The Fiber Optic Update

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Technology

They've ALL Got It All Wrong - And They Confuse A Lot Of People - YOU CANNOT STRIP THE CLADDING OFF GLASS FIBER!!!

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:

- really! - on the FOA Guide page on optical fiber.

No wonder everyone is confused. Practically every drawing shows the core and cladding being separate elements in an optical 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?

Bottom Line:

• Most diagrams of fiber construction are wrong - showing core and cladding as separate - but they are one solid peice of glass.
• You cannot strip the cladding from glass fibers.

Connector Loss For Splice-On Connectors

FOA received a call from a contractor working on a network. His subcontractor doing termination presented data on terminations using mechanical splice-on connectors where he claimed the TIA standard for these connectors was 0.75dB for the connector PLUS 0.3dB for the splice, for a total of 1.05dB. He wanted to know if this were true.

No, it is not true. These connectors have an internal splice to a stub fiber already glued in the ferrule and factory polished. The loss of the connector used to terminate a fiber must include the splice since it is the termination method and there is no way to test it separately from the connector itself.

Cable Bend Radius

All fiber optic cables have specifications that must not be exceeded during installation to prevent irreparable damage to the cable. This includes pulling tension, minimum bend radius and crush loads. Installers must understand these specifications and know how to pull cables without damaging them.

And When An Installer Gets it Wrong

There are two problems here, one visible and one hidden. The visible one is the pulley mounted on the side of the truck used to change the direction of the cable to allow using the capstan mounted on the rear of the truck. The cable is being bent about 120 degrees over a pulley that appears to be about 120mm (5 inches) diameter. That's a radius of 60mm or 2.5 inches. That pulley looks like a stringing block uses for stringing ropes when pulling in power lines.

We believe the cable was a 864 fiber ribbon cable with a diameter of 24mm (0.92") with a minimum bend radius of 360mm or 14".  That means the pulley the cable is being pulled over is ~1/6th the size it should be - shown by the dotted red circle above.

The second problem is the angle of the cable coming out of the manhole. It is exiting a conduit and being pulled almost straight up out of the manhole. If there is no hardware in the manhole, the cable is being pulled over an edge exiting the conduit or the manhole, bending with a very, very small radius.

One can only speculate about the possible damage to a cable when treated like this. What comes to mind first is broken fibers, and that is a possibility. But bending this tightly can also damage the cable structure, including the fiberglass stiffeners, strength members and jacket. Compromising the integrity of the cable reduces its protection for the fibers. Even the fiber ribbons can be delaminated and fibers put under stress. A cable pulled under these circumstances can have damage along the entire length, not just a point where it was kinked.

What should have been done on this pull? The 120mm/5" pulley should have been replaced with one at least 6 times larger. The truck could have been further from the manhole (and maybe turned to be inline with the pull) so the angle of the cable exiting the conduit was less. Hardware should be attached to the conduit to provide a proper bend radius for the cable as it exited the conduit and the cable should have been protected if it contacted the edges of the manhole..

• All cables have specifications for minimum bend radius
• Violating this spec may permanently damage the cable
• Bend radius is generally 20X cable diameter under tension - 10X after installation
  

Optical Loss: Are You Positive It’s Positive?

FOA created this short movie on the FOA Guide page explaining dB showing how a power meter shows loss when a cable is stressed to induce loss:



As the fiber is stressed, inducing loss, the power level goes from -20.0 dBm to --22.3 dBm.That's a more negative number. (-22.3dB) - (-20.0dB) = -2.3dB That's basically 4th grade math.

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

• Loss in dB is a negative number
• Instruments that measure loss do not display negative signs with loss
• Gains are displayed with a negative sign

dB or dBm -Still Confusing 4/2020 -

The second most missed question on FOA/Fiber U online tests concerns dB, that strange logarithmic method we use to measure power in fiber optics (and radio and electronics and acoustics and more...). We've covered the topic several times in our Newsletter but there still seems to be confusion. So we're going to give you a clue to the answers and hopefully help you understand dB better.

These are all correct statements with the percentage of test takers who know the answer is correct.

The most answered correctly: dBm is absolute power relative to 1mw of power (78.8% correct. Does "absolute" confuse people? It's just "power" but absolute in contrast to "relative power" which is loss or gain measured in dB.)

