit that I know of no recommended procedures or standards for the removal of abandoned cable.

Cable “mining” often refers to the removal of underfloor cable, may apply to cables in trays and is hard to get information online - search for “Cable mining” and you get links to sellers of cables designed to be installed in mines (coal, salt, minerals, etc.) underground!

Why We Warn You To Be Careful About Fiber Shards

Photo courtesy  Brian Brandstetter,  Mississauga Training Consultants
1-844-440-0047
www.fiberoptictraining.com

Fiber/Cable   

Fiber Optic Color Codes Reference Chart
Q: Has anyone made a fiber optic pocket reference chart that has cable color orders, frequencies, or other commonly used info on it?
A: The FOA has a page on its Online Guide that covers color codes (https://foa.org/tech/ColCodes.htm). It is the most popular page in the FOA Guide! It includes a print your own pocket color code chart and one for your smartphone or tablet - works great with a smartphone.

Multimode In Premises Cabling
Q: I wonder when/if single mode fiber will start invading the enterprise. There's a whole ecosystem, of course, in addition to physical fiber cabling.  Switches, server connections, protocols, etc. But I'm wondering if you see the industry moving towards some set of standards using single mode?
A: Today, singlemode transceivers are as cheap as multimode for 10G and cheaper at higher speeds. Indoor cell systems (DAS) use singlemode. FTTH PONs (passive optical networks using singlemode) are being used for LANs because they are cheaper too. Both technology and costs point to the advantages of SM. Multimode is the historical design and it's hard to change. But structured cabling standards (TIA-568, ISO 11801)  include singlemode and POLs (passive optical LANs.)

Storing Fiber Optic Cable On Reels
Q: is there a "standard" for how to store a fiber optic cable reel?
A: This is another detail that has not in my knowledge ever been included in a standard. However manufacturers usually put a note on the reel to keep it upright - standing on the edges of the spool sides, not flat on one side of the spool.  If the fiber is to be stored for a period of time, it should be stored in a cool dry place and the ends sealed with electrical tape.
(Photo storing cable on reel)

Midspan Drop Cables
Q: I am working on a project that has 5 sections, consisting of 5 miles each section, CCTV, detectors, DMS connected by 192 count fiber.  We were directed to use the consultants plans from the first section as a guide for uniformity for the remaining contracts.  The attached fiber detail shows a 4 fiber drop cable going to the ITS device.  I was thinking to take all 12 fibers to the device and back for redundancy?  Also, if we did use the 4 fiber drop cable, I didn’t understand why they would splice the other 10 thru cables and instead leave them intact? Is there a preferred method for a drop cable to a device or just preferences?
A: We are not sure why they do it the way they do. Perhaps the designer was not familiar with midspan access which would preclude having to make the other splices. Using a 12 fiber drop cable would be more expensive and perhaps unnecessary unless the device being connected is in a location where a small cell site might be located. They may also have uses for those other fibers that require a connection through the drop point.  We”d suggest to the designer that midspan access might allow saving the 10 splices at each drop.

Fiber Flexibility and Longevity
Q: When I think of glass, I think of a material that is not very flexible. If you try to bend most glass, it will break. So it is rather remarkable  that you can bend a fiber and not crack it, even though the strands are quite thin. Perhaps it's not a good idea to bend fiber too sharply? I was talking to the people who maintain the fiber network at the university here. They tell me they have a problem when fiber gets to be about 15 years old, it will start to become brittle. If you flex it, it will crack or break. Is this a common problem? How long can fiber be expected to last before it becomes brittle? Is fiber that is manufactured more recently have a longer life-span?
A: Fiber is quite flexible. One demo I did when I taught classes was to walk up to a large window and push on it, telling everyone to watch the reflections to see how the glass flexed. I can flex quite a lot. Like most materials, as it gets smaller, it can bend more easily because the stress is less across the cross sectional area. Consider a bar of steel 1” in diameter compared to a piano wire or banjo string. Most things break because either they are overstressed or there is some impurity in the material that focuses the stress and the crack propagates from there. Ever cut glass? You scratch it and stress it along the scratch line and its maps off - called cleaving - where the stress concentrates along the induced fault. Fiber is extremely low in impurities - a matter of how it’s made from raw materials, not melted sand like most glass - that’s part of what makes them have such low loss (efficient transmission) and high strength. A glass fiber is much stronger than steel of the same size. Corning explains it here.
 The brittleness of older fiber is due to the migration of moisture up the cable into the glass. The H2O becomes the OH radical which interacts with the glass to reduce its strength. Most cable companies say their cable today protects the fiber well enough that it should last 40 years, but where fiber is terminated or spliced and exposed to the air, it can get brittle and be hard to handle in 10 years or more. Older fiber had shorter lifetimes simply because we learned to make fiber coatings and cables better at sealing fiber from the ambient atmosphere.

Fiber Choice for LANs
Q: Many manufacturers or suppliers worldwide emphasize the use of OM4 multimode optical fiber for the LAN. Does single-mode fiber not provide greater bandwidth than multimode? Do they imply that single mode optical fiber should only be used for long distance applications and not in LAN environments?
A: Multimode fiber is acceptable for LANs up to 10 gigabits/second and up to 550 meters depending on the type of fiber and Ethernet version. See this page for a complete list of network specifications.Higher versions of multimode fiber OM2-OM3-OM4 have higher bandwidth capability. OM5 is a version of MO4 that also supports wavelength division multiplexing with VCSEL sources in the extended wavelength 850-950nm range. OM1 is a earlier fiber with a different core size that has not been designed into new systems for almost 20 years. LANs can use singlemode fiber for all versions. Singlemode has longer distance capability (up to 40km) and virtually infinite bandwidth. See the singlemode specification in the link above. Singlemode is also used in passive optical LANs that can be much cheaper to build than conventional networks. See this page  for information on optical LANs (OLANs) including passive OLANs based on FTTH GPON technology.

Fiber Lifetime
Q: I am often ask how long the fiber we are deploying today will last or be useable , I typically say something like it will last at least 20 years and that no one really knows how long it can be used.   What is the oldest fiber optic network or longish segment that is still in production that you know of?
A: Current cables are probably good for 40 years or so. Today there is some fiber being used by telcos from the late 1980s and lots form the late 1990s and early 2000s. Lots of OPGW (optical power ground wire) is in use up to 30 years old. Some of this old fiber is being used at 10Gb//s. But remember that fiber from 20 or 30 years ago may have limitations on bandwidth, since both chromatic and polarization mode dispersion has been reduced in newer fibers for higher speed networks. And spectral attenuation of older fibers may be higher and have the water peak at 1383nm that can affect wavelength-division multiplexing systems. But the weak point may not be the cable or fiber, but the splice and termination points where bare fibers may be exposed to the elements. It's not uncommon to find these fibers have become brittle and are hard to work with. What we always tell people is if it’s working, leave it alone. If you want to upgrade to higher bit rate systems, use fiber characterization to determine if the fibers are capable of use at higher speeds.

