Двухслойные оптические волокна: структура, типы и применение.

Double clad fiber (DCF) is a class of optical fiber with a structure consisting of three layers of optical material instead of the usual two. The inner most layer is called the core. It is surrounded by the inner cladding, which is surrounded by the outer cladding. The three layers are made of materials with different refractive indices. There are two different kinds of double clad fibers. The first was developed early in optical fiber history with the purpose of engineering the dispersion of optical fibers. In these fibers, the core carries the majority of the light, and the inner and outer cladding alter the waveguide dispersion of the core guided signal. The second kind of fiber was developed in the late 1980s for use with high power fiber amplifiers and fiber lasers. In these fibers, the core is doped with active dopant material; it both guides and amplifies the signal light. The inner cladding and core together guide the pump light, which provides the energy needed to allow amplification in the core. In these fibers, the core has the highest refractive index and the outer cladding has the lowest. In most cases the outer cladding is made of a polymer material rather than glass.

In double clad fiber for dispersion compensation, the inner cladding layer has lower refractive index than the outer layer. 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). This type of 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 standard singly clad fiber. Since the dispersion of such double clad fibers can be engineered to a great extent, these fibers can be used for the compensation of chromatic dispersion in optical communications and other applications.

In modern double clad fibers for high power fiber amplifiers and lasers, the inner cladding has a higher refractive index than the outer cladding. This enables the inner cladding to guide light by total internal reflection in the same way the core does, but for a different range of wavelengths. This allows diode lasers, which have high power but low radiance, to be used as the optical pump source. The pump light can be easily coupled into the large inner cladding, and propagates through the inner cladding while the signal propagates in the smaller core. The doped core gradually absorbs the cladding light as it propagates, driving the amplification process. This pumping scheme is often called cladding pumping, which is an alternative to the conventional core pumping, in which the pump light is coupled into the small core. The invention of cladding pumping by a Polaroid fiber research team (H. Po, et al.) revolutionized the design of fiber amplifiers and lasers. Using this method, modern fiber lasers can produce continuous power up to several kilowatts, while the signal light in the core maintains near diffraction limited beam quality. The shape of the cladding is very important, especially when the core diameter is small compared to the size of the inner cladding. 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 in the cladding miss the core and hence cannot be used to pump it. In the language of geometrical optics, most of the rays of the pump light do not pass through the core, and hence cannot pump it. Ray tracing, simulations of the paraxial propagation and mode analysis give similar results.

In general, modes of a waveguide have "scars", which correspond to the classical trajectories. The scars may avoid the core, then
the mode is not coupled, and it is vain to excite such a mode in the double clad fiber amplifier. The scars can be distributed more or less uniformly in
so called chaotic fibers have more complicated cross sectional shape and provide more uniform distribution of intensity in the inner cladding, allowing efficient use of the pump light. However, the scarring takes place even in chaotic fibers.

[[Image:ModesSpiralDCF4. jpg|400px|thumb|right|Modes of spiral shaped double clad fiber. Although this theorem is formulated for the core less domain, it prohibits the modes avoiding the core. A mode avoiding the core, then, should be similar to that of the core less domain. Stochastic optimization of the cladding shape confirms that an almost circular spiral realizes the best coupling of pump into the core.

Tapered double clad fiber (T DCF) is a fiber whose outer and inner claddings and core diameters vary smoothly with length. The core at narrow side of T DCF supports propagation of the fundamental mode only, whereas at the wide side the core is able to guide many modes. However, it was shown experimentally that light launched into the narrow end of a T DCF propagates into the wide core without any changes of mode content. As a result, at the wide (substantially multimode) end of T DCF light propagates only in the lowest order mode with excellent beam quality. Thus, tapered fiber is a unique and easy way to implement fundamental mode regime propagation (and amplification) in a multimode fiber.

[[Image:FillingFactor. png|thumb|Estimates of the pump efficiency in a double clad fiber with (blue) and (red), discussed in The estimate with filling factor gives an estimate of how short an efficient double clad fiber laser can be, due to reduction in size of the inner cladding.

For good cladding shapes the filling factor , defined above, approaches unity; the following enhancement is possible at various kinds of tapering of the cladding; non conventional shapes of such cladding are suggested. Planar waveguides with an active gain medium take an intermediate position between conventional solid state lasers and double clad fiber lasers. The planar waveguide may confine a multi mode pump and a high quality signal beam, allowing efficient coupling of the pump, and diffraction limited output.