This one is answered correctly less than half the time: dBm is absolute power like the output of a transmitter. (41.5% correct, see comment above.)

This one does often get answered correctly: The difference between 2 measurements in dBm is expressed in dB. (23.8% correct)

Here is an example of a power meter measuring in dBm and microwatts (a microwatt is 1/1000th of a milliwatt.)

What's That Fiber?

A FOA Newsletter reader sent FOA these microscope photos of two MM (multimode) fibers, asking what was the difference with the one on the right. It is a bend-insensitive (BI) fiber and compared to the regular graded-index MM fiber you readily notice the index "trench" around the core that reflects light lost in stress bends right back into the core. You can read more about bend-insensitive fiber in the FOA Guide.

While researching the answers to the question above, we talked to Phil Irwin at Panduit. He mentioned that you could see the structure of BI fiber and sent along this photo:
   
At the left, you can see the gray area surrounding the core, shown in the drawing in the right as the yellow depressed cladding region.

If you want to try to see it yourself, it's not easy. Phil tells us that OFS fiber is the easiest to see, Corning a bit more difficult. You need a good video microscope. You may need to vary the lighting and illuminate the core with low level light.

The Perils Of 2-Cable Referencing

FOA received an inquiry about fluctuations in insertion loss testing. The installer was using a two cable reference method for setting a "0dB" reference where you attach one reference cable to the source, another to the meter and connect them to set the "0dB" reference. The 2-cable reference method is allowed by most insertion loss testing standards, along with the 1- and 3- cable reference methods, although each gives a different loss value.


3 different ways to set a 0dB reference for loss testing

When a 1-cable reference is used, one sets a reference value at the output of the launch cable and measures the total loss. With a 2-cable reference, a connection between the launch and receive reference cables is included in making the reference, so the loss value measured will be lower by the amount of that connection loss. The 3-cable reference includes two connection losses so the loss will be lower still.


The problem with the two cable reference is the uncertainty added by including the connection between the two reference cables when setting the

• The value of loss you measure depends on how you set your "0dB" reference - more reference cables means less loss.
• Connections between reference cables when setting a 0dB loss add uncertainty to measurements

Troubleshooting With A VFL:  Fibers Damaged In Splice Trays

Is this a trend? Twice in one week, we have inquiries from readers with problems and both were traced to fibers cracked when inserted in splice trays. The photo below shows one of them illuminated with a VFL. This was the same issue we found in the first field trial of a VFL more than 30 years ago that led to its popularity in field troubleshooting.

• VFLs are invaluable troubleshooting tools for finding cable faults
• But only work close by - 3-4km range max

How "Fast" Is Fiber?

We've probably all heard the comment that fiber optics sends signals at the speed of light. But have you ever thought about what that speed really is? The speed of light most people think about is C = speed of light in a vacuum = 300,000 km/s = 186,000 miles/sec. But in glass, the speed is reduced by about 1/3 caused by the material in the glass. The light is slowed down and the amount is defined as the index of refraction of the glass. V= speed of light in a fiber = c/index of refraction of fiber (~1.46) = 205,000 km/s or 127,000 miles/sec. So in glass, the "speed of light" is about 2/3 C, the speed of light in a vacuum.  And the difference in speed in different materials is what makes fiber work - causing "total internal refraction".

One of the FOA instructors sent us this question:  "I work with at Washington Univ with an engineer who works for an electrical utility. He asked a question about the speed of signal transmission over fiber optics, single mode, at top of towers. They need signal to be sent in 18 millisecs for relays to function properly. Is there a problem over a distance of 150 miles?"



Let’s do a calculation:

C = speed of light in a vacuum = 300,000 km/s = 186,000 miles/sec
V= speed of light in a fiber = c/index of refraction of fiber (~1.46) = 205,000 km/s or 127,000 miles/sec
 
150 miles / 127,000 miles/sec = 0.00118 seconds or ~1.2 milliseconds

Another way to look at it is 127,000 miles/sec X 0.018 seconds (18ms) = 2,286 miles

So the fiber transit time is not an issue. The electronics conversion times might be larger than that.