Cable Bend Radius
Q: We are working on project where we need to know difference between short term and long term bend radius for fiber optic cable?
A: The bend radius for cables is generally specified under two conditions - under stress, e. g. when being pulled, it is a radius 20 times the cable diameter. Relaxed, after installation, it is a radius 10 times the cable diameter. The relaxed specification, 10X, is considered a long term specification. Some of the new high fiber count cables have different specifications, sometimes 15X or 20X under either condition. Check with the manufacturer for their specific cable. See this article on bend radius in the FOA Guide.

Old Multimode Fiber
Q: We have old multimode fiber and we are still connecting more equipment to it over greater distances and need some way to insure the equipment will work.
A: I do not know of any simple formula for figuring this out. Last time I remember such a formula from around 2000 done for Gigabit Ethernet with VCSELs and the equation reminded me of the quantum mechanics course I took in physics. The problem is you have two bandwidth factors, modal dispersion and chromatic dispersion. Modal dispersion is highly dependent on mode fill, e.g. the metric “encircled flux” was developed to define the mode fill of multimode fiber with VCSEL sources for simulations to estimate bandwidth. Chromatic dispersion is dependent on the fiber spec and the spectral width of the source which is better with lasers and LEDs.
What generally happens is the standards group developing the network standard, especially IEEE 802.3 for Ethernet, runs the numbers and specifies a maximum distance for the particular network and its speed. FOA has a table of these specs here: Specifications for fiber optic links and systems, including FTTx

105 Micron Fiber?
Q: I have a customer asking about 105um fiber. Does it exist? What is it basically used for? After FOA suggested a clarifying question to the customer: The fiber is for Power over Fiber(PoF). The construction is 105um fiber with 125um cladding. The question or assumption would be – The termination would be the same as 50,62.5 or 8.3um with a 125um cladding?
A: The people who do laser surgery and power over fiber use special step index fibers and SMA connectors. The power density can be very high so the heat can build up in the cable. SMA connectors or the metal ferrule swaged-on connectors are often used for their all-metal construction with the ability to withstand heat and sometimes the need to be drilled for special fiber diameters. Because of the high power, the polish needs to be low reflectance, so we’d recommend using a wet polish and end with a very fine polishing film - 0.3 microns or so. Like polishing SM for DWDM.
Cleanliness is very important for these applications. I remember a call from a doctor doing laser surgery who kept ruining cables because they were dirty and the high power literally exploded the dirt and pitted the ends. When that happens, sometimes they can be polished out but often they are ruined. The same thing happened to the 120 inch telescope at Lick Observatory when Joe Wampler tried using it to laser range to the retroreflector Apollo 11 left on the moon. Exploding dust pitted the aluminization on the mirror.

Replacing OM1 MM Fiber
Q: We are an automation system integrator in South Africa. We have a client that has multimode 62.5/125 fibre optic plant wide. None of the runs between components are longer than 2km. We intend to upgrade the technology from a proprietary communication protocol to a standard ethernet protocol at 100 MHZ. The fibre to copper convertors we will be using are using 1300nm light source and have a Fibre Optic Link Budget of 12.8dB for 62.5/125 um and 9.8dB for 50/125 um. The client has been advised to replace the multimode 62.5/125 with multimode 50/125 cabling and we need to know if this is really a requirement.
A: Do you know how old the fiber is? It should be what we called FDDI grade 62.5/125 fiber with a loss of ~1dB/km and a bandwidth of 500MHz-km at 1300nm. A 2km link should have a loss of 2dB for the fiber and ~0.5dB/connection - well under the power budget of the link. 100Mb/s Ethernet variants were designed for 2km or more on this fiber. There is no reason to upgrade at this time, 50/125 fiber would not be needed until Gigabit Ethernet was desired.

Can A Fiber Optic Cable Catch "Fire"
Q: While working on a cut-over of a dwdm circuit something has happened that I am now looking for an explanation. A transmission fiber emitted "fire" on that same fiber! We did not see any optical light we saw fire. About 5 cm of fiber burned and remained smoke. What phenomenon occurred? Are the dwdm amplifiers so powerful enough to generate fire? Unfortunately I could not get into the station to take the model of the equipment.
A: We’ve heard of high power WDM systems exploding dirt of the endface of fiber connectors and damaging them, but this is a new one. We contacted several technical people in fiber companies and found that this can happen if there was a crack in the fiber in the cable near the connector or lots of reflection perhaps caused by a very dirty connector that allowed the very high power to heat the cable enough for combustion. DWDM with many multiplexed signals and a fiber amplifier creates a lot of power confined to a very small core of the singlemode fiber. That power can ignite the acrylate coating on the fiber.

What's The Lifetime Of Fiber?
Q: "The utility I work for has some FO cable installed, some 20 years+ and I am wondering what is industry standard for the useful life of a cable? This is from an asset management point of view. I realize that FO cables can and do last for decades, especially if the work on them is minimized (ie. splicing for repair or relocation), but what would you consider a good book value for useful life?"
A: Cable manufacturers have generally made fiber optic cable for a lifetime of 20+ years and in the last decade or so we’ve been told that 40 years is reasonable for a cable. But that means the cable will retain its specifications for that time frame. Networks, however, do not stand still. In the last 20 years, network speeds have increased by up to 100 times. In the same time period, the fiber has been engineered to accommodate longer and faster links. 20+ year old fiber was installed when speeds were around 1Gb/s, where dispersion was not an issue, nor was dense wavelength division multiplexing (DWDM) being widely deployed. So if you are using fiber at lower speeds, the current cables are probably fine. You might have trouble splicing older fibers in closures because the exposed fibers do tend to get brittle.
If you want to upgrade to faster speeds or DWDM, the older fibers will need testing - we call if fiber characterization - and here is a page in our FOA Guide about it: Fiber Characterization and Testing long haul networks (CD, PMD, Spectral Attenuation)

Differences In Fibers
Q: What is the difference between OM3 and OM4 type fibers and G.654/G.655? They seem to be rated for the same  amount of GBs (10-400) and the only difference seems to be the multi-mode nature of OM3/4 vs. the single mode nature of G.654/655. Can they both be used in long haul communications if laser optimized?
A: OM3 and OM4 fibers are both 50/125 micron fiber but have different bandwidth capability. OM3 is rated at 1500MHz-km while OM4 is rated 3500 MHz-km. OM4 is an evolution of OM3 where design and manufacture allow more bandwidth. More bandwidth translates into slightly longer link lengths in faster networks, ~1-10Gb/s. For example, Ethernet at 10Gb/s will go 300m on OM# and 450m on OM4, which can be important if it is being chosen for a enterprise network backbone.
The differences in G.654 and G.655 are more complicated. G.654 is singlemode fiber optimized for use at 1550nm for long distance use. G.655 is “non-zero dispersion shifted” fiber tweaked for dense wavelength division multiplexing (DWDM) to prevent secondary problems with high power and closely spaced wavelengths of DWDM and fiber amplification. It’s the kind of fiber used in long submarine cables.

