An optical-fiber-based light source that generates light via third-harmonic (TH) generation is disclosed. The light source includes a pump light source that provides pump light having a fundamental mode of wavelength .lamda..sub.P. The light source also includes an optical fiber optically coupled to the pump light source. The optical fiber's core refractive index profile has (i) a central region with a refractive index value .DELTA..sub.1, (ii) a first annular region immediately surrounding the central region and having a refractive index value .DELTA..sub.2, and (iii) a second annular region immediately surrounding the first annular region and having a refractive index value .DELTA..sub.3. The optical fiber satisfies the conditions .DELTA..sub.2<.DELTA..sub.1 and .DELTA..sub.2<.DELTA..sub.3, and max{.DELTA..sub.1,.DELTA..sub.3}-.DELTA..sub.2>1.2%. The pump light produces TH-generated light having a higher-order mode of wavelength .lamda..sub.TH=(1/3).lamda..sub.P and that is outputted at the optical fiber output end. The pump light fundamental mode overlaps the TH-generated-light higher order mode over the length of the optical fiber to provide a conversion efficiency of 1% or greater.