A Double Clad Fiber(DCF) is a glass fiber consisting of a core glass and two cladding glasses: The inner clad glass, which encloses the core glass, and the outer clad glass, which encloses the inner clad glass.
All three glasses have different refractive indices. The core glass corresponds to that of the monomode fiber. The inner clad glass has a lower refractive index compared to the outer clad glass.
Double Clad Fibers are dependent on the refractive indices of the two clad glasses for their transmission properties. They have two wavelengths without dispersion and lower dispersion values over the entire wavelength range compared to single mode fibers. They also have low losses at tight bends.
Fiber amplifiers using neodymium or ytterbium doped double clad glass fibers can amplify the output power of infrared lasers to powers of several kilowatts. They are ultrafast amplifiers with large amplifier bandwidth that can amplify ultrashort laser pulses.