Arsenic Coated Cable?
Q:  I was told a contractor installed arsenic coated fiber optic cable because they didn’t want animals to chew through it. Is this true?
A: Some cable has chemicals put in the jacket to make it taste bad to rodents. We have not found any one who claims to use arsenic, in fact, we could find no references to what kinds of chemicals are used.

Why Do Cables "Go Bad"?
Q: It’s been my observation over ~15 years of building and managing fiber channel storage area networks that from time to time cables will fall out of transmission spec.  In terms of communicating with non-storage people, they in essence, “go bad”.  Other than possible damage due to physical disruption of a cable, or contamination at the connectors usually caused by a human being unplugging/replacing, has it been your observation that MM cables can “go bad”?

A: There are some possible causes of problems over time. We know of connectors that fail for several reasons.

• The biggest cause is with prepolished/splice connectors with mechanical splices. the assumed problem is the index matching get goes bad, but that’s highly unlikely. It’s usually the crimp fails and the fiber pulls out, especially if it has any stress on the fiber.
• Adhesive connectors can have a bond between the connector and fiber fail, more likely on anaerobic connectors.
• Any stress on the fiber at the connector is bad. Patchcords should not be left hanging on racks but dressed into horizontal racks below each patch panel.
• Residual stress in cables can be a problem - tension or tight bends - and they may get worse over time.
• Moisture is always a worry. It takes years to show up, but indoor cables are not protected from moisture like OSP cables.
• Of course, transceivers fail too - electronics are generally very reliable but do deteriorate over time and cause failures.

Armored Indoor Cable?
Q: Can I get an indoor armored 8 core fiber optic cable?
A: Most cable manufacturers make indoor armored cable using corrugated wrap armor to protect cables from crushing loads from other cables especially in under floor installations.

More Than “Single” Mode?
Q: We're now using SM fibre so it looks like we don't need mandrels in the Ref Lead at the Light Source.  The info I have is that we need to make a couple of air coils 35mm to 50mm in diameter.  Why? 
A: When you launch from a pigtial laser source through a connector into a reference cable, you do have several modes being propogated. It usually takes 100m or so for the second or third order modes to attenuate. So the coil causes them to be attenuated by the stress enough to no longer be significant - it’s a mode filter just like MM. If you do not do this, you will measure higher loss in the fiber and at connections near the source. Since most SM has traditionally been long distance, the effect was small or ignorable, but with short links, it can be significant.
Followup Q: But how do we explain multiple modes in Single Mode fibre?
A: When you get the core of the fiber down to ~5-6 times the wavelength of the light, it no longer acts like geometric optics (like MM fiber). Some of the light can travel outside the core (see the note on “waveguide dispersion”here http://www.thefoa.org/tech/ref/testing/test/CD_PMD.html). At launch, significant amounts of power are at higher angles creating short lived modes that are highly attenuated.


Replacing OM1 MM Fiber
Q: We are an automation system integrator in South Africa. We have a client that has multimode 62.5/125 fibre optic plant wide. None of the runs between components are longer than 2km. We intend to upgrade the technology from a proprietary communication protocol to a standard ethernet protocol at 100 MHZ. The fibre to copper convertors we will be using are using 1300nm light source and have a Fibre Optic Link Budget of 12.8dB for 62.5/125 um and 9.8dB for 50/125 um. The client has been advised to replace the multimode 62.5/125 with multimode 50/125 cabling and we need to know if this is really a requirement.
A: Do you know how old the fiber is? It should be what we called FDDI grade 62.5/125 fiber with a loss of ~1dB/km and a bandwidth of  500MHz-km at 1300nm. A 2km link should have a loss of 2dB for the fiber and ~0.5dB/connection - well under the power budget of the link. 100Mb/s Ethernet variants were designed for 2km or more on this fiber. There is no reason to upgrade at this time, 50/125 fiber would not be needed until Gigabit Ethernet was desired.

Old Fiber
Q: We are looking at a company’s fiber network which has been laid at various points in time over numerous years.  In this process, we are trying to identify the changes that were made to either/both the glass fiber and the cladding.  Are there different generations of what was industry standard in creating the fiber?  For example, are you able to identify the difference in a fiber that was laid in 1980 versus one laid today?  Was the cladding the same size/thickness etc. in 1980 as it is today or has this been modified/improved upon over the years?  In all, we are trying to find what modifications have been over the years and how this may improve the life of the network and its capabilities. 
A: This is a common problem today. Many network operators are evaluating their fiber networks for upgrades, hampered by the fact that few are properly documented. Below is a timeline that should answer your questions. What many network owners are doing now is testing their cable plants - a process called Fiber Characterization.  There are contractors who do this service.

How to Clean POF (plastic optical fiber)
Q: I heard that plastic fibres such as PMMA can suffer damage from cleaning from an alcohol solution. Are there alternate cleaning solutions available for these types of fibres."
A: You can use a 10/90 mix of  isopropyl alcohol/water. Typically use with a lint free swab. (from out POF consultants)

Older Fiber?
Q: I have some 62.5 mm and sm inside fiber plant over 20 years old.  When is a good time to upgrade?
A: When you need to or have to. If it's working OK, there is no need to upgrade!

Do You Strip The Cladding For Termination Or Splicing? NO! They've ALL Got It All Wrong

We 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."

I'll 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:

EVERY 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, no surprise, 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 fiber.

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 splice.

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 magnification.



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?

Termination/Splicing    

MM Splice-on Connector On Singlemode Cable
Q: I encountered a situation where a MM mechanical connector was used on a SM fiber and passed on an OTDR test. The client and I are interested in understanding how these connectors could have passed?
A: The joint between a multimode and singlemode fiber should have vrey high loss, ~17-20 dB, depending on the mode fill of the MM fiber. However the short length of the MM fiber, ~10mm, might not be enough to cause the modes to fill in the short fiber in the connector, resulting in relatively low loss.
Eric Pearson, one of the most knowledgeable people on connectors expressed this idea then tested it with 100m singlemode connected to a second singlemode cable. The second singlemode cable has an OM3 LC unicam connector, An EXFO ftb-400 OTDR indicates a 2.09 dB drop. That is way too much to pass a test but nowhere near the loss that could be expected from the MM/SM joint. See the OTDR trace below.

