A double-clad, gain-producing optical fiber comprising: a core region (10.1) having a longitudinal axis (10.5), a cladding region (10.2, 10.3, 10.4) surrounding said core region (10.1), said core and cladding regions (10.1, 10.2, 10.3, 10.4) configured to support and guide the propagation of signal light between 1030-1180 nm in a fundamental transverse mode in the direction of said axis (10.5), said cladding region (10.2, 10.3, 10.4) including a trench region (10.2) surrounding said core region (10.1), an inner cladding region (10.3) surrounding said trench region (10.2), and an outer cladding region (10.4) surrounding said inner cladding region (10.3), said outer cladding region (10.4) having a refractive index below that of said inner cladding region (10.3), said inner cladding region (10.3) having a refractive index between that of said outer cladding region (10.4) and said core region (10.1), and said trench region (10.2) having a refractive index below that of said inner cladding region (10.3), at least said core region (10.1) including at least one gain-producing species that provides gain to said signal light when suitable pumping energy is applied to said fiber, and said core and cladding regions (10.1, 10.2, 10.3, 10.4) being configured so that said fundamental mode is guided predominantly in said core region (10.1) and the difference in refractive index Δn tr between said trench region (10.2) and said inner cladding region (10.3) is, in absolute value, less than the difference in refractive index Δn core between said core region (10.1) and said inner cladding region (10.3), wherein at least said core region (10.1) is doped with Yb, wherein the radius of said core region (10.1) is approximately 4.6 to 8.7 μm, the difference in refractive index Δn core between said core region (10.1) and said inner cladding region (10.3) is approximately 1.1 to 2.8 x 10 -3, the difference in refractive index Δn tr between said trench region (10.2) and said inner cladding region (10.3) is approximately -0.15 to -0.8 x 10 -3, and the minimum width W tr of said trench region (10.2) is approximately 2.5 to 10.0 μm, yielding a mode field diameter of approximately 10 to 16μm for signal light between 1030-1180 nm. a core region (10.1) having a longitudinal axis (10.5), a cladding region (10.2, 10.3, 10.4) surrounding said core region (10.1), said core and cladding regions (10.1, 10.2, 10.3, 10.4) configured to support and guide the propagation of signal light between 1030-1180 nm in a fundamental transverse mode in the direction of said axis (10.5), said cladding region (10.2, 10.3, 10.4) including a trench region (10.2) surrounding said core region (10.1), an inner cladding region (10.3) surrounding said trench region (10.2), and an outer cladding region (10.4) surrounding said inner cladding region (10.3), said outer cladding region (10.4) having a refractive index below that of said inner cladding region (10.3), said inner cladding region (10.3) having a refractive index between that of said outer cladding region (10.4) and said core region (10.1), and said trench region (10.2) having a refractive index below that of said inner cladding region (10.3), at least said core region (10.1) including at least one gain-producing species that provides gain to said signal light when suitable pumping energy is applied to said fiber, and said core and cladding regions (10.1, 10.2, 10.3, 10.4) being configured so that said fundamental mode is guided predominantly in said core region (10.1) and the difference in refractive index Δn tr between said trench region (10.2) and said inner cladding region (10.3) is, in absolute value, less than the difference in refractive index Δn core between said core region (10.1) and said inner cladding region (10.3), wherein at least said core region (10.1) is doped with Yb, wherein the radius of said core region (10.1) is approximately 4.6 to 8.7 μm, the difference in refractive index Δn core between said core region (10.1) and said inner cladding region (10.3) is approximately 1.1 to 2.8 x 10 -3, the difference in refractive index Δn tr between said trench region (10.2) and said inner cladding region (10.3) is approximately -0.15 to -0.8 x 10 -3, and the minimum width W tr of said trench region (10.2) is approximately 2.5 to 10.0 μm, yielding a mode field diameter of approximately 10 to 16μm for signal light between 1030-1180 nm.