I used to explain to classes that light travels about this fast:

300,000 km / sec
300 km / millisecond
0.3km /microsecond or 300m / microsecond
0.3 m per nanosecond - so in a billionth of a second, light travels about 30cm or 12 inches

Since it travels slower by the ration of the index of refraction, 1.46, that becomes about 20cm or 8 inches per nanosecond.

That is useful to know since an OTDR pulse 10ns wide translates to about 200cm or 2 m pr 80 inches (6 feet and 8 inches), giving you an idea of the pulse width in distance in the fiber or an idea of the best resolution of the OTDR with that pulse width. 

Bottom Line

• Fiber is "fast" because of its bandwidth capability
• Light travels in fiber at the speed of light
• But the speed of light in glass is only 2/3 as fast as the speed of light in air or a vacuum

What Does A FTTH ONT Look Like Today?

That's all there is to the ONT that goes into the home. The arrow points to the 1310 TX/1490 RX transceiver for SC-APC connectors.

Note: That device is now priced at $10-20US. The incredibly high volume of FTTH components has drive costs of these devices down to incredibly low levels. Today media converters for SM fiber are as priced the same as MM. Multimode fiber is beginning to look obsolete.

Components

Here are several technologies that have continued growing in importance in the fiber optic marketplace  -  components that every tech needs to learn about and become familiar with their use.

How Many Fibers? - What's The Optimal Cable Size?

The idea often arises to reduce the number of fibers in a cable and therefore reduce the cable cost, assumed to be important on long cable runs. But is the cost of fiber such a big part of the cost of the cable plant? We decided to analyze cable costs for standard loose tube cable capable of being pulled into conduit for underground or lashed to a messenger for aerial installation.

Gathering data was not easy, but we found several large, reputable US distributors who listed prices for several types of loose tube singlemode OSP cables from top cable makers. All prices are for small quantities (km, not 10s or 100s of km).  Prices are how they were quoted, in $US per foot, so our readers outside the US should feel free to convert into another currency and meters.

This graph shows what we found:

Micros: Microcables, Microducts and Microtrenching

144 fiber Corning MiniXtend cable is smaller than a pencil

MIcrocables, microducts and microtrenching - three technologies that have more in common than the prefix "micro" are gaining in acceptance along with blown cable, the obvious method of installation using them. Smaller is always better when it comes to crowded ducts, especially in cities where duct congestion is a problem in practically every city we have contact with.

Bottom Line:

• Like everything else, cables keep getting smaller
• Work well with microducts and microtrenching
• Installers need to become familiar with "blown cable" technology
  
• They are already accepted in the marketplace
  

Fiber Ducts

With the demand for more fiber for smart cities services like small cells and smart traffic signals, not to mention a ton of other smart cities services, installing more cables in current ducts - without digging up streets - is a major interest. Sometimes it's possible to install microducts in current ducts with a cable and blow in a new microcable. Sometimes it's worth it to pull an older cable out and install a new microduct that will accommodate 6 cables, making room for future expansion. The makers of the fabric ducts, Maxcell, can even show you how to remove the ducts in conduit without disturbing the current cables and pull in fabric ducts to install more cable.

Comparison of MaxCell ducts to rigid plastic duct



Microducts are small ducts for blowing in cable. In the size of a traditional fiber duct, you can get 6 microducts for 6 288 fiber cables.


Microducts And Microtrenching


Nearly invisible microtrenching

If you have to trench, microtrenching is probably the best choice for cities and suburbs. Rather than digging wide trenches or using directional boring (remember the story about the contractor in Nashville, TN using boring to install fiber who punctured 7 water mains in 6 months?), microtrenching is cheaper, faster and much less disruptive.

All of this implies that contractors are willing to invest in new machinery and training, sometimes an optimistic assumption. Microtrenching machines and cable blowing machines are available for rent, but personnel must be trained in the design of networks using these technologies and operating the actual machinery in the field. That's still a considerable investment.

• Cables and ducts are getting smaller allowing more and more fibers in the same space
• Microtrenching allows "construction without disruption"

High Fiber Count Cables

More manufacturers are introducing high fiber count cables - 864, 1728, 3456 or even 6,912 fibers. Like this one from Prysmian with 1728 fibers: The applications were first in large-scale data centers but are also seeing use in dense urban centers to support FTTH and cellular small cell systems.



