The first cables introduced were 1728 fibers, then 3456 fibers and finally 6912 fibers. To make these cables, manufacturers introduced several innovations in fiber and ribbon technology. First, to pack such a high density of fibers required using bend-insensitive fiber to prevent losses due to stress on the fibers. Then the buffer coating was reduced from 250 microns to 200 microns or even less. Since ribbons were desirable from the standpoint of splicing cables with so many fibers, but normal "hard" ribbons limited the design of cables, manufacturers went to a ribbon design that was flexible, allowing ribbons to be rolled up to fit more ribbons into the cable design. Some use construction similar to loose tube cables, others use ribbon designs including some slotted core, and some are new and unique.


OFS Rollable Ribbon

 

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 with splicer manufacturers.
• 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.

High Fiber Count Cable Designs

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

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Ribbon Splicing For High Fiber Count Cables

High Fiber Count Cables - Installation

Continuing our ongoing research on high fiber count cables,FOA was 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

(C)2020, The Fiber Optic Association, Inc.