Connector Mating Adapter Loss
Q: When looking in data sheets on duplex adapters, it’s telling me that it has a Insertion Loss of about 0,2dB. Is that common? Only thought it was the connector that had a loss.
A: Technically, a single connector or mating adapter does not have any loss. It’s not “connector loss” but “connection loss” defined as the insertion loss when two connectors are mated, and for most connectors that requires a 3rd component, a mating adapter to align the ferrules. (Some connectors like MPOs have their own alignment mechanism so the mating adapter merely holds the two connectors - one with pins and one with holes - together.)
The connector manufacturer’s specification for “Connector loss” is the loss of their connector mating to a reference connector with a mating adapter. Connectors are graded in ISO/IEC standards, but not TIA, and the best connectors are ~0.2dB loss when mated to another of the same grade.
If a mating adapter manufacturer is quoting loss, one assumes they mean their adapter with two of the best connectors will have a connection loss of 0.2dB.
Mating adapters for 2.5mm ferrule connectors - FC, SC and ST - have a split sleeve alignment bushing that is the critical element. They have been made with molded glass-filled plastic, phosphor bronze and ceramic. In our opinion/experience, the plastic ones are only good for multimode fiber and wear out in ~10 insertions, discoloring connector ferrules and leaving dust scraped off the plastic on the ferrule ends. The metal ones are good for SM or MM and hundreds of insertions, but they tend to wear and leave marks on the ceramic ferrules. The ceramic ones are recommended for SM and for testing as they work best and last practically forever.


Maximum Fusion Splice Loss
Q: We have set 0.4 dB as our max for all losses per splice and my counterparts argue that customer quality will not suffer with a 1.0db-1.5db loss at a splice. What do you think?
A: We would argue that the issue with high loss splices is more one of reliability. Most fusion splices of singlemode fiber are 0.05 to 0.1 dB A splice that has more than ~0.2dB loss probably has some inclusion (dirt that got on the fiber after cleaving) or an air bubble with means the splice is deficient in strength and may fail over time. If the network is operating at high power with WDM and fiber amplifiers, the inclusions or bubbles may produce heat which can cause failures. At very high speeds or using coherent communications over long lengths, it might affect dispersion.

Loss For APC vs UPC Connectors
Q: I was wondering if there will be a standard connector loss for a UPC connector and a different lower value for an APC connector.. ex. upc has 0.5dB while APC is 0.3dB.
I would like to make all connectors uniform on a new network infra to avoid mismatch and causing any possible damage on the equipment when APC will be plugged into to a flat.
A: There is really no statistical difference between APC and UPC connector loss. The lower reflectance of the APC actually reduces loss since the reflectance represents a factor in connection loss, This issue of connector grades has been discussed at international standards committees for years. ISO/IEC wants to have grades of connectors, rated for connector loss in ranges from 0.1 to 1dB, but I do not think it’s standardized. I recommend using 0.3-0.5dB for loss budgets, where in OSP networks it matters little, since there are only a few connections and fiber and splice loss is a larger factor.
Keeping UPC and APC connectors straight is easy - APCs are Green, UPCs are blue. Everybody just needs to be taught that!

Fusion Splice-On Connectors (SOCs) (From an FOA Instructor)
Q: A question came up from one of our students regarding splice on connectors.  Is there a TIA or other standards body that addresses this issue? We are used to the 0.75 dB loss for a mated pair, however, when this mated pair has two fusion splices that terminate the connector, is there a recommendation? 
One could make the argument that it does not make any difference as the other alternative is splicing a pigtail for termination of a cable.  This pigtail splice is normally included in the link loss budget calculation.   So similarly, with a splice on connector it is the same as splicing on a pigtail.
A: There are no specific TIA or IEC specs that address these splice-on connectors or pigtails. If you used TIA numbers and included the splice and connector it would be 1.05dB - 0.75dB for the connection and 0.3dB for the splice, that’s mated to a factory adhesive/polish connector.   Or if it were two similar connectors, 1.35dB. 
Everybody, including the people in TIA standards groups, know those numbers are too high for most single ferrule connectors. They keep them at 0.75dB for prepolished/splice connectors (w/ mechanical splices) and array connectors (MPOs) which have somewhat unpredictable performance. Internationally, IEC has created grades of connectors from ~0.3 to over 1dB. The newer mechanical splice connector kits now use the Chinese copied cleavers which are super - at least the few we have tested - and the connectors are now much lower loss and consistent.
SOCs (fusion splice-on connectors) are spec’ed as the total termination and are generally just as good as the typical adhesive polish connector - 0.5dB is plenty of margin for a those mated to a factory adhesive/polish connector.
Spliced on pigtails are generally considered a termination and the splice is not broken out - like a long SOC. But I cannot guarantee everybody thinks that way. But a fusion splice is typically <0.1dB anyway.

APC or UPC?
Q: Can my otdr test upc connectors?  Can I test this distribution without problems?

A: APC and UPC connectors are not compatible. APCs are green, UPCs are blue. See https://foa.org/tech/ColCodes.htm To test UPC connectors you would need a launch cable that is UPC on the OTDR end and APC to to mate to an APC connectors. But if I look behind these green APC mating adapters, I seem to see blue connectors - blue is UPC not APC.  (add photo)

PON Troubleshooting
Q: I have question about DBM IN GPON system with splitter 1:64. I spliced 2 fiber from splitter to customer going through 4 splicing points and when I measured the loss at the end (customer) the #2 fiber was fine  but the #1 fiber was down 12 dB. I checked the fiber with OTDR without splitter and it looks fine. What you think is the problem?
A: You need to test the splitter itself to make sure all ports are good.

Splice Loss
Q: We are installing 216 fiber aerial cable for 12km with 2 splicing points. We use 3 different fusion machines and they report that all splices are 0.00db. But when we check using OTDR we get above 0.04db. The question is how can we get below 0.04db splicing loss?
A: The loss results from both fusion splicers and OTDRs are estimates, with considerable uncertainty. The splicing machines estimate based on the optical images of the fibers. The OTDR estimates loss based on fiber backscatter and may give significant differences depending on the direction of test. The differences you quote are within the uncertainty of the two instruments.

APC Connectors
Q: Why NOT make the use of APC connectors the new standard for all adds, moves and changes to any campus, MDU or similar application using single mode cable?
A: There is absolutely no reason not to use APC connectors other than the cost is slightly higher and one must be careful if they are used in a cable plant that also has PC or UPC connectors because they are incompatible. We recommend them all the time for short links like data centers, passive optical LANs and FTTH where runs of singlemode fiber are short. In fact they are very common in these networks today.

