Why Network Designers Specify Particular Brands of Optical Fiber for Their Systems

People sometimes wonder why network designers specify particular brands of optical fiber for their systems, such as Corning SMF-28e. Usually, the designer wants to guarantee the highest optical performance in the network for his customer and insure fiber compatibility. Incidentally, all of the cables provided by FIS/TLC contain genuine Corning fiber.

On a large network installation, there may be several installation contractors working on the project. Using compatible fiber is essential for ensuring that the part of the network you install will work correctly with the rest of the system. Before you bid a job, make sure that your cable vendor can provide the correct fiber.

Singlemode Compatibility Issues

There are several domestic fiber manufacturers that produce "standard" SMF-28e singlemode fiber. These companies spec their fiber as being fully compatible with or equivalent to "standard" singlemode fiber. However, if you are adding "off-shore fiber" to an existing network, you may run into some compatibility issues. These include:

• Operating Wavelengths: Corning has enhanced their product to carry wavelengths from 1280-1700nm. However, some "off-shore" fiber still has an attenuation peak in the 1400-1500nm window. These fibers won't be compatible with many of the new Fiber-to-the-Home networks that need to operate at around 1490nm.
  
  
• Index of Refraction (IR): The IR of some "off-shore" fibers is slightly different from the IR of Corning fiber. This difference can cause additional optical loss in a system where the two different fibers meet. This can also cause confusing OTDR results. Under certain conditions, a slight difference of IR between two spliced fibers can produce a "gainer" on an OTDR trace. A "gainer" is an OTDR splice event that appears to gain optical power through the splice instead of showing a slight loss.
  
  
• Splicing: Fusion splicing an "off-shore" fiber to a Corning fiber may require you to set custom splicing parameters in order for your fusion splice machine to fuse properly.

Multimode Considerations

Compatibility problems are not confined to singlemode fiber. There is an issue with multimode fiber regarding "On-Center Laser Launch."

Newer Corning InfiniCor multimode fibers are designed to work in laserbased protocols, such as Gigabit Ethernet, without requiring mode-conditioning patchcords. These new fibers allow you to launch a laser directly into the center of the fiber core without inducing modal dispersion at fast transmission speeds. However, older multimode fibers, and some "off-shore" fibers, won't work in a gigabit network without some sort of off-center launch mode conditioning. If a non-compatible multimode fiber is placed directly into a gigabit network, the network's transmission rate will drop significantly.