results in a corresponding increase in the fiber length unfortunately for high power amplifiers this increase in length concomitantly increases the amount of undesired length dependent nonlinear effects for this reason it is desirable to guide the pump in the same spatial region as the signal to increase the overlap in conventional large mode area lma fibers with low brightness pumps this is typically not possible because the core has a relatively low numerical aperture na to reduce the number of guided modes and maintain a large mode area in such designs the pump guide e g cladding has different characteristics and therefore must be in a different spatial region that the signal guide e g core for hom propagation this need not be so because the mode is robustly guided and resistant to mode coupling and because it can be excited with high extinction the na of the guide can be large in fact the pump guide and the hom guide can be in the same region turning to the drawings fig4 a shows a cross sectional image 410 of a fiber or waveguide that can propagate hom signals the fiber comprises a central core not visible in the image 410 and an inner cladding 430 that surrounds the core radially exterior to the inner cladding 430 is a ring of air holes 440 which in turn is surrounded by a silica ring 450 while not shown a polymer layer surrounds the entire structure the refractive index profile of such a fiber is shown in fig3 the inner cladding 430 is supported by a thin silica web which defines the air holes the thickness.