Cleaning Connector Protective Caps
Q:How do you clean LC Fiber Optic end caps (the cap that covers the cleaned fiber cable)? Is there a tool for that?
A: We assume you are talking about the small plastic protective caps on the connector ferrule. There is a joke in the industry that goes “there’s a reason they call them "dust caps’” they’re often full of dust.” The problem is these are plastic molded parts that are made by the billions for various purposes - some just fit fiber optic connectors. They come out of the molding machine and are dumped in barrels. No provision is made to keep them clean, plus they will have some mold release chemicals inside them that can attract or hold dust. Even static electricity is a problem.
We know no way to clean them nor to keep them clean. We recommend using them to protect the connector ferrule - in fact we’re trying to get people to call them “protective caps” - but after they are removed and before use (connecting to another cable or a transceiver or testing them) they need inspection and cleaning.
See these pages in the FOA Guide: Microscope Inspection And Cleaning of Fiber Optic Connectors  Cleaning Fiber Optic Connections

Directional Splice Loss
Q: I have a customer that is splicing a fiber distribution hub to their fiber plant.  The fiber distribution hub utilizes 100FT long fiber stubs of SMF G.657.A1 and the fiber plant uses SMF G.654.D.  The project has a contract fusion splice passing spec of 0.2dB loss, averaged bi-directional and also a one-way <0.3dB loss (either direction) specification; using an OTDR for measurements.
From my research, if the splices OTDR’s test results for the 2 directions are -0.2dB / +0.6 (average of +0.2), the network is not actually seeing a +0.6dB loss; but this is how the OTDR interprets the backscatter information… the OTDR being somewhat confused due to the bend insensitive fiber characteristics.
A: Correct - the directional differences are due to the mode field diameter variations in the two fibers.  G.654 is a large MFD fiber, ~12.5microns, compared to ~9 microns for G.657.A fiber. The OTDR measures based on backscatter which will be very different for the two fibers.

Connector Mating Adapters
Q: I am looking for a standard that describes the value of parameters (IL, RL) and class (if we can talk about class) for FO mating adapters.
A: Mating adapters are part of the connection but like each connector they only contribute to the total loss of the connection and cannot be separated from the other two when talking about loss. They can be specified by mechanical dimensions and materials. For example on 2.5mm ferrule connectors (SC, ST, FC and the obsolete FDDI and ESCON duplex connectors) the mating adapters have had alignment sleeves made of molded glass-filled thermoplastic, phosphor-bronze and ceramic. The plastic ones are cheap but wear out quickly - 10 insertions will leave plastic dust all over the mating connectors. Phosphor-bronze mating adapters last longer - maybe 500 cycles. The ceramic sleeve ones last almost indefinitely. We know this because we were in the test equipment business for 20 years (we started FOTEC in 1980 and sold it to Fluke in 2000) and we tested these mating adapters for longevity with reference test cables used in insertion loss testing. We had many calls from techs with problems caused by the adapters with plastic sleeves. So the way we know the mating adapters are graded is by alignment sleeve materials.

APC Connectors
Q: With a fiber optic pathway that has multiple patching points...if the end user requires APC connections, isn't it only important to have those angled connectors at the end/equipment connections with UPC being acceptable throughout the middle part of the link?
A: Reflectance at the connection is the issue, of course.
Reflectance near a transmitter can affect the laser transmitter causing nonlinearities or noise in the device. That’s always been a major concern.
The second issue is reflectance causing background noise in the link. If you have ever seen a ghost on an OTDR, you have seen a reflectance at a connection that is bouncing back and forth in the fiber and is of high enough amplitude that you can see it at the source. Of course if it reflects back and forth in the fiber link, it will also show up at the receiver end, becoming noise and/or distorting the receiver pulses. In a bidirectional single-fiber network like a PON, it affects receivers at both ends.
Some refer to this as multipath interference. It is being studied by international standards groups but nothing has been published on it as far as I know.
We are familiar with a link that was ~1km of SM fiber with hand-polished ST connectors at several connections. The link had acceptable loss for all the fibers in the cable but none would work with electronics. Replacing the connectors with fusion spliced pigtails cleared the problem up immediately. Was the problem
With that background, I would answer you question this way. APC connections at each end of the link will effectively stop any reflectance issues going back and forth in the whole link. Using UPC or PC connectors in the link with reflectance better than -40dB are unlikely to cause problems. (Keep them clean of course since dirty connectors show high reflectance.) If the links are very short (<1km), the fiber will not attenuate any reflectance substantially, so short SM links (FTTH and passive OLANS for example) often use APC connectors everywhere.
And a final practical issue - mixing APCs and PC connectors is very bad, perhaps damaging the surfaces. If you do mix them in a link, you must train personnel how to handle them. If you have patch panels with PCs and equipment with APCs, for example, you have to ensure the patch cords are color coded properly (blue = PC, green = APC) and everybody knows not to mix them

Connector Loss At Patch Panel
Q: If I have two SC connections in a cabinet eg one incoming cable jumperd to an out going cable. Should I be looking for a loss of no more than .75db across the two of them as per TIA-568
A: TIA 568 has included a connector loss of 0.75dB for decades. Even the committee is aware that this is a bogus number for most connectors but they leave it in because the manufacturers of MPO connectors need it to comply with the standard.
SC connections should be ~0.2-0.3dB if the connectors are good and properly cleaned. Now in the patch panel you describe, each of the two connections should be in that range for a total loss of 0.2-0.6dB. TIA would allow 0.75dB for each connection or 1.5dB total.

Differences Between Singlemode or Multimode Connector Mating Adapters
Q: What is the difference between singlemode and multimode bulkhead/adapters (mating adapters). My understanding is you cannot use the singlemode with the multimode and visa versa.
A: There are 3 types of adapters - rated for SM or MM - based on the alignment sleeve material.
-Plastic (glass filled thermoplastic) alignment sleeves are cheap, not very precise and wear quickly (you can see ceramic ferrules get dirty using them) - only good for multimode and one or two insertions - not recommended
-Metal (phosphor bronze) alignment sleeves are better with good alignment but still wear some - OK for MM, some are rated for singlemode (check before you buy), and are OK for most uses but will wear out if used for repetitive testing
-Ceramic alignment sleeves are the best and most expensive. They are very precise in alignment and last for a long time. Recommended for all singlemode and all testing purposes.
Don’t use MM adapters for SM but SM adapters are OK for MM.

