An optical waveguide amplifier assembly comprising: a signal source configured to provide signal light at a wavelength of between 1050nm and 1120nm; and a very large mode area, VLMA, active optical waveguide coupled to the signal source, the VLMA active optical waveguide comprising: a core configured to guide the signal light, wherein the core is doped with neodymium ions at a concentration of at least 0.1% by weight providing a net optical absorption of at least 3dB/m for pump light at a wavelength of 795 to 815nm or 883 to 887nm; a first cladding surrounding the core, configured to guide the pump light at the wavelength of 795 to 815nm or 883 to 887nm, wherein the first cladding has a refractive index structure to confine the signal light within the core, wherein in operation, the signal light is amplified by the neodymium ions when a population inversion is created in the neodymium ions upon absorption of the pump light; and a second cladding surrounding the first cladding, having an effective refractive index lower than an effective refractive index of the first cladding, configured to confine the pump light to the first cladding and the core; wherein the refractive index structure of the first cladding is such that a fundamental spatial mode of the signal light has an area of at least 500 square micrometers, wherein the VLMA active optical waveguide has a length of no greater than 2m, wherein a population inversion density of the neodymium ions satisfies a level such that a gain resulting in the amplification of the signal light is linear up to a value of 24 dB. a signal source configured to provide signal light at a wavelength of between 1050nm and 1120nm; and a very large mode area, VLMA, active optical waveguide coupled to the signal source, the VLMA active optical waveguide comprising: a core configured to guide the signal light, wherein the core is doped with neodymium ions at a concentration of at least 0.1% by weight providing a net optical absorption of at least 3dB/m for pump light at a wavelength of 795 to 815nm or 883 to 887nm; a first cladding surrounding the core, configured to guide the pump light at the wavelength of 795 to 815nm or 883 to 887nm, wherein the first cladding has a refractive index structure to confine the signal light within the core, wherein in operation, the signal light is amplified by the neodymium ions when a population inversion is created in the neodymium ions upon absorption of the pump light; and a second cladding surrounding the first cladding, having an effective refractive index lower than an effective refractive index of the first cladding, configured to confine the pump light to the first cladding and the core; wherein the refractive index structure of the first cladding is such that a fundamental spatial mode of the signal light has an area of at least 500 square micrometers, wherein the VLMA active optical waveguide has a length of no greater than 2m, wherein a population inversion density of the neodymium ions satisfies a level such that a gain resulting in the amplification of the signal light is linear up to a value of 24 dB. a core configured to guide the signal light, wherein the core is doped with neodymium ions at a concentration of at least 0.1% by weight providing a net optical absorption of at least 3dB/m for pump light at a wavelength of 795 to 815nm or 883 to 887nm; a first cladding surrounding the core, configured to guide the pump light at the wavelength of 795 to 815nm or 883 to 887nm, wherein the first cladding has a refractive index structure to confine the signal light within the core, wherein in operation, the signal light is amplified by the neodymium ions when a population inversion is created in the neodymium ions upon absorption of the pump light; and a second cladding surrounding the first cladding, having an effective refractive index lower than an effective refractive index of the first cladding, configured to confine the pump light to the first cladding and the core; wherein the refractive index structure of the first cladding is such that a fundamental spatial mode of the signal light has an area of at least 500 square micrometers, wherein the VLMA active optical waveguide has a length of no greater than 2m, wherein a population inversion density of the neodymium ions satisfies a level such that a gain resulting in the amplification of the signal light is linear up to a value of 24 dB.