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. Chaotic fibers 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. Spiral shape An almost-circular shape with small spiral deformation seems to be the most efficient for chaotic fibers. In such a fiber, the angular momentum of a ray increases at each reflection from the smooth wall, until the ray hits the "chunk", at which the spiral curve is broken (see figure at right). The core, placed in vicinity of this chunk, is intercepted more regularly by all the rays compared to other chaotic fibers. This behavior of rays has an analogy in wave optics. In the language of modes, all the modes have non-zero derivative in vicinity of the chunk, and cannot avoid the core if it is