Thinking Small: Fiber Optic Microcables

Fiber optic microcables

Microcables are becoming very popular, and it’s important to understand their purpose, applications and installation.

These smaller fiber optic cables are designed for applications where large numbers of fibers are required and space is at a premium. In particular, microcables have been used extensively in data centers and metropolitan networks.

Large data centers need many fiber connections; the biggest data centers will have 100,000 to 1 million interconnects. If the data center is using multimode fiber—considered obsolete by most large data center operators—a single server/switch rack may need 2,000 fibers for interconnection. That’s because multimode fiber lacks sufficient bandwidth, so each connection requires 8–20 fibers depending on the connection speed. Single-mode fiber needs only two fibers per connection since the multiplexing is done by wavelengths on each fiber, but all of those connections still mean many fibers.

Another application that requires many fibers is metropolitan networking. Within cities today, we find fiber-intensive applications, such as fiber to the home, small cells, smart streetlights, metro Wi-Fi systems, and surveillance cameras. In the future, we will find vehicle to infrastructure, vehicle to vehicle and the internet of things. Cities are notorious for having inadequate conduit space and an aversion to having the streets dug up for each new application.

The secret, of course, is to make smaller cables with more fibers. To do this, manufacturers took advantage of a relatively new development in fiber optics: bend-insensitive fiber. Since this fiber is much less sensitive to the stress caused by cabling, fibers can be packed much more densely into the cable. In addition, by reducing the fiber coating from 250 microns to less than 200 microns, a bundle of fibers takes up much less space.

The differences between conventional and microcables are substantial. A traditional 144-fiber loose-tube cable is around 15–16 millimeters (mm) in diameter while a comparable microcable is only about 8 mm in diameter—half the diameter and about one-third the weight of the conventional cable. The smaller size allows for larger fiber counts.

I’ve seen special indoor loose-tube cables with 1,728 fibers that are less than 1 inch in diameter. Several manufacturers have cables in their catalogs with more than 3,000 fibers, and I heard recently about cables with more than 8,000 fibers for use in data centers.

As with regular fiber optic cables, some sizes seem to hit a “sweet spot.” With microcables, it seems to be the 288-fiber loose-tube cable. It’s less than 10 mm in diameter, and the cable is installed in a special 12-mm duct called, appropriately, a microduct, by blowing the cable into the duct.

Microcables are available for both premises and outside plant installations. Indoors, in areas such as data centers, they can be placed in cable trays like regular fiber optic cables. Outdoors or indoors, their small size allows a different installation technique where the cable is “blown” into micro ducts, plastic tubes much smaller than conventional fiber innerducts or conduits.

The cable is not really blown into the duct but floated on air to reduce friction then pushed into the duct. The cable is not exactly blown into the duct like it is with blown fiber. Blown fiber, which has been around since the 1980s, literally blows special fibers into small plastic ducts instead of using cables.

Cable is too heavy to blow into ducts, but a high-pressure air stream blowing into the duct reduces the friction on the cable to near zero (like the puck on an air hockey table). The cable can be pushed into the duct by the blowing machine for long lengths. Depending on the cable, duct and blowing machine, a distance of up to 2 kilometers is possible.

Microducts are available in many designs to fit different applications. There may be only one tube or six tubes or more. The multiple tube designs can be in a round or linear form. The round duct can be installed by pulling into conduit like regular innerduct or a single cable. The linear duct can be installed by microtrenching—where a shallow, narrow groove is sawed in roadways, sidewalks or even lawns—probably the installation method that allows for the least disruption in urban environments.

In next month’s column, we’ll discuss microtrenching and cable installation methods in general.

About the Author

Jim Hayes

Fiber Optics Columnist and Contributing Editor
Jim Hayes is a VDV writer and trainer and the president of The Fiber Optic Association. Find him at www.JimHayes.com .

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