These cables use bend-insensitive fibers to allow high density of fibers without worrying about crushing loads affecting attenuation. Most also use fibers with 200 micron buffer coatings instead of 250 micron buffer coatings to allow even higher density. Many, or even most, use ribbons of fiber, either the conventional hard ribbons or the newer flexible ribbons, since, as we show below, the time to splice even a 1728 fiber cable is extremely long unless ribbon splicing is used.

High Fiber Count Cables may not be for everyone. Maybe only for a very few. A single cable that has as many fibers as 12-144 fiber cables (1728 fibers) in a cable with a diameter of only twice that of a conventional 144 fiber cable can present challenges.

• First of all, the cost - it's high. You do not want to waste cable at this price. Engineering the cable length precisely will save lots of money.And it's worse for higher fiber counts.
  
• Likewise, making mistakes when preparing the cable for termination can be expensive.
• The cable may require special preparation procedures to separate fibers for termination. Most use new methods of identifying cables and bundles.
  
• Besides skill, the tech working with high fiber count cables needs lots of patience.
• Splicing multiple cables at a joint can get complicated keeping all fibers straight.
  
• These cables will generally use 200 micron buffered fiber and often a flexible ribbon instead of a typical rigid ribbon structure to reduce fiber sizes. This may complicate splicing as the methodology to splice the flexible fibers and splice 200 micron fibers to regular 250 micron fibers is a work in progress.
• Splicing 200 to 250 micron fibers may be easier with the flexible ribbon designs which make it easier to spread fibers to the same spacing.
  
• We've heard the splicing time for flexible ribbons is about 50-100% longer than that of conventional rigid ribbons. So if you use that table below, you may need to increase your ribbon splicing estimates when working with flexible ribbons.

Corning generously sent FOA some samples of 1728 and 3456 "RocketRibbon^TM" cable. We took some photos and must admit that these cables are fascinating updates on the traditional fiber optic cables.

These are the tubes of ribbons from these cables. Each of those tubes of ribbons has the equivalent of 24 ribbons of 12 fibers each (actually 8 X 12 fibers and 8 by 24 fibers stacked up) for 288 fibers total. The 1728 fiber cable has 6 tubes and a center foam spacer, with 144 ribbons total. The 3456 fiber version has 12 tubes and no spacers, 288 fiber ribbons total.

What amazes us is the density of fibers.



We calculated the "fiber density" of this 3456 fiber cable based on 200 micron buffered fibers and determined that 54% of the cable is fiber. Compare that to a typical 144 fiber loose tube cable, which is about 14% fiber or a 144 fiber microcable which is about 36% fiber.

Looking at the end of this cable reminded us of nothing so much as this PR photo from AT&T from their intro of fiber in 1976:



Not the fiber, the dense cable of copper pairs!

Of course the cable is much lighter than copper but much heaver than you are used to with fiber - it weighs 752 kg/km or about 1/2 pound per foot. And it's stiff. Very stiff. The minimum bend radius is 15 times the cable diameter or 480mm (~19 inches), about a meter or yard in diameter.

As we noted in the photo above, Ian Gordon Fudge of FIBERDK taught some data center techs how to handle a 1728 fiber Sumitomo cable with a slotted core. Ian sent FOA this photo to illustrate the number of fibers in the cable he was using for training. Impressive!



Here is the slotted core that separates the flexible fiber ribbons

:



More on high fiber count cables and our continuing coverage. 

High fiber count cables are all ribbon cables, some with hard ribbons and some with flexible ribbons, All require ribbon splicing because of the construction and the time it would take to terminate them. This is a table of estimated termination times. Is that realistic? We've heard the flexible ribbons may take

High Fiber Count Cables - Continued Updates - Installation

Continuing our ongoing research on high fiber count cables, last month we were invited to visit Corning's OSP test and training facility to experience the processes of installing these cables for ourselves. We had the opportunity to handle some of these cables ourselves and see how experienced techs worked with this cable.

