The Design of Fiber

Core and Cladding

An optical fiber consists of two different types of highly pure, solid glass to form the core and cladding. A protective acrylate coating (see Figure 1) then surrounds the cladding. In some cases, the protective coating may be a dual layer.

Figure 1. Core and Cladding

 

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 and installation/termination processes.

Single-Mode and Multimode Fibers

There are two general categories of optical fiber: single mode and multimode (see Figure 2).

 

 

Figure 2. Single-Mode and Multimode Fibers

Multimode fiber was the first type of fiber to be commercialized. It has a much larger core than single-mode fiber, allowing hundreds of rays or 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 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. Single-mode fibers are designed to maintain the integrity of each optical signal over longer distances, allowing more information to be transmitted.

Its tremendous information-carrying capacity and low intrinsic loss have made single-mode fiber the ideal transmission medium for a multitude of applications. Multimode fiber is used primarily in systems with short transmission distances (under 2 km), such as premises communications and private data networks.

Optical Fiber Sizes

The international standard outer cladding diameter of most single-mode optical fibers is 125 microns (µm) for the glass and 245 µm 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 µm in diameter. Multimode fibers, with core sizes of 50 to 100 µm in diameter, are used for specific applications, such as short-distance transmission of data. With its greater information-carrying capacity and lower intrinsic loss, single-mode fiber is typically used for longer-distance and higher-bandwidth applications. (see Figure 3).

 

Figure 3. Optical Fiber Sizes

3. OVD Process

Optical fiber manufacturing consists of three primary steps: laydown, consolidation, and draw.

Laydown

In the laydown step, a soot preform is made from ultrapure vapors as they travel through a traversing burner and react in the flame to form fine soot particles of silica and germania (see Figure 4).

Figure 4. OVD Laydown Process

 

The OVD process is distinguished by the method of depositing the soot. These particles are deposited on the surface of a rotating target rod. The core material is deposited first, followed by the pure silica cladding. As both core and cladding raw materials are vapor-deposited, the entire preform is extremely pure.

Consolidation

When deposition is complete, the target rod is removed from the center of the porous preform, and the preform is placed into a consolidation furnace. During the consolidation process, the water vapor is removed from the preform. This high-temperature consolidation step sinters the preform into a solid, dense, and transparent glass. The hole left by the target rod disappears completely; there is no hole in the finished fiber.

The Draw

The finished glass preform is placed in a draw tower and drawn into a continuous strand of glass fiber (see Figure 5).

Figure 5. Optical Fiber Drawing Process

First, the glass blank is lowered into the top of the draw furnace. The tip of the blank is heated until a piece of molten glass, called a gob, begins to fall from the blank—much like hot taffy. It pulls behind it a thin strand of glass, the beginning of an optical fiber.

The gob is cut off, and the fine fiber strand is threaded into a tractor assembly. Then, as the diameter is monitored, the assembly speeds up or slows down to control the size of the fiber's diameter precisely.

The fiber progresses through a laser-based, on-line monitor that measures the diameter hundreds of times per second to ensure specified outside diameter. Next, the primary and secondary coatings are applied and cured, using ultraviolet lamps.

At the bottom of the draw, the fiber is wound on spools for further processing. Fiber on these spools is proof-tested to ensure the strength of each fiber, cut to length, and measured for performance of relevant optical and geometrical parameters. With a unique identification number that encodes all relevant manufacturing data (including raw materials and manufacturing equipment), each fiber reel is placed into protective shipping containers. Finally, the fiber is prepared for shipment to customers worldwide.