Polishing Films
Q:  Are there different grades (micron) polishing films/papers for multimode and single mode fiber cables in ODF termination ? If yes, What are the grades polishing papers for multimode 50/125 um and 62.5/125 um fibers.
A: The polishing of MM and SM fiber is indeed different. Both start with an “air polish” with 12micron alumina polishing film to remove the protruding fiber. Then the polishing continues on a soft polishing pad (3mm 80 durometer rubber).
MM uses a 3micron alumina polishing film polished dry then a final 0.3micron alumina film polish. See http://www.thefoa.org/tech/ref/termination/ConnHints.html
SM is usually done with a wet polish using as special polishing slurry and diamond polishing film. The diamond film will polish both the ferrule and the fiber to get the best end finish. See http://www.thefoa.org/tech/ref/termination/sm.html
There are even more pages of information on the FOA Guide at http://www.thefoa.org/tech/ref/contents.html#Components

MPO Connector Loss
Q: Is there a current standard, for maximum allowable loss, for MPO fiber connectors? If so… what is the standard # from EIA/TIA? (Was it amended in 568B, since they were introduced?) Would it be similar to standard connectors @ 0.75dB Max allowable loss?
A: The MPO is covered under the TIA 568 standard. All fiber optic connectors are the same - 0.75dB.
There are discussions being held at TIA and ISO/IEC on using a different method of specification, statistical in nature, that says X% would be less than YdB in several stages from 0.1-0.2 to over 1dB, but it’s led to some headed discussions.
MPOs for MM are probably no less than 0.5dB and SM are near the 0.75dB mark. At least the SM ones are APC (usual 8 degrees, but still a flat polish).
I’ve recently learned that MPOs are polished for fiber protrusion to try to get fiber contact, but the evenness along the line of fibers is harder to control.


image from SUMIX showing protruding fibers in MPO connector

Fusion Splicing Regular And Bend-Insensitive Singlemode Fiber
Q: Would fusion splicing single mode bend insensitive to standard fiber with same core cause a numerical aperture mismatch? We are seeing loss but it’s hard to tell from what. Going from a drop to BI inside cabling. Any direction is appreciated!

A: There is a lot of controversy in this area and has been for some time. The issue is mode field diameter(MFD)  differences between regular and bend-insensitive (BI) fiber caused by the low index trenches around the core that are used to limit bend insensitivity.

Some (maybe most) manufacturers make BI SMF to match MFD of their regular SMF, since a common use is splicing BI SMF pigtails onto regular SMF. With so much BI SMF fiber being used in microcables and high fiber count cables, the opposite situation could be an issue also.

Another factor at play here is the fusion splicing program. The different structures of the fiber may need special programming in the fusion splicer to get heat and feed right for the two different fibers.

FOA hopes to have some independent data on this topic soon. FOA Master Instructor Joe Botha has done tests before on splicing dissimilar fibers when BI fiber first became available (read the report here)  and has planned a more extensive set of tests to update that data for more recent fibers soon.

Fusion Splicing Live Fibers
Q: Is it safe to fusion splice a live fiber, or is there a chance that
the light from the arc will damage the detectors in the modules at the end (20km-rated SM for us).
A: I have never heard of this being a problem. The amount of light coupled into the fiber from the splicing would be very small compared to a properly coupled laser. When a cable is broken you might be splicing the fibers that are live without knowing which are live and not caring. On your newer splicers this is not a problem. On the older splicers with the LID system you would have to reduce the power to get a good splice which they would do by putting a bend in the Fiber.

Maximum Fusion Splice Loss
Q: We have set 0.4 dB as our max for all losses per splice and my counterparts argue that customer quality will not suffer with a 1.0db-1.5db loss at a splice. What do you think?
A: We would argue that the issue with high loss splices is more one of reliability. Most fusion splices of singlemode fiber are 0.05 to 0.1 dB A splice that has more than ~0.2dB loss probably has some inclusion (dirt that got on the fiber after cleaving) or an air bubble with means the splice is deficient in strength and may fail over time. If the network is operating at high power with WDM and fiber amplifiers, the inclusions or bubbles may produce heat which can cause failures. At very high speeds or using coherent communications over long lengths, it might affect dispersion.

How Long Does Termination Take?

FOA received a request from a consultant recently wondering if we had information on the termination times for fiber optic cables. After some looking in our archives, we realized we had a document online that compared times for various fiber optic termination processes. The paper was written after several FOA instructors did a comprehensive time and motion study on termination processes. The document is about 15 years old but still relevant.

You can read it here in the FOA Online Guide.

"Connector Loss" or "Connection Loss"
Q: I have always counted the loss of a connector as .75 dB (568B-3) and 1.5 for a mated pair. Is that correct?
A: While the industry always says "connector" loss, it is actually "connection" loss. As we explain in the page on termination and splicing (http://www.thefoa.org/tech/ref/basic/term.html) When we say "connector" loss, we really mean "connection" loss - the loss of a mated pair of connectors, expressed in "dB." Thus, testing connectors requires mating them to reference connectors which must be high quality connectors themselves to not adversely affect the measured loss when mated to an unknown connector. This is an important point often not fully explained.  In order to measure the loss of the connectors you must mate them to a similar, known good, connector. When a connector being tested is mated to several different connectors, it may have different losses, because those losses are dependent on the reference connector it is mated to."
The TIA spec of 0.75dB is for a mated pair of connectors. If you have been passing connectors tested @ 1.5dB loss....you may have some very bad connectors in your cabling!

Test & Measurement

• At the top of the list is "insertion loss" testing which uses a light source and power  meter to test the fibers in the same way that a communications system transmits over the fiber. It is a simple test and the equipment needed is inexpensive.
• Techs will also use a microscope to inspect the fiber optic connectors for dirt and damage, a big issue for fiber.
• The instrument called an "OTDR" takes a snapshot of the fiber using a technique like radar. Most outside plant cables are tested with an OTDR and the data ( the snapshots are called "traces") stored for future reference. OTDRs are more expensive and require more training to use properly.

A: 

Differences in OTDR Traces
Q: What causes the differences in otdr traces for fibre cores that are in the same cable?
A: Several things can cause the fibers  in a cable to have differences in their OTDR traces:
Differences in fiber from different production batches including fiber that may come from different preforms.
Differences in stress on the fiber caused by inconsistent cable design and manufacture
Of course differences in splices including stress on fibers in a splice closure and terminations including stress on fibers in racks and panels.

Test Source Variations
Q: When we plug in the patch cable to use as a reference to get ready for our insertion loss test, our power meter gives us different readings each time we plug in and unplug the cable.  Is this normal?
A: If you plug a reference cable into a source, it is likely to have some variation in coupling each time so the power out of the cable is different. Once you plug the cable into the source and a reference set with the meter, you should not remove the cable until you finish testing.


Test Source Modulation Options
Q: Our light source user’s manual listed the following: “In the actual project, it is necessary to load the audio carrier in the optical signal to identify the optical fiber. The equipment contains three carrier frequencies, which are 270Hz, 1KHZ, and 2KHz.” Can you tell me if I need to be concerned about setting this to a certain value?
A: Sources often have an option to modulate the source for use with another instrument called a “fiber identifier.” Then it offers the output as DC or CW (a steady unvarying signal) or modulated at 270HZ,1 or 2 kHz. When testing loss the DC or CW setting should be used.