Once you get a chance to handle this cable and see how big, stiff and heavy it really is, you understand that it's quite different from any fiber optic cable you have worked with, with the possible exception of some hefty 144/288 fiber loose tube cable that's armored and double jacketed. With a bend radius of 15X the diameter of the cable, the minimum bend radius of a 1728 fiber cable is 15" (375mm) and that's a 30" (750mm - 3/4 of a meter) diameter. Just the reel it's shipped on is outsized - it should have a ~750mm (30 inch) core and will be probably ~1.8m (6 feet ) in overall diameter. 3300 feet (1km) of this cable will weigh 550-750kg (1200-1700 pounds.) and the reel will weigh another ~300-400kg (700-900 pounds). Will that fit on your loading dock? Can you handle a ton of cable? (Metric or English)

I tried bending one of the 1728 fiber cables and (with the manufacturer’s OK) tried to break it. The 1728 fiber cable I was bending took an enormous amount of muscle to bend, and when I got down to about an 8 inch radius, it broke, with a sound like a tree limb of similar diameter cracking. In the field, that would have been an expensive incident.

The stiffness of these cables affects the choice of other components and hardware. You will not fit service loops into a typical handhole, you need a large vault like the one shown in the photos taken at Corning. You will also need close to 100 feet (30m) of cable for a service loop. You may need to figure 8 the cable on an intermediate pull and that will require lots of space and a crew to lift the cable to flip it over.

This 1728 fiber cable is stiff, does not easily twist and only bends in one direction because there are stiff strength members on opposite sides of the cable. Placing it into a manhole or vault and fitting service loops into it is not easy. In this case, it helped to have two people and one was the trainer. You need to have a "feel" for the cable - how it bends and twists - to make it fit. The limits of bend radius, stiffness and unidirectional bending makes it necessary to work carefully with the cable to fit loops into the vault. Sometimes it's necessary to pull a loop out and try in a different way to get it to fit. But it can be done as you see at the right.


 

Pulling the cable out of conduit in the vault without damaging it also requires care. You can see in the back the orange duct coming into this vault. When pulling the cable, it's important to not kink the cable while pulling it out of a duct. A length of stiff duct can be attached to the incoming duct to limit bend radius. Capstans, sheeves and radius cable sheaves need to be chosen to fit the required cable bend radius. A a radius cable sheave with small rollers can damage the cable under tension and are bot a good choice unless the rollers are used with a piece of conduit to just set the bend radius.

Corning also showed us a new feature of their RocketRibbon Cables. A high fiber count cable has a lot of fibers, even a lot of ribbons, so identifying ribbons can be a problem. In addition to printing data on each ribbon, Corning now tints the ribbons with color codes to simplify identification. Great idea.

Tight Fit: 6912 Fiber Cable Pulled in 1.25 inch Conduit

Furukawa Electric Co., Ltd. (FEC) conducted an experiment in its Mie, Japan facility to demonstrate the installation of a 6912-fiber optic cable with an outer diameter of 1.14 inches (29 mm) in a 696 foot (200m) long conduit with three 90 degree curves and an inner diameter of 32mm. The conduit used was a standard product installed in conventional data center campuses. Engineers confirmed a maximum pulling tension of 84 pounds (372N), well below the maximum pulling tension of 600 pounds (2700N) specified for the cable.

 

The cable was installed in a 1.25 inch (32mm) conduit with a maximum length of 1,411 feet (430m) in a North American data center campus in 2020 to support live traffic. The high fill ratio in this application is not typically recommended for Outside Plant (OSP) cable installation. However, in this application, the end-user was willing to accept the installation risk in return for maximum fiber density. The installation demonstrated that FEC’s 6912 fiber optic cable can be successfully installed into 1.25 inch (32mm) conduit using appropriate tools, work procedures, and optimum installation conditions.

“The FEC 6912 fiber optic cable at least doubled the fiber count possible in a 1.25 inch conduit, compared to competing available designs,” said Ichiro Kobayashi, General Manager of optical fiber & cable engineering department, FEC.

• High fiber count cables allow extremely high fiber counts in small cable sizes, perfect for dense applications in data centers and metro areas
• With so many fibers, ribbon splicing is the only sensible way to splice them
• Ensure you splicing machines can handle 200micron buffer fibers
• Because bend radius limits are so high, they require special consideration for installation and storage - BIG manholes for example

Cable Marking Mystery

FOA has been reaching out to people at cable companies to see if anyone has a definitive answer as to what this symbol means, and the answer comes from Rodney Casteel and his engineers at Commscope.

