Fiber-Optic Technology
The Design of Fiber Core and Cladding
An optical fiber consists of two different types of highly pure, solid glass, composed to form the core and cladding. A protective acrylate coating (see Figure 1) then surrounds the cladding. In most cases, the protective coating is a dual layer composition.
A protective coating is applied to the glass fiber as the final step in the manufacturing process. This coating protects the glass from dust and scratches that can affect fiber strength. This protective coating can be comprised of two layers: a soft inner layer that cushions the fiber and allows the coating to be stripped from the glass mechanically and a harder outer layer that protects the fiber during handling, particularly the cabling, installation, and termination processes.
Single-Mode and Multimode Fibers
There are two general categories of optical fiber: single-mode and multimode (see Figure 2).
Multimode fiber was the first type of fiber to be commercialized. It has a much larger core than single-mode fiber, allowing hundreds of modes of light to propagate through the fiber simultaneously. Additionally, the larger core diameter of multimode fiber facilitates the use of lower-cost optical transmitters (such as light emitting diodes [LEDs] or vertical cavity surface emitting lasers [VCSELs]) and connectors.
Single-mode fiber, on the other hand, has a much smaller core that allows only one mode of light at a time to propagate through the core. While it might appear that multimode fibers have higher capacity, in fact the opposite is true. Singlemode fibers are designed to maintain spatial and spectral integrity of each optical signal over longer distances, allowing more information to be transmitted.
Optical Fiber Sizes
The international standard for outer cladding diameter of most single-mode optical fibers is 125 microns (ìm) for the glass and 245 ìm for the coating. This standard is important because it ensures compatibility among connectors, splices, and tools used throughout the industry.
Standard single-mode fibers are manufactured with a small core size, approximately 8 to 10 ìm in diameter. Multimode fibers have core sizes of 50 to 62.5 ìm in diameter.
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