Directional Testing
Q: I have taught for several institutions and throughout all my years of doing this I was always taught that when testing for insertion loss and back reflection for singlemode cable links that testing bi-directional is an imperative.  Recently when I was attending a meeting involving members of the military along with folks involved in the development of the military manuals, it was mentioned that with singlemode testing that bi-directional testing is not necessary.
A; First of all, there is a directional difference in splice or connector loss - and maybe reflectance (that term is now almost universally used in place of “back reflectance” which is a poor term since a reflection is always back) - as long as two different fibers are being spliced. Fiber geometry is the main difference - mode field diameter in SM and core diameter in MM - but it can also be a matter of the fiber composition.This happens if two different manufacturers’ fibers are joined or bend-insensitive (BI) fiber with differences in depresssed cladding geometry are joined.
If you test bidirectionally with sufficient accuracy, you can see the difference. It’s an OTDR trace that most people are familiar with  -  if you see a gainer, you shoot the other direction and average to get the “actual” splice loss. When splicing or connecting different types or manufacturers SM fiber you may see directional differences of up to 0.3dB or more. Same for MM, not only for differences in core diameter but also for connecting BI to non-BI fibers. While those OTDR measurements are actually differences in backscatter levels, they are indicative of real differences in connector loss or splice loss in opposite directions.
Are those differences enough to be of concern? That’s a judgment call.
We’ve found most people do their bidirectional testing all wrong. With an OTDR, you should disconnect the instrument, not your launch and receive cables, and take just the instrument to the other end. Disconnecting the launch and receive cables changes the fibers at the connections to the cable under test and you lose the connection you want to test from the opposite direction.
Bidirectional testing with a test source and power meter is more confusing. You have to do your “0 dB” reference, check the launch and receive reference cables, measure the cable under test, then move just the source and meter to the other end, test the cable under test again, then disconnect and then measure the output of the source to get the “0 dB” reference used in that direction. That’s confusing!
Even if you do test bidirectionally, you do not get the “actual” connection or splice loss like everybody says - you get the average of the two directions. Unless you are willing to do a lab setup and some careful testing, that’s the best you can do.
Now we are into measurement uncertainty. If the measurement uncertainty is around the same as the typical bidirectional difference, does bidirectional testing gain you that much?
That’s a judgement call.

The FOA book on testing goes into this - a complete chapter is devoted to measurement uncertainty.





When To Test Fiber
Q: Should testing of the fiber plant be done before the Optical Network Terminal is installed?
A: Fiber optic testing is generally done when the cable plant is installed to confirm proper installation and check that the performance is adquate for the electronics planned for use on it.
There are several processes - first an overview of testing:
Check cable before installing - continuity if it looks OK, OTDR testing if the reel is damages
Test installed cable when splicing - check the fusion splicer estimate of loss and do OTDR testing if questionable and inspect every connector as termination is done to confirm the connector is good.
Test cable plant after splicing and termination - end-to-end insertion loss and OTDR testing for longer OSP (outside plant) links.
New singlemode cable plants for high speeds may need "fiber characterization” - adding in CD (chromatic dispersion), PMD (polarization mode dispersion) and SA (spectral attenuation for DWDM wavelengths) testing. If one is considering upgrading a cable plant that is already installed or has been used, these same tests should be done - inspection/cleaning, insertion loss, OTDR.
When the system is installed, one should know it should work because of the testing done during installation. One should inspect and clean patchcords before installation and test them if suspect. In fact any connector needs inspection/cleaning before hooking up equipment. Dirt is the biggest problem with fiber optic systems.

Negative Loss?
Q: We were sent this OTDR trace and asked why some traces showed negative loss - gains.

A: The issue with this OTDR test of a factory-manufactured patchcord was not straightforward. To summarize, the patchcord was plugged directly into the OTDR port, with no launch cable. The OTDR has no reference for measuring the loss of the first connector on the cable nor the second connector since neither launch or receive cables were used. Without a launch cable, the OTDR is trying to get information from the connection to the instrument itself which is basically impossible since it’s suffering from overload caused by the test pulse - even with APC connectors. The OTDR (Yokogawa) is one of the cleanest OTDRs at the interface (we have one on loan at FOA right now) but it’s still not designed to measure that loss. Furthermore, using the instrument interface to plug in every connector will wear that connector in the unit out quickly and require servicing by the factory. The second issue is the difficulty of measuring on short cables like this. Note the vertical digital resolution of the display and think about the location of the second marker - it’s measured dB value will jump around as it goes from digital segment to the next digital segment. When you use a launch cable and measure the loss of the connectors using the “4-point” measurement - also called “least squares approximation” in the FOA Guide to OTDRs. That will overcome the digitization errors as well as the settling times of the pulses.

FOA consistently makes the point that the most important part of a network design or installation is the documentation. It tell the installers what to build and records their test results. It helps troubleshoot faults if a cable plant problem arises and restoration becomes necessary.

Documentation is also a big help when testing with an OTDR, as two technical inquiries this month showed. One concerned the testing of a cable still on the reel before installation. In this case, just knowing the exact length of the cable or fiber would be helpful.

Here is the first trace sent to FOA:

There was nothing wrong with the fiber.

The second trace was for a longer fiber, still a cable on a reel before installation:

The question was why was there a "gainer" in the middle of the fiber. The trace sent to FOA showed a noisy trace with a "gainer" at around 5500 feet. Again there seems to be no problem with the fiber, but the OTDR setup caused the noisy trace and that "gainer" was because the noise exceeded the threshold for events set in the OTDR. Redoing the trace with a longer test pulse or longer integration time should reduce to noise. In this case, just knowing the fiber was continuous with no splices tells you a gainer (caused by the joint between two different fibers) was not the cause of the event. And looking at the noise on the fiber says the OTDR is either not properly set up or defective.

Optical Loss: Are You Positive It’s Positive? A question we get often.

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!


Well the real problem is that to understand this you need to understand logarithms and that’s Algebra II*, way beyond fourth grade addition and subtraction. You see dB is defined as a logarithmic function:



With logarithms, if the ratio of measured power to reference power is greater than 1, e.g. measured power is more than reference power, the log is positive. If the ratio of measured power to reference power is less than 1, e.g. measured power is more than reference power, the log is negative. If the ratio is 1, the log is 0.

Since the logarithm for optical power ratio is base 10 and then multiplied by 10, each change of 10 in the ratio of the measured and reference power becomes a change of 10dB. E.g. +10 dB is a factor of 10 (10 times log10 10 which is 1), +20dB is a factor of 100  (10 times log10 100 which is 2), +30dB a factor of 1000  (10 times log10 1000 which is 3)and so on. Negative dB means division, so -10 dB means a factor of 1/10th  (10 times log10 0.1 which is -1), -20dB a factor of 1/100th  (10 times log10 0.01 which is -2) and so on. 0 dB means the measured power to reference power ratio is 1 – they are equal.

Let’s try a graphic explanation of this equation. Take a look at this “semi-log” graph (logarithmic on the x axis and linear on the y axis) of dBm vs optical power in the range commonly used for fiber optics and calculated with our equation above. Remember 0 dBm means all power is referenced to 1 milliwatt optical power.