"The handset symbol is mandatory for cables “suitable for direct burial applications” per ICEA 640 and Telcordia GR-20.  I think this handset symbol started a long time ago so data cables could be identified if they were dug up. My guess is the first standard to mandate this was Telcordia GR-20 Issue 1 back in the 80s."

From ICEA-640:




And Bellcore/Telcordia GR-20:

New Connectors

SENKO CS and SN
In the FOA Newsletter for January 2018, we featured the SENKO CS connector, a miniature duplex connector using two 1.25mm ferrules, but much smaller than a duplex LC. The CS is sell on its way to becoming standardized with a FOCIS (fiber optic connector intermateabliity standard), but on the SENKO web page, there is another new connector, the SN, that makes the SC look huge! The big difference is the vertical format that allows stacking connectors very close. That can allow transceivers to have more channels, a big plus for data centers. Here is more information on the SENKO CS and SN connectors.


SENKO


Comparison of SENKO CS (L) and SN (R) connectors with duplex LC.

US Conec MXC and MDC Connectors
The R&M and 3M expanded beam multifiber connectors reminded us that US Conec introduced the

MXC connector over 5 years ago, using similar technology for up to 64 fibers per connector. The MXC is on the US Conec website, but seems to be aimed at board level connections, not far off its original purpose as a connector for silicon photonic circuits. But when we checked the US Conec website, there was a connector name we dis not recognize, the MDC. The MDC (below) is a vertical format duplex connector using 1.25mm ferrules that looks similar to the SENKO SN above. Here is information on the US Conec MDC duplex connector.

US Conec

Its All About The Data Center
Just like the high fiber count cables discussed above, the CS, SN and MDC connectors are aimed at high density cabling and transceivers for data centers. All three are specified for the new QSFP-DD pluggable transceiver multi-source agreement.


Bottom Line:

• Like everything else, connectors keep getting smaller
• Too early to determine if they will be accepted in the marketplace and can compete with LCs

Splice-On Connectors

Termination has been seeing greater acceptance of the SOC - splice-on connector - using fusion splicers. It's popularity started in data centers for singlemode fiber where the number of connections is very large so the cost of a fusion splicer is readily amortized and the speed of making connections is the real cost advantage. The performance of SOCs is much better than prepolished/splice (mechanical splice) connectors simply because of the superiority of a fusion splice and the cost of the SOCs are much less since they do not have the complex mechanical splice in the connector.

• Splice-On Connectors (SOCs) are easy to install, low loss and low cost
• Less hardware than pigtail splicing
• Premises or OSP

Splice-On Connector Manufacturers and Tradenames   7/2020

Installation

Midspan Access - Simplifying Installation Of Drops

Many installations involve dropping a small fiber count cable from a large backbone cable. Backbone cables of 144-288 fibers are common and larger ones are becoming more common too. Drop cables are often only 2-14 fibers, meaning most fibers are continuing straight through the drop point. Midspan access involves opening the cable by removing the jacket and strength members, opening the buffer tube and splicing only the fibers being dropped at that point. The untouched buffer tubes from the opened cable are carefully rolled up and stored in the same splice closure as the fibers that will be separated and spliced to a drop cable.



If there is a method of splicing only the 4 drop fibers instead of the 144 fibers, we will only have 4 splices instead of 144 or 146 depending on the architecture of our system. The difference is according to how the drop is configured.


If you are building a star network where every drop links back to the origin of the network, you will splice 4 fibers in the cable to the drop cable, leaving 4 splices on 4 fibers (instead of 144 splices if the backbone cable is cut and respliced.


If you are building a ring network, you may only be splicing two fibers going to the drop and two others that are continuing along the ring network.

All this may seem obvious but in actual practice requires some knowledge, skills and careful workmanship. To do a proper job. Fortunately, manufacturers of cables and tools have good information available online on how to do it, and FOA Master Instructor Joe Botha has provided FOA with a application note on how midspan access is done in his classes also.

