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Matched-clad Fiber versus Depressed-clad Fiber (Video)

Matched-clad is the simplest single mode fiber design. Matched-clad fiber results in a constant refractive index profile throughout the cladding, or from the edge of the core.

This produces a fiber consistency that is especially important when different fibers need to be joined together in an exiting network. Matched-clad fiber typically results in slightly lower fiber attenuation and a larger mode-field diameter than its depressed-clad fiber counterpart.

Depressed-clad fiber is usually produced from the inside vapor deposition (IVD) or modified chemical vapor deposition (MOCVD) process. It is called depressed because the index of refraction of the inner cladding region is lower than the rest of the cladding. A depression occurs where the two indices of refraction do not match. This is where the glass tube and the deposited glass meet. This depression is caused by the addition of dopant chemicals to the cladding just prior to depositing the core material.

The type of cladding used affects bend losses. The smaller the mode-field diameter, the smaller the bending loss. This is true for both matched-clad and depressed-clad fibers.

However, for the same mode-field diameter (MFD), induced losses are greater in depressed-clad fiber for bend diameters greater than 50 mm. These types of bends are usually encountered in splice enclosures and in cables. Thus, one would expect to find that standard production depressed-clad fibers have a somewhat smaller mode-field diameter to achieve satisfactory cable-fiber performance. The smaller MFD is designed into a depressed-clad fiber.

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