Dmitrii Kouznetsov and Jerome V.Moloney

“ The laws of conservation established for the paraxial equation of diffraction give certain integral relations for the derivatives of modes at the boundary. Such relations are formulated as theorems. These theorems show ways to non-traditional design of single-mode fiber amplifiers with a multimode pump. In particular, the conservation of momentum can be applied to the design of the slab-pumped fiber amplifier. The conservation of angular momentum predicts high efficiency of coupling of pump into a doped core embedded in aspiral-shaped double-clad fiber. ”

“ It was shown that a small asymmetric distortion of an otherwise circular cladding significantly improves the efficiency of coupling of the incoherent light of the pumped cladding into the core. Such an improvement is comparable to or even greater than the improvement due to a strong double-cut of the cladding [6-9] . The spiral is not a closed curve so, the spiral-shaped boundary of the region must have a strong local deformation somewhere. Overlap of modes with the relatively small core is especially good, when this core is located close to this region of deformation. ”

“ The core provides for discrimination of modes of the signal; the core cannot be too large, unless the tolerance interval of the step of index of refraction becomes very small. The design of inner cladding of the double-clad fiber amplifier should provide the effective coupling of most of pump into relatively small core. The coupling of the pump cladding modes to the core is especially poor in the case of a fiber with circular symmetry [1-3] ; most of modes are unusable: they avoid the core. ”