The basic process is simple. We will look at a loose tube cable but processes exist for ribbon cables also. You remove the jacket of the cable for a specified length according to the cable type and splice closures used. After removing the cable jacket, you remove unnecessary strength members, leaving enough of the stiff central member on both ends to attach to the splice closure. Identify the tube with the fibers to be spliced to the drop cable and set aside while carefully coiling the other tubes for storage in the closure.

To open the buffer tube, you need a midspan access tube that shaves off a section of the tube to allow removal of the fibers without damaging them. Here two types of Miller tools that shave the tube:

 

Nanotrenching Failure In Louisville, KY

Google FIber tried a new way to install cable in Louisville, KY, that turned out to be a very expensive failure. Nanotrenching is what some call very shallow trenching for installing fiber optic cable - see the photo below - and filling with rubber cement. It did not work.

Google Fiber ending service in Louisville

Chris Otts, WDRB Louisville,
Feb 7, 2019

LOUISVILLE, Ky. (WDRB) – Google Fiber is leaving Louisville only about a year after it began offering its superfast Internet service to a few neighborhoods, citing problems with the method it used to build the network through shallow trenches in city streets.

The shut off will happen April 15, said Google Fiber, a unit of Silicon Valley tech giant Alphabet, in a blog post Thursday.

Google Fiber has served about a dozen cities, and Louisville is the first it has abandoned."

Shortly after Google announced Louisville as a possible location in 2015, the Metro Council passed a utility pole ordinance at Google’s behest, then spent $382,328 on outside lawyers to defend the ordinance in lawsuits from AT&T and the cable company now called Spectrum.

Mayor Greg Fischer said in early 2016 that Louisville’s landing Google Fiber was “huge signal to the world.”

Louisville’s public works department allowed Google Fiber to try a new approach to running fiber – cutting shallow trenches into the pavement of city streets to bury cables.

It led to a lot of problems, including sealant that popped out of the trenches and snaked over the roadways.



Louisville street,

• Before you try some new idea, ask some experienced installers what they think

Manhole/Handhole Size

• Manholes need to be big enough for the cables they must contain
• They usually aren't!

Installation - Cleaning

Bad Advice

Was this perhaps another early April Fools' joke...like this one we ran several years ago about the wrong way to clean connectors:

Why You Clean Connectors Before You Make Connections

Brian Teague of Microcare/Sticklers send us this series of photos showing what happens when you make connections with dirty connectors. It speak for itself!

How To Backfill A Trench For Underground Construction

FCC Adopts One Touch Make Ready (OTMR) Rules For Utility Poles

Is OTMR A Good Idea?

OTMR has the potential to speed deployment of new communications networks if handled properly. However, one hopes the installers doing OTMR know what they are doing. We've heard so many horror stories about botched installations, cut fiber and power cables, punctured water mains and gas lines done by inept contractors that we just hope this doesn't cause even more trouble.

For example, here are 2 poles in the LA area where small cells are being installed. Can just any contractor handle OTMR on these poles?

 

Bottom Line

• OTMR may be problematic if contractors doing installation are not competent

Fiber Optic Testing

Test Sources For Multimode Fiber Testing

One of our schools recently asked up for recommendations on test sources for multimode fiber, wondering if the sources should be a LED or laser. Multimode test sources are always LEDs and these sources should be always used with a mode conditioner, usually a mandrel wrap. See here. This is how all standards for testing multimode fiber require test sources.

Years ago, as systems got faster and LEDs were too slow at speeds above a few hundred Mb/s. Fortunately 850nm VCSELs were invented to provide the solution for faster transmitters. But VCSELs were not good for test sources. They had variable mode fill and modal noise, so testers continued using LEDs for test sources, but with mode conditioners like the mandrel wrap that filtered out higher order modes to simulate the mode fill of an ideal VCSEL

The bigger issue with MM fiber is whether to test at both 850 and 1300nm. In the past, we did both because there were systems that used 1300nm LEDs or Fabry-Perot lasers for sources because the fiber attenuation was lower at 1300nm than 850nm. As network speeds increased to 1Gb/s and above, bandwidth became the limiting factor for distance, not attenuation.  VCSELs only worked at 850nm and all systems in MM basically have been switched to 850nm VCSELs.

