4. OVD Benefits

The benefits of fiber produced by the patented OVD process include enhanced reliability, lower attenuation, and solid geometrical and optical consistency.

Matched-Clad Fiber Consistency

Fibers are made of a core and cladding glass, each with slightly different compositions. The manufacturing process determines the relationship between those two glasses. The OVD process produces matched-clad fiber, the single-mode fiber design that allows for the most consistent fiber (see Figure 6).

 

Figure 6. Matched-Clad Fiber Design

 

The OVD process produces well-controlled fiber profiles and geometry, both of which lead to a more consistent fiber. Fiber-to-fiber consistency is especially important when fibers from different manufacturing periods are united to form an optical system.

Depressed-Clad Fiber Profile

The inside vapor deposition (IVD) or modified chemical vapor deposition (MCVD) process produces what is called depressed-clad fiber because of the shape of its refractive index profile.

Figure 7. Depressed-Clad Fiber Design

 

Depressed-clad fibers are made with two different cladding glasses that form an inner and an outer cladding region. The inner cladding region adjacent to the fiber core has an index of refraction that is lower than that of pure silica, while the outer cladding has an index equal to that of pure silica. This depressed region is typically a design requirement as a result of using the IVD process.

Questions of Strength

One common misconception about optical fiber is that it must be fragile because it is made of glass. In fact, research, theoretical analysis, and practical experience prove that the opposite is true. While traditional bulk glass is brittle, the ultrapure glass of optical fibers exhibits both high tensile strength and extreme durability.

How strong is fiber? Figures like 600 or 800 thousand pounds per square inch are often cited, far more than copper's capability of 100 pounds per square inch. That figure refers to the ultimate tensile-breaking strength of fiber produced today. This is fiber's real, rather than theoretical, strength, which is 2 million pounds per square inch.

ABCs of Fiber Strength

The actual strength of optical fiber is determined by the depth of inherent microscopic flaws on its surface. These microscopic flaws exist in any fiber. As in a length of chain, the weakest link (or, in fiber's case, the deepest flaw) determines the ultimate strength of the entire length of fiber.

The OVD process offers a significant benefit in the area of fiber strength. Because OVD does not start with a bulk-glass or externally manufactured rube, every millimeter of the fiber is made from the ultrapure vapor-deposition process and contains fewer surface flaws.

Many fiber manufacturers tensile-load, or proof-test, fiber after production. This process eliminates the largest flaws, thereby ensuring a strength level to specification.

Life Expectancy

Fiber is designed and manufactured to provide a lifetime service of 20 years or more, provided it is cabled and installed according to recommended procedures. Life expectancy can be extrapolated from many tests. These test results, along with theoretical analysis, support the prediction of long service life. Environmental issues are also important to consider when evaluating a fiber's mechanical performance.

Bending Parameters

Optical fiber cable is easy to install because of its light weight, small size, and flexibility. Nevertheless, some people new to fiber express concern over the precautions required to avoid too-tight bends, which can cause loss of light or premature fiber breakage.

Experience and testing show that bare fiber can be safely looped with bend diameters as small as two inches, the recognized industry standard for minimum-bend diameter. Splice trays and other fiber-handling equipment, such as racks, are designed to prevent fiber-installation errors.