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Fiber-Optic Technology

4. OVD Benefits

Fiber produced using the OVD process is purely synthetic, exhibits enhanced reliability, and allows for precise geometrical and optical consistency. The OVD process produces a very consistent “matched-clad” fiber.

OVD fibers are made of a core and cladding glass, each with slightly different compositions. The manufacturing process provides the relationship between these two glasses. A matched-clad, single-mode fiber design allows for a consistent fiber (see Figure 6).

Figure 6. Index Profile of a 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 joined, through splicing and connectorization, to form an optical system.

Depressed-Clad Fiber Profile

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

Figure 7. Index Profile of a Depressed-Clad Fiber Design

Depressed-clad fibers are made with two different cladding glasses that form an inner and an outer cladding region. The outer cladding consists of a glass from a substrate tube that is generally purchased from an outside supplier, as opposed to the OVD method, where all of the glass is made synthetically within the fiber manufacturer’s control. 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. Hence, the index of the glass adjacent to the core is depressed.

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 strength of fiber produced today. Fiber’s real, rather than theoretical, strength is 2 million pounds per square inch.

ABCs of Fiber Strength

The depth of inherent microscopic flaws on its surface determines the actual strength of optical fiber. 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 the chain. The flaws are distributed along the fiber length – the larger the flaw, the more distance between them along the fiber.

Many fiber manufacturers tensile-load, or proof-test, fibers after production. This process eliminates proof-test size flaws and larger, thereby ensuring that the flaws of most concern are removed and creating a minimum design strength for the fiber.

Life Expectancy

Fiber is designed and manufactured to provide a lifetime of service, 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 and reliability performance.

Bending Parameters

Optical fiber and cable are easy to install because it is lightweight, small in size, and flexible. Nevertheless, precautions are needed to avoid tight bends, which may cause loss of light or premature fiber failure.

Experience and testing show that bare fiber can be safely looped with bend diameters as small as two to three inches, depending on allowable optical loss. Splice trays and other fiber-handling equipment, such as racks, are designed to prevent fiber-installation errors such as this.

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