estimate above applies in this optimistic case. In particular, the higher the step in refractive index from inner to outer cladding, the better-confined the pump is. As a limiting case, the index step can be of order of two, from glass to air. 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. Alternative structures 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. Notes and references Optical fiber