Double-clad fiber
From Wikipedia, the free encyclopedia
In fiber optics, a double-clad fiber (or doubly clad fiber) is a single-mode optical fiber that has two claddings. This type of fiber is also called depressed-inner-cladding fiber and W-profile fiber (from the fact that a symmetrical plot of its refractive index profile superficially resembles the letter W).
Each cladding of a double-clad fiber has a refractive index that is lower than that of the core. Of the two claddings, inner and outer, the inner cladding has the lower refractive index. A double-clad fiber has the advantage of very low microbending losses. It also has two zero-dispersion points, and low dispersion over a much wider wavelength range than a singly-clad fiber.
This type of fiber is often used for fiber lasers and optical amplifiers. In this case, the core is doped and acts as the gain medium, and the inner cladding acts to carry the pump light that maintains the population inversion in the core.
The shape of the cladding can be important. Circular symmetry in a double-clad fiber seems to be the worst solution for a fiber laser; in this case, many modes of the light through the cladding miss the core and hence cannot be used to pump it. So-called "chaotic fibers" have more complicated shape and provide more uniform distribution of intensity of most of cladding modes, allowing efficient use of pump. Designers of the double-clad fiber lasers need to find a reasonable compromise between simple shape and efficient pumping of the core.
The efficiency of absorption of pumping energy in the fiber is an important parameter of a double-clad fiber laser. In many cases, this efficiency can be approximated as
where
- S is area of cross-section of the cladding
- r is radius of the core (which is supposed to be circular)
- α is absorption of pump in the core
- L is length of the double-clad fiber and
- F is a dimension-less adjusting parameter, which is sometimes called the "filling factor"; 0 < F < 1. The filling factor may depend on both initial distribution of the pump launched, the shape of the cladding and the position of core in it. In particular, for chaotic fibers, F approaches unity. The numerical analysis with propagation of waves, expansion by modes or the geometric-optical ray tracing can be used in order to estimate F.
Double-clad fibers can also be used for the compensation of chromatic dispersion in optical communications and other applications.