We also used to test at both wavelengths because if a fiber was stressed, the bending losses were higher at 1300nm, so you could determine if a fiber had problems with stress. But since MM fiber has all gone to bend-insensitive fiber, that no longer works and the need or reason to test at 1300nm went away. It has not been purged from all standards yet however.

To complicate things, standards say that you should not use bend-insensitive fiber for test cables (launch or receiver reference cables) because they modify modal distribution, but it’s a moot point - practically all MM fiber is bend-insensitive so you have no choice but to use it. And most links will have BI to BI connections that should be tested. But we checked with some technical contacts and there are no specifications for BI fiber mandrels as mode conditioners.

Best solution - 850 LED with a mode conditioner on non-BI fiber (if you can find it - see above).

Bottom Line

• Multimode fiber needs testing with a 850nm LED source
  

Power Budgets and Loss Budgets

• A power budget is the amount of loss the link electronics can tolerate - transmitter to receiver. You use this to compare to the cable plant link loss budget when designing a cable plant to ensure the link will work on the cable plant design.
• The link loss budget is the estimated loss of the fiber optic cable plant including the loss of the fiber, splices and connections. You compare that to the power budget to ensure the link will work on the cable plant being designed, then again after installation to compare to test results.

Consider this diagram:

 
At the top of the diagram above is a fiber optic link with a transmitter connected to a cable plant with a patchcord. The cable plant has 1 intermediate connection and 1 splice plus, of course, "connectors" on each end which become "connections" when the transmitter and receiver patchcords are connected. At the receiver end, a patchcord connects the cable plant to the receiver.

Definition: Connection: A connector is the hardware attached to the end of a fiber which allows it to be connected to another fiber or a transmitter or receiver. When two connectors are mated to join two fibers, usually requiring a mating adapter, it is called a connection. Connections have losses - connectors do not.

Below the drawing of the fiber optic link is a graph of the power in the link over the length of the link.  The vertical scale (Y) is optical power at the distance from the transmitter shown in the horizontal (X) scale. As optical signal from the transmitter travels down the fiber, the fiber attenuation and losses in connections and splice reduces the power as shown in the green graph of the power.

Comment: That looks like an OTDR trace. Of course. The OTDR sends a test pulse down the fiber and backscatter allows the OTDR to convert that into a snapshot of what happens to a pulse going down the fiber. The power in the test pulse is diminished by the attenuation of the fiber and the loss in connectors and splices. In our drawing, we don't see reflectance peaks but that additional loss is included in the loss of the connector.

Power Budget: On the left side of the graph, we show the power coupled from the transmitter into its patchcord, measured at point #1 and the attenuated signal at the end of the patchcord connected to the receiver shown at point #2. We also show the receiver sensitivity, the minimum power required for the transmitter and receiver to send error-free data. The difference between the transmitter output and the receiver sensitivity is the Power Budget. Expressed in dB, the power budget is the amount of loss the link can tolerate and still work properly -

Margin: The margin of a link is the difference between the Power Budget and the Loss of the cable plant.




Determining The Power Budget  For A Link

Question: How is the power budget determined? Well, you test the link under operating conditions and insert loss while watching the data transmission quality. The test setup is like this:


Connect the transmitter and receiver with patchcords to a variable attenuator. Increase attenuation until you see the link has a high bit-error rate (BER  for digital links) or poor signal-to-noise ratio (SNR for analog links). By measuring the output of the transmitter patchcord (point #1) and the output of the receiver patchcord (point #2), you can determine the maximum loss of the link  and the maximum power the receiver can

From this test you can generate a graph that looks like this:

A receiver must have enough power to have a low BER (or high SNR, the inverse of BER) but not so much it overloads and signal distortion affects transmission. We show it as a function of receiver power here but knowing transmitter output, this curve can be translated to loss - you need low enough loss in the cable plant to have good transmission but with low loss the receiver may overload, so you add an attenuator at the receiver to get the loss up to an acceptable level.

You must realize that not all transmitters have the same power output nor do receivers have the same sensitivity, so you test several (often many) to get an idea of the variability of the devices. Depending on the point of view of the manufacturer, you generally error on the conservative side so that your likelihood of providing a customer with a pair of devices that do not work is low. It's easier that way.

Safety On The Job

Why We Warn You To Be Careful About Fiber Shards

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

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