As you move to the right, power increases and the value in dBm gets more positive – that would be gain. So from 1mw to 10mw, we see a gain from 0dBm to +10dBm or 10dB, a positive change.

As you move to the left, to lower optical power, as would be loss, the dBm value gets more negative. From 1mw to 100microwatts (that’s 1/10mw), we go from 0dBm to -10dBm, or -10dB; that negative change indicating a loss of 10dB.

That shows gain is positive dB and loss is negative dB. Now we’re getting to the fourth grade math.*

How about an example? Let’s say we decide to test a singlemode cable plant. We start with a laser source and launch cable which we measure our reference level for loss with a power meter to have an output of 0dBm. That’s 1 milliwatt of power, about the normal output of a fiber optic laser. After we attach the cable plant to test and a receive cable to our power meter, we measure 3dB loss.

What power did we measure? The power must be lower, of course, since we have loss, and 3dB is approximately a factor of 2, so the power the meter measured is 1mw divided by 2 = 1/2milliwatt or 0.5mw. Since our power meter is measuring in dBm, it will read minus 3dBm (-3 dBm), since lower optical power is always more negative. If it read +3dBm, the power measured would be 2mw and that would be a gain from our reference (0dBm) which we know is incorrect – passive cable plants are not fiber amplifiers.

Here is the graphical version of this loss test:

 

And then there is 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.

No question – loss means a more negative power reading in dB and a negative number in dB indicates loss.**

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? 

Perhaps we should blame accounting.

Suppose you have a company that has $1million in sales and $900,000 in expenses. What’s the profit? It’s $1,000,000 - $900,000 = $100,000. That’s a profit, right?

But suppose your company has $1million in sales and $1,100,000 in expenses. What’s the bottom line? It’s $1,000,000 - $1,100,000 =  - $100,000. Wait a minute, that is a negative number – that’s not a profit, it’s a loss.

So in accounting, profits are positive numbers and losses are negative numbers when we do the math, but when we talk about loss, we don’t say we have a loss of “-$100,000,” we just have we have a loss of $100,000. Then we’ll put that number in parentheses when we publish our P&L like this ($100,000) and hope it doesn’t get noticed by investors, but you know it will.

Loss and gain in fiber optic measurements are similar. If you are using a separate source and power meter, loss will be a negative number and gain will be a positive number. But because of convention, we sometimes drop the signs when we report the values because loss always means the optical power measurement was negative and gain means the optical power measurement was positive. But maybe that’s not what the convention has evolved to.

Optical loss test sets (OLTS) aren’t designed to measure and display optical power, just loss. The actual power measured is lost in the algorithms used for calculating loss based on the “0dB” reference power and the measured loss. Long ago, most OLTS measured loss and displayed it as a negative number, but some companies who got into the fiber optic test equipment business from other test businesses arbitrarily decided to display loss as a positive number, and today most OLTS do show loss as a positive number. But when the instrument sees a gain, which it can do if improperly used, it therefore displays a negative number, which can be very confusing to a trained fiber tech who understands fiber optic power and loss measurements.

OTDRs do the same thing. I looked at traces from a half-dozen OTDRs and all showed loss as a positive number and gain as a negative number. And yes, when you have a gainer in one direction, they show it as a negative number. Telling them that is wrong will fall on deaf ears, I’m afraid.

The same article/blog post goes on to discuss optical return loss and reflectance, which has similar issues but they get it more or less right, which is confusing. Why can they understand that more negative numbers for reflectance means lower power in the reflectance but claim the opposite for insertion loss?

The “less right” is that with most OTDRs reflectance of an event and optical return loss (ORL) are not the same thing. ORL is the summation of all reflectance events and fiber backscatter from the entire length of fiber.

And, please, please stop saying “back reflection;” a reflection always goes back toward the source so the term is redundant and was dropped from fiber optics years ago.

*So the problem is not simply fourth grade math, it also involves a bit of convention and tradition and marketing. And it requires understanding logarithms that create the negative number of loss. That’s more like Algebra II or 7th grade math, and here is a good tutorial from Kahn Academy on that: https://www.khanacademy.org/math/algebra2/exponential-and-logarithmic-functions/introduction-to-logarithms/v/logarithms

And more basic, here is a tutorial on adding and subtracting negative numbers https://www.khanacademy.org/math/cc-seventh-grade-math

** 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.

The FOA has an explanation of dB on our online Guide and a couple of graphics that illustrate what happens with loss.

Note: 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!

Education/Learning

Have An Old Cell Phonet? Use it to test for infrared light in fiber optics. See the video on

 

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The FOA was chartered to "promote professionalism in fiber optics through education, certification and standards." Our focus on creating a professional workforce to properly design, install, maintain and repair communications network infrastructure has led us to work with groups in many different areas of technology that use fiber optics, way beyond the basic telecom applications that most of us think of first. FOA has probably worked with most of the potential applications of fiber optics, but we're always learning about new ones!
In addition, we get lots of calls and emails from our members looking for information about where the jobs are and how to train for them. FOA has created three ways to help you find jobs, train for them and apply for them.

Where Are The Jobs In Fiber Optics?
FOA has created a 20 minute YouTube video that talks about all the applications for fiber optics, what jobs are involved and the qualifications for the workers in the field. Besides telecom and the Internet, we cover wireless, cable TV, energy, LANs, security, etc. etc. etc. It's a quick way to get an overview of the fiber optic marketplace and we give you an idea of where the opportunities are today.

Watch the new FOA YouTube Video: Where Are The Jobs In Fiber Optics?

What Training Is Needed For The Jobs In Fiber Optics?
As you will learn from the video described above, the jobs in fiber optics are quite diverse. FOA has investigated these jobs to understand the needs of workers for those jobs and, when necessary, create curriculum and certifications to properly train workers. For example, the FOA FTTx certification was developed at the request of Verizon who needed specialized installers for their FiOS program. Now we are working with the industry on the OLAN (Optical LAN) program (see below).
We have summarized the jobs and required training in a new web page that has two uses - 1) If you have FOA certifications, what jobs are you specifically qualified for? - 2) If you are working in a specialized field or want to get a job in that area, what training and certifications will qualify you for those jobs?
What Training And Certifications Are Needed For Jobs In Fiber Optics? 

How To Find And Apply For Jobs In Fiber Optics
We get many questions from CFOTs, students at FOA-Approved schools and others contemplating getting into the fiber optic business regarding jobs in fiber optics - and how to find them - so we’ve created a new web page to share some information we've gathered about jobs in our industry. The information is designed to help you understand what jobs are available in fiber optics, how to find them and apply for them.
If you are looking for a job in fiber optics, here is the FOA's guide to jobs. 

We hope you find this useful. FOA tries to find new to increase the professionalism in our industry and helping qualified people find jobs is our highest priority - read the article below to see why! If you have feedback on how we can help you and our industry, contact us at info@thefoa.org.